The concept of resistant starch (RS) has evoked new interest in the bioavailability of starch and in its use as a source of dietary fiber, particularly in adults. RS is now considered to provide functional properties and find applications in a variety of foods. Types of RS, factors influencing their formation, consequence of such formation, their methods of preparation, their methods of estimation, and health benefits have been briefly discussed in this review.

Resistant Starch–A Review

Starch is the most widespread and abundant storage carbohydrate in plants. We depend upon starch for our nutrition, exploit its unique properties in industry, and use it as a feedstock for bioethanol production. Here, we review recent advances in research in three key areas. First, we assess progress in identifying the enzymatic machinery required for the synthesis of amylopectin, the glucose polymer responsible for the insoluble nature of starch. Second, we discuss the pathways of starch degradation, focusing on the emerging role of transient glucan phosphorylation in plastids as a mechanism for solubilizing the surface of the starch granule. We contrast this pathway in leaves with the degradation of starch in the endosperm of germinated cereal seeds. Third, we consider the evolution of starch biosynthesis in plants from the ancestral ability to make glycogen. Finally, we discuss how this basic knowledge has been utilized to improve and diversify starch crops.

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Starch: its metabolism, evolution, and biotechnological modification in plants.

Plants, green algae, and cyanobacteria synthesize storage polysaccharides by a similar ADPglucose-based pathway. Plant starch metabolism can be distinguished from that of bacterial glycogen by the presence of multiple forms of enzyme activities for each step of the pathway. This multiplicity does not coincide with any functional redundancy, as each form has seemingly acquired a distinctive and conserved role in starch metabolism. Comparisons of phenotypes generated by debranching enzyme-defective mutants in Escherichia coli and plants suggest that enzymes previously thought to be involved in polysaccharide degradation have been recruited during evolution to serve a particular purpose in starch biosynthesis. Speculations have been made that link this recruitment to the appearance of semicrystalline starch in photosynthetic eukaryotes. Besides the common core pathway, other enzymes of malto-oligosaccharide metabolism are required for normal starch metabolism. However, according to the genetic and physiological system under study, these enzymes may have acquired distinctive roles.

From bacterial glycogen to starch: understanding the biogenesis of the plant starch granule.

Improving starch for food and industrial applications.

▪ Abstract This review describes and discusses the implications of recent discoveries about how starch polymers are synthesized and organized to form a starch granule. Three issues are highlighted. 1. The role and importance of ADPglucose pyrophosphorylase in the generation of ADPglucose as the substrate for polymer synthesis. 2. The contributions of isoforms of starch-branching enzyme, starch synthase, and debranching enzyme to the synthesis and ordered packing of amylopectin molecules. 3. The requirements for and regulation of the synthesis of amylose.

The synthesis of the starch granule

The present invention relates to plant cells and plants, which are genetically modified, wherein the genetic modification leads to the expression in plastids of such plant cells and plants of an enzyme having the activity of a mutansucrase. Furthermore, the present invention relates to means and methods for the manufacture of such plant cells and plants. Plant cells and plants of this type synthesise a modified starch. The present invention therefore also relates to the starch synthesised by the plant cells and plants according to the invention as well as to methods for the manufacture of the starch and to the manufacture of starch derivatives of this modified starch.

Transformed Plant Expressing a Mutansucrase and Synthesizing a Modified Starch

This paper describes the comparison of potato amylose-free starches obtained by two different approaches: mutation induction of the gene encoding the key-enzyme in amylose biosynthesis Granule Bound Starch Synthase (GBSS) and antisense inhibition of the expression of GBSS, with that of normal amylose containing potato starch. One of the most prominent differences between the amylose-free and amylose-containing starches was their phenotype after staining with iodine. The amylose-free starch obtained via mutation induction stained completely red whereas the amylose-free starch obtained by the antisense approach always had a very small dark-blue staining core in a further entirely red staining granule. Amylose-containing starch stained blue with iodine. Differences were also observed with regard to the absence of GBSS protein and activity in the amylose-free potatoes. Furthermore the lambda max and the blue value of the amylose-free starches was markedly different as would be expected since they almost completely lacked (< 0.1%) amylose in both types of amylose-free starches. Determination of a number of different chemical parameters including particle size distribution, ion content, as well as phosphate levels of the starch indicated that these were not different in the amylose-free containing starches as compared to the normal amylose containing starches. The absence of amylose resulted in improved gel stability and paste clarity of gels even after prolonged durations at 5°C and at room temperature.

