y one is familiar with the souring of milk, but perhaps it is not so
generally known that there are great differences in the results obtained
in accordance with the conditions under which the souring takes place.
The skilled butter-maker, by keeping the milk in a cool and cleanly
dairy, obtains a sour milk of a characteristic and agreeable aroma and
taste, which beneficially affect the flavour of the butter produced. On
the other hand, if milk is kept in hot and dirty surroundings, the
development of acidity is accompanied by different bad tastes and
odours, and it becomes unfit for use as a food. In the first case, the
conditions are favourable to the maximum production of the lactic acid
bacteria, and these occupy the field, and largely prevent the
development of the other bacteria which are present--the survival of the
fittest in the struggle for existence. In the second case, the impure
surroundings swarm with the germs of many kinds of putrefactive
bacteria, and the high temperature assists these to gain the upper hand.
Again, the survival of the fittest, in the particular conditions. Even
in cool and cleanly surroundings injurious taints may develop,
especially if the milk has previously been subjected to a journey by
road or rail, as is the case in the modern creamery system, where the
farmers deliver their milk to a central creamery, where it is made into
butter. In such establishments it is the regular practice to kill the
germs, lactic and others, existing in the milk, by heating it to a high
temperature. This process is called pasteurising, after the great French
chemist and bacteriologist who invented it. Pure lactic cultures are
added to the pasteurised milk, and the souring process is under exact
control, with the result that butter of uniform flavour and quality is
produced. The same method is made use of in making the special sour milk
described in this book, with, of course, modifications in the apparatus
employed, to suit the smaller scale in which the manufacture is
conducted.

The ash is the mineral matter which is left when milk, previously dried,
is burnt in a crucible. It is a complex mixture, and, as we have seen,
it amounts to about 0.7 per cent. of the milk. The process of burning
destroys all the organic matter, and, at the same time, alters somewhat
the state of combination of the inorganic or mineral elements. Attempts
have been made from the analysis of the ash to reconstitute the
compositi