What is Milk ? Milk is the lacteal secretion obtained from milking of healthy milch animals -whole, fresh, clean and free from colostrum. Milk is an ideal food. It supplies many body building proteins; bone forming minerals and health-giving vitamins and furnishes energy giving lactose and milk fat. It is an excellent source of calcium and phosphorus, both of which together with vitamin D are essential for bone formation. All of these properties make milk an important food for pregnant women, adults, invalids, school going children, old persons and patients alike. How to define clean Milk Comes from healthy, milch animals. Should be devoid of bad smells, dirt and filth. Should contain relatively small number of bacteria. Should be free from disease causing pathogens. Should have normal composition and acidity. Sources of Contamination and prevention mechanisms Animal body The sources of contamination of milk on the farm can broadly be categorized as: Interior of the Udder Exterior of the Udder The interior reflects the health of the animal, while the exterior depends on hygienic conditions and practices on the farm, equipment used, persons, water etc. 1. Interior of the Udder The animal by itself is one of the most significant contributors of microorganisms in milk. Microorganisms enter the udder through the duct at the teat tip. During the milking process, bacteria are present in the largest number at the beginning and they gradually decrease. This is mainly due to mechanical dislodging of bacteria, particularly in the teat canal where the number is probably highest. Discarding the first few streams of milk results in a lower count of microbes in milk. Most of the bacteria are excreted in the fore milk. When the animal is suffering from some disease, the causative organisms are also likely to come into milk depending upon the severity of infection. The most common disease in milch animals is mastitis and the primary causative organism; Streptococcus agalactiae is commonly present, even though no clinical evidence of mastitis is present in the cows. The other common mastitis causing organisms potentially harmful to humans are Staphylococcus aureus and Escherichia coli, occasionally Streptococcus uberis and Streptococcus dysgalactiae. Some of the other organisms encountered occasionally are Listeria sps, Leptospira sp, Bacillus cereus, Pasteurella multocida, Clostridium perfringens, Actinomycetes, Coxiella burnetti and the Yeast Cryptococcus neoformans. The other organisms, which can come through the animal, include, Mycobacterium tuberculosis, Brucella abortus, etc. Control Animals should be maintained in a clean, dry environment, free from dust and dirt. Milk from the first few streams should be discarded. Milk from the infected udder should be discarded. Dairy animals should be periodically vaccinated against susceptible diseases. 2. Exterior of the Udder Number and type of organisms associated with udder vary, depending on the type and amount of soil. Udder and teat become soiled with dung, mud, bedding material, such as saw dust, straw etc. With heavily soiled udder teats the counts may be 1 lakh cfu/ml. Bedding material has a higher number of bacteria in winter. Psychrotrophs, coliforms and Bacillus sp are the main organisms in descending order as per numbers. In summer since the cows are returned to pasture, the number of bacteria in bedding declines. Udder micro flora is not affected very much by washing. Sodium hypochlorite washing and accompanied by drying help in reducing the number.1. Teat surface and animal body2. Teat surface Teat surface may also contain clostridia spores that are usually found in cow's fodder, bedding and faeces. Prevention strategies Prevent regular soiling of teat surface. Wash with disinfectant. Quaternary ammonium compounds can act effectively, but presence of organic matter interferes with action of Chlorine and it is also an irritant to hands. Drying of teats before milking. A fresh cloth should be used for every cow and it should be moistened in sanitizers after each use. Paper towels are preferable. Coat of Cow The Coat serves as a vehicle to contribute bacteria directly to milk. Clipping of hair around udder, flanks and tail reduces the count of bacteria in milk. The Coat may indirectly contribute organisms into the air of the barn, especially Bacillus sp. The Coat may carry bacteria from stagnant water pools especially ropy milk organisms. Coliforms may gain from soil and manure. Prevention strategies Periodic clipping of hair and regular brushing of coat should be done. Machine milking practice should be adopted. Milking barn and the environment The housekeeping practices in the milk producing area also decide the level of contamination. Accumulation of mud, animal urine and faeces, left-over straw and feed in the milking barn can directly or indirectly contaminate the milk through air. The following should NOT be done as they contaminate milk in the milking barn. Sweeping right before milking Handling hay and feed just before milking Brushing animals just before milking Having dusty bedding material Allowing dust and dirt to accumulate on the wall and the ceiling. Prevention The environment in the milking barn should be clean. Aerial contamination can be prevented by the use of small top