Yogurt is among the trickiest of dairy products; it appears to be so straightforward, yet its production and the halachic questions it generates rise to the greatest heights of complexity.

Let’s take a look at basic production and then address some little-known halachic considerations.  

Ingredients and Production    

In order to produce yogurt, one must of course start with milk. But this is not so simple. In order to achieve the right balance of fat and solids and the desired product texture, non-fat dry milk and whey protein may be added to the milk, creating a milk blend, whose ingredients now become a kashrus concern.

Stabilizers are often added to the milk blend as well. Stabilizers can come from gums, starches, and pectin - and they also very often come from gelatin. Thus do we have another kashrus concern.   

The milk blend is then pasteurized (to 185 F degrees and held there for 30 minutes, or to 200 F degrees and held there for 10 minutes!), homogenized and cooled, after which it is inoculated with lactobacillus bulgaricus and streptococcus thermophiles cultures. These cultures cause the milk’s lactose (sugar) to ferment into lactic acid, which acts on the milk to lower its pH, thereby causing the milk to clot into a yogurt gel and attain a distinct flavor.     

Some yogurt also contains probiotic cultures, which can boost the body’s immune system and contribute to gastrointestinal health as well as to the body’s ability to digest lactose. But these cultures are not necessary in order to create yogurt.

Although the cultures used for yogurt production are inherently kosher, they can often be manufactured in non-kosher environments, and their source plants thus require tight kashrus controls and solid certification.

After inoculation with cultures, the milk blend is held for several hours at 108 F degrees until the pH reaches 4.5, during which time fermentation, gelling and development of flavor occur.

The product, which can now justly be called yogurt, is then cooled to 46-47 F degrees, halting the fermentation process.      

Afterwards, fruit base is commonly added. Fruit base often contains carmine, a non-kosher deep red color derived from insects. Other highly sensitive ingredients may also be used in fruit base production. For these reasons, fruit base manufacturers require reliable kosher certification.

Greek yogurt has been a boon both for the dairy industry as well as for kashrus. The reason for the latter is that instead of being thickened with stabilizers, Greek yogurt achieves its thick consistency by being strained (usually via centrifuge) to remove moisture; stabilizers are therefore not typically used, and there is thus one fewer kashrus concern.

There are other assorted permutations of yogurt (set-style yogurt, Swiss style/stirred yogurt, etc.); the differences between these products reflect variations in processing but are not material for kashrus purposes.

Is Yogurt a (Halachic) Cheese?

The OU, and most national kosher agencies, follow the p’sak of Rav Yosef Eliyohu Henkin zt”l that only cheese which is enzymatically coagulated, via rennet, is subject to the special halachic stringencies of gevinah and is hence only kosher when made as gevinas Yisroel (i.e. full-time onsite hashgochoh). Thus, cheddar, mozzarella, feta, parmesan and all other rennet-set cheeses require hashgochoh temidis for production. However, acid-set cheeses, such as cottage cheese and cream cheese, in which rennet is not present or is not the main coagulant, and the product is instead formed through acidification of milk, are not subject to the special halachic stringencies of gevinah, according to this approach. Rav Henkin maintained that acid-set cheeses are kosher so long as their ingredients and processing equipment are kosher, and lack of hashgocho temidis does not render them non-kosher/gevinas akum. (As we noted previously, even those who consume cholov stam may not consume gevinas akum; the heter of cholov stam does not permit cheese made without a mashgiach temidi.)

Nonetheless, many poskim take the stricter approach and rule that all cheeses, including acid-set cheeses, become non-kosher/gevinas akum absent hashgocho temidis at manufacture. This is the position of the Chochmas Odom and Aruch Ha-Shulchan, and Igros Moshe hesitated to be lenient on this matter.  

What about yogurt? According to the stricter approach, must yogurt be made as gevinas Yisroel?

Although the question may sound very strange, it is quite valid, due to the fact that cheese curd (of both rennet-set and acid-set cheese) is a matrix of milk’s casein protein – and the same is true for yogurt curd. Since the body, the curd, of both cheese and yogurt are formed from casein matrices, should yogurt not also be encumbered by the requirements of gevinah, if one adopts the position that such requirements pertain to acid-set cheeses?

