Module 1. Hygienic milk

Lesson 2

SOURCES OF CONTAMINATION IN MILK

2.1 Introduction

Milk when secreted into an uninfected animal’s udder is sterile and invariably, it becomes contaminated during milking, cooling and/or storage. It is an excellent medium for the growth of bacteria, yeasts and moulds that are the common contaminants of any food material. Their rapid growth, particularly at high ambient temperatures can spoil the milk for liquid consumption and for manufacturing dairy products. This can be avoided to a greater extent by adopting the basic rules of clean milk production.

2.2 Sources of Microbial Contamination of Milk

Microbial contamination of milk can be from the internal and/ or external sources (Fig. 2.1) that are described in the following section.

2.2.1 Interior of udder

Varying numbers of bacteria are found in aseptically drawn milk with the reported counts of <100-10,000 CFU/ml from normal udder, but an anticipated average is 500-1000 CFU/ml in advanced countries. Microorganisms enter the udder through the duct at the teat tip that varies in length (from 5-14 mm) and its surface is heavily keratinized. This keratin layer retains the milk residues and exhibit antimicrobial activity. (Fig. 2.2)

During progress of a milking, bacteria are present in the largest numbers at the beginning and then gradually decrease.  This is mainly due to the mechanical dislodging of bacteria, particularly in teat canal, where the numbers are probably highest.  Because of this discarding of first few streams of milk helps in lowering the counts of microbes in milk. Milk from different quarters also vary in numbers.Different species of bacteria that are found in milk, as it comes from udder are very limited as given in Table 2.1.

Table 2.1 Presence of different microbial groups in raw milk

Though micrococci are slow growing, but if allowed to grow, they cause acid formation and proteolysis.  These are mostly non-pathogenic. Streptococci are less frequent than micrococci. Streptococcus agalactiae may be present even in non-clinical mastitis and thus it appears to be a natural inhabitant of udder. Among Gram positive rods, Corynebacterium bovis has been found in large numbers. It is non-pathogenic, but if grown causes rancidity. If an animal is infected from mastitis, microbial contamination from within the udder of animal contributes notably to the total numbers of microbes in the bulk milk, when compared with the milk originated from a healthy animal. The influence of mastitis on the total bacterial count of milk depends on the type of the infecting microbe. Most common microbial agents of mastitis in milch animals are given in (Fig 2.3) are Staphylococcus aureus, Streptococcus agalactiae, Streptococcus dysgalactiae, Streptococcus uberis, Escherichia coli and Corynebacterium pyogenes.

Fig. 2.3 Most common microbial agents of mastitis

2.2.2  Exterior of udder

In addition, to the udder infections, unclean udder and teats of animal also contribute significantly to the total bacterial counts of milk. The microbes that are naturally associated with the skin of the animals as well as those derived from the environment, where the cow is housed and milked are predominant in the milk. The environmental conditions such as soil, manure, mud, feed or bedding; determines what kind of microbes will dominate in milk.

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 1,00,000 cfu/ml. The bedding material in winter has high number of bacteria, mainly psychrotrophs, coliforms and Bacillus spp. Udder microflora is not affected much by simple washing. Economy washing with sodium hypochlorite accompanied by drying, helps in reducing the number of microbes. Different category of microbes that occurs in the exterior of udder are

· Predominantly micrococci and coagulase negative staphylococci exist.

· Next, on the teat surface are faecal streptococci, but Gram negative bacteria including coliforms are less.  Coliforms do not survive well on teat surface.

· Aerobic thermoduric organisms are entirely Bacillus spp. The more frequent are B. licheniformis, B. subtilis, B. pumilis and less frequent ones are B. cereus, B. circulans and B. firmus.

· Teat surface may also contain clostridial spores that are usually found in cows fodder, bedding and faeces.

Psychrotrophic and thermoduric bacteria predominate on the teat surfaces. The psychrotrophs that can grow at 7ºC and below are mostly Gram negative rods, and the major ones are Pseudomonas fluorescens, followed by Alcaligenes, Flavobacterium and coliforms (Fig. 2.4). On the other hand, thermodurics on teat surfaces are often bacterial spores (a dormant and non-reproductive structure; highly resistant to radiations, desiccation, lysozymes, high temperature, starvation and disinfectants) that are typically found in the soil (Fig. 2.5). When these spores enter the bulk milk, they may survive during pasteurization and cause a number of post-pasteurization problems.

