Sunday, January 30, 2011

DEXTRIN

DEXTRIN:
      
               The term dextrin denotes a starch partly or completely broken down to water soluble bodies. White dextrin is a product in which the solubilising process is incomplete, leaving a high percentage of unconverted starch :It can be made from any type of starch .british gum is a light yellow product obtained by roasting starch at 150 deg. C.for some hours, after moistening with 0.2-0.4% nitric acid; this results in very considerable conversion of starch. It is almost always made from maize starch only. Dextrin proper is a dark co-loured,completely soluble,and completely converted form of British gum; it can be made from any type of starch.

The commercial products consists of a mixture, in varying proportions, of starch ,soluble starch, dextrin, and dextrose. They are insoluble in 50% and in 100% alcohol. The moisture of a good quality is not more than 8% and ash less than 0.5%. Only the starch free dextrins give a clear solution in water.

For most purposes in textile printing a good dark dextrin may be used at 8 IB .per gallon to replace gum senegal at 12 IB .per gallon, but the shades are somewhat yellower, as steaming "fixes" some of the yellow colouring matter , especially in wool and silks .for thickening strongly alkaline colours a maize starch gum should be used .


The analysis consists of extracting dextrose and dextrin by cold water, the residue beidue being dried and weighed as unconverted starch. the extract is boiled with fehiling's solution (i) as it stands ,and (ii) after hydrolysis with acid ; the former value gives dextrose, and the difference between the two values is calculated as dextrin.