CHEESE TECHNOLOGY

Module 3. Quality of milk in relation to cheesemaking

Lesson 7
MICROBIOLOGICAL QUALITY OF MILK

7.1 Introduction

Cow milk is the commonly used source for cheesemaking all over the world. The microbiological quality of raw milk reaching the cheese factory is controlled by the following ‘H’ factors.

(i) Health of the milch animal.

(ii) Hygiene during milk production (hygiene of farm, personnel and equipments/utensils).

(iii) Handling and refrigeration.

Clean milk secreted from the udder of the healthy animal contains only few numbers of microorganism (< 10,000/ml). Milk obtained from mastitis cow may contain high numbers of microorganism as well as leucocytes depending on the severity of infection. Maintenance of hygienic practices during milk production at farm is an important aspect of clean milk production. Cleaning and sanitization of milk contact surfaces directly determine the extent of contamination of milk after it has been drawn from the udder.

The post production hygienic handling of milk and proper refrigeration can check the proliferation of microorganisms in milk before cheesemaking. Milk should be held at around 4°C during transport an in cheese plant.

7.2 Microorganisms in Raw Milk

The presence and multiplication of saprophytic and pathogenic bacteria in raw milk might change the milk composition and produce toxins, and influence the quality and safety of the milk and milk products. Moreover, flavor of the raw milk may be adversely influenced and heat-stable bacterial enzymes may continue to act in products particularly during long storage and adversely affect the stability of milk and milk products. The pathogenic bacteria include ‘classical’ microorganisms and ‘emerging pathogens’. At present Salmonella, pathogenic Escherichia coli strains, Yersinia enterocolitica, Staphylococcus aureus, Listeria monocytogenes, Campylobacter jejuni are the most important.

According to the main points of attack on the major milk constituents, the saprophytic bacteria are subdivided as follows:

a) Microorganisms degrading lactose are classified as glycolates, e.g. Streptococci, Lactobacilli and Coliforms

b) Microorganisms degrading proteins are classified as proteolytes, e.g. Pseudomonas, Enterobacteriaceae, and aerobic sporeformers.

c) Microorganisms degrading lipids are classified as lipolytes, e.g. Pseudomonas, Micrococci, Aeromonas and Corynebacteria.

The effect of growth on saprophytic bacteria in milk may be important in three ways as follows:

a) The change in milk composition may interfere with manufacture, if fermentation is involved in the manufacture process, and this may affect the yield and quality of the product.

b) The flavor of the raw milk may be adversely influenced (e.g. rancidity) and this may directly affect the flavor of the product e.g. Cottage cheese.

c) Heat-stable bacterial enzymes may continue to act in the product, particularly during long storage, and adversely affect the stability and/or flavor of cream and UHT milk.

Milk is generally held at refrigerated condition for 1-3 days before processing. The milk stored under such conditions contains predominantly psychotropic bacteria (over one million/ml). The common genera encountered are Pseudomonas, Aeromonas, Alcaligenes, lactic acid bacteria, gram positive sporeformers, coryneform group, enterococci and coliforms. The psychotropic bacteria may further increase in number due to proliferation, especially when the temperature of the refrigerated milk increases. These conditions facilitate the release of some heat stable (surviving pasteurization) enzymes like proteinases, lipases, and phospholipases in milk which leads to proteolysis and lipolysis. Lipolysis of milk leads to rancidity that inhibits growth and activity of lactic culture because of lower surface tension or specific toxic effects of certain free fatty acids (C₈-C₁₂ fatty acids).

7.3 Somatic Cells

The milk used for preparation of cheese should be from healthy animal with a somatic cell count of < 50,000/ ml. If raw milk contains > 50,000 somatic cells/ ml results in phagocytosization of lactic acid bacteria (LAB) leads to slow starter activity in cheese vat, increase in rennet clotting time causing decreased curd firmness and a loose final body and texture.

7.4. Antibiotic Residues

In lactating cows, antimicrobial agents are used mostly for the therapy of mastitis but also of other diseases (e.g. laminitis, respiratory diseases, metritis). Antimicrobial agents administered to cows in the course of lactation can pass to milk in various levels and inhibit starter activity.

7.5 Disinfectants and Preservatives

Occasionally, chemical sanitizers may contaminate milk, usually as a result of human error. Quaternary ammonium compounds (QAC’s) present more potential problems, because they maintain activity in milk, and LAB are sensitive to low concentrations. The amount of chemical sanitizer that might enter milk through lack of rinsing should not be sufficient to cause culture inhibition. However, problems can be encountered when sanitizer solution is not drained from tanks or trucks.

7.6 Bacteriophages

Bacteriophages (phages) are viruses that infect bacteria. Bacteriophagic infection of starter cultures can result in failure of the fermentation and loss of product. Despite implementation of control measures, bacteriophagic infection still causes production problems in the modern dairy fermentation industry.

7.7 Raw Milk Associated Inhibitors

Lactic starter cultures grow more slowly in raw than in heated milk; a phenomenon caused by the presence of natural inhibitors. The lactoperoxidase system is the most significant microbial inhibitor in raw milk, but the presence of agglutinins is an important problem in acid-coagulated cheeses. Other naturally occurring microbial inhibitors in milk include lysozyme and lactoferrin.