Production and Recovery of Lactic Acid for Polylactide - An Overview

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Production and Recovery of Lactic Acid for Polylactide—An OverviewA. N. Vaidya a; R. A. Pandey a; S. Mudliar a; M. Suresh Kumar a; T. Chakrabarti a; S. Devotta a

a National Environmental Engineering Research Institute, Nehru Marg Nagpur, India

Online Publication Date: 01 September 2005

To cite this Article Vaidya, A. N., Pandey, R. A., Mudliar, S., Kumar, M. Suresh, Chakrabarti, T. and Devotta, S.(2005)'Production andRecovery of Lactic Acid for Polylactide—An Overview',Critical Reviews in Environmental Science and Technology,35:5,429 — 467

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Critical Reviews in Environmental Science and Technology, 35:429–467, 2005Copyright © Taylor & Francis Inc.ISSN: 1064-3389 print / 1547-6537 onlineDOI: 10.1080/10643380590966181

Production and Recovery of Lactic Acidfor Polylactide—An Overview

A. N. VAIDYA, R. A. PANDEY, S. MUDLIAR, M. SURESH KUMAR,T. CHAKRABARTI, and S. DEVOTTA

National Environmental Engineering Research Institute, Nehru Marg Nagpur, India

In the recent past the ultimate disposability of synthetic plastics hasbeen a greater environmental concern, and it has triggered theR&D efforts in the designing of material with an environmentallyfriendly life cycle by integrating material design concepts with ul-timate disposability, resource utilization, and conservation. Tra-ditionally, all plastics have been manufactured from nonrenew-able petroleum resources, and these plastics are nonbiodegradable.Conventional disposal methods include incineration and securedlandfill, which are associated with many environmental problems,such as production of dioxins. The continued depletion of landfillspace and problems associated with incineration have led to thedevelopment of biodegradable plastics such as polylactides (PLA),which are manufactured from lactic acid that in turn is producedfrom starch. Although production processes for lactic acid and PLAare well known, very few processes have been commercialized andstill the cost of PLA is not competitive with synthetic plastics. Thecrux of the PLA production technology is the fermentative produc-tion of optically active lactic acid and its recovery. Many processesare reported in the literature and through patents for the recoveryof optically active lactic acid and still offer an extensive scope forresearch and development. This article critically reviews the pro-duction and recovery processes for lactic acid and PLA production.

KEY WORDS: biodegradable plastic, fermentation, lactic acid, lifecycle, PLA, recovery

Address correspondence to A. N. Vaidya, National Environmental Engineering ResearchInstitute, Nehru Marg Nagpur 440 020, India. E-mail: [email protected]

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430 A. N. Vaidya et al.

I. INTRODUCTION

Growing environmental concerns, regulations, and social demands through-out the world have triggered a paradigm shift in the industry to develop theprocesses and materials compatible with the environment. This necessitatesattention to designing materials with an environmentally friendly life cycle,integrating material design concepts with ultimate disposability, resource uti-lization, and conservation. In the recent past the ultimate disposability ofsynthetic plastics has been a great environmental concern. Traditionally, allplastics are manufactured using crude oil as the very basic material, andthis is a nonrenewable resource. Manufacturing of conventional plastics inpetrochemical industries consumes more than 270 million tonnes of oil andgas every year worldwide. Known global reserves of crude oil are expectedto run dry in the near future, but the economic impact of their depletioncould be realized much sooner. Therefore, an emergent need has been re-alized for shifting the policies toward minimizing the resource consumption,and employing alternative renewable resources for fuels and chemicals in-cluding synthetic plastics. As a result, research in the recent past is directedtoward the development of alternative processes for the production of plas-tics utilizing renewable raw material sources and thereby conserving nonre-newable resources and minimizing the environmental deterioration causedby the production and disposal of synthetic plastics. Most of the syntheticplastics that are in wide use are either nonbiodegradable or partially degrad-able. The existing disposal methods, therefore, include secured landfill andincineration. The disposal of plastic materials by incineration generates airpollutants/wastes of environmental concern, namely, dioxins, toxic hydrocar-bons, and so on. The continued depletion of landfill space is posing severerestriction on the secured landfill disposal of plastic. Therefore, developing abiodegradable plastic is viewed as an environmentally and ecologically soundapproach. Plant materials, such as corn, potato peels, sago, and others, oreven waste from food processing products can also be used for the pro-duction of biodegradable plastics. These raw materials for the biodegradablepolymers contain starch-based constituents, which are fermented to producethe lactic acid. The lactic acid is further converted into lactide, which serves asa monomer for production of polylactide (PLA), a prominent biodegradablepolymer. The production of PLA encompasses:

� Utilization of renewable resources that leads to conservation of thenonrenewable resource base.

� Production of biodegradable materials that follow the concept of “cradleto grave.”

The primary focus of this article is to reveal the present statusof lactic acid and PLA production and its use. The article covers the


Production and Recovery of Lactic Acid for Polylactide 431

history of development of PLA: types of raw materials used for produc-tion of PLA via lactic acid, production and recovery processes available,environmental benefits derived from the use of PLA, and probable im-pacts of PLA on the environment. Based on the information available,the future research needs in different areas pertaining to PLA are alsoenvisaged.

II. PROCESSES FOR PRODUCTION OF LACTIC ACID

A. Chemical Processes for Lactic Acid Production

General properties of lactic acid are given in Table 1. Lactic acidcan be prepared by a variety of chemical reactions. Many of thesereactions are of scientific interest only, and very few have beencommercialized. These reactions can be classified broadly in sixcategories:

� Hydrolysis of lactic acid derivatives, for example, esters or nitriles.� Hydrolysis of other 2-substituted propionic acids.� Decarboxylation of certain derivatives of 2-methylmalonic acid.� Reduction.� Oxidation.� Rearrangement and disproportionation.

TABLE 1. Properties of Lactic Acid

Product identificationCAS no. 50-21-5, 79-33-4 (L), 10326-41-7 (D)EINECS no. 200-018-0Formula CH3CH(OH)COOHMol. wt. 90.08H.S. code 2918.11Toxicity Oral rat LD50: 3543 mg/kgSynonyms 2-hydroxypropanoic acid;

1-hydroxyethanecarboxylic acid;ethylidenelactic acid;alpha-hydroxypropionic acid

Physical and chemical properties (99%)Physical State Colorless to slightly yellow, syrupy liquidMelting point 17◦CBoiling point 122◦CSpecific gravity 1.2Solubility in water MiscibleNFPA ratings Health 3, Flammability 1, Reactivity 1Flash point 112◦CStability Stable under ordinary conditions



Out of these, only the synthesis from lactic acid derivatives has beencommercialized.

Lactic acid can be liberated from most of its derivatives by suitable treat-ment, however, there is no practical advantage, as lactic acid is the rawmaterial for these derivatives. Lactonitrile and lactic acid nitrate are the im-portant exceptions, as these are not produced from lactic acid. Lactonitrileis produced from acetaldehyde and hydrogen cyanide as an intermediate inthe production of acrylonitrile, and it is further hydrolyzed to lactic acid asfollows:

CH3 CHOAcetaldehyde

+ HCNHCl−→ CH3 CHOH CN

lactonitrile(1)

CH3 CHOH CN + 2H2OHCl−→ CH3 CHOH COONH4

Ammonium lactate(2)

→ CH3CHOHCOOHlactic acid

Unreacted or waste lactonitrile may be used as a raw material. In 1963,Monsanto Chemical Company installed the first commercial the plant inUnited States that produced technical-grade lactic acid (55 to 85% w/w).In another route, propene is converted into α-nitropropionic acid by nitricacid in the presence of oxygen. α-Nitropropionic is further hydrolyzed tolactic acid. Reactions involved can be expressed as:

CH3CH2CH3 + HNO3 + O2 → CH3CH (NO2) COOHα-Nitropropionic acid

(3)

CH3CH (NO2) COOH + H2O → CH3CHOHCOOHLactic acid

(4)

The commercial details of these reactions are not available.

B. Fermentation Processes for Lactic Acid Production

Industrial production of lactic acid, especially where pure optical isomers arerequired, is presently carried out predominantly by fermentation process. Theproduction process can be divided into:

� Actual production of lactic acid by fermentation of a carbohydrate source.� Downstream processing of the fermentation broth to obtain pure lactic

acid.



