The main lactic acid formers are homofermentative lactic acid bacteria such as Streptecoccus thermophilus, Lactococcus lactis, Lactococcus lactis subspp cremoris. Lactic acid vary in rate of acid production. The rate is influenced by temperature, pH, antibiotics, bacteriophage, stimulants, inhibitory compound, milk composition, available nutrients, condition of the culture, strain compatibility and strain dominance. The term starter culture refers to a culture of lactic acid bacteria in food which is used to induce a lactic acid fermentationto produce fermented products.the particular lactic acid bacteria required in any given starter culture depend on the purpose of use. In cheese making an active production of lactic acid is an essential requrement and cultures may consist of Lactococcus lactis, Lactobacillus helveticus, Lactobacillus bulgaricus, of combination of these with or without aroma forming bacteri. Microbial spoilage in cheese is generally limited because of the combined effect of acid, salt and low moisture. Fresh cheese may be spoiled by gram (-) psychrotrophic bacteria (Flavobacterium, Psedomonas or Alcaligenes), coliforms, yeast and molds that enter as post- pasteurization contaminants. The most common pathogens in cheese are Staphylococcus aureus, Listeria monocytogenes, Escherichia coli, Salmonella, Brucella.

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