Physicochemical changes to starch granules during micronisation and extrusion processing of wheat, and their implications for starch digestibility in the newly weaned piglet

Abstract

Two trials were performed to assess changes in the physicochemical properties of precisely processed (micronised v. extruded) wheats, prior to inclusion in piglet diets. The in vitro data obtained were subsequently related to biological responses of newly weaned piglets over 14 days. The effects of the severity of micronisation (Trial 1) or extrusion (Trial 2) on the nutritional value of two wheats (varying in endosperm texture) were examined. Extrusion, in contrast to micronisation, drastically disrupted the structural properties of wheat starch granules through melting of crystallites and macromolecular degradation of starch polysaccharides. These structural changes strongly improved the hydration characteristics of starch and its digestibility. The amount of starch digested in vitro was approximately 0.20, 0.70 and 0.90 for the unprocessed, micronised and extruded samples, respectively. This enhanced in vitro digestibility correlated well with, and helped to explain, the significant improvement in the apparent digestibility of starch at both the 0.5 region (mean coefficients for extruded wheat were 0.869 and 0.959 v. raw 0.392; P 5 0.017) and 0.75 region (extruded 0.973 v. raw 0.809; P 5 0.009) of the small intestine, when compared with piglets fed an unprocessed wheat diet. Extrusion and, to a lesser extent, micronisation lessened the reduction in apparent starch digestibility on day 4 post-weaning, typically seen at the 0.5 intestinal region in piglets fed an unprocessed wheat diet. Processing variables influenced both in vitro and in vivo data, with for instance, a positive relationship between specific mechanical energy (SME) input during extrusion and starch digestibility at the 0.5 region. The higher digestibility coefficient observed at the 0.5 region for the high SME diet suggests enhanced digestion and more rapid release of starch. However, it remains to be seen whether a diet containing rapidly digestible, as opposed to slowly digestible, starch is more beneficial for piglets. This rate of starch breakdown in the piglet is an important finding, which may have implications in helping to alleviate the post-weaning growth check, particularly in the absence of in-feed antibiotic growth promoters. Processing did not appear to offer any benefit over unprocessed wheat with regard to daily live-weight gain or the apparent digestibility of nitrogen in the small intestine over the 14-day period. Based on the enhanced in vivo starch digestibility, performance might be improved over a longer period, although future studies are required to confirm this. Precise processing variables for raw materials must be stated in all animal trials.

Predicting in vivo starch digestibility coefficients in newly weaned piglets from in vitro assessment of diets using multivariate analysis

“…None of the diets contained antibiotic growth promoters or zinc oxide. Two wheat cultivars were used throughout the trials, which were of similar genetic background, identical chemical composition (including protein levels), grown under the same agronomic conditions, but differing in endosperm texture (hard (11·2) and soft (6·3) as measured with the Bran & Luebbe Infra-analyser 260 NIR (4) ). Two batches of soft wheat were used in the research programme; the first batch for trials 1, 2 and 3, the second batch for trials 4 and 5.…”

“…Five piglet trials examined the extent to which in vivo apparent starch digestibility coefficients in the small intestine (taken proportionally at 0·5 and 0·75 along from the gastric pylorus to the ileo-caecal valve) as well as other biological responses (2,4) were affected by diets differing in the source of starch. Starch sources and exact processing conditions used for each trial are summarised in Tables 2, 3 and 4.…”

“…However, progress in human food science has shown that processing causes a continuum of complex structural changes to the starch component of cereals (commonly termed 'starch conversion' (12) ). The nature and extent of these changes depend on the type of processing, the processing variables used and the physico-chemical properties of the raw cereal before processing (3,4,13,14) . These changes in starch structure result in a wide range of hydration characteristics, which have been shown to influence starch digestibility in the small intestine of the young piglet (2 -4) .…”

“…In young piglets, this variable has been shown to be considerably reduced in both the mid (0·5) and distal (0·75) regions of the small intestine (regions taken proportionally at 0·5 and 0·75 along from the gastric pylorus to the ileo-caecal valve, respectively) in the period following weaning, when the animal is introduced to a diet based on raw cereals (2,3) . This reduction in starch digestion can, however, be lessened when cereals are processed, with extrusion under controlled conditions appearing to be a more favourable treatment than micronisation (4) . The digestibility of starch at weaning may be an important consideration in helping to alleviate, in part, the post-weaning growth-check commonly observed.…”

Impact of particle size, thermal processing, fat inclusion, and moisture addition on starch gelatinization of broiler feeds

“…Gelatinization is an irreversible process: water diffuses into the starch granule, hydrogen bonds are disrupted, the granule swells and loses shape, and amylose begins to leach (Holm et al, 1988;Lund & Lorenz, 1984;Moritz et al, 2005;Coral et al, 2009). White et al (2008) reported that the changes in the optical properties of starch can be monitored by the disappearance of 'Maltese crosses' and the disappearance of A-type diffraction patterns in the X-ray diffractograms when wheat granules are submitted to extrusion. The temperature at which starch loses its crystallinity and the material Impact of Particle Size, Thermal Processing, Fat Inclusion, and Moisture Addition on Starch Gelatinization of Broiler Feeds diffuses from the granules is called the "gelatinization temperature".…”