Function of the Fractions in Starch Paste Behavior

The various industrial uses of starch depend in large measure on the properties of the individual fractions. A starch paste may function in two ways it may be used for its flocculating, its adsorptive, or its gel qualities or it may act as a sol or protective colloid. These properties derive directly from the A and B fractions, respectively. In many instances, it appears that only one action is required, and that the presence of the other starch component may even be detrimental. Thus, Hixon and Sprague have reported superior qualities for waxy maize starch in those applications for which stability and protective colloid action are requisite. 

Boiled starch pastes tend to gel on cooling. This action must be attributed primarily to the linear A-fraction which has been partially leached from the swollen granules and which subsequently sets up an interlacing network within and about the granules. The various starches exhibit different gelation tendencies thus, corn starch (especially when defatted) gives a firm gel, while potato and tapioca pastes are weak and slimy. These variations may be due in part to differences in content 

In this survey, gelation is considered as a rapid and random association of linear molecules, while retrogradation is comparatively slow and crystalline. The differentiation is an arbitrary one, since the same forces are responsible. Thus corn and wheat starches are prone to retrograde as compared with tapioca and potato, while waxy maize is completely stable. 

When the A-fraction from corn starch is potentiometrically titrated with iodine, its affinity for the latter is sharply reduced in the presence of small amounts of fatty acid. Thus the iodine adsorption of recrystallized A-fraction (originally 18.7%) is reduced to 12.4%, 3.5% and 0% by the addition respectively of 2%, 5% and 10% of palmitic acid. Raw corn starch contains approximately 0.66% of fatty acids, corresponding to approximately 2% on the basis of the linear A-fraction. Thus, a third of the linear component in raw com starch is inactivated. In a sense, Taylor and coworkers were correct in assuming an association between a-amylose and fatty acid, but they erred in presuming the combination to be an ester. 

The higher fatty acids act as fractional precipitants for the A-frac-tion. Thus when an autoclaved sol of defatted corn starch is cooled 

Dunlap and C. O. Beckmann Paper presented at the Pittsburgh meeting of the American Chemical Society, September, 1943. 

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