Milk is the most basic source of all that is dairy. Milk is also pretty basic from a kashrus perspective; so long as it is not cholov beheimah temei’ah (milk from a non-kosher species) or cholov akum (milk which is unsupervised or of unverified origin), milk is always kosher.

Thus, most dairy materials made directly from milk would appear to be simple from a kashrus standpoint.

However, there is a second, perhaps more common source of dairy materials and by-products: cheese. The cheese-making process yields by-products which serve as essential and very common dairy materials and components.

Thus, there is a conceptual bi-directional chain of dairy materials manufacture: milk is developed or elevated to cheese, and cheese becomes a source of dairy components and by-products.

Although milk may appear to be a simple and innocuous item, its constitution is extremely complex. This complexity attests to the Creator’s purposeful planning, such that each facet of milk serves a specific goal and is clearly intended to be used for special and often sophisticated processes. Let’s take a look.

Milk is a wondrous compound of water, fat, proteins, sugar, minerals and bacteria. Milk’s water – referred to in technical dairy literature as its aqueous phase – is the venue where the balance of the above materials is suspended.

Fat floats through the aqueous phase in the form of globules; these fat globules miraculously do not stick together, as they are covered with a membrane which prevents their adhesion. The protein which is suspended in milk’s aqueous phase takes two general forms – casein and whey. Casein is the predominant protein in milk, and it is present as microscopic micelles – particles. Casein micelles are hydrophobic – they do not like to be in water; they therefore seek to separate from the aqueous phase, and – if possible – they would prefer to bunch together and not be intermingled amidst the milk. If they could, casein micelles would aggregate and form their own clusters, leaving behind the milk that suspends them. However, in order that this not occur and that milk serve as a stable and rich form of drinkable protein, Hashem did two things to the casein micelles: (1) He endowed them with a negative electric charge, so that they repel each other and therefore remain scattered in milk, and (2) He covered them with a hairy coating called kappa casein, which further prevents casein’s aggregation and separation from milk.

Raw milk is commonly separated into its water and fat segments. When put through a separator, raw milk yields cream – milk-fat, industrially termed sweet cream – and skim milk, which is merely milk without fat.

Skim milk is often condensed, meaning that its excess moisture is removed. Similarly, skim milk can be run through ultrafiltration equipment to remove its proteins, which are then concentrated and used to fortify high-protein foods.

Skim milk powder – referred to in the dairy industry as “NFDM” (non-fat dry milk) – is produced by drying skim milk into powder in spray dryers. Whole milk, too, can be converted into powder, but this is less common, as the fat content of whole milk makes it more difficult to dry and less desirous from a nutritional standpoint. (Whole milk powder is of special utility in the manufacture of upscale milk chocolate, which uses whole milk to attain a rich, creamy texture.)

Thus far, there would appear to be few kashrus issues. So long as the raw milk is kosher, one would assume its derivatives to be kosher as well. The truth is that this apparently naןve concept is largely true, as most derivatives of milk face no kashrus challenges. However, some important exceptions must be noted.

Some dairy plants, particularly in Europe, enrich milk power with fat and protein from various sources. This is a real concern, and milk power thus needs reliable kashrus verification.

Furthermore – and this applies in the United States as well – spray drying equipment used for milk powder production is sometimes shared with other varied materials. This author is familiar with spray-dry firms which use the same equipment to dry milk powder, non-kosher cheese, whey coated with lard, grain liquids, and other foods. Although this does not reflect the majority of cases, the kashrus of milk powder obviously needs tight monitoring. This is particularly true when it comes to whole milk powder, as it is often dried on smaller dryers called roller dryers; roller dryers often service a large spectrum of varied food manufacture.

Condensed skim milk is also of slight concern as well, for the equipment used to remove its excess liquid can potentially be used for non-kosher processing as well. This author has experienced such occurrences.

Cheese is made by separating milk’s casein micelles and forming them into a curd – an aggregate or structure of casein. There are two methods by which cheese curd is formed.

One is by neutralizing the negative charges of casein micelles, enabling the micelles to bunch together. The second method is to remove the micelles’ hairy kappa casein layer that disables the casein from bunching together, such that the gelatinous under-layer of casein micelles becomes exposed and the micelles literally stick together.

The above two processes represent the two basic methods of cheese-making (and the two principal categories of cheese). Acid-set cheese is produced by acidifying milk to a pH of 4.6, such that the casein’s negative charges are removed and they can aggregate and be separated from milk in bunches. This acidification is accomplished by milk’s bacteria converting the milk’s sugar – called lactose – into lactic acid; this occurs naturally when milk is warm. Alternatively, lactic acid cultures (bacteria) can be added to milk, where these cultures convert the milk’s lactose into lactic acid to acidify the milk; or – as is done with ricotta cheese – milk can be dosed with vinegar or other acids in order to generate direct acidification. (The conversion of lactose into lactic acid, precipitated by bacteria present in milk, resulting in cheese, is a wondrous testament to the Divine scheme. The entire system of cheese development was pre-ordained by milk naturally containing the elements of cheese production and these elements reacting together in beautiful orchestration.)

