Improving starch for food and industrial applications
Introduction
Starch is the most abundant storage reserve carbohydrate in plants. It is found in many different plant organs, including seeds, fruits, tubers and roots, where it is used as a source of energy during periods of dormancy and regrowth. Many of these starch-storing organs — for example, the grains of maize and rice or the tubers of cassava and potatoes — are staple foodstuffs in the human diet. Increasingly, starch is also used as a renewable raw material, as a source of energy after conversion to ethanol, and for many different industrial applications [1]. Starch is a versatile and useful polymer not only because it is a cheap, natural material but also because of the ease with which its physicochemical properties can be altered through chemical or enzyme modification and/or physical treatment. In recent years, our understanding of starch structure and our knowledge of the enzymes that are involved in starch biosynthesis has increased greatly, many of the genes that encode these enzymes have been cloned 2., 3.•. Most crops are now amenable to transformation by Agrobacterium tumefaciens, and so it has become possible, using genetic modification techniques, to alter the expression of individual starch biosynthetic genes and to study the effect of such changes on starch structure. There is now great potential for creating designer starches that have novel functionalities, and this review describes some of the recent advances in this field. I focus on functionality, concentrating on tuber starches from potato but describing developments in the use of cereal starches where appropriate.
Section snippets
Overview of starch structure and use
Starch is a relatively simple polymer composed of glucose molecules that are linked together in two different forms. Amylose, which makes up 20–30% of normal starch, is an essentially linear molecule in which the glucose units are joined end-to-end by α1-4 linkages. Amylopectin is the major component of starch (comprising 70–80%) and is a much larger branched molecule in which about 5% of the glucose units are joined by α1-6 linkages. Starch synthases catalyse the addition of glucose units from
Amylose-free (waxy) starches
Mutation of the Waxy locus [5], which encodes the GBSS protein, creates a starch that has no amylose. waxy mutants of maize, barley, sorghum and amaranth have been known for many years, but only waxy maize is grown on a commercial scale. The lack of amylose in waxy corn starch means that it gelatinises easily, yielding clear pastes that will not gel. It is used as a stabiliser and thickener in food products and as an emulsifier for salad dressings. A waxy wheat mutant was bred recently by
High-amylose starches
High-amylose starches are of great interest [17]. Commercially, at least 20 000 hectares of maize with amylose levels within starch of 50%, 70% and more recently 90% [18] are grown in the USA under contract each year. In the food industry, the high gelling strength of these starches makes them especially useful for producing sweets. The film-forming ability of these starches keeps the coating on fried products crispy and reduces their fat uptake upon cooking. High-amylose starches can be
Altered amylopectin structure
Amylose synthesis requires just a single gene, whereas amylopectin synthesis requires the concerted action of several enzymes, including starch synthases, branching enzymes and debranching enzymes, each of which has multiple isoforms [31•]. It is not surprising, therefore, that attempts to create novel amylopectin-type functional starches by downregulating single enzymes have not been very successful. For instance, dramatic reductions in SBEI in potato have no effect on amylopectin content or
Phosphate content
The only naturally occurring covalent modification of starch is phosphorylation, and the level of phosphorylation strongly influences the physical properties of starch [40]. Potato starch has the highest level of phosphate among commercial starches, and this is in part responsible for the high swelling power and stable-paste properties of this starch. The enzyme responsible for the incorporation of phosphate groups into starch was discovered in potato [41], and has recently been identified as
Granule size and number
Very little is known about what controls the size of starch granules and there is a huge variation in the granule sizes of starches from different species. Granule size is an important factor for many applications; for instance, in determining noodle quality [45]. Many of the modified potato starches that are produced by the downregulation of starch biosynthetic genes have altered granule morphology and number 15.••, 35., 36., 37.. Studies of cereal mutants have recently suggested that
Other uses
There are many other potential uses of starch but space constraints prevent me from discussing these in detail. Recent examples include their use as delivery vehicles that protect pharmaceutically active proteins from digestion [48], as microencapsules for small molecules [49] and as biodegradable films 50., 51.. Advances are also being made in processing starch by the expression of thermostable starch-degrading enzymes, which enable the complete digestion of the starch within hours of
Conclusions and future advances
Even with the remarkable advances that have been made in the past ten years, we still have an incomplete understanding of starch biosynthesis and there are many gaps in our knowledge of the relationship between the structure and function of starches. Nevertheless, some novel starches with significantly enhanced functionality have been produced. We can only hope that they will get through the regulatory process and be used to provide consumers with a choice of improved-quality products. A method
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
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of special interest
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of outstanding interest
Acknowledgements
I wish to thank my former colleagues at Unilever and National Starch for their contributions to the work on potato starch modification, and I wish to dedicate this review to the memory of Niklas Holmberg. I am grateful to Julie Ralfs for the photo in Figure 3 and to Jutta Tuerck for helpful comments. I apologise to colleagues whose work I have not cited because of space limitations.
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