Granular potato starch and amylopectin potato starch were methylated in aqueous suspension to molar substitutions (MS) up to 0.29. A method was developed to determine the MS of both branched and linear regions. After exhaustive digestion of the methylated starches with -amylase, the highly branched fraction with a degree of polymerisation (DP) > 8 was separated from the linear oligomers by selective precipitation of the former in methanol. The substitution levels of branched and linear regions were determined. It was found that methylation takes place preferably at the branched regions of amylopectin and that amylose is higher substituted than linear regions of amylopectin. The distribution of methyl substituents in trimers and tetramers was determined by FABMS and compared to the outcome of a statistically random distribution. The results provided evidence for heterogeneous substituent distributions. Quanti®cation of the degree of heterogeneity of the branched and linear regions showed a much larger deviation from random distribution in the linear regions. #

Granular potato starch and amylopectin potato starch were methylated in aqueous suspension with dimethyl sulfate to molar substitution (MS) up to 0.29. The percentage of amorphous starch compared with crystalline domains increased with increasing MS. Prolonged treatment of these methylated starches with hydrochloric acid below the swelling temperature resulted in the release of D-glucose and small D-glucose-oligomers from the amorphous domains. The granular structure was maintained during the acidic treatment, indicating that the crystalline lamellae were less affected by acid. The amorphous domains contained on average about twice as many substituents per glucose unit as the remaining crystalline network. The distributions of methyl substituents in trimers and tetramers, released from amorphous domains and prepared from crystalline fractions, were determined by FABMS and compared to the outcome of a statistically random distribution. Quantification of the degree of heterogeneity of the thus-obtained trimers and tetramers showed a much larger deviation from random substitution in the crystalline fractions compared with the amorphous ones. These results are in agreement with our previous study that describes substitution patterns in branched and linear regions of methylated starch granules.

Granular potato starches were methylated in aqueous suspension with dimethyl sulfate to molar substitution (MS) values up to 0.29. Fractions containing mainly amylose or amylopectin were obtained after aqueous leaching of the derivatised starch granules. Amylopectin in these fractions was precipitated with Concanavalin A to separate it from amylose. Amylose remained in solution and was enzymatically converted into D-glucose for quantification, thereby taking into account the decreased digestibility due to the presence of methyl substituents. It was found that the MS of amylose was 1.6-1.9 times higher than that of amylopectin in methylated starch granules. The distributions of methyl substituents in trimers and tetramers, prepared from amylose-or amylopectin-enriched fractions, were determined by FAB mass spectrometry and compared with the outcome of a statistically random distribution. It turned out that substituents in amylopectin were distributed heterogeneously, whereas substitution of amylose was almost random. The results are rationalised on the basis of an organised framework that is built up from amylopectin side chains. The crystalline lamellae are less accessible for substitution than amorphous branching points and amylose.

No abstract

One of the major techniques to modify starch is crosslinking [1,2,3]. Cross-linking reinforces the granule in such a way that on swelling the integrity of the granule is maintained [2]. The technique is used when a stable, highviscosity starch paste is needed and particularly when the dispersion is to be subjected to high temperature, shear or low pH [3]. Cross-linked starches thus provide high-viscosity thickeners, a short salve-like paste texture as well as resistance to viscosity breakdown and to loss of texture in acidic media. Cross-linking occurs upon treating starch with multifunctional reagents, thus introducing intermolecular bridges. The very low degree of cross-linking makes direct chemical analysis of the product very difficult [4]. Therefore, characterization and quality control is usually based upon the measurement of physical properties such as viscosity, swelling power, solubility pattern and resistance to shear [3]. Physical properties are often measured by using the Brabender Viscograph [2,3], an widely-spread apparatus used in the starch and food industry. The viscograph applies a well defined heating program to a starch/water suspension and registers the resistance to a fixed rotation speed. The Brabender viscogram gives valuable information for product quality assessment, however, usual Brabender methods without cooling take 50 min analysis time [5]. Because the production time of cross-linked starch is only several hours, the Brabender Viscograph is less suited for process control.The Rapid Visco Analyser (RVA) is a viscograph which applies a heating program to a starch/water system comparable to the Brabender Viscograph. However, because of the small sample volume (28 ml in comparison with 450 ml) the RVA can heat the starch suspension faster, resulting in an overall program of 10 min. The speed does not affect the accuracy, because in major parts of the RVA viscogram the relative standard deviation is within 2 % [5,6], which is comparable with the Brabender Viscograph [6]. Since the RVA is fast, it is interesting to find out in what way RVA and Brabender are related.