Modification of food-processing properties of fruit

Wheat is a cereal commonly used to make bread and a number of end-products such as cakes, cookies or pastry. The adequacy of the wheat grains used for each purpose is related to two main properties of the grain that have been consequently targeted as susceptible to being modified by genetic engineering. One is the gluten protein composition that may eventually determine the viscoelasticity (elasticity and extensibility) of the dough. The other is the grain hardness that has a direct effect on the milling and baking properties of the grain.

What is known as gluten is in fact a complex mixture of up to 50 proteins most of them classified as prolamins. Among these, a group is assembled in high molecular weight (HMW) polymers whose subunit composition determines some properties of the dough, including its elasticity. The entrapment of in CO₂ the formed network favours a porous structure that determines the viscoelasticity of the dough. Since wheat is hexaploid, up to six possible genes for the HMW subunits may be expected. However, it has been demonstrated that the modification of one or two genes may be sufficient to change the properties of the dough prepared from the grain. Thus, wheat transformed with one or two HMW subunits produced grains that caused a stepwise increase in dough elasticity (Barroet al., 1997).

Recent studies have demonstrated that two wheat proteins, named puroindolines A and B, acting together in a 1:1 ratio, control to some extent wheat grain softness. In cereals with a hard texture, such as corn and rice, these two proteins are not present in the grain. To genetically modify the texture of these cereals, transgenic plants expressing the wheat puroindoline genes were obtained. The result was that rice grain softness has been increased significantly (Krishnamurthy and Giroux, 2001).

Potato constitutes a basic food in the diet of many Western countries where it is commonly consumed as potato chips. When harvested, potato tubers are frequently stored in the cold to prevent sprouting. Under this condition part of the starch of the tuber is converted into hexoses. The increased hexoses induce the sweetening of the tuber and have an adverse effect on the quality of the processed chips. The reason is that an excess of hexoses react with the amino acids during frying causing an undesirable browning of the chips. Hexoses increase in the cold stored tubers results from an imbalance between the rate of their production from starch and the rate of their degradation in the glycolytic pathway. The accumulated hexoses may be metabolically converted into sucrose that is eventually split into glucose and fructose by the action of the enzyme invertase. Several strategies have been attempted with variable success to keep the hexose level low in the cold stored tubers. The latest is based on a gene encodinga protein that inhibits the invertase enzyme (Greiner et al., 1999). Thus,in the presence of this protein, hexoses would not be produced from the hydrolysis of sucrose. Transgenic potato plants over expressing this invertase inhibitor gene reduced by 70% the hexose accumulation after the cold treatment and, as expected, browning of the chips was also prevented. Most importantly, this occurred without any changein the starchquality or quantity.

Around 30% of the starch produced in plants is used for direct human and animal consumption. The ratio between the two main components of this polymer, linear amylose and branched amylopectin, determinesits applicability. There as on is that the amylose : amylopectin ratio influences the physico chemical properties of the starch. This ratio fluctuates among different crops, between 20–30% for amylose to 70–80% for amylopectin. There has been recently a successful report on the variation of this ratio in a transgenic potato plant (Schwall et al., 2000). The species was transformed with the antisense of two genes encoding two isoforms of a starch branching enzyme. The result was that amylopectin was practically absent as a component of the starch.