Optimization of sequential freeze-infrared drying process of jujube slices

Jujube is a very popular fruit in China. Jujube slices is a kind of dried snack with high vitamin C content. In order to decrease drying time and improve quality, the effects of various drying methods (e.g. freeze drying, infrared drying, hot air drying and microwave vacuum drying) on drying characteristics and quality of jujube slices were explored and compared. However, the above mentioned single drying technologies of jujube have low energy efficiency and drying rates, and also can't guarantee the color and nutrient retention of products. In current work, a sequential combined drying method for jujube slices drying was developed. With a moisture content (MC) of 52.06%±0.50% (wet basis) were dried in a sequential freeze-drying and infrared drying combined (FD-IRD) from conversion moisture 40%, 35%, 30%, 25%, 20% separately to 10% MC. The conversion moisture, infrared temperature, and slice thickness were selected as influencing factors, drying time and VC retention rate were used as evaluation indicators, and the response surface was adopted. The experiment optimizes the sequential freeze-infrared combined drying process parameters of jujube slices and compares them with single infrared drying. Results showed that: 1) the drying time of freeze and hot air drying is longer than other two methods, which are 8.5 h and 5.75 h respectively, while the shortest is 0.83 h for microwave vacuum drying, and the second shortest is 2.5 h for infrared drying. 2) Freeze-dried products obtained better quality but with average crispness, while infrared-dried products are better than hot air and microwave vacuum-dried products in terms of color, texture (hardness/crispness) and microstructure, and have the best crispness. 3) Conversion moisture content, infrared temperature and slice thickness have significant effects on the combined freeze-infrared drying process of jujube slices (P<0.05), with the main order of effects on drying time are conversion moisture content, infrared temperature and slice thickness, and the main order of effects on VC retention are infrared temperature, conversion moisture content and slice thickness. 4) The response surface method was used to determine the optimal process parameters: conversion moisture content of 34%, infrared temperature of 64 ℃, and slice thickness of 5 mm, at which point the drying time was 3.62 h and the VC retention rate was 68.92%. 5) The quality of freeze-infrared combined drying products was better than that of infrared drying, and the drying time was 57.6% shorter than that of freeze-drying, and the VC retention rate was 34.6% higher than that of infrared drying. This paper shows that the combined freeze-infrared drying shortens the drying time and ensures the drying quality at the same time, which can provide a new combined drying technology and theoretical basis for the drying and processing of jujube slices.