Origins of Cheese Flavor - ACS Symposium Series (ACS Publications)

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Chapter 9

Origins of Cheese Flavor

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Michael H . Tunick Dairy Processing and Products Research Unit, Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 600 East Mermaid Lane, Wyndmoor, PA 19038

The flavor of cheese is affected by many factors, including milk quality stemming from the quality of the milk, processing parameters such as pasteurization and addition of salt, and enzymatic and chemical reactions that occur as the cheese ages. Lactose and citrate are metabolized by lactic acid bacteria to form a number of important compounds, including acetoin, 2,3-butanediol, and diacetyl, which generate buttery, cheesy flavors. Proteolysis of casein by coagulant, plasmin, and other enzymes leads to the production of acids, alcohols, aldehydes, amines, and amino acids, which bring about alcoholic, fatty, and green flavor notes. The breakdown of aromatic, branched-chain, and sulfur-containing amino acids also produces flavor compounds, many of which are undesirable. Triacylglycerols are lipolyzed into fatty acids, which impart pungent, cheesy flavors. Fatty acids can then be converted into methyl ketones, secondary alcohols, lactones, esters, and other compounds, which are responsible for earthy, floral, fruity, and rancid flavors. An array of compounds contributes to the unique flavor characteristics of each cheese variety.

© 2007 American Chemical Society

In Flavor of Dairy Products; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

155

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156 The many types of cheese in the world provide an abundance of flavors. Starting with a bland product, milk, a cheesemaker adds starter cultures (selected lactic acid bacterial strains added to the milk), enzymes, and other chemicals (such as NaCl), and adjusts manufacturing procedures to produce any one of hundreds of cheese varieties, each with a characteristic flavor. Flavor in cheese arises from compounds in milk (transferred directly from diet, synthesized in mammary tissue, or added to the milk), control of manufacturing conditions (pasteurization, heat, etc.), and enzymatic and non-enzymatic chemical reactions. Flavors continue to form during storage, resulting from metabolism of lactate, lactose and citrate, proteolysis of caseins, and lipolysis of triacylglycerols. Compositional changes occur after milk is separated into curd and whey, the whey is drained off, and the curd is transformed into cheese. Cheddar cheese provides a representative example of these changes (Table I).

Table I. Percentages of components in bovine milk and fresh Cheddar cheese (/, 2).

Milk Cheese

Lactose

Citrate

4.8 0.8-1.0

0.18 0.2-0.5

Protein 3.2 25

Fat 3.3 33

Hundreds of compounds have been detected in cheese, many of which contribute to flavor (5), often in a synergistic manner. The purpose of this chapter is to review the processing parameters and the classes of compounds that lead to these flavors.

Processing Parameters Processing parameters affecting cheese flavor include the condition of the milk, pasteurization, homogenization, starter cultures, non-starter lactic acid bacteria, coagulant, whey syneresis, salt, surface microorganisms, and ripening time, temperature, and humidity.

Milk Changes in cows' diet will affect the flavor of milk and therefore the flavor of cheese. A common management practice that keeps feed consistent

In Flavor of Dairy Products; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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157 throughout the year is the use of silage, which consists of alfalfa, barley, corn, vetch, and wheat that has been allowed to ferment anaerobically and then fed to cows. In contrast, cows on pasture eat what they prefer. Some compounds from grasses, flowers, and other plants ingested by cows on pasture either pass directly into milk or are converted in the cow into other compounds (4). Many of these compounds impart desirable flowery flavors to cheese (5). When live pasture plants are consumed by cows, the plants activate the lipoxygenase system, which may be part of an injury response (6). Lipoxygenase activity breaks down lipids and carotenoids to products that affect cheese flavor (5). Nonanal, 2-nonenal, 2,4-decadienal, and methyl jasmonate are some of the compounds generated in this fashion from unsaturated fatty acids in plants. Citronellol, carvone, and geranyl acetate are precursors or degradation products of carotenoids (5). Linalool and a-pinene are terpenes often found in artisanal Alpine cheese (7). Research on the effects of pasture plants on cheese is in its infancy many of the world's cheese varieties are made from pasture-fed animals, but the flavor origins of these products and their concentrations in cheese have barely been examined. Urbach has reviewed the effects of feed on flavor (4). The health of the cows will also have an effect on cheese, as high somatic cell counts resulting from mastitis and poor nutrition will adversely affect coagulation and proteolysis (8).