Several microorganisms have been isolated and used in the productionof lactic acid from the genera Lactobacillus, Streptococcus, and Pediococcus.These organisms exhibit maximum productivity only within a very narrowpH range. The fermentation processes are associated with the production oflactic acid as well as other organic acids, which lower the pH of the fermen-tation broth continually. Therefore, it is necessary during fermentation notonly to maintain optimum temperature but also to maintain a pH at constantvalue, preferably in the range of 5.5 to 6.5. This is achieved by addition ofbases such as hydroxides or carbonates of alkali or alkaline earth metals, orammonia. Thus, the main constituent of fermentation broth is a salt of lacticacid, and not pure lactic acid apart from salts of other organic acids, unre-acted materials, nutrients, microorganisms, and so on. Such a fermentationbroth cannot be utilized for the further use, and therefore additional process-ing steps are required to obtain pure lactic acid. The recent research workin lactic acid production comprises mainly downstream processing, and var-ious downstream processes have been developed and invariably patented(Vickroy, 1985; Chahal, 1990).

I. RAW MATERIALS FOR LACTIC ACID PRODUCTION

Raw materials for lactic acid production should meet certain purity require-ments besides being available at reasonable prices, since these are criticalfor the further purification of the produced lactic acid. The choice of rawmaterials largely depends on the intended application and the respectivecosts of product purification. Various readily available mono- and disaccha-ride materials are traditional substrates for lactic acid manufacturing. Theseare

� Glucose (dextrose) and glucose syrups of varying qualities as end productsof starch conversion processes applying enzymes such as glucoamylases.

� Maltose as a product of specific enzymatic starch conversion applying α-amylases and β-amylases from barley malt or other sources.

� Sucrose as end product, intermediate products (syrups, juices), and by-product (molasses) of beet and cane sugar production.

� Lactose as a constituent of whey as the natural substrate of most lactic acidbacteria.

Starch-based raw materials can be obtained from corn (maize), potatoes,wheat, tapioca (cassava), and other plants. Starch cannot be utilized by com-mon lactic acid bacteria, except Lactobacillus amylophilus and L. amylovorus.Starch hydrolysis can be performed by either chemical or enzymatic meth-ods, with chemical methods being abandoned for some time. Since maltoseis the main product of enzymatic hydrolysis, only a limited number of or-ganisms are suitable of the fermentation process. Starch liquefaction and



saccharification are achieved, at present by the combined action of differentα-amylases of bacterial origin and of fungal glucoamylases.

Sucrose-derived raw materials are also suitable as raw materials for lac-tic acid fermentation, and molasses is the cheapest raw material. Most ofthe known homofermentative organisms of Lactobacillus can utilize sucrose,with L. bulgaricus and L. helveticus being notable exceptions.

Lactose poses a limitation as a raw material due to its low concentrationin the most readily available whey, and in addition expensive purificationprocedures have impeded the successful utilization of whey. The fact thatwhey represents a considerable environment load has stimulated the de-velopment of modern technologies of whey concentration and fractionation,such as ultrafiltration and electrodialysis. These, in turn, have exerted a strongstimulus on fermentation research laboratories, as may be seen from the re-cent literature (Roy et al., 1986; Boyaval et al., 1987; European Patent 265409,1988; Leh and Charles, 1989a, 1989b; Kulozik et al., 1992; Chiarini et al., 1992;Boergardts et al., 1994).

2. MICROORGANISMS USED IN LACTIC ACID PRODUCTION

Substantial information related to the genetics of lactic acid bacteria andpossible applications of genetic engineering is available (Devos and Vaughan,1994; Fitzsimons et al., 1994; Wel et al., 1995). Although all prerequisites fordeveloping a recombinant DNA system have been thoroughly investigated,industrial production of lactic acid apparently still relies on producer strainsthat are selected empirically, on the basis of raw material.

Lactic acid bacteria utilize either the well-known EMP pathway of glu-cose metabolism to produce lactic acid as the major end product, or usepathways of pentose metabolism resulting in the formation of lactic acid plusother products such as acetic acid, ethanol, and CO2. In 1919, Orla-jensenproposed the terms “homofermentative” and “heterofermentative” for the re-spective organisms. The industrial production of lactic acid is mostly carriedout by homofermentative bacteria; however, the homofermentative behaviorof certain strains may change to heterofermentative under the growth-limitinglevels of carbon source (De Vries et al., 1970). The temperature range for op-timal growth of mesophilic lactic acid bacteria is from 28 to 45◦C and that ofthermophilic lactic acid bacteria is 45–62◦C. Organisms operating at highertemperatures are preferred because of reduced risk of contamination duringfermentation; however, most of the microbial strains are sensitive to pH. Ac-cording to Buchta (1994), lactic acid fermentation is strongly inhibited at pH5 and ceases at pH values below 4.5. Lactic acid bacteria differ in their abilityto produce D-(−)-, L-(+)-, and DL-lactic acid, depending upon the presenceof respective lactate dehydrogenases and racemases (Garvie, 1980; Teuber,1993).



Lactic acid bacteria have complex nutrient requirements. These com-prise many of the known vitamins, amino acids, and even small peptides(Barton-Wright, 1952). Therefore the fermentation media needed for lacticacid production is quite complex and expensive, too. In order to createreproducible conditions, it is recommended that one maintain cultures ofproducer organisms on standard culture media, such as MRS medium, whichis available from several sources (Merck 106600500, 106610500; Oxoid CM395; Difco 0881).

For glucose as a raw material, any homofermentative member of thegenus Lactobacillus may be used. The preferred organism, however, is Lac-tobacillus delbrueckii, now designated as Lactobacillus delbrueckii ssp. Del-brueckii (Teuber, 1993; Hammes et al., 1991). The organism can also utilizesucrose and is thus suitable for the fermentation of sucrose or molasses, butit cannot utilize lactose. In contrast, Lactobacillus delbrueckii ssp. Bulgari-cus utilizes lactose but not sucrose and has been used for the conversionof lactose, that is, whey. Lactobacillus helveticus, on the other hand, utilizeslactose but not sucrose, and thus represents another useful organism for theconversion of whey. Lactobacillus delbkueckii ssp. lactis, formerly known asL. lactis, may be used in fermentation of glucose, sucrose, and lactose. Withrespect to starch, which cannot be fermented by lactic acid bacteria and hasto be hydrolyzed prior to fermentation, the choice of suitable organisms de-pends on the way of starch hydrolysis: If α- and β-amylases (barley malt) areused, the resulting carbon source will be mainly maltose. Organisms capableof fermenting maltose are L. delbrueckii ssp. lactis and some strains of L.delbrueckii ssp. delbrueckii (Teuber, 1993). In the case of saccharificationwith α-amylases and glucoamylases, resulting in the production of mainlyglucose and glucose oligosaccharides, suitable strains will be those alreadydescribed. One exception should be mentioned: Lactobacillus amylophilus(Nakamura and Crowell, 1979) and Lactobacillus amylovorus (Nakamura,1981) have been described to actively ferment starch, and this has led toalternative processes of industrial lactic acid production (Cheng et al., 1991;Zhang and Cheryan, 1994). Irrespective of the raw material used, it is de-sirable to select strains with high rates of productivity. Since productivity isstrongly dependent on the hydrogen concentration of the fermentation broth,it might be expected that adaptation to higher concentrations of lactic acidwould increase the performance of a given strain. Increased yields have, infact, been claimed following this procedure (U.S. Patent 4,794,080, 1988).Application of chemical mutagens (e.g., ethyl methane sulfonate) has beenreported to yield mutant of L. delbrueckii (ATCC 9649) with improved tol-erance to higher acid concentrations and higher productivities (Demirci andPometto, 1992).

In recent years, research efforts in microbiology have concentrated onthe development of improved microorganisms for the production of lactic



acid. The targets are increased growth rates, improved pH resistance, andincreased yields of lactic acid. These targets are partially achieved by ef-forts through genetic engineering, control of metabolic activity by appro-priate culturing techniques, and development of acid resistant mutants. Anotable patent (U.S. Patent 6,660,515, 2003) in this regard provides meth-ods of enhancing the growth rate and/or controlling the metabolic activityof lactic acid bacteria, which are defective in their pyruvate metabolism.There are also starter culture compositions comprising such defective lac-tic acid bacteria as helper organisms and lactic acid bacterial starter culturestrains. Useful helper organisms are Lactococcus strains, which are defec-tive with respect to pyruvate formate-lyase (Pfl) and/or lactate dehydroge-nase (Ldh) activity. The helper organisms may overexpress a gene codingfor an NAD+-regenerating enzyme such as NADH oxidase, encoded the bynox gene. Accordingly, invention relates in a first aspect to a method ofenhancing the growth rate and/or controlling the metabolic activity of alactic acid bacterial strain, comprising cultivating the strain in associationwith a lactic acid bacterial helper organism that is defective in its pyruvatemetabolism.

