Modified starches production methods

Starch is an abundant, inexpensive polysaccharide that is readily available from staple crops such as com or maize and is thus is mostly important as food. Industrially, starch is also widely used in papermaking, the production of adhesives or as additives in plastics. For a number of these applications, it is desirable to chemically modify the starch to increase its hydrophobicity. Starch modification can thus prevent retrodegradation improve gel texture, clarity and sheen improve film formation and stabilize emulsions [108], This may, for example, be achieved by partial acetylation, alkyl siliconation or esterification however, these methods typically require environmentally unfriendly stoichiometric reagents and produce waste. Catalytic modification, such as the palladium-catalyzed telomerization (Scheme 18), of starch may provide a green atom-efficient way for creating chemically modified starches. The physicochemical properties of thus modified starches are discussed by Bouquillon et al. [22]. 

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 Derivatives. The most important ester is starch acetate it is obtained according to the same method as that leading to cellulose acetate. The gradual substitution of acetate groups for hydroxyls decreases the hydrophilic-ity of modified starch even at low degrees of modification, the hydrosolubility in hot water disappears and products with D.S.> 1.5 become soluble in organic solvents. 

The method of preparation was modified somewhat by Alexander, Dimler and Mehltretter, a product of higher purity thereby being obtained. For the pyrolytic vacuum distillation, the same apparatus was employed as for the preparation of levoglucosan and l,6-anhydro- 8-D-glucofuranose from starch. The use of an electrically heated jacket instead of a burner for heating the flask resulted in better control of the reaction. In addition, the dextrorotation of a solution of the sirupy distillate was about twice that reported by Hann and Hudson for their product from the same weight of galactose. 

The latter method has the advantage of recycling the natural or waste product. Significant examples are cotton modified with amines [3, 4], starch utilized in xanthate form [5, 6, 7], thiol type keratine gel prepared from the wool [8, 9,10], wool simply reduced [11] or modified with amine groups [12, 13, 14] and finally polysaccharides such as cellulose [15, 16, 17], glucose [18] or polyosides [19] modified with thiol or amine groups. 

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