Rapid Visco Analyser in the management of dysphagia

This article is adapted from a paper by Yun Liu (1), Naomi So (2) and Peter Sopade (1)

(1) Centre for Nutrition and Food Sciences, The University of Queensland, St Lucia Qld 4072, Australia, email: p.sopade@uq.edu.au
(2) School of Land, Crop and Food Sciences, The University of Queensland, St Lucia Qld 4072, Australia


Dysphagia or swallowing impairment, dysfunction or difficulty is a symptom of many disorders and diseases, and can lead to complications such as malnutrition, dehydration, chest infection, aspiration, pneumonia, and mortality. It affects all age groups, and it can be managed with ready-to-use thickened fluids or by adding powdered commercial food thickeners to drinks to control their viscosity. (Sopade et al., 2007).

The health and safety issues associated with thickened fluids emphasise the need to measure their rheological properties properly to effectively manage dysphagia. The RVA involves in-situ mixing, is relatively simple to operate and uses inexpensive accessories (So, 2007), making it suitable for assessing the flow properties of fluids used in managing the condition.

Materials And Methods

Three commercial food thickeners marketed in Australia were used:

  • NovartisTM: Resource Thicken-UpTM (Novartis Nutrition Inc. Police Road, Mulgrave, Vic. 3170), modified maize starch
  • QuikThikTM: QuikThikTM (Steggall Nutrition Pty. Ltd., Enoggera, Qld 4051), xanthan gum
  • SupercolTM: (Chipmonk Pty Ltd., Nambour, Qld 4560), guar gum

For each thickener, dispersions at four solids contents (%, w/w) were prepared in water and allowed to fully hydrate and equilibrate overnight. Each dispersion (25 g) was put in a canister and analysed in an RVA-4 at 30oC for 10 min. at four rotational speeds (200, 160, 100, and 50 rpm). These speeds correspond respectively to shear rates (s-1) of 16.8, 33.5, 53.6, and 67.0 (Lai et al. (2000)).

SupercolTM was studied at an additional solids content at speeds 200 and 100 rpm. Replicate dispersions were prepared, and all analyses were done in a randomised full factorial experimental design (3 x 4 x 4 x 2 + 2 x 2 = 100). Analysis of variance (ANOVA) and significant tests were performed using Minitab ver. 15.

Results And Discussion 

Figure 1a shows typical RVA viscograms of the SupercolTM at five solids contents, rotational speed 100 rpm (calculated shear rate 33.5s-1). RVA viscosity increased as the solids content was increased for all thickeners.

Figures 1b–1d show the sensitivity of the RVA to the concentrations of NovartisTM, QuikThikTM and SupercolTM, in water.

Figures 1b and 1c show that the thickened fluids reduced in viscosity (shear thinning) as the shear rate was increased.

Figure 1d shows the relationship between solids content of NovartisTM, QuikThikTM and SupercolTM, and the interpolated viscosity at a shear rate of 50s-1, which is usually assumed to be the swallowing shear rate. 

Conclusion

The RVA can be used to assess the viscosity of thickened fluids used in dysphagia management, at relevant solids contents and shear rates. (Estimated swallowing shear rate, 50s-1, is equivalent to an RVA stirring speed of 150 rpm.)

The RVA can be used to assess the three common categories of thickened fluids: Level 150 – mildly thick (nectar, ¼-thick); Level 400 – moderately thick (honey, ½-thick); Level 900 – extremely thick (spoon-thick, pudding-thick, full-thick), widely used to manage dysphagia.

The RVA can be used with other dispersing media such as juices, milk, tea, and coffee which are commonly used to prepare thickened fluids for dysphagic individuals.

The RVA viscosity of the thickened fluids studied exhibited the same relationships with the rate of shear and solids content as those obtained with rheometry, however, the RVA offers advantages such as in-situ mixing, relatively simple operation and inexpensive accessories. 

Acknowledgements

The authors are grateful to the manufacturers of the ready-to-use thickened fluids and food thickeners and Dr Julie Cichero of the School of Medicine, The University of Queensland, St Lucia Qld 4072, Australia.

References

Lai, K. P., Steffe, J. F. and Ng, P. K. W. (2000). Average shear rates in the rapid visco analyser (RVA) mixing system. Cereal Chemistry 77: 714–716.
So, N. (2007). Effects of premix conditions on (RVA) rheology of food thickeners used in managing dysphagia. Unpublished Honours thesis. School of Land, Crop and Food Sciences, The University of Queensland, St Lucia, Australia