milk pails and milking machines. Milking utensils Improperly cleaned 'milk contact surfaces' like milking equipment including buckets, pails, cans, bulk tanks etc. are the only source of contamination in milk after it leaves the udder. The most hazardous situation arises when the milking utensils are not thoroughly cleaned after use and the milk solids with some moisture are left on the surfaces. These allow growth of microorganisms and heavily contaminate the fresh milk, which comes in contact with these utensils later on. When mastitis is prevalent in the herd, the utensils may contain Streptococcus agalactiae, Staphylococcus aureus, E. coli or Corynebacterium sp. When hot cleaning of equipment is done, the predominating organisms are thermoduric, mainly Bacillus sp. and Micrococcus. When cold cleaning is followed, heterogeneous microfloras are present. Control The milking utensils and equipment should be cleaned and sanitized before and after milking. The tanks used for bulk transport of milk should be cleaned and sanitized immediately after the unloading of milk. Milker When hand milking is done, the contribution from milker is high. Milker, with infected wounds on hands, can contribute pathogenic Streptococcus & Micrococcus Activities like sneezing, coughing etc. increase the risk of contamination. During wet hand milking process lubricant enters milk and adds bacteria from the hands and teats. Pathogens causing typhoid, paratyphoid, dysentery, scarlet fever, septic sore throat, diphtheria, cholera etc. are contributed from humans. Action of milker may also dislodge dust and dirt and increase air contamination. Control The hands of the milker should be clean and he may dip his hands in antiseptic solution before milking. Unhealthy milkmen should be avoided. Bad habits like coughing and sneezing should be prevented within the milking barn. Milkers should do their health checkup at every 6 months' interval, as a routine practice. They should receive vaccines against cholera, typhoid, dysentery etc. which generally gets transmitted from human beings to milk. Water Supply Water used should be potable and good in terms of bacteriological quality. Direct sources of contamination are: Storage tanks, not protected from rodents, birds, insects and dust. Hoses Water troughs Untreated water supplies from bore wells, lakes and rivers may be contaminated at source with faecal Streptococci, Coliforms, G-ve rods, Lactic acid bacteria, Bacillus sp., and Corynebacterium sp. Control - Water used for cleaning should be free from faecal contamination and water may be chlorinated before use. Chlorination - with hypochlorites is recommended. Milking Machine Milking machines are used when large number of animals are to be milked at a time. When they are used, proper care is required not only in their cleaning but also in their use, because improper use may damage the udder and thereby increase risk of contamination. When pipelines are reused for direct milk collection, their improper cleaning and sanitation add to the microbial load of milk. The types of microorganisms coming from milking machines are similar to those coming from other equipment. Control - Milking machines should be cleaned and sanitized by using iodine sanitizers as a routine practice. Storage and Transport Mainly the psychotropic and thermoduric bacteria contaminate the milk through unclean milk cans and bulk milk transport tankers. Among the psychotropic genera in stored milk, Pseudomonas sps. flourescens generally predominates. Control - Milk should be stored and transported between the temperatures of 0-5° C. Facts to know about milk contamination Once the milk has left the animal, many avenues of subsequent contamination may be operative. When milk is held at ordinary temperature, it curdles or putrefies within a few hours. The bacteria commonly found may grow rapidly and bring about undesirable changes when milk is held at the ambient atmospheric temperature. Among these, the acid forming bacteria, or those fermenting lactose into lactic acid are important groups of Streptococcus: one of these is found in milk and other dairy products subjected to relatively high heat; these form acid and gas with objectionable proteolysis (Streptococcus liquifaciens); another is typical lactic acid organism of butter cultures (Streptococcus lactis, Streptococcus cremoris); it is frequently found in stagnation and surface water; and the third one causes rapid curdling of milk. These are also gas-forming (Aerobactor aerogenes, Bacillus polymyxа, Clostridium butyricum), ropy or slimy milk-forming (Alcaligenes viscosus), sweet-curding (Bacillus cereus) bacteria. Milk also contains yeasts (Saccharomyces delbruekii, Candida mycoderma) and moulds (Cladosporium, Penicillium, Rhizopus) which bring about its fermentation. Emphasis has been placed on cold-loving bacteria which can grow at refrigeration temperatures and are not reduced by pasteurisation. They are troublesome because they produce a high bacterial count during storage and may also create a ropy milk condition. They survive even UHT processing and can only be detected by using relatively high incubation temperatures. Source : Department of Animal Husbandry and Dairying Ministry of Fisheries, Animal Husbandry and Dairying Government of India