BOSWORTH, A. W., Chemical studies of Camembert cheese, N.

Y. Exp. Sta. Tech. Bul. 5, 1907.

BOSWORTH, A. W., and L. L. VAN SLYKE, Preparation and composition of basic calcium caseinate and paracaseinate, Jour. Biol. Chem. Vol. 14, pages 207-210, 1913.

DUCLAUX, eMILE, Action de la presure sur le lait, Compt.

Rend. Acad. Sci. 98, pages 526-528, 1884.

HAMMARSTEN, OLOF, Zur Kenntnis des Caseins und der Wirkung des Labfermentes, Nova. Acta Regiae Soc. Sci.

Upsaliensis in Memoriam Quattuor Saec. ab Univ., Upsaliensi Peractorum, 1877.

HAMMARSTEN, OLOF, Ueber das Verhalten des Paracaseins zu dem Labenzyme, Zeit. physiol. Chem. 22, pages 103-126, 1896.

HAMMARSTEN, OLOF, A text book of physiological chemistry, from the author's 7th German edition, 1911.

HOSL, J., Unterschiede in der tryptischen und peptischen Spaltung des Caseins, Paracaseins und des Paracaseinkalkes aus Kuh- und Ziegenmilch, Inaug. Diss.

Bern., 31 pp., 1910.

KIKKOJI, T., Beitrage zur Kenntniss des Caseins und Paracaseins, Zeit. physiol. Chem. No. 61, pages 130-146, 1909.

LINDET, L., Solubilite des alb.u.minoides du lait dans les elements du serum; retrogradation de leur solubilite sous l'influence du chlorure, Bul. Soc. Chim. (ser. 4) 13, pages 929-935.

LINDET, L., Sur les elements mineraux contenus dans la caseine du lait, Rep. Eighth Internat. Congr. of Applied Chem. 19, 199-207, 1912.

LOEVENHART, A. S., Ueber die Gerinnung der Milch, Zeit.

physiol. Chem. 41, pages 177-205, 1904.

PETRY, EUGEN, Ueber die Einwirkung des Labferments auf Kasein, Beitrage z. Chem. Physiol. u. Path. 8, pages 339-364, 1906.

ROBERTSON, T. BRAILSFORD, On the influence of temperature upon the solubility of casein in alkaline solutions, Jour. Biol. Chem. 5, pages 147-154, 1908.

SCHMIDT-NIELSON, SIGVAL, Zur Kenntnis des Kaseins und der Labgerinnung, Upsala lakaref. Forh. (N. F.) No. 11, Suppl.

Hammarsten Festschrift No. XV, 1-26; through Jahresb. u.

d. Fortschr. d. Thierchem. No. 36, pages 255-256, 1906.

SPIRO, K., Beeinflussung und Natur des Labungsvorganges, Beitrage z. Chem. Physiol. u. Path. 8, pages 365-369, 1906.

VAN DAM, W., Ueber die Wirkung des Labs Auf.

Paracaseinkalks, Zeit. physiol. Chem. No. 61, pages 147-163, 1909.

VAN HERWERDEN, M., Beitrag zur Kenntnis der Labwirkung auf Casein, Zeit. physiol. Chem. 52, pages 184-206, 1907.

VAN SLYKE, L. L., and A. W. BOSWORTH, I. Some of the first chemical changes in Cheddar cheese. II. The acidity of the water extract of Cheddar cheese, N. Y. Exp. Sta.

Tech. Bul. 4, 1907.

VAN SLYKE, L. L., and A. W. BOSWORTH, Composition and properties of some casein and paracasein compounds and their relations to cheese, N. Y. Exp. Sta. Tech. Bul. 26, 1912.

VAN SLYKE, L. L., and A. W. BOSWORTH, Method of preparing ash-free casein and paracasein, Jour. Biol. Chem. Vol.

14, pages 203-206, 1913.

VAN SLYKE, L. L., and A. W. BOSWORTH, Preparation and composition of unsaturated or acid caseinates and paracaseinates, _Ibid._ Vol. 14, pages 211-225, 1913.

VAN SLYKE, L. L., and A. W. BOSWORTH, Valency of molecules and molecular weights of casein and paracasein, _Ibid._ Vol. 14, pages 227-230, 1913.

