Cheese is one of the oldest human foods and is developed approximately 8,000 years ago. About 2,000 distinct varieties of cheese are produced throughout the world, representing approximately 20 general types. Cheese is regarded as an important fermented food and the manufacture is one of the classical examples of food preservation since ancient period.
Preservation of the most important constituents of milk (i.e., fat and protein) as cheese exploits two of the classical principles of food preservation, i.e., lactic acid fermentation and reduction of water activity through removal of water and addition of sodium chloride. The establishment of a low redox potential as a result of bacterial growth contributes to the storage stability of cheese.
Cheeses are classified based on texture or hardness:
(1) Soft cheese (cottage, cream, brie),
(2) Semi soft cheese (Muenster, Limburger, blue), and
(3) Hard cheese (cheddar, Colby, Swiss).
The cheese production is carried out from lactic acid fermentation of milk. By the enzymatic activity of rennin the coagulation of milk protein and formation of curd will take place. After the curd is formed it is heated and pressed to remove the watery part of the milk, salted and then ripened. The organisms responsible for the manufacturing of cheese are Lactobacillus lactis, Propionibacterium sps. and Penicillium sps.
The basic principles of cheese manufacturing involves the removal of water from milk with a consequent six to tenfold concentration of the protein, fat, minerals and vitamins by the formation of a protein coagulum that then shrinks to expel “whey”. The processes involved are- acidification, coagulation, cooking, salting, dehydration or syneresis, moulding (or shaping) and pressing, packaging and maturation or storage.
The manufacture of cheese involves the following steps:
Pasteurisation of the milk kills nearly all the microorganisms present in the milk including the pathogenic bacteria that cause diseases, such as, tuberculosis and leptospirosis and other undesirable microorganisms, such yeasts and coliforms that may alter the cheese characteristics by producing carbon dioxide and undesirable proteolysis.
Acidification of the milk is important for the proper release of whey from the cheese curd and to control the growth of many undesirable bacteria. It is usually done by the addition of lactic acid bacteria that convert lactose to lactic acid. The lactic acid bacterium in this case is regarded as starter culture.
The large volumes of starter culture is required for cheese making are made in special bulk starter fermentation pots in which the milk is heat treated to destroy unwanted bacteria, spores and phages and cooled to about 22°C, a temperature suitable for starter growth. The frozen starter is mixed in and fermentation continues for about 6 to 16 hours.
The amount of starter required varies for the different cheese varieties but, for Cheddar cheese, this is normally between 1.25 and 2.0% of the cheese milk. The amount of lactic acid produced and the moisture in the finished cheese regulate and control the subsequent rate of the biochemical changes that take place during the ripening or maturation of the cheese.
Coagulation of milk is an important step in cheese manufacturing. Here the casein fraction of the milk to form a gel that can be achieved by lowering the milk pH and the addition of “rennet”, a mixture of specific proteolytic enzyme. The most commonly used rennet contains the enzyme chymosin, an enzyme obtained from calf abomasums, animal sources and microorganisms or as the recombinant product.
The four main groups of caseins in milk are the αsl-, αs2-, β-and k-caseins. These are phosphoproteins held together by microclusters of calcium and phosphate and exist in milk as micelles of about 100 nm in diameter containing hundreds of molecules of each type of casein. The more hydrophobic regions of these phosphoproteins are believed to be located inside the micelle with the more hydrophilic regions of K-casein on the outside.
The negatively charged carboxy-terminal of the K-casein molecules is thought to protrude ‘hair-like’ structures from the micelle and repel other casein micelles (charge stabilisation). The addition of rennet leads to the partial proteolysis of K-casein by cleavage at the Phe105-Met106 bond.
The release of the hydrophilic carboxy- terminal peptide (glycomacropeptide) results in destabilisation of the micelles which become less negatively charged and more hydrophobic. These micelles then aggregate (in the presence of calcium and at a temperature above 15°C) to form a coagulum. A rennet coagulum consists of a continuous matrix of strands of casein micelles, which incorporate fat globules, water, minerals and lactose and in which microorganisms are entrapped.
