Types of microorganisms and their activity in milk
The numbered list below identifies seven types of bacteria according to how they change the properties of milk. Often these changes are negative (spoilage) but as we will see in later sections, many of these bacteria are important to the development of cheese flavour. Before proceeding to the list, please note the following definitions:
- Psychrotrophic refers to microorganisms which are able to grow at temperatures less than 7C. Cold milk storage and transport selects for psychrotrophic bacteria which are often proteolytic and lipolytic. Common psychrotrophic bacteria in milk are species of Micrococci, Bacilli, Staphyloccoci, Lactobacilli, Pseudomonas, and coliforms. Pseudomonas species are the most common and typically have the most impact on quality. At temperatures of 2 - 4C, bacterial growth in milk is mainly due to strains of Pseudomonas flourescens. Little growth occurs at temperature less than 2C.
- Spore forming bacteria are able to exist in a highly stable form called 'spores'. In the spore state, these bacteria are able to withstand greater extremes of acidity, temperature and desiccation.
- Enzymes are biological catalysts that accelerate the rates of biochemical reactions. Bacterial enzymes are most significant to milk spoilage and cheese ripening but it is important to distinguish between the enzyme and the bacterial source. For example, many psychrotrophic bacteria produce heat stable enzymes which remain active in milk and cheese even after the bacteria are killed by pasteurization.
Keeping the above definitions in mind, note the following types of microorganisms, grouped according to their impact on milk quality.
(1) Lactic acid bacteria which ferment lactose to lactic acid and other end products. Lactic acid bacteria (LAB) important to cheese making will be described further in Cultures. For now note the following:
- As noted earlier, LAB are able to readily metabolize lactose so they have some competitive advantage over other microorganisms.
- Notwithstanding, their ability to metabolize lactose, LAB prefer temperatures greater than 30C, so, depending on initial relative counts, psychrotrophic bacteria including some coliform and pseudomonas bacteria are able to outgrow LAB at room temperature.
(2) Proteolytic bacteria which degrade protein and cause bitterness and putrefaction. Most important in cheese milk are species of:
- Pseudomonas which are psychrotrophic and produce heat stable lipases.
- Bacillus which form heat stable spores and survive pasteurization
(3) Lipolytic bacteria which degrade fats and produce lipolytic rancidity. Again, the most common example in milk is the genus Pseudomonas. Several psychrotrophic species of Pseudomonas produce heat stable lipases as well as proteases.
(4) Gas producing microorganisms which cause cheese openness, floating curd in cottage cheese, and gassy milk.
- Yeasts are always present in milk and are common contaminants during the cheese making process. They may cause 'yeast slits' in cheese and contribute to ripening of surface ripened cheese.
- Coliform bacteria are always present in milk but their numbers can be minimized by good sanitation. Also, coliform bacteria compete poorly with lactic acid bacteria, so their numbers rapidly decrease in the presence of a rapidly growing lactic acid culture.
- Clostridium tyrobutyricum is a thermoduric (survives pasteurization) spore forming organism of legendary fame among cheese makers. C. tyrobutyricum causes gas formation (carbon dioxide) during the later stages of ripening of Swiss and Dutch type cheeses. The resulting craters and cracks in the cheese are called 'late gas defect'. European cheese makers frequently check raw milk for thermoduric and/or spore forming bacteria to estimate potential for late gas defects. Five hundred spores per litre of milk are sufficient to cause late gas defect.
- Propioni bacterium produces the desirable gas formation in Swiss type cheese.
- Some lactic cultures, called heterofermentative, also produce carbon dioxide. See Cultures.
(5) Ropy bacteria cause stringy milk due to excretion of gummy polysaccharides. Usually ropy bacteria such as Alcaligenes viscolactis are undesirable. However, in some fermented dairy products, ropy lactic acid bacteria such as certain subspecies of Lactococcus lactis are used to develop texture.
(6) Sweet curdling bacteria produce rennet-like enzymes which may coagulate milk. Common examples are the psychrotrophic spore formers Bacillus subtilis and Bacillus cereus.
(7) Numerous off flavours have been associated with specific milk contaminates. Some examples are:
- Malty: S. lactis var maltigenes
- Bitter: see (2) Proteolytic bacteria
- Rancid: see (3) Lipolytic bacteria
- Unclean: coliform bacteria
- Fishy: Pseudomonas
- Fruity: Pseudomonas