Rennet cheese ripening

Rennets. The use of rennets in cheesemaking is the principal application of proteinases in food processing and is second only to amylases among industrial applications of enzymes. The sources of rennets and their role in milk coagulation and cheese ripening are discussed in Chapter 10 and will not be considered here. 

The activity of rennet in some maturing cheese is essential for normal cheese ripening. The practice of using less rennet in making cheese with concentrated milk has shown that the cheese does not develop characteristic sharp flavors (Chapman et al. 1974). The role of rennet in flavor development of cheese is to produce peptides that are degraded subsequently by the bacterial flora of the cheese. 

Cheese is ripened for 6 months to 1 year or longer at 5° to 15°C and 70-75% relative humidity. Cheese ripening is a complex process involving a combination of chemical, biochemical, and physical reactions. Proteolytic enzymes, e.g., rennet and lactic starter culture enzymes, hydrolyze caseins to produce flavor compounds and proper body. Lipase and lactase enzymes also hydrolyze their respective substrates to produce a large number of characteristic flavor compounds (Reiter and Sharpe 1971 Harper 1959 Law 1981 Schmidt etal. 1976), including free fatty acids, methanethiol, methanol, dimethyl sulfide, diacetyl, acetone, and others (Moskowitz 1980). 

During cheese ripening, the population of starter bacteria generally decreases while the number of non-starter lactic acid bacteria (NSLAB) generally increases these changes are well documented for many full-fat rennet-curd cheese varieties, (e.g., Cheddar) (Cromie et al., 1987 Jordan and Cogan, 1993 McSweeney et al., 1993 Lane et al., 1997 Haque et al., 1997 Beresford and Williams, 2004). 

Cheese Ripening. Rennet plays a major role in the texture and flavor development of cheese during the ripening process. Besides the rapid cleavage of the key phenylalanine-methionine bond to coagulate milk, chymosin has been shown to hydrolyze at least 22 other bonds in the casein molecules. The favored amino acids at the point of cleavage are leucine, isoleucine and phenylalanine. 

Both proteolysis and lipolysis are involved in the cheese ripening process. The rate and extent of their interactions are influenced by the rennet preparation used, characteristics of the starter culture, pH, moisture range, salting practices, temperature, and the activity of adventitious microorganisms present in or on the cheese. 

Harding and Prucha worked, on cheese ripening at Geneva, N. Y., 1900-1910, and concluded that the digestion of toe casein of cheese during ripening is due to toe collective action of native enzymes of milk, bacterial enzymes, and rennet action. 

Lactate. Hard rennet cheeses represent a selective habitat for propionic acid bacteria, since they contain lactate formed as the end product of lactose fermentation by lactic acid bacteria. Unlike many other bacteria, propioni-bacteria can utilize lactate efficiently, which is the reason why propioni-bacteria are so abundant in hard, ripened cheeses. Propionic acid bacteria use lactate best in the presence of yeast extract (Antila, 1954), but even higher stimulatory effect is exerted by cell-free extracts of lactic acid bacteria. Streptococcus thermophilus and Lactobacillus spp. (Hietaranta and Antila, 1953). Lactate as a carbon source supports higher growth rates of propionic acid bacteria than lactose (El-Hagarawy et al., 1954). 

Proteolysis has three phases (see Section 2.4.5.2.1) proteolysis in milk before cheese manufacture due to indigenous milk protease (plasmin) activity, the enzymatically induced coagulation of the milk in rennet cheeses (hydrolysis of K-casein by rennin) and proteolysis dining ripening of most cheeses, which is the most important reaction having a major impact on flavour and texture. 

The strength of the rennet-induced gel is also adversely affected by heat treatment of the milk, again presumably because the whey protein-coated micelles are unable to participate properly in the gel network. Gels from severely heat-treated milk have poor syneresis properties, resulting in high-moisture cheese which does not ripen properly. Syneresis is undesirable in fermented milks, e.g. yoghurt, the milk for which is severely heat-treated (e.g. 90°C x 10 min) to reduce the risk of syneresis. 

