Vacuum Freeze Drying Of Yunnan Small Instant Coffee With Prefabricated Pores

Yunnan small grain instant coffee is produced by drying the beans after they are selected,cleaned,roasted,ground and extracted.Among them,there are two drying methods for instant coffee,namely spray drying and freeze drying.The high temperature of spray drying in the drying process is not conducive to the retention of heat-sensitive aromatic components and harmful substances produced by biochemical reactions under high temperature.Therefore,the freeze drying method is basically adopted for fine coffee,which has good nutritional value and economic value.At present,freeze-dried coffee has also appeared in the domestic market,but its origin is mainly in Taiwan Province of China and other foreign regions;At present,the drying of coffee in mainland China is mainly based on hot drying,and the production of freeze-dried coffee has not yet reached a scale.Therefore,the development of cold processed instant coffee is very necessary.The traditional vacuum freeze-drying method for liquid materials is to freeze liquid materials directly into"saturated"liquid materials,that is,there are no pores inside the material.The sublimation stage of the drying process only occurs at the sublimation interface of the material,which leads to the problems of long drying time and high energy consumption.Therefore,under the background of energy crisis and the realization of"two-carbon strategy",increasing drying rate and reducing drying energy consumption are urgent technical problems in vacuum freeze drying field.In order to solve this technical problem,aiming at the energy-consuming link of deep processing of instant coffee,the innovative method of"intensive freeze drying of prefabricated porous unsaturated materials"was applied in this study,and the method of making ice cream with liquid nitrogen was used to prepare unsaturated materials of instant coffee with prefabricated pores for vacuum freeze drying.Establish a single factor experimental scheme.The temperature of drying chamber,drying pressure and specific surface area of sample were used as test factors to determine the range of vacuum freeze-drying process parameters.Then,Box-Behnken design in response surface method was used to analyze the single factor drying test and select the optimal combination of three factors:drying temperature,drying pressure and specific surface area of sample.Furthermore,sensory evaluation,unit energy consumption and drying time were used as the optimal evaluation indexes.Through the above tests,the following conclusions are drawn:(1)The drying test was conducted under the same conditions.For saturated Yunnan small grain instant coffee,the drying time of unsaturated Yunnan small grain instant coffee was shortened by 30%and the unit energy consumption was reduced by 28%.(2)According to the results of single factor experiment,the vacuum freeze drying process of dried unsaturated Yunnan small grain instant coffee is always in the stage of slowing down.At 25℃,the drying rates are 12.2%/h,11.5%/h,10.3%/h,8.2%/h,5.9%/h,4.8%/h,3.6%/h,2.8%/h,2%/h and 1.1%/h respectively.This is because the unsaturated material has initial pores inside,and the ice crystals can sublimate rapidly in the initial drying stage.Therefore,the drying process is always in the deceleration stage.(3)According to the drying characteristics of unsaturated Yunnan small grain instant coffee at different drying temperatures,when the drying temperature is 65℃,the drying rate is the highest,which are 16.5%/h,14.5%/h,12%/h,8.5%/h and 4.1%/h respectively,and the water content decreases faster.According to the drying characteristics of unsaturated Yunnan small grain instant coffee under different specific surface areas,when the specific surface area of the sample is 4.67cm~2/g,the drying rate is the highest,which are 15.6%/h,14.1%/h,12%/h,8.5%/h,4.6%/h,2%/h,respectively,and the water content decreases faster.According to the drying characteristics of unsaturated Yunnan small grain instant coffee under different drying pressures,the drying time is shortened by 30min with the increase of pressure of 22Pa,which indicates that increasing pressure can slightly shorten the drying time of unsaturated materials.(4)The specific surface area of the sample had significant effects on sensory scores and unit energy consumption(P<0.01).When the specific surface area of the sample was about 4.2cm~2/g,unsaturated Yunnan small instant coffee with good sensory quality could be obtained,and the drying efficiency could be improved to some extent.Drying temperature,drying pressure and specific surface area of sample had significant effects on unit energy consumption(P<0.01).Among them,the influence of each factor on unit energy consumption in vacuum freeze drying process of unsaturated Yunnan small grain instant coffee is as follows:specific surface area of sample>drying pressure>drying temperature;Drying temperature and specific surface area of samples had significant effects on drying time(P<0.01).(5)In order to shorten drying time,reduce unit energy consumption and obtain Yunnan small-grain instant coffee with good sensory quality,the drying process parameters are comprehensively optimized as follows:Under the conditions of drying temperature 57.97℃,drying pressure 50.23Pa,sample specific surface area 4.13cm~2/g,the sensory score of unsaturated Yunnan small instant coffee is 7.73,unit energy consumption is 2.77k J/g,and drying time is 278.64min.