110 Milk and Its Products 
but almost any others, will readily live and grow 
should they gain access to milk: Nearly all forms 
of bacteria are sensitive to conditions of tempera- 
ture. The range of temperature in which they 
thrive the best and grow most rapidly is rather 
narrow, though there is a considerable range above 
or below, in which they will still grow and develop. 
The temperature at which any given germ will grow 
most rapidly is called the optimum temperature, and 
the optimum temperature varies widely with the 
various classes of organisms, though by far the 
larger number of bacteria find their optimum point 
between 75° to 100° F., and a higher temperature 
not only stops their growth, but if sufficiently high 
kills them outright. A temperature of 185° F. kills a 
large number of germs. Very few are able to live 
above a temperature of 180°, and none can with- 
stand the temperature of boiling water, 212°, for 
more than a few minutes. If heat is accompanied 
by moisture it is much more effective, so that heat 
applied in the form of live steam is the best means 
of destroying the life of these germs. Under the’ 
influence of cold the germs become inactive, and 
some kinds are killed by a sufficient degree of cold, 
but very many kinds are able to withstand any 
degree of cold possible to be produced for long pe- 
riods of time. 
Under certain conditions bacteria are able to as- 
sume an inactive condition, or spore form. In do- 
ing this, the protoplasm shrinks into a hard, glisten- 
ing mass, and contracts toward one end of the cell, 
