Starch alkali number

Alkali Liability" of Starch or "Alkali Number" of Starch. See under Starch Alkali Metal Amides, such as potassamide (KNHj) and sodamide (NaNH2),can be obtained by the action of pure iiq NH3 on these metals. The blue solas of the metals thus formed are the result of the reaction. ... 

Fig. 5.—Variation of alkali number of potato starch as a function of duration of milling (—) under nitrogen, (-) under oxygen, and (- -) in air (from Ref. 45).

A simplified alkali lability method has been employed in the author s laboratory, whereby the rate of alkali degradation is estimated from the amount of acidic substances produced by alkali degradation. This is expressed as alkali number, and represents the ml. of 0.1 N acid formed from 1 g. of the starch substance under specified conditions of alkali digestion. It is entirely an arbitrary value, and cannot be construed as an absolute measurement of aldehyde content or of molecular weight. It serves only to indicate whether the aldehyde content of the starch has remained constant, or whether it has increased or decreased. Farley and Hixon have found that the alkali number detects hydrolytic changes in the starch before they are measurable by copper or ferri-cyanide reduction. 

Alkali lability supplied the first positive evidence of chemical differences between the starch fractions as separated by butyl alcohol. The alkali number of defatted com starch is 10-11 values for the crude A and B fractions are 25 and 5.5, respectively. The observed alkali lability of the original starch coincides with the value calculated from the percentage composition and alkali numbers of the indi idual fractions (viz., 23% X 25 -H 77% X 5.5 = 10). These values are in agreement with the presumed structure of the A and B fractions as linear and branched glucose polymers respectively. Alkali decomposition of the A-fraction should be more extensive than with the B-fraction, since the latter possesses a lower aldehyde content. Also, a branched structure may impede the decomposition in hot alkali. 

Potato starch has an alkali number of approximately 7 while values for the A and B fractions are 10 and 6, respectively. Hence there must be positive structural differences between the A-fractions of corn and potato starches. Alkali lability and ferricyanide reducing values indicate a larger molecular weight for the linear component of potato starch. 

When starch is treated with hot alkaline solutions, it behaves in the manner to be expected of a material composed of an alkali-labile fraction and an alkali-stable fraction. Starches from different sources vary in the relative amounts of these two components, and, therefore, starches can be characterized by their alkali lability. Alkali lability of a starch can be expressed as its alkali number, which is an empirical expression of the acids produced from the terminal, reducing group by the action of alkali. The alkali number (and, therefore, the alkali lability) increases progressively during acid hydrolysis. - Hence, the alkali number is an index of molecular size for linear molecules. 

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