+232. Milk-sugar.+--The milk-sugar (lactose) is changed into lactic acid by the lactic acid-forming organisms, within the first few days after the cheese is made. This acid is combined with the other constituents as fast as it is formed. After a few days, the milk-sugar will have entirely disappeared from the cheese.[104] The relation between the milk-sugar and lactic acid is very close. It is necessary that milk-sugar be present in order later to have the lactic acid develop. +233. The salts.+--Just what changes the salts[105] undergo or how they combine with the other compounds is not definitely known. It is supposed that the calcium salts first combine with the phosphates and later, as the lactic acid is formed, they combine with the lactic acid, forming a calcium lactate. +234. Gases.+--In the process of cheese ripening, gases are formed, the commonest being carbon dioxide.[106] Exactly how this gas is formed is not known. It may be due to the formation of lactic acid from the milk-sugar or to the living organisms in the cheese. +235. Casein or proteins.+--Complex ripening changes in the cheese take place in the casein compounds or proteins. Because of the complex chemical nature of the proteins and the various agents acting on them, it is difficult to follow these changes. This has led to different opinions regarding the ripening process. The various compounds thought to be formed from the casein or proteins are as follows:[107] _Paracasein_ (formed by the action of the rennet on the casein). Insoluble in brine and warm 5 per cent salt brine. _Protein._ Soluble in warm 5 per cent salt brine. _Protein._ Insoluble in warm salt brine or water. _Paranuclein._ A protein soluble in water and precipitable by dilute hydrochloric acid. _Caseoses and proteoses._ Protein derivations soluble in water and not coagulated by heat. _Peptones._ Protein derivations simpler than the proteoses, soluble in water and not coagulated by heat. _Amido acids._ Protein derivations soluble in water, least complex except ammonia. _Ammonia._ The simplest protein derivations. From the discussion of the constituents in the milk and cheese, it is evident that practically all the principal ripening changes are concerned with those taking place in the proteins. +236. Causes of ripening changes.+--Authorities disagree as to the exact agents which cause the ripening changes. Some think they are due to the action of the enzymes in the rennet and those secreted in the milk. Others hold that these changes are due entirely to bacterial action. A combination of the two seems probable. The action of the rennet extract renders the casein insoluble and in the ripening process the proteins become soluble, the degree depending on the length of time the cheese is ripened. The amount of water-soluble proteins and protein derivatives is used as a measure of the extent of cheese ripening, considered from a chemical standpoint.