Optimization of fixation process by electromagnetic heat roller for green tea

Abstract: Fixation is the key procedure of green tea processing, which significantly impacts the color, aroma and taste of green tea. Because traditional roller fixation machine was heated by firing coal, firing gas or electricity, there are many defects in thermal efficiency and temperature-control stability. Electromagnetic heat roller fixation machine was directly heated by electromagnetic heat technology, which significantly increases the energy conversion rate and heat energy utilization rate. The thermal efficiency of electromagnetic heat roller fixation machine is 50%~60%, the temperature drift range can be controlled under ±3℃, and it can synchronously measure and control the temperature of three parts (front, middle and back). The structure composition, operational principle and technical characteristics of a new-style electromagnetism heating roller fixation machine (Patent number: ZL201020157446.2) were introduced in this study. Then with the materials of fresh tea leaves (from one bud and one leaf to one bud and two leaves), the parameters of electromagnetic heat fixation processes of green tea were optimized.Based on the results of single factor experiments, the orthogonal experiments of four factors including front part temperature (240℃, 270℃ and 300℃), middle part temperature (220℃, 250℃ and 280℃), back part temperature (190℃, 220℃ and 250℃) and fixation time (90 s, 105 s and 120 s) were carried out. According to the weight coefficients of tea sensory test, tea polyphenols (25%), amino acids (25%), soluble total sugar (20%), catechins (20%) and chlorophyll (10%) were used as the evaluating indicators of green tea quality in the orthogonal experiments. Through comprehensive analysis, A2B3C1D1 (270℃, 280℃, 190℃, 90 s) was preliminarily considered as the optimum fixation parameters. But, due to that the temperatures between middle part and back part was too large, it may easily result in distortion of the roller and decreasing the homogeneity of temperature control and life length of the machine. To synchronously consider the mechanical function and the green tea quality, A2B2C1D2 (270℃, 250℃, 190℃, 105 s) was compared with A2B3C1D1. The results showed that there was no significant difference in chemical component contents and sensory quality, except that the aroma of green tea with A2B3C1D1 was higher than that with A2B2C1D2. Comprehensive analysis of chemical components contents of green tea, working performance and economic benefits, the appropriate fixation parameter of electromagnetic heat roller fixation machine is A2B2C1D2. Then the fixation with electromagnetic heat roller was compared with other fixation ways (traditional electroghermal heat roller fination, microwave fixation and high temperature hot wind fixation), the results showed that electromagnetic heat roller fixation exhibited high-efficiency (146 kg/h), low-cost (0.38 yuan/kg), easy control, high-stability and low-pollution advantages. And the green tea processed from electromagnetic heat roller had best sensory quality, the fixing leaves showed light fresh green, rich aroma. This study will help to enhance the fixation technology of green tea processing.