1 Juny, 1899.] QUEENSLAND AGRICULTURAL JOURNAL. 27 
CHANGES IN MILK. 
CAUSES OF THE CHANGES. 
Tur following may give some little insight into the changes which take place 
in that important and distinctly unstable commodity, milk. Even the towns- 
man, who from one year’s end to another does not set eyes on the cow, knows 
that it is necessary to his comfort to have milk delivered at the door of his 
house twice daily, and at the same time has usually an idea, in common with the 
housewife, that the said milk will not “keep” for any length of time. 
In the ordinary course of things, this change. although apparently brought 
about by certain conditions of warmth and eration, or lack of the same, has as 
its root a cause which is dependent on these various conditions as to how its 
effects develop themselves. 
The most noticeable thing about the milk when it is turning “ bad” is that 
it becomes sour in smell and to the taste, and eventually turns into a mass of 
jelly-like clots floating in a thin, watery liquid. 
CAUSE OF THE CHANGE. 
The normal souring of milk is primarily caused by the presence of certain 
little living organisms termed by the scientist “bacteria.” Of these bacteria there 
are many different kinds, producing varying results; but the special group of 
bacterial organisms which, in the natural outcome of things, produce sour milk 
in this fashion are known as lactic acid bacteria, their method of procedure being 
to change the lactose, or natural sugar of milk, into lactic, or milk, acid, and the 
presence of this acid so acts upon the casein, or coagulable portion of the milk, 
which otherwise is in solution, as to cause it to clot or curdle. If milk in such 
a condition is left to itself for any length of time, other species of bacteria 
acting upon it, each in their own way, cause it eventually to putrefy and 
decompose. 
When some of these other kinds of bacteria obtain access to the milk in 
the early stages, curdling, if it takes place, may be due to their action ; but they 
usually work, not, like the lactic acid bacterium, by manufacturing acid from the 
milk sugar, but by producing a rennet-like substance, which by its fermenting 
nature causes the casein to coagulate. 
An instance of this nature is seen in butyric fermentation. Milk which 
has been boiled will sometimes coagulate under the action of this ferment, and 
when the milk changes in this way it becomes not acid, but slightly alkaline, in 
reaction. The changes are found to be similar to those effected by rennet, but a 
bitter taste is developed, and at last the coagulum clears, and butyric acid is 
formed. . 
DISEASES IN MILK. 
Certain bacteria more approaching in nature the last-mentioned organism, 
and which are foreign to the milk, cause what are known as the various diseases 
of milk. A short summary of these is here given. 
BITTER MILK, 
Milk which possesses a bitter flavour may have derived it from the food of 
the cow, or bitterness may arise from the cow giving the milk being in bad 
health; but in some cases it cannot be traced to either of these causes, and is 
shown to be due to the presence of organisms which attack and change the 
nature of the casein, producing certain substances known as peptones, which 
probably impart the bitter taste. 
Bacteria of this group are very tenacious of life, and so is explained the fact 
that boiled milk is especially liable to be attacked by them, the process of boiling 
having killed the lactic acid bacteria, which have considerably less power. of 
resisting the heat. In fact, all bacteria, we may take it, are destroyed under 
these circumstances ; but some of them, among which are those at present under 
consideration, can produce seeds, or, more correctly speaking, spores, which 
possess in a remarkable degree a heat-resisting strength, so that, passing 
