THE CARBOHYDRATES AND FATS 



11 



several substances like acetic acid, lactic acid, and inosite, which 

 are not carbohydrates. 



The formulae given above are merely empirical ; and there is no 

 doubt that the quantity n in the starch group is variable, and often 

 large ; hence the name polysaccharides that is given to the group. 

 Kesearch has, moreover, shown that the glucoses are either aldehydes 

 or ketones of hexatomic alcohols having the general formula 

 C6H8(OH)6. Thus dextrose is the aldehyde of sorbite, le^Tilose the 

 ketone of mannite, and galactose the aldehyde of dulcite. The 

 amyloses maybe regarded as the anhydrides of the glucoses [wCgH, gOg 

 — 71H2O = (CgHioOs),]. Thesucroses are condensed glucoses — i.e. 

 they are formed by the combination of two molecules of glucose with 

 the loss of one molecule of water (CgHjaOg + CeHioOs — HjO 

 =C,2H2.20,,) ; hence the term disaccharide. The following are the 

 chief facts in relation to each of the principal carbohydrates : — 



MONOSACCHAKIDES 



Dextrose or Grape Sugar. — This carbohydrate is found in fruits, 

 honey, and in minute quantities in the blood (0'12 per cent.) and 

 numerous tissues, organs, and fluids of the 

 body. It is the form of sugar found in 

 large quantities in the blood and urine 

 in the disease known as diabetes. 



Dextrose is soluble in hot and cold 

 water and in alcohol. It is crystaUine 

 (see fig. 1), but not so sweet as cane sugar. 

 When heated with strong potash certain 

 complex acids are formed which have a 

 yellow or brown colovir. This constitutes 

 Moore's test for sugar. In alkaline solu- 

 tions dextrose reduces salts of silver, 



bismuth, mercury, and copper. The reduction of cupric to cuprous 

 oxide constitutes Trommefs test, which has been already described 

 at the head of the lesson. On boihng it with an alkaUne solution of 

 picric acid, a dark red opaque solution due to reduction of the picric 

 to picramic acid is produced. Another important property of grape 

 sugar is that under the influence of yeast it is converted into alcohol 

 and carbonic acid (CeHiaOg = '^.G^B.^O + 2CO2). 



Dextrose may be estimated by the fermentation test, by the 

 polarimeter, and by the use of Fehling's solution. The last method 

 is the most important ; it rests on the same principles as Trommer's 



Fig. 1. — Destrose crystals. 



