Quality yogurt is the base for frozen yogurt.
Yogurt
Frozen yogurt starts with the yogurt. Frozen yogurt makers first pasteurize cow's milk by heating it in a double boiler or water jacket to about 185 degrees F. These heating methods will heat the milk evenly without burning it. Pasteurizing the milk will kill bacteria that might make consumers sick. Then they allow it to cool to 110 degrees F in a water bath. They ferment the milk by adding bacteria, such as streptococcus thermophilus or lactobacillus bulgaricus, to it and maintain the milk's temperature at 110 degrees F. The bacteria will eat the sugar called lactose that is present in the milk, which will curdle it and produce lactic acid. This lactic acid will make the milk thick and tangy. The longer the milk is allowed to ferment, the thicker and tangier the yogurt will become. For frozen yogurt, milk is allowed to ferment for about six to eight hours, and then it is chilled.
Add-ins
Once the yogurt is chilled, frozen yogurt makers can strain off any liquid that may have separated out. This will prevent the frozen yogurt from having too many ice crystals once it is frozen. They will be able to mix in any add-ins at this point, including sweeteners such as sugar or honey, and flavoring such as vanilla, fruit extracts or purees. Some frozen yogurt makers might add eggs or egg yolks for texture and to help the mixture freeze more quickly. They might also add gelatin as a stabilizer to help ensure that the frozen yogurt keeps a smooth consistency, even if it's subjected to temperature fluctuations that might otherwise produce ice crystals. Stabilizers will also help slow melting.
Freezing
The yogurt mixture is frozen in a special machine that also churns the mix as it freezes. Churning the frozen yogurt will help bring up large ice crystals that might form as the water in the ingredients begin to freeze. It also incorporates air into the mix, which will add volume and lighten the frozen yogurt's texture. Depending on the type of machine, freezing the mixture can take anywhere from 3 minutes to 30. The yogurt makers freeze the mixture just until it's still soft enough to be distributed into containers. Between half and two-thirds of the water will freeze during this time. Then it is quickly frozen solid to ensure that the water still unfrozen does not collect and form large ice crystals. Once the frozen yogurt is completely frozen, it's ready for distribution and consumption.
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