食用菌恒温干燥过程中MRI成像及水分迁移变化

为了研究干燥过程对草莓脯水分迁移及品质的影响，对渗透脱水后的草莓脯采用恒温（60 °C-12 h，干至6 h翻面）与分阶（65 °C-2 h，60 °C-2 h，55 °C-2 h；翻面；65 °C-2 h，60 °C-2 h，55 °C-2 h）2种方式烘干。采用低场核磁共振技术（Low-field nuclear magnetic reasonance，LF-NMR）研究草莓脯中水分分布和迁移规律，并对2种干燥方式得到的草莓脯理化（质构、V_C、总酚）和感官品质（风味、组织形态、口感、色泽）进行了比较。结果表明：随着干燥时间延长，草莓脯干基含水量逐渐降低，但分阶干燥的速率高于恒温干燥。LF-NMR的T₂反演图谱及峰面积的变化显示渗透脱水后的草莓中自由水为水分主要形态，还有少量结合水和不易流动水，2种干燥方式下草莓脯的T₂值均随干燥时间延长逐渐减小，水分流动性降低，但至干燥结束时，恒温干燥和分阶干燥的总弛豫峰面积分别下降至初始水平的31.3%和26.4%，分阶干燥较恒温干燥具有更高的干燥效率。核磁共振成像（magnetic reasonance imaging，MRI）显示在干燥过程中，草莓中水分最初向破损的果蒂端富集并散失，中期则以经果皮失水为主，最后水分主要残留在髓部。从成像结果看，2种干燥方式的主要区别在于恒温干燥的草莓脯在干燥6 h时存在明显的均湿现象，即水分在果脯内重新均匀分布，而在分阶干燥中未出现此过程。另外，分阶干燥后的草莓脯V_C和总酚含量显著高于恒温干燥（P<0.05），为恒温干燥的1.89倍和1.14倍，而粘力、硬度、咀嚼性则显著低于恒温干燥（P<0.05）；分阶干燥的草莓脯质地、外观、口感以及整体评分均显著高于恒温干燥（P<0.05），为恒温干燥的31.3%、41.3%、40.2%。总体而言，分阶干燥的干燥效率更高，并且获得的草莓脯品质更优。

Stability of microencapsulated strawberry flavour by spray drying,freeze drying and fluid bed

In order to discover the moisture distribution and migration during the drying process of strawberries at constant or gradient temperatures and the effect on the quality of dried strawberries, the strawberries which had experienced osmotic dehydration were dried under constant temperatures (60 °C-12 h, flip it over when dry to 6 h) and gradient temperature (65 °C-2 h, 60 °C-2 h, 55 °C-2 h, flip it over, 65 °C-2 h, 60 °C-2 h, 55 °C-2 h) during hot air drying. Low-field nuclear magnetic resonance (LF-NMR) was used and the physicochemical (texture, V_C, total phenols) and sensory qualities (flavor, texture, taste, color) of the preserved strawberries obtained by the two drying programs were compared. The results showed that the water content of the dry base of strawberries at both programs decreased with time extending gradually, but the rate of gradient temperature was higher than that of constant temperature drying. From the T₂ inversion spectra and peak areas of LF-NMR, it illustrated that the free water was the main form of water in the osmotically dehydrated strawberries, and there were also a small amount of bound water and immobile water. With the drying procedure proceeded, the T₂ peak of the strawberry gradually decreases, which indicates a decrease in the mobility of the water. However, the total relaxation peak area of constant temperature drying and gradient drying was reduced to 31.3% and 26.4% of the original at the end of the drying programs, which showed that the gradient drying had higher drying efficiency. Magnetic resonance imaging (MRI) showed that the water in strawberries was firstly enriched to the broken tip of the fruit and dispersed, then in the medium of the drying process, the peel took the main position for the water evaporation, and finally some water left in the pith. From the imaging results, the main difference between the two drying programs was that there was an obvious homogeneous wetness phenomenon in the strawberries after 6 h drying i.e. the water was redistributed evenly in the fruit at that time, while this process did not appeared in the gradient drying. In addition, the V_C and total phenolic contents of dried strawberries were significantly higher (P<0.05) using the gradient program than those under constant temperature drying, which were 1.89 and 1.14 times higher (P<0.05). Moreover, significantly lower (P<0.05) values of the viscosity, hardness, and chewiness were obtained by the persevered strawberries. For the sensory scores, the gradient program resulted in considerably better (P<0.05) texture, appearance, taste, and overall ratings for dried strawberries than constant temperature drying which were 31.3%, 41.3%, and 40.2%. Overall, the drying efficiency of step drying was higher and the quality of the dried strawberries obtained was better.