The production of rennet-coagulated cheeses can, for convenience, be divided into two phases (1) conversion of milk to curds and (2) ripening of the curds. 

Syneresis. Renneted milk gels are quite stable if undisturbed but synerese (contract), following first-order kinetics, when cut or broken. By controlling the extent of syneresis, the cheesemaker can control the moisture content of cheese curd and hence the rate and extent of ripening and the stability of the cheese - the higher the moisture content, the faster the cheese will ripen... 

While rennet-coagulated cheese curd may be consumed immediately after manufacture (and a little is), it is rather flavourless and rubbery. Consequently, rennet-coagulated cheeses are ripened (matured) for a period ranging from about 3 weeks for Mozzarella to more than 2 years for Parmesan and extra-mature Cheddar. During this period, a very complex series of biological, biochemical and chemical reactions occur through which the characteristic flavour compounds are produced and the texture altered. 

Parmesan or Grana, as it is known in Italy, is a group of very hard bacteria-ripened, granular-textured cheeses made from partially skimmed cow s milk. They originated in Parma, near Emilia, Italy, hence the name. Special lipolytic enzymes derived from animals are used, in addition to rennet, to produce the characteristic rancid flavor. 

Starter cultures of heat-resistant lactobacilli and S. thermophilus are added, along with rennet, to form the curds. Manufacture and salting of the cheeses take about 20 days, with 12-15 days for brining. They are then stored in cool, ventilated rooms to ripen in one or two years. A fully cured Parmesan keeps indefinitely, is very hard and thus grates easily, and is used for seasoning. Low moisture and low fat contents contribute to its hardness. Parmesan cheese made in the United States is cured for at least ten months. 

Rao, K. S. N., Krishna, N., Nand, K., Srikanta, S., Krishna-Swamy, M. A., and Murthy, V. S. 1979. Changes during manufacture and ripening of Cheddar cheese prepared with fungal rennet substitute of Rhizopus oligosporus. Nahrung 23, 621-626. 

Rennet is inactivated at the high cooking temperatures used in Swiss and Mozzarella but is still active in Cheddar curd cooked to 39 °C (Matheson 1981). Residual rennet activity has implications for the subsequent ripening of the cheese. 

Penicillium caseicolum produces an extracellular aspartyl proteinase and a metalloproteinase with properties very similar to those of the extracellular enzymes produced by P roqueforti (Trieu-Cout and Gripon 1981 Trieu-Cout et al. 1982). Breakdown of casein in mold-ripened cheese results from the synergistic action of rennet and the proteases of lactic streptococci and penicillia (Desmazeaud and Gripon 1977). Peptidases of both lactic acid bacteria and penicillia contribute to formation of free amino acid and nonprotein nitrogen (Gripon et al. 1977). 

Part of the process to make cheese involves the flocculation of an electrostatically stabilized colloidal O/W emulsion of oil droplets coated with milk casein. The flocculation is caused by the addition of a salt, leading to the formation of networks which eventually gel. The other part of the process involves reaction with an enzyme (such as rennet), an acid (such as lactic acid), and possibly heat, pressure and microorganisms, to help with the ripening [811]. The final aggregates (curd) trap much of the fat and some of the water and lactose. The remaining liquid is the whey, much of which readily separates out from the curd. Adding heat to the curd (-38 °C) helps to further separate out the whey and convert the curd from a suspension to an elastic solid. There are about 20 different basic kinds of cheese, with nearly 1000 types and regional names. Potter provides some classification [811]. 

Various physico-chemical changes occur in the structural components of the para-casein matrix during maturation these changes are mediated by the residual rennet, microorganisms and their enzymes, and changes in mineral equilibrium between the serum and para-casein matrix. The type and level of the physico-chemical changes depend on the cheese variety, cheese composition and ripening conditions. These may include ... 

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