The laws of kashrut dictate that the products of non-kosher animals are not kosher, and vice versa. (Which animals are kosher?)
The eggs (or other products) of non-kosher birds or fish are not kosher. Caviar, therefore, must come from a kosher fish and this requires reliable supervision. Commercial liquid eggs also require supervision.
Eggs of kosher fowl that contain a blood spot must be discarded, and therefore eggs should be checked before use.
Rabbinic law requires that there be supervision during the milking process to ensure that a kosher animal is the source of the milk.
Following the opinion of many rabbinic authorities, the policy of the OU is that in the United States, the Department of Agriculture’s regulations and controls are sufficiently stringent to ensure that only cow’s milk is sold commercially. These government requirements fulfill the rabbinical requirement for supervision.
However, some individuals are more stringent and only consume milk that was produced with full-time supervision. This is known as Chalav Yisrael.
All cheeses require kosher certification, including hard cheeses (Swiss, cheddar, etc.) and soft cheeses (cottage, farmer, pot, and cream cheese). Hard cheese produced with kosher ingredients and a full-time supervisor is known as Gevinat Yisrael.
Rennet, an enzyme processed from the stomachs of unweaned calves, is often used in the production of hard cheese as a curdling and coagulating agent. Kosher hard cheese is produced with microbial rennet, which is derived from kosher sources. Because hard cheese is typically made with animal rennet, the Sages decreed that even when animal rennet is not used, a full-time supervisor must be present to guarantee the kosher integrity of the product. Click here to learn more about Gevinat Yisrael.
Dairy products that are processed further, such as butter, yogurt, or ice cream have more complex kashrut requirements.