PRACTICAL PAPERS—BUTTER FACTORIES. 
251 
but in tlie comparative absence of air, as when growing in milk, it decom¬ 
poses the sugar, and a lactic acid is the chief result of this metamorphosis. 
If milk, which by short exposure to the air has had the microscopic germs 
of the ferment-plant sown in it, be then excluded from the air as much as 
possible, the plant, in its growth, is necessitated to decompose the milk 
sugar, and hence the milk rapidly sours. On the other hand, exposure to 
the air supplies the ferment-plant with free oxygen, and the milk remains 
sweet for a longer period. Such is the theory of the change. That low 
temperature should prevent souring, is in analogy with all we know of 
chemical changes. 
“ Stirring the cream does not promote souring, but rathers hinders it by 
increasing access of air ; it may be advantageous in making the souring 
uniform. 
Fig. 37. Millan & Son’s Milk Pails. 
“ When to Churn .—Although the milk should never be permitted to sour 
before skimming, the cream, on the other hand, should become slightly 
acid before churning. To accomplish this end most expeditiously, the tem¬ 
perature may be raised slightly ; keeping it in the warm milk room will 
usually suffice ; but previous to putting it in the churn it should be again 
cooled, according to the quality of the cream. If it be rich summer cream , 
55° is most favorable ; if it be the product of short, poor feed, or of straw, or 
of roots, or if the cows have been exposed to cold storms, and under the 
necessity of exhausting the oleine of the butter, the temperature must be 
raised to correspond ; and it may even require so much heating that little 
of the butter flavor will remain. For all ordiuary cases the range may be 
set down as from 55° to 65°.” 
MILK PAILS. 
The importance of using milk pails that will not absorb 
