148 



KNOWLEDGE 



[August 1, 1891. 



and cheese we are principally concerned with the various 

 proportions in which these articles contain the three con- 

 stituents, fat, caseine, and milk sugar. Our mode of 

 treatment of the subject precludes us from attempting 

 a detailed explanation of the chemical nature of each of 

 these ingredients ; we merely draw attention to the fact 

 that fats and sugars possess distinct and valuable nutri- 

 tive properties (chiefly as fattening and warming diet), 

 wliilst caseine, which is the only one of the three con- 

 taining the element nitrogen (which constitutes about 15 

 per cent, of the caseine) helps to form tissue, its presence 

 in cheese giving that article its value as a substitute for 

 meat diet. The composition of the milk of cows only 

 varies within somewhat narrow limits. Change of diet 

 affects rather the quantity than the composition of the 

 secretion, milk in this respect resembling the blood rather 

 than certain other animal fluids. It is true that the diet 

 affects the flavour (as in the case of a turnip flavour from 

 swedes if eaten shortly before milking time), but this is 

 apparently due to the presence of a very small quantity of 

 a strongly-tasting material, the percentage composition of 

 the milk being practically unaffected. Below are given 

 representative analyses of — 



(1) Ordinary country milk. 



(2) Of town mUk from stall-fed cows, which is generally 

 somewhat richer in fat. 



Country MUk. Town (.«tall-f(><l) Milk. 



Water 87-55 % ... 85-94 % 



Fat 3-09 ... 4-00 



Caseine 4-01 ... 5-01 



Milk sugar ... 4-63 ... 4-31 



Ash -72 ... -74 



100-00 



100-00 



We have said that the composition of cows' milk varies, 

 though only within somewhat narrow limits. Eich milk 

 contains more fat, poor milk more water ; the dissolved 

 substances (caseine, milk sugar, and the inorganic sub- 

 stances), which are often referred to as the inilk aolith not 

 lilt constitute a nearly constant quantity. The determina- 

 tion of the milk solids not fat affords a means of 

 ascertaining if the milk has been sophistkatnl, that is to 

 say, has undergone artificial alteration of its composition. 

 Thus a milk with a somewhat high percentage of water 

 woidd pass as a naturally poor milk if the solids not fat 

 were in the usual proportion to the total weight of the 

 milk, but if the amount of these solids is too low the 

 conclusion can be drawn that the milk has been watered. 

 Skimming the cream or fat from the surface of the milk 

 would make the percentage of solids not fat too high. 

 It is true that it would be possible to combine the two 

 processes of skimming and wateriug so as to leave im- 

 altered the percentage of solids not fat, but in this case 

 the ratio of water to fat would be greater than is admitted 

 by the known range of variation in the composition of the 

 natural milk. This device of combined skimming and 

 watering is said to be sometimes resorted to in order to 

 baffle the inspector and his lactometer. The specific gra^-ity 

 of milk is about 1 -03 (water being taken as tmity). The fa't 

 is Ughter than water, therefore skimming raises the specific 

 gravity. Ordinary well water is lighter than milk (owing 

 to the presence of the solids in solution in the watery fluid 

 of the milk), hence watering lowers the speciiic gravity. 

 Though the lactometer — which indicates the density by 

 the depth to which it sinks— is de\-ised to detect either 

 mode of sophistication, yet a skilful combination of the 

 two wiU bafQe the instrument. As we have shown, how- 

 ever, analysis is capable by an indirect method of detecting 



this mode of sophistication. It is, nevertheless, unfortunate 

 that there is no iJirect test known by which added water 

 can be discriminated from that of the natural milk. It is 

 probable that the adulteration of milk with impure water 

 has sometimes resulted in the propagation of disease. 



When milk is left to stand, the globttles of fat, being 

 lighter than the liquid in which they float, gradually rise 

 to the surface, a layer of cream being formed. The rich- 

 ness of the cream thus produced varies greatly, the per- 

 centage of fat being sometimes less than 20, sometimes 

 more than 40. The watery fluid of the cream has nearly 

 the same composition as that of ordinary mUk, and by the 

 determination of the proportion of water to milk solids not 

 fat the analyst is enabled to judge if the article has been 

 treated by watering. 



In cream the globules of fat are more closely aggregated 

 than in milk, but when the cream is subjected to prolon'ged 

 agitation, as is done in the churn, the globules actually 

 cohere, and a solid mass separates fi-om the butter-milk, 

 which consists principally of water, caseine, and milk- 

 sugar. The salt, which is added as a preservative, and 

 for flavouring purposes, j)asses largely into the butter- 

 milk, that which remains in the butter being kept in solu- 

 tion by the water present. Good butter usually contains 

 about 14 per cent, of water, 2 per cent, of salt, and a very 

 small quantity of caseine, the remainder, or more than 

 80 per cent., being fat. Its value as a food depends on 

 the proportion of fat, but the importance of freeing the 

 butter as much as possible from water arises also from the 

 circumstance that water in an undue proportion not only 

 lessens the nutritive value but prevents the butter from 

 keeping. Although rich butter consists principally of 

 milk fat, the fat by itself would prove unpalatable. 



Fats other than milk fat, chiefly the fat of meat, are 

 now largely employed for the j^reparation of a substitute 

 for butter. They possess some of the valuable qualities of 

 milk fat, but are inferior in flavour, and less easily digested. 

 Margarine is sometimes made wholly fi-om these fats, whilst 

 the more expensive kinds contain milk fat, sometimes in 

 quantity equal to that of the foreign fats. Formerly 

 the detection of foreign fats in butter could not be made 

 with certainty, but as the practice of adulterating butter 

 in this way became more general, new methods of analysis 

 were elaborated, and now the detection of foreign fats 

 can be made readily and with certainty. Any material 

 containing an admixture of foreign fats is, legally, mar- 

 garine, and its exposure for sale without the weU-known 

 label is punishable by fiae. The exammation of "reputed 

 butter" now forms a considerable part of the public 

 analyst's work. A usual method of ijrocedure is to deter- 

 mine, by a quick process, the specific gi-avity of the melted 

 fat. If the specific granty is below that of butter fat, a 

 chemical analysis is made. The principal tests are those 

 depending on the fact that milk fat yields about ten times 

 as much butyric acid as other fats, and that the ordinary 

 margarine fats yield a larger proportion of the so-called 

 umaturiittd acids. These have the power of combining 

 with iodine, and their presence may be detected by the 

 iodine-ahsorjitiiDi test, which consists in determining the 

 change of strength which takes place in a standard iodine 

 solution when left in contact with the material. 



In the process of butter-making, as we have seen, almost 

 aU the caseine and the milk sugar are left in the butter- 

 milk. In Germany, milk sugar is extracted from butter- 

 milk for the purpose of adding it to ordinary cows' milk 

 intended as food for infants. In this way a milk is 

 obtained which approximates closely to the natm-al food of 

 children, which contains a larger proportion of sugar than 

 the milk of the daii-y. 



