Attempts have been made to overcome the casein difficulty in cows' milk by the addition of citrate of soda. In 1893 Wright drew attention to the importance of the calcium of cows' milk in producing the dense curd. Human milk contains 03 per cent of lime, while cows' milk contains .17 per cent.
The chief base in human milk is potash, while in most other milks it is lime. This excess of lime in cows' milk can be dispensed with without injury to the nutrition of the infant, as the lime is presumably there with a view to the calf's walking powers being called into play almost immediately after birth, and an infant does not walk for a year. Rennet coagulation is delayed and curdling becomes less and less firm as an increasing proportion of the lime salts of the milk becomes precipitated as insoluble salts. Wright found that the addition of a solution of citrate of soda sufficed to prevent any coagulation by rennet, by precipitating the lime as an insoluble salt, and he suggested the use of citrate of soda as a means of "humanizing" cows' milk for infants. The chemical changes involved are not exactly known, but are thus explained by Poynton. The caseinogen of milk is acid, and in the process of clotting combines with the calcium salts in the milk to produce the thick casein clot (calcium casein). Sodium citrate, when added to the milk, combines with the caseinogen, and a sodium compound is formed (sodium casein). The latter is of lower molecular weight than the calcium compound and hence of less density. The calcium salts of the milk combine with the citric acid to produce calcium citrate, which is absorbed into the system and so the lime is not lost to the economy. The density and toughness of the clot varies inversely with the amount of citrate of soda used, so that by increasing the amount a very fine coagulum is produced. For ordinary cases Poynton uses a solution of the strength of one grain of citrate of soda in a drachm of water. Of this 1 drachm is added for each ounce of milk in the food as prepared for use. Thus if an infant is taking 20 oz. of whole milk in the day, it will receive 20 grains of citrate of soda. In cases in which a finer curd seems to be required he uses two or even three grains of citrate of soda to the ounce of milk. The amount of citrate of soda required by the infant can be determined after trial with a small dose by the moaunt of curd passed in the motions. When the solution is prepared in bulk, say 12 oz., a few drops of chloroform should be added to prevent a mould forming. The citrate of soda may also be obtained in the form of tabloids of one, two, or three grain strength, for convenience in travelling. Poynton has recommended his method chiefly in connexion with the weaning of infants, and for the correction of milk dyspepsia, but it has also enabled him to increase the amount of milk which can be taken in the twenty-four hours. He finds that the milk thus treated is palatable, is somewhat constipating, is well tolerated by most infants, and is followed by no bad effects, so far as he has seen. The method is a cheap one, citrate of soda costing about 2d. an ounce, and it can be carried out in every domestic circle. Citrated milk has also been used as a method of feeding healthy infants by other physicians, who report favourably on its use.
The advantage claimed here is the diminished denseness of the casein and the increased digestibility of the milk. A sufficient time has not elapsed to test thoroughly the results of this method. The addition to the diet of citrate of soda in the doses prescribed is not probably injurious in itself, although such chemical treatment of milk should preferably be avoided. As Poynton states, citrates are a normal constituent of cows' milk and are not known to act injuriously on the body. There is no evidence that the stomach is not called on to do a proper amount of work in digesting the milk, and the excessive dilution which is called for by some forms of modification is here unnecessary. We think that this method is worthy of a full trial at the hands of the profession, both as regards healthy infants and those who do not take kindly to diluted cows' milk.