What has been, said of pasteurised milk will hold in general for evaporated and condensed milks. The general methods of preparation of these products have been described (p. 277). Condensed (sweetened) milk is subjected to about the same degree of heat as that used in the "hold " method of pasteurisation, while evaporated milk is usually sterilised by heating under pressure. Both products are free from bacterial contamination but are probably deficient in C, and should therefore be supplemented by additional supplies of this vitamin.102 Hess103 states that sweetened condensed milk has been found to retain the larger part of its antiscorbutic factor, while on the other hand Oiorgi104 has described an epidemic of scurvy coincident with the introduction of condensed milk. This contradiction may perhaps be reconciled by the statement of Hume104a that there is no destruction of vitamin in full-cream sweetened condensed milk prepared by concentration at a low temperature in vacuo, so that if the condensed milk is diluted to its original volume by addition of three volumes of water the vitamin content may be said to be adequate, but that if it is diluted eight times or more, according to instructions, its vitamin content may be inadequate, although, owing to its higher sugar content, it still gives an adequate supply of calories.

The nutritive value of dried milk has been very fully reported on by Coutts and Winfield.105 There are various methods of preparing this product, in some of which the milk is evaporated at a comparatively low temperature, while in others it is exposed to a high temperature, but for a very short time. Usually the milk is either passed in a thin layer over the surface of a heated cylinder, evaporation being hastened by a current of hot air, or forced, after partial evaporation, in the form of a very fine spray, into hot dry air. As an example of the first process, that of Just-Hatmaker is described by Coutts as follows:

Two hollow metal cylinders arranged to revolve in opposite directions are mounted so as almost to touch one another. At one time the process was carried out with the cylinders performing six or seven complete revolutions per minute, but the speed has been increased in later models to 14 times a minute, or even faster. The cylinders are heated internally by steam at a pressure of two to three atmospheres, so that their surface temperature is considerably over 100 C.

The liquid milk runs from an elevated container into the hollow between the two revolving cylinders, the result being a constant rapid ebullition leading no doubt to a certain concentration of the milk. The milk then spreads in a thin film over the surface of the cylinders, and the water is rapidly evaporated, leaving a thin sheet of dried milk having the appearance of tissue paper. When the cylinders have completed about two-thirds of a revolution, knife-blades, adjusted so as to touch the surface of the cylinders, scrape off the film of dried milk and the thin pellicle falls into a receptacle below. This is afterwards broken up and passed through a sieve, thus giving a uniform, finely granular powder.

102 See Hart, Steenbock and Smith, 1. c.

103 Hess, J. Am. Med. Ass. 76, 603, 1921.

104 Oiorgi, Pediatria, 29, 66,1921.

104a Hume, Bioohexn. J. 15, 163, 1921.

105 Coutts and Winfield, Local Govt Bd. Food Reps. No. 24, London, 1920.

The second method is used in the preparation of Trumilk,106 in which the milk is first partially condensed in vacuo and then without being cooled is filtered and forced in a mist-like spray into a hot chamber through which air is circulating. The milk falls to the floor in a fine powder.

On account of their convenience in transportation and preservation, milk powders are coming into extensive use by bakers and confectioners, for whose purposes they are said to be fully equal, if not superior, to whole milk. They are also prepared for the use of travellers and campers in regions where fresh milk is not available, and as an ingredient of special foods for invalids and children. With regard to its bacterial content, dried milk compares very favorably with fresh milk, although it is not to be regarded as sterile.

Dried milks may be either full cream, half cream, or skimmed milks. In what follows unless otherwise specified full cream milk is referred to.

The concentration of the solids in dried milk is regarded roughly as eight times that in fluid milk, and the proportion in which the constituents are present is estimated to be as follows:107

Fat...........................................25-28 per cent.

Protein ......................................25-28 " ".

Lactose....................................... 84-40 " ".

Ash.......................................... 6-7 " ".

Water ........................................ 5-7 " ".

Coutts108 states that the albumin and globulin are at least partly coagulated. The caseinogen is not coagulated, but is probably altered in some way, the general concensus of opinion being that it is more digestible as found in dried milk than as in fresh milk. Any alteration which occurs in the fat or the lactose appears to be insignificant. The soluble salts of calcium are transformed, to some extent at least, into insoluble salts, but these are still available for nutritive purposes.109

106 Merrell-Soule, Eng. Pat. 19,193, 1906.

107 Lane-Claypon, p. 206.

108 Coutts, 1. c, p. 28.

109 Daniels and Loughlin, J. Biol. Cham. 44, 896, 1920.

With regard to the vitamin content of dried milk, it might be expected that this product would not be inferior to evaporated milk in that respect, and might be superior, owing to the short period of exposure to heat. The experiments of Osborne and Mendel already referred to (p. 270) in which a diet of 60 per cent milk powder, 12 per cent starch, and 28 per cent lard was used, indicate that there is at least a fair amount of both A and B present, but when Winfield110 attempted to keep rats on an exclusive diet of dried milk he found that while growth was normal for some weeks, often for as long as three months, it gradually became retarded and eventually ceased entirely, although the animals apparently remained in perfect health for the full period of the experiment, 16 months or more. As already pointed out (p. 271), it seems improbable that this failure in growth was due to lack of the recognized vitamins, since the fault was corrected by the addition of starch and lard, which are supposed to be free from vitamins. Moreover, although growth ceased there was no evidence of xerophthalmia, neuritis, or any of the other symptoms generally observed on a vitamin-poor diet.

Use of fresh milk instead of milk powder did not induce renewed growth in the animals which had ceased to grow, but when a bread and milk ration was given growth recommenced.111