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about 40 per cent. This should mean, then, that the net effect of the coking operation is exothermic to this extent. This exothermicity presumably represents the net heat effect of fusing the coal, the evaporation of various hydrocarbons, some decomposition, and heats of formation of carbon dioxide, carbon monoxide, methane, and other compounds. It is possible to calculate roughly the heat effect of coking from the composition of the products formed. This, however, is qualitative only, since the mechanism of the decomposition is not definitely known. Furthermore, undoubtedly the heat evolved would vary with the temperature since the character of the tar and other products of coking is dependent upon the temperature of carbonization. This test would give a figure of about 450 B. t. u. per lb. of dry coal cok’ed as the exothermicity of the coking process. Bearing in mind that the oxygen content of this coal mixture is about 5 per cent, this figure for the heat liberated qualitatively agrees well with results of work done independently by Mahler and Euchene by quite different methods. It should be noted,
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however, that this value of 450 B. t. u. per lb. is not exact since it is obtained as a difference between relatively large numbers, and consequently a small percentage variation in either of these values will produce a large percentage variation in the difference between them. However, the value is qualitatively correct, and is included here in view of the need for more knowledge‘of the heat requirements for coking different coals, particularly the heat required as the oxygen content varies. If values with varying oxygen content were known, such information should prove of great help in determining the coking qualities and the heat requirements of new coal mixtures.
ACKNOWLEDGMENT It is desired to acknowledge here the great interest and the untiring enthusiasm of the students in the School of Chemical Engineering Practice who did the actual experimental work and to whose efforts is due the success of the work.
Milk and Ice Cream as Fatty Foods1 By Jerome Alexander 50 EAST 41sT ST., NEWYORK,N . Y .
T
H E IMPORTAXCE of fat in milk is evident from the fact that its minimum percentage is fixed by law; but this importance far transcends the mere legal 3 per cent or so, because milk contains about 88 per cent of water, which means that the fat constitutes over 25 per cent of the milk solids. The fine subdivision of milk fat, coupled with its low melting point, are factors which would insure its easy digestion and assimilation, were it not that the protein constituents of the milk, the milk curds, must be also reckoned with. The clotting or curding of the fluid milk in the stomach is due to two factors-first, the acidity of the gastric juice, and second, the activity of the digestive enzymes. The speed of clotting (or even its total inhibition) is a matter of great importance, because the casein coagulum mechanically entraps or entangles all the fat, making a fatty or greasy curd which tends to repel the acid protein-dissolving gastric juice, and thus remain partially or entirely undissolved before being discharged into the duodenum. The constituent primarily responsible for the curding of milk is its casein, an easily coagulable colloid. The motion of its particles and their clumping under the influence of acid or rennin are readily discernible in the ultramicroscope. I n all milks the casein is stabilized or protected (in the colloidal sense) by the other main protein constituent, lactalbumin, and the behavior of the milks of different animals is greatly influenced by the extent of this protection-i. e., by the ratio between casein and the lactalbumin.2 The following table, giving average compositions, brings out this point which is often concealed by combining the casein and lactalbumin under single heading, “total protein.” KINDOB
MILK Ass Woman. cow
....... .. ... ......
~
Casein 0.67 1.03 3.02
Lactalbumin 1.55 1.26 0.53
Fat 1.64 3 .‘78 3.64
Protective Ratio 2.31 1.13 0.14
1 Presented before the Division of Agricultural and Food Chemistry at the 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 t o 8, 1922. 2 Alexander, KoZZoid-Z., 1909-1910: J . A m . Chem. SOC., 3%(19101, 680; Alexander and Bullowa, Arch. Pediatrtcs, a7 (1910), 18; 3. Am. Med. Assoc., 1910.
The values of the three milks for infant feeding parallel their protective ratios; thus, asses’ milk is recognized as a refuge in digestive disturbances where neither cows’ milk nor even mothers’ milk is tolerated. Mere dilution of cows’ milk to make its total protein content equal that of mothers’ milk does not aid, even though the disparities of fat and sugar content be equalized; for such dilution does not change the protective ratio. Almost every successful formula for modifying cows’ milk for infant-feeding involves the addition of something which increases the protective ratio. Dextrinized barley gruel is popular now, but gelatin, gum arabic, Iceland moss, malt extract, and even beer are among the additions that have been successfuly used. Sodium citrate, used in some formulas, acts as a colloidal protector, while limewater simply acts as a mild alkali inhibiting the action of the digestive acid. The mechanism of the curding of milk by digestive enzymes seems to be the same as that of rennin coagulation-the enzymes split up the protective substances, thus reducing the protective ratio, or even producing substances which in themselves have a positive coagulating e f f e ~ t . ~Thus, lactalbumin, when digested with pepsin, becomes a coagulant to colloidal silver chloride. Thus, it is obvious that efficient colloidal protection is of prime importance in insuring the proper digestion of milk, because by inhibiting the coagulation of the casein or delaying it so that peptic digestion and peristalsis permit only small curds to form, the formation of large, greasy, indigestible curds is prevented. All this applies with especial force to ice cream, where the fat content greatly exceeds that of milk; and here, too, the experience of the housewife, as well as of the manufacturer, shows the advantage of adding gelatin, eggs, etc., which increases the protective ratio. Many persons who cannot take milk alone can take milk with eggs. The protective ratio should d S 0 be of importance in artificial milks and cheese, which offer a field for research along this line. 8
Alexander, “The Rennin Coagulation of Milk,” 8th Intern. Cong.
A p p l . Chem., 6 (1912), 12.
