Heat pump drying properties of chili and optimization of technical parameters

Abstract: Chili is a kind of spicy vegetable in the genus Capsicum member of the nightshade family, with the essential condiment components of polyphenols, carotenoids, capsaicinoids, vitamin C and E and other nutrients. Although it is easy for fresh chili into mildew condition, dried chili can not only be preserved for a long time, but also increase economic benefits. However, the ordinary drying method for chili, especially sun-drying is restricted by natural conditions, and chili is easy to be polluted by pests, microorganisms, and dust. Among different techniques of drying worldwide, heat pump drying technology is commonly used in chili drying and it has the advantages of improving the quality, saving energy and improving efficiency in the process of drying. In this paper, we designed a fruit and vegetable drying integrated machine for the experiments at College of Engineering, Nanjing Agricultural University with Xuzhou Haitao Refrigeration Company. The effects of the first stage temperature, the second stage temperature, the material thickness and the interaction of these 3 factors on comprehensive score, unit energy consumption and drying time were investigated. These parameters were optimized through the response surface method with 3 factors as well as 3 levels. Linear weighting method was used to analyze comprehensive multi-objective optimization, and regression equations of 3 indices were developed based on the experimental data. The experimental results revealed that the effect of temperature on the drying rate is important, a higher temperature means a higher drying rate, and in the temperature range of 55-65 ℃, there is a significant increment in drying rate. Similarly, the material thickness also has a great impact on the drying rate. If the material is thicker, then the drying rate is lower; and if the thickness is more than 50 mm, it results in the obvious decrease of drying rate. The first stage temperature, the second stage temperature, and material thickness have significant impacts on comprehensive score, unit energy consumption and drying time. The major factor affecting unit energy consumption was the first stage temperature (P<0.05). Lower first stage temperature could reduce unit energy consumption. The major factor affecting comprehensive score and drying time was the second stage temperature (P<0.05). Lower second stage temperature was beneficial to the quality of dry chili and reduced time consumption during the drying. The quality of dry chili is excellent when the second stage temperature is about 60 ℃. The material thickness had an obvious influence on the comprehensive score, unit energy consumption and drying time (P<0.01). The quality of chili was better when the material thickness was about 45 mm. When the first stage temperature, the second stage temperature, and material thickness were 48 ℃, 61 ℃ and 44 mm, respectively, the Composite quality score was 8.64, the unit energy consumption was 92.05 kJ/kg and the drying time was 896.02 min. These were the optimal results got from the 3 evaluation indices. The research provides a theoretical basis for the application of heat pump drying technology in chili drying.