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Papers by Stephen Homer

The influence of microstructure and mechanical properties on the in vitro digestibility of 15% wh... more The influence of microstructure and mechanical properties on the in vitro digestibility of 15% whey protein isolate (WPI) gels was investigated. Gels were prepared via heat set gelation at three pH values (pH 3, 5 and 7), which produced gels with distinct microstructures and mechanical properties. The gels were minced to simulate an oral/chewing phase, which led to the formation particles of various sizes and textures. The minced gels were passed through either an Infogest (pre-set pH of 3) or Glass stomach (dynamic pH) protocol. Gels were digested in the gastric phase for up to 120 min, at which point the extent of digestion was measured by the amount of filterable nitrogen passing through a sieve. The digesta from both gastric methods were passed through an in vitro simulated intestinal phase. A strong link was found between the elasticity of the initial gel and the gel particle size following simulated oral processing, which significantly (p < 0.01) affected the rate of digest...

Food Hydrocolloids

The structure and rheology of whey proteins at a concentration of 10 % has been investigated as a... more The structure and rheology of whey proteins at a concentration of 10 % has been investigated as a function of pH. The turbidity of whey protein isolate (WPI) solutions was measured and DSC thermogr ...

Journal of Cereal Science

Abstract High amylose wheat (HAW) starch has been the focus of a number of nutritional studies, b... more Abstract High amylose wheat (HAW) starch has been the focus of a number of nutritional studies, but there is limited information around its effect on the mechanical properties of wheat flour dough. This study investigated the size, shape and packing volume of HAW starch and their effect on the microstructure and rheology of dough. Four flour blends were formulated by adding vital wheat gluten and either HAW or commercial wheat starch to HAW flour to achieve a constant 14% protein content, but varied amounts of HAW starch. A large number of small and irregularly shaped HAW starch granules resulted in a high packing volume per gram of starch. Confocal laser scanning microscopy of optimally mixed doughs correlated the degree of starch granule aggregation with the level of HAW starch in the bi-continuous dough network. Small deformation rheology demonstrated that increased quantities of HAW starch in the dough increased the elastic modulus G′ values. Uniaxial extension measurements highlighted a synergy between HAW starch and sources of gluten proteins resulting in increased strain hardening. The impact of HAW starch on dough rheology was attributed to its irregular shape and large number of small granules leading to greater granule-granule interactions.

Food Hydrocolloids

Abstract The effect of oscillatory shear during heat-induced gelation of whey protein isolate has... more Abstract The effect of oscillatory shear during heat-induced gelation of whey protein isolate has been investigated. For each gel sample, a single oscillating strain was applied during the gelation process from within the range of 0-1.0. A strain sweep was then used to evaluate the linear viscoelastic region as well as the fracturing properties of each gel. The application of strains lower than ∼0.01 during gelation did not affect the storage modulus while larger strains resulted in lower storage moduli in the linear viscoelastic region. Furthermore, gels produced under small ( 0.01) amplitude strain were characterised by a two-step fracture pattern. Between the fracture steps, strain hardening behaviour was observed. Confocal laser scanning microscopy was used to identify structural differences between the gels. Greater inhomogeneity was found in gels produced under large amplitude compared to small amplitude strain. It is suggested that localised redistribution of aggregates due to shear during gelation increases the average pore size and possibly creates two distinct types of aggregate structure with differing moduli. The combined effect of heating rate and oscillatory strain was also investigated. We postulate that the mechanism underlying our observations is generic to many gel systems.

Carbohydrate Polymers, 2013

The effect of NaCl on the thermal behaviour of wheat starch was investigated with particular focu... more The effect of NaCl on the thermal behaviour of wheat starch was investigated with particular focus on starch at low moisture contents (25-45 wt%). Increasing the level of NaCl reduced the starch peak viscosity (in 90% water) as measured by RVA and shifted all of the thermal peaks (up to 120°C) to higher temperatures as observed by DSC. Above a moisture content of 45%, the temperature difference of the first thermal transition of starch in the presence of 2% NaCl and in the absence of NaCl was found to be constant. In the absence of NaCl, the peak temperature of gelatinisation (Tp) increased by 12°C (from 62 to 74°C) as the water content was reduced from 35% to 25%. In the presence of 2% NaCl, the variation in Tp due to changes in water content was significantly reduced. At NaCl concentrations greater than 2% (w/w total), the Tp of the starch remained constant irrespective of water content. Evidence of this effect was observed in situ using confocal microscopy. In the presence of 2% NaCl, images taken at elevated temperatures show little difference in the extent of starch swelling at 25% compared to 45% water content. However, in the absence of NaCl, significantly more swelling was observed at 45% than at 25% water content. With increasing NaCl concentration, the interaction of starch and NaCl became dominate. Thus the on-set of the thermal transitions of starch granules is primarily controlled by the amount of NaCl present, and secondarily by the water content which becomes dominant when the NaCl concentration is low.

Carbohydrate Polymers, 2014

The impact of heating rate on the glass transition (T g) and melting transitions observed by diff... more The impact of heating rate on the glass transition (T g) and melting transitions observed by differential scanning calorimetry (DSC) on starch and a starch/gluten blend (80:20 ratio) at low moisture content was examined. The results were compared to those determined by thermo-mechanical analysis (TMA). Comparison with dynamic mechanical thermal analysis (DMTA) and phase transition analysis (PTA) is also discussed. Higher heating rates increased the determined T g as well as the melting peak temperatures in both starch and the starch/gluten blend. A heating rate of 5 • C/min gave the most precise value of T g while still being clearly observed above the baseline. T g values determined from the first and second DSC scans were found to differ significantly and retrogradation of starch biopolymers may be responsible. T g values of starch determined by TMA showed good agreement with DSC results where the T g was below 80 • C. However, moisture loss led to inaccurate T g determination for TMA analyses at temperatures above 80 • C.

The influence of microstructure and mechanical properties on the in vitro digestibility of 15% wh... more The influence of microstructure and mechanical properties on the in vitro digestibility of 15% whey protein isolate (WPI) gels was investigated. Gels were prepared via heat set gelation at three pH values (pH 3, 5 and 7), which produced gels with distinct microstructures and mechanical properties. The gels were minced to simulate an oral/chewing phase, which led to the formation particles of various sizes and textures. The minced gels were passed through either an Infogest (pre-set pH of 3) or Glass stomach (dynamic pH) protocol. Gels were digested in the gastric phase for up to 120 min, at which point the extent of digestion was measured by the amount of filterable nitrogen passing through a sieve. The digesta from both gastric methods were passed through an in vitro simulated intestinal phase. A strong link was found between the elasticity of the initial gel and the gel particle size following simulated oral processing, which significantly (p < 0.01) affected the rate of digest...

Food Hydrocolloids

The structure and rheology of whey proteins at a concentration of 10 % has been investigated as a... more The structure and rheology of whey proteins at a concentration of 10 % has been investigated as a function of pH. The turbidity of whey protein isolate (WPI) solutions was measured and DSC thermogr ...

Journal of Cereal Science

Abstract High amylose wheat (HAW) starch has been the focus of a number of nutritional studies, b... more Abstract High amylose wheat (HAW) starch has been the focus of a number of nutritional studies, but there is limited information around its effect on the mechanical properties of wheat flour dough. This study investigated the size, shape and packing volume of HAW starch and their effect on the microstructure and rheology of dough. Four flour blends were formulated by adding vital wheat gluten and either HAW or commercial wheat starch to HAW flour to achieve a constant 14% protein content, but varied amounts of HAW starch. A large number of small and irregularly shaped HAW starch granules resulted in a high packing volume per gram of starch. Confocal laser scanning microscopy of optimally mixed doughs correlated the degree of starch granule aggregation with the level of HAW starch in the bi-continuous dough network. Small deformation rheology demonstrated that increased quantities of HAW starch in the dough increased the elastic modulus G′ values. Uniaxial extension measurements highlighted a synergy between HAW starch and sources of gluten proteins resulting in increased strain hardening. The impact of HAW starch on dough rheology was attributed to its irregular shape and large number of small granules leading to greater granule-granule interactions.

