CEREAL PROCESSING, PARTICLE SIZE DISTRIBUTION AND STARCH DIGESTIBILITY IN ANIMAL FEED

Ghaid Al Rabadi (2010). CEREAL PROCESSING, PARTICLE SIZE DISTRIBUTION AND STARCH DIGESTIBILITY IN ANIMAL FEED PhD Thesis, School of Animal Studies, The University of Queensland.

       
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Author Ghaid Al Rabadi
Thesis Title CEREAL PROCESSING, PARTICLE SIZE DISTRIBUTION AND STARCH DIGESTIBILITY IN ANIMAL FEED
School, Centre or Institute School of Animal Studies
Institution The University of Queensland
Publication date 2010-11
Thesis type PhD Thesis
Supervisor Prof. Mike Gidley
Dr Barbara Williams
Dr Wayne Bryden
Total pages 193
Total colour pages 6
Total black and white pages 187
Subjects 07 Agricultural and Veterinary Sciences
Abstract/Summary Abstract Starch in grains represents the main source of dietary energy for monogastric production animals. However, utilization of this energy is not complete due to factors related to grain source and in vivo gastrointestinal conditions. Incomplete starch digestion is a significant economic issue for the pork industry in Australia and prior evidence suggests that there is room for improvement. The general hypothesis of this thesis is that milled grain particle size fractions obtained by sieving posses a significantly different and wide range of functional and compositional properties in relation to digestion and hydrothermal processing. To identify these properties, sorghum and barley grains were milled separately using a 4 mm hammer-mill and then the ground cereals were segregated by size using a series of analytical sieves. Generated different particle sizes were investigated for their properties before and after cooking using extrusion processing. Sorghum was used as an example of a high starch content grain with low fibre content and barley was used as an example of a grain which contains high levels of fibre. Both grains are widely used as major feed ingredients in Australia, particularly in Queensland. Particle Size Effect on Digestibility The effect of milled uncooked grain particle size on the kinetics of enzymatic starch digestion and starch sources was examined. The in vitro starch digestion for different particle sizes were well fitted by simple first order kinetics for both barley (r2 range 0.90-0.99) and sorghum (r2 range 0.87-0.99). The digestion rate coefficients for different particle sizes decreased with increasing particle size, and were shown to be well fitted by an inverse square relationship for both barley (r2=0.99) and sorghum (r2=0.98). This supports the hypothesis that the rate-determining step in starch digestion is the diffusion of enzyme through the grain fragment. Quantitatively, the estimated (apparent) diffusion coefficient was estimated for both barley and sorghum (1.7 and 0.76 x 10-7 cm2 s-1, respectively). The apparent diffusion coefficient of á- amylase enzyme in grains was lower by an order of magnitude than that obtained in water, thus quantifying the barriers to amylase diffusion caused by grain structure. Particle Size Distribution in Milled Grain The contribution of individual size fractions to the overall hydration, rheological, and enzyme susceptibility properties of milled grain was evaluated. Sieve fractions were obtained from hammer-milled barley and sorghum grains (eight sieves, 0.125 mm to 2.8 mm opening size). Different particle size fractions showed a significant effect on digestion and hydrothermal properties. The extent of starch digestibility varied between different particle size up to 10 fold between the smallest and the largest particle size. When the results obtained for the unfractionated milled grain were compared to the weight average values (calculated from fraction yields and property values for each size fraction), water absorption index (WAI), water solubility index (WSI), and starch digestibility values were not significantly different from values obtained for non-fractionated ground grains of both barley and sorghum. Generally, hydration and digestibility results can be attributed to particle size and composition (starch or soluble fibre) of individual fractions. Response to hydrothermal treatment was different between sorghum and barley. In sorghum, viscosity profiles for the fractions were controlled by starch swelling which became increasingly limited as particle sizes increased. However in barley, viscosity profiles for segregated fractions did not show typical starch-based behaviour and were most likely controlled by the soluble fibre component. The results show the possibility of predicting and manipulating hydration and digestibility