SPOILAGE OF MILK AND DAIRY PRODUCTS Milk microbiology

SPOILAGE OF MILK AND DAIRY PRODUCTS

Milk microbiology üIn addition to being a nutritious food for humans, milk provides a favorable environment for the growth of microorganisms. üYeasts, moulds and a broad spectrum of bacteria can grow in milk, particularly at temperatures above 16°C. üMicrobes can enter milk via the cow, air, feedstuffs, milk handling equipment and the milker. üOnce microorganisms get into the milk their numbers increase rapidly.

üIt is more effective to exclude microorganisms than to try to control microbial growth once they have entered the milk. Microbial growth üControlled by cooling the milk because most microorganisms reproduce slowly in colder environments. üCooling milk also slows chemical deterioration by minimizing the enzymatic activity. üThe temperature of freshly drawn milk is about 38°C. Bacteria multiply very rapidly in warm milk and milk sours rapidly if held at these temperatures.

Natural souring of milk may be advantageous… §In butter-making, the acid developed assists in the extraction of fat. §The low p. H retards growth of lipolytic and proteolytic bacteria and therefore protects the fat and protein in the milk. §The acidity of the milk also inhibits the growth of pathogens. §It does not, however, retard the growth of molds.

§The initial microflora of raw milk reflects directly microbial contamination during production. §The initial bacterial count of milk may range from less than 1000 cells/ml to 106/ml. §High counts (more than 105/ml) are evidence of poor production hygiene. §Rapid tests are available for estimating the bacterial quality of milk.

Types of microorganisms (grouped according to their impact on milk quality) (1) Lactic acid bacteria which ferment lactose to lactic acid. (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. (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. § Example: Clostridium tyrobutyricum causes gas formation (carbon dioxide) during the later stages of ripening of cheeses. § The resulting craters and cracks in the cheese are called 'late gas defect'.

Milk pasteurization v. Pasteurization is the process used to destroy bacteria in milk. v. In pasteurization, the milk is heated to a temperature sufficient to kill pathogenic bacteria, but well below its boiling point thereby extends the storage stability of the milk. v. Numerous time/temperature combinations are recommended but the most usual is 72°C for 15 seconds followed by rapid cooling to below 10°C.

v. The lower temperature used for batch pasteurization (where the milk amount is very less) means that a longer time is required to complete the process 30 minutes at 63°C. v. Milk is normally pasteurized prior to sale as liquid milk. v. Pasteurization is used to reduce the microbial counts in milk for cheese making, and cream is pasteurized prior to tempering for butter making in some factories.

Effects of pasteurization on milk §Pasteurization reduces the cream layer, since some of the fat globule membrane constituents are denatured. §This inhibits clustering of the fat globules and consequently reduces the extent of creaming. §However, pasteurization does not reduce the fat content of milk.

§Pasteurization has little effect on the nutritive value of milk. There is some loss of vitamin C and B group vitamins, but this is insignificant. §The process kills many fermentative organisms as well as pathogens. §Although pasteurized milk has a storage stability of 2 to 3 days, subsequent deterioration is cause by putrefactive organisms. §Thus, pasteurized milk will putrefy rather than develop acidity.

Milk sterilization §In pasteurization, milk receives mild heat treatment to reduce the number of bacteria present. §In sterilization, milk is subjected to severe heat treatment that ensures almost complete destruction of the microbial population. §Time/temperature treatments of above 100°C for 15 to 40 minutes are used. The product has a longer shelf life than pasteurized milk.

§Another method of sterilization is ultra-heat treatment, or UHT. In this system, milk is heated under pressure to about 140°C for 4 seconds. §The product is virtually sterile. However, it retains more of the properties of fresh milk than conventionally sterilized milk.

Microbial tests pasteurized milk for üTests are available microbiological quality. to raw know the and milk üBacteria, coliform and somatic cell counts are frequently used. Bacteria Count §The total bacteria count is the number of bacteria in a sample that can grow and form countable colonies on Standard Methods Agar after being held at 32°C for 48 hours.

Coliform Count ØThe coliform count is the number of colonies in a sample that grow and form distinctive countable colonies on Violet Red Bile Agar after being held at 32°C for 24 hours. ØColiforms are generally only present in food that has been fecally or environmentally contaminated.

Somatic Cell Count ØSomatic cells are blood cells that fight infection and occur naturally in milk. ØThe presence of mastitis (an infection of the mammary gland) in the cow will increase the somatic cell count. ØThe somatic cell count can be determined by direct microscopic examination or by electronic instruments designed to count somatic cells.

Antibiotics in milk üAntibiotics are used to treat mastitis infections. üCows under antibiotic treatment for mastitis infections may have antibiotic residues in their milk. üMilk containing antibiotic residues is not used for human consumption. üThe legal standard defined by the Food and Drug Administration (FDA), requires that milk contain no detectable antibiotics. üRegulatory action is taken against the farm with the positive antibiotic test.

Other methods to prevent/control microbial contamination v. Dehydration v. Using of antimicrobial agents v. Using food additives v. Using food stabilizers and emulsifiers