Food processing and industrial uses

Starch is the primary storage compound in tubers and starchy foods are the world’s most abundant staples. It is the most important source of calories in the animal and human diet and provides a starter material for the preparation of more than 500 different commercial products. The physical properties of starch vary with plant source but there are considerable opportunities to generate novel starches for use in food and non-food market sectors. Genetic engineering has already generated novel potato starch, including high amylopectin starch (with no apparent yield penalty) through the down-regulation of the granule bound starch synthase gene which controls amylose synthesis (Visser et al. 1991; Kossman and Lloyd, 2000 and references therein). High amylose starch is also in great demand by the starch industry for its unique functional properties, but very few high amylose crops are available. Scwall et al. (2000) showed that concurrent down-regulation of two starch branching enzymes, A and B, in potato tubers modifies both starch grain morphology and composition and produces a significant increase in amylose content.

Stark et al. (1992) increased the starch content of tubers by expressing an E. coli glgC16 gene which encodes for the enzyme ADPglucose pyrophosphorylase.

The corresponding potato enzyme resides in the starch granule and plays a key role in starch biosynthesis. The E. coli enzyme is not regulated by the same fine control mechanisms which operate on the endogenous potato enzyme and is therefore able to increase the production of ADPglucose which becomes incorporated into the growing starch granule. Tuber starch content can be increased by up to 25% in some glgC16 expressing lines but the response appears to be genotype dependent. These high starch potatoes also accumulate lower levels of reducing sugars (glucose and fructose) in stored tubers which is highly relevant to the requirements of the processing sector. The processing industry requires low reducing sugar levels in tubers as these sugars are primarily responsible for non-enzymic browning through a typical Maillard reaction which occurs at the temperatures required to generate potato chips (crisps) and French fries.

Ideally, the industry would like to store tubers at low temperature ca. (4°C) to minimise sprout growth and eliminate the need to use chemicals to suppress the sprouting process. However, low temperatures induce glucose and fructose accumulation. Success in minimising sugar accumulation using transgenic approaches have come from the use of the glgC16 gene and from modifying the expression of genes in pathways of primary carbohydrate metabolism, e.g., by minimising the conversion of sucrose to glucose and fructose by expressing invertase inhibitor protein (Greiner et al. 1999). More detail on the control of sugar accumulation starch biosynthesis and potato quality can be found in Davies and Viola (1992), Davies and Mackay (1994), Davies (1996) and Davies (1998).