BIOCHEMISTRY OF POSTHARVEST STORAGE

Submitted to Dr. Jayanthi Tokkas, Department of Biochemistry

Submitted byNaresh Kumar, 2013BS07DM.Raghavendra, 2013BS09D

Contents

Biochemistry of post harvest storage

The factors affecting storage life

Temperature Water loss Mechanical damage Decay of storage

Post harvest factors influencing quality

Supplemental treatments applied to the commodity

Supplemental treatments involved in manipulation of environment

Composition and compositional changes

Effect of ascorbic acid on post harvest storage Carbohydrates Proteins Lipids Organic acid Pigments Phenolic compounds Volatiles Vitamin minerals

Biological factors involved in post harvest deterioration of fruits

Respiration Ethylene production Transpiration/ water loss

Biochemistry of post harvest storage

Fresh fruits and vegetables play a very essential role in human nutrition and health, especially as sources of vitamins, minerals, and dietary fibre. Nutritional value varies greatly among commodities and cultivars of each commodity. Using plant breeding and biotechnology approaches it is possible to develop genotypes that have enhanced nutritional quality and improved flavour quality to encourage consumers to eat more fruits and vegetables. Climatic conditions especially temperature and light intensity, have a strong effect on the nutritional quality of fruits and vegetables. Maturity at harvest and harvesting method influences the commodities quality and extent of physical injuries. Delay between harvest and consumption or processing can result in losses of flavour and nutritional quality. The magnitude of these losses increases with exposure to temperatures, relative humidity and concentrations of oxygen, carbondioxide and ethylene outside the ranges that are optimum for each commodity during the entire post harvest handling system. Furthermore, processing and cooking methods can greatly affect the nutritional value of fruits and vegetables.

Maturity at harvest is the most important factor that determines storage life and final fruit quality. Immature fruits are more subject to shrivelling and mechanical damage and are of inferior quality when ripened. Over ripe fruits are likely to become soft and mealy with insipid flavour soon after harvest. In general, fruits become sweeter, more colourful, and softer as they mature.

The factors affecting storage life

The natural limits to the post-harvest life of all types of fresh produce are severely affected by other biological and environmental conditions:

Temperature. An increase in temperature causes an increase in the rate of natural breakdown of all produce as food reserves and water content become depleted. The cooling of produce will extend its life by slowing the rate of breakdown.

Water loss. High temperature and injuries to produce can greatly increase the loss of water from stored produce beyond that unavoidably lost from natural causes. Maximum storage life can be achieved by storing only undamaged produce at the lowest temperature tolerable by the crop.

Mechanical damage. Damage caused during harvesting and subsequent handling increases the rate of deterioration of produce and renders it liable to attacks by decay organisms. Mechanical damage to root crops will cause heavy losses owing to bacterial decay and must be remedied by curing the roots or tubers before storage.

Decay in storage. Decay of fresh produce during storage is mostly caused by the infection of mechanical injuries. Furthermore, many fruits and vegetables are attacked by decay organisms which penetrate through natural openings or even through the intact skin. These infections may be established during the growth of the plant in the

field but lie dormant until after harvest, often becoming visible only during storage or ripening.

Post harvest opportunities for enhancing the quality and quantity of essential nutrients present in fruits and vegetables include:

1. Increasing overall consumption of fruits and vegetables.2. Improving bioavailability of nutrients.3. Increasing levels of essential nutrients through fortification methods and4. Reducing nutrient losses.

Postharvest factors influencing quality

The brief overview of how post harvest factors influence produce quality and its maintenance.

Temperature and relative humidity: keeping the fruits and vegetables within their optimum ranges of tempartures and relative humidity is the most important factor in maintaining their quality and minimizing postharvest losses.

1. Above freezing point, (for non-chilling sensitive commodities) and minimum safe temperature (for chilling sensitive commodities) every 100C increase in temperature accelerates deterioration and the rate of loss in nutritional quality by 2-3 fold.

2. Delays between harvesting and cooling or processing can result in direct losses (due to water loss and decay) and indirect losses (losses in flavour and nutritional quality).

3. Distribution chain: the weakest link in the postharvest handling chain of fresh fruits and vegetables is the home handling system; greatest potential for improvement includes development of more sophisticated home-handling equipment and transfer of handling knowledge to the consumer.

Classification of fruits according to their optimum storage temperatures and

potential storage life

Supplemental treatments applied to the commodity

1. Curing of root vegetables, cleaning, sorting to eliminate defects, sorting by maturity/ripeness stage, sizing, waxing, treatment with fungicides for decay control, heat treatments for decay and/or insect control, fumigation for insect control, irradiation for preventing sprouting or insect disinfection, and exposure of fruits to ethylene for faster and more uniform ripening.

2. In most cases, these treatments are useful in maintaining quality and extending postharvest-life of the produce.

3. Irradiation at doses below 1 kilogray has no significant effects on nutritional quality of fruits and vegetables.

4. Ethylene treatment shortens the time between harvest and consumption which can result in maintaining a higher concentration of ascorbic acid in tomatoes.

