Rennet-Curd Cheese

Cheese is essentially a concentrated protein gel, which occludes fat, moisture, and other materials. Gelation of the milk is brought about either by  

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  

The hydration and swelling of the casein matrix has a major influence on the structure of the fat phase and the cooking properties of the cheese, as discussed in Sections 11.3.1.2 and 11.9. 

The enmeshed fat globules occupy the spaces between the protein strands and may be considered to impede physically the aggregation of the para-casein matrix, to a degree dependent on their volume fraction and size distribution. Consequently, a higher fat level leads to slower syneresis during manufacture (Dejmek and Walstra, 2004), and an increase in the level of MNFS in the cheese (Tunick et al., 1995 Poudaval and Mistry, 1999 Fenelon and Guinee, 1999) the increase in MNFS has a major impact on cheese yield and quality, as discussed in Sections 11.4, 11.6-11.9. 

Some clumping and/or coalescence of fat globules generally occur in most cheese varieties. Evidence for the clumping of fat globules in Cheddar cheese has been demonstrated clearly by both SEM and TEM 

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The FDM content is between 42—56% for most of the rennet-curd cheese varieties (e.g., Cheddar, Gouda, Blue, Brie), but varies from 33%> in Grana Padano and low-moisture part-skim Mozzarella to 70% in Cam-bazola (see USDA, 1976 Holland et al., 1991 Robinson, 1995 Kosikowski... 

Hydrolysis of the casein micelle-stabilizing K-casein by the action of selected acid proteinases (rennets), and the resultant slow quiescent aggregation of the destabilized micelles in the presence of calcium ions ( 3 mM) at 30-36°C (e.g., for most rennet-curd cheeses such as Cheddar, Mozzarella and Gouda)... 

Acidification to the isolectric pH of casein using lactic acid bacteria or food-grade acids/acidogens, at 20 40 C. and resultant slow quiescent aggregation of the sensitized casein micelles e.g., for cream cheese. [A combination of acidification and rennet-hydrolysis (a smaller quantity of rennet than for rennet-curd cheeses, e.g., 5-100 versus 900-1000 chymosin units per 100 L milk) is normally used for low-fat acid-curd cheeses such as Quark and related varieties (Schulz-Collins and Senge, 2004)]... 

The effects of fat content on the microstructure of Cheddar (Mistry and Anderson, 1993 Bullens et al., 1994 Baer et al., 1995 Bryant et al., 1995 Desai and Nolting, 1995 Metzger and Mistry, 1995 Drake et al., 1996a,b Guinee et al., 1998, 2000a) and Mozzarella (McMahon et al., 1996, 1999) have been evaluated in a number of studies but little information is available on effect of fat content on the structure of other rennet-curd cheese varieties. 

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). 

A reduction in fat level results in a decrease in the moisture-to-protein ratio, as reflected by the lower content of MNFS (Figure 11.1 Table 11.1). However, the flowability of rennet-curd cheeses is positively correlated with the content of MNFS (Riiegg et al., 1991 McMahon et al., 1993), an effect which may, in part, be due to the concomitant increase in casein hydration and the lubrication effect of moisture. 

Farkye, N.Y. 2004. Acid- and acid/rennet-curd cheeses. Part C Acid-heat coagulated cheeses. In Cheese Chemistry, Physics and Microbiology, Vol. 2, Major Cheese Groups, 3rd edn (P.F. Fox, P.L.H. McSweeney, T.M. Cogan, T.P. Guinee, eds.), pp. 343-348, Elsevier Academic Press, Amsterdam. 

Cheddar. However, acid-curd cheeses are relatively low in calcium compared to rennet-curd cheeses such as Cheddar (ca. 0.75% Ca) and Swiss (ca. 0.95% Ca) (Table III). 

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. 

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. 

The word curdling comes from curd, the coagulated solid formed by adding acid or rennet to milk. Curds are the precursors of most cheeses. 

The most abundant milk protein is casein, of which there are several different kinds, usually designated a-, (1-, and K-casein. The different caseins relate to small differences in their amino acid sequences. Casein micelles in milk have diameters less than 300 nm. Disruption of the casein micelles occurs during the preparation of cheese. Lactic acid increases the acidity of the milk until the micelles crosslink and a curd develops. The liquid portion, known as whey, containing water, lactose and some protein, is removed. Addition of the enzyme rennet (chymosin) speeds up the process by hydrolysing a specific peptide bond in K-casein. This opens up the casein and encourages further cross-linking. 

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... 

Coagulant. Most of the coagulant is lost in the whey but some is retained in the curd. Approximately 6% of added chymosin is normally retained in Cheddar and similar varieties, including Dutch types the amount of rennet retained increases as the pH at whey drainage is reduced. As much as 20% of added chymosin is retained in high-moisture, low-pH cheese, e.g. Camembert. Only about 3% of microbial rennet substitutes is retained in the curd and the level retained is independent of pH. 

To increase curd elasticity and improve eye formation, the milk used to produce Swiss cheese must be clarified. Standardization of the fat content of the milk after clarification ensures uniform composition. Rennet and lactic acid from the bacteria cause casein coagulation. Swiss cheeses made in the United States are cured for three to four... 

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. 

M. pusillus var. Lindt protease has given satisfactory results as a chymosin substitute in the manufacture of a number of cheese varieties, but not all varieties of M. pusillus var. Lindt are capable of producing acceptable cheese (Babel and Somkuti 1968). The clotting activity of M. pusillus var. Lindt protease is more sensitive to pH changes between 6.4 and 6.8 than chymosin, but is much less sensitive than that of porcine pepsin (Richardson et al 1967). The same authors reported that CaCL added to milk affected the clotting activity of M. pusillus var. Lindt rennet more than it did that of chymosin rennet. They also reported that this rennet was more stable than chymosin between pH 4.75 and 6.25. M. pusillus var. Lindt rennet is not destroyed during the manufacture of Cheddar cheese, although less than 2% of the enzyme added to the milk remains in the curd. Nearly all of it is found in the whey (Holmes et al. 1977). Mickelsen and Fish (1970) found M. pusillus var. Lindt rennet to be much less proteolytic than E. parasitica rennet but more proteolytic than chymosin rennet on whole casein, a8-casein and /3-casein at pH 6.65. 

The casein micelles become surrounded by whey proteins and cannot interact with one another, thus reducing whey syneresis. This results in a soft curd that retains more moisture. The yield of cheese is increased due to the incorporation of whey proteins and the higher moisture content. Overheated milk requires longer rennet coagulation times. If milk is heated for 30 min at 75° C, it will not clot at all (Ustu-nol and Brown 1985). 

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. 

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