Yogurt Production
Bacterial Cultures
The main (starter) cultures in yogurt are Lactobacillus bulgaricus andStreptococcus thermophilus. The function of the starter cultures is to ferment lactose (milk sugar) to produce lactic acid. The increase in lactic acid decreases pH and causes the milk to clot, or form the soft gel that is characteristic of yogurt. The fermentation of lactose also produces the flavor compounds that are characteristic of yogurt. Lactobacillus bulgaricus andStreptococcus thermophilus are the only 2 cultures required by law (CFR) to be present in yogurt.
General Manufacturing Procedure
General Yogurt Processing Steps
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 85°c for 30 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
While the milk is being heat treated, it is also homogenized. Homogenization is a process in which the fat globules in milk are broken up into smaller, more consistently dispersed particles. This produces a much smoother and creamier end product. In this machine, the milk is forced through small openings at a high pressure and fat globules are broken up due to shearing forces.
Fermentation
When homogenization is complete, the milk is cooled to between 43-46° C and the starter cultures are 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.the incubation process is done either in a large tank of several hundred gallons andThe 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. For swiss style yogurt the fruit is blended with the fermented, cooled yogurt prior to packaging.
9. Package
The yogurt is pumped from the fermentation vat and packaged as desired.