Pasteurization and Homogenization Pasteurizing milk above 75°C denatures whey proteins, primarily 0lactoglobulin, which liberates sulfhydryls and volatile sulfides, especially H S (9). These compounds give the milk a cooked flavor that can carry over to the cheese. Pasteurizing at 62°C for 30 min is enough to kill many indigenous microflora while not imparting a cooked flavor to the resulting cheese, but this length of time is longer and less economical than the more common 71°C for 15 s (10). Pasteurization at high temperatures also causes heat-induced interactions of whey proteins with casein, which changes the characteristics of proteolysis by limiting accessibility of proteases and peptidases to casein micelles (//). In addition, pasteurization inactivates indigenous microflora and several milk enzymes, causing the resulting cheese to contain fewer flavor compounds than raw milk cheese because of decreased proteolysis and lipolysis (12). Some enzymes survive pasteurization and are present in the milk at the beginning of cheesemaking. Nonstarter lactic acid bacteria may also enter the curd from the environment and increase the extent of proteolysis, often detrimentally (14). Plasmin, an enzyme found in milk and not inactivated by pasteurization, also contributes to proteolysis. Cheesemilk is not usually homogenized because the fat globules are reduced in size, which alters texture and functionality of the resulting cheese. Small, homogenized fat globules are subject to a higher level 2

In Flavor of Dairy Products; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

158 of lipolysis, which may lead to rancid flavors (8) as discussed below. Increased lipolysis may be desirable in some cases; Feta and Blue cheese are often homogenized, and cream is sometimes homogenized before adding to cheesemilk.

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Starter Cultures Starter cultures include lactococci, lactobacilli, and leuconostocs, and are specifically selected for their ability to ferment lactose to lactic acid. These cultures are added at the start of cheesemaking in order to lower the pH of the milk before the coagulation step. Adjunct cultures are added at the same time; these are additional starters intended to produce particular flavors and may include propionibacteria, which produces the openings in Swiss cheese, and Penicillium roqueforti mold, which is used in Blue and Brie manufacture. Lipase enzymes are added in the manufacture of Parmesan and Romano, enhancing lipolysis. Lactic acid bacteria influence the rate and extent of acid development, which controls the dissociation of colloidal calcium phosphate; these factors affect the rate of proteolysis and flavor production (75). Live starter cultures probably contribute little to flavor development once the cheese is made, but the enzymes they release upon cell death are responsible for proteolysis and some lipolysis (8).

Coagulant Milk is coagulated by the addition of food-grade acid, which may impart its own flavor to cheese, or rennet, which is a mixture of proteolytic enzymes. Acid addition is not normally used for ripened cheese. Rennet, traditionally extracted from calf stomach, is now more economically and more commonly obtained from microbial sources such as Rhizomucor miehei, and from microbial fermentation. Chymosin, the primary enzyme in rennet, cleaves the K-casein that stabilizes the casein micelle, causing the micelles to aggregate into a curd. Rennet survival in curd increases as the temperature of cooking decreases, leading to more proteolysis in the resulting cheese. Chymosin preferentially hydrolyzes ctsi-casein, but R. miehei enzymes also attack a - and P-casein, which leads to greater peptide formation and additional flavors (13). s2

Syneresis Whey imparts a tart flavor that may be objectionable in many cheese varieties. Around 90% of coagulated milk consists of whey, and cheesemakers

In Flavor of Dairy Products; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

159 try to remove much of it so they can isolate the curd. Syneresis (whey expulsion) decreases the moisture in the curd, which slows the growth of bacteria and therefore reduces proteolysis. Syneresis increases with the temperature and time at which the curd is coagulated and cooked. Cheesemakers also increase syneresis by reducing the size of the cubes into which the curd is cut, thus increasing their surface area (75). Whey proteins, which account for 20% of milk protein, comprise only 3-6% of the protein in cheese.

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Salt and Coloring Before cheese is packaged, NaCl is added to modify salty flavor and to regulate the growth of microorganisms. NaCl is added in dry form to the curd, or the cheese is immersed in brine. Starter culture activity and metabolism of lactose are inhibited when the salt-in-moisture concentration goes above 5% (