Market Milk

Module 5. Physico-chemical, microbiological and nutritional properties of milk

Lesson 14
MICROBIOLOGY OF RAW MILK

14.1 Introduction

Microorganisms are microscopic, mostly single-celled organisms capable of rapid reproduction under proper growth conditions. Some of them are helpful and serve useful functions such as producing chemical changes that are desirable in the production of fermented dairy products such as cheese and yogurt. Others can cause milk and its products to spoil and make them inedible, thus causing the dairy industry great losses. Microorganisms termed as pathogens can be harmful to health and are important from the safety and public health point of view.

Milk in the mammary gland is normally sterile though bacteria are present in mammary glands of unhealthy animals. Once the milk leaves the udder, microflora from the exterior of the udder, coat of the animal, atmosphere, utensils and workers easily pass into the milk. Therefore, even under extremely clean conditions, freshly drawn milk may have a few thousand bacteria. Conditions of storage, pre-processing activities and unhygienic practices may further add to the bacterial load. The microbial content of milk is a very good indicator of the sanitary quality and conditions of production.

14.2 Thermophilic Bacteria

Thermophilic bacteria can grow in milk held at elevated temperatures (55°C or higher), including pasteurization temperatures. The species that grow in higher temperatures include Bacillus species which enter milk from various sources in the farm, or from poorly cleaned equipment in the processing plant. When the milk is held at high temperatures for long periods, these bacteria rapidly increase in numbers and may cause flavour defects, curdling or problems related to legal standards for microbiology. Thermophilic bacteria are enumerated by Standard Plate Count with plate incubation at 55°C.

14.3 Thermoduric Bacteria

Thermoduric bacteria can survive exposure to temperatures considerably above their maximal temperature for growth. In the dairy industry, the term is applied to those organisms which survive, but do not grow, at pasteurization temperature. They usually include species of Micrococcus, Streptococcus, Lactobacillus, Bacillus and occasionally some gram-negative rods. The sources of contamination are poorly cleaned and sanitized utensils and equipment on farm and processing plants. These bacteria contribute to significantly higher Standard Plate Count on pasteurized milk. The thermoduric count has been used in the dairy industry primarily as a test of hygienic practices and sanitary conditions maintained. It is also a means for detecting sources of contamination in products.

14.4 Psychtrophic Bacteria

The terms psychrotropic or psychrophilic mean ‘cold-loving’ and microorganisms which play a significant role in biological processes in low-temperature environments are classified under this group. Psychrophilic species such as Pseudomonas, Flavovacterium, Alcaligenes, Acinetobacter, and Bacillus can grow rapidly at 7°C and below. They are generally non-pathogens and are rarely present in the udder. They can cause off-flavours (fruity, stale, bitter, putrid, rancid) in dairy products. The counts of these bacteria in milk depend upon sanitary conditions prevailing during production and the time-temperature combination of milk storage before processing. The effect of psychotropic bacteria on the shelf life of pasteurized milk is largely dependent on number present after packaging, the rate of growth, the storage period, and the biochemical activity of the organisms.

14.5 Lactic Acid Bacteria (LAB)

When milk is held at ordinary temperature, it curdles or putrefies within a few hours owing to the rapid growth of bacteria. The lactic acid forming bacteria (those fermenting lactose into lactic acid) are important because they sour the milk. Some acid-forming bacteria maybe useful, such as those used as starter cultures in cultured dairy products such as dahi, yogurt and cheese and some harmful, such as those causing curdling and spoilage of milk. Some of the bacteria significant in dairy industry are listed in Figure 14.1.

Fig. 14.1 Lactic acid bacteria significant in dairy industry

(**Old names in brackets)

14.6 Spoilage Organisms

Microbial spoilage of milk denotes the degradation of protein, carbohydrates, and fats by microorganisms and/or their enzymes. Coliforms (e.g. Escherichia coli) are facultative anaerobes that grow optimally at 37°C. They also can cause rapid spoilage of milk because they are able to ferment lactose with the production of acid and gas, and are able to degrade milk proteins. As they are destroyed by pasteurization treatment, their presence in processed milk is indicative of post-pasteurization contamination. Psychotropic organisms play a major role in the spoilage if milk. Although most psychrotrophs are destroyed by pasteurization, some like Pseudomanas sp. Alcaligenes are of economic value, since they produce a high bacterial count during storage and may also create a ropy milk condition. Pseudomonas fluorescens, Pseudomonas fragi can produce proteolytic and lipolytic extracellular enzymes which are heat stable and capable of causing spoilage. They are capable of producing heat resistant proteases and lipases, which are capable of surviving even UHT processing. Some organisms that can survive pasteurization temperatures and grow at refrigeration temperatures are Bacillus, Clostridium, Corynebacterium, Arthrobacter, Lactobacillus, Microbacterium, Micrococcus and Streptococcus. These organisms may be detected by their characteristic morphology or by plate count incubated at refrigeration temperatures.

Some other groups of acid-forming bacteria that are significant in dairy field include two important groups of Streptococcus. One of these groups found in high heat-processed milk and milk products produces acid and gas with objectionable proteolysis (Streptococcus liquifaciens). The second group causing rapid curdling of milk include gas-forming (Aerobactor aerogenes, Bacillus polymyxa, Clostridium butyricum), ropy or slimy milk-forming (Alcaligenes viscosus) or sweet-curdling (Bacillus cereus) bacteria. Milk may also be fermented by yeasts (Saccharomyces delbrueckii, Candida mycoderma) and moulds (Cladosporium, Penicillium, Rhizopus).

14.7 Pathogenic Organisms

Clean milk production practices, hygienic handling and storage of milk, thermal processing practices such as pasteurization have decreased the incidence of milkborne diseases such as tuberculosis, brucellosis and typhoid fever. Milk is pasteurized primarily to eliminate any pathogenic organism which may be present so as to make it safe for human consumption. However, pathogens such as Bacillus cereus, Listeria monocytogenes, Yersinia enterocolitica, Salmonella spp., Escherichia coli O157:H7 and Campylobacter jejuni are known to survive pasteurization. Molds (Aspergillus, Fusarium and Penicillium) may produce mycotoxins which can be a health hazard. These organisms may enter into milk from an infected animal or by contamination from various sources such as infected food handlers.

Selected Readings

Anantakrishnan, C.P., P. N. Padmanabhan, P.N., Singh, R.B. 1994. Dairy Microbiology. Sri Lakshmi Publications. , India.
Robinson, R.K. 2002. Dairy Microbiology Handbook. John Wiley and Sons, New York.