This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
Kefir contains three varieties of ferments which produce complex fermentation processes resulting in the formation of alcohol, lactic acid, modified albumins, and peptones. The casein of milk is in great part digested by them, and the portion which remains is precipitated in flocculi instead of heavy curds. The milk sugar is almost completely converted into alcohol and carbonic-acid gas, which bubbles to the surface. Kefir has a sour taste, due to the organic acids which it contains.