It is otherwise known as shrinking; of the coagulum is largely the result of continuing rennet action. It causes loss of whey and is accelerated by cutting, stirring, cooking, salting or pressing the curd, as well as the increasing amount of acid produced by the starter and gradually increases during cheese making. As a result, the cheese curd contracts and moisture is continuously expelled during the cooking stages.
Salt is added to cheese as a preservative and because it affects the texture and flavor of the final cheese by controlling microbial growth and enzyme activity. The salt can be added either directly to the curd after the whey is run off and before moulding or pressing into shape, or by immersing the shaped cheese block in salt brine for several days following manufacture. Salt is also involved in physical changes in cheese protein solubility and conformation, which influence cheese rheology and texture. Another important function of salt in cheese is as a flavor or a flavour enhancer.
(a) Heat Treatments:
The application of heat to cheese curd at any of several different times during the manufacture of particular cheese varieties is to selectively stop the growth of certain types of bacteria and consequently influence the maturation pathway of the cheeses. It also alters the composition and texture of the cheese by increasing the syneresis without increasing the acidity.
Stretching the curd is an important operation for several kinds of cheese. Traditionally the curd was immersed in hot water and the fluid mass of cheese was pulled into strands to align the protein fibres and then poured into a container to cool. It was then immersed in brine. Large scale production means that special machines are used for stretching.
It is a mild form of stretching in which the cheese curd is piled up and held warm so that it flows under the force of gravity. It is periodically turned to flow again. The pH of the curd falls during this process and whey continues to exude. Again, in large scale manufacture, this is done in large machines.
The formation of the final cheese shape into spheres, flattened spheres, discs, cylinders or rectangular blocks is traditional but for some varieties it affects the maturation pathway. Some cheeses are pressed in moulds (made of plastic or stainless steel) under the whey for a short time whereas others are compressed at high pressures for several hours.
The ripening of cheese involves three major biochemical reactions:
Lactose is metabolized to lactic acid, which may then be catabolized (broken down into smaller molecules), to form acetic and propionic acids, carbon dioxide, esters and alcohol by the enzymes of the microorganisms in the milk.
The lipids are broken down to form free fatty acids that may be then catabolised to for ketones, lactones and esters by natural milk enzymes and those that are added to create the flavor some variety of cheese.
Proteins (caseins) are gradually broken down to form peptides and amino acids by the enzymes of the coagulant, the natural milk enzymes and the enzymes of the starter bacteria and other added microorganisms, e.g., moulds such as Penicillium camemberti used in the manufacture of Camembert and Penicillium roqueforti used in the manufacture of blue-veined cheeses such as Roquefort and Stilton.
The enzymes of these mould species typically result in a high level of proteolysis in these cheese types. The activities of the coagulant enzyme, the amount of enzyme remaining in the curd and the amount of proteolysis are dependent on the amount of acid produced in the initial stages of cheese making. The pH also controls the level of moisture, which in turn affects proteolysis in the cheese.
The final pH of the curd and the rate of pH decline determine the extent of dissolution of colloidal calcium phosphate from the curd. This modifies the susceptibility of the caseins to proteolysis during manufacture and influences the rheological properties (such as texture) of the cheese.
The breakdown of the proteins to peptides (proteolysis) transforms the rubbery and flavorless cheese curd into a cheese that has a desirable texture and flavor. Further proteolysis produces amino acids and the further biochemical glycolysis and hydrolysis result in the formation of amines, aldehydes, alcohols and sulphur compounds that add to the flavor of the cheese.
Many cheeses are made and matured in large blocks and they are exported as such. When they are to be sold in supermarkets, they are usually cut into appropriate size blocks and either shrink wrapped in an atmosphere of carbon dioxide, which dissolves into the body of the cheese, or vacuum sealed in a special “top- and-bottom” “webbed” package. The subsequent anaerobic environment prevents mould growth on the cheese surface. Many cheeses are ready for sale at maturation and are packaged in special aerating wrapping and in porous boxes.