If the milk is stirred too long or after it begins to thicken, the result is a granular sort of curd, and there will be an abnormally large loss of fat in the manufacturing process. The addition of the rennet and subsequent stirring require the exercise of great care and constant attention to details. The cheese-maker can do nothing else for those few minutes. When through stirring, it is a good plan in cold weather to cover the vat with a cloth as this will keep the surface of the curd warm. In summer the same cover will keep out the flies. _Causes of a delayed coagulation:_ (1) Weak rennet extract or too small an amount. (2) Low temperatures due to inaccurate thermometers. (3) Pasteurized milk. (4) Presence of abnormal bacterial ferments. (5) Presence of preservatives. (6) Heavily watered milk. (7) Use of badly rusted[29] cans. (8) Milk containing small amounts of casein or calcium salts. _Causes of uneven coagulation:_ (1) Uneven temperature of the mix in the vat, due to lack of agitation. (2) Uneven distribution of the rennet extract. (3) Adding rennet to vat too soon after heating, while the sides and bottom are still hot, causes curd to stick to sides and bottom of the vat making cutting difficult. (4) Sloshing after the milk begins to thicken breaks the curd and causes it to whey off. +98. The curdling period.+--The time allowed for rennet action also affects the texture of the curd. The enzymes of rennet (rennin and pepsin) do not cease acting with the thickening of the milk. In many cheeses, the handling process begins as soon as the curd has become solid enough to split cleanly before a finger thrust into it. If let stand further, the same curd mass will continue to harden with the progressive separation of whey; this shows first as drops ("sweating") on its surface, which then increase in number and size until they run together and form a sheet of whey. The limit of such action is difficult to measure. The solidifying process ceases in a period of hours. The further action of the enzymes is digestive in character and goes on slowly. It requires a period of weeks or even months to accomplish measurable results at the working temperatures in use in the trade. Other ripening agents with more rapid action intervene to shape the final result. It follows that the rennet factor in the ripening changes found at the end of the period is almost negligible for most varieties of cheese, although it appears to be measurable in some varieties.