The truth is that this is a machlokes poskim; the various mekoros are cited by Rabbi Zushe Blech in footnote 28 in The Dairy Industry: A Halachic Primer (Daf Ha-Kashrus 5:10 - Iyar 5757/May 1997) and by Rabbi Chanoch Bleier on pp. 260-261 in Cholov Yisroel K’hilchoso. (See also Chelkas Binyomin YD 115:2, in Biurim.) Although there is an opinion that the requirements of gevinah apply only to those foods that are called “cheese” (and hence is yogurt exempt), most do not hold this way; rather, they base their opinion on whether the product’s structural quality is halachically that of cheese.

This is where the uniqueness of yogurt makes its mark. Unlike cheese, in which the goal in manufacturing is to expel excess moisture (whey) from curd – as the product will otherwise be liquidy - the goal in yogurt manufacture is to retain moisture, so that a somewhat fluid consistency is achieved. The technical term used for the process of whey expulsion is syneresis, which means the separating out of liquid when a gel is formed. Cheese production needs syneresis, but yogurt production must avoid it.

How is syneresis prevented in yogurt production? The key is in the very unique pasteurization that milk must undergo prior to being made into yogurt. Unlike standard pasteurization of milk for cheese production, which occurs at 161 F degrees for 15 seconds (this is called High Temperature/Short Time [HTST] Pasteurization, or at 145 F degrees for 30 minutes, which is called Batch Pasteurization), milk for yogurt production must be pasteurized at 185 F degrees for 30 minutes or at 200 F degrees for 10 minutes.

The function of this extreme pasteurization is to denature a certain whey protein found in milk, called beta-lactoglobulin, which causes this whey protein to attach itself to the exterior of casein particles (“micelles” in technical lingo), so that the casein micelles will be partially covered with these whey proteins and will have little room to cluster together tightly to form cheese curd when the milk acidifies. The result is that casein micelles in yogurt do not bond in a cluster, as they do in cheese production, but instead bond in broad chains, trapping whey inside. This is why yogurt does not as readily undergo syneresis, as its casein chains entrap large amounts of whey - unlike with cheese, whose casein is in a cluster formation. (Some syneresis is unavoidable, and that is why yogurt typically needs stabilizers, so as to assure textural consistency.) This distinction between the casein formations of cheese and yogurt is the reason that cheese has so much more protein than yogurt, as cheese is made of casein clusters, whereas yogurt is made of casein chains filled with liquid whey, the latter of which contain far less protein than casein.   

Hence, yogurt curd is indeed made of casein that bonds together, but the bonds are noticeably different than those of cheese. From this derives the halachic question regarding yogurt: does its unique casein structure qualify as gevinah? Yogurt has some characteristics of cheese, yet it lacks other characteristics of cheese; this impacts how yogurt is to be viewed through the lens of Halacha.     

Greek Yogurt and Cheese

And what about Greek yogurt, whose whey is mostly strained out? Some have argued that even if yogurt is not halachically deemed gevinah, Greek yogurt should nonetheless be considered acid-set gevinah, as its loss of whey renders it concentrated casein, just like cheese. This notion is questionable, as Greek yogurt’s curd is initially coagulated as regular yogurt, and it is only afterwards that the whey is strained out. It is the ma’aseh gibun/ha’amodoh (act of coagulation), when an acid or enzymatic reaction forms the product’s casein bonds, that determines its status as gevinah. Since the curd of Greek yogurt did not undergo a ma’aseh gibun/ha’amodoh that created gevinah, it is quite difficult to argue that the product attains gevinah status afterwards when it is strained into Greek yogurt.

Furthermore, production of milk protein concentrate (MPC), which is 80% casein and 20% whey protein separated out of milk, does not involve acid or enzymatic coagulation into curd structures and is instead performed through ultrafiltration. Just like MPC is not gevinah, as it lacks acid or enzymatic coagulation into curd structures, so too would it appear that Greek yogurt, which likewise is an agglomeration of milk protein whose greater density is not the result of enzymatic coagulation but rather of straining, should not be considered gevinah. As explained above, the casein in milk which undergoes the exceptional pasteurization needed for yogurt is blocked from clustering into gevinah matrices; when this milk is made into yogurt and then strained, it still cannot cluster into gevinah matrices, and thus is not a form of gevinah, and is halachically similar to MPC.

Milk’s incredibly complex structure and amazing functionality are among the great Nifa’os Ha-Borei, the wonders of the Creator in the natural universe.  

We can now return to our title and confidently affirm: yogurt is not so simple.