(1) Three processes. There are three processes whereby milk may be
pasteurized, all of which are effective in eliminating pathogens: vat pasteurization, high-
temperature, short time (HTST) pasteurization, and ultra-high-temperature (UHT)
pasteurization. Vat pasteurization utilizes a temperature of 145F (63C) for 30 minutes,
whereas the HTST method heats the milk to 161F (72C) for 15 seconds.
Ultra-high-temperature utilizes a temperature of 203-308F (95-153C) for 3
seconds or less. All are followed immediately by rapid cooling.
(2) Importance of sanitary handling. The sanitary handling of milk, along
with pasteurization, is most important in preventive medicine. Organisms killed by
pasteurization include those of tuberculosis, typhoid fever, bacillary dysentery, diptheria,
and scarlet fever. Of course, it is extremely important that the milk not be contaminated
after completion of the pasteurization process.
a. Lactic Acid Bacteria.
(1) Bacteria most commonly found in milk. Bacteria invade milk usually as
dust-borne contaminants; hence, almost any type may be present. By far the most
abundant bacteria making up the normal flora of milk belong to the family
Lactobacillaceae. The organisms in this family are frequently referred to as lactic acid
bacteria. The members of Lactobacillaceae are nonmotile, anaerobic rods or cocci.
(2) Two groups of lactic acid bacteria. Milk constitutes a typical enrichment
culture medium, and so only the most suited types will predominate at any one time.
Usually, the first organism to flourish in milk is Streptococcus lactis. These organisms
have rather complex nutritional requirements, and all require varying numbers of amino
acids and vitamins for growth. In addition, all lactic acid bacteria require a fermentable
carbohydrate (lactose) as their source of energy. They ferment the lactose principally to
lactic acid. As the pH drops, other species, such as Lactobacillus casei and L.
acidophilus may replace Streptococcus lactis as the predominant type. L. casei and L.
acidophilus and other lactic acid bacteria will further the fermentation process.
Conventionally, these organisms are divided into two groups. Those that essentially
produce only lactic acid from fermentable carbohydrates are known as the
homofermentative lactic acid bacteria. Those that produce acetic acid, ethanol, and
carbon dioxide, as well as lactic acid from a fermentable carbohydrate are called
heterofermentative lactic acid bacteria.
b. Souring and Curd Formation. Milk, whether raw or pasteurized, will sour on
standing due mainly to the production of lactic acid by Streptoccus lactis or by the
Lactobacillus species. These organisms do not produce disease in man but ferment the
carbohydrate lactose. The bacteria break lactose down into the simple sugar glucose
and finally convert glucose to lactic acid, thus lowering the pH of milk (souring). When
the pH is lowered from 6.8 to around 4.5, the protein casein in milk becomes insoluble
and forms a lumpy precipitate (curd).