Novel Technology for Cryopreservation of Food by Ultrasonic Irradiation

冷冻可以减缓或阻止保存食物的一些生物反应。冷冻对于保持食物的味道是致命的，这也是事实。在冷冻过程中，细胞外和细胞内的冰形成、水渗透细胞膜、细胞变形等行为都发生在微观尺度上，并带来与蛋白质滴注和变性相关的严重伤害。主动控制结晶的方法是一种很有前途的低温保存技术。本课题的目的是研究超声波辐照对生物材料冷冻过程中结冰的影响。在实验中，琼脂凝胶和/或马铃薯组织在28kHz的超声照射下冷冻。用荧光指示剂对冰晶进行了温度测量和显微观察。首先，随着超声功率的增加，样品的冷却速度加快，因为声流和声空化引起的流动干扰增加。其次，在快速冷却条件下，随着超声功率的增大，冰结构尺寸减小。在此基础上，利用冻结锋处的局部冷却速率分析了超声对冻结的影响。结果表明，在相同的局部冷却速率下，超声照射下形成的冰晶比无超声照射下形成的冰晶要小。因此，声流不仅可以提高局部冷却速率，还可以通过其他相关现象对冰的形成产生影响。讨论了超声功率和冷却速度与生物组织超声结晶机理的关系。

Freezing can slow down or stop some biological reactions for preserving food. It is also true that the freezing is lethal to keeping the taste of food. During freezing, the extra- and intra-cellular ice formation, osmotic water permeation through cell membrane, deformation of the cell and other behavior occur at microscale and they bring serious injuries connecting to drip and denaturation of protein.A method to actively controlling crystallization is one of promising technique for cryopreservation. The object of this project is to study the effects of ultrasonic irradiation on ice formation during freezing of biological materials. In the experiments, agar gel and/or potato tissue was frozen under irradiation of ultrasound at frequency of 28kHz. The measurements of temperature and the microscopic observation of ice crystals by using fluorescent indicator were carried out.Firstly, the cooling rate of sample augmented according to the increase of the ultrasonic power, since acoustic streaming and flow disturbance due to acoustic cavitation increased. Secondly, it was found that ice structure size decreased with increasing ultrasonic power in case of rapid cooling. Furthermore, the ultrasonically induced effects on freezing was analyzed using local cooling rate defined at freezing front. It was clarified that smaller ice crystals were formed by irradiating ultrasound, comparing with case without ultrasound, under being the same local cooling rate. Therefore, there are effects on ice formation through not only enhancing local cooling rate due to acoustic streaming, but also other relevant phenomena. The mechanism of sonocrystallisation in biological tissue was discussed in relation to ultrasonic power and cooling rate.