The cheese-making process

There exist more than 2000 names of cheeses, although sometimes similar products have different names. Different types of cheese can be classified in many ways, but the most commonly used classification is based on cheese hardness (consistency) or ripening method. Thus, cheeses are classified as:

• very hard (30 to 35 percent moisture, ripened by bacteria Romano, Parmesan);

• hard (35 to 40 percent moisture, ripened by bacteria without eyes such as Cheddar, colby, provolone or with eyes such as Swiss, Em-mentaler);

• semisoft (40 to 45 percent moisture, ripened principally by bacteria (Muenster, asigo) or by bacteria and microorganisms on surface (limburger, Port du Salut) or by blue mould in the interior (Roquefort, Gorgonzola, Stilton);

• soft (ripened by surface microorganisms with 45 to 52 percent moisture such as Brie, Cammembert or unripened with 52 to 80 percent moisture such as cottage, cream, pizza).

The cheese-making process consists of removing the major part of the water contained in fresh fluid milk while retaining most of the solids. Since storage life increases as water content decreases, cheese-making can also be considered a form of food preservation through the process of milk fermentation. The fermentation of milk into finished cheese requires several essential steps: 1) preparing and inoculating the milk with lactic-acid — producing bacteria, 2) curdling the milk, 3) cutting the curd, 4) shrinking the curd (by cooking), 5) draining or dipping the whey, 6) salting, 7) pressing, and 8) ripening. These steps begin with four basic ingredients: milk, microorganisms, rennet, and salt.

Inoculation and curdling. Milk for cheese-making is supposed to be of the highest quality. Good farm sanitation and pasteurization or partial heat treatment are important to the cheese-making process, for the natural microflora presented in milk frequently include undesirable types called psychrophiles. In addition, the milk must be free of substances that may inhibit the growth of acid-forming bacteria (e.g., antibiotics). Milk is often pasteurized to destroy pathogenic microorganisms and to eliminate spoilage and defects induced by bacteria. However, since pasteurization destroys the natural enzymes found in milk, cheese produced from pasteurized milk ripens less rapidly and less extensively than most cheese made from raw or lightly heat-treated milk.

During pasteurization, the milk may be passed through a standardizing separator to adjust the fat-to-protein ratio of the milk. In some

cases the cheese yield is improved by concentrating protein in a process known as ultrafiltration1. The milk is then inoculated with fermenting microorganisms and rennet which promote curdling.

Different fermenting microorganisms carrying out the anaerobic conversion of lactose into lactic acid, the type of organisms used depends on the variety of cheese and on the production process. Rennet is an enzymatic preparation that is usually obtained from the fourth stomach of calves. It contains a number of proteolytic (protein-degrading) enzymes, including rennin and pepsin. Some cheeses, such as cottage cheese and cream cheese, are produced by acid coagulation alone. In the presence of lactic acid, rennet, or both, the milk protein casein clumps together and precipitates out of solution; and it is this process that is known as curdling, or coagulation. Coagulated casein assumes a solid or gellike structure (the curd), which traps most of the fat, bacteria, calcium, phosphate, and other particulates. The remaining liquid (the whey) contains water, proteins resistant to acidic and enzymatic denaturation (e.g., antibodies), carbohydrates (lactose), and minerals.

Lactic acid produced by the starter culture organisms has several functions. It promotes curd formation by rennet, which activity requires an acidic pH, causes the curd to shrink, enhances whey drainage, and helps prevent the growth of undesirable microorganisms during cheese-making and ripening. In addition, acid affects the elasticity of the finished curd and promotes fusion of the curd into a solid mass. Being released by the bacterial cells, enzymes influence flavour development during ripening.

Salt added to the curd not only enhances flavour, but it helps to withdraw the whey from the curd and inhibits the growth of undesirable microorganisms as well.

Cutting and shrinking. Being formed, the curd is cut with fine wire "knives" into small cubes approximately one centimetre square and the following gentle heating causes its shrinking. The moisture content and the final consistency of the cheese are determined by the degree of shrinkage. Whey removed by draining or dipping may be further processed to make whey cheeses (e.g., ricotta) or beverages, or it may be dried in order to preserve it as a food ingredient.

