The grades of skimmed-milk cheese vary between rather wide limits--from those made entirely of skimmed-milk to those made of milk from which only a small amount of fat has been removed and which are almost like whole-milk cheese. Because of the gradations of skimmed-milk cheese, it is difficult to make anything but general statements and to base comparisons with whole-milk cheese. CHAPTER XIV _CHEDDAR CHEESE RIPENING_ Freshly made Cheddar cheese is hard, tough and elastic and lacks characteristic cheese flavor. In this condition it is called "green," unripe or not cured. Before the cheese is ready to be eaten, it passes through a complex series of changes which are collectively known as ripening. In the ripening process the texture becomes soft and mellow and the characteristic cheese flavors develop. Cheese ripening must be considered from two view-points, first, the changes taking place inside the cheese and secondly the outside conditions necessary for ripening. Some of the chemical changes during ripening are known, while others are not understood. The different agents causing ripening, and the constituents of the milk, will be discussed. +231. Fat.+--Numerous investigations have been made to ascertain what chemical changes the fat undergoes in the ripening process. Suzuki,[103] in studying the fat, found no enzyme capable of producing lactic acid or volatile fatty acids. However, these acids were found in increasing amounts during the ripening process and after the lactose had disappeared. Acetic and propionic acids reached a maximum at three months and then decreased, while butyric and caproic acids continually increased during the experimental period covered. Formic acid was detected in the whole-milk cheese only at the five and one-half month stage. In the judgment of the experimenter the principal source of acetic and propionic acids was probably lactates. Traces of these acids may have had their origin in protein decomposition or further fermentation of glycerine. The principal sources indicated for butyric and caproic acids were fats and proteins. The distillate from the experimental cheese was designated "flavor solution" and contained alcohols and esters, giving a close resemblance to the cheese aroma. The "flavor solution" from the mild whole-milk cheese contained esters made up largely of ethyl alcohol and acetic acid, while from the more pungent skimmed-milk cheese the esters were largely compounds of ethyl alcohol and caproic and butyric acids. The alcohol may have come from the lactose fermentation. It appears to be an important factor in flavor production. The agencies operative in the production of volatile acids and syntheses of esters are as yet undefined.