The Making of Great CheesesThe first steps in making great cheeses involve building the right physical andchemical environment to support microbial ripening

Paul S. Kindstedt

Cheese making transforms milk into either freshor ripened cheese, with the aging process rangingfrom weeks to years. The fırst days establish thechemical characteristics of the cheese before itbegins to age, setting the stage for microbial rip-ening. Of the plethora of organisms in newcheese, some remain viable and may proliferateduring aging, others will be suppressed, while stillothers may be suppressed initially and then fa-vored or vice versa, depending on the chemicalenvironment to which they are subject. To add tothis complexity, the chemical environmentwithin the cheese may change dramatically asripening progresses.Much of this complexity canbe reduced to a few scientifıc principles thatcheese makers vary systematically to achieve awide range of outcomes. The historical develop-ment of distinctly different cheese varieties in-volved modulating these basic principles.

The goal of this two-part series is to paint aconceptual picture in which the microbiology ofcheese making is seen to fıt closely with other,basic cheese making practices and the scientifıcprinciples that underpin them. In part one, wefocus on the early steps in making cheese, wherethe chemistry and physical properties are set forthe subsequentmicrobiologically driven ripeningphase.

The Basics of Milk Coagulation

Milk coagulation is the key fırst step in whichcasein, the major protein in milk, and fat com-bine to form curds, while most of the water andlactose form whey. Cheese makers of old discov-ered three ways to coagulate milk, and their usegave rise to three distinct families of cheese: acidcoagulated, acid-heat coagulated, and rennet co-agulated. Cheeses from the fırst two families arealmost always consumed fresh because of theirhigh moisture content and, in the case of acid-

heat coagulation, high pH, that make them proneto spoilage and rotting as they age. In contrast,rennet coagulation yields cheeses with muchlower moisture content that support desirablemicrobial transformations during aging.

Rennet enzymes, which derive from a varietyof animal, plant, and microbial sources, are as-partic proteinases that preferentially hydrolyzeand destabilize casein, leading it to coagulate.About 80% of the protein in milk consists ofcasein molecules that form spherical macromo-lecular micelles that each contains about 10,000molecules. Casein micelles also contain abun-dantly dispersed nanocrystals of calcium phos-phate.

Rennet coagulation is a two-step process thatbegins with the enzymes hydrolyzing casein, es-pecially along the micelle surface. This activitytriggers a nonenzymatic phase, during which thespherical micelles progressively lose their abilityto interact with water molecules, forcing them tointeract instead with one another to form aggre-gates and chains. As coagulation progresses, themicellar chains increase in length and thickness,interlocking to form a three-dimensional, net-like matrix that entraps water and other majorcomponents of milk, including lactose, fat, wheyproteins, and minerals (Fig. 1).

A critical feature of rennet curd is its capacity,

SUMMARY

➤ Cheese makers follow seven early steps to coagulate milk into curd,expelling whey and minerals while incorporating salt into the newly made,unripened cheese.

➤ By controlling the conditions at each of these steps, cheese makers buildthe right moisture, pH, and salt to support microbial, enzymatic, physical,and biochemical changes that occur as the cheese ripens.

➤ The last cheese-making step, affinage or finishing, which may take weeks oryears, very much depends on setting the correct chemical environmentfrom the outset.

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called syneresis, to contract and expel whey. In-deed, cheesemakers take pains to control synere-sis precisely so as to achieve a wide range ofmoistures levels within their cheeses, opening theway to producing an extraordinary diversity ofripened cheeses. Thus, the discoveries of variousmeans to control whey expulsion through inno-vations in cutting, cooking, pressing, and saltingare major milestones in the history of rennet-coagulated cheese making.

Rennet coagulation produces curds that arerich in calcium phosphate, whose capacity to ab-sorb and neutralize hydrogen ions explains itshigh buffering capacity. However, as lactic acidaccumulates when starter cultures digest lactose,that buffering capacity decreases when micellarcalcium phosphate dissolves into the whey and islost from the cheese. Therefore, the greater therate of lactic acid production during cheese mak-ing, the greater the loss of calcium phosphate tothe whey, and the lower the buffering capacityretained in the cheese. Buffering capacity is ex-tremely important because it helps to determinethe cheese pH at the start of ripening. Cheeses in

which acid forms slowly are rich in calciumphos-phate and retain a strong buffering capacity.

Thus, it is possible to produce rennet-coagu-lated cheeses that range widely in both moisturecontent and initial pH by modulating the rates atwhich whey is expulsed and lactic acid is pro-duced. This wide compositional latitude makespossible an extraordinary diversity of ripeningoutcomes. Indeed, the overwhelming majority ofcheeses are rennet-coagulated types.

