These early experiments were too unsophisticated to withstand modern statistical analysis but this does not deny their historical value. Reference can, however, be made to more recent and precise experiments which have compared untreated with heat-treated milk.

There is approximately a 10% loss of vitamins BI, B6, B12 and folate and a 25% loss of vitamin C although some workers have noted higher losses of vitamin C. Greater losses of vitamins occur with more severe heat treatment .

The proteins in milk are of two kinds - casein and whey. Caseins are remarkably heat stable but the whey proteins, which are of much higher nutritional value, are denatured by heat treatment (5,8). The degree of denaturation varies depending on the temperature and time of heat exposure - 10% during pasteurisation, 70% during ultra heat treatment. Homogenisation has a further destabilising effect.

Several experiments have reported adverse effects of heat treated whey proteins on baby pigs and calves. Although no such effects have been reported for humans and it is generally assumed that such denaturation is of no practical significance, some workers argue that the effects of the cross-linking of whey proteins caused by heating may be detrimental to the consumer, possibly via an effect on nutritional value and also perhaps by the increased potential to trigger some form of allergic reaction.

Vitamins and minerals can be bound to proteins and this binding can facilitate their absorption from the digestive system. Pasteurisation destroys the ability of certain proteins in milk to bind the important vitamin folate and hence help its absorption. Heat treatment might also cause a similar inactivation of other protein carriers, for example those for zinc and vitamin B12.

anaphylactic allergy
atopic allergy.
In the first instance, heat treatment does diminish, but does not completely destroy, the allergenic properties of milk (9, 16). In the second type of allergy, atopic, it was found by one researcher that heat processing may render milk more harmful to atopic individuals. The B-lactoglobulin from fresh raw cow's milk had a lower allergy-causing reactivity than that from pasteurised or otherwise heat-processed milk.

One doctor has even gone so far as to suggest that the response of the body to heat-denatured milk protein may contribute to the development of atherosclerosis. He has produced evidence linking the introduction of the Holder pasteurisation technique and its geographical distribution to the incidence of heart disease. Such results, though interesting, should be treated cautiously, since they are statistical associations and not evidence of cause and effect. Whilst others have not found supporting evidence for this theory, it is clear that more critical research is necessary before heat treatment is universally enforced.

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).

The majority of infections attributed to milk during this period were outbreaks of salmonella food poisoning and campylobacter infection, with a few isolated cases of other infections. For the decade 1971 - 1980, there were 86 outbreaks of infection attributed to untreated cow's milk with 1096 people affected.

In 1981, 21 outbreaks from consumption of unpasteurised milk were reported with 294 people affected and in 1982, 18 outbreaks with approximately 612 people suffering. The question is whether these figures represent a true increase in the numbers of people infected by consuming unpasteurised milk, and if so, why, or whether they are the result of increased reporting of this type of infection, with possibly undue blame put on milk.

Furthermore, "in the incidents reported to the Communicable Disease Surveillance Centre, the food vehicle of infection is only infrequently confirmed epidemiologically or microbiologically. Therefore food items reported to be vehicles of infection should be viewed cautiously". For example, in one report from this Centre, unpasteurised milk was the "suggested" vehicle in 21 outbreaks of salmonellosis in 1982 whereas in another report published two weeks later, the figure given was 15 and it was admitted that the causative organism was isolated from milk, milk stocks, or both in only 10 of these outbreaks.

In the period 1950 - 1982 there were 172 outbreaks attributed to untreated milk and 6 deaths, which is only a small proportion of the total numbers over that period. In 1982 salmonellosis attributed to untreated milk affected 412 people, only 3.2% of the total number of 12,684 who were affected from all causes. During the period 1950 - 1980 this figure was only approximately 1%. When considered in terms of 'incidents' of food poisoning, less than 3% of the total for 1982 were conclusively linked to untreated milk .

Over the 31 years from 1951 - 1982 only 6 deaths were attributed to infections from untreated milk. Yet in one year alone, 1982, there were 67 deaths from other types of bacterial food poisoning.