83. Marschall rennet test.—To make a Marschall rennet test, 1 c.c. of rennet extract is measured, with the 1 c.c. pipette, and placed in the bottle. Care should be exercised to rinse out the pipette. The bottle is then filled to the mark with cold water. After the milk has been heated to the setting temperature, 84°-86° F., the cup is filled with milk and set on the edge of the vat so that the milk running out through the hole in the bottom of the cup will flow into the vat. Just as the surface of the milk reaches the 0 mark on the cup, the diluted rennet extract is added and thoroughly mixed with the milk, using the small spatula to stir it. The rennet and milk should be mixed until it has run down at least one-half space on the scale in the cup. As the rennet begins to coagulate the milk, it runs slower from the hole in the bottom of the cup, until it finally stops. When it stops, the point on the scale indicated by the surface of the coagulated milk is noted. The test is recorded by the number of spaces the surface of the milk lowers from the time the rennet is added until it is coagulated. This test depends on three factors: the strength of the rennet extract, the temperature of the milk, the acidity of the milk. The more acid, the quicker the milk will coagulate. To measure any one of these factors, the other two must be constant. The variable factor is the acidity of the milk. This test will not indicate the percentage of acid in the milk, but is simply a comparative test to be used from day to day; for example, if the rennet test to-day shows three spaces, and the operator makes that milk into cheese and the process seems to be normal, it shows that for good results in this factory, milk should be ripened to show three spaces every day. If the next day the milk showed four spaces, it should be allowed to ripen more until it shows three spaces. If it shows only two spaces, this indicates that the milk has too much acid development or is over-ripe. A cheese-maker will have to determine at what point to set his milk, because the test will vary from one factory to another.
84. Comparison of acid and rennet test.—Each of these tests has its advantages and disadvantages. The advantage of the acid test is that it can be made as well of warm as cold milk. This is of great importance in determining whether the milk delivered by any patron is too ripe to be received. The acidity of other materials, such as whey and starter, can be determined as well as that of milk. The disadvantages are that it is difficult to get the alkali solution of the proper strength and the solution is liable to deteriorate on standing. It requires a careful exact operator to make the test.
The advantages of the rennet test are that it is easy to make, and it requires no materials that are hard to replace. The disadvantage is that the milk must be warmed to the same temperature before a comparative test can be made. The size of the outlet in cups varies. It does not indicate the percentage of acid present in the milk. It is simply a comparative test. To obtain the best result, both tests should be used in conjunction.
85. Control of acid.—The control of acidity in curd and cheese is dependent on the control of the moisture or water-content. The control of both factors is very important in relation to the quality[27] of the cheese. Often acidity is spoken of when moisture is really intended, and vice versa. The close relation between the moisture and acidity is due to the presence of the milk-sugar in solution in the milk-serum which becomes the whey of cheese-making. Water or moisture in cheese consists of the remnant of this whey which is not expelled in the making process. During manufacture and the ripening process, the milk-sugar is changed to lactic acid. A cheese may be sweet when first made and after a time become sour because it contains too much moisture in the form of whey. Excess of whey carries excess of milk-sugar from which fermentation produces intense acidity.
Various tests have been devised to determine the amount of acid developed at the different stages of manufacture. These tests are described on page 61. By the use of such tests, the development of acid during the manufacturing process can be very accurately determined. There is no quick, accurate test to determine the amount of moisture in the curd. The cheese-maker has to rely on his own judgment, guided largely by the appearance, feeling and condition of the curd.
After the rennet extract has been added, all control of the acid development is lost. The cheese-maker can determine rather accurately how fast the acid will develop during the ripening of the milk. This shows the importance of the proper ripening. The amount of acid developed during the different stages of the manufacturing process can be approximately followed with the various acid tests. The manufacturing process should then be varied to obtain the proper relation between the moisture and the acid present. The only time that the acidity may be controlled is when the milk is being ripened. If too much acid is developed before the rennet is added, there is apt to be too much acid at each stage of the manufacturing process. This is liable to hurry the cheese-making process and to cause a loss, both in quality and quantity of cheese, and may cause a high acid or sour cheese. If sufficient acid is not developed at the time the rennet is added or if the milk is not sufficiently ripened, the acid is liable not to develop fast enough so that there will not be sufficient at each step in the cheese-making process. Such a cheese is called "sweet." There are several conditions which will cause an over-development of acid. Such a cheese is called "acidy" or "sour." These factors are within the control of the cheese-maker, hence should be avoided. A sour cheese shows lack of skill and care on the part of the cheese-maker.
Conditions causing an acidy or sour cheese:
Receiving sour or high acid milk at the cheese-factory.
Use of too much starter.
Ripening the milk too much before the rennet is added.