Cheese is created when the casein (a protein) separates from the other parts of the milk.  The two basic ways to make this happen are with rennet or with acid.  The rennet or acid causes the casein and some other items to separate from the rest of the milk (a.k.a. the whey), and the newly formed item – known as curd – is further processed to become what we know as cheese.  Although both methods remove casein from milk, they don’t work in the same manner,¹ and the curds produced by the two methods are quite different from one another.   As a rule, milk curdled with rennet creates “hard” cheeses such as mozzarella, Muenster, pasteurized process (i.e. American cheese) and Swiss cheese, and acid-set cheeses are typically “soft” cheeses such as cottage cheese and cream cheese. 

Traditionally, rennet was derived from the calf stomachs, and Chazal forbade a non-Jew’s cheese as gevinas akum because of a concern that the cheese might be set with rennet from an animal that didn’t have shechitah (i.e. a neveilah).²  It’s clear that this prohibition includes all of the rennet-set cheeses, but there is much discussion in the Poskim as to whether it includes acid-set cheeses as well. 

Quite a number of Poskim hold that the issur of gevinas akum includes all forms of cheese including acid-set cheeses.³  However, the accepted custom in the United States is to follow the lenient opinion which argues that acid-set cheeses were never included in the gezairah because those cheeses curdle without rennet (and sometimes without the addition of any coagulant at all – see below) such that there’s no reason to be concerned that neveilah rennet will be used.⁴

Acid-set cheese that uses rennet

Modern methods of cheese production have raised a further question within the lenient opinion.  Nowadays, it is quite common for manufacturers of acid-set cheese to add a bit of rennet into the milk to speed up the cheese-making process and to produce a somewhat firmer end product.  Does that change the cheese’s status to that of rennet-set cheese?  Iggeros Moshe [who explains the lenient opinion without wholeheartedly accepting it] rejects this for two reasons:

1. So little rennet is used that it has no affect on the finished product other than to speed up a process that would happen naturally.
2. Even if the rennet plays some role in the cheese’s coagulation, it is a supporting role which qualifies as זה וזה גורם and can be discounted.

Discussions with professional cheese makers supports this distinction, as they tell us that rennet-set cheeses typically use 70-90 ml. of rennet per 1,000 pounds of milk, while acid-set cheeses will use about 1-2 ml. of rennet for the same quantity of milk.  [More details on this are presented below].  These experts further say that the 1-2 ml. of rennet used couldn’t possibly cause true coagulation of that much milk and would just create a bit of gelling.  This surely qualifies for Iggeros Moshe’s second reason and possibly even for the first.

Based on this line of reasoning, non-Jewish companies regularly produce cottage cheese and other acid-set cheeses and are certified as kosher without any form of gevinas Yisroel, even though some rennet is used in the process.

Different types of acid-set cheese

In the context of understanding these issues it is worthwhile to subdivide acid-set cheeses based on how they are acidified and whether there is any separation of curd, as follows:

• Milk which is acidified through the addition of vinegar, lemon juice, or some other acid, is most similar to rennet-set cheese in that “something” is added to the milk which causes it to change into cheese.  As such, the strict opinion cited above has their strongest argument in this case.
• A somewhat similar case is milk which is acidified via the introduction of lactic acid producing cultures/microorganisms.  In this case, the culture doesn’t directly affect the cheese but it creates a byproduct which causes the cheese to form (much like rennet does).
• However, some cheeses form through a mere souring of the milk (as a result of sitting unrefrigerated for some time) without anything added to it.  This case is least similar to rennet-set cheese. 
• In many forms of acid-set cheese (e.g. farmers and cottage cheese) the finished cheese is curd, much like rennet-set cheeses are.  However, other cheeses such as sour cream include the entire milk rather than just the casein/curd.  In this way, sour cream is so different than rennet-set cheese, that it’s not clear if any Poskim would hold that it requires gevinas Yisroel. 

From this perspective, the word “yogurt” includes two distinct types of cheese. “Strained yogurts” are similar to the farmers and cottage cheese discussed above in that the finished product is just curd, with the whey being strained/filtered out.  Other yogurts include all of the elements present in the milk (in a congealed form) with no separation of curd and whey to speak of.⁵  As noted, it’s not clear if any Poskim would rule that the latter form of yogurt requires gevinas Yisroel, and the Acharonim who ruled that “yogurt” must be gevinas Yisroel were likely discussing strained yogurts.

Skyr

The above issues are somewhat more complicated when dealing with Skyr which contains the relatively small amount of rennet associated with acid set cheeses, but cannot possibly take on its true identity without that rennet. 

For hundreds of years, the people of Iceland have consumed a low-fat, high-protein, yogurt-like food known as Skyr (pronounced “skeer”), and in recent years they have begun selling this item in the United States.  Without rennet, Skyr is a watery yogurt beverage, but it is so commonly made with rennet to thicken it, that an expert who once saw a recipe that appeared to not contain rennet was sure that it was a mistake.

Skyr contains too little rennet to independently coagulate the milk and therefore appears to qualify for the lenient logic proposed by Iggeros Moshe and presented in the text above.  On the other hand, the underlying assumption of the lenient opinion is that the prohibition of gevinas akum doesn’t apply to cheeses which don’t require the addition of rennet; if so, since rennet is required to give Skyr its authentic form one could argue that it should require gevinas Yisroel and the secondary issue of what role the rennet plays should be unimportant.⁶  Rav Schwartz was inclined to be lenient on this issue as in essence Skyr is an acid-set cheese, and the rennet merely plays a minor role in the cheese’s final form.

See the footnote regarding whether one can infer a lenient position on Skyr from the common custom to not require gevinas Yisroel on bakers cheese.⁷

Rennet use in different cheeses

In the preceding pages we’ve seen that the common custom is that acid-set cheeses do not require gevinas Yisroel even though they may contain a minimal amount of rennet.  Which cheeses qualify for this leniency?  How can one tell whether the amount of rennet used is “minimal” or not?  The following chart shows that the amount of rennet used in rennet-set cheeses is so much greater than acid-set cheeses, that by merely checking the cheese’s recipe one can easily know whether it does or doesn’t require gevinas Yisroel. 

The chart is based on information given to the cRc by David P. Brown, Senior Extension Associate at the Department of Food Science of Cornell University in Ithaca, New York, in November 2007.  He cautioned that although the numbers given appear to be very exact, in fact the amount used varies up and down based on the production procedure used in the plant.  Nonetheless, the difference in the amount of rennet used in rennet set cheeses as compared to acid set cheese is so great, that minor fluctuations from plant to plant are insignificant.