General Functions of Cheese Cultures
Lactic acid bacteria and other microorganisms are present as 'contaminants' in cheese milk and further environmental contamination takes place during cheese manufacture. Provided the milk is not chilled, it is possible to make cheese without any additional cultures, but normal practice is to add domestic cultures for the manufacture of cheese from both raw and pasteurized milk. Culture, then, refers to prepared inocula of bacteria, yeast and moulds which are added to cheese milk and cheese. In the broadest terms cultures have two purposes in cheese making: (1) to develop acidity; and (2) to promote ripening. Lactic acid cultures contribute to both of these functions, while numerous special or secondary cultures are added to help with the second function.
Development of Acidity
Raw milk at warm temperature will support a variety of micro-organisms in succession as the pH changes over time (see illustration on the right). In controlled conversion of milk to fermented dairy products, a primary component of fermentation is development of acidity by lactic acid bacteria. Acid development in cheese making is absolutely essential to cheese flavour, cheese texture and cheese safety. Acid is required to:
- Assist coagulation. Lower pH results in faster coagulation and in acid coagulated cheese is the only factor which induces coagulation.
- Promote syneresis. This is a most critical means of controlling moisture content. Acidity (specifically reduced pH) causes the protein matrix in the curd to contract and squeeze out moisture. That process of contraction is called syneresis.
- Prevent growth of pathogenic and spoilage bacteria. Proper rate and extent of acid development is the most important principle with respect to quality and safety of natural cheese. I would argue that with the exception of noncultured cheese varieties such as ricotta, proper culture growth and acid development is equal in importance to pasteurization with respect to safety.
- Develop cheese texture, flavour and colour. The following general associations are relevant to most cheese varieties.
- high pH produces soft, soapy, fruity and bitter cheese
- low pH produces cheese with brittle texture and mottled colour
- Growth factors produced by lactic cultures are required for other non-starter microorganisms which contribute to the desired flavour and body of cheese
- Enzymes (both lipases and proteases) produced by lactic cultures contribute to interior ripening of cheese and are important to both flavour and texture development.
- Special or secondary cultures are responsible for eye development, surface ripening etc. See Section 7.5.