Microbiology of Raw Milk

Microbiology of Raw Milk

While many microorganisms are helpful and their growth desirable in the production of fermented dairy products such as cheese and yogurt, several others cause milk and milk products spoil. Few of such microbes are pathogens and are harmful to health.

Milk in the mammary gland is sterile but once the milk leaves the udder, microflora from the exterior of the udder, coat of the animal, atmosphere, utensils and workers enter and contaminate milk. So freshly drawn milk may contain a few thousand bacteria.  The microbial load of milk increases during further processing – storage activities and unhygienic practices increase the microbial load. Milk is an excellent medium for microbial growth due to its high water activity, moderate pH (6.4–6.6) and rich content of nutrients. So high hygienic standards are to be adopted during the production and processing of milk. The microbial content of milk indicates the sanitary quality and conditions of milk production.

The organisms most commonly isolated from freshly drawn milk are Micrococci, Streptococci, Corynebacterium bovis, etc. Higher microbial counts are observed in situations of mastitis. Some animals may be not having acute mastitis, but may be subclinically infected without any signs of infection yet the causative organism can be present in the milk.  The most important organisms casuing mastitis are Staphylococcus aureus, Escherichia coli, Streptococcus agalactiae, Strep. dysgalactiae, Strep. uberis, Pseudomonas aeruginosa and Corynebacterium pyogenes.

The udder exterior and its immediate environment may get contaminated with organisms from the cow’s general environment. Such contamination from bedding and manure can be a source of human pathogens such as E. coli, Campylobacter, Salmonella, Bacillus species, C. butyricum and C. tyrobutyricum.

Milk-handling equipment such as teat cups, pipework, milk holders and storage tanks are also the principal source of the micro-organisms in raw milk. If poorly cleaned equipment are used milk residues left on surfaces will allow microbial growth and can contaminate subsequent batches of milk.

In most developed countries milk is chilled immediately it is collected from the animal and is held at a low temperature thereafter. But many psychrotrophic species, such as Pseudomonas, Acinetobacter, Alcaligenes, Flavobacterium, psychrotrophic coliforms, Aerobacter spp., and Gram-positive Bacillus spp. survive the low temperature.

Thermophilic bacteria such as Bacillus can grow in milk held at elevated temperatures, including pasteurization temperatures. Such bacteria may enter milk from various sources in the farm, or from poorly cleaned equipments used.

Thermoduric bacteria can survive but do not grow when exposed to higher temperatures. Examples are species of Micrococcus, Streptococcus, Lactobacillus, Bacillus and occasionally some gram-negative rods which generally come from poorly cleaned and sanitized utensils and equipments on farm and processing plants.

Lactic Acid Bacteria (LAB), which ferment lactose into lactic acid grow rapidly when milk is held at ordinary temperature and curdles or putrefies the milk within a few hours. They are important because they sour the milk. Some acid-forming bacteria such as those used as starter cultures in cultured dairy products such as dahi, yogurt and cheese are useful, and some such as those causing curdling and spoilage of milk are harmful.

Microbial spoilage of milk denotes the degradation of protein, carbohydrates, and fats by microorganisms and/or their enzymes. Coliforms such as Escherichia coli which are facultative anaerobes that grow optimally at 37°C 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. Psychrotropic organisms like Pseudomanas also play a major role in the spoilage if milk. Alcaligenes produce a high bacterial count and create a ropy milk condition. Pseudomonas fluorescens and Pseudomonas fragi produce proteolytic and lipolytic extracellular heat stable proteases and lipases capable of causing spoilage.

Streptococcus liquifaciens found in high heat-processed milk and milk products produces acid and gas with objectionable proteolysis.

Some bacteria causing rapid curdling of milk include gas-forming bacteria such as Aerobactor aerogenes, Bacillus polymyxa, Clostridium butyricum, etc.

There are ropy or slimy milk-forming bacteria such as Alcaligenes viscosus or sweet-curdling bacteria such as Bacillus cereus.

Milk may also be fermented by yeasts such as Saccharomyces delbrueckii, Candida mycoderma, etc. and moulds such as Cladosporium, Penicillium, Rhizopus, etc.

Milk may contain infectious pathogens that may cause milkborne diseases such as tuberculosis, brucellosis and typhoid fever.  Milk may also contain fungi such as Aspergillus, Fusarium and Penicillium which produce mycotoxins and cause health hazard. These organisms may enter into milk from an infected animal or by contamination from various sources such as infected food handlers.  Clean milk production practices, hygienic handling and storage, thermal processing practices such as pasteurization, etc help to decrease the incidence of such milk borne diseases. Some pathogens such as Bacillus cereus, Listeria monocytogenes, Yersinia enterocolitica, Salmonella spp., Escherichia coli O157:H7 and Campylobacter jejuni survive pasteurization.

Milk contain number of antimicrobial features such as Moderate pH, High levels of protein, carbohydrate and fat, Lactoferrin, Lysozyme, Lactoperoxidase, Immunoglobulin, etc which protect the udder from infection and protect the newborn calf.

Lactoferrin is an iron-binding protein that have antibacterial activity.  Most micro-organisms need iron for growth and lactoferrin inhibit the growth of bacteria or kill them by depriving them of iron. It is effective against Escherichia coli O157:H7, Listeria monocytogenes, and other foodborne pathogens and spoilage organisms.

Lysozyme is an enzyme present in the milk.  Lysozyme kills bacteria by disrupting the glycosidic bond between the two components of peptidoglycan, a constituent of the bacterial cell wall.  Lysozyme possesses antibacterial activity against a number of bacteria. It functions in association with lactoferrin or immunoglobulin A and is effective against Escherichia coli and in association with ascorbate and peroxide it inhibits some species of salmonellae.

Lactoperoxidase has antibacterial activity when combined with hydrogen peroxide and thiocyanate. It catalyses the oxidation of thiocyanate by hydrogen peroxide and the reaction produces short lived oxidation products such as hypothiocyanate which can kill Gram negative bacteria and inhibit Gram positive bacteria by damaging the bacterial cytoplasmic membrane. Hydrogen peroxide is generated in milk by endogenous enzyme activity or by the aerobic metabolism of lactic acid bacteria.

Since the concentration or activity of these antimicrobial factors are very low, they do not have any influence in the keeping quality or safety.  Also sometimes the antimicrobial activity is antagonized by other milk constituents such as citrate and bicarbonate, etc.

References

• Food Microbiology, Third Edition,  Martin R. Adams and Maurice O. Moss University of Surrey, Surrey, Guildford, UK.
• 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.
• http://ecoursesonline.iasri.res.in/mod/page/view.php?id=147900
• http://www.milkfacts.info/Milk%20Microbiology/Antibacterial%20Properties.htm