What is starch?

Starch occurs as water-insoluble, semi-crystalline, osmotically-inert granules within the amyloplasts of tubers and consists of two types of glucose polymers, that is, amylose and amylopectin.

• Amylose is an essentially linear (un-branched) α-1, 4 linked glucose polymer and stacks to form tightly packed granules which are insoluble and hard to digest enzymically.  The higher the amylose content of starch the greater its resistance to digestion due to these tightly packed granules.  Amylose forms a helical complex with iodine giving a characteristic blue colour.
• Amylopectin is a much larger molecule consisting of α-1, 4 linked chains with α-1, 6 branchpoints.  It cannot form tightly packed granules and is thus easier to digest.  Amylopectin binds weakly with iodine and the complex typically gives a red/brown colour.

Potato tuber starch usually consists of ca. 80% amylopectin and 20% amylose although mutants and transgenics have been generated with almost 100% amylopectin or with ca. 70% amylose contents.  This (along with other starch properties) changes the degree of resistance to enzymic digestion and will therefore influence the Glycaemic Index (GI) of the starch.  The relationship between rate of starch digestion and GI has been established (O’Dea et al., 1981; Jenkins et al., 1982; Heaton et al., 1988; Bornet et al., 1989; Englyst et al., 1992; Granfeldt et al., 1992). 

Importance and uses of potato starch

Starch typically accounts for 75% of man's daily calorific intake (Duffus, 1984) and is therefore an important component of the diet.  The biosynthesis of starch represents one of the most agronomically important activities carried out by a range of plant taxa. Its uses are wide and varied.  For example, purified starch is used in food, adhesives, paper manufacture, textiles, cosmetics and biodegradable plastics and serves as the substrate for the production of glucose and fructose syrups. With regard to food  starches are traditionally used to improve properties such as texture, appearance, mouth-feel and shelf-life.

Starch structure is an important determinant of end use.  For example, starches with no amylose (the so-called 'waxy' starches) are particularly suited to use in the food industry as thickeners and gel-forming agents because of their ability to form clear, stable gels in solution. This is due to the fact that highly branched amylopectin has a limited capacity for hydrogen bonding (cf. amylose).  Conversely, starches with high amylose contents are better suited to use in the plastics and adhesives industries where they form hydrogen-bonded, insoluble aggregates when retrograded in solution. Retrogradation is a term used to describe the structural changes that occur in starch when it is boiled in solution, cooled and stored. Starch that re-crystallises upon cooling (and remains crystallised upon further heating) is termed retrograded.

Starch and its impact on texture

Texture plays a key role in the acceptability of food by consumers. The most important factor in texture is starch content, as this affects the way potato cells separate on cooking. For example, boiled potato texture has been shown to be significantly correlated with dry matter content, starch peak viscosity and degree of retrogradation. High starch content creates a dry texture synonymous with floury potatoes, which disintegrate during boiling and mashing.

Starch, Glycaemic Index (GI) and health

The Glycaemic Index is a ranking of carbohydrates based on their immediate effect on blood glucose (blood sugar) levels. High GI foods are believed to contribute to the development of insulin resistance, type II diabetes and obesity. High GI foods flood the body rapidly with sugar, and it is these 'bursts' which contribute to insulin resistance. Conversely, low GI foods contain carbohydrates that break down slowly and so raise blood sugar at a steady rate that sustains energy and helps the body feel full for longer.

The potato generally has one of the highest GI values of any food.  Given the popularity of the potato in the diet (contributing to ca. 15-20% of total starch intake cf. bread which contributes ca. 35%), a low GI variety would be beneficial. The nature of the cooking process and the composition of the food matrix consumed with a potato diet can influence the GI value for potato.

Objective of the work

The importance of GI is now well established but the identification of appropriate raw materials which have the necessary attributes for the production of lower GI foods is an important goal.  The objective of the work is to assess genetic variation in the GI potential of potato and to assess, temporally, changes in GI during tuber development, storage etc.  Basic information on starch structural and rheological properties may also help explain the different textural characteristics of potato cultivars.

The parameters that will be measured in comparative analysis include:

• starch and soluble sugar content
• amylose:amylopectin ratio
• amylopectin chain length distribution
• viscometric analysis
• phosphate content.

Also in vitro systems will be used to assess GI potential and to cross relate this with starch structural components. This will be augmented by studies with human volunteers.

Literature cited

• Bornet, F.R.J., Fontveille, A.M., Rizkalla, S., Colonna, P., Blayo, A., Mercier, C. and Slama, G. 1989. Insulin and glycaemic responses in healthy humans to native starches processed in different ways: correlation with in vitro α-amylase hydrolysis. American Journal of Clinical Nutrition 50, 315-323.
• Duffus, C.M. 1984. Metabolism of reserve starch. In: Lewis DH (ed). Storage Carbohydrates in Vascular Plants. Cambridge University Press, Cambridge, 231-252.
• Englyst, H.N., Kingman, S.M. and Cummins, J.H. 1992. Classifications and measurement of nutritionally important starch fractions. European Journal of Clinical Nutrition 46, Suppl S33-S50.
• Granfeldt, Y., Bjorck, A.I., Drews, A. and Tovar, J. 1992. An in vitro procedure based on chewing to predict metabolic response to starch in cereal and legume products. European Journal of Clinical Nutrition 46, 649-660.
• Heaton, K.W., Marcus, S.N., Emmett, P.M. and Bolton, C.H. 1988. Particle size of wheat, maize, and oat test meals: effects on plasma glucose and insulin responses and on the rate of starch digestion in vitro. American Journal of Clinical Nutrition 47, 675-682.
• Jenkins, D.J.A., Ghafari, H., Wolever, T.M.S., Taylor, R.H., Jenkins, A.L., Barker, H.M., Fielden, H. and Bowling, A.C. 1982. Relationship between rate of digestion of foods and post-prandial glycaemia. Diabetologia 22, 450-455.
• O’Dea, K., Snow, P. and Nestel, P. 1981. Rate of starch hydrolysis in vitro as a predictor of metabolic responses to complex carbohydrate in vivo. American Journal of Clinical Nutrition 34, 1991-1993.