Cross-Bonded Starches

Cross-bonded starches can also be manufactured by reaction with trimetaphosphates (115), but these require more vigorous conditions than phosphoms oxychloride. Typically, a starch slurry and 2% trimetaphosphate salt react at pH 10—11 and 50°C for 1 h. 

Figure 8-35 Viscosity and Granule Appearance in the Viscoamylograph Test of a 5% Suspension in Water of Waxy Corn with 1 Cross-Bond per 100,000 Glucose Units. A = viscosity curve, B = granule appearance. Source Reprinted from L.H. Kruger and R. Murray, Starch Texture, in Rheology and Texture in Food Quality, J.M. deMan, P.W. Voisey, V.F. Raspar, and D.W. Stanley, eds., 1976, Aspen Publishers, Inc.

The starch content of a natural product or of a man-made product may be required. The latter products might contain modified starches, and a limited number are permitted for food purposes acetate, adipate, succinate, oxidized, hydroxypropyl. For nonfood applications cationic and carboxymethyl starches are in common use. Polarimetric methods, when applied to such modified starches, reveal that calcium chloride dissolution is better than the Ewers method. Even with cross-bonded starches, which are difficult to solubilize, complete dissolution is achieved within 30 min with calcium chloride solution. Recovery, based on a universal optical rotation of 203°, is over 95%. While this application of the method needs further verification, several laboratories in an ISO work group report similar findings. 

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. 

Type IV resistant starches are produced by promoting the modification of the granular structure that results in glycosidic linkages other than a-1, 4, or 1-6. This category also includes cross-bonded starches that are resistant to amylases. The health implications of the different types of resistant starches are discussed in Chapter 17. 

Cellulose. Cellulose or starch xanthate cross-linked by titanates can adsorb uranium from seawater (536). CarboxymethylceUulose cross-linked with TYZOR ISTT is the bonding agent for clay, talc, wax, and pigments to make colored pencil leads of unusual strength (537). 

Starch molecules have many exposed O—bonds, so this phosphorylation reaction results in multiple phosphate groups attached to each starch molecule. The remaining —OH group on each phosphate can condense with an O— H bond on another starch molecule. This cross-linking of starch chains gives the desired thick consistency of puddings and pies. 

Repeat the activity using warm water instead of acetone. When water is used, the starch peanut dissolves and the polystyrene peanut does not. The acetone and polystyrene do not exhibit hydrogen bonding, while starch peanuts and water do exhibit hydrogen bonding. This illustrates the general principle of like dissolves like. Not all the polystyrene dissolves because of the presence of cross-linked units that form the polystyrene. 

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