(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.
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.
99. Cutting or breaking[30] the curd.—As soon as curd is formed, separation of whey begins upon the surface and perhaps around the sides of the vessel. This is accompanied by shrinkage and hardening of the mass. If the curd remains unbroken, the separation is extremely slow. In cheese-making practice, such curd masses may be dipped at once into hoops as in Camembert, dumped in mass into cloths for drainage as in Neufchâtel or, as in the larger number of cheeses, cut or broken in some characteristic manner. After the curd mass is firm, the rate at which subsequent changes take place depends largely on the size of the particles into which the curd is cut. The smaller the particles, the quicker the water is expelled. Consequently the development of the acidity and other changes take place more slowly. For this reason the curd should be cut into pieces of uniform size. If the work is not properly performed, the pieces of curd of various sizes will be at different stages of development. The fine particles will be firm and elastic while the larger particles are still soft and full of whey and may be developing too much acid. The knives should be inserted into the curd obliquely so that they will cut their way into the curd and not break it. The horizontal knife is used lengthwise of the vat and cuts the curd into layers of uniform thickness. The perpendicular knife then is used lengthwise and crosswise of the vat. It first cuts the curd into strips and then into cubes. The knives may have wire blades or steel blades, some operators preferring one and some the other. Whichever is used, the blades should be close enough together to give the fineness of curd desired.
After the knife passes through, the cut faces quickly become covered with a smooth coating, continuous over all exposed areas. This surface has the appearance of a smooth elastic coating or film. This can be seen by carefully breaking a piece in the hand. It is this film which holds the fat within the pieces of curd. If the film is broken, some of the fat globules are lost because the rennet extract acts only on the casein and that in turn holds the fat. All the fat globules which come in contact with the knives as they pass through the curd will be left between the pieces of curd and will pass off in the whey. If care is exercised in cutting, the loss of fat will be confined to what may be called a mechanical loss. This is similar to the loss of the sawdust when sawing a board. This loss in American Cheddar is about 0.3 per cent and cannot be avoided. If it is greater than this, it is due to negligence on the part of the cheese-maker or the poor condition of the milk. The cutting of the curd into small pieces may be considered a necessary evil. If the moisture could be expelled from the whole mass without disturbing it, this fat loss could be prevented. The cutting, breaking or turning should be done with the greatest care, that the loss may be as small as possible.