28.4 Starch
28.4.1 Introduction
Starch consists of a chain of D-glucopyranosyl units. It is used in the manufacture of textiles, paper, adhesives, insecticides, paints, soaps, explosives, and derivatives as dextrins, nitrostarch and corn sugar.
Chemical formula- C6H10O5
Starch gets hydrolysed by acids, alkalis and enzymes giving dextrin, dextrose. Starch forms gels with water within several minutes at 60-80°C.
The major source of starch is maize kernels.
Process flow sheet: Illustrated in Figure.
Figure 28.2 Manufacture of starch
28.4.2 Functional role of various processes
(a) Air cleaner:-
- • At first, maize kernel containing 60-65% starch is introduced in air cleaner.
• Air is passed through the bottom of cleaner.
(b) Steeped tank:-
- • Air cleaned maize kernel is passed to steeped tank to make it soft.
• 50-55°C water and 0.15-0.3% SO2 is added.
• Here SO2 acts as a bacteriostatic.
• Steeping is done to yield high production and quality of starch.
• The residence time is 40-50 hours.
(c) Grinding mill:-
- • The soft grains are passed through coarse grinding mill to rupture the cells.
• The floating germ oil from the tank is removed.
(d) Buhrstone mill:-
- • The kernel is then wet ground in buhrstone mill.
• This mill completely disrupts the cells of endosperm and release starch granules.
(e) Nylon screen:
- • The wet ground kernel is then passed through nylon cloth.
• Water washes the starch through the screens.
• The fibre and hulls is left over on the nylon screen.
(f) Two stage centrifuge:
- • The gluten is water insoluble protein.
• Gluten is a light fraction which is separated in two-stage centrifuge.
(g) Rotary filter:
- • Water starch mixture is then passed then passed to continuous rotary filter.
• Starch is separated as filter cake which is of yellow colored and contains high amounts of protein.
• Other products can also be made by hydrolysis of starch.
(h) Dryer:
- • Starch as filter cake is dried and powdered in dryer with the introduction of steam in dryer and produced as pearl starch.