COMPOSITION OF FOODS 133 



but in the case of hay, straw, and roots other matters 

 are also present (p. 136). 



The " soluble carbohydrates " mentioned in the table 

 include not only the soluble sugars, mucilage, &c., 

 but also starch, pectin, pentosans,^ and a considerable 

 part of the cellulose ; these latter are not soluble in 

 water, but are dissolved in the process of boiHng with 

 weak acid and alkali employed by the analyst to 

 separate the coarse fibre. The soluble carbohydrates 

 of a food analysis are thus a mixture of very various 

 constituents, which have probably not the same feed- 

 ing value. Unfortunately the exact composition of 

 these mixed carbohydrates has been but little studied. 



The ^^ fibre " left by the process of extraction em- 

 ployed in food analysis varies much in constitution ; 

 among its ingredients are the typical cotton cellulose, 

 oxycellulose, and lignin. Oxycellulose forms the chief 

 constituent of the fibre of gramineous hay and straw. 

 As already mentioned, the proportion of fibre by no 

 means represents the whole of the cellulose, a part 

 being reckoned as soluble carbohydrate according to 

 the present method of food analysis. 



We will now consider the average composition of the 

 various foods mentioned in the table on p. 130. 



* Pentosans are generally insoluble in water, but dissolve in alkali. 

 When heated with dilute acids they are converted into pentoses — that 

 is, sugars containing a multiple of five atoms of carbon in their mole- 

 cule. These sugars are not fermentable, and are very imperfectly 

 burnt in the animal system ; they have thus little nutritive value. 

 The amount of pentosans present is usually calculated from the 

 quantity of furfural produced when the food is boiled with hydro- 

 chloric acid. The quantity thus calculated may, however, be 

 considerably in excess of the truth, as oxycellulose also yields fur- 

 fural. The group of furfural yielding bodies is thus of very mixed 

 character, and is best described as furfuroids. 