Supplemental treatments involving manipulation of the environment:

1. Responses to atmospheric modification vary greatly among plant species, organ type and developmental stage, and duration and temperature of exposure.

2. Maintaining the optimum ranges of oxygen, carbondioxide and ethylene concentrations around the commodity extends its postharvest life by about 50-100% relative to air control.

3. In general, low O2 atmospheres reduce losses of ascorbic acid in fresh produce. Elevated CO2 atmospheres upto 10% also reduce ascorbic acid losses, but higher CO2 concentrations can accelerate these losses.

4. Exposure to ethylene can be detrimental to the quality of most vegetables.

Composition and compositional changes

The flesh of young developing fruits contain very little sugar and large amounts of starch, acid and phenolics present make them inedible as the fruits approach maturity, flesh celss enlarge considerably and sugar content increases while starch, acid and phenolic contents decrease. In addition, certain volatile compounds develop, giving the fruit its characteristic aroma. Chlorophyll degradation (loss of green color) and synthesis of carotenoids (yellow to orange colors) and anthocyanins (red and blue colors) take place both in the skin and the flesh with fruit ripening. All fruits soften as they ripen due to changes in cell wall composition and structure.

Metabolic changes in post harvest fruits and vegetables

DEGRADATIVE SYNTHETICDestruction of chloroplast Synthesis of carotenoids and anthocyaninsBreakdown of chlorophyll Synthesis of flavour volatilesStarch hydrolysis Synthesis of starchOrganic acid catabolism Synthesis of ligninOxidation of substrate Preservation of selective membranesInactivation of phenolic compounds Interconversion of sugarsHydrolysis of pectin Protein synthesisBreakdown of biological membranes Gene transcription

Cell wall softening Formation of ethylene biosynthesis pathway

Effect of Ascorbic acid on post harvest storage: Many harvesting and postharvest handling procedures influence the nutritional quality of fruits and vegetables. Much of the available information is about AA, which appears to be the most sensitive to postharvest losses. Oxidation can occur in the presence of catalysts, oxidase enzymes, or as a result of heat during processing. Therefore, vitamin C losses continue through postharvest handling, processing, cooking, and storage of fruits and vegetables. Additional research is needed to investigate the effects of postharvest handling procedures on nutrients in fresh fruits and vegetables. Also, the possible effects on nutritional quality of those procedures that have not been evaluated need to be elucidated. Any new harvesting or postharvest handling method should be evaluated as to its potential impact on nutritional quality before being recommended for use.

Carbohydrates: they are the most abundant and widely distributed food component derived from plants. Fresh fruits vary greatly in their carbohydrate content, with a general range being between 10% and 25%. The structural framework, texture, taste and food value of a fresh fruit is related to its carbohydrate content. Sucrose, glucose and fructose are the primary sugars found in fruits. Sucrose content ranges from a trace in cherries, grapes and pomegranates to more than 8% in ripe banana and pineapples. Such variation influences tastes. Apart from other polysaccharides present in fruits include cellulose, hemicelluloses, pectin and lignin which are formed mainly in cell walls and vary greatly among commodities. The transformation of insoluble pectins into soluble pectins is controlled, for the most part by the enzymes pectinesterase and polygalacturonase. Reduced activities of these two enzymes have been associated with reduced juiciness and poor texture in peaches that were ripened after storage at 1oC for more than three weeks.

Proteins: fruits contain less than 1% protein (9-20% in nuts eg., walnut). Changes in the level and activity of proteins resulting from permeability changes in cell membranes may be involved in chilling injury. Enzymes which catalyse metabolic processes in fruits, are proteins that are important in the reactions involved in fruit ripening and senescence. Some of the enzymes important in fruit quality include

Lipids: lipids constitutes only 0.1-0.2% of most fresh fruits, except for avocados, olive and nuts; however lipids are very important because they make up the surface wax, which contribute to fruit appearance, and cuticle, which protects the fruit again water loss and pathogens. The degree of fatty acids saturation establishes membrane flexibility , with greater saturation resulting in less flexibility. Denaturation of fatty aicds can occur upon chilling in chilling sensitive fruits, in which case membrane undergo a phase change ( Liquid crystalline to solid gel ) at chilling temperatures, resulting in disruption of normal metabolism.

Organic Acids: they are important intermediate of metabolism which provides energy for maintenance of cell integrity. Most fresh fruits are acidic such as lemon s and limes constitute 2 to 3 percent of fresh weight of acids. Acid content usually decreases during ripening due to utilization of organic acids during respiration or their conversion to sugars. Malic acid and citric aicds are the most abundant in fruits except grapes in which tartaric acid are important in most cultivars and kiwi fruits which has abundant quinc acid.