In another recent patent (U.S. Patent 6,645,754, 2003), mutants of lac-tic acid bacteria including Lactococcus lactis that are defective in pyru-vate formate-lyase production and/or in their lactate dehydrogenase (Ldh)production and methods of isolating such mutants or variants are pro-vided. The mutants are useful in the production of food products or inthe manufacturing of compounds such as diacetyl, acetoin, and acetalde-hyde and as components of food starter cultures, in particular Lactococ-cus lactis DN223 deposited under accession number DSM 11036. Accord-ingly, the invention provides a method of isolating a pyruvate formate-lyase (Pfl) defective lactic acid bacterium, with the method comprising thesteps of

1. Providing a wild-type lactic acid bacterial strain that under aerobic con-ditions is not capable of growth in the absence of acetate in a mediumnot containing lipoic acid, but that is capable of growth is such mediumunder anaerobic conditions.

2. Selecting from said wild-type strain a mutant that under said conditionsessentially does not grow in the absence of acetate.

In U.S. Patent 6,475,759 (2002), a process for producing lactic acid,which includes incubating the acid-tolerant homolactic bacteria in nutrientmedium to produce a fermentation broth with high levels of free lactic acid,is provided. An isolate of the acid-tolerant homolactic bacteria capable ofproducing high levels of free lactic acid is also provided. The invention bySato and his coworkers (U.S. Patent 5,597,716, 1997) relates to a process



for producing D-lactic acid and L-lactamide, comprising a culture broth ofa microorganism capable of asymmetric hydrolysis of DL-lactamide belong-ing to the genus Alcaligenes, Pseudomonas, Agrobacterium, Brevibacterium,Acinetobacter, Corynebacterium, Enterobacter, Micrococcus, or Rhodococ-cus, a material obtained therefrom or an immobilized material thereof to acton DL-lactamide, and recovering the resulting D-lactic acid and the remainingL-lactamide. This invention enables sufficient production of D-lactic acid andL-lactamide by the microorganism used in the study. (U.S. Patent 5,597,716,1997).

3. ORGANISMS OTHER THAN LACTIC ACID BACTERIA

A limited number of other microorganisms are capable to produce largeramounts of lactic acid from common carbon sources. The best known isRhizopus. Foster (1949) has recommended the use of Rhizopus for commer-cial lactic acid production (U.S. Patent 2,132,712, 1938; Prescott and Dunn,1959) because this organism is able to convert several sugars with apprecia-ble yields into synthetic fermentation media. Thus, downstream processingwas considered to be easier than fermentation by the more fastidious lacticacid bacteria. So far, although the process is patented (U.S. Patent 3,125,494,1964); commercial use of this potential technique has not been accomplished.Recently, a process has been patented in which conversion of starch by Rhi-zopus oryzae in single-step fermentation process with high lactic acid yieldsis claimed (U.S. Patent 4,963,486 A, 1990). Several reports have been pub-lished on a group of spore-forming bacteria, which have long been knownas lactic acid producers:

� Sporolactobacillus inulinus culture produces D-(−)-lactic acid (U.S. Patent3,262,862, 1966; Japan Patent 61058588 A2, 1986; European Patent 190770A2, 1986; Kobayashi and Tanaka, 1988).

� Bacillus coagulans (10 strains listed) and lactic acid production by this or-ganism (Blumenstock, 1984) have been patented (German Patent 4000942A1, 1990).

Owing to their common properties, both organisms have been studiedextensively, especially with regard to their phylogenetic relationships (Suzukiand Yamasato, 1994). Crabtree negative organisms such as Kluyveromyces,Pichia, Hansenula, and Candida are used to produce selected organic prod-ucts such as lactic acid. The organisms are cultured in a first culture mediumthat includes glucose, under conditions that promote cellular respiration. Theorganisms are then cultured under a second set of conditions that promoteproduction of the selected organic product. The organisms preferably containan exogenous lactate dehydrogenase gene (U.S. Patent 6,485,947, 2002).



4. BIOCHEMISTRY OF LACTIC ACID PRODUCTION

Review of the literature on fermentative production of lactic acid revealsthat, whatever the starting material, ultimately lactic acid is produced fromdextrose through pyruvic acid, and the reaction is

bacteria−→ CH3COCOOHPyruvic acid

−→ CH3CHOHCOOHLatic acid

(5)

If polysaccharides are to be used as raw material, then they have to behydrolyzed to mono- or disaccharide, as mentioned earlier.

The various fermentation reactions involved can be expressed as

StarchHydrolysis−→ n(C6H12O6)

Dextrose

Bacteria−→ 2nCH3CHOHCOOHLactic acid

(6)

Starch → n(C22H22O11)Maltose

→ 4nCH3CHOHCOOHLactic acid

(7)

(8)

(9)

It is interesting to note that dextrose can also be chemically broken downto lactic acid in the presence of KOH at high temperature, according to thefollowing reaction:



(10)

At present, lactic acid production from fermentation of starch is com-monly used for commercial purpose. Dairy wastes contain lactose and sugar,and beverage industry wastes contain sucrose. Therefore, if proper fermen-tation processes are developed to produce lactic acid from waste sucrose orlactose, a twofold objective of rendering wastes into a resource and minimiz-ing environmental problems can be achieved.

III. DOWNSTREAM PROCESSING OF LACTIC ACID

Downstream processing of lactic acid broth is extremely complicated andinvolves a number of steps. Different methods for purification of lactic acidhave been mentioned in the literature and patents. Review of this literatureclearly reveals the fact that no single method could be referred to as a stan-dard method. However, the recovery and purification of the lactic acid canbe broadly classified in to the following steps. Each step can have multiplealternatives and combinations thereof.

� Separation of biomass and other solids from the broth.� Acidification of broth with strong acid to liberate the lactic acid.� Removal of salt from the lactic acid solution or removal of lactic acid from

the broth or splitting of lactate salt. The general alternatives are:

� Precipitation of salt of cation with strong acid.� Liquid–liquid extraction with simultaneous lactate salt formation with

strong base and back-extraction of lactic acid with water.� Simultaneous acidification and esterification with alcohol followed by

back-extraction with water.� Direct removal of lactic acid from broth by advanced separation methods

such as adsorption, membrane separations, and ion exchange.

� Concentration of lactic acid.

Separation of biomass and other solids can be achieved by any conven-tional method. Acidification of broth is necessary to break the lactate into



lactic acid and cation. This can be achieved below a pH of 3.86 as the pKa

value for lactic acid is 3.86 and above this pH lactate is virtually present asa salt. If calcium is used as the cation for maintaining the pH during fermen-tation, then acidification with sulfuric acid results in precipitation of calciumsulfate. Likewise, a number of processes can be used to remove cations.For example, if sodium is used as the cation in fermentation, then it canbe removed by ion exchange. Removal of lactic acid can also be effected byesterification of lactic acid with alcohols, by liquid–liquid extraction, or by ad-sorption. Membrane processes, distillation, evaporation, crystallization, andso on can achieve the concentration of the separated lactic acid. Separationand purification can also be achieved by chromatography and ion exchange.

