Lastly, while the heating process is of course the essential feature of efficient pasteurisation, it must not be forgotten that rapid and thorough cooling is almost equally important. As we have seen, pasteurisation differs from complete sterilisation in that it leaves behind a certain number of microbes or their spores. Cooling inhibits the germination and growth of this organismal residue. If after the heating process the milk is cooled and kept in a refrigerator, it will probably keep sweet from three to six days, and may do so for three weeks.
Before leaving this subject we may glance for a moment at the bacterial results of pasteurisation and sterilisation. The chief two of these are the enhanced keeping quality and the removal of disease-producing germs. The former is due in part to the latter, and also to the removal of the lactic acid and other fermentative bacteria. As a general rule these bacteria do not produce spores, and hence they are easily annihilated by pasteurisation. True, a number of indifferent bacteria are untouched, and also some of the peptonising species. The cooling itself contributes to the increased keeping power of the milk, especially in transit to the consumer.
Pasteurised milks have the following three economical and commercial advantages over sterilised milks, namely, they are more digestible, the flavour is not altered, and the fat and lact-albumen are unchanged. Professor Hunter Stewart, of Edinburgh, about two years ago, compiled from a number of experiments the following instructive and comprehensive table (page 212).
It will be admitted that this table exhibits much in favour of pasteurisation; yet the crucial test must ever be the effect upon pathogenic bacteria. Flügge has conducted a series of experiments upon the destruction of bacteria in milk, and he states that a temperature of 158° F. (70° C.) maintained for thirty minutes will kill the specific organisms of tubercle, diphtheria, typhoid, and cholera. MacFadyen and Hewlett have demonstrated,[62] by sudden alternate heating and cooling, that 70° C. maintained for half a minute is generally sufficient to kill suppurative organisms and such virulent types of pathogenic bacteria as Bacillus diphtheriæ, B. typhosus, and B. tuberculosis.
| No. of Experiments. | Average No. of Microbes spe cc. in Milk before Treatment. | Temperature and Duration of Pasteurisation in Minutes. | No. of Microbes per cc. in Pasteurised Milk after 24 Hours. | Soluble Albumen in Fresh Milk, per cent. | Soluble Albumen in Pasteurised Milk, per cent. | Taste of Pasteurised Milk |
| 5 | 136,262 | 10' 60° C. | 1722 average | 0.423 | 0.418 | Unaffected |
| 4 | 53,656 | 30' 60° C. | 1 sterile 3 averaged 955 | 0.435 | 0.427 | " |
| 12 | 78,562 | 10' 65° C. | 6 sterile 3 averaged 686 | 0.395 | 0.362 | Not appreciably affected |
| 12 | 132,833 | 30' 65° C. | 9 sterile 3 averaged 233 | 0.395 | 0.362 | " |
| 13 | 49,867 | 10' 70° C. | sterile | 0.422 | 0.269 | Slightly boiled |
| 9 | 38,320 | 30' 70° C. | " | 0.421 | 0.253 | " |
| 2 | 77,062 | 10' 75° C. | " | 0.380 | 0.070 | Boiled |
| 3 | 48,250 | 30' 75° C. | " | 0.380 | 0.050 | " |
| 1 | 1,107,000 | 10' 80° C. | " | 0.375 | 0.000 | " |
| 1 | 1,107,000 | 30' 80° C. | " | 0.375 | 0.000 | " |
Respecting the numerical diminution of microbes brought about by pasteurisation and sterilisation, respectively, we may take the following two sets of experiments. Dr. N. L. Russell[63] tabulates the immediate results of pasteurisation as follows:
| Unpasteurised. | Pasteurised. | |||||
| Minimum. | Maximum. | Average. | Minimum. | Maximum. | Average. | |
| Full cream milk. | 25,300 | 18,827,000 | 3,674,000 | 0 | 37,500 | 6,140 |
| Cream, 25%. | 425,000 | 32,800,000 | 8,700,000 | 0 | 57,000 | 24,250 |
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: