Carbohydrate Polymers 86 (2011) 1732–1738 Contents lists available at ScienceDirect Carbohydrate Polymers j ourna l ho me pag e: www.elsevier.com/locate/carbpol Diffusion and rheology characteristics of barley mixed linkage -glucan and possible implications for digestion Kinnari J. Shelat a , Francisco Vilaplana a , Timothy M. Nicholson b , Michael J. Gidley a , Robert G. Gilbert a,∗ a The University of Queensland, Centre for Nutrition & Food Sciences, Queensland Alliance for Agriculture and Food Innovation, Brisbane, Qld 4072, Australia b The University of Queensland, Centre for High Performance Polymers, School of Chemical Engineering, Brisbane, Qld 4072, Australia a r t i c l e i n f o Article history: Received 24 May 2011 Received in revised form 5 July 2011 Accepted 5 July 2011 Available online 12 July 2011 Keywords: Diffusion (1,3 1,4)--Glucan Viscosity Size exclusion Fluorescence recovery after photobleaching a b s t r a c t -Glucan is one of the most studied soluble dietary fibres, and is known for its positive effects on human health such as lowering glycemic responses and reducing serum cholesterol levels. Viscosity and diffusion phenomena are thought to play an important role in imparting these beneficial effects through interac- tions with digestive enzymes and bile salt micelles in the digestive tract. Correlations between viscosity, probe diffusivity, and molecular structure for three barley -glucans are studied here to enhance under- standing of the molecular basis for these nutritional effects. Fluorescence recovery after photobleaching (FRAP) is used to measure the diffusion coefficients of a dextran probe similar in size to both digestive enzymes and bile salt micelles in -glucan solutions. Diffusion coefficients are found to decrease with an increase in the viscosity, but showed systematic deviations from Stokes–Einstein behaviour, similar to those found for cereal arabinoxylans, and thus indicating that bulk viscosity measurements cannot be reliably used as sole indicator of diffusion processes, due to local aggregation and microviscosity effects. The diffusion coefficient values are 10–100 times slower than predicted for diffusion in the absence of -glucan, consistent with a functional role in retarding digestion and absorption processes in the small intestine. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Cell wall polysaccharides such as mixed-linkage (1,3; 1,4) - glucans and arabinoxylans are the major source of soluble dietary fibre in cereals. They are a major component of cell walls in the starchy endosperm of cereals such as oat, barley, rye and wheat (Cui & Wang, 2009; Lazaridou & Biliaderis, 2007). -Glucan is one of the most studied soluble dietary fibres, mainly due to its wide range of applications in the food industry (Brennan & Cleary, 2005) and demonstrated beneficial effects to human health (Brownlee, 2011; Jenkins et al., 1978; Wood, 1994). Barley mixed-linkage -glucan consists predominantly of linear chains of cellotetraosyl units randomly connected by cellotriosyl units (Gomez, Navarro, Manzanare, Hortab, & Carbonell, 1997a, 1997b; Woodward, Fincher, & Stone, 1983). The physicochemical proper- ties of -glucans are governed by their structural aspects, primarily average molecular weight (and probably molecular weight dis- tributions) and the ratio of cellotetraosyl (DP3) and cellotriosyl ∗ Corresponding author. Tel.: +61 7 3365 4809; fax: +61 7 3365 1188. E-mail address: b.gilbert@uq.edu.au (R.G. Gilbert). units (DP4) (Lazaridou & Biliaderis, 2007). The water solubility of -glucan largely depends on -(1–3) and -(1–4) linkages, and the presence of -(1–3) linkage increases the flexibility of the chain enhancing water solubility (Bulinga, Brant, & Fincher, 1986; Lazaridou & Biliaderis, 2007). It is now established that soluble dietary fibres such as -glucan can play an important role in improving human health and help to prevent diseases such as type2 diabetes and cardiovascular disor- ders (Brownlee, 2011). It has been observed that the presence of -glucan affects digestion by increasing viscosity of gut content (Dikeman et al., 2006), potentially resulting in slowing down the absorption of nutrients through the unstirred mucosal layer (Lund, Gee, Brown, Wood, & Johnson, 1989). For instance, it was demon- strated that in healthy human volunteers, oat -glucan reduced the postprandial glucose response to an oral glucose load simi- larly to guar gum (Wood, 1994). Blackburn and Johnson (1981) reported reduced blood cholesterol correlated with increased vis- cosity in rat models on diets containing guar galactomanan. Jenkins et al. (1978) observed high viscosity dietary fibre had positive effect on decreasing postprandinal glucose level in human subjects. In human studies, reduction of molecular size (and hence viscosity) of oat -glucan has been shown to limit the lowering of serum LDL cholesterol (Wolever et al., 2010), confirming that physicochem- ical properties of -glucan need to be taken into account when 0144-8617/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbpol.2011.07.004