A. Precipitation of Salt of Cation With Strong Acid

The general trend in lactic acid production practices is to use bases of alkalineearth metals to control the pH, and in such cases precipitation of metal cationby strong mineral acid is the most common downstream processing option.Rauch et al. (1960) have described product recovery and purification in suchcases. Accordingly, the fermentation liquid is heated to dissolve all calciumlactate and treated with stoichiometric amounts of sulfuric acid. The resultingcalcium sulfate is separated by filtration with thorough washing of the filtercake. The combined liquids are vacuum evaporated. Residual amounts ofgypsum precipitating in the concentrated lactic acid solution are filtered offtogether with coloring substances, which may be adsorbed onto activatedcarbon. Further purification is achieved by passing the solution through ion-exchange columns and by treatment with hydrogen peroxide or potassiumpermanganate. The resulting solution yields rather pure lactic acid (foodquality) with a concentration of 80%. Improvements have been reported us-ing zinc lactate or magnesium lactate instead. Purifications with magnesiumlactate have been recommended in fermentations using crude carbohydratesources such as molasses because of improved crystallization properties (U.S.Patent 3,429,777, 1969). In another approach to obtain high-purity lactic acid,as described in European Patent (EP) 849252 (1988), multiple crystallizationsare generally carried out, first of the calcium lactates in order to remove thesoluble impurities from the fermentation medium, and then of the calciumsulfates liberated after treatment with sulfuric acid. The first disadvantage ofthis method is the high sulfuric acid consumption and above all the produc-tion of large amounts of gypsum, which poses serious problems in terms ofwaste treatment. The second disadvantage is the complexity and the highnumber of steps required to obtain high-purity lactic acid. Other methodshave therefore been proposed, leading to the crystallization of salts of lac-tic acid. For example, U.S. Patent 5,641,406 (1997) describes, after the stepinvolving the precipitation of calcium lactate with sulfuric acid and the treat-ment with ferrocyanide or hexaferrocyanide salts to remove the copper and



iron ions, the depolarization of the “crude” lactic acid thus obtained withactivated charcoal, and after the subsequent purification steps to remove allthe residual salts, concentration by evaporation, and hence crystallization ofthe lactates. Here again, this process suffers due to a large number of purifi-cation steps and the handling of toxic chemicals. The latest patented processfor the preparation of high-purity lactic acid from an aqueous solution con-taining lactic acid in the form of salt(s) is characterized in that the aqueoussolution is treated with a strong acid in order to liberate lactic acid in the freeform, and to produce corresponding salts of the strong acid, the salts of thestrong acid are crystallized by evaporative crystallization and the lactic acidis recovered in the free form in solution (U.S. Patent 6,384,276, 2002).

A general schematic for all such processes is shown in Figure 1.

FIGURE 1. Schematic for recovery of lactic acid from lactate.



B. Liquid–Liquid Extraction With Simultaneous Lactate Salt FormationWith Strong Base and Back-Extraction of Lactic Acid With Water

Solvent extraction of lactic acid has been proposed as an effective down-stream process in many patents and publications. Liquid–liquid extraction oflactic acid involves addition of suitable solvent to the dilute aqueous solu-tion of acid, resulting in distribution of lactic acid in the solvent phase andaqueous phase. The fundamental requirement of a good extractive solvent isa high distribution coefficient, which is the ratio of the lactic acid concentra-tions in solvent phase to aqueous phase. Other important requirements arehigh selectivity for lactic acid, low viscosity, low miscibility with water, higherdensity difference between aqueous and solvent phase, thermal stability, andlow toxicity to microorganisms in fermentation broth.

Three major types of extractant solvents have been suggested. These are:

1. Conventional oxygen-bearing hydrocarbons, such as octanol and methylisobutyl ketone (MIBK).

2. Phosphorus-bonded oxygen-bearing solvents, such as tributyl phosphate.3. High-molecular-weight aliphatic amines, such as dodecyl amine.

It has been reported that distribution coefficients for the first type ofextractants are very low, while those for the second type extractants are nothigh enough to extract lactic acid efficiently. Extractants from the third type,high-molecular-weight amines, are the most effective ones. The high basic-ity of amines results in reactive extraction of lactic acid, which increases theextraction efficiency substantially. Any solvent that has a high enough capac-ity for lactic acid to be economic, such as amyl alcohol or a tertiary amine,will also coextract some water, salts, and other organic acids. Thus, extrac-tion alone does not economically produce lactic acid of high enough pu-rity. Several extractants such as isopropyl ether (U.S. Patent 1,906,068, 1931)α, ω-diamino-oligoethers (Miesiac et al., 1992), isobutanol (German Patent3415141, 1985), and trialkyl tertiary amines in an organic solvent (U.S. Patent4698303, such as) di-n-octylamine and n-hexane (Hano et al., 1993), havebeen described. As a novel improvement, the application of liquid mem-branes should be mentioned (Chaudhari and Pyle, 1992a, 1992b; Scholleret al., 1993; Lazarova and Peeva, 1994). Bar and Geiner (1987) studied thefeasibility of extracting lactic acid from aqueous solution by means of a long-chain trialkyl amine of low basicity, such as tridodecylamine, using varioustridodecylamine solutions in n-dodecanol. It was found that extraction of lac-tic acid with a long-chain trialkyl amine such as tridodecylamine was effectiveonly at a pH that is lower than the pKa of lactic acid, which is 3.86. At sucha low pH, however, the lactic acid-fermenting microorganisms such as, forexample, Lactobacillus delbrueckii or Lactobacillus acidophilus are severelyinhibited. In addition, the extraction of lactic acid with amines produces the



salt of lactic acid with amine, and not pure lactic acid. Obviously, lactic acidand amine should be recovered back from the salt, and several methodsare suggested for this salt splitting. These methods include back-extractionwith water, back-extraction with another amine, thermal splitting, and inertgas-induced splitting. Applying these methods, pure lactic acid is obtainedand the recovered amine is recycled back to extraction. In effect, extractionof lactic acid with amines becomes at least two or many times a multistepdownstream process.

U.S. Patent 5,132,456, 1992, describes a process for recovering carboxylicacid from a carboxylic acid-containing aqueous feed stream having a pH closeto or above the pKa level of the acid. The recovery process involves whatmay be described as a cascade-type acid withdrawal operation, in which thebasicity of the extractant is increased stepwise. In the second stage, carboxylicacid is back-extracted either by ammonia or low-molecular-weight tertiaryamine, referred to as secondary extractant, which results in formation oflactate salt with ammonia or tertiary amine. This salt is then thermally split toyield back pure acid and secondary extractant. Applying this process to lacticacid involves the formation of salts of lactic acid with strong bases havinga pKa value of about 9–11. Thus, the decomposition of these salts into freelactic acid is energy-intensive. The patent also mentions the use of Alamine336 (tricaprylylamine) for the extraction of, among others, lactic acid from anaqueous solution, but no yields are mentioned. With the extraction of evensmall quantities of lactic acid from a fermentation broth, the pH of the brothrises rapidly to above 7, while the pKa of an extractant based on Alamine336 is less than 6. As shown in the patent, the uptake of carboxylic acidsfrom aqueous solutions drops rapidly with an increase of pH. It is thereforeinherent that the lactic acid uptake, if any, is negligible. It is further noted thatupon heat treatment and concentration of an ammonium lactate, crystallinelactic acid does not precipitate; instead, the viscosity of the solutions increasessteadily as a result of self-association of the acid. Baniel et al. (U.S. Patent4,275,234, 1981), found that the extracted acid can be recovered from theacid comprising extractant by back-extraction with water. They have alsofound that if the back-extraction is conducted at a temperature higher thanthat of the extraction, the concentration of the acid in the back-extract (theaqueous product of the back-extraction) could be significantly higher thanthat of the aqueous feed to the process. Yet if the concentration of the acidin the feed is very low, the concentration of the back-extract could still betoo low. That is particularly true when the feed consists mainly of the saltof the acid rather than the free acid. Acidulating neutral fermentation liquorsby the addition of acids for recovery via ester formation or other methodsresults in the formation of by-product salts, such as gypsum in the case ofcalcium lactate acidulation with sulfuric acid or sodium, or ammonium sulfatein others. Reagents are consumed, and disposal of undesired by-products isrequired. Efforts have recently been made to recover the lactic acid from



fermentation liquors without formation of by-product salts. Such a processis like salt splitting processes. In some recently published patents, extractionis applied to salt splitting. Thus, in U.S. Patent (5,132,456, 1992), a stronglybasic extractant extracts part of the lactic acid from the neutral solution,which results in a lactic acid-loaded extractant and a basic solution. Thisbasic solution, which still comprises most of the lactic acid values, couldbe recycled as a neutralizing medium to the fermentation. In U.S. Patent5,510,526 (1996) the extraction of the acid is conducted under CO2 pressure,so that bicarbonate is formed. The latter can be used as a neutralizing agent inthe fermentation. In order to limit the CO2 pressure to an economic one andstill achieve high yields, the extractant used should be quite strong. In fact,any extraction-based salt splitting process that avoids the production of by-product salts would require a strongly basic extractant. These extractants areusually composed of an amine as the main active component. The preferredamines are chosen from the group of primary, secondary or tertiary amines,with a total number of at least 18 carbon atoms. Mostly preferred are tertiaryamines. A diluent is usually used to achieve the required physical properties.The basicity of the extractant is easily adjusted by adding a polar solventto the extractant. Such polar solvents enhance the extraction efficiency ofthe amine, and are usually referred to as enhancers. Alkanols provide veryefficient enhancers. The basicity of the extractant is thus adjusted by theamount of the enhancer in the extractant, or, more precisely, by the enhancerto amine molar ratio. In the strongly basic extractants used in salt splittingprocesses, the enhancer to amine molar ratio is usually at least 1:1, and inmany cases is higher than that.