Ripening. Most cheese is ripened for varying amounts of time in order to bring about the chemical changes necessary for transforming fresh curd

into a distinctive aged cheese. These changes are catalyzed by enzymes from three main sources: 1) rennet or other enzyme preparations of animal or vegetable origin added during coagulation, 2) microorganisms that grow within the cheese or on its surface, 3) the cheese milk itself.

The ripening time may be as short as one month, as for Brie, or a year or more, as in the case of sharp Cheddar.

The ripening of cheese is influenced by the interaction of bacteria, enzymes, and physical conditions in the curing room. The speed of the reactions is determined by temperature and humidity conditions in the room as well as by the moisture content of the cheese. In most cheeses lactose continues to be fermented to lactic acid and lactates, or it is hydrolyzed to form other sugars. As a result, aged cheeses such as Em-mentaler and cheddar have no residual lactose.

In a similar manner, proteins and lipids (fats) are broken down during ripening, the degree of protein decomposition, or proteolysis, affecting both the flavour and the consistency of the final cheese. It is especially apparent in Limburgerand some blue-mould ripened cheeses. Surface-mold ripened cheeses, such as Brie, rely on enzymes produced by the white Penicillium camemberti mould to break down proteins from the outside. When lipids are broken down (as in Parmesan and Romano cheeses), the process is called lipolysis.

The eyes, or holes, typical of Swiss-type cheeses such as Emmen-talerand Gruyere result from a secondary fermentation that takes place when, after two weeks, the cheeses are moved from refrigerated curing to a warmer room, where temperatures are in the range of 20° to 24°C. At this stage, residual lactates provide a suitable medium for propionic acid bacteria (Propionibacterium shermanii) to grow and generate carbon dioxide gas. Eye formation takes three to six weeks. Then, warm-room curing is stopped and the cheese is moved back to a cold room, where it is aged at about 7°C for 4 to 12 months in order to develop its typical sweet, nutty flavour.

The unique ripening of blue-veined cheeses is due to from the mould spores Penicillium roqueforti or P.glaucum which are added to the milk or to the curds before pressing and are activated by air. Air is introduced by "needling" the cheese with a device that punches'' about 50 small holes into the top. These air passages allow mould spores to grow vegetative cells and spread their greenish blue mycelia, or thread-like structures, through the cheese. Penicillium moulds are known to be rich in proteolytic and lipolytic enzymes, and as a result a variety of trace compounds, such as free amines, amino acids, carbonyls, and fatty acids affecting the flavour and texture of the cheese are produced during the ripening process.

Surface-ripened cheeses like Gruyere, Port du Salut, and Limburger derive their flavour from both internal ripening and the surface environment. For instance, the high-moisture wiping of Gruy6re surface gives that cheese more flavour than its Emmentaler counterpart. Specific organisms, such as Brevibacterium linens, in Limburger cheese cause a reddish brown surface growth and the breakdown of protein to amino nitrogen. Although the resulting odour seems offensive to some extent, but the flavour and texture of the cheese are sure to please tastes of many consumers.

Not all cheeses are ripened. Cottage, cream, ricotta, and most moz-zarella cheeses are ready for sale as soon as they are made, these cheeses having sweet, delicate flavours and are often combined with other foods.

Answer the questions:

1. What makes butter perfect food?

2. How is butter manufactured?

3. Why is it necessary to add salt to butter?

4. What does the colour of butter depend on?

5. What are the by-products of butter manufacture?

6. How many types of cheese are known nowadays?

7. What are the four basic ingredients for cheese-making?

8. What are the eight steps of cheese-making process?

9. Why is the cheese-ripening step known to be the most risky?

10. What does the term "inoculation" mean?

11. How can the curdling process be described?

12. What is the role of lactic acid produced by starter culture organisms?

13. What are the sources of enzymes used in cheese ripening?

14. How does the ripening time vary with different kinds of cheese?

15. How can cheeses be classified?