| China accounts for over 90% of the world’s total production of star anise and is the only country in the world that can produce and supply raw materials on a large scale.Yunnan is the second largest province in China in terms of star anise cultivation area.At present,the processing mainly relies on scattered farmers and the use of small workshops to process the sun-drying method in hot water,but the drying process is greatly affected by the weather,and rainy weather is very likely to lead to moisture,mould and rot.Therefore,in this paper,by studying the purpose of shortening the drying time and ensuring the quality of the dried star anise,we set up a joint microwave heat pump drying test bench independently and used a joint microwave heat pump drying method to dry star anise,2.1,2.6,3.1,3.6m/s)on the quality of star anise.The main findings were as follows:(1)The drying characteristics of star anise were analysed by single-factor tests on the samples,combining rehydration ratio,sensory evaluation,volatile oil extraction rate and trans-anisidine brain concentration as the indexes for investigation.The microwave power becomes high,causing the water inside the star anise to drain rapidly to the surface and the moisture content to change more quickly,so the drying time required is shorter.Compared to the traditional way of drying star anise,combined microwave heat pump drying has saved drying time.When the heat pump temperature reaches 45°C it takes 660 min to dry to a safe moisture content,while at 65°C it only takes 210 min,a difference of more than 3times the drying time.Increasing the air speed can also greatly reduce the drying time and increase the drying rate.However,high air speed will also have an impact on the drying rate,so the appropriate increase in drying temperature,microwave power and air speed can improve drying efficiency and achieve the effect of shortening the drying time.The use of combined microwave heat pump drying does shorten the time by 1/3 compared with the traditional drying method,which can improve the drying rate and save time cost.(2)The drying quality of the samples was studied by single-factor test,with the four quality indicators of rehydration ratio,sensory evaluation,volatile oil extraction rate and way aniseed brain content as reference,to lay the foundation for the next orthogonal optimization test.The study concluded that: the three level values with the highest overall microwave power rating were selected as 280 W,420W and 560 W,with ratings of 0.4646,0.5583 and 0.4951;the three level values of 55℃,60℃ and 65℃ were selected as the highest comprehensive rating of heat pump temperature,with ratings of 0.5735,0.7475 and0.6157 respectively;the three level values of 2.1m/s,2.6m/s and 3.1m/s were selected as the highest comprehensive rating of heat pump air speed,with ratings of 0.7259,0.7914 and 0.7192 respectively;(3)To the purpose of the strength of the influence of each factor on the drying quality,the orthogonal test method was used to study the combined drying of star anise by microwave heat pump by means of extreme difference analysis and analysis of variance(ANOVA)on the test indexes respectively.The results showed that microwave power,heat pump temperature and heat pump air speed had significant effects on the rehydration ratio,volatile oil extraction rate and trans-anise brain content,while the sensory evaluation was significantly affected by heat pump temperature and insignificantly affected by microwave power and heat pump air speed.The results showed that the selected factors influenced the rehydration ratio in the order of heat pump temperature > microwave power > heat pump air speed.The order of strength of the factors on the sensory evaluation was heat pump temperature > heat pump air speed > microwave power.The order of influence of the selected factors on the volatile oil extraction rate was microwave power > heat pump air speed > heat pump temperature.The order of influence of the selected factors on the content of trans-anisidine was microwave power > heat pump temperature > heat pump air speed.Finally,the degree of influence of each factor was combined by the integrated equilibrium method,which in turn determined the optimal combination of sample quality.The optimum process conditions for combined microwave and heat pump drying were:560W microwave drying for 12 min during the microwave drying stage,followed by a heat pump drying stage with a heat pump temperature of 60°C and a heat pump air speed of2.1m/s.The best overall quality of samples was obtained after 4h of drying,and the quality indexes met the national standards.The test volatile oil extraction rate of this solution reached 7.8% and the trans-anisidine brain content reached 94.9%. |
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