Coagulum proteins have high nutritional value. Their taste and texture vary depending on their source and preparation methods. Protein coagulum is generally susceptible to undesirable microbial growth and thus has a shorter life span. Among the available food preservation methods, freezing has been highly used. The main challenge of this method is, however, the formation of ice associated with degradation of the quality, taste, color, and appearance of the preserved food mostly due to the crystallization of water and solute. This has led to the development of different techniques to reduce the extent of degradation and damage caused by freezing.
Supercooling has exhibited great potential for improving the quality and easy handling of refrigeration storage. Under atmospheric pressure, supercooling does not require high-pressure equipment. However, supercooling is generally unstable and the slow cooling may accelerate the growth of ice crystals. Moreover, supercooling freezing has not been fully explored in the literature despite its promising potential of advancing different freezing and storage conditions.
In a recent paper published in the International Journal of Refrigeration, Rajib Poudyal Lochan (Ph.D. Student), Professor Toru Suzuki and Professor Manabu Watanabe from Tokyo University of Marine Science and Technology in collaboration with Dr. Rika Kobayashi from Nihon University studied carefully the effect of various conditions of freezing on protein coagulum. These conditions involved the presence/absence of supercooling, storage temperature and freezing rate after the release of supercooling. Specifically, they focused on the effect of different freezing and storage conditions on the physical properties of firm tofu, a type of agglomeration protein coagulum. In a one-month storage test experiment comprising of different storage temperatures, different quality parameters: texture, drip loss, and microstructure were analyzed and compared with conventional freezing methods.
Both freezing temperatures and freezing rates affect the structural damage of food. All three main parameters discussed demonstrated a close relationship. For non-supercooling samples like slow freezing followed by storage at -20 °C (NS-20) and rapid freezing followed by storage at -20 °C (NR-20), long and large ice crystals that resulted in many drip losses were formed. The drip loss increased during the one-month experiment duration. These samples were characterized by spongy and hard to fracture texture. For the supercooled samples, both microstructure and textural quality were observed to be almost the same at different storage temperatures. For instance, both super cooled slow freezing followed by storage at -20 °C (SR-20) and SR-80 showed no change in the growth of ice crystal during the entire one-month storage period. The two conditions exhibited a similar trend in hardness, drip loss and textural profile properties which were likened to that of the fresh sample.
A significant difference was noted between the conventional freezing and supercooling freezing in terms of ice crystals and behavioral nature during storage. Both SR-80 and NR-80 demonstrated high quality after the frozen storage. However, SR-80 showed limited practical capabilities as compared to SR-20, which was highly preferred. The remaining conventional freezing showed low quality compared with supercooling freezing considering the drip loss, microstructure, and texture.
In summary, supercooling freezing, especially the SR-20, emerged as a better storage method for momen tofu. The temperature was quickly reduced after the breakthrough of supercooling to ensure high quality even after thawing. According to Professor Manabu Watanabe, their study provides useful insights that will pave way for research into better preservation methods for other types of protein coagulum.
Poudyal, L.R., Kobayashi, R., Suzuki, T., & Watanabe, M. (2019). Effect of different freezing and storage condition on the physical properties of protein coagulum (Firm Tofu). International Journal of Refrigeration, 107, 11-19.