Concerning these figures little comment is necessary. But here again, we may remember that this rapid multiplication continues only up to a certain point, after which competition brings about a marked reduction.
The effect of temperature and time has been illustrated by Dr. Buchanan Young's recent researches, laid before the Royal Society of Edinburgh. He estimated that in the Edinburgh milk supply three hours after milking there were 24,000 micro-organisms per cc. in winter; 44,000 in spring; 173,000 in late summer and autumn. Again, he found that five hours after milking there were 41,000 micro-organisms per cc. in country milk, and more than 350,000 micro-organisms per cc. in town milk. Many London milks would exceed 500,000 per cc.[56]
There is no standard or uniformity in the numerical estimation of bacteria in milk. A host of observers have recorded widely varying returns due to the widely varying circumstances under which the milk has been collected, removed, stored, and examined. Nor is it possible to establish any standard which may be accepted as a normal or healthy number of bacteria, as is done in water examination. Bitter has suggested 50,000 micro-organisms per cc. as a maximum limit for milk intended for human consumption.
But owing to differences of nomenclature and classification, in addition to differences in mode of examination at present existing in various countries, it is impossible to state even approximately how many bacteria and how many species of bacteria have been isolated from milk. Until some common international standard is established mathematical computations are practically worthless. They are needlessly alarming and sensational. And it should be remembered that great reliance cannot be placed upon these numerical estimations. They vary from day to day, and even hour to hour. Furthermore, vast numbers of bacteria are economic in the best sense of the term, and the bacteria of milk are chiefly those of a fermentative kind, and not disease-producers.
Kinds of Bacteria in Milk. It is clear from the foregoing that the only valuable estimation of bacteria in milk is a qualitative one. The kinds commonly found may be classified thus:
1. Non-pathogenic; fermenting and various unclassified micro-organisms.
2. Pathogenic; tuberculosis, typhoid, cholera, scarlet fever, diphtheria, and suppurative diseases have all been spread by the agency of milk.
1. The Fermentation Bacteria
At the most we can make a merely provisional classification of these processes. Many of them are intimately related. Of others, again, our knowledge is at present very limited. It may be advisable, before proceeding, to consider shortly what are the constituents of milk upon which living ferments of various kinds exert their action. A tabulation of the chief constituents would be as follows:
| Ordinary fresh milk = 100 per cent. |  | (1) Water | 87.5 | per | cent. |
| (2) Milk-sugar | 4.9 | " | " | ||
| (3) Fat | 3.6 | " | " | ||
| (4) Proteids (casein, etc.) | 3.3 | " | " | ||
| (5) Mineral matter | 0.7 | " | " | ||
| —— 100.0 | |||||