退火对冷冻干燥固态果蔬食品模拟体系冰晶结构和质构的影响

Effects of annealing on the ice crystal structure and texture of the solid food simulated system for freeze-dried fruit and vegetable

为探究冷冻过程中退火条件对真空冷冻干燥固体果蔬食品模拟体系冰晶结构、干燥特性、微观结构和质构的影响,该试验通过建立真空冷冻干燥果胶-蔗糖凝胶体系模拟果蔬天然网络结构,考察了不同退火条件对冷冻模拟体系的冰晶结构、冷冻干燥模拟体系的微观结构和质构等品质的影响。结果表明,退火温度范围和退火次数是影响冰晶结构、干燥过程和产品质构的重要因素。研究发现,冷冻模拟体系冰晶的直径均值、冰晶比例随退火温度范围增加和退火次数增加而降低;模拟体系的冷冻干燥速率随退火温度范围和退火次数增加而加快;冻干模拟体系的硬度和脆度随退火温度范围和退火次数增加而降低。采用0℃退火3次条件下,干燥时间最短约为14.0 h,此时模拟体系冰晶分形维数为1.616,冰晶直径极差为0.98 mm,直径切尾均值为0.16 mm,冰晶比例为75.00%,在脆度阈值为0.784 N时脆度保留效果最好,峰个数为4.33。相关性分析结果表明,模拟体系的干燥时间与冰晶直径分布的离散程度、直径均值呈正相关,与分形维数呈负相关,与冰晶的直径均值和退火到达温度呈正相关。该研究明确了退火条件对模拟体系冰晶形态和冻干后多孔结构的影响,可为真空冷冻干燥食品预冻过程中控制冰晶形成提供理论依据。

Vacuum freeze-drying(FD)can be performed using sublimation,in order to produce a high-quality product with a long shelf-life and crispness texture.But the ice crystals are formed in the process of freezing,due to the long drying time and high energy consumption during vacuum FD.It is a high demand to control the ice crystal formation for energy-saving food products with the desired sensory properties.This study aimed to examine the effect of annealing temperature(-18,-4,0,and 4℃)and cycles times(1-3)on the ice crystal structure and distribution,drying kinetics,microstructure,and texture of the pectin-sucrose simulated system.The pectin-sucrose solid system with the porous scaffolds was established to simulate the skeleton structure of natural fruits and vegetables.Simultaneously,some properties were also similar to those of fruits and vegetables,such as the thermal physical and mechanical properties.An investigation was made to determine the ice crystal distribution and morphology of the frozen simulated system,while the microstructure and texture of the freeze-dried simulated system.The results show that the ice crystal structure,drying characteristics,and product texture depended mainly on the annealing temperature and cycles.Observations of ice crystals showed that the size of ice crystals increased with the annealing temperature and cycles.The ice crystal images were evaluated to analyze the fractal dimension and ice crystal diameter distribution.The fractal dimension,mean diameter,and proportion of ice crystals decreased with the increase of annealing temperature and cycles.The large ice crystals of the simulated system accelerated the drying process in the sublimation drying.Moreover,the drying rate of the food simulated system increased with the decrease of fractal dimension.In addition,the hardness and crushing work of freeze-dried samples decreased with the increase in annealing temperature and cycles.The 0.784 N threshold crispness values showed no significant difference between the annealing(-18,-4,and 0℃)and untreated samples.The increasing annealing time and cycles resulted in a faster initial drying rate and shorter FD time.Especially,the 0℃-3 annealing treatment took the shortest drying time(14.0 h),which was 4%shorter than that of the 0℃-1sample(14.6 h),12%shorter than that of the 18℃-3 sample(16.0 h),and about 16%shorter than that of the untreated group(16.6 h).The ice crystal morphology of the 0℃-3 annealing treatment food simulated system showed that the fractal dimension was 1.616,the maximum range was 0.98 mm,the trimmed mean diameter is 0.16 mm,and the proportion of ice crystal was 75.00%.This treatment led to a higher retention of crispness(4.33)than the samples treated with 4℃.The pore structure of the 0℃-3 annealing treatment freeze-dried sample showed a larger pore area than that in the lower annealing temperature samples.Correlation analysis showed that the drying time was significantly positively correlated with the fractal dimension and ice crystal numbers,while negatively correlated with the diameter range and proportion of the ice crystal,and annealing temperature.In conclusion,annealing can be expected to improve the drying rate and texture properties of the fruit and vegetable simulated system.The fractal dimension and diameter distribution of ice crystals can also be evaluated for the drying characteristics and texture change during freezing.The finding can provide a strong reference to control the ice crystal formation in the process of vacuum FD fruits and vegetables.