Composition of milk ppt8/1/2023 Acid curd: hard curd with clear supernatant (whey).Multiple changes can occur over the observation period. Observe daily for seven days for alkaline reaction (litmus turns blue), indicator reduction, acid clot, acid reaction (litmus turns pink), rennet clot, and peptonization.Inoculate with 4 drops of a 24-hour broth culture.Powdered skim milk (100 g), litmus (0.5 g), sodium sulphite (0.5 g), per 1000 mL, pH 6.8. Additionally, some organisms reduce litmus, in which case the medium becomes colorless in the bottom of the tube. Some bacteria hydrolyze casein, causing the milk to become straw colored and resemble turbid serum. With some organisms, the curd shrinks and whey is formed at the surface. If sufficient acid is produced, casein in the milk is coagulated, solidifying the milk. A variety of different biochemical changes result.įermentation of lactose is demonstrated when the litmus turns pink as a result of acid production. Litmus milk then forms an excellent differential medium in which microorganisms can metabolize milk substrates depending on their enzymatic complement. To distinguish among the metabolic changes produced in milk, a pH indicator, the oxidationreduction indicator litmus, is incorporated into the medium. The major milk substrates capable of transformation are the milk sugar lactose and the milk proteins casein, lactalbumin, and lactoglobulin.
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