A general schematic for liquid-liquid extraction is shown in Figure 2.

C. Simultaneous Acidification and Esterification With AlcoholFollowed by Back-Extraction With Water

Another method of lactic acid purification is esterification with alcohol, anddistillation of the volatile ester as a most effective separation step to yieldpure products (U.S. Patent 2,350.370, 1943; British Patent 90322, 1962; CzechPatent 104398, 1962). Forming lactic acid ester with alcohol, purifying theester by distillation or extraction, and then converting the ester back to lacticacid can produce high purity lactic acid. Since the pKa of lactic acid is 3.86,which is much lower than the near-neutral pH of fermentation, mostly lac-tate salts exist in the broth. Therefore, substantially complete recovery andpurification of lactic acid require acidulation with a strong acid that releaseslactic acid from its salt to react with alcohol. Esterification is performed incountercurrent operation with concomitant separation of the volatile esterand subsequent de-esterification with water in a second stage to yield freelactic acid and alcohol to be reintroduced into the system. This method is



FIGURE 2. General schematic for liquid–liquid extraction.

said to yield lactic acid of optimum purity without any waste product tobe disposed of. Disadvantages of this method are the expensive equipmentand technical problems in handling a fluid with higher contents of inorganiccompounds.



In lactic acid purification, it is known that lactic acid can be reactedwith high excesses of methanol to produce methyl lactate, with methanol andwater being drawn overhead to drive the reaction. An alternate method, usingmethanol as the esterifying alcohol, involves removing the ester continuouslyfrom the top to drive the reaction. The scheme involves bubbling excesshot alcohol, such as methanol vapor, through the lactic acid solution at atemperature above the boiling point of alcohol, whereby the produced lactateester is removed with the alcohol vapors and water. Approximately 9 moles ofmethanol are required to remove 1 mole of lactic acid from an 82% solution.Dramatically larger quantities of methanol are required for more dilute lacticacid feed solutions. This may be acceptable if a highly concentrated purelactic acid feed solution is used. However, the disadvantage of this method isthat there is little liquid alcohol or liquid ester present in the reaction broth,leaving behind a thick residue of impurities and partially reacted material,with limited yield in a given cycle. A solution to these problems was given inU.S. Patent 5,210,296 (1993) by the use of a process consisting of continuousacidification of an aqueous solution containing ammonium lactate in thepresence of an alcohol, having four to five carbon atoms, being used as adiluent, with sulfuric acid (or any other strong acid), removal of water fromthe acidified mixture by distillation of the water/alcohol azeotrope, and, ina simultaneous or sequential manner, removal of the produced ammoniumsulfate crystals (or salts of strong acid produced), distillation, and hydrolysisof the liberated lactic acid ester to produce a free lactic acid having a purityof more than 99.5% (U.S. Patent 5,210,296, 1993). However, the difficulty ofthis process lies in particular in the need to remove the ammonium sulfate. Itis mentioned that it is imperative to use alcohols having four to five carbonatoms (namely, n-butanol in this case) in order to obtain sufficiently coarseammonium sulfate crystals to facilitate their separation by simple filtration ofthe reaction medium. Gabriel et al. (U.S. Patent 1,668,806, 1928) disclose thecomposition of matter of 1-butyl lactate and its preparation. They prepared 1-butyl lactate by dehydrating 70% lactic acid with excess 1-butanol at 117◦C,followed by the addition of HCl catalyst, and refluxing and esterificationwith addition of excess 1-butanol and drawing a 1-butanol water azeotropeoverhead. Nakanishi and Tsuda (Japanese Patent 46/30176, 1971) studied theproduction of 1-butyl lactate by extraction of an acidified crude fermentationbroth with 1-butanol, followed by esterification of the extract phase. BASF(EP 159-285) considers the production of isobutyl lactate by extraction of anacidified crude fermentation broth with isobutanol, followed by esterificationof the extract phase, which was then distilled in vacuum to give purifiedisobutyl lactate.

In WO 93/00440, assigned to DuPont Corporation, a process is describedthat comprises the steps of: (1) simultaneously mixing a strong acid, an alco-hol, and a concentrated fermentation broth that contains mainly basic salts oflactic acid, which react to form a crystal precipitate comprising of the basic



salts of the strong acid and an impure lactate ester of the alcohol; (2) re-moving water from the mixture as a water/alcohol azeotrope, which can beaccomplished either sequentially or substantially simultaneously with step 1;(3) removing the crystal precipitate from the mixture; (4) distilling the impurelactate ester to remove the impurities; and (5) recovering the high-purity ester.

It can be seen that the processes of the esterification suffer in practicefrom succession of numerous and cumbersome steps that make the purifica-tion of lactic acid from an aqueous solution containing lactic acid in the formof salt(s) long and tedious. A general flow diagram for lactic acid recoveryby esterification is shown in Figure 3.

FIGURE 3. Schematic for recovery of lactic acid by esterification.



D. Direct Removal of Lactic Acid from Broth by Advanced SeparationMethods Such as Membranes, and Ion Exchange

More recently, ion exchange, hitherto only used in later purification steps,has been proposed for primary separation of lactic acid from fermenta-tion broths (German Patent 4000942 A1, 1990; Japanese Patent 01091788,1989; U.S. Patent 5,068,418, 1991; European Patent 517242 A2, 1992; JapanPatent 04320691 A2, 1992; Evangelista et al., 1994). It is claimed that thismethod makes the production of food-grade, heat-stable lactic acid possi-ble without the problem of waste disposal as in the calcium and sulfuricacid procedure. Ion-exchange methods normally involve exchange of cationthat has been used for the maintenance of pH during fermentation (U.S.Patent 6,280,985, 2001). A process involving ion exchange has been referredto for extracting pure lactic acid from fermentation broth by ion-exchangechromatography on a strongly acidic cation exchanger, preferably in H+

form. In a first step, the ammonium lactate coming from the fermentationis converted into the free acid by genuine ion exchange. This conversion ispreferably affected on a weakly acidic cation exchanger in H+ form (U.S.Patent 5,641,406, 1997). A process combining ion exchange and solvent ex-traction has been developed by Eyal et al. (U.S. Patent 6,320,077, 2001) forthe recovery of purified lactic acid values from an aqueous feed solutioncontaining lactic acid, lactic acid salt, or mixtures thereof. It includes: (1)bringing said feed solution into contact with a substantially immiscible an-ion exchanger to form a substantially water-immiscible phase comprising ofan anion exchanger–lactic acid adduct; (2) effecting a condensation reac-tion in the said substantially water-immiscible phase between a carboxylicmoiety of said lactic acid adduct and a moiety selected from a hydroxylmoiety and a primary or secondary amine moiety to respectively form a lac-tic acid ester or amine product; and (3) separating the formed lactic acidproduct from the anion exchanger. (U.S. Patent 6,160,173, 2000). Similarlogic is applied for the processes wherein adsorption is used instead ofion exchange. Schematics for lactic acid recovery by ion exchange and ad-sorption are given in Figures 4. The commercially available ion-exchangeresins such as Reillex 425 and Reillex HP (Reilly Industries, Inc., USA),Dowex MWA-1 and Dowex 66 (Dow Chemical Company, Midland, MI), andDuolite A561 and AmberliteIRA-67 (Rohm and Hass Corp., USA) are generallyused.

Recently, electrodialysis has been proposed for purification of lactic acid.Currently this process has two disadvantages: high cost and a product ofintermediate purity.

Other methods used for recovery and purification of lactic acids aremembrane processes. Membrane processes may include microfiltration, ul-trafiltration, nanofiltration or reverse osmosis or combinations thereof (U.S.



FIGURE 4. General schematic of lactic acid separation by ion exchange.

Patent 5,250,182, 1993). Overall application of membrane processes in lacticacid recovery is schematically represented in Figure 5.

It is evident from the preceding discussion that there is an unsatisfiedneed for a simpler and cheaper process that permits the separation, concen-tration, and purification of a high-purity lactic acid with an excellent yieldfrom an aqueous solution containing lactic acid in the form of salt(s).