Pigments: these are chemicals responsible for skin and flesh colors which undergo changes during storage these include

1. Loss of chlorophyll which is influenced by pH changes , oxidative changes and chlorophyllase action.

2. Synthesis or revelation of carotinoids ( Yellow and Orange Color)3. Development of anthocynin which are fruit specific for example apple has

Cyanidin 3-arabinoside, grape has Malvidin 3-glucoside etc.

Beta carotene which is nutritionally important or are very stable and remain intact in fruit tissue even when extensive senescence occurred. Wheras anthocynin occur as glycosides are unstable and hydrolyzed by phenol oxidaze which gives brown oxidation products.

Phenolic Compound: Total phenolic content is high in immature fruit then mature fruits. It includes chlorogenic acids , catechin, epicatechin, leucoanthocyanidins, flavanols and phenols. Chlorogenic acid (ester of caffeic acid) occurs widely in fruits and is the main substrate unvolved in enzymatic browning of cut, or otherwise damaged, fruit tissues when exposed to air. Normally phenolic compounds are separate from the PPO (Polyphenoloxidase). Enzyme in the intact cells of plant tissue. Once the tissue is damaged, PPO and the phenolic compounds that it acts on are decompartmentalized and the above reactions occur, leading to tissue browning. Astringency is directly related to phenolic content, and it usually decreases with fruit ripening because of conversion of astringent phenolic compounds from the soluble to the insoluble, nonastringent form. Loss of astringency occurs via 1. Binding or polymerization of phenolics 2. Change in molecular size of phenolics 3. Change in hydroxylation pattern of phenolic compounds.

Volatiles: they are responsible for the characteristic aroma of fruits. They are present in extremely small quantities. The total amount of carbon involved in the synthesis of volatiles is <1% of that expelled as CO2. The major volatile formed in climacteric fruits is ethylene. Ethylene does not have a strong aroma and does not contribute to typical fruit aromas. Volatile compounds are largely esters, alcohols, aids, aldehydes and ketones (low molecular weight compounds). Very large numbers of volatile compounds have been identified in fruits, and more are identified as advances in separation and detection techniques and gas chromatographic methods are made; however only a few key volatiles are important for the particular aroma of a given fruit.

Vitamins: the water soluble vitamins include vitamin C, thiamine, riboflavin, niacin, vitamin B6, folacin, vitamin B12, Biotin, and pantothenic acid. Fat soluble vitamins include vitamins A,D,E,K, fat soluble vitamins are less susceptible to postharvest losses.

Minerals: Important fruit minerals include base-forming elements (Ca, Mg, Na, K) and acid forming elements (P,Cl,S). Minerals present in microquantities include Fe, Cu, Co, Mn, Zn,I and Mo.High nitrogen content is often associated with reduced soluble solids content, lower acidity and increased susceptibility to physiological disorders in fruits.

High potassium content is often associated with increased acidity and improved color of fruits. High calcium contents reduce CO2 and C2H2 production rates, delay ripening, reduce the incidence of physiological disorders, and extend the storage life of apples and other fruits. High phosphorous content may result in decreased acidity in some fruits.

Biological factors involved in post harvest deterioration of fruit

Respiration: it is the process by which stored organic material are broken down into simple end products with a release of energy. Oxygen is used in this process and carbondioxide is produced. Food value (energy value) for the consumer is lost; it has reduced flavour quality, with sweetness especially being lost; and salable dry weight is lost (especially important for commodities destined for dehydration). The energy released as heat, which is known as vital heat, affects postharvest technology considerations such as estimations of refrigeration and ventilation requirements. The rate of deterioration (degree of perishability) of fruits is generally proportional to their respiration rate.

Ethylene production: ethylene the simplest of the organic compounds affecting the physiological processes of plants is a natural product of plant metabolism and is produced by all tissues of higher plants and by some microorganisms. A a plant hormone,ethylene regulates man aspects od growth development, and senescence and is physiologically active in trace amounts (less than 0.1 ppm). Ethylene synthesis starts with the aminoacid methionine, which is energized by ATP to produce S-adenosyl methionine (SAM). The key enzyme in the pathway, ACC synthase, converts SAM to 1-aminocyclopropane-1carboxylic acid (ACC), which is converted to ethylene by the action of ACC oxidase.

Classification of fruits according to the ethylene production

Transpiration or water loss: water loss is the main cause of deterioration because it results not only in direct quantitative losses (loss of salable weight), but also in losses in appearance (wilting and shrivelling), textural quality (softening, flaccidity, limpness, loss of crispness and juiciness) and nutritional quality. The dermal system i.e., cuticle, epidermal cells, stomata, lenticels and trichomes govern the regulation of water loss in the commodity.

Transpiration rate is influenced by internal or commodity factors (morphological and anatomical characteristics, surface to volume ratio, surface injuries and maturity stage) and external or environmental factors (temparature, relative humidity, air movement and atmospheric pressure). Transpiration (evaporation of water from the plant tissues)is a physical process that can be controlled by applying treatments to the commodity (e.g., waxes and other surface coating or wrapping with plastic films) or by manipulation of the environmental (eg., maintenance of high relative humidity and control of air circulation).