2.2.3 Coat of cow

The coat serves as a vehicle to contribute bacteria directly to milk. The hairs around udder, flanks and tail contribute to the higher bacterial count in milk.  The coat may indirectly contribute microbes into air, especially Bacillus spp. The coat may carry bacteria from the stagnant water pools, especially ropiness causing milk microbes.

2.2.4 Animal shed and surroundings

Milk produced on farms with poor hygiene practices may undergo significant spoilage and have a shorter shelf-life, when compared to milk produced under hygienic conditions. Microbes associated with the bedding materials include:

· Coliforms

· Spore-formers

· Staphylococci

· Streptococci

· Other Gram negative bacteria

2.2.5 Milking staff

The staffs involved at different stages of milk production plays a pivotal role in maintaining hygiene and preventing milk contamination. The hand contacts or dislodging of dust and dirt particles by milker may add varieties of microbes to milk. Risks of contamination from milker are definitely higher, when cows are hand-milked in comparison to when they are machine-milked. Soiled clothes and hands increase the risk of contamination of milk and milking equipments many folds. Milker with infected wounds on hands contributes pathogenic Streptococcus spp. and micrococci. If wet hand milking is practiced, the microorganisms present in lubricants like fore-milk, water or saliva of the milker and bacteria from hands and teats will enter the milk.

The common microbial pathogens from humans causing diseases such as typhoid, paratyphoid and dysentery may contaminate the milk. Microbial pathogens causing scarlet fever, septic sore throat, diptheria, cholera etc. contaminate the milk.

2.2.6 Milking equipment (storage containers and transportation systems)

Improperly cleaned milking and cooling equipments are one of the main sources of milk contamination. Milk residues left on the equipment contact surfaces supports the growth of a variety of microbes. Although natural inhabitants of the teat canal, apex and skin; microorganisms associated with contagious mastitis do not grow well on these equipments, it is possible that certain strains associated with environmental mastitis may grow to a significant level. Since, it is very difficult to remove all milk residues and deposits from the milk contact surfaces of milking equipments; hence equipment with smooth surfaces and minimal joints should be used. The tanker and collecting pipes are also the potential sources of contamination, if not adequately cleaned. In addition, biofilms can easily build up on the enclosed, hard to clean surfaces (Fig. 2.6).

Unclean or improperly cleaned milk cans and lids if they are still moist, results in multiplication of thermophilic bacteria like Bacillus cereus. Improperly sterilized milking machines contain thermoduric micrococci, Bacillus spp. and Microbacterium spp. predominantly compared to coliforms and streptococci. Rubber hoses predominantly contribute to pseudomonads rather than thermodurics.

2.2.7 Water supplies

At dairy-farms, the water can be a predominant source of microbial contamination. Water used in production should be of good bacteriological quality. Inadequately or uncleaned, storage tanks, untreated water supplies from natural sources like bore wells, tanks and rivers, may also be contaminated with the faecal microbes (e.g. Coliforms, Streptococci and Clostridia). In addition, a wide variety of saprophytic bacteria (i.e. Pseudomonas, Coliforms, other Gram negative rods, Bacillus spores, Coryneform bacteria and lactic acid bacteria) may also be present in water and may contaminate the milk potentially. The warm water used for udder washing is potent source of Pseudomonas and Coliforms which may even cause mastitis.

2.2.8 Airborne contamination

Aerial contamination of milk by bacteria is insignificant, in comparison to microbes with those that are derived from the teat surfaces. The microbial counts of air in sheds rarely exceed 200 cfu/l. Micrococci account for >50% of the aerial microflora. Air contains dust, moisture and bacteria; hence its entry should be minimized in milk. Micrococci, Coryneforms, Bacillus spores, streptococci, and Gram negative rods are the major genera present in air. In general, more air incorporated into milk leads to the faster growth of bacteria. Following are some of the practices that increase aerial counts in milk:

· Sweeping of floors just before milking process

· Handling hay and feed shortly before milking process

· Brushing of animals prior to milking process

· Having the dusty bedding materials for animals

· Allowing dust and dirt to accumulate on  the walls or ceiling of sheds