【作者】 张波；

【导师】 王绍金；

【作者基本信息】 西北农林科技大学 ， 农业机械化工程， 2017， 硕士

【摘要】 核桃(Juglans regia L.)是胡桃科家族中的一员,在中国得到广泛的种植,核桃的果仁含有大量的抗氧化物质和不饱和脂肪酸等营养成分,是一种营养价值极高的保健食品。在核桃的工业化生产过程中,干燥是一个极其重要的工序,对日后核桃的深加工和贮藏都有深远的影响。射频(RF)加热作为一种新型的快速加热技术,为核桃的工业化干燥提供了新思路。本文以核桃为研究对象,进行了射频干燥工艺研究,主要的研究内容为:(1)应用食品加工领域常用的6 kW,27.12 MHz的射频加热系统将含水率为20%(d.b.)的带壳核桃干燥到含水率为8%(d.b.)左右,确定了其干燥过程中的含水率变化趋势和干燥曲线等干燥参数。(2)通过与热风干燥和真空干燥方式进行比较,分析了不同干燥方式对核桃提取油的酸值、过氧化值、皂化值、碘值以及脂肪酸的组成,总抗氧化能力和总酚含量的影响。(3)进行了核桃的解吸吸附特性研究,绘制了解吸吸附等温线,并进行了系统的分析。主要结论如下:(1)在核桃的射频热风联合干燥试验过程中,极板间距18.0 cm和50°C热风提供了一个比较合适的升温速率,是最优的射频干燥参数,并被用于进一步干燥试验。将核桃水分含量干燥到8.0%(d.b.),单独使用热风干燥和射频热风联合干燥分别需要240和100分钟。在选定的数学模型当中,射频联合热风的干燥曲线与Page模型的拟合度最好。射频联合热风干燥的核桃在干燥和加速贮藏试验过程中的过氧化值、自由脂肪酸和颜色值指标与热风干燥都没有发生明显变化(P>0.05)。射频热风联合干燥的核桃各项品质指标都在工业生产要求范围之内,并且要优于单独热风干燥的核桃品质。(2)在热风、真空和热风辅助射频三种干燥方式中,射频热风联合干燥核桃的时间最短(138 min),这个结果比第二章中多层核桃干燥试验的时间(100 min)有所增加,但样品上升的温度有所降低。试验结果表明,真空和热风辅助射频处理对核桃样品干燥过程和贮藏期间的总抗氧化能力和总酚含量影响很小。并且射频热风联合干燥处理的核桃样品品质要优于其他两种干燥方式。(3)在相同温度和水活度条件下,核桃壳的平衡含水率要明显高于核桃仁。由于核桃仁和核桃壳成分差异,两者的等温线类型也不同。核桃仁和核桃壳的解吸吸附等温线存在着明显的滞后现象,并且相同温度下,核桃壳的滞后现象要更明显。以水活度小于0.6为贮藏安全界限,贮藏时必须将核桃仁和核桃壳的含水量分别降至4.35%和9.31%(d.b.)以下。更多还原

【Abstract】 Walnut(Juglans regia L.),a member of the Juglandaceae family,is mostly cultivated in China.Walnut kernels are nutrient-rich food due to their high polyunsaturated fatty acids and antioxidants.In industrial process of walnuts,drying is a very important process for walnut storage in the future.Radio frequency(RF)heating is a rapid dehydration technique that can be applied for drying walnuts.In this paper,the following experiments of RF drying walnuts were conducted:(1)A pilot-scale,27.12 MHz,6 kW RF unit was used to reduce the whole walnut moisture content from 20% to 8.0%(d.b.),and determine drying curve and protocol parameter.(2)Analyze and compare the effects of hot air,vacuum and hot air assisted RF drying on the drying characteristics and product quality of in-shell walnuts with regard to acid value,peroxide value,saponification value,iodine value,fatty acid compositions,total phenolics concentration and total antioxidant capacities.(3)Study desorption adsorption characteristics of walnuts,desorption adsorption curve and lag phenomenon.The main conclusions are as follows:(1)An electrode gap of 18.0 cm combined with hot air temperature of 50 °C provided an acceptable heating rate and stable sample temperatures during RF drying.Total drying times to reduce the whole walnut moisture content from 20 to 8.0% on dry basis required 240 and 100 min using hot air drying and hot air-assisted radio frequency drying,respectively.The drying curve for hot air-assisted RF drying was fitted to five different kinetic models,and the best one was the Page model.Quality analysis showed that there were no significant differences in peroxide value,free fatty acid,and colour between RF treatments and untreated controls during drying and storage periods(P > 0.05).(2)Compared with vacuum drying and hot air drying,hot air assisted RF drying had the shortest drying time(138 min).RF drying single layer walnuts needed more time than multiple layers walnuts(100 min),but the temperature of sample was lower.Both the reduced power assay and free radical scavenging capacity tests showed that air-assisted RF drying and vacuum drying had little effect on the total antioxidant capacity and total phenol content of walnuts during the drying process and storage.Hot air assisted RF treated walnut quality was better than that of the other two drying methods.(3)At the same temperature and water activity conditions,walnut shell moisture content should be significantly higher than walnut kernel.Walnut kernel and shell desorption adsorption isotherm type was different,because of the composition and construction differences in walnut kernel and shell.There was a significant lag phenomenon in the desorption adsorption isotherm both of walnut kernel and shell,but walnut shell lag phenomenon was greater.The water activity of 0.6 as a safety storage limit,during storage,the water content of walnut kernel and shells must be reduced to below 4.35% and 9.31%(d.b.),respectively.更多还原