Experimental Research On Drying Characteristics And Quality Of Jujube Powder By Two Drying Methods

In order to develop jujube products and solve the problem of slow selling,the semi-dried Jun jujube and fresh Jun jujube were used as experimental material to be made into jujube powder by solar drying and vacuum freeze-drying separately and jointly.The sensory evaluation,nutritional composition test and micro-structure analysis of the dried jujube powder were carried out.The best experimental conditions will be selected to provide guidance for the industrial production of preparation of jujube powder.Firstly,the drying characteristics of jujube blocks with different thicknesses and jujube pulp with different liquid heights were studied in the glass greenhouse solar drying device and vacuum freeze dryer.The results of drying with jujube blocks separately showed that:in the scope of the experiment,the drying characteristics of fresh jujube blocks and semi-dried jujube blocks were the same,that was,the solar drying time of jujube blocks with 4 mm thickness was the shortest(semi-dried jujube: 6 h;fresh jujube: 10 h);during vacuum freeze-drying jujube blocks,when the thickness of jujube blocks was 6 mm,the vacuum degree was 10 Pa and the shelf heating temperature was 45 ?,which had the shortest freeze-drying time(11.5 h),and the mass drying rate of jujube blocks of the same thickness increased with the increase of shelf heating temperature.The experimental results of vacuum freeze-drying jujube pulp showed that when the concentrations of jujube pulp were 20%(semi-dried jujube)and 50%(fresh jujube),the liquid height of jujube pulp was6 mm,the vacuum degree was 10 Pa and the shelf heating temperature was 45 ?,which had the shortest freeze-drying time(semi-dried jujube: 21 h;fresh jujube: 20 h);the mass drying rate of jujube pulp with the same liquid height decreased with the increase of shelf temperature and the mass drying rate decreased with the increase of liquid height when the shelf heating temperature was the same;the results of solar-vacuum freeze-drying showed that: the conversion points of the drying stage of solar-vacuum freeze-drying were 30%(semi-dried jujube)and 60%(fresh jujube);wherein in the vacuum freeze-drying stage,with the vacuum degree being 10 Pa,the mass drying rate of jujube pulp made by semi-dried jujube decreased with the increase of liquid height which was at the same shelf temperature;the mass drying rate of jujube pulp made by fresh jujube which was at the same liquid height decreased with the increase of shelf temperature;the freeze-drying timeof the concentrated jujube pulp was the shortest(semi-dried jujube: 15.5 h;fresh jujube: 19h)when the liquid height was 4 mm and the shelf heating temperature was 55 ?.Then the quality of pre-dried jujube powder prepared was studied.The results showed that: the sensory score of jujube powder obtained by solar drying with 4 mm thickness of jujube block was the highest when semi-dried jujube was used as experimental material;with the vacuum degree being 10 Pa and the shelf heating temperature being 45 ?,the sensory score of jujube powder obtained by freeze-drying of jujube blocks with 6 mm thickness and jujube pulp with 6 mm liquid height was the highest;the sensory score of jujube powder obtained by vacuum freeze-drying jujube pulp was the highest when fresh jujube was used as experimental material;under the same drying conditions,compared with fresh jujube,the sensory evaluation of jujube powder obtained by semi-dried jujube was better than that of fresh jujube;the total sugar content of jujube powder obtained by solar-vacuum freeze-drying jujube pulp was the highest,and the vitamin C retention rate of jujube powder obtained by vacuum freeze-drying jujube blocks(semi-dried jujube)and jujube pulp(fresh jujube)was the highest;the jujube powder obtained by vacuum freeze-drying was porous and honeycomb,so the micro-structure was the best.Based on the analysis of drying characteristics and sensory evaluation of semi-dried jujube and fresh jujube,in the scope of the experiment,the optimum thickness of solar drying semi-dried jujube blocks was 4 mm.The optimum experimental conditions of vacuum freeze-drying were as follows: the vacuum degree was 10 Pa,the shelf temperature was 45 ?,the thickness of jujube blocks and liquid height of jujube pulp were both 6 mm.The best experimental conditions of solar-vacuum freeze-drying were as follows: firstly,jujube pulp whose liquid height was 4 mm was dried from the concentration of 20%(semi-dried jujube)and 50%(fresh jujube)to(30%±1%)and(60%±1%)by solar drying,then continued to be dried in the vacuum freeze dryer with the shelf temperature being55 ? and vacuum degree being 10 Pa.To sum up,compared with the jujube powder prepared by fresh jujube,the drying time of jujube powder prepared by semi-dried jujube was shorter and the sensory evaluation and micro-structure were better meanwhile.So the quality of jujube powder prepared by semi-dried jujube was better.