lactic acid

Lactic acid is what two bacteria make when they are set loose on milk sugar together. Lactobacillus delbrueckii subspecies bulgaricus and Streptococcus thermophilus feed each other as they work, the first breaking milk protein into peptides the second needs, the second returning compounds the first grows on, and between them they turn lactose into the acid that sours yogurt and ayran. In this library it is the cold tang served beside abo shalhoob, the brighter substitute standing in for jameed's fermented sourness in the jameed-free mansaf, and, in a kiwi smoothie, a sourness that comes with its own shortcut for digestion.

The acid itself, C₃H₆O₃, is a hydroxy acid with a chiral centre, and what it does to milk is a matter of pH. Fresh milk sits near pH 6.8; as the bacteria drop it toward 4.6 it crosses the isoelectric point of casein, where the protein micelles lose the surface charge that keeps them apart and lock into the network felt as set yogurt, the tang and the texture the same event. Jameed, the sun-dried fermented yogurt a traditional mansaf is built on, pushes the same acid further: one analysis measured its titratable acidity at 6.6 per cent, and a six-month storage trial watched the pH of a sheep-buttermilk version fall from 4.98 to 3.36 as the salt and protein concentrated.

The digestive part is where the yogurt helps itself: lactase-nonpersistent people tolerate yogurt better than milk because the bacteria that soured it carry their own beta-galactosidase into the gut, packaged inside intact cells that survive the stomach and reopen in the less acidic small intestine. It is established enough that the European Food Safety Authority has authorised the claim, on condition that a product still carries at least a hundred million live cultures per gram when eaten; heat-treating a yogurt after fermentation, a common shelf-stability step, kills the bacteria and voids it. In this library lactic acid is traced in three recipes, always from yogurt or ayran.