VAN SLYKE, L. L., and A. W. BOSWORTH, Composition and properties of the brine-soluble compounds in cheese, Jour. Biol. Chem. 14, pages 231-236, 1913.

VAN SLYKE, L. L., and E. B. HART, A study of some of the salts formed by casein and paracasein with acids; their relations to American Cheddar cheese, N. Y. Exp. Sta.

Bul. 214, 1902.

VAN SLYKE, L. L., and E. B. HART, Casein and paracasein in some of their relations to bases and acids, American Chem. Jour. 33, pages 461-996, 1905.

VAN SLYKE, L. L., and E. B. HART, Some of the relations of casein and paracasein to bases and acids, and their application to Cheddar cheese, N. Y. Exp. Sta. Bul. 261, 1905.

CHAPTER IV

_LACTIC STARTERS_

Acidity in cheese-making arises almost exclusively from the lactic acid produced from the fermentation of milk-sugar (lactose) by bacteria.

Hydrochloric acid is used in the Wisconsin[22] process of making pasteurized milk cheese and sometimes for making skimmed-milk curd for baking purposes. It is regularly used in precipitating casein not for food but for manufacturing purposes.

+56. Acidifying organisms.+--Many species of bacteria have been shown to possess the power to produce lactic acid by fermenting lactose. In practice, however, the cheese-maker seeks to control this fermentation by the actual introduction of the desired organisms and by the production of conditions which will insure this dominance through natural selection. For this purpose the initial souring for most types of cheeses is produced by some variety of the species originally described by Esten[23] and commonly referred to as _Bacterium lactis-acidi_, but variously named as _B. acidi-lactici_, _Streptococcus lacticus_, _B. guntheri_ by different authors. Organisms of this series dominate all other species in milk which is incubated at 70 F. They produce a smooth solid ma.s.s without a sign of gas holes and without the separation of whey from the curd, and develop in milk a maximum acidity of about 0.90 of one per cent when t.i.trated as lactic acid. (For t.i.tration see Chapter V.) This species is usually present in small numbers in fresh milk. There are many varieties or strains of the species with differing rates of activity and measurable differences in acid produced but with approximately the same qualitative characters.

Most commercial starters for cheese- and b.u.t.ter-making belong to this group of species, although special mixtures with other organisms are prepared for special purposes. In addition to this group, most varieties of cheese contain some members of the colon-aerogenes group. When the milk is in proper condition, the activity of this group should be held in check by the early and rapid development of acid. Free development of members of this group usually shows itself in the presence of gas holes in the curd.

+57. Starter.+--The practice of using pure cultures in cheese-making has brought about the development of factory methods of producing day by day cultures of the organisms desired, in quant.i.ties sufficient to inoculate the total quant.i.ty of milk used in manufacture. For this purpose milk is mostly used and the product is known as "starter." For cheese-making purposes, a starter is a substance used in the manufacture of dairy products having a predominance of lactic acid-forming microorganisms in an active state. There are two general cla.s.ses of starter: (1) Natural starter; (2) commercial starter.

+58. Natural starter.+--Milk, or other similar substance, which has become sour or in which large numbers of lactic acid-forming organisms are present, is called a natural starter when used in the manufacture of dairy products. To secure clean-flavored milk, the cheese-maker usually selects the milk of some producer who usually brings good milk and allows it to sour naturally for use the next day. There is often a variation from day to day in the milk delivered by the same producer, so that the cheese-maker is not certain of a uniform quality in his fundamental material. While the lactic acid-forming organisms are developing, other organisms may also be present in numbers sufficient to produce bad flavors. If a starter has any objectionable flavor, it should not be used. Natural starters very commonly develop objectionable flavors which at first are very difficult to recognize. When natural starters with objectionable but not easily recognizable odors are used, the effect may be seen on the cheese. Milk, sour whey and b.u.t.termilk are materials commonly used as natural starter. A common difficulty in skimmed-milk cheese is caused by the use of b.u.t.termilk as a starter.

+59. Commercial starter or pure cultures.+--The alternative practice consists in the introduction of pure cultures of known strains of lactic bacteria into special milk to make the starter. Since these cultures must be prepared by a bacteriologist, commercial laboratories have developed a large business in their production. Many such commercial brands are manufactured under trade-marked names. Some of these cultures represent races of lactic bacteria cultivated and cared for efficiently, hence uniformly valuable over long periods of time. Others carelessly produced are worthless, or even a peril to the user.