The second method of cheese-making involves the use of an enzyme to remove kappa casein and expose casein’s gelatinous under-layer, enabling the micelles to stick together and form a curd. To do this, rennet must be used. Rennet is a enzyme extract that occurs ‘naturally’ as a lining in the fourth stomach of calves. (The function of rennet – called rennin in its original state – is to convert milk that calves drink into protein-rich cheese while yet in their stomachs, quickly building muscles. Such nifla’os Ha-Borei – wonders of the Creator!) When rennet cleaves off a significant portion of kappa casein (which is rennet’s only use in nature), rennet-set cheese curd is formed by the casein micelles that bond together and separate from the aqueous phase.

Rennet cheese-making requires the use of bacterial cultures in order to acidify the milk to prepare it for rennet to act with greater ease. The cultures, which result in a souring of the milk, also help determine the final taste of the cheese.

Temperature likewise plays an important role in cheese production. Rennet works faster with heat application, and heat assists in casein separation as well.

Acid-set and rennet-set cheeses have major differences. Acid-set cheeses, such as cottage, quark, farmer’s and cream cheese, have a course curd, as their kappa casein is still intact, and the gelatinous layer of casein is not exposed. Thus, these cheeses lack a cohesive texture; they are merely bunches of casein micelles along with fat, sugar and some water taken from the milk. Acid-set cheeses therefore drip when lifted, as they are not one unit. (They are thus often referred to as ‘soft cheeses’, as they lack firmness or solidity.) Rennet-set cheeses, however, have a rubbery or smooth consistency, as their gelatinous casein surfaces stick together to form a lattice, where fat and water are entrapped. (These cheeses, such as cheddar, mozzarella, gouda, Swiss and edam are popularly referred to as ‘hard cheeses’, due to their cohesive and often firm structure.)

As was described above, both types of cheese production involve casein precipitating from milk. When the casein micelles coagulate, the liquid, casein-deficient liquid that remains is called whey. Since milk consists mostly of aqueous phase, the majority of milk in a given cheese production results in whey.

Whey proteins are an increasingly valued nutritional material. They are collected as follows:

Whey, like milk, is put through a separator, where its fat is removed. (This fat, called whey cream, is often mixed with sweet cream at dairy plants.) The residual whey liquid can then be filtered, where its protein is extracted and often then concentrated. This concentrate is called whey protein concentrate, or wpc. Lactose is also frequently filtered out of whey, as are the minerals found in milk.

One more point about whey: Some Italian-style cheeses are cooked and stretched in a bath-like vessel after they are formed; this cooking and stretching endows the curd with an elastic texture that enables it to melt smoothly and remain intact when heated. The water in which these cheeses are cooked (called cooker water) is commonly salvaged and put through a separator to remove its fat (called cooker cream), which is compatible with whey cream and may be mixed with it; the remaining liquid is often subsequently added to whey.

Now, to the kashrus (finally!). Halacha stipulates that cheese made by nochrim is non-kosher. (Gem. AZ 35a; Rambam Hil. Ma’achalos Asuros 3:13; Tur and Shulchan Aruch YD 115:2) This cheese, termed gevinas akum, was prohibited for one or more possible reasons as enumerated in the Gemara (AZ ibid., 35b); the reason adopted by the Rambam and Shulchan Aruch (and likely the Rif – AZ d.h.r. 13) is that advanced by Shmuel, who explained that fear of cheese coagulation via non kosher-slaughtered (neveilah) veal rennet was what precipitated the gezeirah (ban). Although the amount of rennet used to make cheese is miniscule, since rennet turns milk into a firm curd, it is a davar ha-ma’amid (ingredient that creates form) and is therefore never botel (nullified). (Shulchan Aruch YD 87:11)

The truth is that standard cheeses made in the US and UK do not use animal-based rennet, with few exceptions; microbial (artificial) rennet has become the norm in these nations. (Organic cheese is made with meat-based rennet, as artificial microbial rennet is deemed not natural and it thus does not meet organic standards.) Does the gezeirah of gevinas akum apply to hard cheeses that do not use real rennet?

The Rambam (ibid. with Kesef Mishneh), Shulchan Aruch (YD 115:2) and seemingly the Tur (YD ibid.) rule that the answer is yes, as do latter poskim. (Chochmas Odom 67:7, Aruch Ha-Shulchan YD 115:16.) The Ge’onei Narvona (see Tos. AZ 35a, d.h. ‘Chada’) permitted cheeses of akum in locales that do not use animal rennet, but their position was largely not adopted.

