Kosher yogurt differs from other yogurt only in that the process of making the yogurt is in accordance with Jewish dietary laws. These laws are a set of dietary restrictions followed by many people of the Jewish faith. Though the main ingredient in yogurt, milk, is kosher so long as it comes from a kosher animal, such as a cow, goat or sheep, the cultures used to turn this milk into yogurt and the additives used to give it its consistency are often derived from non-kosher sources. Kosher yogurt must also be processed on equipment that is never used for non-kosher foods.
In order for yogurt to receive kosher certification, a rabbi must inspect the ingredients and processes used. The rabbi does not make the yogurt kosher but only confirms that no non-kosher ingredients have been used and that the equipment used has not been used to process non-kosher food. There are a few different organizations that offer kosher certification, and each may follow slightly different procedures when evaluating the kosher status of a food such as yogurt.
Yogurt contains a number of different ingredients, all of which must be reviewed for compliance with kosher rules before it can be considered kosher yogurt. The live culture used to turn milk into yogurt must be grown using nutrients taken from kosher sources in order for the resulting yogurt to be considered kosher. Cream used to add to the yogurt's fat content must be separated from milk using a method other than whey cream processing, and non-fat dried milk added to lower the yogurt's fat content cannot be dried in the same facility as non-kosher meats or cheeses. Any thickeners or stabilizers used in the yogurt, such as gelatin, must be gathered from kosher animals or plant sources.
Additionally, kosher yogurt needs to be manufactured on equipment that is only used for kosher foods during all parts of the process. The bacteria grown to add live cultures to the milk must be grown in containers that are only used for kosher-fed bacteria and the yogurt must be cultured in vats that are only used for kosher yogurt. If there is fruit added to the yogurt, it must also be processed on equipment that is not used for non-kosher foods. The difficulty of following kosher practices throughout the process of making yogurt often makes this yogurt difficult to find it in stores, though there are a few brands that follow kosher practices.