properties of ground grains simply by indentifying the particle size distribution of ground grains. Extrusion Processing of Milled Grain Extrusion processing was used in attempts to increase the rate and extent of starch digestibility as well as to characterize processing parameters and extrudate properties. Extrusion processing of three levels of particle size after sieve fractionation (fine, medium and coarse fractions) affected processing parameters (torque, specific mechanical energy [SME], die pressure), extrudate properties (WAI, WSI, expansion index, pasting profiles, and the kinetics and extent of starch digestion). The differences in response to processing parameters and extrudate properties was mainly due to the differences in viscosity during extrusion, while extrusion temperature (100°C versus 140°C maximum barrel temperature) had a smaller effect. The different particle size fractions of sorghum and barley both before and after extrusion showed a good fit to first-order kinetics of starch digestion, with all r2 values above 0.95. Results of the extrusion experiment suggested that the extent of swelling and gelatinisation during the extrusion may have been further improved by incorporating a pre-conditioning stage prior to extrusion. Particle size had a significant effect on the fractional digestion rate for both grains, but extrusion temperature had no effect. Based on fractional digestion rate values, extrusion processing at low temperature (100°C) could be an alternative method to fine milling for achieving a high rate and extent of starch digestion. Feed Conversion in Pigs A novel processing procedure, based on separating the coarse fraction after hammer milling and then regrinding this fraction, was used in an attempt to improve growth performance in pigs offered barley and sorghum based diets. The pig feeding trial examined the effect of (a) grain type (barley; sorghum), (b) particle size (ground grain; ground grain where the coarse fraction was separated and re-ground then added back to the fine fraction), and (c) diet form (mash, pellet), on feed conversion ratio (FCR), rate of growth (ROG) and average daily intake (ADI). Particle size had the largest effect on FCR in pigs. Feeding pigs a barley or sorghum based diet in the mash form after regrinding the coarse fraction improved FCR in sorghum by 10% and in barley by 7.8% compared to diets prepared without regrinding the coarse fraction. The form of the diet showed a smaller effect on FCR. In the pellet form, pigs fed sorghum or barley based diets after regrinding the coarse fraction improved FCR in sorghum by 5.1% and in barley by 4.8% compared to diets prepared without regrinding the coarse fraction. Within grain type, effects of particle size or diet form on ROG and ADI in barley were not statistically significant, although their ratio (FCR) did show significance. In sorghum, pigs offered diets in the ground-mash form had the highest ADI. Pigs offered ground-mash form (ground grain where the coarse fraction was separated and re-ground then added back to the fine fraction) had the highest ROG. The results from this investigation suggest that regrinding the coarse fraction after sieve separation can be used to maximise feed conversion ratio (FCR) for both barley and sorghum based diets. In contrast to single stage grinding through a smaller screen size, the regrinding method used in this work did not result in major increases in fractions with sizes below 0.5 mm and particularly below 0.25 mm. This is important as these very small particles cause dust problems in handling and on ingestion. Desired hydration properties, chemical composition, response to hydrothermal treatment, and extent of starch digestion could be manipulated by modifying the distribution of particular size fractions of ground grains. Extrusion of medium and coarse size fractions can enhance digestibility with efficient utilization of thermal energy compared to extrusion of ground but unfractionated grains. This could be a good processing strategy as this work has found that the rate and extent of starch digestion is relatively high for uncooked small particle size fractions. Alternatively, regrinding the coarse fraction after separation could be an alternative method to conventional steam pelletizing for increasing starch digestibility and energy delivery to growing pigs.
Keyword Grains; barley; sorghum; starch digestibility; kinetics; physiochemical properties;extrusion ; particle size; growth performance.
Additional Notes colour pages 23-23, 40, 44, 51, 171 landscape pages 69, 96, 98

 
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