Microbiological quality of milk for cheese production

This bachelor thesis is focused on microbiological quality of milk for cheese production. Besides that, it informs about the composition of milk and the processes in cheese production. The composition of milk differs between animal species. Lactose is the main sugar of milk, with content about 4-8 %. The majority of fat found in milk is triacylglycerol type. Casein is the most important protein, which occurs in three fractions (alpha, beta a kappa). Milk also contains various minerals and vitamins. The most important mineral is calcium, which plays an important role in cheese production. The milk composition is a suitable environment for the growth of microorganisms (Pseudomonas, Enterobacter, etc.), that can contaminate raw milk. Despite this risk, the raw milk is commonly used for cheese production, because it gives the cheese better structure and taste compared with the treated milk. However, the majority of cheeses are manufactured from the treated milk. In most cases, the milk is treated by heat, especially by pasteurization and ultrahigh temperature. These procedures kill microorganisms in milk. In some cases, milk can be contaminated again after the treatment or by inactive spores that survived the treatment. They can become active under favourable conditions, for example during cheese making. Cheeses can be divided into groups by several criteria (type of coagulation, water content or fat content in dry matter). Starting cultures are used during cheese making from treated milk. Thermophilic starting culture Lactobacillus casei is used for manufacturing of soft cheeses and cheeses with high-heated curd. Cheeses with holes are made by fermentation (Propionibacterium freudenreichii). Mesophilic bacteria Lactobacillus lactis are used for production of cheeses with low-heated curd. There are many other cultures used in cheese making. Another important process during a cheese making is coagulation. During this process, the milk fat is coagulated by enzymes (chymosin, pepsin). After the coagulation, the curd is cut and forms soft grains, which exude whey. The grains are often heated for the better whey secretion. Salt is used during the cheese production to slow the activity of microorganisms and enzymes and it also gives better taste to the final product. Cheese is then let to age, during this period of time, biochemical processes occur and unique characteristics of the cheese are gained