1.Biochemical activities
If allowed to stand under condition that permit bacterial growth, raw milk of a good sanitary quality will rapidly undergo a series of chemical changes. The principal change is lactose fermentation to lactic acid.
This change is brought about by acid uric lactic organisms, especially Strepotococcus lactis and certain lactobacilli. These include two distinct biochemical types, homo-and heterofermentative. In homofermentation lactic acid is the major product of lactose fermentation.
Heterofermentative organisms, however, produce lactic, acetic, propionic, and some other acids, and some alcohols and gases such as CO2 and H2 Organisms continue to form lactic acid until the concentration of acid is itself too great for the organisms to remain live. Microbacteria, micrococci, coliforn, etc. also ferment lactose to lactic acid and other products
As the acidity continues to increase and reaches a pH of 4.7, it eventually causes a precipitation of casein. Organisms capable of metabolizing lactic and other acids develop especially acid uric, yeasts and moulds.
The acidity of milk is diminished and the alkaline products of protein decomposition such as amines, ammonia and the like are produced.
This is accomplished by many species of the genera Bacillus, Clostridium, Pseudomonas, Proteus and numerous other forms.
The action of microorganisms does not involve fat as readily as it does lactose and protein. Lipolysis results from the action of lipase produced by bacteria such as Pseudomonas, Achromobacter and by some yeasts and moulds.
Fat is hydrolysed to glycerol and fatty acids. Some of the fatty acids, for example, butyric and caproic acid give milk products, distinctive and usually rancid, odours and flavours.
Several microorganisms also bring about certain objectionable changes in the milk which may not be harmful to health. Rapines in milk is sometimes encountered. The milk become ropy or slimy and may be pulled out into long threads.
It is produced by several organisms but the most important species is Alcaligenes viscolactis. A rapid fermentation of lactose in milk is sometimes observed and is known as stormy fermentation. This is brought about by Clostridium perfringens.
The curd becomes torn to shreds by the vigorous fermentation and gas production. Several organisms have been isolated from milk which impart brilliant colours. Pseudomonas syncyanea imparts blue colour, pseudomonas synxantha yellow colour and Serratia marcescens red colour to the milk.
2.Temperature response
Microorganisms found in milk can also be described according their optimum temperature for growth and heat resistance. This is a very practical consideration since milk is preserved by employing low temperatures to prevent changes due to microbial activi1y'and by high temperatures to reduce microbial population and destroy pathogens. All the four types of microorganisms i.e. psychrophilic, mesophilic, thermophilic and thermoduric are found in milk.
Psychrophiles grow at temperatures just above freezing and at refrigeration tempratures. They produce a wide variety of spoilage defects. The defects may result in the production of many "off" flavours and odours. The most commonly encountered psychrophilic bacteria are members of the genera pseudomonas, achromobacter, Vibrio, Flavobacterium, Alcaligenes, Listeria monocytogenes, Yersinia enterocolitica, some E. coli strains and some Bacillus strains. They are killed in the pasteurization process, but are sometimes found in pasteurized milk. The contamin¬ation takes place after pasteurization from equipment, cans, bottles, and water.
The most important mesophilic bacteria are streptococci, lactobacilli and coliforms, which produce acid and gas and off flavours. They are killed in the pasteurization process
Thermophilic bacteria grow well at the temperature used in pasteurization, especially when the low temperature holding method is followed. Most thermophilic forms are found in two genera, Bacillus and lostridium.
Thermoduric organisms are regarded as those which survive pasteurization but do not grow at pasteurization temperatures. The most common thermoduric bacteria are found in the genera Microbacterium, Corynebacterium, Micrococcus, Streptococcus and Bacillus.
Excessive numbers of thermoduric bacteria in milk make it difficult to meet the grading standard.
3.Ability to cause infection and disease
The disease organisms present in milk may be derived from (1) diseased animals or (2) persons collecting and handling milk: Thus the danger is due to the inoculum and not to the growth of organisms in the milk.
The health of animal is an important factor. Several diseases of cattle including staphylococcal and streptococcal infections, tuberculosis, brucellosis, salmonellosis, Q fever and Foot and mouth disease may be transmitted to man.
The organisms causing these diseases may get into the milk either directly from the udder, or indirectly from infected body discharges, which may drop, splash, or be blown into the milk.
Some of the important diseases of human origin that have been transmitted by milk are (1) typhoid fever (2) diphtheria, (3) scarlet fever, (4) dysentery (5) septic sore throat and (6) poliomyelitis.
It is also possible for humans to infect animals. For example, mastitis may be caused by a variety of organisms, including Staphylococcus aureus. The infecting organism, in some cases, has been traced to humans.
Titany answered the question on April 25, 2022 at 07:11