Abstract

The measurement of digesta viscosity is difficult due to settling and compositional changes that occur during digestion. The current work determined whether the relative viscosity (ηr) of digesta could be accurately determined from DMC, ɸmax, and ɸ using the Maron-Pierce equation with ɸmax being derived from the aspect ratios (Rs) of the particulate fraction.

The relationships between ηr, DMC, R, ɸ, and ɸmax were verified using data obtained from suspensions of plant fibres in a Newtonian liquid (70% aqueous fructose solution). The plant fibres were used after in-vitro digestion; in all cases the

viscosity of the fibre suspension was similar to that of digesta from the small intestine of the pig.

The concentration of solids in all suspensions was expressed as DMC and ɸ, which were quantified using a combination of centrifugation, washing, drying, and gas pycnometer. The Rs of spherical glass beads and the food fibres after in-vitro

digestion varied between 1 and 6, determined using imaging analysis. Viscosity measurements were carried out in a rotation mode using cup and vane geometry at a shear rate of 100 s-1. The relationship between the relative viscosity (ηr), and the ratio ɸ/ɸmax was fitted using the Maron-Pierce equation (Equation 2.3).

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127 Multiple stepwise regressions showed that the Maron-Pierce equation characterised the viscosity of digesta well, and strongly suggests that digesta behaves as a particulate suspension especially when ɸ ≤ 0.20 or ɸ/ɸmax ≤ 0.6. The predicted ηr and the measured ηr of the food fibres using Maron-Pierce equation were similar.

The ɸmax was the only unknown in the Maron-Pierce equation and it is difficult to determine for a suspension with solid particles of different R. Therefore, a simpler empirical linear equation relating ɸmax with R for the food fibres and glass bead suspensions was derived from Equation 2.6. It was found that ɸmax could be accurately predicted from ɸmax = 0.528 - 0.042 R with an R2 = 0.99.

5.2. Introduction

Digesta from the small intestine comprises a suspension of macerated plant material (Lentle & Janssen, 2011) in a liquid with water like properties. The solid food residues are suspended in a continuous liquid phase containing various secretions from the gut (Lentle et al., 2008; Shelat et al., 2015). As the digesta traverses the small intestine, digestible solid food components and various liquids are reabsorbed into the blood stream, and the less digestible solids accumulate in the digesta. Therefore, the volume fraction of liquid decreases in proportion to the volume fraction of solid particles and the viscosity of the ileal digesta increases when it traverses the colon (McRorie et al., 2000).

The effect of solid particles in the solid phase on the viscosity of digesta is not well understood (Lentle et al., 2005; Takahashi et al., 2004). Solid particles in the

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128 digesta vary in physical properties such as WHC, density, distribution of particle volume, size, and shape (Chapter 4); and chemical properties, such as content of cellulose, hemicellulose and lignin (Chapter 4). The density and chemical properties of these particles remained similar after in-vitro digestion, but not the particles

volume distribution (Chapter 4). Viscosity of dog ileal digesta decreases from 17 Pa.s to 0.0008 Pa.s measured at 1 s-1, after removal of solid particles by centrifugation at 12,000 g (Dikeman, Barry, et al., 2007); whereas the liquid phase

of chicken cecal digesta was 0.0006 Pa.s (Razdan & Pettersson, 1996). These results suggested that the viscosity of a suspension which has similar solid particles to digesta is dependent on the solid volume fraction (ɸ) of particles suspended in the continuous phase (Lentle & Janssen, 2010).

Generally, the concentration of solid particles in digesta is reported as DMC (Piel et al., 2005; Takahashi et al., 2004; Takahashi & Sakata, 2004). The DMC of the intestinal contents of chickens increased by 17.5% as it traversed from the fore gut to the colon and the viscosity of digesta increased from 1.2 Pa.s to 73 Pa.s measured at a shear rate of 1 s-1 (Takahashi et al., 2004). Viscosity of digesta does not increase linearly with DMC, but in a power law function with DMC (Lentle, Stafford, et al., 2009). This relationship resembles that of solid particle suspensions in which the viscosity is proportional to ɸ/ɸmax (Chapter 2.3.2.2., Equation 2.3, the Maron-Pierce equation (ηr = (1-ɸ/ɸmax)-2)). The Maron-Pierce equation was derived for solid spherical particles suspended in a Newtonian liquid (monodisperse system) and where the whole suspension is Newtonian measured over the high shear or low shear plateaus (Figure 2.11) (Krieger & Dougherty, 1959). However, solid particles

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129 of digesta may be elastic and are not spherical and for this reason this section of the work seeks to determine if the Maron-Pierce equation can be used to model the viscosity of digesta.

Digesta contains solid particles of varying shape, in which this variation could be simplified in a single measurement known as aspect ratio (R) (Chapter 2, Section 2.3.2.3.). It is considered as a polydisperse system where the smaller particles are trapped in voids between larger particles (Lentle & Janssen, 2008, 2010; Takahashi & Sakata, 2004). Differences in R values of solid particles in a suspension may give different arrangement and packing in a polydisperse suspension and hence the ɸmax of the solid particles in the suspension (Probstein, Sengun, & Tseng, 1994; Wierenge & Philipse, 1996). The relative viscosity (ηr = ηa/ηs, where ηa is apparent viscosity and ηs is the viscosity of the liquid phase in which the particles are suspended), of a polydisperse suspension is similar to that of a monodisperse suspension when measured in the high shear region (≥ 100 s-1) (Probstein et al., 1994).

The R values of solid particles in digesta vary with diet (Guerin et al., 2001; Lentle & Janssen, 2010), and they range from 2 to 8 as quantified by image analysis software for animals fed with a high plant fibre diet (Jalali, Nørgaard, Weisbjerg, & Nadeau, 2012; Jalali, Nørgaard, Weisbjerg, & Nielsen, 2012; Krämer et al., 2013). The ɸmax of a particulate suspension can be estimated from the average of the R values of the solid particles it contains (Pabst, Berthold, et al., 2006) by substituting these R values into the Equation 2.6 (Pabst, Berthold, et al., 2006).

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130 The objective of this study was to determine whether the whole digesta resembles a particulate suspension and whether the relative viscosities of digesta could be accurately determined from ɸmax and ɸ using the Maron-Pierce equation where ɸmax was derived from the aspect ratios (R) of the particulate fraction using empirical data.