Food Hydrocolloids

Abstract The effect of oscillatory shear during heat-induced gelation of whey protein isolate has... more Abstract The effect of oscillatory shear during heat-induced gelation of whey protein isolate has been investigated. For each gel sample, a single oscillating strain was applied during the gelation process from within the range of 0-1.0. A strain sweep was then used to evaluate the linear viscoelastic region as well as the fracturing properties of each gel. The application of strains lower than ∼0.01 during gelation did not affect the storage modulus while larger strains resulted in lower storage moduli in the linear viscoelastic region. Furthermore, gels produced under small ( 0.01) amplitude strain were characterised by a two-step fracture pattern. Between the fracture steps, strain hardening behaviour was observed. Confocal laser scanning microscopy was used to identify structural differences between the gels. Greater inhomogeneity was found in gels produced under large amplitude compared to small amplitude strain. It is suggested that localised redistribution of aggregates due to shear during gelation increases the average pore size and possibly creates two distinct types of aggregate structure with differing moduli. The combined effect of heating rate and oscillatory strain was also investigated. We postulate that the mechanism underlying our observations is generic to many gel systems.

Carbohydrate Polymers, 2013

The effect of NaCl on the thermal behaviour of wheat starch was investigated with particular focu... more The effect of NaCl on the thermal behaviour of wheat starch was investigated with particular focus on starch at low moisture contents (25-45 wt%). Increasing the level of NaCl reduced the starch peak viscosity (in 90% water) as measured by RVA and shifted all of the thermal peaks (up to 120°C) to higher temperatures as observed by DSC. Above a moisture content of 45%, the temperature difference of the first thermal transition of starch in the presence of 2% NaCl and in the absence of NaCl was found to be constant. In the absence of NaCl, the peak temperature of gelatinisation (Tp) increased by 12°C (from 62 to 74°C) as the water content was reduced from 35% to 25%. In the presence of 2% NaCl, the variation in Tp due to changes in water content was significantly reduced. At NaCl concentrations greater than 2% (w/w total), the Tp of the starch remained constant irrespective of water content. Evidence of this effect was observed in situ using confocal microscopy. In the presence of 2% NaCl, images taken at elevated temperatures show little difference in the extent of starch swelling at 25% compared to 45% water content. However, in the absence of NaCl, significantly more swelling was observed at 45% than at 25% water content. With increasing NaCl concentration, the interaction of starch and NaCl became dominate. Thus the on-set of the thermal transitions of starch granules is primarily controlled by the amount of NaCl present, and secondarily by the water content which becomes dominant when the NaCl concentration is low.

Carbohydrate Polymers, 2014

The impact of heating rate on the glass transition (T g) and melting transitions observed by diff... more The impact of heating rate on the glass transition (T g) and melting transitions observed by differential scanning calorimetry (DSC) on starch and a starch/gluten blend (80:20 ratio) at low moisture content was examined. The results were compared to those determined by thermo-mechanical analysis (TMA). Comparison with dynamic mechanical thermal analysis (DMTA) and phase transition analysis (PTA) is also discussed. Higher heating rates increased the determined T g as well as the melting peak temperatures in both starch and the starch/gluten blend. A heating rate of 5 • C/min gave the most precise value of T g while still being clearly observed above the baseline. T g values determined from the first and second DSC scans were found to differ significantly and retrogradation of starch biopolymers may be responsible. T g values of starch determined by TMA showed good agreement with DSC results where the T g was below 80 • C. However, moisture loss led to inaccurate T g determination for TMA analyses at temperatures above 80 • C.