The ripening of cream is essentially a bacteriological and chemical phenomenon. The main process in the ripening is the lactic acid fermentation. Although it has not been proved that the lactic acid fermentation is responsible for all the changes in cream ripening, it is so closely associated with them that in practice ripening is governed by controlling this fermentation. This is done by controlling the temperature and using starters.
A starter is a material containing bacteria, used to inoculate a dairy product. It may be employed to inoculate milk for cheese-making or cream for butter-making. The bacteria are presumably of a desirable kind and are present in great numbers. Whole milk, skim milk, butter-milk, cream, whey, and sugar solutions are used for growing the bacteria. The germs are usually lactic acid bacteria, but some cultures are said to contain flavor producing bacteria.
Bouska, F. W.
"The use of starters in butter-making,"
Bulletin: Vol. 9
, Article 1.
Available at: https://lib.dr.iastate.edu/bulletin/vol9/iss103/1