The quality of milk used in Cheddar manufacture influences the cheese yield, the flavor, and the texture character of the final product. The most important milk constituents in terms of cheese yield and texture are the fat and casein content of the milk; the microbiological quality of the milk (6,7) and the presence of indigenous milk enzymes (8) have an impact on the ultimate flavor of the cheese.
The breed of cow, stage of lactation, and the type and level of feeding all influence milk composition (9). Seasonal changes in milk composition result mainly from lactational and dietary effects. The principal determinants of yield and quality of cheese are the fat and casein content of the milk. The sum of these two components is the principal determinant of cheese yield, and the ratio influences both the quality and composition of the final Cheddar. Milk for Cheddar manufacture in large commercial creameries is delivered by tanker from individual farms and is bulked and stored in silos prior to standardization and pasteurization. Extended storage of raw milk prior to manufacture can result in growth of psychro-tropic organisms. These produce heat-stable lipolytic enzymes that survive pasteurization and can have a negative influence on flavor development during ripening (6,7). Seasonal variation in milk composition gives rise to significant changes in the balance of fat and protein and as a consequence of this, milk for Cheddar manufacture is generally standardized so that a casein to fat ratio of 0.70 is achieved. Failure to standardize can result in reductions in yield efficiency, lack of moisture control during manufacture, and production of substandard cheese that does not meet required specifications for fat in dry matter.
Following standardization, milk for Cheddar manufacture is pasteurized. Pasteurization effectively destroys pathogenic nonsporing bacteria, but spore-forming bacteria such as Clostridium tyrobutyricum can survive and potentially give rise to defective flavors and gas production in Cheddar in the later stages of maturation.
The basic steps in Cheddar manufacture are outlined in Fig. 1. The temperature profiles shown are typical, but processing times are related to scale of manufacture and differ among various commercial systems and traditional production. Manufacture of Cheddar
cheese is initiated by coagulation of starter acidified milk with chymosin or a suitable chymosin substitute (10,11). The chymosin destabilizes the colloidal suspension of casein micelles by hydrolysis of n-casein (11). The release of the hydrophilic glycomacropeptide reduces steric hindrance and the surface micelle charge that results in micellar aggregation to form a gel (11). The resulting coagulum, which in Cheddar manufacture is formed at a temperature between 30 and 32°C and a pH of 6.4, comprises a continuous casein network in which fat is entrapped. The coagulum is then cut into small cubes and heat is applied with stirring to facilitate expulsion of moisture from the curd. As the temperature during scald is increased from 30 to 39°C over a period of 45 min, the curd particles begin to contract and expel moisture by a process termed syneresis.
Moisture expulsion is further enhanced by continued acid production by the starter and maintenance of the temperature at 39 °C while stirring continues for a further 45 min. At an appropriate level of acidification and moisture reduction, the whey is removed from the curds by drainage. The pH of the whey at pitching is approximately 6.18 in traditional Cheddar manufacture. Approximately 90% of the starter culture added during Cheddar manufacture is retained in the curd following whey drainage.
Following drainage, the process of cheddaring begins. During the cheddaring period, the curds are encouraged to fuse by application of pressure. Acid development by the starter proceeds rapidly due to the high concentration of starter in the curd. Under the influence of heat, acid, and pressure, the curd particles mat together to form a solid mass. A central channel is cut in the fused curd to allow whey drainage in traditional manufacture. The curd mass is then cut into slabs which are turned and stacked at regular intervals in a controlled and incremental manner over a period of about 90 min. In modern mechanized systems, the process of cheddaring is highly automated and is achieved on a continuous belt system enclosed in tunnels (12). The perforated belt system allows whey drainage as the curd particles mat together and incorporates a mechanism for turning the curd.
Once the appropriate level of moisture, acidification, and structure is obtained the cheddared curd is milled and dry-salted. Addition of salt facilitates further reduction of moisture level in curd and inhibits acid production by the starter. In traditional manufacture the salted curd is transferred to molds for overnight pressing. However, in highly automated manufacturing systems the milled curd particles are vacuumed into tower block formers (12). The tower acts as a pressing system by virtue of the weight of curd in the vertical column. Whey is continually siphoned off as the curd mass proceeds down the tower. A guillotine system at the base of the tower called a block former cuts a 20 kg block from the stack at regular intervals and the block is transferred to a vacuum packing system. The pressed cheese is matured at temperatures ranging from 5 to 12°C. An important factor in the control of Cheddar quality is the rate and extent of acid production in the vat because this largely determines the final pH and basic structure of the cheese (13). The pH at which the whey is drained from the curd is a critical control point in determining Cheddar quality because this influences the calcium content of the final cheese and the proportion of chymosin retained in curd. Stringent monitoring and control of starter activity during manufacture is essential.
