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Answer on Microbiology Question for Raquel Smedley

Question #10538
how do variations in the yogurt making process temperature affect the process, and the final product
Expert's answer
During yogurt fermentation when pasteurization and homogenization are complete, the milk is cooled to between 109.4-114.8° F (43-46° C) and the fermentation culture is added in a concentration of about 2%. It is held at this temperature for about three to four hours while the incubation process takes place. During this time, the bacteria metabolizes certain compounds in the milk producing the characteristic yogurt flavor. An important byproduct of this process is lactic acid.

Heat Treatment

• typically 85 °C for 30 min.


(1) to destroy all pathogenic bacteria,

(2) to inactivate all the enzymes that may be present in milk, including lipase,

(3) to destroy most of the spoilage-causing bacteria, including thermodurics,

(4) to denature whey proteins, β-lactoglobulin (90%) and α-lactalbumin ( 60 %).

• Increased hydrophilic properties of the casein leads reduced syneresis, forms stable coagulum.

• The hydration of the protein decreases& when milk is heated over 85 C

• the heated milk forms a smooth gel-like structure at pH=4.6 (isoelectric point of casein , casein proteins carry no electrical charge). Depends on at what temperature milk was heated yogurt will have different structure and properties. Most of all type of yogurt will be determined by the appropriate combination of heating and cooling of milk (set, stirred, drinken yogurts).

& Without heat treatment, the shelf-life of yogurt is 4–5 weeks at 4 °C.

• Heating destroy most of the starter bacteria and yeast&molds: the shelf-life of the product can be extended to 8 weeks

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