The Basic Early Steps of Cheese Making

Rennet-coagulated cheeses can prove challeng-ing because of the complex and unforgiving na-ture of the fınal ripening process. For ripening togo well, the newly made cheese needs to be at theright pH and have the correct moisture and saltcontents. They set the stage for the complex phys-ical, biochemical, and microbiological changesthat follow.

Producers fırst need to expel the correctamount of whey from the curd to achieve a mois-ture content ranging from 60 to 30%, depending

FIGURE 1

Diagram of coagulation mediated through the action of rennet enzymes that cleave casein at the casein micellesurface. This causes the micelles to aggregate in the form of a net-like matrix that transforms the liquid milk to agel.

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on the cheese variety being made. Second, theproducer needs to control the rates of acidifıca-tion and curd demineralization so that the buff-ering capacity of the newly made cheese is com-patible with a pH ranging from 5.4 to 4.6. Finally,salt must be incorporated into the cheese at thecorrect rate to a content varying from 0.5 to 4.0%or higher. In short, the fırst day of cheese makingis a series of steps to dehydrate and demineralizethe rennet coagulum and then to add salt to thecurd.

The pH, moisture, and salt contents of theunripened curd collectively shape the chemicalenvironment of the cheese. The chemical envi-ronment and physical parameters such as tem-perature, humidity, exposure to oxygen and airmovement, and physical handling such as rub-bing, brushing, and turning determine whichbacteria, yeasts, and molds within the cheese andon its surface will be favored or suppressed, aswell as the timing and sequence in which theyproliferate and die off. The microbiological pro-gression, in turn, profoundly affects the develop-ment of cheese flavor, aroma, texture, and ap-pearance.

The chemical environment also determineswhich of themany enzymes within the cheese areswitched on or suppressed, and sometimes theirspecifıcities. Finally, the chemical environmentand microbiological and enzymatic changes alsodetermine physical and chemical changes, espe-cially those that involve casein-water interactionssuch as structural swelling and casein solubliza-tion. Therefore, if the moisture, pH, or salt con-tents fall outside their target ranges, the cheesecannot ripen along the intended path, thus lead-ing to a different cheese with different character-istics.

More Detailed Look at FirstSteps in Cheese Making

The basic steps used to make rennet-coagulatedcheeses have changed little over thousands ofyears. Cheese makers of old learned to controlmoisture, pH, and salt contents by varying con-ditions during different steps and by using inno-vative equipment and techniques to producecheeses that were suited to their needs. The basicsteps of rennet-coagulated cheese making in-clude setting, cutting, cooking, draining, knit-ting, pressing, salting, and fınishing, or affınage.

The earliest step, called setting, includes inoc-

ulating milk with a starter culture containing lac-tic acid bacteria (LAB), incubating briefly, addingrennet, and then allowing more time for the milkto coagulate. In ancient practice, LAB enteredmilk as adventitious contaminants and grewquickly at ambient temperatures.

By the end of the 19th century,microbiologistsbegan isolating and identifying the sources forrennet as well as the microorganisms responsiblefor fermenting milk during cheese making. Soonthereafter, microbiologists began characterizingin considerable detail many LAB strains for theircontributions to cheesemaking. After identifyingthe best-acting and reliable LAB strains, microbi-ologists developed means for freezing or freeze-drying them, and then preparing precise blendsto deliver specifıc outcomes.

First and foremost, the role of the starter LABis to deliver a predictable rate of acidifıcationduring cheese making. Starter LAB strains usedfor making cheese fall into two broad categoriesbased on temperature sensitivity. Those calledmesophiles include Lactococcus lactis subsp. lac-tis, Lactococcus lactis subsp. cremoris, while oth-ers, called thermophiles, include Lactobacillusdelbrueckii subsp. bulgaricus, Lactobacillus helve-ticus, Lactobacillus delbrueckii subsp. lactis, andStreptococcus thermophilus. The mesophiles areinactivated at temperatures higher than 40°C,whereas the thermophiles remain active to about65°C.

Renneting follows the addition and ripeningof the starter culture. Generally, enough rennet isadded to cause the liquidmilk to forma gelwithin60minutes. After the gel forms, it needs cutting toexpedite the separating of whey from the curd(Fig. 2). Cutting greatly expands the curd surfacearea, acting as a gateway for whey release. Thegreater the surface area to volume ratio of the cutparticles, that is, the smaller the curd particles, thegreater the release of whey, and the lower themoisture content of the cheese.

The step after cutting—cooking—includesheating and stirring the curds and whey. If thisstep involves no heat, the cheese is called un-cooked. Cooking to higher temperatures, longertimes, and with more stirring allows the curd tocontract and expel more of the whey. Moreover,cooking influences how rapidly LAB strains pro-duce lactic acid and that, in turn, affects syneresisbecause curd particles contract and expel wheymore readily as the pH decreases. The effects ofcooking temperature can be complex, affecting

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not only lactic acid production rates but also curddemineralization and buffering capacity. Thus,cooking times and temperatures need to be deli-cately balanced to shrink, dehydrate, acidify, anddemineralize the curd particles as they are pre-pared for ripening.

