Studies On The Microwave Vacuum Drying Of Lotus-seed

Drying technology is played the most important roles in lotus-seed processing industry. The microwave-vacuum drying (MVD) technology was applied to lotus-seed drying; the drying and quality characteristics of lotus-seed were systematically studied in this paper. On the basis, quadratic rotation-orthogonal combination design was used for experimental design, combined with response surface methodology to determine the optimal procedure parameters of lotus-seed drying. The results as following:1. Studied on the drying characteristics of MVD lotus-seedConditions of raw material: Fresh lotus-seed, at second or third stage maturity, with no color protection.The MVD was resulted in substantial saving of processing time, lower of drying temperature, and improved quality of lotus-seed. The effects of microwave power, on/off ratio were significant: the higher the power and the on/off ratio were, the bigger the drying rate was, the shorter the drying time was; The vacuum was higher, then the drying time was shorter, but the effect was inconspicuousness.Two drying periods, namely accelerated drying and decelerated drying were found in MVD lotus-seed, with no significant constant drying period. The accelerated drying period was short, and when the microwave power, vacuum and on/off ratio were higher, the decelerated drying period was shorter. In MVD lotus-seed, the central and surface temperatures were basically similar, and far lower than microwave drying simply.2. Studied on the quality of MVD lotus-seedThe results of electrical microscope slice showed that the cells were single regularly stacked, and a mass of pores were existed in intercellular. The starch was partly gelatinized, and still some starch granules integrated existed. The unique phenomenon"cave"was formed, easy for water migration and good quality keeping when rehydration.The results of PPO activity research showed that PPO activity was inhibited within 2 min. That means the effect of enzymatic browning could be effectively controlled by MVD in a short time.The results of shrinkage and rehydration research showed that lower shrinkage and higher rehydration could be achieved by increasing microwave power, vacuum pressure and on/off ratio. But when the power reached 4 kW, the shrinkage rage was no longer lower and the rage got lower. While vacuum pressure was -0.08MPa, the rehydration was the biggest, but when it reached -0.09 MPa, the rehydration rage was no longer bigger.The results of color research showed that higher WH could be achieved by increasing microwave power, vacuum pressure and on/off ratio. When microwave power was at 3 kW, WH was quite close to the fresh, but it reached 4 kW, WH got lower. When the vacuum pressure was -0.09 MPa, WH was quite similar to the fresh.3. Studied on the process optimization of MVD lotus-seedThe quadratic rotation-orthogonal combination design was used for experimental design, and the dynamic mathematical model was established as follows:(microwave powerX1, vacuum pressureX2, on/off ratioX3, drying time Y1, shrinkage Y2, rehydration Y3, whiteness Y4) Y1=14.08696-3.687769X1-2.020287X2-2.316503X3-1.6875X1X2+1.1875X1X3 Y2= 39.08823-2.97355X2-1.98288X3+4.19479X12 +2.15833X22 +1.65098X32 Y3=154.3661+2.214661X1-0.604753X2+1.780317X3-1.577267X12-1.665654X22- 0.4X2X3-1.488879X32 Y4= 67.82598+1.666092X2-3.704279X22-2.735536X32The response surface methodology and weight value analysis was used to determine the optimal procedure parameters of MVD lotus-seed. When comprehensively considering the costs and the quality of MVD lotus-seed, the optimal parameters of microwave power, vacuum pressure and on/off ratio should be kept at 3 kW, -0.086 MPa, 3.5 (94s-on/26-off).