Yogurt sell in Marzhe was slimy and with curds formed in it and it’s containers were bloated with noticeably swelling at the lid. Yogurt spoilage is usually causes by high pH value (intrinsic factor) and temperature (extrinsic factor).
Yogurt, one of the fermented milk products, is made by inoculating certain bacteria (starter culture), usually Streptococcus thermophilus and Lactobacillus bulgaricus, into milk. After the milk is incubated at approximately 450C for 2 to 3 hours or until firm; the milk is coagulated by bacteria-produced lactic acid.
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Yogurt is of low pH value and high lactic acid concentration and are a highly selective environment favoring the growth of yeasts as spoilage micro-organisms. Yeasts are the most important spoilage organisms and are most commonly associated with fermentation leading to gas production. Such spoilage is readily recognized by " doming" of the foil lids and even burst containers. Oxidative yeasts are also important spoilage in yogurt. Growth is limited by availability of oxygen and may restricted to the air/yogurt interface. Moist, flat colonies, or film of growth (slime) may develop. Thin walled polystyrene containers however permit sufficient air to enter the pack to support the growth of oxidative yeasts in the bulk of yogurt. In such circumstances oxidative yeasts predominate near the pack walls and fermentative yeasts in the center of the pack.
The titratable acidity of a good finished product is around 0.85-0.90% while the titratable acidity of Marzhe’s yogurt is 1.8-2,0% which is way too high. This is due to over production of lactic acid. This causes the yogurt to curd and makes it sour.
Another extrinsic factor that causes food spoilage is the temperature of the cold room that store yogurt. The set temperature of the cold room is around 4+20C but the temperature shoot up as high as 80C at around 11th – 14th May, 25th – 27th May and 14th – 18th June. This may cause many psychotropic strains among the yeast and moulds to activate. Although psychrotrophs rarely survive in commercial operations, their presence in a pasteurized product almost certainly represents post-pasteurization contamination. This can occur during cooling after sealing or contaminated storage tank during incubation.
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