FIGURE 5. General schematic for lactic acid separation by membranes.

IV. COMMERCIAL PROCESSES FOR LACTIC ACID PRODUCTION

A. Classical Calcium Lactate Process

Considering most of the information just described, a basic protocol for man-ufacturing lactic acid in a classical way is described in Figure 6.

FIGURE 6. Block diagram for calcium lactate process.



The raw material (glucose, sucrose) is brought to a sugar concentrationof 120–180 g L−1. Complex nitrogen sources such as mixture of inorganicN-compounds such as ammonia and ammonium phosphates with complexorganic materials such as corn steep liquor, yeast extracts, peptones andother protein digests, malt sprouts, and so on, yielding nitrogen concen-trations between 1 and 10 g L−1, are added. Fermentation is carried outin reactor volumes of even more than 100 m3. One important factor is thematerial of construction, since lactic acid is known to be highly corrosive.Corrosion of the vessels has to be prevented not only to protect the reactorbut also to avoid contamination of the fermentation fluid by soluble com-pounds (heavy metals etc.) that would complicate the further purificationsteps. In the current industrial scenario, suitably lined concrete or stainless-steel vessels are preferred. Conventional stirrers perform gentle agitation.As the fermentation is conducted at temperatures greater than 40◦C, heat-ing has to be provided in the first stages, and cooling as soon as the heatis generated by the fermentation itself. Maintaining relatively high temper-atures of upto 50◦C for L. delbrueckii or similar strains reduces the prob-ability of contaminations by, for example, butyric acid forming anaerobicbacteria.

Sterile calcium carbonate, preferably as powdered chalk, is added ei-ther at the beginning or in increments during the fermentation to keep theconcentration of free lactic acid as low as possible. As mentioned earlier,pH values should be maintained between 5.5 and 6.0. Active fermentation iscompleted after 2–6 days, depending on the concentration of the used car-bon source. In calcium lactate fermentations, the upper limits of sugar con-centration are determined by the solubility of the resulting calcium lactate,which at higher concentrations tends to precipitate from the fermentationbroth. It has been claimed, however, that the application of higher sugarconcentrations (e.g., 260 g L−1) would be feasible in fermentation with acertain CaO dosage to adjust the pH to 6.3, causing continuous precipitationof calcium lactate. This would shift the equilibrium of the reaction to thedirection of product formation. A 99.6% conversion based on reducible sug-ars over 3 days was reported with this protocol (Poland Patent 144390 B2,1985).

The reactions involved in calcium lactate process are as follows:

2CH3CHOHCOOHLactic acid

+ CaCO3Calcium carbonate

→ (CH3CHOHCOO)2CaCalcium lactate

+ H2O + CO2

(11)

(CH3CHOHCOO)2Ca + H2SO4Sulfuric acid

→ 2CH3CHOHCOOHLactic acid

+ CaSO4gypsum

(12)

Usually, conversion yields of 85–95% (calculated on the basis of sug-ars) of the theoretical accessible values are reported. Amounts of up to 2%of acetic acid and propionic acid as by-products may be due to temporary



switches to heterolactic phases of fermentation by deviations from optimumconditions of pH or substrate concentrations due to incomplete mixing.The protocol is not limited to the use of calcium carbonate but also ap-plies to use of hydroxides and carbonates of other alkali or alkaline earthmetals.

B. Ammonium Lactate Proces

Several workers have tried the usage of ammonia as a neutralizing agent inthe fermentative production of lactic acid. Ammonia reacts with lactic acid toform ammonium lactate and thereby reduces the acidity of the fermentationbroth. The industrial fermentations are carried out using ammonia liquor, andthe rest of the process steps are similar to those of the calcium lactate process.Recovery of lactic acid from ammonium lactate is effected by acidulation withsulfuric acid and subsequent crystallization of ammonium sulfate salt, or byesterification of free lactic acid by alcohol and subsequent back-recovery bywater.

The involved reactions are expressed as follows:

CH3CHOHCOOHLactic acid

+ NH4OHAmmonia

→ CH3CHOHCOONH4Ammonium lactate

(13)

CH3CHOHCOOONH4 + H2SO4 → CH3CHOHCOOH + NH3 (14)

C. Continuous Fermentation Process

In continuous fermentation processes, considerably higher productivities areachieved, and thus they have been performed in various forms. Early studiescomprised experiments using cell suspensions (Childs and Welsby, 1966),eventually with cell recycling (Vick et al., 1983). In fermenter systems withhigh flow rates, productivities reached up to about 50 g L−1 h−1 (Richteret al., 1987). Such high productivities, of course, were considered to be in-teresting for the conversion of whey, especially in places with high accumu-lation of whey. Several authors have reported continuous fermentations ofwhey permeates with high productivities (Boyaval et al., 1987; Mehaia andCheryan, 1987a; Aeschlimann and Von Stockar, 1990; Krischke et al., 1991;Kulozik et al., 1992; Boergardts et al., 1994). Some aspects of integrated pro-cesses have been discussed in a textbook published recently (Chmiel andPaulsen, 1991). Combinations of these processes with electrodialysis havealso been described (Hongo et al., 1986; Czytko et al., 1987; Yao and Toda,1990).

In all these methods, the lactic acid production is strongly dependenton the growth of the bacterial population. Lactic acid bacteria are known torequire a rich medium for growth, as their capacity to synthesize the growth



factors is very small. Often the costs of nutrients are more than the sugar feed-stock. In addition, part of the nutrients not bound to the growing biomassremain in the product, thus lowering its purity. It has been recently reportedthat in the preparation of lactic acid, the actual fermentation reaction andthe culturing of producer organisms can be separated into discrete produc-tion and refreshing cycles. In the culturing stage, that is, refreshing cycle, therich nutrient medium is passed through the bioreactor for a few hours. Afterthe culturing stage, pure feedstock solution that reacts into lactic acid canbe passed through the bioreactor. A carbohydrate, such as starch, or otherpolysaccharide, such as polydextrose or inulin, or sucrose, lactose, or glu-cose, or other mono-, di-, or oligosaccharides or a mixture of these may beused as feedstock (U.S. Patent 5,932,455, 1999, U.S. Patent 4698303, 1987).

D. Application of Immobilized Cells

An appreciable amount of work has been devoted to studies of lactic acidproduction with immobilized cell systems (Linko et al., 1984; Mehaia andCheryan, 1987b; Boyaval and Goulet, 1988; Bassi et al., 1991), but industrialapplications have not been realized so far (Venkatesh et al., 1993).

V. BRIEF HISTORY OF DEVELOPMENT OF POLYLACTIC ACID(PLA)—A BIODEGRADABLE PLASTIC

The use of lactic acid and lactide to manufacture biodegradable polymers iswell known in the medical industry. Such polymers have been used in themaking of biodegradable sutures, clamps, bone plates, and biologically activecontrolled-release devices. The processes developed for the manufacture ofthe polymers to be used in medical industry included techniques appropriateto the need for high purity and biocompatibility in the final polymer product.In addition, these processes were developed to yield small quantities of poly-mers with high costs, with less emphasis given to cost and yield. However,viable and competitive processes for the continuous manufacture of puri-fied lactide and lactide polymers from lactic acid, having physical propertiessuitable for replacing the present petrochemical-based plastics used in nonmedical applications, were not developed till the 1990s. It is a well-knownfact that lactic acid undergoes a condensation reaction to form polylacticacid when water is removed by evaporation or other means. However, theresulting polylactic acid was found to be a low-molecular-weight polymerwith very limited application based on physical properties. The low molecu-lar weight of the polymer was attributed to the competing depolymerizationreaction in which the cyclic dimer of lactic acid, referred to as lactide, is gen-erated. The rate of polymerization reaction gradually reduces as the polymer



