Yogurt is a sour milk product. Its sour taste is attributed to the presence of lactic acid yielded by bacteria through fermentation. Bacteria that are commonly used in yoghurt production are Streptococcus thermophilus and Lactobacillus bulgaricus. These bacteria have the ability to break down sugars in the milk producing lactate or lactic acid as by-product.
Below is a step-by-step procedure in the production of yogurt. Find out from the procedure why yogurt has low fat content.
Note: Practice aseptic technique. Wash hands before proceeding and wear lab gown, head cap, latex gloves and if necessary, facial mask to prevent unwanted contamination of the pre-yogurt mixture.
Ingredients (good for one group)
1. 500 mL homogenized pasteurized milk
(to be bought and brought as 1L pack good for two groups)
to be provided:
2.15 grams skim milk powder (SMP)
3.50 grams refined sugar
4.40 ml starter culture bacteria from natural yoghurt prepared as follows :
3% skim milk powder in distilled water, yoghurt or yakult, incubated at 43o to 46o for 3 to 4 hours till clotted
1.In a clean container, carefully transfer the milk and carefully dissolve the SMP and sugar. Cover with aluminum foil and heat the mixture at 90oC (water bath) for 10 minutes. 2.Cool down to 50oC in a bath of ice water.
3.Meanwhile shake the culture of the starter bacteria to free it from lumps and carefully add (inoculate) to the milk. Make sure that the temperature of the container is cool enough to touch. Mix to evenly distribute the starter culture. 5.Transfer the inoculated milk into the plastic container provided. Seal the lid with masking tape and label the container properly (group number, section and instructor). 6. Incubate the milk at 43oC to 46oC in an incubator overnight. At this point this will be taken cared of by the lab technicians and will be chilled at 4C thereafter.