As regards the later effect of the process, he states that in fifteen samples of pasteurised milk examined from November to December nine of them revealed no organisms, or so few that they might almost be regarded as sterile; in those samples examined after January the lowest number was 100 germs per cc., while the average was nearly 5,000. With the pasteurised cream a similar condition was to be observed. Dr. Hewlett[64] defines pasteurisation briefly as heating the milk to 68° C. for twenty or thirty minutes, and this treatment he quotes as destroying 99.75 per cent. of the total number of organisms. Bitter's table of results at 158° F. bears out the same: BEFORE PASTEURISATION. AFTER PASTEURISATION. No. of Bacteria in 10 Drops. No. of Bacteria in 10 Drops. 1. 102,600 2-3 2. 251,600 30-40 3. 25,000 3-5 4. 37,500 2-5 5. 94,000 2 BACTERIA IN MILK PRODUCTS Cream is generally richer in bacteria than milk. _Set_ cream contains more bacteria than _separated_ cream, but germs are abundant in both. Yet whilst it is true that cream contains a large number of bacteria, it must be pointed out that the butter fat in cream is a less suitable food for organisms than is the case with milk. Hence the fermentative changes set up in cream are of less degree than in milk, particularly so if separated from the milk. Butter-milk and whey vary much in their bacterial content. Butter necessarily follows the standard of the cream. But as the butter fat is not well adapted for bacterial food, the number of bacteria in butter is usually less than in cream.[65] Moreover, they are soon reduced both in quality and quantity. Butter examined after it is several months old is often found to be almost free from germs; yet in the intervening period a variety of conditions are set up directly or indirectly through bacterial action. _Rancid_ butter is partly due to organisms. _Putrid_ butter is caused, according to Jensen, by various putrefactive bacteria, one form of which is named _Bacillus fœtidus lactis_. This organism is killed at a comparatively low temperature, and is therefore completely removed by pasteurisation. _Ill-flavoured_ butter may be due to germs or an unsuitable diet of the cow and a retention of the bad quality of the resulting milk. _Lardy_ and _oily_ butters have been investigated by Storch and Jensen and traced to bacteria. Lastly, _bitter_ butter occasionally occurs, and is due to fermentative changes in the milk. Butter may also contain pathogenic bacteria, like tubercle. The _B. coli_ can live for one month in butter.