chain length increases, and ultimately it equals the rate of depolymerization,which effectively limits the molecular weight of the polymer. It was also ob-served that high-molecular-weight polymer could be manufactured from thelactide that is generated by the depolymerization of lactic acid. Lactic acidexists in two optically active isomers, L and D. It also exists as a recemicmixture where D-lactic acid and L-lactic acid are present in equal propor-tion. Depending on the type of lactic acid, L- or D- or LD-actide is generated.The chiral purity of the lactic acid is important with respect to the needs ofindustrial applications. For polylactic acid applications, the chiral purity ofthe lactic acid has a strong influence on the properties of the polymer. Thechiral purity of the polymer controls the ability of the polymer to crystal-lize. In some instances, polymers with controlled amounts of crystallinity aredesired in order to get polymer properties that are advantageous in an indus-trial application—for example, to raise the heat distortion temperature of thepolymer. Other advantages of controlled polymer crystallinity relate to thestorage, transfer, and processing of polylactic acid resins into fibers, nonwo-ven fabrics, films, and other end products. Lactic acid currently used in foodapplications has chiral purity requirements greater than 95%, generally witha preference for the L form. The chiral purity of lactic acid is also importantfor end products such as pharmaceuticals and other medical devices wherelactic acid is a starting material. The term “95% chiral purity” means 95% ofthe lactic acid/lactate content is one the of two possible enantiomers. It wassoon recognized that high-molecular-weight polylactide of desired physicalproperties could be manufactured by purification of lactide prior to polymer-ization. The purification of lactide could be carried out using solvent extrac-tion and recrystallization of lactide. However, these processes were knownto have poor yields and were associated with substantial loss of material inrecrystallization steps. These facts imposed limitations on the commercial-ization of these processes. The real breakthrough in lactide polymerizationwas achieved in the 1990s, when P. Gruber developed a continuous pro-cess for lactide preparation, purification, and subsequent polymerization topolylactide (U.S. Patents 6,326,458, 2001; and 5,357,035, 1994). Cargill, Inc.,(Minneapolis), USA, has commercialized the process . In recent years manymultinationals are in the process of commercialization of PLA production. Achronological account of polylactide development on an international levelis presented in Table 2.

Polylactic acid is a multifunctional thermoplastic that can be processedinto staple fibers (e.g., carpet fibers), spinning fibers in woven applicationsto replace (or in blends with) cotton, wool, and polyesters, extruded filmsfor wrappings, injection and thermo-molded products such as polyethylene,propylene, and styrene foam products, and thermo-formed plastics such aseating utensils, coatings, and others. Polylactic acid is completely recyclableand is the only major polymer that slowly but totally biodegrades duringcomposting. The use of polylactic acid as a mass polymer, until now, has been



TABLE 2. Polylactic Acid Development

Researcher/institution/industry,international status Year Salient development

Carothers 1932 Polymerization of lactic acid insolvent under high vacuumproduced polymer with too lowmelting point

Bonsigore P.V. et al. 1992 PLA as alternative binder forcellulosic nonwovens

University of Tennessee,Knoxville

1993 Spun-laid and melt-blownnonwoven based on PLA

Kanebo (Japan) 1994 and 1998 Poly-L-lactide Lactron©R fiber andspun-laid nonwoven (2000 tpa to3000 tpa)

BBA France 1997 Disclosed non woven webs andlaminates made of 100% PLA

Galactic Laboratories (Belgium) 1999 Excellent overview of polylactic acidpolymers concluding that therewould be 3,90,000 tonnes of PLApolymers production by 2008 at aprice of $2/kg

Cargill Dow Polymers LLC 2000, 2003 Production of 4000 tpa of PLApolymer–Eco PLATM (now NatureworksTM), 140,000 MT/annum

NKK (Japan), Kuraray (Japan),Dai-Nippan, Ink Polymers,Showa Polymers, ShimadzuCorp., and Mitsui Totasue,Shinawa (Japan)

2001 LACEATM, HaibonTM, Lactron©R-PLAbased polymers

limited due to the high costs associated with its production, primarily energycosts, making it uncompetitive with similar nonbiodegradable petroleum-based polymers and polyesters.

VI. PRODUCTION PROCESSES FOR POLYLACTIDE POLYMER

The diagrammatic representation of PLA production in general is depictedin Figure 7. There are two major routes of producing polylactic acid directlyfrom the lactic acid monomer. The first route involves the removal of waterof condensation by using a solvent under high vacuum and temperature.This approach is currently used, for example, by Mitsui Toatsu Chemicalsto produce a low- to intermediate-molecular-weight polymer. In an alterna-tive route, which is considered to be the classical approach of producingpolylactic acid, water is removed under milder conditions directly from lacticacid, without using a solvent, to produce a cyclic (ring closing) intermediatedimer referred to as “dilactic acid.” This dimer is then purified under vacuumdistillation and then “ring-opening” polymerization is accomplished using



FIGURE 7. Block diagram for the production of polylactic acid (PLA), a green polymer.

heat, without solvent, to produce the polylactic acid. This “ring-opening”method of producing polylactic acid is currently used worldwide and is thesubject of many patents and other literature. This process, however, suffersfrom long reaction times and high temperatures and the formation of a num-ber of side reactions and by-products. It usually results in a low chemicalyield of 50% to 55% for the polylactic acid polymer (on the basis of lacticacid).

Recently a third route of producing polylactic acid has been patentedand is now being commercially practiced by Cargill. This process relieson the initial production of an impure and low-molecular-weight polylac-tic acid/polylactide polymer (sometimes referred as oligomer) as a feedstockin the production of polylactic acid. This impure polymer must then be de-polymerized using additional energy steps in order to achieve a more purepolylactic acid/polylactide polymer. These steps are also energy-intensive



FIGURE 8. Production of polylactide (PLA) polymer from lactic acid.

and therefore result in a high production cost associated with producingpolylactic acid. The diagrammatic representation of this process is shown inFigure 8.

Yet another process of producing dilactic acids or dimers and subse-quently producing polylactic acid avoids such energy-intensive steps as de-scribed in the Cargill process. This particular method uses aminium lactatesalt (crystal) instead of an impure polylactic acid as a starting material for the



production of dilactic acids or dimers. It describes the use of organic amines(technically called heterocyclic amines, for example, Piperazine) within thelactic acid fermentation broth to produce aminium lactate (salts). Thoughaminium lactate salts are referred to specifically, other salts such as ammo-nium lactate salts may also be produced and used in such a process. Lactatesalts have lower melting points, from 80 to 150◦C, and can dissociate in thepresence of catalysts (acetonitrile, dioxan, ethylene glycol monoethylether,dimethyl sulfoxide-d6) and low heat to form dilactic acids or dimers. Thisprocess completely avoids the need to first produce impure polylactic acidpolymers as the feedstock in order to produce such dilactic acids. In thisprocess, however, ultrafiltration and electrocoagulation are used to concen-trate and extract the lactic acids and lactate salts. The fallacy of this process,for large-scale processing, lies in the use of the organic amines within thefermentation broth and the use of ultrafiltration membranes that require highpressures to remove and separate out the cell mass from the lactic acid salt.Once the lactic acid is separated from the cell mass, electrocoagulation isthen used to bring about the separation or breakdown of the lactic acid fromthe amine salt in order to concentrate it to a minimum of a 45–85% pure lacticacid. The purer lactic acid is then recontacted with the organic amine onceagain, for example, Piperazine, to form the Piperazine salt once again. TheKamm process, as described, requires unnecessary steps of forming the saltfrom the lactic acid in order to achieve a higher concentration of the lacticacid (45–85%), which then must be recrystallized to form the salt that mustbe restructured to form the dilactic acid or dimmer. This process results in theproduction of impure dilactic acids and lactate salts (as an interim step), andthe impurities in the lactic acid produced during fermentation within this pro-cess limit the achievable polymer length. It has recently been reported thatthe lactate salts of the Kamm process can, under certain conditions, becomea low-cost and low-energy starting material for the production of polylacticacid (U.S. Patents 6,667,385 [2003], 6,569,989 [2003], 6,277,951 [2001]).

VII. INTERNATIONAL STATUS

Most of the developed countries have gone for production of PLA on acommercial scale. Cargill (USA), Minneapolis, MN, Ecochem, Wilmington,DE, Kanebo (Japan), BBA (France), Di-Nippon (Japan), and Ink Chemical,Showa Polymers, Shimadzu Corporation, and Mitsui Toatsu (Japan) are ma-jor international corporate bodies that have gone into the production of PLA.The other industries in these countries process the produced PLA to differentcommercial items. Thus, the production of PLA is in an advanced phase ofcommercialization. However, the cost of the produced items based on PLAis 10- to 12-fold more than for plastic items produced by the conventionalpetrochemical-based polymers. The efforts are now directed to minimize the



cost of production and processing of PLA for commercial use. The com-mercial items made from PLA include packaging materials, computer cases,paper coatings, fibers, garbage bags, and automobile parts.

