Infrared Freeze-drying Of Characteristic Vegetables And Its Mechanism Of Loss Reduction

Vegetables are rich in nutrients and are an important source of essential vitamins,minerals,trace elements and dietary fiber.However,the water content of vegetables is high and the shelf life after harvest is short,which makes vegetables are extremely prone to wilting and corruption.Drying is an important way to extend the shelf life of vegetables.At present,the products produced by the more mature hot air drying technology are inferior,while the traditional vacuum freeze-dried(TFD)products have low drying efficiency and high energy consumption despite good quality.With the improvement of people's living standards,the international market is increasingly demanding high-quality,high-efficiency,low-energy dry products.Research on the reduction of energy consumption and quality loss in vegetable drying is of great significance to the development of the dehydrated vegetable industry.To this end,an infrared freeze-drying(IRFD)platform was set up in this subject to study the loss reduction effect and mechanism of IRFD of three specialty vegetables.The main research content of this paper are as follows:The infrared radiation(IR)was used to replace the traditional vacuum freeze-drying heating source.An IRFD platform was built and the infrared absorption characteristics and the penetration of infrared radiation of three kinds of vegetables:Pakchoi petioles(leafy vegetables),vegetable sweet potato(roots)and vegetable Cordyceps militaris(edible fungus)were studied.A nickel-chromium wire quartz tube was selected as the infrared freeze-dried radiator.According to the uniformity of the temperature field distribution and the radiation intensity,four single-powered 450 W or 10 single-intensity 100 W infrared tube rows were selected as the radiation source.The optimal radiation distance was 70 mm.In the whole infrared spectrum,water exhibits strong absorption and weak scattering characteristics of radiant energy,while the main components of the three characteristic vegetables are water,accounting for more than 80%.Therefore,their optical properties are mainly determined by water.Infrared absorption spectrum showed that Pakchoi petioles,vegetable sweet potato and Cordyceps militaris were capillary porous materials with similar absorption spectra.At the wavelengths of 3,6,7.5 and 10?m,the three characteristic vegetables have strong absorption peaks.The penetration of infrared radiation to the three featured vegetables was weak,and only the temperature at the depth of 2 mm was higher than the surface temperature,indicating that the infrared radiation was acting in the shallow layer.When the power of the infrared lamp increased from 100 W to 450 W and the peak wavelength decreased from 5.78?m to 3.85?m,the penetrability of the three featured vegetables remained basically unchanged,indicating that the penetrability of the infrared radiation on the three materials was not obviously dependent on the peak wavelength.The relationship depending on the peak wavelength is not obvious.However,under the condition of 450 W infrared lamp drying,Pakchoi petioles and Cordyceps militaris would cause the obvious increase of color difference and shrinkage of products.Therefore,100 W infrared lamp was selected to dry these two vegetables,while 450 W infrared lamp was used to dry vegetable sweet potato.The effects of ultrasonic pretreatment on infrared freeze-drying of three characteristic vegetables and their mechanisms were studied.When the ultrasonic power was 200,400 and600 W,the infrared freeze-drying time of Pakchoi petioles,vegetable sweet potato and vegetable Cordyceps militaris decreased by 7.34%~15.60%,3.33%~13.33%and3.85%~12.82%,respectively.However,the content of chlorophyll and ascorbic acid in Pakchoi petioles was significantly decreased after ultrasonic pretreatment(p<0.05).At the same time,the content of cordycepin in vegetable Cordyceps militaris pretreated by ultrasonic wave also decreased significantly(p<0.05).For vegetable sweet potato,ultrasonic pretreatment could significantly increase the content of total phenols,flavonoids and?-carotene in the samples after infrared freeze-drying(p<0.05).In terms of mechanism,the application of ultrasonic wave could increase the freedom of water in the sample,making it easier to remove water.In addition,ultrasonic treatment also caused changes in the microstructure of samples.The formation of micro-pore channels and expansion of pores were conducive to the formation of larger ice crystals.In the subsequent infrared freeze-drying,sublimation process could be accelerated and drying time was shortened.However,ultrasonic pretreatment would cause the decrease of nutrient quality of Pakchoi petioles and vegetable Cordyceps militaris.Therefore,in the subsequent drying process,the ultrasonic pretreatment was carried out only in vegetable sweet potato drying process.The loss reduction effect of IRFD was evaluated.Compared with TFD,IRFD could significantly improve the drying efficiency and shorten the drying time of three featured vegetables at the same temperature.Among the five empirical drying mathematical models,the Midilli model had the best fitting effect on the moisture content curve of materials under different freeze-drying conditions(R~2 of Midilli model was above 0.99,RMSE and?~2 were lower).From the comprehensive analysis of temperature,water content,color difference,shrinkage and microstructure,it could be seen that the evenness of IRFD dried three characteristic vegetables was slightly lower than that of TFD,but the difference was not obvious.That is,IRFD dried products still have a good evenness.Based on the changes of the central temperature and water content of Pakchoi petioles,vegetable sweet potato and vegetable Cordyceps militaris in freeze-drying process,the conversion points of sublimation/desorption drying of three kinds of characteristic vegetables were determined as the sample center temperature reaching 0?,and the moisture content was 10%.By comparing the heating and cooling rates of IRFD and TFD under no load,it was found that the heat source of IRFD was far less inert in temperature change than that of TFD.In freeze-drying,IRFD was more conducive to large-scale continuous production.The sensory quality of IRFD products was similar to that of TFD products at the same drying temperature.The sensory scores of the three characteristic vegetables dried by infrared atmospheric drying(IRD)were significantly lower than those of IRFD and TFD,and were at the level III of the evaluation grade.The hardness and crispness of IRFD dried Pakchoi petioles,vegetable sweet potato and vegetable Cordyceps militaris were higher than TFD and have better texture characteristics.In addition,the total phenols,?-carotene,DPPH clearance and reducing force in IRFD dried vegetable sweet potatoes were significantly higher than those in TFD(p<0.05).For Pakchoi petioles and vegetable Cordyceps militaris,IRFD and TFD were slightly different in their nutritional and antioxidant capacities at different freeze-drying temperatures,but the difference was not significant(p>0.05).In general,compared with TFD,IRFD had a certain redction effect on the drying quality loss of these three characteristic vegetables.Compared with TFD at the same temperature,the energy loss of IRFD on Pakchoi petioles,vegetable sweet potato and vegetable Cordyceps militaris were 15.72%~19.00%,47.82%~50.52%and 11.88%~18.37%,respectively.Although the drying energy consumption required by IRD was the lowest,its sensory quality,nutritional characteristics,antioxidant capacity and retention of characteristic flavor of dried products were quite lower than that of IRFD and TFD.The mechanism of loss reduction effect of IRFD was clarified.In IRFD,the mass transfer and heat transfer directions of the materials were consistent,which was beneficial to accelerate the freeze-drying process.Compared with TFD,IRFD could significantly improve the drying efficiency of three characteristic vegetables and shorten the drying time when the temperature was the same.In addition,at the same temperature,the IRFD had a higher effective moisture diffusion coefficient than the TFD.This further clarified that IRFD drying efficiency was higher than TFD from the perspective of mass transfer kinetics.The effective water diffusivity of the three kinds of characteristic vegetables had extremely significant(p<0.01)or significant(p<0.05)negative correlation with drying time and energy consumption.At the same time,the effective water diffusion coefficient and drying time were also significantly correlated with the nutritional characteristics and antioxidant properties of the three featured vegetables.Therefore,the reduction of loss of infrared freeze-drying was mainly based on the improvement of effective water diffusion coefficient and the shortening of drying time in the freeze-drying process.