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.