Cheddar cheese is matured for periods of from 3 to 18 months, but extended maturation periods of up to 2 years or more may be used occasionally in production of specialist cheeses. The length of the maturation period determines the flavor and texture character of the cheese. Cheddar produced to a mild recipe would be marketed at 3 months when the flavor is mild and creamy and the young cheese has a pliable texture. A strong flavor intensity, and a texture that is firm and crumbly would be characteristic of a cheese produced to a mature recipe, which would be ripened in excess of 12 months.
Compositional standards for Cheddar stipulate that the cheese should contain 39% or less moisture and more than 48% fat in dry matter. Cheddar produced for the mature or extra-mature market would be expected to have a moisture content of 33-35%, a fat in dry matter content of 52-54%, a salt content of 1.6-1.8%, and a pH of 4.95-5.25. A higher-moisture cheese, 37-38%, would be ripened for 3-4 months and marketed as mild Cheddar. Grading systems based on compositional parameters have been used for many years to predict quality (13), but more recently attribute grading by sensory profiling flavor character has been used to supplement compositional grading. The attribute grading system has two key elements: a descriptive analysis to determine the sensory attributes of the cheeses and a fitness-for-purpose judgment to determine whether the attribute profile meets the final Cheddar specification (14).
Flavor and texture in Cheddar develop over an extended maturation period. Proteolysis is the most extensive biochemical event occurring during ripening and lipolysis is limited in contrast (10). The proteolytic process is initiated by the hydrolysis of as1-casein by chymosin originating in the coagulant and the degradation of h-casein by the micelle-associated plasmin (10). Approximately 6% of the chymosin added to milk for Cheddar manufacture is retained in the curd. Chymosin retention in Cheddar is determined by the pH of whey at drainage and activity during ripening is controlled by the pH of the cheese (10). Residual chymosin in Cheddar rapidly hydrolyzes as1-casein at the Phe23-Phe24 bond producing as1-casein (f1-23) and as1-casein (f24-199) (15). Further degradation of the as1-casein (f24-199) peptide by chymosin has been shown to generate the peptides as1-casein (f33-199), as1-casein (f102-199), and as1-casein (f110-199). Further hydrolysis of the as1-casein peptides is achieved by action of plasmin and the lactococcal cell envelope associated proteinase.
The indigenous milk proteinase, plasmin, is stable to pasteurization and is almost completely retained in curd during Cheddar manufacture to give a concentration of approximately 3-4.5 Ag g—1 curd (10). The main role of plasmin in Cheddar ripening is in the degradation of h-casein. h-Casein is more resistant to hydrolysis than a-s1 casein, and only 50% of h-casein is degraded in mature Cheddar (15). The primary cleavage sites of plasmin on h-casein are Lys28-Lys29, Lys105-Gln 106, and Lys107-Glu108. Hydrolysis at these sites yields the peptides h-casein (f29-209), h-casein (f106-209), and h-casein (f108-209) (13). Plasmin is also responsible for the cleavage of the as1-casein (f24-199) at the Lys103-Tyr104 and Lys105 — Val106, yielding as1-casein (f104-199) and as1-casein (f106-199) (15).
The initial degradation of the caseins in the first weeks of ripening results in the softening of the rubbery texture of young Cheddar curd, but the large peptides generated are tasteless. Further degradation of the casein-derived peptides by the starter and nonstarter lactic acid bacteria (NSLAB) results in the formation of small peptides, amino acids, and volatile compounds derived from amino acid catabolism that contribute to flavor.
Development of Cheddar flavor cannot be associated with an individual potent chemical but results from the generation of a number of components during ripening. The production of these flavor compounds must be balanced, and excessive production of one component can lead to development of off-flavors (16).
Peptides and amino acids contribute to the background savory flavor of Cheddar, but specific peptides can impart undesirable bitter flavours. Volatile aroma compounds have potential to add specific aroma and flavor notes to Cheddar. These are derived mainly from catabolism of amino acids, but also originate from lipolysis. Origins of flavor components in Cheddar have been studied extensively and several comprehensive reviews have been published (16-18).
Volatile components contributing to flavor include fatty acids, ketones, lactones, esters, and volatile sulfur compounds. 3-Methylbutanoic acid, derived from catabolism of leucine, has a rancid, cheesy, sweaty odor that may contribute to mature Cheddar aroma. Enhancement of this component in young Cheddar using a-ketoacids has been shown to be associated with an increase in the intensity of Cheddar aroma (19). Butyric acid, derived from lipolysis, has a cheesy, sweaty odor and is considered to be an important component of Cheddar flavor. Methional present in Cheddar has a boiled potato-like aroma; methane-thiol, DMDS, and DMTS add garlic notes to the flavor of mature Cheddar. The aldehyde 3-methylbutanal at high concentrations imparts unclean harsh flavor in Cheddar but at low levels gives a pleasant fruity flavor. Ethylbutyrate, if present at excessively high levels, may cause a fruity flavor defect in Cheddar. Indole and skatole, products of the catabolism of phenylalanine and other aromatic amino acid metabolites, are responsible for unclean and barnyard flavors in Cheddar.
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