红枣片冷冻-红外分段组合干燥工艺优化

Optimization of sequential freeze-infrared drying process of jujube slices

为开发一种提质增效的红枣片干燥工艺,比较了单一干燥(冷冻干燥、红外干燥、热风干燥和微波真空干燥)对红枣片干燥特性及品质的影响,选用冷冻与红外干燥分段组合的方法干制红枣片,以干燥时间和维生素C保留率为评价指标,采用三元二次通用旋转组合设计优化红枣片冷冻-红外组合干燥工艺参数,并与红外干燥、冷冻干燥产品的干燥时间和品质进行对比分析。结果表明:1)冷冻与热风干燥的干燥时间最长,微波真空干燥最短,红外干燥次之;2)冷冻干燥产品品质较好,但酥脆性一般,红外干燥产品在色泽、质构(硬/脆度)、微观结构方面均好于热风和微波真空干燥产品,且酥脆性较好;3)转换含水率、红外温度和切片厚度对红枣片冷冻-红外组合干燥过程有显著影响(P<0.05),对干燥时间影响主次顺序依次为转换含水率、红外温度、切片厚度,对维生素C保留率影响主次顺序依次为红外温度、转换含水率、切片厚度;4)采用响应曲面法优化与试验验证确定出较佳工艺参数为:转换含水率34%、红外温度64℃、切片厚度5 mm,此时,干燥时间3.62 h,维生素C保留率68.92%;5)冷冻-红外组合干燥产品品质优于红外干燥,干燥时间比冷冻干燥缩短57.6%,维生素C保留率比红外干燥提高了34.6%。结果表明冷冻-红外组合干燥缩短了干燥时间同时保证了干燥品质,可为红枣片干制加工提供一种新的组合干燥技术和理论依据。

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.