Changes in quality characteristics of tofu with freezing treatment of soybeans

Abstract

The objective of this study was to investigate freezing treatment for different time effect on the quality characteristics of tofu. Soybean was freezing at -18 °C for 5 h and 24 h. There were significant changes in water absorption index (WAI), water solubility (WS) and swelling power (SP) of soybean flour (p < 0.05). The yield and moisture of tofu made from soybean with freezing treatment were lower than tofu made from unfrozen soybean (control). It may be ascribed to the denser and more compact structure, which made water easily release from the curd during pressing. Tofu made from frozen soybean showed higher protein content and lower fat content than control. -18 °C, 5 h freezing treatment maintained water entrapped in tofu as well as control. However, freezing for 24 h made the syneresis of tofu increased significantly (p < 0.05). All tofu samples showed no significant different in color. Texture analysis of tofu made from frozen soybean showed higher hardness, gumminess, chewiness than control. Freezing treatment also made the microstructure of tofu more uniform than control. But after 24h freezing, hardness decreased and microstructure of tofu appeared to form pores again which means the decrease of quality. The sensory evaluation showed that mouth-feel decreased with increasing of freezing time. All of tofu samples had similar flavor score. Tofu made from 5h-frozen soybeans had the highest firmness score. After 24 h freezing, there was a trend of decrease for firmness. Among all of tofu samples, tofu made from soybean with -18 °C, 5 h freezing treatment showed the highest score of overall acceptability. The results indicated that freezing treatment for different time effect on the quality of tofu and tofu made from 5h-frozen soybeans showed the best quality in both physical properties and sensory evaluation.