Comparison Between Amylose-free and Amylose Containing Potato Starches

One isoform of the branching enzyme (BE; EC 2.4.1.18) of potato (Solarium tuberosum L.) is known and catalyses the formation of α-1,6 bonds in a glucan chain, resulting in the branched starch component amylopectin. Constructs containing the antisense or sense-orientated distal 1.5-kb part of a cDNA for potato BE were used to transform the amylose-free (amf) mutant of potato, the starch of which stains red with iodine. The expression of the endogenous BE gene was inhibited either largely or fully as judged by the decrease or absence of the BE mRNA and protein. This resulted in a low percentage of starch granules with a small blue core and large red outer layer. There was no effect on the amylose content, degree of branching or λmax of the iodine-stained starch. However, when the physico-chemical properties of the different starch suspensions were assessed, differences were observed, which although small indicated that starch in the transformants was different from that of theamf mutant.

Introduction of sense and antisense cDNA for branching enzyme in the amylose-free potato mutant leads to physico-chemical changes in the starch

Cassava is a poor man's crop which is mainly grown as a subsistence crop in many developing countries. Its commercial use was first as animal feed (also known as tapioca), but has shifted since the late sixties to a source of native starch. The availability of native starches, which on the one hand do not require substantial chemical derivatisation and on the other hand have improved properties, would make cassava also for small farmers a potentially attractive cash crop. Since breeding is difficult in this polyploid, vegetatively propagated, crop a transgenic approach would be ideal to improve certain characteristics. We have created a cassava genotype producing amylose-free starch by genetic modification. The absence of amylose increased the clarity and stability of gels made with the transgenic starch, without requiring treatment with environment-unfriendly chemicals such as epoxides (propylene oxide, ethylene oxide) and acetic anhydride, which are normally used to improve stability. The amylose-free starch showed no changes in particle size distribution, chain length distribution or phosphorous content when compared to amylose-containing starch, but the granule melting temperature was increased by almost 2°C. Furthermore, the amylose-free cassava starch shows enhanced clarity and stability properties. These improved functionalities are desired in technical applications in paper and textile manufacturing, but also in the food industry for the production of sauces, dairy products and noodles.

Improved cassava starch by antisense inhibition of granule-bound starch synthase I

In this paper some physico-chemical characteristics of amylose-free potato starch are described and compared to those from amylose containing potato starches. The availability of these high-amylopection or amylose-free starches has opened up new possibilities to assess the relationship between structural characteristics and physico-chemical properties of potato starch. At the same time it should be possible to determine more precisely the role of the amylose in the starch on several properties and characteristics. In all different analyses it was clear that amylose-free starch displayed unique properties. Not only was the response to heating much faster; it also occurred at much higher temperatures as demonstrated by viscographs, differential scanning calorimetry and swelling experiments. Rheological analyses demonstrated that gels formed from amylose-free starch were less rigid, smoother and shorter. The breaking of amylose-free gels furthermore required much less strength than that of amylose-containing gels. This aspect was also evident when threads were made from both starches; amylose-free threads were much shorter before breaking than threads made from amylose-containing starch.

Luc C. J. M. Suurs

Papers

Transformed Plant Expressing a Mutansucrase and Synthesizing a Modified Starch

Comparison Between Amylose-free and Amylose Containing Potato Starches

Introduction of sense and antisense cDNA for branching enzyme in the amylose-free potato mutant leads to physico-chemical changes in the starch

Improved cassava starch by antisense inhibition of granule-bound starch synthase I

Some Physicochemical Properties of Amylose‐free Potato Starch