Abstract

Media summary

Key words

GI, glycaemic index, wheat grain screenings.

Introduction

The composition of starch in food is partly responsible for the glycaemic index (GI), which has become an important measure of the ability of a food to cause a spike in blood glucose and, in sufferers of diabetes, the dangerous post-consumption glucose low. Rapidly-digestible starch has been positively correlated with GI, whereas slowly-digestible starch is negatively correlated (Englyst et al., 1999; Araya et al., 2002; Englyst et al., 2003). Therefore, agricultural commodities (especially grains) with a higher proportion of starch present as the slowly-digestible fraction are preferable for the production of low GI foods. While many comparisons have been made between different foods in terms of GI (Foster-Powell et al., 2002) and studies have indicated how environmental conditions can affect total starch content in wheat (Jenner et al., 1991), there is no information available on how environmental conditions may alter the proportions of slowly- or rapidly-digestible fractions, and hence the GI.

Jenner et al. (1991) reported that stressful growing conditions (e.g. drought or high temperature) caused a decrease in total starch deposition and often resulted in a greater proportion of small or shrivelled grain. The aim of this study was to determine if shrivelled grain differs in starch composition to plump grain grown under the same conditions.

Methods

Grain

Three historical wheat varieties (Baroota Wonder, Federation, and Gabo) were grown in multiplication plots at the same site near Roseworthy, SA, in 1997 under rainfed conditions, with annual rainfall of 447 mm. Grain was harvested with a small plot harvester and stored at 4⁰C before three samples were prepared for comparison, i.e. a composite comprising plump, shrivelled and broken grain; plump grains that would not pass through a 2 mm sieve; shrivelled grain which was manually sorted from the broken grains present in the screenings. Samples were air dried for 48 hr and then ground for 10 seconds using an IKA mill.

Starch analysis

The rapidly- and slowly-digestible starch fractions were determined in duplicate based on the enzymatic method of Englyst et al., (1992). Samples (approximately 1 g) plus sodium maleate buffer (pH 6.0) and alpha-amylase, invertase and amyloglucosidase, were horizontally shaken (with glass balls) at 37°⁰C. Sub-samples were taken at 20 min (to determine rapidly-digestible starch) and 120 min (to determine slowly-digestible starch) and mixed with ethanol. Glucose contents were determined using the Megazyme glucose assay, incubating with the reagent for 20 min at 40°C and reading absorbance at 510 nm.

Free glucose was determined based on Englyst et al., (1992) by placing samples (approximately 1 g) plus sodium maleate buffer (pH 6.0) in 100°C water bath for 30 min, then shaking horizontally (with glass balls) at 37°C after the addition of invertase. A sub-sample was mixed with ethanol, and glucose content determined.

Rapidly-digestible starch (g/100 g) was calculated as the glucose determined in the 20 min sample minus free glucose, and multiplied by 0.9 to convert from free glucose to anhydroglucose as present in starch. Total digestible starch (g/100 g) was calculated similarly, but using the glucose determined in the 120 min sample instead of the 20 min sample. Slowly-digestible starch is the difference between the total digestible starch and the rapidly-digestible starch.

Data were analysed using 2-way analysis of variance (without blocking) in Genstat 6.

Results

The shrivelled grain contained significantly less rapidly-digestible starch than both the plump and the composite samples (Table 1). However, there was no significant difference between the grain sizes in terms of slowly-digestible starch. This resulted in a lower proportion of the digestible starch present as rapidly-digestible starch in the shrivelled grain. Free glucose was less than 2 % in all samples.

Table 1. Starch composition of shrivelled wheat grain, compared with plump and composite samples. Standard errors are included in the parentheses.

There was no significant difference in rapidly-digestible or slowly-digestible starch between varieties (0.05<P<0.1), but the proportion of total digestible starch present as the rapidly-digestible fraction was significantly greater (P<0.05) in Baroota Wonder than Federation, with Gabo intermediate (Figure 1). There was no significant interaction between variety and grain size.

Figure 1. Rapidly-digestible starch (RDS) expressed as a percentage of the total digestible starch for plump and shrivelled wheat and a composite sample, for varieties Baroota Wonder (BW), Federation (F) and Gabo (G).

Discussion

These results indicate that shrivelled grain would have a lower GI than either plump grain or a composite sample grown under the same conditions. However, other factors than carbohydrate composition, such as rate of gastric emptying and fibre content of the meal contribute to the GI value of food (Foster-Powell et al., 2002). Consequentially, the lower GI value of shrivelled grain could only be validated by in vivo trials. If such trials were positive, an opportunity might exist for the use of shrivelled grain in the production of low GI foods, rather than its current use as a low value product, such as chicken feed (or equivalent).

Grain affected by drought, high temperature or frost can contain a greater proportion of shrivelled grains than grain produced in more ideal conditions. It is possible that grain produced in drier climates, where protein content is increased due to a reduction in starch deposition (Jenner et al., 1991), may also have a similar starch profile to shrivelled grain, but this needs to be investigated.

The varietal difference in the proportion of total digestible starch present in the rapidly-digestible fraction indicates that there is potential for identifying certain varieties of wheat with lower GI so that the best grain for low GI products could be sourced (e.g. in this study, shrivelled Federation (Figure 1)). However, to be useful this will have to be shown in varieties that are currently grown.

Conclusion

Due to the lower proportion of rapidly-digestible starch in shrivelled grain, there is potential to value-add shrivelled grain for use in the production of low GI foods.

Acknowledgements

Funding for this work was provided by the Australian Government Department of Agriculture, Fisheries and Forestry in the form of a 2003 Science and Innovation Award for Young People in Agriculture, Fisheries and Forestry.

References

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Englyst HN, Kingman SM and Cummings JH (1992). Classification and measurement of nutritionally important starch fractions. European Journal of Clinical Nutrition 46, S33-S50.

Englyst KN, Englyst HN, Hudson GJ, Cole TJ and Cummings JH (1999). Rapidly available glucose in foods: an in vitro measurement that reflects the glycemic response. American Journal of Clinical Nutrition 69, 448-454.

Englyst KN, Vinoy S, Englyst HN and Lang V (2003). Glycaemic index of cereal products explained by their content of rapidly and slowly available glucose. British Journal of Nutrition 89, 329-340.

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