curd at a desired rate. =Ripening of cheese.= The curd at the time it is put to press is tough and rubbery, and has none of the characteristic flavor of cheddar cheese; it is also quite insoluble and indigestible. Before the cheese is fit to eat it must pass through a complex series of changes which are collectively known as _ripening_. In these changes there is not only a breaking down of the casein into soluble compounds, which process makes the cheese soft and plastic under pressure, but the characteristic flavor is developed in greater or less degree. A very considerable part of the cheese thus becomes soluble in water, and it is much more easily digested than in an unripened condition. The different factors that are operative in the ripening changes are not yet fully known, but in recent years as a result of scientific study, material progress in the study of the changes has been made. =Rennet.= The commercial rennet extract when in condition for use contains very few bacteria. A preservative, boric acid, is added by the manufacturer to restrain the bacteria, otherwise the extract would soon be unfit for use. The bacteria in the commercial rennet extract are too few to be of any importance whatever in the ripening process. Rennet extract contains an enzyme, rennin, that causes the milk to curdle; also another enzyme, pepsin, that exerts a digestive action on the curdled casein. Pepsin is always found in the stomach juices of all animals, but no digestive action takes place, unless the reaction is distinctly acid, as is the ease under normal conditions, since hydrochloric acid is excreted by the walls of the stomach. Outside of the stomach, the same conditions must obtain with reference to the presence of acid, if pepsin is to exert a digestive effect. In the cheese curd, the milk sugar is rapidly changed into lactic acid by the action of the bacteria. This gives the proper chemical reaction for peptic action, and the enzyme is then able to act on the paracasein, the nitrogenous part of the cheese. If milk contains no acid-forming bacteria, conditions will not permit of peptic action, and as a consequence, the ripening processes do not take place. If the sugar is fermented by some organism that does not form acid, as the lactose-fermenting yeasts, the cheese does not ripen. The lactic bacteria are therefore an essential factor in inaugurating the ripening changes in all types of rennet cheese.