Introduction

Recently there has been increasing interest in the effects of fibre on glycaemic control and satiety. The efficacy of these fibres in-vivo appears to be related to their effect on luminal viscosity (Brennan et al.1996). Previous work has indicated that measuring viscosity in-vitro may provide beneficial information in predicting the impact of fibres in the human digestive tract (Gee & Johnson.1985). However, little has been published about the effects of food processing and gastric conditions on fibre viscosity.

In this work, the Rapid Visco Analyser (RVA) was used to evaluate the effect of different fibre sources on viscosity in both starch and sugar systems. The fibre – starch/sugar systems were heated and cooled to represent food processing conditions and were then subjected to simulated gastric conditions to determine the effect of both processing and gastric conditions on the viscosity of the fibres.

Materials & Methods

Ingredients

Six (6) fibres were obtained from commercial suppliers for evaluation. These were guar gum, inulin, pectin, xanthan gum, oat fibre and psyllium husk. The starch used was commercial wheat starch and the sugar was dextrose. Hydrochloric acid (HCl) was injected to simulate gastric conditions.

RVA Analysis

RVA analysis was conducted using a Series 4 Rapid Visco Analyser (RVA) (Newport Scientific, Warriewood). The profile consisted of heating to 90°C, cooling to 37°C followed by an injection step and further stirring at 37°C. The analysis profile measured the peak viscosity during heating, the viscosity after cooling and the final viscosity after stirring with HCl added.

The fibres were evaluated at a 1% addition level (0.25g fibre/25ml distilled water). To ensure even dispersion the fibres were blended with starch or dextrose (1.75g) prior to analysis. All analyses were conducted in triplicate using both starch and dextrose in a randomised order.

Results & Discussion

RVA Results using Wheat Starch

The results using the wheat starch base (Figure 1) showed great variability in viscosity across the different fibre types. Guar gum was noted to have the highest viscosity during heating and cooling and increased in viscosity with acid addition.

Xanthan gum (Figure 1) had a similar viscosity to other fibres during heating and cooling but showed a large increase in viscosity after acid addition. All of the other fibre types showed an increase in viscosity after heating, cooling and HCl addition, as compared to the wheat starch control, however the increase in viscosity was much smaller than with guar and xanthan gum.

Figure 2: Viscosity of dextrose & fibre after heating, cooling & HCl addition

RVA Results using Dextrose

The results using dextrose (Figure 2) showed some interesting contrasts to the wheat starch samples. As for the wheat starch, with the dextrose the guar gum was noted to significantly increase in viscosity during heating, cooling and after HCl addition. Xanthan gum showed a slight decrease in viscosity after cooling with dextrose, but still achieved a high viscosity with HCl addition. All of the other fibre samples showed a decrease in viscosity when HCl was added as opposed to the increase seen in the wheat starch samples. This was most significant with the psyllium husk which showed a much larger increase in viscosity with the dextrose after cooling but showed a large drop in viscosity when the HCl was added. This result is significant as it indicates that the efficacy of psyllium husk may be affected by the form in which it is consumed (e.g. in a starch based product versus a glucose drink).

Conclusions

The results of this work highlight that different fibre sources have different responses to food processing and also show great variability in viscosity under simulated gastric conditions. Some fibres, such as guar gum and xanthan gum, increase in viscosity during food processing and in simulated gastric conditions whereas others, such as psyllium husk, are viscous in food products but lose their viscosity in gastric conditions. Addition of fibres to starch rich foods versus sugar rich foods is also expected to affect gastric viscosity as is seen by the differences in viscosity responses using starch versus dextrose. Although this work was conducted using a simplified model of the digestive system the results do suggest that the RVA might be a useful tool for the in-vitro prediction of gastric viscosity.

Acknowldegements

We would like to thank the Grain Foods CRC for their support with this project.

References

Brennan, C. S., Blake, D.E., Ellis, P.R. & Schofield, J.D. (1996) Journal of Cer. Sci. 24: 151-160.

Gee, J. M. & Johnson, I. T. (1985) Journal of Sci. Food & Agric. 36: 614 -620.