MICROBIOLOGY Study Guide - 2012

CHAPTER 15. FOOD MICROBIOLOGY

15.1. MICROBIOLOGY OF MILK AND DAIRY PRODUCTS

15.1.2. Microflora of pasteurized milk

Pasteurization is the heat Treatment of milk at temperatures below its boiling point (ranging from 65 to 95 °C). The choice of Temperature and holding time depends on the type of product being processed and the equipment used. The pasteurization regime must ensure the maximum preservation of the original properties of milk, as well as its nutritional and biological value. The MAIN OBJECTIVES OF milk pasteurization are:

✵ destroying pathogenic microorganisms to ensure the hygienic safety of milk for the consumer;

✵ reducing the total microbial load of the milk;

✵ inactivating the native Enzymes of raw milk.

Depending on technological requirements, the dairy industry employs the following milk pasteurization regimes:

✵ for drinking milk — 76 ± 2 °C with a holding time of 20—25 s;

✵ for cottage cheese production — 78 ± 2 °C with a holding time of 15—20 s;

✵ for cheese production — 72 ± 2 °C with a holding time of 15—20 s;

✵ for fermented milk beverages — 82 ± 2 °C with a holding time of 10—15 min or 94 ± 2 °C with a holding time of 2—8 min;

✵ for starter culture preparation — 95 ± 2 °C with a holding time of 20—30 min.

Ultra-pasteurization of milk is carried out at 140 °C with a holding time of 2—4 s.

The degree of microbial destruction during pasteurization depends on the initial count and predominant species COMPOSITION OF THE Raw milk microflora, its preliminary clarification by centrifugation, as well as the temperature and duration of heating.

The primary criterion for the hygienic reliability of pasteurized milk is the heat treatment regime that ensures the destruction of the most resistant non-spore-forming pathogen—the tubercle bacillus. Indirect indicators of pasteurization efficiency include the inactivation of the enzyme phosphatase found in raw milk and the destruction of coliforms. Since phosphatase inactivation and the death of coliforms occur at higher temperatures than the destruction of the tubercle bacillus, these tests are used to characterize the safety of pasteurized milk. For example, at 75 °C, tubercle bacilli are destroyed in 10—12 s, whereas phosphatase is inactivated only after 23 s. Pasteurized milk sampled from the cooling section of the pasteurizer must be free of coliforms in 10 cm³. Pasteurization efficiency depends on the temperature and duration of thermal exposure, the initial microbial contamination, the qualitative composition of the raw milk microflora, and, above all, the presence of heat-resistant microorganisms. The microflora that survives pasteurization is referred to as residual microflora. The predominant group of residual microflora consists of enterococci (Enterococcus faecalis), while micrococci and bacterial spores of the genera Bacillus and Clostridium may be detected in small amounts.

After pasteurization and cooling, as milk is transferred to bottling machines, it can be additionally contaminated by microorganisms present on the surfaces of pipelines and equipment. Microorganisms that enter the milk after pasteurization are called secondary microflora. Coliforms, psychrotrophic Bacteria from rinsing Water, lactic acid bacteria, and putrefactive bacteria can be transferred from equipment into the milk. Under poor sanitary conditions, secondary contamination can increase the total bacterial count in pasteurized milk by 3—4 times, and the coliform count by 10—100 times.

According to Federal Law No. 88, pasteurized milk in consumer packaging must meet the following microbiological standards:

✵ total plate count (TVC) — not more than 5 x 10⁴ (in cans and tanks — not more than 2 x 10⁵ per 1 cm³);

✵ coliforms must not be present in 0.1 cm³ (in cans and tanks — in 0.01 cm³);

✵ S. aureus must not be present in 1 cm³ (in cans and tanks — in 0.1 cm³);

✵ pathogens, including salmonellae and Listeria monocytogenes, must be absent in 25 cm³.

When stored beyond permissible shelf lives, pasteurized milk may undergo microbial spoilage, manifested by changes in flavor or texture.

1. Milk coagulation without an increase in acidity can be caused by the growth of mesophilic spore-forming putrefactive bacteria of the species B. subtilis, B. mesentericus or thermophilic bacteria of the species B. coagulans, B. circulans.

2. Acid coagulation of milk occurs due to unsealed packaging, violation of the pasteurization regime, or contamination of the finished product with lactic acid bacteria after pasteurization. This defect is caused by the growth of lactic acid bacteria.

3. A bitter taste is caused by The formation of bitter Peptides during the Hydrolysis of milk Proteins under the action of proteinases from enterococci and thermophilic spore-forming bacteria — Bacillus circulans, В. coagulans, В. stearothermophilus.

4. A rancid taste appears As a result of The breakdown of milk fat by heat-resistant lipases from psychrotrophic bacteria.