Time and temperature characteristics of feed gelatinization by gradient isothermal hydrothermal treatment

Gelatinization, as a functional property during thermal processing of starch, is applied extensively in starch-based feed industry. Furthermore, time and temperature are two interdependent process parameters in feed thermal processing including conditioning. In order to explore the sensitivity of feed to time and temperature in the process of gelatinization and to grasp the regularity of feed gelatinization and denaturation, the formula feed for finishing pigs was taken as the research material in this study, and the proximate composition of the feed, including moisture, starch, crude protein, ash, NDF, ADF, was determined. Based on viscosity characteristics of feed gelatinization, apparent viscosities of samples were measured during different hydrothermal treatments by RVA (rapid visco analyzer). Temperature scanning tests for feed- deionized water suspension were performed from 25 to 95℃ at three heating rates of 5, 10, 15℃/min, and the derivative of viscosity was calculated for analysis of critical temperatures. Then, for gradient isothermal heating of feed- deionized water suspension within 64 to 95℃ and holding time of 1, 3, 5 and 10 minutes respectively, a programmed stepwise temperature increase at constant heating rate was written to the custom configuration in RVA. The viscosity gradient and viscosity difference (Δμ value) before and after isothermal heating were used to determine the temperature thresholds. The results showed that: the gelatinization behavior of feed was affected by heating rate (P<0.05). When the heating rate increased from 5 to 15 ℃/min, the peak viscosity increased from 295 to 364 mPa·s and the temperature on the onset of gelatinization increased from 71.90 to 72.85 ℃. A fast heating rate would increase the lag between the measured and the actual temperature of samples. Three temperature thresholds of 72, 78 and 86 ℃ were obtained, which corresponded to the irreversible swelling of amorphous regions, the disruption of crystalline structure and the disintegration of amylose-lipid complexes of the starch source in feed. Therefore, three temperature thresholds, i.e. three nodes, divide the growth trend of viscosity into four stages during whole gelatinization process of feed. The temperature dependence and time dependence of feed gelatinization were confirmed. Temperature dependence meant that even if the feed was heated for an infinite length of time, only reaching a certain temperature would the feed gelatinized completely, while the time dependence referred to the spontaneous gelatinization of feed heated at a constant temperature, even if the temperature was not raised any more. Feed in the initial stage of gelatinization showed a strong temperature dependence, but in the late stage of gelatinization, a longer time of constant temperature heating could also promote the completion of the gelatinization process, showing a significant time dependence. When the hydrothermal treatment temperature was below 88 ℃, excessive constant temperature or heating treatment time would reduce the maximum viscosity of feed-deionized water suspension during gelatinization process, which showed shear thinning phenomenon of feed gelatinization. According to the analysis in this paper, in the setting of conditioning parameters of formula feed for finishing pigs, it is advisable to choose a higher conditioning temperature than the initial gelatinization temperature, and delaying the ripening retention time could bring better conditioning effect. The results provide a new idea for the study of feed gelatinization process and also a reference for optimizing the thermal processing treatment such as conditioning of formula feed.