Experimental Study Of Herring Slice Freeze Drying Efficiency Improvement

This study regards the middle of fresh herring as research object, effect of different freeze-drying conditions on freeze drying efficiency was studied.Firstly ,the characteristics of the vacuum freeze-drying process, as well as the structure and component of herring were introduced. The main issues exist in food freeze-drying process are the long drying time, and large energy-consuming. The theory of mass transfer and heat transfer consistency was proposed in order to shorten the drying time.The eutectic point temperature range of herring was measured with differential scanning calorimeter. The freezing temperature of herring was identified as -30℃. The water content of the selected herring was measured as 71% by STATORIUS moisture analyzers MA100. The freeze-drying experiments in different testing conditions were performed, the freeze-drying curves of freezing and sublimation process were drawn.The freeze-drying experiments with different freezing rate were taken. The curves of the fish slices quality and water content changes in sublimation drying process were drawn. The faster the freezing rate was, the slower the drying rate was. The sublimation rate can be increased from 25% to 50% with fish slice thickness from 4mm to 10mm using slower freezing rate. Herring slices with a slower freezing rate were more conducive to the steam escape from herring muscle, thus had a faster drying rate. The reason was that the ice crystals were larger while frozen with a slower rate, so the diffusion resistance was smaller, and the drying rate was faster.The freeze-drying efficiency difference between two cutting modes, such as parallel to and perpendicular to the herring muscles, under different freezing rates was studied. The curves of the herring slice quality and water content vibration in sublimation drying process were drawn. The freeze drying rate can be increased from 7.8% to 9.5% with fish slice thickness from 4mm to 10mm using parallel cut of fish slice. The result showed that sample wth cutting in a parallel direction was more benefit for the vapor escaping from herring slices, shortened the drying time and save energy.Effect was overturned before sublimation on freeze drying was studied. The curves of the herring slice quality and water content vibration in sublimation drying process were drawn. The sublimation rate of fish slice with 4mm thickness can be increased by 13.5% with overturn before sublimation, this trend became significant with the fish slice thickness increase. The results showed that the operation of overturning the beef was good for the vapor escape. The reason was that the ice crystal were gathered in the bottom of herring slice in freezing process, when herring slice was overturned before sublimation drying, it would be benefit for the water escape.By comparison, when freezing rate was slow, the ice crystals were larger, is conducive to the diffusion of water vapor; parallel to the direction of the muscle fibers beef is more benefit to the mass transfer than the slices perpendicular parallel to the direction of the muscle fibers. The consistency of heat and mass transfer can be obtained using the opposed placing pattern between freezing process and sublimation process, thus the mass transfer among sublimation process could be improved.According to the previous results above, the efficiency of herring slices freeze-drying can be improved by reducing the freezing rate, overturning the slices before sublimation and using cutting mode parallel to the muscle fiber direction. The drying rate can be shortened, the drying efficiency can be improved and energy consumption can be reduced.