Rennet.
—As has been said in the description of cheese making, the material which is most useful in the precipitation of the curd is rennet. The rennet is the secretion of the stomach of various animals, that of the calf being most highly priced for cheese making. The fourth stomach of the animal is the one which is used in the manufacture of rennet. The aqueous extract made from these stomachs contains a ferment which has the property of coagulating casein in a very high degree. One part of good rennet preparation from healthy stomachs of calves will coagulate 1000 parts of milk. In former days rennet was freshly made and used at the factories. At the present time it is largely prepared on a commercial scale and sold to the cheese maker. It is highly important that the rennet used in cheese making should be of the best quality, as an inferior grade gives a bad taste and color to the cheese. Just as in the manufacture of fermented beverages and making of bread the character of the yeast is a dominant factor in the nature of the finished product, so it is even to a greater degree in the case of rennet. Those who purchase the rennet already made should therefore be certain it is of a quality to give the desired character to the cheese. The greater the amount of milk fat in milk the larger the proportion of rennet, since the milk fat protects to some extent the casein from the action of the ferment. Experience has shown also that during the summer the rennet acts more readily upon the milk, probably due to the higher temperature. Care should be taken to avoid the use of any excess of rennet, since anything more than the amount necessary to conduct the coagulation is apt to add an unpleasant flavor to the cheese. The curd also in such cases is less cohesive and makes a tougher and drier product which does not lend itself so readily to the ripening process. For this reason the rennet which is to be used should always be tested in small quantities of milk beforehand in order that the proper proportion to be used may be known so that the process in a large way may be conducted with certainty and not by guess. (“British Dairy Farming,” by Jas. Long.)
Rennet is sometimes treated with borax to preserve it during transit. In such cases the borax may not all be removed by the whey and is consequently found in ripened cheese. Its introduction in this way should be avoided.
Coagulating the Milk by Rennet Extract.
—This process is often termed by the cheese makers “setting the milk with rennet.” The milk which is used for the purpose of cheese making should be, in the technical language of the cheese maker, “ripe,” that is, containing a sufficient quantity of lactic acid. The principal method of producing the proper amount of lactic acid in milk is by keeping it warm, namely, at a temperature of about 84 degrees. At this temperature the most favorable conditions exist in milk for the rapid growth of the lactic acid ferments. If the natural ferments which produce lactic acid are not in sufficient quantity in the original milk it is better, rather than to wait too long a time, to start the development of the lactic acid by adding an artificial ferment. Lactic ferments are specially prepared for this purpose, or some previously ripened milk may be added to the mass. This is called a “starter.” From two to five pounds of “starter” are usually required for each one hundred pounds of milk. The degree of ripening is ascertained by measuring the quantity of lactic acid present. The proper condition of the milk is tested by means of a rennet preparation and if the milk will coagulate, when thus tested, in about one minute or a little more it is an indication that a sufficient amount of acid has been developed to add the rennet for the proper coagulation of the milk. It is important to have the milk in just the right condition in order that the proper operations in cheese making may go on uniformly. Care must be taken, however, not to have too much lactic acid in the milk. For instance, 0.2 of one percent is too great, and such a milk is very liable to give trouble in subsequent operations. In the curding of milk by rennet the temperature should be kept between 82 and 86 degrees. The amount of rennet extract, of course, varies with its character and strength, and this is best determined by the cheese maker’s experimenting in order that the proper quantity to be added to the great mass of milk may be known beforehand. A sufficient quantity of rennet extract should be used to curdle the milk in fifteen or twenty minutes for a quick-curing cheese, and in thirty to forty minutes for a slow-curing cheese. The rennet extracts in common use are added at the rate of from one-half to five ounces for 1000 pounds of milk. Before adding, the extract should be diluted with from 20 to 40 times its volume of water at a temperature of from 85 to 90 degrees. The rennet thus diluted acts with uniformity on the milk, preventing the production of curd of a lumpy character. Previous to adding the rennet extract the mass of milk is thoroughly stirred in order to mix the fat therewith and the dilute rennet added evenly and slowly with constant stirring which is continued for several minutes. A gentle stirring of the surface of the milk should be continued until the curd is at least half formed, in order that the fat may not separate. After the stirring is finished, a cloth is placed over the top of the vat to keep the surface of the milk from cooling, and the milk is then left undisturbed until the coagulation is complete. The coagulation goes on gradually until the whole mass of milk is one solid coagulum produced by the changing of casein into paracasein.
Cutting the Curd.
—In order that the whey may be separated it is necessary that the curd be cut into pieces. The smaller the pieces of curd, the more rapidly will the whey escape. As soon as the curd is formed it shows a tendency to contract and this tends to force out the whey. By cutting the extent of the surface from which the whey can exude is amplified and the rapidity of the process is enormously increased. The time at which the curd is to be cut is one of great importance and is determined by the skill and experience of the cheese maker. If the curd is cut when it is too soft there may be large loss of fat and a decreased yield of cheese. If the curd is too hard the whey is more difficultly removed and the quality of the cheeses is not so fine. The following test is used to determine when the curd is in the right condition to cut. The end of the index finger is inserted obliquely into the curd half an inch or more and then slowly raised toward the surface. If the curd breaks apart with a clean fracture without leaving any particles on the finger and the whey which exudes from the broken surface is clear and not milky it shows the proper time has come for cutting. Specially devised knives are used for cutting the curd, which leave it in small cubes of about one-half inch surface. Skill in the use of the cutting knife is important and can only be acquired by proper experience.
Heating the Curd.
—As soon as the curd is cut the whey begins to go out of it and the curd settles to the bottom of the vat, the whey being of a higher specific gravity than the curd. After the pieces of curd sink to the bottom the surface easily reunites and, when broken apart, additional fat is lost. As soon, therefore, as the curd is cut the whole mass is kept in gentle motion by hand stirring or with a wire basket designed for the purpose, care being taken to avoid breaking or comminuting the cubes. When properly stirred the whey appears clear and is free of small particles of curd.
The curd contracts and hardens during this process, and soon reaches a condition when the surface does not adhere so readily. The vat should be kept warm during the process of separation of the whey, the temperature being raised to about 90 degrees and finally, toward the last, to 98 degrees, about blood heat.