Chymosin.

Chymosin and pepsin are aspartic proteases with overlapping acid pH optima. Calf rennet is a mixture of the two, with the pepsin fraction rising as the animal ages. Pepsin cleaves more broadly, so a rennet high in pepsin gives more general protein breakdown alongside the specific milk-clotting cut.

Chymosin makes a single cut in kappa-casein, but the destabilised micelles only aggregate into a curd once calcium ions bridge them. Without adequate ionic calcium the cut still happens and no gel forms. This is why calcium chloride is routinely added to pasteurised milk before rennet.

Chymosin is occasionally included in acid-protease blends alongside pepsin and fungal proteases to cover a wider range of cleavage sites at gastric pH. Its own specificity is narrow, so it contributes a particular cut rather than broad proteolysis. Read the pairing as complementary specificity rather than as more of the same activity.

Chymosin is active in acid and loses activity as pH climbs toward neutral. Betaine hydrochloride is used in supplements to lower gastric pH, which keeps acid proteases in their working range. The relationship is about the conditions the enzyme needs, not about a combined effect on any outcome.

Bicarbonate raises gastric pH, and chymosin activity falls sharply as pH rises above its acid optimum. Taking an alkalising agent alongside an acid protease works against the enzyme. This is straightforward enzyme chemistry and worth flagging on any product that contains both.

Kappa-casein is chymosin's natural substrate, and the enzyme cleaves it at a single bond between phenylalanine 105 and methionine 106. That cut strips the stabilising hair layer from the casein micelle and lets the micelles aggregate. Any casein-containing matrix will gel in the presence of active chymosin and calcium.

Products aimed at dairy handling sometimes carry both, with lactase acting on the milk sugar and chymosin on the casein fraction. The two work on entirely separate substrates and do not interfere with each other. Their pH preferences differ, so a single format cannot give both enzymes ideal conditions.

Chymosin is the dominant gastric protease in the newborn ruminant, where its role is to clot milk and slow its passage out of the stomach so digestion has more time. Colostrum and early milk are the substrate that system evolved around. In adult humans chymosin secretion has largely given way to pepsin, so the pairing is physiologically interesting rather than a practical adult combination.