Dairy products are not commonly thought of as being unacceptable for Passover. They do not contain fermented grain or legumes (ever heard of pasta ice cream or butter made with peas?), nor are they manufactured in facilities which are out-of-bounds for Passover (such as bakeries). Why, then, does the OU grant and require Passover certification for dairy products?
Cheese
All hard cheese and most soft cheese contain bacterial cultures, and hard cheese is coagulated via rennet (usually microbial). These cultures and enzymes are often fermented in environments and on surfaces which are offensive to Passover production, and nutrients used in fermentation are also frequently not acceptable for Passover. (Blue cheese mold is grown on bread, as are many other enzymes and cultures.) The OU assures that all Passover-approved rennet and cultures are supplied exclusively with Passover-approved nutrients, grown in kosher for Passover environments and contain no non-Passover contents. Many cheese factories add vinegar to their cheese vats to balance pH. Passover-approved cheese (and whey, which comes from cheese production) must be supervised so as to assure that any vinegar used is kosher-for-Passover as well.
Butter
Butter is principally made from cream, which is churned into butter clumps. Both fresh cream and whey cream can be used. Fresh cream is inherently kosher for Passover, whereas whey cream is a derivative of whey and is subject to the same Passover concerns as whey itself. It is critical that all cream sources are evaluated for Passover production and that whey cream be certified for Passover prior to use in Passover-approved butter.
Butter often contains starter distillate — a “buttery” flavorant. Starter distillate is composed of a blend of dairy substances, frequently including whey and whey byproducts. So, too, butter facilities often use cultures in their specialized products. The OU must assure that all such butter additives are acceptable for Passover.
Yogurt
Yogurt shares the same essential contents as soft cheese, but it also usually has added flavors and fruit fillings, as well as starch (for texture). Flavors and fillings can be processed on equipment which is not kosher-for-Passover, and they can even contain chemical additives which are truly not acceptable for Passover use. Regular food-grade starch is very often grain or corn-based,and it therefore cannot be used in Passover products. Yogurt also commonly contains carrageenan gum, which is likely to be standardized with non-Passover dextrose. Other yogurt stabilizers — most often gum-based — are spray dried into powder on dryers used very frequently for all sorts of stuff (including fermented grain). Corn-based sweeteners (corn syrup and solids) are the norm for yogurt, but they are not acceptable for Passover. Thus, flavors, fillings, starch, gum and other stabilizers must all be Passover-certified, and Passover-approved sweeteners must be substituted for corn-based ones. Of all kosher-for Passover dairy foods, yogurt clearly presents the plurality of ingredient concerns.
Milk
By law, all milk sold on a retail level must contain vitamins. Such vitamins can be unacceptable for Passover use. In the case of fresh, unflavored milk the OU assures that all added vitamins are Passover-certified.
Flavored milk features flavors, stabilizers and starch which can be wheat or corn-based and processed in spray-dryers used for Passover-offensive materials (see above). Such milk is almost always sweetened with corn syrup. Again, Passover dairy supervision is most surely necessary.
Passover Dairy Production
Aside from ingredient issues, kosher-for-Passover dairy certification involves equipment concerns, as production equipment is often exposed to non-Passover materials. (Aside from all of the non-Passover components enumerated above being used year-round on such equipment, industrial equipment is often shared with other production. Butter can be pasteurized in the same equipment used for chocolate milk; whey can be dried into powder in spray-dryers used for wheat germ, and so forth.)
The OU therefore needs to kosherize (kosher-sanitize) such equipment for Passover dairy production. In such cases, the on-site rabbinic field representative supervises kosherization prior to Passover production.
We need full-time supervision for manufacture of Passover products which are identical to non-Passover products to assure that the non-Passover ingredients are not used. Without full-time rabbinic supervision during production, one would never know if the item is kosher for Passover or not, and it could therefore not be certified for Passover use.