+81. The acid test+[26] is made by t.i.trating a known amount of milk (Fig. 9) against an alkali solution of known strength, using phenolphthalein as an indicator. The object of the indicator is to tell the condition of the milk, whether it is acid, alkaline or neutral. The indicator does not change in an acid solution but turns pink when the solution is or becomes alkaline. To make the test, a known quant.i.ty of the material to be tested is placed in a white cup, and to this several drops of indicator are added. As an indicator, a 1 per cent solution of phenolphthalein in 95 per cent alcohol is commonly used. As an alkali solution, sodium hydroxide (NaOH) is used in the standardized strength usually either tenth (N/10) normal or twentieth (N/20) normal. This solution should be obtained in some one of the standardized forms commercially prepared. The alkali is added, drop by drop, from a graduated burette until a faint pink color appears. This shows that the acid in the milk has been neutralized by the alkali. The amount of alkali that has been used can be determined from the burette. Knowing the amount of milk and alkali solution used, it is easy to calculate the amount of acid in the substance tested. The results are usually expressed either as percentages of lactic acid or preferably as cubic centimeters of normal alkali required to neutralize 100 or 1000 c.c. of milk. This kind of test is on the market under different names, such as Mann's, Publow's, Farrington's and Marschall's.

[Ill.u.s.tration: _FIG. 9._--An acid tester.]

+82. Rennet tests.+--Several rennet tests have been devised, but the one most widely used is the Marschall (Fig. 10). This consists of a 1 c.c.

pipette to measure the rennet extract, a small bottle in which to dilute the extract, a special cup to hold the milk and a spatula to mix the milk with the rennet extract. This cup has on the inside from top to bottom a scale graduated from 0 at the top to 10 at the bottom. There is a hole in the bottom to allow the milk to run out.

[Ill.u.s.tration: FIG. 10.--Marschall rennet test.]

+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 s.p.a.ce 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 s.p.a.ces 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 s.p.a.ces, 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 s.p.a.ces every day. If the next day the milk showed four s.p.a.ces, it should be allowed to ripen more until it shows three s.p.a.ces. If it shows only two s.p.a.ces, 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 quant.i.ty 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.

Removing the whey before the curd is properly firmed, hence leaving it with too much moisture.

Development of too much acid in the whey before the whey is removed.

Improper relationship between the moisture and acidity at the time of removing the whey.

_Conditions causing deficient acid:_

Adding the rennet before sufficient acid has developed.

Not using sufficient starter.

Not developing sufficient acid in the whey.

+86. Acidity and rennet action.+--The rennet extract acts only in an acid medium. The greater the acid development, within certain limits, the faster the action of the rennet. If enough acid has developed to cause a coagulation of the casein, the rennet will not coagulate the milk. This is one reason why Cheddar cheese cannot be made from sour milk.

+87. Acidity and expulsion of the whey.+--The contraction of the curd and expulsion of the whey are so closely related that they may be treated under the same heading. The more acid, the faster the whey separates from the curd, other conditions being uniform. The relation of acidity and firmness of the curd to temperature of the curd is another important factor in the successful manufacture of cheese. The higher the acidity, the faster the temperature of the curd can be raised without any harmful effects. If the temperature is raised too fast in relation to the acidity, the film surrounding each piece of curd will become toughened so that the moisture will not be able to escape. When this condition exists, the curd will feel firm but when the pieces are broken open the inside is found to be very soft. This results in a large loss later or may cause a sour cheese. It usually causes an uneven texture and color in the cheese.

+88. Acidity in relation to cheese flavor.+--Just what part the acid plays in the development of cheese flavor is not known. If a certain amount of acid is not present, the characteristic cheese flavor does not develop. If too much acid is developed, it gives the cheese a sour flavor which is unpleasant. If sufficient acid is not developed, the other undesirable factors seem to be more active, causing very disagreeable flavor and may cause the cheese to putrefy. A cheese with a low acid usually develops a very mild flavor, and if carried to extremes, as in the case of some washed curd cheese, the true cheese flavor never develops.

+89. Acidity in relation to body and texture of cheese.+--If a cheese is to have a close, smooth, mellow, silky body and texture, a certain amount of acid development is necessary. If too much acid is developed, the body and texture will be dry, harsh, sandy, mealy, corky. If the acid is not sufficient the cheese may be soft or weak bodied, and is usually characterized by "Swiss curd holes," which are s.p.a.ces of various sizes usually more or less round and very shiny on the inside.

+90. Acidity in relation to cheese color.+--An over-development of acidity affects the color of a cheese. If this development of acidity is uniform throughout the cheese, it causes the color to become pale or bleached. If this development is uneven, due to the uneven distribution of moisture, the color will be bleached in spots, causing a mottled effect.

+91. Control of moisture.+[28]--The cheese-maker must use skill and judgment in regulating the amount of moisture in relation to the firmness of the curd and the acid. Since there are no quick accurate tests to determine the amount of moisture, this is left entirely to the judgment of the operator. Certain methods of handling the curd reduce the moisture-content, while others increase it. The cheese-maker must decide how to handle the curd. If the curd becomes too dry, methods should be employed to increase the moisture, and vice versa.

