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

Abstract: 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 (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℃ 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℃ 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.