利用LF-NMR研究牛肉粒微波干燥过程中水分迁移和分布变化

Low field-NMR in measuring water mobility and distribution in beef granules during microwave drying process

水分是食品体系特别是干制品中重要的组成部分。干制品中水分的含量、分布和存在状态的差异会对其风味、质地、稳定性及货架期产生显著影响。所以研究食品干燥过程中水分的分布和迁移变化规律具有重要意义。利用低场核磁共振(Lowfield-nuclear magnetic resonance,LF-NMR)技术研究一种牛肉粒在高火、中高火、中火和中低火微波干燥过程中水分的分布和迁移变化规律。T2弛豫测试结果显示,牛肉粒中的水分可以分为结合水、不易流动水和自由水3个组分,在不同功率干燥的过程中,不易流动水能够部分转化为结合水,转化而来的结合水弛豫时间较原组织结合水增加;核磁共振成像(Magnetic Resonance Imaging,MRI)结果直观地显示出,微波干燥是一个均匀的干燥过程,牛肉粒内部和外部同时失水,且微波干燥功率越高,失水速率越快。中高火微波干燥时,水分含量高(20.87%)而水分活度低(0.676),故为牛肉粒最佳干燥功率。

Water is the most important component in food systems especially in dried food. Different moisture content, distribution and existence of water in foods that will have significant effect on the flavor, texture, stability and shelf-life of dried food products. Therefore, the study of water mobility and distribution in dried food during drying process is of important significance. Low field-nuclear magnetic resonance （LF-NMR） was employed in this study to evaluate water mobility and distribution in beef granules during microwave drying process at high power, medium high power, medium power and medium low power respectively. T 2 relaxation test results showed that bound water, immobilized water and free water werethe three existing states of water in beef granules; immobilized water was transformed into bound water along with the increasing of relaxation time during drying at different conditions. The MRI results directly showed that the microwave drying is a well-distributed drying process, the water-desorbing rate of outer layer is equivalent to inner layer during microwave drying process, what’s more, the higher the microwave power the quicker the water-desorbing rate. Medium high power was the best drying power for beef granules which has the largest moisture content （20.87%） and lowest Aw value （0.676）.