This machlokes hinges on whether or not the gezeirah of gevinas akum was declared as a davar she-b’minyan (a rule that applies to all cases, regardless of circumstances). The Maggid Mishneh (Rambam ibid. hal. 14) posits that the gezeirah was made on cheese of akum irrespective of its rennet source, as the prohibition was all-inclusive. The Ge’onei Narvona held that the inapplicability of the gezeirah to certain circumstances permit cheeses which do not fit the ban, and these talmidei chochomim therefore permitted gevinas akum in their region, where cheese was made with flowers (likely thistle buds, as are used in some Portuguese cheeses).

What about acid-set cheese? Does the gezeirah of gevinas akum apply to it?

Rav Yosef Eliyahu Henkin held not, and Rav Moshe Feinstein explained the rationale for this (Igros Moshe YD 2:48), writing that since acid-set cheese does not use rennet, and – in fact – it can be produced without any added ingredients by allowing milk to acidify on its own, there are grounds to say that such cheese it totally outside the definition of cheese upon which Chazal declared their ban. Most national kashrus agencies adopt this position. On the other hand, the Chochmas Odom (53:38) and Aruch Ha-Shulchan (ibid.) hold that all cheese of any type is subject to the rule of gevinas akum – period. Although this position carries much weight, it is clear that the very distinct methods of cheese-making as detailed above may bear consideration in favor of the lenient position.

(Reb Moshe (ibid.) also argued that rennet occasionally added to acid-set cheeses to hasten their production does not pose a gevinas akum problem, as the rennet is not essential and cannot form these cheeses independently. Based on this logic, a recent meeting of kashrus agencies with poskim concluded that baker’s cheese, which needs trace amounts of rennet to form, is not gevinas akum, as baker’s curd is acid-set, and the rennet on its own cannot coagulate this cheese.)

To be kosher, cheese must be gevinas Yisroel. The Remo (YD 115:2), Noda B’Yehuda (ShuT. Tin. OC s. 37) and many poskim adopt the position of the Rambam in the Peirush Ha-Mishnayos (AZ ch. 2) which requires a Yisroel to oversee the cheese-making process. This assures that only kosher rennet is used. (A Teshuvas Ha-Rashbo that concurs with this is noted in one very late source, but this author was unable to locate or verify it.) However, the Shach (YD 115 s.k. 20), Gro (ibid. s.k. 14), Chochmas Odom (67:7) and – quite apparently – the Rambam in the Yad (ibid.), as well as the Tur (ibid.) and the Mechaber in Shulchan Aruch (ibid.), posit that a Yisroel must actually add the rennet to the milk. That is, a Yisroel must make the cheese. (The Aruch Ha-Shulchan (ibid. s. 19) concurs with the Remo but advises that one follow the Gro’s opinion.) The Gro derives this from a reading in the Tosefta (AZ 5:5) which compares cheese to pas and bishul akum (bread and cooked foods of nochrim), which are permitted only when a Yisroel actually participates in their production; overseeing by a Yisroel is insufficient. In practice, most accepted kashrus agencies (including the OU) require a mashgiach to both oversee and add the rennet for kosher cheese productions, fulfilling all requirements. (Those kashrus agencies which are not widely accepted often certify cheese based on the approach of the Ge’onei Narvona, providing little or no on-site supervision during cheese-making.)

Despite the above, poskim rule that whey from gevinas akum is kosher. This is because Chazal were gozair (decreed a ban) on the cheese; the whey, which represents the portion of milk that did not become cheese, remains permissible.

Nonetheless, whey can very easily manifest kashrus problems. This can occur if the cheese is made with non-kosher rennet or other non-kosher ingredients (such as non-kosher cultures, wine vinegar, or lipase – an enzyme that decomposes cheese fat and is commonly derived from animal tongue-roots). So, too, if the cheese vat is hot (yad soledes bo), as is the case with many varieties of Swiss, parmesan and Scandinavian cheeses, the non-kosher flavor of the hot curd transfers into the whey, which contacts the curd in the vat. Furthermore, cooker water from Italian cooked gevinas akum cheeses is non-kosher, and its incorporation into whey renders the whey of like status.

For these reasons, whey and its derivatives are very kosher-sensitive, and this sensitivity affects many basic milk products. Whey cream, lactose, whey protein concentrate and milk minerals, all of which are extracted from whey, suddenly become of concern, as the whey from which they are sourced must be kosher. Sweet cream, culled from fresh milk, also becomes of concern, as it is interchangeable with and often contains blends of whey cream and cooker cream. Butter, which results from churning cream to clump together its fat while excess fluid is removed, is very much affected as well, as butter’s cream can be in the form of whey cream or its blends.

The wondrous technology of milk and cheese attests to so much Divine Hashgacha, while the halachic issues engendered necessitate advanced and aggressive kashrus hashgacha.