Starch alkali-lability

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. 

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. 

Through alkali lability evaluation, it becomes apparent that chemical degradation is occasioned by many of the methods employed in the past to solubilize the starch preparatory to fractionation. Thus, prolonged dry-grinding of starch in a pebble mill causes a progressive rise in alkali lability and eventually yields a product which is colored red by iodine. It is quite possible that sufficient local heat is generated by impact of... 

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. 

Samec and coworkers have differentiated between the fractions on the basis of phosphorus content. Undoubtedly, organic phosphate is associated primarily with the B-fraction of potato starch, contributing polarity to that component. In the case of corn starch, the distinction is not as sharp, since the total phosphorus content is very much lower. Also, it is possible to separate the B-fraction of corn starch into subfractions of high and low phosphorus content, and these do not differ materially in. solubility behavior or alkali lability. Thus phosphorus is believed to constitute only a minor difference between the fractions. 

Fig. 15.— Effect of Temperature and Time of Roasting on the Alkali-lability Value of Starch. (See Fig. 10 for notation.)...

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. 

No dialdehyde groups are detectable in modified celluloses obtained by oxidation of cellulose with persulfate, hypobromite, hypochlorite, permanganate, nitric acid, or dichromate. " Alkaline-hypochlorite oxidized starches, which have already been partially degraded by alkali, are more resistant to alkaline degradation than are unmodified starches, and the alkali-lability slowly decreases with increasing oxidation because of the resulting oxidation of reducing ends."... 

Fig. 1.—Formation of a White Dextrin from Corn Starch Changes in (1) Viscosity, in Centistokes, (2) Alkali Lability, and (3) Solubility, %, as a Function of the Conversion Time. (Redrawn from Ref. 5.)...

Of the esters, starch phosphate is produced by reaction with phosphorus oxychloride, polyphosphates, or metaphosphates a cross-bonded product results. Total degree of substitution is determined by measuring the phosphorus content, and the mono- to disubstitution ratio can be calculated by potentio-metric titration. Allowance is made for the natural phosphorus content of the starch. Treatment of starch with acetic anhydride produces starch acetate, which has improved paste stability over native starch. The acetyl group is very labile, and hydrolyses readily under mild alkaline conditions. When a known amount of alkali is used, the excess can be titrated and the ester function measured. This is not specific, however, and a method based on an enzymatic measurement of the acetate has been developed in an ISO work group. The modified starch is hydrolyzed under acidic conditions, which releases acetic acid and permits filtration of the resulting solution. Acetic acid is then measured by a commercially available enzyme test kit. Both bound and free acetyl groups can be measured, and the method is applicable... 

Starch adipate, a cross-bonded starch for food use, is made by reaction with adipic anhydride, which is formed from adipic acid in the presence of excess acetic anhydride. It is also a labile ester, and after hydrolysis with alkali, followed by acidification, is extracted with ethyl acetate and silylated. Gas chromatographic analysis is performed on a capillary column of fused silica coated with dimethyl siloxane, film thickness 5 pm. Pimelic acid is the internal standard. The method cannot differentiate between mono- and disubstitution. Alkyl succinate substitution can be determined using the same procedure as for adipate. 

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