Gas production (Strong fermentation of milk):.Similarly, Clostridium produce butyric acid.For example: coliform produce acetic acid, formic acid, ethanol, CO 2, H 2 etc.Bacteria other than lactic acid bacteria produce acid specially if conditions are unfavorable for lactic acid bacteria.Pasteurization of milk kills more active acid forming bacteria but permit survival of thermoduric lactic acid bacteria such as Enterococcus, Streptococcus thermophilus, Lactobacillus, etc.Little souring occurs in milk held at refrigeration temperature.At higher temperature, 37-50 oC, Streptococcus thermophilus and Streptococcus faecalis may produce 1% acid and it may be followed by Lactobacillus which produces more acid.At temperature of 10-37 oC, Streptococcus lactis is most likely to cause souring with possible growth of Coliform, Enterococci, Lactobacillus and Micrococcus.Many lactic acid bacteria, coliform and other bacteria ferment sugar of milk and produce acid.Evidence of souring of milk are sour flavor and then coagulation of milk to form solid like curd.Therefore, different types of microorganisms grow in it and cause spoilage.Milk is an excellent culture media for growth of many microorganisms. Microbial Spoilage of milk and mik products :
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