Effects of pulsed vacuum steaming on the heating rate and quality of Gastrodia elata

Abstract: A dried Gastrodia elata (G. elata) is an important traditional Chinese herb. The steaming step is used to process the fresh G. elata into dried one. However, the small heating rate and low equipment automation occurred in the existing technology of atmospheric pressure steaming for G. elata. In this study, a novel technology of pulsed vacuum steaming was applied to steam G. elata, and the related steaming parameters were investigated, including the effects of pulsation ratios (vacuum time (s): atmospheric pressure time (s) 20:130, 25:130, 30:130, 20:110 and 20:150) and the number of pulsation cycles (1-4) on the heating rate, the mass loss rate, viscosity, PPO relative activity and active ingredients ((gastrodin, GA), (p-hydroxybenzyl alcohol, HA), (parishin A, PA), (parishin B, PB)) for the longitudinal bisection of G. elata, as well for the G. elata root. The variations in quality were studied compared with the samples after the atmospheric pressure steaming. The maximum average heating rate of G. elata was 4.40 ℃/min when the pulsation ratio was 20:130 at the same number of pulsation cycles, about 27.54% higher than that of the atmospheric pressure steaming (3.45 ℃/min) under the vacuum degree of 85 kPa and atmospheric absolute pressure of 110 kPa. Since the masses of all G. elata reduced after steaming, the rate of mass loss increased with the number of pulsation cycles. Compared with the fresh G. elata, the PPO activity decreased, while the viscosity increased after the pulsed vacuum steaming. Since the gelatinization occurred during steaming, the increase rate of the G. elata viscosity was related to the gelatinization degree of G. elata starch. At the same central temperature, the inhibition effect of the pulsed vacuum steaming was better than that of the atmospheric pressure steaming, reducing by 11.2%. Because a complex phase transformation also occurs, the GA can be obtained by breaking ester bond during the steaming process of G. elata. Therefore, the GA increased, while PA decreased after steaming. However, the contents of HA and PB in G. elata were not changed regularly due to the complex transformation of internal components. The maximum average heating rate of the G. elata can be achieved under the optimum pulsation ratio of 20:130 at the same pulsation times. The maximum increase rate was 192.48 % for the GA at the pulsation ratio of 20:130 for three cycles. When the G. elata was steamed at the same core temperature, the increase rate of the GA after the vacuum pulsation steaming (58.07%) was 77.97% higher than that of after atmospheric pressure steaming (32.63%). To the same steaming extent, the number of pulsating cycles that needed for the G. elata root was twice that for the longitudinal bisection of G. elata, while the same core temperature was observed for the maximum GA content of G. elata root compared with that of the longitudinal bisection. Therefore, the quality of the G. elata can be preserved better when the core temperature of the GA root reaches 77-78 ℃ under the pulsation ratio of 20:130. This finding can provide a sound technical support for the pulsed vacuum steaming of G. elata.