VIII. PLA AND THE ENVIRONMENT

PLA is a green plastic that is produced from renewable resource such as plantstarch. It is biodegradable and is likely to minimize the disposal problems.Thus it can be viewed as an environmentally friendly plastic. The life cycleof PLA is shown in Figure 9. However, its real impact on the environmentshould be assessed in detail with regard to:

� Energy requirement for production and processing of green plastics.� Substitution of nonrenewable resource base with renewable resource base.

FIGURE 9. Life cycle of PLA polymer.



� Establishment of balance between production of green plastic and ecosys-tem through the principle of “cradle to grave” without affecting theenvironment.

A. Energy Requirement for Production and Processing of PLA

The energy requirement for producing plant-derived plastics gives rise toa considerable environmental concern, as the process consumes 19 timesmore electricity, 22% more steam, and 7 times more water than the chemicalmethod of manufacturing polystyrene. Fossil crude oil is the main resourcefor conventional plastic production, but making plastics from plant materialdepends mainly on coal and gas, which are used to power the corn farmingand corn processing industries for production of PLA. Any kind of plant-based method, therefore, involves switching from a less abundant fuel (oil)to more abundant one (coal). Such a shift is considered to be a step towardsustainability. Major concern in this context is that all fossil fuels used tomake PLA from renewable raw materials (corn) are combusted to generateenergy, whereas the petrochemical-based processes incorporate a significantportion of fossil resource into the final product.

Burning of more fossil fuels will cause global climatic problems by in-creasing greenhouse gases such as CO2. Naturally, the level of emission asso-ciated with the combustion of fossil fuel such as sulfur dioxide is also likelyto be enhanced. This gas contributes to acid rain and therefore is of concern.Thus, switching from conventional plastics to green plastics requires specialattention to improve air quality and to curtail global warming by reducingcarbon dioxide and other gases in the atmosphere.

The environmental benefit of producing plastic from renewable re-sources is overshadowed by increase in the energy consumption and gasemissions. PLA seems to be the only plant-based plastic that has a chanceof becoming competitive in this regard. In spite of the advantages ofPLA over other plant-based green polymers, it’s production is likely toemit more greenhouse gases than by petrochemical-based conventionalplastics.

In analyzing the energy requirement for production of green plasticsusing the route of PLA production and processing, one can depend on therenewable energy source that can be derived from burning of plant materialor biomass. This may supply and act as an additional source of energy forthe processing of PLA. Emissions generated in this way may be viewed morefavorably than CO2 released by fossil fuels. Burning the carbon content inthe corn stalks would not increase net CO2 in the atmosphere because newplants growing in the following seasons would absorb an equal amount ofCO2 gas. This is the reason why plant-based plastics do not increase the CO2

level and dioxins when they are incinerated, as in the case of conventionalplastics.



Monsanto and Cargill Dow, USA, have formulated strategies for derivingenergy from biomass. Monsanto proposes to burn all the corm stover thatremains after extraction of plastic, to generate electricity. Thus, it seems thatutilization of biomass-derived electricity is possible and more than enoughto meet the power requirement of PLA extraction.

B. Substitution of Nonrenewable Resource Base With RenewableResource Base for Maintaining the Ecological Balance

The PLA polymers are derived from the plant-based materials. Therefore, it iscertain that one conserves the petroleum crude by using the plants, which areabundant in starch/sugar. However, the conservation of nonrenewable feed-stock and energy source with renewable is totally dependent on the extentof fuel energy input and improvement in the production process of PLA withminimum input of energy that is associated with lesser generation of green-house gases without disturbing the existing ecological balance. This requiresextensive research and development (R&D) for improving the productionprocess of PLA and plastics using these polymers. Thus, it is certain that theconservation of nonrenewable resources is possible only if production of PLAis achieved through economically and environmentally sound processes, byadopting appropriate balance strategies for minimizing the consumption offossil fuel sources.

C. Establishment of a Balance Between Production of Green Plasticsand Ecosystem by Adoption the Principle of “Cradle to Grave”

Complex polymeric plastic materials with specific and desirable propertiesand derived from petrochemical feedstock are nonbiodegradable. This resultsin the disturbance of the ecosystem through accumulation in the environ-ment, and therefore the need for green plastics was realized. The elementalconstituents of green plastics, especially carbon, are processed by the carboncycles of the ecosystem without getting accumulated. Carbon in the form ofatmospheric CO2 and compost and manure aris taken up by the plants andreduced to carbohydrates through photosynthesis. In the case of ultimate dis-posal, carbohydrate/starch/sugar is recycled back for the production of greenplastics. This process of recycling and reuse in the ecosystem is called theprinciple of “cradle to grave” (Figure 10). The PLA even can be fermented tolactic acid, and the rate for conversion of polylactide with maximum recycleand reuse can be followed. However, the number of times of recycling ofPLA for reproduction of green polymers is yet to be studied in detail. Thus,after some time of recycling, the PLA-based items have been processed forbiodegradability. The preliminary studies have indicated that PLA is largelyresistant to attack by microorganisms until and unless it is hydrolyzed atelevated temperature to reduce the molecular weight before biodegradation



FIGURE 10. “Cradle to grave” concept for PLA.

commences. Claims of biodegradability can therefore only be made wherecomposting infrastructure facility exists. The data from Cargill Dow, USA,shows that composting at 60◦C causes hydrolytic degradation of PLA, whichover 10 days depolymerizes and embrittles the polymer sufficiently for it tofragment. Complete biodegradation to CO2 occurs over the next 30–40 days(http://www.Nonwore.co.uk). Cargil Dow pledges to support the develop-ment of composting infrastructure in those countries that do not have one.This requires extra expenditure before adopting the process for productionof green plastics. Otherwise, this will create a solid waste disposal problem.Therefore, the need exists for designing a suitable and efficient compostingsystem for the green plastic materials prior to switching over from conven-tional plastics to green ones.

Unfortunately, no single strategy can overcome all the environmental,technical, and economic limitations of the various manufacturing approaches.Conventional plastics require fossil fuels as a raw material, but PLA does not.



The conventional plastics are not biodegradable, but they have broader rangeof material properties when compared to PLA. Biodegradability, helps to re-lieve the problem of solid-waste disposal, but degradation gives off green-house gases, thereby compromising air quality. Although PLA productionuses fewer fossil resources than its petrochemical counterparts, it still re-quires more energy and emits more greenhouse gases during manufacture.

IX. DISCUSSION

The production of green plastics, energy requirement for production, con-servation of resources, and the environment are very complex issues. Thechoice will ultimately depend on how to prioritize the depletion of fossil re-sources, emissions of greenhouse gases, land use, solid-waste disposal, andprofitability—all of which are subject to their own interpretation, politicalconstituencies, and value systems. Regardless of the particular approach tomaking plastics, energy use and the resulting emissions constitute the mostsignificant impact on the environment.

In light of this fact, it is proposed that any scheme to produce plas-tics should not only reduce greenhouse gas emissions but should also go astep beyond that, to reverse the flux of carbon into the atmosphere. Accom-plishing this goal will require finding ways to produce nondegradable plasticfrom resources that absorb carbon dioxide from the atmosphere, sequester-ing the carbon in the ground instead of returning it to the atmosphere. Somebiodegradable plastics may also end up sequestering carbon, because land-fills, where many plastic products end up, typically do not have the properconditions to initiate rapid biodegradation.

If things are viewed in the context of a developing country like India,the energy crisis is very deep and the shift from conventional fossil energysources to renewable energy is very difficult due to constraints of funds andinfrastructure facilities. Therefore, it is very difficult to adopt these processesfor production and use of green plastic-based materials for conservation ofnonrenewable resources by adopting the principle of “cradle to grave.”

X. RESEARCH NEEDS

It is quite evident from this article that production of PLA on a commercialscale, although fairly established, needs substantial improvements in produc-tion processes, especially in lactic acid recovery and purification stages, tomake it competitive with petrochemical-based plastics. The areas that warrantspecial research attention are:

� Development of acid resistant microbial strains to enhance the lactic acidyields and to minimize the chemical consumption.



� Development of processes, especially membrane processes, for the con-tinuous removal of lactic acid from fermentation broths.

� Development of bioreactor systems for fermentation, specifically thenfixed film systems or immobilized culture systems, to enhance the tol-erance of microorganisms to acid shock loads.

� Development of the recovery and purification processes that include min-imum numbers of steps and consume minimum energy.

� Minimizing the wastes in order to make PLA truly eco-friendly.

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Production and Recovery of Lactic Acid for Polylactide - An Overview

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