Whey

Milk protein comes in two families. About four fifths of it is Casein, which curdles. The remaining fifth stays dissolved in the liquid, and that liquid is whey.

The US regulation puts it plainly: whey is “the liquid substance obtained by separating the coagulum from milk, cream, or skim milk in cheesemaking.” It is a cheese by-product that turned into an industry, which is why it now appears in protein powders, biscuits, bread, crisps, infant formula, processed meat and a great deal else.

The good news first

This is one of the few ingredients on this site where the label is genuinely on your side.

Milk is a declarable allergen in every jurisdiction with allergen rules, and whey is milk. It cannot appear under a name that conceals it, and in most markets it must be emphasised in the ingredient list or repeated in a “contains” statement. If you are avoiding dairy because of an allergy, whey is not the ingredient that will catch you out.

That is worth stating clearly, because most of these pages exist to explain a label failure and this one does not.

The two things it gets confused with

Whey is not lactose. Lactose is milk sugar. Whey is milk protein plus whatever else is dissolved in that liquid, which does include lactose. Someone who is lactose intolerant reacts to the sugar; someone with a milk allergy reacts to the protein. Whey protein isolate has had most of the lactose removed and is still entirely unsuitable for a milk allergy. The two conditions are different and this ingredient sits across both, which is why it generates so much confusion. The same boundary is set out in more detail on the page on casein and lactose.

Whey is not casein. They are the two protein fractions of the same milk. A product can contain one without the other, so “casein-free” does not mean whey-free and neither means milk-free. For an allergy this distinction is mostly academic, since a milk allergy usually means avoiding both, but for anyone reading a label against a specific instruction it matters.

The part the label does not cover

Whey comes out of cheesemaking, and most cheese is made with Rennet. So whey usually carries the rennet question along with it: if the cheese it came from was set with animal rennet, that is part of its history, and no whey label records which.

For a milk allergy this is irrelevant. For anyone avoiding animal ingredients beyond dairy itself, or keeping halal or kosher, it is the reason whey is on this list at all. The ingredient is transparent about being milk and silent about everything upstream of that.

Vegetarian, mostly

Whey itself involves no slaughtered animal. Whether the cheesemaking behind it did depends on the coagulant, which is the same fork described on the rennet page.

Protein powders are where this comes up most, because they are bought deliberately by people who read labels closely. A whey protein marked vegetarian has usually been made from cheese set with fermentation-produced or microbial coagulant, and the brands that do this generally say so, since it is a selling point rather than a secret.