95. Strength of coagulating materials.—Rennet and pepsin preparations vary in strength and in keeping quality. With a particular stock, changes go on to such a degree that the last samples from a barrel of rennet are much weaker than the earlier ones. Each sample, barrel, keg or bottle should be tested before used. In continuous work the results of each day's work furnish the guide for the next day's use of a particular lot of rennet.
96. Amount of coagulating materials to use.—For most varieties of cheese, sufficient rennet extract or pepsin is added to the milk to give a firm curd in twenty-five to forty minutes. Of the ordinary commercial rennet extract, this requires from two and one-half to four ounces to one thousand pounds of milk. This gives a maximum of one part rennet for each four to six thousand parts of milk. The great strength of the rennet extract is thus clearly shown.
97. Method of adding rennet.—Before rennet is added to the milk, it is diluted in about forty times its volume of cold water, which chills the enzyme and retards its action until it can be thoroughly mixed with the milk. If the material is added without such dilution, the concentrated extract produces instant coagulation in the drops with which it comes in contact, forming solid masses from which the enzyme escapes only slowly to diffuse throughout the mass. Uniform coagulation thus becomes impossible. After the rennet extract has been diluted with cold water, it should be distributed the entire length of the vat in an even stream from a pail. It should then be mixed with the milk by stirring from top to bottom for about three to four minutes. For this purpose, either a long-handled dipper or a wooden rake may be used. A dipperful should be drawn from the gate and stirred into the vat, otherwise the milk in the gate will fail to coagulate properly because the rennet diffuses too slowly to reach and affect all the milk at that point. The milk should be stirred on the top, preferably with the bottom of a dipper, until signs of coagulation begin to appear. This stirring keeps the cream from rising. There are various ways or signs to indicate when the coagulation has gone to the stage at which the mix is about to become thick: (1) The milk becomes lazy or thicker as the finger is passed through it; (2) bubbles caused by moving the finger remain on the milk longer, usually until one can count ten when ready to thicken.
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.