Question

3. How does the consistency of the milk change during the production of yogurt? What facilitates this change?

Answer 1

This is because of the fermentation process of a bacteria in milk that turns the sugar in the milk "lactose" in Lactic Acid.

As soon as the lactic acid increases, the consistency thickens and sets as it turns to yogurt.

Yogurt is a fermented milk product that contains the characteristic bacterial cultures Lactobacillus bulgaricus and Streptococcus thermophilus. All yogurt must contain at least 8.25% solids not fat. Full fat yogurt must contain not less than 3.25% milk fat, lowfat yogurt not more than 2% milk fat, and nonfat yogurt less than 0.5% milk.

General Manufacturing Procedure

The following flow chart and discussion provide a general outline of the steps required for making yogurt.

General Yogurt Processing Steps

Adjust Milk Composition & Blend Ingredients

Pasteurize Milk

Homogenize

Cool Milk

Inoculate with Starter Cultures

Hold

Cool

Add Flavors & Fruit

Package

1. Adjust Milk Composition & Blend Ingredients

Milk composition may be adjusted to achieve the desired fat and solids content. Often dry milk is added to increase the amount of whey protein to provide a desirable texture. Ingredients such as stabilizers are added at this time.

2. Pasteurize Milk

The milk mixture is pasteurized at 185°F (85°C) for 30 minutes or at 203°F (95°C) for 10 minutes. A high heat treatment is used to denature the whey (serum) proteins. This allows the proteins to form a more stable gel, which prevents separation of the water during storage. The high heat treatment also further reduces the number of spoilage organisms in the milk to provide a better environment for the starter cultures to grow. Yogurt is pasteurized before the starter cultures are added to ensure that the cultures remain active in the yogurt after fermentation to act as probiotics; if the yogurt is pasteurized after fermentation the cultures will be inactivated.

3. Homogenize

The blend is homogenized (2000 to 2500 psi) to mix all ingredients thoroughly and improve yogurt consistency.

4. Cool Milk

The milk is cooled to 108°F (42°C) to bring the yogurt to the ideal growth temperature for the starter culture.

5. Inoculate with Starter Cultures

The starter cultures are mixed into the cooled milk.

6. Hold

The milk is held at 108°F (42°C) until a pH 4.5 is reached. This allows the fermentation to progress to form a soft gel and the characteristic flavor of yogurt. This process can take several hours.

7. Cool

The yogurt is cooled to 7°C to stop the fermentation process.

8. Add Fruit & Flavors

Fruit and flavors are added at different steps depending on the type of yogurt. For set style yogurt the fruit is added in the bottom of the cup and then the inoculated yogurt is poured on top and the yogurt is fermented in the cup.

9. Package

The yogurt is pumped from the fermentation vat and packaged as desired.

Answer 2

The transformation of milk into yogurt is due to a fermentation process, mediated by two types of bacteria: Lactobacillus bulgaricus and Streptococcus thermophilus. Ideally yogurts should contain approximately equal amounts of these bacteria.

These bacterial microorganisms use lactose, milk sugar, as a source of carbon and energy through a process of lactic fermentation. This transformation results in the production of lactic acid, responsible for the acidic character of this food. The increase in lactic acid converts milk consistency into yogurt consistency. This process occurs when the temperature of the milk is around 40 ° C.  

This process is facilitated in situations of high temperature and absence of oxygen. Because the responsible bacteria are anaerobic beings.