"The Effect on 'Anti-Infective' Agents"

Untreated milk contains varying amounts of anti-microbial proteins and other anti-infective agents which are designed to protect the young animal from infectious disease. This can happen even if the milk is contaminated - for example, investigators reported a "low incidence of enteric infections in suckled infants under appalling hygenic conditions amongst South American Indians, although the milk was frequently harbouring bacteria". In an experiment involving newborn guinea pigs, untreated fresh cow's milk was found to be the most effective in decreasing colonisation by Escherichia coli, followed by pasteurised milk, then boiled milk, frozen milk was the worst. In this experiment the animals fed untreated milk also showed striking differences in the skin and coat compared to those fed pasteurised or boiled milk.

It is not only newborn animals which may benefit from this protection. Studies have shown that consumption of untreated milk by various tribes favoured the suppression of infection . Animal experiments have demonstrated a suppressing effect of milk, although this remained to some extent even after pasteurisation.

In addition to the fact that untreated milk contains anti-microbial agents which function after its ingestion, it also has components which inhibit the proliferation of bacteria before it is consumed. In one sampling study, bacterial counts were measured in 48 samples, the bacterial counts did not increase significantly over the two-day period and in 5 of these the count actually fell. Rigorous testing by the Milk Marketing Board's Central Testing Laboratories make it unlikely that antibiotics contaminating the milk might have contributed to the decline in bacterial count. Whilst some organisms may have continued to grow, the number dying was greater, resulting in an overall decline. In another experiment, in milk held at 4C the bacterium Campylobacter jejuni (a potential cause of acute gastro-entritis) "died most rapidly in unpasteurised milk and was inactivated at an intermediate rate in sterile milk". If untreated milk is of high microbiological quality, it is obviously not the breeding ground for bacteria that many people would have us believe.

Once milk has been pasteurised or otherwise heat-treated, the ability to restrict bacterial growth is lost or severely curtailed. The enzymes and other infective agents are destroyed to varying degrees. This means that bacteria contaminating milk after pasteurisation (e.g. from filling machines, from the air, or even entering under the bottle cap) can grow more rapidly than they would in untreated milk. Pasteurisation of human milk for hospital milk banks has been reported in some instances to contribute to outbreaks of digestive upsets in infants. The doctors writing this report concluded that "the evidence suggests that pasteurisation not only eliminates pathogenic bacteria but also damages bacteriostatic mechanisms, so making the milk more susceptible to later contamination". They concluded that "pasteurisation of donated breast milk is unnecessary and it is not recommended" and that "untreated breast milk can be safely stored at 4 - 6 C for 72 hours".

Conditions of hygiene in many of the larger dairies are fortunately extremely good but nevertheless post-pasteurisation contamination remains a problem and some of the contaminating micro-organisms can grow quite well at refrigeration temperatures. In a recent report from the National Institute for Research in Dairying, 146 out of 158 samples of pasteurised milk from 50 dairies were contaminated with these kinds of bacteria. Although not pathogenic they do contribute to spoilage. In most cases, the bacteria were present at a very low and insignificant level so that the keeping quality of the milk was still very good. Some, however, had bacterial counts sufficiently high that the effective shelf-life was very much shorter.

Yet another group of bacteria, the so-called thermodurics, can survive pasteurisation. The numbers present in bottled milk can vary quite widely and the presence of those which grow well at low temperatures affects the shelf-life of the pasteurised product (28). In addition, laboratory studies have shown that when cow's milk is pasteurised, a germinant for certain bacterial spores can be produced (29).