Characterization of gelatinization kinetics and particle structure of feed under limited moisture condition

Abstract: The gelatinization of starch in feedstuffs as a functional property is applied extensively in feed thermal processing, and the moisture, temperature and time are the important technological parameters affecting the gelatinization of feed. In order to explore the gelatinization kinetic parameters and intraparticle structural changes in formula feed during the process of hydrothermal treatment, the heat treatment of formula feed for finishing pigs (moisture-adjusted to 25% and 30%) was carried out at different temperature, i.e. 75, 80, 85, 90 and 95 ℃, for different heating time, i.e. 0.5, 1, 3, 5, 7 and 10 min, respectively, based on the homogeneous plate heating method. The degree of feed gelatinization was quantified by enzymatic method, and polarized light microscopy, X-ray diffractometry, scanning electron microscope were used to characterize the physical and chemical properties of samples after heat treatment, such as, gelatinization kinetic parameters, crystallization characteristics, birefringence and micro morphology. The results showed that full gelatinization did not occur even at high enough temperature under limited moisture condition. The maximum gelatinization degree of feed with 25% and 30% moisture were 0.320 6±0.0162 and 0.668 8±0.015 0, respectively. The gelatinization process of feed followed nonlinear first order kinetics model. The gelatinization degree tended to the asymptotic value of terminal extent of gelatinization, when the heat time reached about 3 min. Compared with the gelatinization activation energy of natural starch in the previous study, the gelatinization activation energy of feed with 25% and 30% moisture in this research, 11 356.58 and 52 705.59 J/mol, obtained from regression of Arrhenius formula was relatively low, and it could only be regarded as "incomplete gelatinization activation energy" under limited moisture condition. Granules with different disappearance degree of birefringence coexisted in the same heat-treated sample system, and the birefringence lost from the center of some granules, indicating that the main microcosmic change of samples after heating treatment was preferential destruction of the structure of the amorphous regions in the center of starch granules. The destruction of intraparticle structure led to the collapse of the particle surface and promoted the leaching of amylose in the amorphous regions of starch granules. Due to the formation of the amylose-lipid complexes, the crystalline phases of samples changed from A-type to V-type. The relative crystallinity of the samples decreased firstly, and then decreased again after recovering slightly at characteristic temperature, because the destruction and rearrangement of the crystalline structure occurred simultaneously but in different degrees during the heat treatment of formula feed. Moisture was the first limiting factor of feed gelatinization, and proper increase of moisture was beneficial to improve feed gelatinization degree and pellet forming quality. The gelatinization of feed developed rapidly within 1-2 minutes of heating treatment, and reached an upper limit for the extent of gelatinization about 3 minutes. Therefore, it is necessary to avoid excessive pursuit of multi-level conditioning to prolong the conditioning time, which will lead to the loss of thermosensitive components in feed. The results of the present work may provide fundamental knowledge of interaction and change mechanism of raw materials during feed thermal processing, which has a potential application value in the optimization of feed conditioning and extruding process.