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周新丽,滕芸,戴澄.超声波平板冷冻提高胡萝卜冻干速率[J].农业工程学报,2017,33(1):256-261.DOI:10.11975/j.issn.1002-6819.2017.01.035
超声波平板冷冻提高胡萝卜冻干速率
投稿时间:2016-07-25  修订日期:2016-08-15
中文关键词:  超声波  成核  干燥  冻干速率  胡萝卜
基金项目:国家自然科学基金(51376132)
作者单位
周新丽 上海理工大学医疗器械与食品学院上海 200093 
滕芸 上海理工大学医疗器械与食品学院上海 200093 
戴澄 上海理工大学医疗器械与食品学院上海 200093 
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中文摘要:为了探究接触式超声波辅助平板冷冻法对固体果蔬冻干过程的影响,该文研究了接触式超声波辅助胡萝卜的平板冷冻过程。首先,利用低温显微镜证实了接触式超声波能够有效促进平板冷冻固体果蔬样品成核,并且冰晶的形态与成核温度呈显著相关性(相关系数0.99),即成核温度越高,冰晶尺寸越大。其次,研究了超声波作用参数(功率,作用时间)对胡萝卜冰晶尺寸以及冻干速率的影响。结果表明,在胡萝卜样品厚度为5 mm,样品冷冻温度为?1℃时施加178.7 W功率的超声波10 s能显著提高(P<0.05)样品的成核温度,使冻干胡萝卜的孔隙当量直径从无超声波辅助条件下对照组样品的 (66.29±3.58) μm提高到了(80.81±3.03) μm,同时干燥至实际含水率为10 %时,升华干燥速率提升了29.1%。该研究为接触式超声波辅助平板冷冻用于果蔬样品的冻干过程提供了有益参考。
Zhou Xinli,Teng Yun,Dai Cheng.Contact ultrasound freezing improving freeze drying rate of carrot[J].Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE),2017,33(1):256-261.DOI:10.11975/j.issn.1002-6819.2017.01.035
Contact ultrasound freezing improving freeze drying rate of carrot
Author NameAffiliation
Zhou Xinli School of Medical Instrument and Food Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China 
Teng Yun School of Medical Instrument and Food Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China 
Dai Cheng School of Medical Instrument and Food Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China 
Key words:ultrasound wave  nucleation  drying  freeze drying rate  carrot
Abstract: Ultrasound can promote nucleation at higher temperatures, resulting in larger ice crystals. Thereby the sublimation time is significantly reduced. From all the researches, either ultrasound is applied in ultrasonic water bath or sample itself is liquid form since the low acoustic impedance between liquid and acoustic waves favors ultrasound transfer. However solid samples have barely been taken into account due to the poor transfer efficiency of ultrasound in food matrix. During the freeze drying process, the freezing is usually realized by shelf cooling in freeze-drier. Thus, using liquid as ultrasound applying media is not applicable. The technology currently is for liquid samples or adopts the immersion freezing process, but the plate freezing process for solid samples (fruits/vegetables) was barely taken into account. In this study, contact ultrasound-assisted plate freezing process was achieved. Firstly, the effect of direct contact ultrasound on inducing nucleation of solid samples was observed in the micro environment. Then a microscopic observation was carried out to examine the effectiveness of direct contact ultrasound on inducing nucleation for solid samples at different supercooling degree. The results showed that crystal morphology and nucleation temperature had significant correlation, namely the ice crystal size increased with the nucleation temperature. For example, under the nucleation temperature of -6 ℃, the average ice diameter was 20.20 μm, while the mean diameter increased up to 33.16 μm when the nucleation temperature was raised to -3 ℃. In addition, a phenomenon was found that there existed a postponement of nucleation after the onset of ultrasound application, in other words, nucleation did not follow the ultrasound triggering immediately. Secondly, the influences of ultrasonic parameters (power, exposure time) on the ice crystal size and freeze drying rate of carrot were investigated. The samples thickness was 5 mm. The results showed that applying 178.7 W power ultrasonic for 10 s at -1 ℃ could significantly increase the nucleation temperature, and make the void size of freeze-dried carrot increase from (66.29±3.58) to (80.81±3.03) μm, and the sublimation time was significantly reduced by 29.1% when the residual moisture was 10%. But it used the appropriate ultrasound irradiation condition to avoid the heat accumulation inside the food matrix. In addition, it was further verified that the primary drying rate was significantly reduced since the bigger crystals were formed in the presence of ultrasound. Therefore, the direct contact ultrasound is a promising technology in solid sample freeze-drying process. Finally, the influence of sample thickness on inducing nucleation was investigated. Under the sample thickness of 5 mm in this experiment, there was no significant difference between the void size of upper and lower part of the freeze-dried cake. But if larger thickness was considered, the freezing rate gradient within the sample may result in crystal size difference in axial direction. Smaller ice crystals may form at the region closer to the cooling plate since it was cooled faster than the interior. Furthermore, with the increase of thickness, the region far away from the vibrating plate may not be subjected to enough ultrasound radiation due to the acoustic attenuation within the sample and thus inducing nucleation would not be effective. Considering both inducing nucleation effectiveness and freeze drying efficacy, a maximum product thickness should not be too thick. These studies can provide a reference for the freeze drying process of samples (fruits/vegetables) by contact ultrasonic vibrating plate freezing.
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