Microorganisms associated with Raw Milk. Morphological & Biocharacter. Significant of Psychrotrophs, Mesophiles, Thermophiles and Thermoduric.

Types of microorganisms and their Activity in Milk

• Psychrotrophic referes to microorganisms which are able to grow at temperature less than 7 degree Celcius. Cold milk storage and transport selects for psychrotrophic bacteria which are often proteolytic and lipolytic. Common psychrotropic bacteria in milk are species of Micrococci, Bacilli, Staphylococci, Lactobacillus, Pseudomonas and Coliforms pseudommonas species are the most common and typically have the most impact on quality. At tenperatures of 2-4% degree Celcius, bacterial groeth in milk is mainly due to strains of Pseudomonas flourescens little growth occurs at temperature less than 2 degree Celcius.

  

  Pseudomonas flourescens microscopic figure

• Spore forming bacteria are able to exist in a highly stable form called 'spores'. In the spore stste, these bacteria are able to withstand greater extremes of activity, temperature and desiccation.
• Enzymes are biological catalysts that accelerates the rates of biochemical reactions. Bacterial enzymes are most significant to milk spoilage and cheese ripening but it is important to distinguish between the enzymes 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.
• Lactic Acid Bacteria are able to readily metabolize lactose so they have some competitive advantages over other microorganisms. Not withstanding, their ability to metabolize lactose. Lactic acid bacteria prefer temperatures greater than 30 degree Celcius, so, depending on initial relative counts, psychrotrophic bacteria including some coliform and pseudomonas bacteria are able to grow lactic acid bacteria at room temperature.
• 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 lipase. Bacillus which form heat stable spores and survive pasteurization.
  
• 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 lipase as well as proteases.
• Gas producing Microorganisms which cause cheese openness, floationg curd in cottage cheese, and gassy milk.
• Yeasts are  always present in milk and are common contaminants during the cheese making process. They 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 number 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 (survuves pasteurizarion) 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 crackes in the cheese are called 'late gas defect'. Five hundred spores per litre of milk are sufficient to cause late gas defect. Propionic bacterium produces the desirable gas formation in Swiss type cheese.
• Ropy bacteria causes 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.
• Sweet curdling bacteria produce rennet - like enzymes which may cogulate milk. Common examples are the psychrotrophic spore former Bacillus subtilis and Bacillus cereus. Numerous off flavours have been associated with specific milk contaminates. Some examples are:

Malty: S. lactic var maltigenes

Bitter: Proteolytic bacteria

Rancid: Lipolytic bacteria

Unclean: Coliform bacteria

Fishy: Pseudomonas

Fruity: Pseudomonas

• Pathogenic bacteria, Pathogens common before 1940 Brucella abortus causes brucellosis in cow and undulant fever in people. Pathogens which emerged during 1940 - 1970 Salmonellosis is an infection caused by many species and strains of species of Slmonella. Salmonellosis is of great concern to the dairy product/industry, especially the cheese and milk powder sectors. Infectious doses can be extremly low, perhaps as low as a single organism. Enteropathogenic E. coli produce enterotoxins, some of which are heat stable. Of numerous species and strains, the most famous is E. coli 01570 H7, which occurs frequently in raw milk. E. coli 0157 H7 is of particular concern because it is quite acid tolerant and is able to grow at refrigeration temperatures. Recent pathogen Listeria monocytogenes is a pschrotropic infectious agent, which requires special caution because it is acid tolerant and more heat stable than most pathogens, although it doesnot survive proper pasteurisation. Bacillus cereus is mainly important as spoilage agent. However, some strains are mainly pathogenic which is problematic because Bacillus cereus forms heat stable spores which survive pasteurization and are able to grow at refrigeration temperatures.

Psychrotrophs in Raw Milk. Significance & Affects.

The role of pseudomonads and primarily P. fluorescens as the predominant psychrotrophs associated with the spoilage of cold stored milk and milk-derived products has been established through a long history of culture-based studies. However, these types of studies are often limited to the phenotypic characterisation of the most abundant isolates following their isolation as pure cultures. The validity of this approach has been questioned by recent findings using nucleotide sequence-based identification methods, which have shown that the diversity of Pseudomonas species involved in milk spoilage is much wider than was previously thought. Moreover, real time molecular analysis of microbial communities has shown that psychrotrophic species other than Pseudomonas predominate in milk during the early stages post-collection, and that the activities of those communities may play a role in the subsequent spoilage of milk.

The emergence of molecular methods has provided a new means with which to obtain an accurate global view of the microbial communities in milk. The application of these methods has revealed potential roles for genera other thanPseudomonas as important agents of milk spoilage at refrigeration temperatures. Moreover, it is likely that the mixed bacterial populations that are often present in the form of biofilms collaborate in the spoilage process via mechanisms based on quorum sensing. The practical implications for these essentially preliminary findings have yet to be elucidated. However, it is clear that the continued study of milk-associated psychrotrophs is required and should be encouraged in order to enhance and improve existing control methods and to help ensure the quality and safety of milk and milk-derived foodstuffs.