These organisms are usually shipped in small quant.i.ties in bottles of liquid or powder, or in capsules of uniform size. The contents may be either the culture medium upon which the organisms grew or inert substance designed merely to hold the bacteria in inactive form. In either solid or liquid form, the producer of the culture should guarantee its activity up to a plainly stated date.

It is the problem[24] of the cheese-maker or b.u.t.ter-maker to increase this small amount of lactic acid-forming organisms to such numbers and in such active condition that it may be used in the factory; while being built up, these organisms must be kept pure. The usual practice is to allow them to develop in some material, usually whole milk or skimmed-milk; dissolved milk powder may be used in the place of milk.

+60. Manufacturer's directions.+--The manufacturer usually sends directions with his starter preparation, telling how it should be used to secure the best result. These directions apply to average conditions and must be varied to suit the individual instances so that a good starter will be the result. The directions usually state the amount of milk necessary for the first inoculation. It is usually a small amount, one or two quarts. After the specific amount has been selected, this milk should be pasteurized.

+61. Selecting milk.+--The milk for use in starter-making should be selected with very much care. Only clean-flavored sweet milk, free from undesirable micro-organisms, should be used in the preparation of starter. The milk is ordinarily chosen from a producer whose milk is usually in good condition. The quality of the milk can be determined by the use of the fermentation test. (See Chapter II.) It is better to choose only the morning's milk for the making of starter, because the bacteria have not had so much opportunity to develop. In no case should the mixed milk be used in the preparation of starter, as this eliminates all opportunity for selection. The flavor of the starter will be the same as that of the milk from which it is made.

+62. Pasteurization+ is the process of heating to a high temperature for a given length of time and quickly cooling. It kills most of the micro-organisms in the milk. In other words, it makes a clean seed-bed for the pure culture. The temperatures of pasteurization recommended for starter-making differ with the authority. A temperature of 180 F. for thirty minutes or longer seems to be very satisfactory, since under these conditions nearly all the micro-organisms in the milk are killed.

+63. Containers.+--Various kinds of containers may be used for starter-making. One-quart gla.s.s fruit jars or milk bottles make very satisfactory containers, because the condition of the starter may be seen at any time. They are also easily cleaned. They have the disadvantage, however, of being easily broken, if the temperature is suddenly changed, or if severely jarred. Tin containers may also be used. Such containers are not easily broken, but they are harder to clean and must be opened to examine the contents; hence the liability of contamination is very much greater.

This small amount of milk may be pasteurized by placing the container in water heated to the desired temperature. A very satisfactory arrangement is to cut of a barrel, and place a steam pipe in it. The barrel can then be filled partly full of water and heated by steam. The bottles of milk to be pasteurized are hung in the water in the barrel. Two or three more bottles should be prepared than it is expected will be used as some of the bottles are liable to be broken while cooling or heating. The bottles should be filled about two-thirds full. This leaves room enough to add the mother starter and later to break up the starter to examine it. It is desirable not to have the milk or starter touch the cover since contaminations are more likely. It is a good plan when pasteurizing to have one bottle as a check. This may be filled with water and left open and the thermometer placed in it. A uniform temperature may be obtained by shaking the bottles.

+64. Adding cultures.+--After being pasteurized, the milk should be cooled to a temperature of 80 F. This is a suitable temperature for the development of the lactic acid-forming organisms. The commercial or pure culture should now be added to the milk at the rate specified in the directions. Care should be exercised in opening bottles not to put the covers in an unclean place. A sterile dipper is a good place to put them. After the culture has been added to the milk, it should be mixed thoroughly by shaking the bottle. This should be repeated every fifteen or twenty minutes for four or five times. This is done to make certain that the culture is thoroughly mixed with the milk. The milk should be placed in a room or incubator as near 80 F. as possible, in order to have a uniform temperature for the growth of the organisms. The bacteria in the pure culture are more or less dormant so that a somewhat higher temperature than the ordinary is necessary to stimulate their activity.

This milk should be coagulated in eighteen to twenty-four hours, depending largely on the uniformity of the temperature during incubation.