Before ripening begins, however, the whey isseparated from the curd particles, which are thenfused, or knitted, to form a larger entity. Early on,cheesemakers developed two approaches for sep-arating whey from curds. The more ancientmethod is referred to as dipping, whereby themixture is scooped or poured from the coagula-tion vessel into a draining vessel (Fig. 3). The

other more recent approach, draining, involvesleaving the curd in the coagulation vat while thewhey is drained off through a valve fıtted with astrainer.

The knitting of curd particles begins as thewhey is removed. Knitting proceeds, often forseveral hours, until the fusing curd forms a con-tinuous mass. Knitting continues the dehydra-tion and demineralization processes that beganduring cooking.

Knitting is often accompanied or followed bypressing—applying pressure to the curd to re-lease additional whey. Early cheese makers usedtheir hands to push or piled stones on the curd

FIGURE 2

Cutting the coagulated milk by hand with wire knives in artisanal cheese making. Cutting creates new surface areathat promotes the separation of whey from the resulting curd particles. (photo courtesy of The Cellars at JasperHill, Greensboro, Vt.)

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mass to remove further whey. Some cheeses arereferred to as unpressedwhen gravity alone is reliedon to knit the curd particles. However, pressinghelps to expel whey and promotes more completefusion of the curd particles, resulting in a moreclosed texture and a surface with fewer openings.

Dry salt can be rubbed onto the surface of anunfınished cheese at this stage, and it dissolvesinto the water phase and gradually diffuses intothe cheese interior (Fig. 4). The cheese also can besubmerged in concentrated salt brine, or the curdcan be broken into particles and mixed with salt

FIGURE 3

Dipping the curds and whey en masse into cylindrical draining forms in artisanal cheese making. Dipping initiatesthe permanent separation of free whey from the curd particles and begins the process of curd knitting. (Photocourtesy of The Cellars at Jasper Hill, Greensboro, Vt.)

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before pressing the now-salted curd into a blockof unripened cheese.

Conclusion

These steps are completed within one or a fewdays, whereas affınage, or fınishing, unfolds overthe next weeks or years. Depending on the cheesevariety being sought, the initially bland cheese ismicrobiologically, enzymatically, physically, andchemically transformed into true greatness.However, unless the cheese maker successfullybuilds the correct chemical composition into thenewly made cheese during those fırst early steps,ripening will either be for naught due to spoilageor will shift away from the intended outcome, forbetter or for worse.Paul S. Kindstedt is a Professor in the Department of Nutritionand Food Sciences at the University of Vermont, Burlington.

Suggested Reading

Dejmek, P., and P. Walstra. 2004. The syneresis of fen-net-coagulated curd, p.71–104. In P. F. Fox, P. L. H.McSweeney, T. M. Cogan, and T. P. Guinee (ed.),Cheese chemistry, physics and microbiology, volume1 general aspects, 3rd ed. Elsevier Academic Press, SanDiego, Calif.

Fox, P. F., T. P. Guinee, T. M. Cogan, and P. L. H.McSweeney. 2000. Fundamentals of cheese science.Aspen Publishers, Inc., Gaithersburg, Md.

Harboe, M., M. L. Broe, and K. B. Qvist. 2010. Theproduction, action and application of rennet and co-agulants, p.98–125. In B. A. Law and A. Y Tamime(ed.), Technology of cheesemaking, 2nd ed., Wiley-Blackwell, Chichester, United Kingdom.

Johnson, M., and B. A. Law. 2010. The origins, develop-ment and basic operations of cheesemaking technol-ogy, p.70–97. In B. A. Law and A. Y Tamime (ed.),Technology of cheesemaking, 2nd ed., Wiley-Black-well, Chichester, United Kingdom.

FIGURE 4

Applying dry salt by hand to the surface of newly made cheese. The salt draws whey out from the cheese body tothe surface as the salt dissolves into the water phase of the cheese and diffuses inwards. (Photo courtesy of TheCellars at Jasper Hill, Greensboro, Vt.)

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Kosikowski, F. V., and V. V. Mistry. 1997. Cheese andfermented milk foods, vol. 1: origins and principles.F.V. Kosikowski L.L.C., Great Falls, Va.

Parente, E., and T. M. Cogan. 2004. Starter cultures:general aspects, p. 123–148. In P. F. Fox, P. L. H.McSweeney, T. M. Cogan, and T. P. Guinee (ed.),

Cheese chemistry, physics and microbiology, volume1 general aspects, 3rd ed. Elsevier Academic Press, SanDiego, Calif.

Walstra, P., J. T. M. Wouters, and T. J. Geurts. 2006.Dairy science and technology, 2nd ed. CRC Press,Boca Raton, Fla.

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