_Causes of excessive moisture:_

Cutting the curd coa.r.s.e.

Cutting the curd after it has become too hard.

Setting the milk at a high temperature.

Use of excessive amount of rennet extract.

Low acid in the curd at the time of removing the whey.

Not stirring the curd with the hands as the last of the whey is removed.

High piling of the curd during the cheddaring process.

Piling the curd too quickly after removing the whey.

Use of a small amount of salt.

Holding the curd at too low a temperature after the whey is removed.

Soaking the curd in water previous to salting.

Allowing the curd to remain in the whey too long so that it reabsorbs the whey.

Heating the curd too rapidly.

_Causes of insufficient moisture:_

Cutting the curd too fine or breaking up the pieces with the rake into too small pieces.

Cutting the curd too soft.

Stirring the curd too much by hand as the last of the whey is being removed.

Developing high acid in the curd at the time of removing the whey.

Insufficient piling of the curd during the cheddaring process.

Using a large amount of salt.

High temperature and low humidity in the curing room.

+92. Relation of moisture to manufacture and quality.+--(1) _Flavor:_ If the cheese contains too much moisture, it is likely to develop a sour or acidy flavor. A cheese with a normally high moisture-content usually ripens or develops a cheese flavor much faster than one with a lower moisture-content, other conditions being uniform. A cheese with a high moisture-content is much more liable, during the curing process, to develop undesirable flavors than is one with a lower moisture-content.

(2) _Body and texture:_ A cheese containing too much moisture is very soft and is difficult to hold in shape. Such a product breaks down very rapidly and is usually pasty and sticky in texture. If too little moisture is present, the cheese is very dry and hard, and cures or ripens very slowly because of the lack of moisture together with milk-sugar from which acid may be formed. Dry cheeses are usually harsh, tough and rubbery in texture. Such cheeses also have poor rinds. (3) _Color:_ If the ideal conditions exist, the moisture will be evenly distributed throughout the cheese. The spots containing more moisture will be lighter in color. If a cheese contains so much moisture that it becomes "acidy," the effect is the same as when too much acid is developed, that is, the color becomes pale from the action of the acid.

(4) _Finish:_ A cheese containing too much moisture is usually soft. A good rind does not form. Such a cheese loses its shape very easily, especially in a warm curing room. (5) _Quality:_ A cheese with a high moisture-content is usually marketable for only a very short period.

Such a product usually develops flavor very quickly in comparison to a dry cheese. It must be sold very soon because if held too long, the flavor becomes so strong as to be undesirable, and objectionable flavors are liable to develop. In some cases, such cheeses rot.

+93. Relation of moisture to acidity.+--From the preceding discussion, it is evident that the relation between the moisture and acidity is very close, in fact so intimate that in some cases it is difficult to distinguish one from the other when the quality of the cheese is considered. The proper relation of the moisture and the acidity determines the quality of the resulting cheese. If too much acid is developed during the manufacturing process, the product will be sour. If too much moisture is retained in the form of whey, the cheese will be sour. The less acid in the curd, the more moisture in the form of whey may be retained in the curd without causing a sour cheese. The proper relationship between the moisture and the acidity must be maintained or a sour cheese will result.

The relation of the moisture to the acidity also has an influence on the curing. If the cheese has a low development of acidity and a low moisture-content, it will cure very slowly. The increasing of either the acidity or moisture usually increases the rate of cheese ripening, other factors being the same.

The relation of the acidity and the moisture is so important that it cannot be neglected without injuring both the quality and quant.i.ty of cheese. This knowledge can be obtained only by experience.

+94. Setting temperature.+--The temperature of renneting makes very much difference in the texture of the product. The enzyme rennin is sensitive to very slight changes in temperature. Below 70 F., its rate of action is very slow. Beginning with approximately 20 per cent of its maximum effectiveness at 70 F. (the curdling point for Neufchatel), it has risen to 65 per cent at 84 F., to 70 per cent at 86 F., as used in Cheddar, to about 80 to 85 per cent at 90-94 F., as used in Limburger.

At 105 F. it reaches its maximum effective working rate to fall from that efficiency to about 50 per cent at 120 F. Curdling at low temperature lengthens the time required for the same amount of rennet to curdle a given quant.i.ty of the same milk. The texture of curd produced at temperatures between 70 F. and 84 F. is soft, jelly-like, friable rather than rubbery. At 86 F. it begins to show toughening or rubbery characters which become very marked at 90 F. to 94 F. as used in Limburger. With the increased vigor of action as it pa.s.ses its maximum rate of action at 105 F., the texture tends to become loose, floccose to granular. Aside from the Neufchatel group, the working range of temperatures for the renneting period runs from about 84 F. to about 94 F., a range of barely 10 F., or the use of 65 per cent to 80 or possibly 85 per cent of the maximum efficiency of the rennet. Within this range of temperature, the curd has the physical characters demanded for making most varieties of cheese.

+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 r.e.t.a.r.ds 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 ma.s.ses from which the enzyme escapes only slowly to diffuse throughout the ma.s.s. 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 pa.s.sed 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:_