The effect of food processing on the glycaemic index

Available carbohydrates are those absorbed via the small intestine and used in the metabolism (Livesey, 2005). Indigestible carbohydrates, on the other hand, are considered to be dietary fibre, which include non-starch polysaccharides (mostly of plant and algal origin), resistant starches (RS), oligosac-charides and sugar alcohols (polyols) (Champ et al., 2003). Older methods for measuring dietary fibre did not measure these indigestible carbohydrates completely, leading to an underestimation of the true content of unavailable carbohydrates in foods, and this may have led to inaccuracies in published GI values for foods (Foster-Powell et al., 2002). However, the majority of commercial foods included in the international GI tables contain low levels of these sources of indigestible carbohydrates.

In addition to these problems with analysis, the amount of RS in foods can be influenced by several factors, including processing and preparation methods. Starch can be indigestible due to its botanical structure, or become resistant during processing by retrogradation (the formation of indigestible crystalline structures). The degree of ripening of fruits and vegetables is another variable. For example, a green banana has a very high content of RS, but only negligible amounts remain after ripening (Brouns et al., 2005).

Today, most of the variables that contribute to differences in GI between foods have been identified, and can be used to optimize the GI of commercial food products (see Table 3.2). Some of these factors are related to the choice of raw material and others to the processing conditions. In general, the structure of the food is important. The gross structure can be influenced by grinding or heat treatment; the more homogenized the food, the higher the GI. Cell wall integrity and/or cellular structure changes during the ripening process, and the GI increases with increased ripeness. With respect to starchy foods, a high degree of crystallinity within the starch substrate will favour a lowered rate of amylolysis, and hence a lower GI. A highly ordered starch structure can be obtained by preserving the starch crystallinity present in native granules, i.e. avoiding gelatinization. In most ready-to-eat food items, the starch crystallinity is generally lost as the commonly applied food processing conditions result in more or less complete gelatinization. A tool to increase the crystallinity of processed foods is to promote retrogradation of gelatinized starch. In this respect, the genotype of the raw material can influence the glycaemic response. In starches, the retrogradation of the amylose component but not the amylopectin component can be readily obtained under commonly used conditions of food processing. This makes starches containing high amounts of amylose particularly interesting in this regard.

An enzymatic barrier may be induced by a highly organized food form such as that found in pasta at the molecular level or at the tissue level in leguminous and kernel-based products. The presence of viscous dietary fibre may also reduce the glycaemic response to a carbohydrate meal; the

Table 3.2 Variables affecting the GI of foods and meals (Arvidsson-Lenner et al., 2004)

Food variable „ ^ r b Effect factors

Structure

Gross structure

Cellular structure (cell wall integritiy)

Starch

Granular structure (intact or gelatinized)

Amylose (unbranched)

Amylopectin (branched)

Other factors

Gel-forming types of dietary fibre

Organic acids

Amylase inhibitor

Fructose: glucose ratio

Grinding, heat treatment Ripeness

Heat treatment Genotype of raw material Genotype of raw material

Genotype of raw material, added fibres Fermentation, added acids Heat treatment Genotype of raw material, type of added sugar

Higher GI when homogenized Higher GI with increased ripeness

Higher GI when gelatinized Lower GI compared with amylopectin Higher GI compared with amylose

Lowers GI

Lowers GI Lowers GI

Lower GI with increased ratio mechanism is likely to be due more to reduced gastrointestinal motility than to a reduced rate of starch digestion. Certain organic acids, such as those produced upon sourdough fermentation, may reduce glycaemia either by reducing gastric emptying rate or by reducing the rate of starch digestion. This effect of organic acids has renewed interest in the nutritional benefits of food fermentation. A reduction of the GI for starchy food products appears to be accompanied by a higher content of resistant starch. Food factors that reduce the rate of starch digestion, such as retrogradation of the amylose component or encapsulation within botanical structures, may render a starch fraction resistant to amylase.

For foods high in simple sugars, GI is strongly influenced by the fructose : glucose ratio; the higher the ratio of fructose : glucose, the lower the GI. The GI of sugary foods can therefore be modified by the choice of raw material or through the type of added sugar. The addition of fructose (GI = 19) will lower the GI of a food, whereas addition of glucose (GI = 100) will elevate it (Bjorck et al., 2000; Arvidsson-Lenner et al., 2004). Fat, by slowing gastric emptying, and protein, by increasing insulin secretion, may both modify the glycaemic response to a carbohydrate food. However, it appears that fat and protein in the amounts found in most foods (with the exception of peanuts and most nuts) do not significantly alter the glycaemic response to the carbohydrate (Wolever et al., 1994).

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