166 PEACTICAL PHYSIOLOGY 



saliva, or of amylopsin, a ferment in the pancreatic juice. It is also 

 produced by the action of malt diastase, and to a certain extent by 

 ordinary hydrolysing agencies acting on starch. 



By moistening barley and allowing it to germinate in heaps at a 

 constant temperature, the starch which it contains is converted into 

 dextrose (see below) and maltose. This change is brought about by 

 the ferment called diastase which exists in barley. The product when 

 dried is called malt. If this be dissolved in water and the yeast plant 

 allowed to grow on the solution, malted liquors, such as beer and ale, 

 are obtained. 



Maltose readily ferments with yeast, being first of all inverted into 

 two molecules of dextrose by the inverting ferment contained in the 

 yeast. 



It reduces metallic oxides in alkaline solution. 



CHAPTER II. 



CARBOHYDRATES CONTINUED. 



III. POLYSACCHARIDES. 



A POLYSACCHARIDE is the condensation product of more than two 

 monosaccharide molecules, and has accordingly the general formula, 

 (C 6 H 10 O 5 ) W , where n stands for a variable number. 1 The'y can be 

 Irydrolysed, the ultimate products being monosaccharides ; polysac- 

 charides (dextrines) of lower molecular weight (i.e. with n of less value), 

 and several disaccharides being developed as intermediate products. 



Thus, when starch is boiled with a weak acid, or is acted on by the 

 ferments ptyalin or amylopsin, it yields at first dextrine a lower 

 polysaccharide and maltose a disaccharide. The former of these is 

 then further hydrolysed to form maltose. Boiling with an acid further 

 produces dextrose. 



The most important members of this group are starch, dextrine, 

 glycogen, cellulose, and gums. They are very widely distributed in 

 vegetables and constitute a most important class of food-stuffs. 



General Characters. They do not form crystals, nor, with few 

 exceptions, are they soluble in cold water. Few possess any sweet 

 taste. They do not reduce metallic oxides in alkaline solution, they 

 do not form osazones, and they cannot be fermented with yeasts. 

 They are precipitated when their solutions are saturated with certain 

 neutral salts, such as ammonium sulphate. They may be sub-divided 

 into three sub-groups, the starches, the gums, and cellulose. 



1 It is impossible to give a definite value to n because the molecular weight is 

 unknown. 



