不同干燥方式对柠檬片干燥特性及品质的影响

为探索不同干燥方式对柠檬片干燥特性及品质的影响,该研究采用真空冻结冷冻干燥、传统冷冻干燥、热风干燥3种方式进行对比干燥试验。结果表明,真空冻结冷冻干燥柠檬片工艺耗时比传统冷冻干燥节省5 h,但是热风干燥2倍以上;耗电量比传统冷冻干燥节省14.27%,但是热风干燥5倍以上;真空冻结冷冻干燥、传统冷冻干燥、热风干燥柠檬片维生素C保存率分别为66.03%、45.45%、19.14%,复水比分别为4.60、3.97、2.24,差异显著(P0.05);热风干燥柠檬片色差值显著(P0.05)高于两组冷冻干燥柠檬片,而两组冷冻干燥柠檬片色差值未见显著差异(P0.05);真空冻结冷冻干燥柠檬片中5种主要挥发性风味化合物保存率显著(P0.05)高于传统冷冻干燥柠檬片,热风干燥柠檬片保存率最低(P0.05)。3组干燥柠檬片残留水均呈现自由水、不易流动水、结合水3个横向弛豫时间峰位,干燥样品残留水分中结合水占比例最高,不易流动水和自由水占比例较小,热风干燥柠檬片横向弛豫时间峰位相对其他两组呈现小幅度的右移趋势;两组冷冻干燥柠檬片感官特征总评分差异不显著(P0.05),热风干燥柠檬片在色泽、质地、柠檬味上远不及两组冷冻干燥柠檬片感官特征丰富。真空冻结冷冻干燥是高附加值柠檬片的首选干燥方法。该研究结果为柠檬片干燥加工技术提升提供参考。

Effect of different drying methods on drying characteristics and qualities of lemon slices

Abstract: Diesel engines are widely applied in the field of transportation and manufacturing because of their dynamic and economic performance. Compared to gasoline engine, the hydrocarbon (HC) and carbon monoxide (CO) emissions from diesel engine are much lower. However, diesel engine emits huge quantities of particulate matter (PM) which pose a great threat to human health and environmental protection. As emission regulations are becoming gradually stricter, it is imperative to stringently control diesel PM emission with a feasible after-treatment technic. The technology of diesel particulate filter (DPF) is considered as the most effective mean to reduce diesel PM emission. The key of DPF technology is regeneration of the DPF, which is to timely remove the carbon deposit captured by DPF. Non-thermal plasma (NTP) technology is a promising method to control diesel emission. Reactive species generated by NTP reactor can activate complicated chemical reactions under common condition. Therefore, NTP technology has been used to remove PM deposited in DPF and become a new research hotspot in the field of DPF regeneration. Other studies have shown DPF could be effectively regenerated by NTP when the regeneration temperature was precisely controlled by an extra heater. In this work, an experimental system of DPF regeneration was constructed to investigate the regeneration effect in which DPF was not heated by an external heat source. Aided by the exhaust waste heat after engine outage, an experimental study on DPF regeneration was conducted by using a dielectric barrier discharge (DBD) NTP reactor. In the process of DPF regeneration, reactive species and PM generated exothermic oxidation reactions. Infrared gas analyzer was used to measure the volume fraction of CO and CO2 that were the main oxidation products in DPF regeneration. Twelve pairs of thermocouples were distributed in the interior of DPF to monitor the temperature change in the regeneration process. Based on NTP technology aided by exhaust waste heat, the regeneration process was investigated by analyzing the concentration change of oxidation products and the temperature change of each measuring point. Engine exhaust pipe was equipped with pitot tubes to measure the exhaust backpressure before and after regeneration. The regeneration effect was evaluated by backpressure variation of DPF after regeneration. In addition, an auxiliary test was conducted to explore the decomposition law of O3 versus temperature, contributing to the analysis of regeneration process. Thermogravimetric analysis (TGA) was performed to compare the physicochemical properties of deposit before and after NTP treatment. Results showed that NTP technology aided by exhaust waste heat exerted a good regeneration effect on DPF without an external heat source, dramatically lowering the backpressure of DPF by 69%. With the decrease of temperature, the decomposition of O3 in NTP was weakened. Therefore oxidation reaction of PM was intensified, causing the rising of the internal temperature of DPF instead of dropping. In the regeneration process, the oxidation area extended from the front of DPF to the back. The most vigorous oxidation reaction occurred at the radial midpoint of DPF and the remnant PM after regeneration on the axial section of DPF showed a ? shape. The soluble organic fraction (SOF) in the remnant deposit reduced after DPF regeneration. NTP treatment reduced the activation energy of PM both in SOF and in dry soot (DS). In the whole regeneration process, the mass of carbon in the PM decomposed was more than 6 g. This work proves the feasibility of the DPF regeneration by NTP without an external heat source, and provides experimental basis for vehicle application of the NTP technology.