Milk is an essential part of the daily diet for many people in the western world. It is used on our breakfast cereal, in our tea and coffee, in cooking, and as the only source of nutrition for infants for the first few months of their life. In the latter case, preferably by the mother’s own supply, otherwise, infant formula produced from cow’s milk.
Milk is also a very nutritious medium for microbial growth, and spoils very quickly if not handled properly after collection. In addition, many pathogenic bacteria can also grow in milk, for example, Listeria monocytogenes, Salmonella spp, and Escherichia coli.
Historically, the lengthy storage of milk could only be achieved by processing it by some form of fermentation in some way; making yoghurt and various soft and hard cheeses, for example.
Modern dairies now do an excellent job in ensuring that milk is safe and relatively stable once it leaves the dairy. Look at the shelflife extension that has been achieved today with milk, if purchased though the supermarket in comparison to doorstep delivery.
This has been achieved through improved processing extension of the chill chain right through to the home fridge (if the shopper does not take too long to get home), and more hygienic packaging, for supermarket purchased milk.
As ever, it is the supermarkets that ensure that hygiene and other issues are considered, in their striving for longer shelf-lives, better quality, and fewer hazards. In the supermarket, milk is packed in plastic or cardboard, not glass, removing the glass hazard, but also allowing improved hygiene through the use of virtually sterile plastic bottles. This can all go wrong, however, if on-farm hygiene and other practices are poor, contaminating the milk with bacteria associated with the cows gut or skin, or from the soil.
HACCP is normally applied to food once it has entered the processing factory. In the dairy industry, however, it is, in many cases, being applied to pre-process production since it has been realised that the earlier in the chain of production it comes into play, the better the overall control can be.
Milk is essentially sterile at the point of production. Problems occur if the animal is diseased, or poor hygienic practices are employed on the farm. Milk can become contaminated from an infected udder, which can be quite common. Surveys in the UK have indicated that approximately one third of dairy cattle suffer from the condition called mastitis. Mastitis can be caused when the micro flora associated with this condition enter the udder usually through the duct at the teat tip. S. aureus in particular is able to colonise the duct itself and other typical micro flora includes Staphylococcus aureus, Streptomyces agalactiae, Streptococcus uberis, and E. coli. Although mastitis causes visible effects, in the earlier, less visible stages it can still cause contamination of the milk.
It has been suggested that the milking parlour equipment is responsible, actually pushing the bacteria into the teat. Once into the milk in this way, bacterial numbers will rise through multiplication in the collection chain. In some surveys S. aureus has been isolated from 86% of milk samples, usually at quite lownumbers (<104cfu/ml in 90% of cases); so it is clearly a very important contaminating organism. Most of the organisms causing mastitis do not grow in refrigerated milk; but they can survive, as can any pre-formed toxins that they may have produced.
Mastitis can be effectively controlled by good hygienic practices and the use of antibiotics. Antibiotics must, however, be used carefully, or other problems can be generated, for example, the transfer of antibiotic resistance to human pathogens, and carryover into the milk, causing slow development of starter cultures in yoghurt and cheese fermentations. The outside of the udder is also a good source of contaminating micro-organisms. Faecal material, mud and soil, plant and bedding material can all stick to the udder, and then poor hygienic practices allow the milk to be contaminated when the milking equipment isattached.
Listeria is one such organism that gets into the milk by this route. It is an environmental bacterium, commonly found in soil. Washing with water jets can be done, but the use of cloths impregnated with an approved disinfectant is more common, since physical abrasion is required to get rid of dirt and faecal matter. Whichever method is used, udders must be properly dried after cleaning and before milking, using disinfected towels. Failure to dry the udder and teats can lead to contamination of the milk with Pseudomonas spp.
Cows tend to be less contaminated in the summer so it is reasonable to conclude that pasture is a less likely source of contamination than bedding. Spore formers such as Bacillus spp. are commonly associated with bedding. After milking, it is usual practice to use an approved disinfectant such as iodine as a teat dip, before returning cows to pasture or their bedding area. Other sources of contamination could include parlour personnel, water, and the air, though these are all much less likely than the above sources.
Potent source of infection
Poorly sanitised parlour equipment can also be a significant source of contamination. Bacteria can divide about every 20-30 minutes under ideal conditions so a few millilitres can be a potent source of infection for fresh milk coming into contact with it. Control measures are very simple, cleaning, cleaning, and cleaning! Bulk tanks are fairly easy to clean and, therefore, tend not to contribute greatly to contamination, in comparison to the pipes leading to the tank from the milking parlour. Because they can be difficult to clean taps, stopcocks, thermometers and all other items that intrude into the smooth design can cause problems.
The key issue is how quickly the milk is cooled to 4°C or less. This is the primary method of controlling growth once the milk is in the tank. An additional factor is the milk collection sequence. Is it collected daily, or every other day by the tanker? This will also affect the growth potential of contaminating cold loving organisms or psychrophiles, because they start to come out of the lag phase and into the exponential phase of growth after about 60-72 hours.
Milk collected every other day will contain organisms entering this phase by the time it enters the dairy. This milk must be proces- sed quickly as its storage potential is greatly reduced. Milk from farms with poor hygiene practices will be undergoing significant spoilage and have a short shelf life relative to milk produced under more hygienic conditions. The tanker and collecting pipes are potential source of contamination, if not adequately cleaned. Biofilms can easily build up on enclosed, hard to clean surfaces. Biofilms are a community of different organisms that gain mutual benefit from each other. They start when food solids adhere to a surface, followed by bacteria that secrete gums and other sticky extra cellular compounds that allow the first bacteria to stick to the surface. They are a major problem within the dairy industry. Cleaning itself can be an issue, if the cleaning and disinfecting agents are not rinsed off of surfaces thoroughly after cleaning is carried out; they can be a source of chemical contamination.
It is common to see a two fold increase in bacterial count from the farm to the dairy, due to the growth of psychrophiles during the journey. Control, therefore, is by good hygiene on the farm, making sure the cow is clean and particularly the udder and teats are thoroughly cleaned before milking. The milk is then cooled rapidly after milking, and the cleaning and disinfecting agents are used carefully.