Abstract
To compare the effects of storage and thawing methods on the quality of high-pressure-frozen tofu, momen-tofu (soybean curd) was frozen at ca. -20°C under 100, 200, 340, 400, 500 or 600 MPa of pressure and then thawed at atmospheric pressure (A, 90 min frozen; B, 90 min frozen and then stored 2 days at -30°C; C, 160 min frozen) or thawed at a high pressure (D, 90 min frozen). The texture and structure (cryo-SEM observations) of the tofu were investigated under each condition.
The rupture stress, strain and pore size of A-D frozen at 100 or 600 MPa all increased. However, the textural quality of tofu frozen at 200-400 MPa was better than of tofu frozen at 100 or 600 MPa regardless of the thawing method used. When tofu was frozen at 200-500 MPa, the ice crystal size was (largest to smallest) B>A and C>D; the pore size and texture of D were the same as those of untreated tofu.
These results show that ice crystals did not grow when tofu was frozen at 200-500 MPa. However, ice crystal growth under reduced pressure at ca.-20°C and growth while thawing at atmospheric pressure due to phase transition were noted.
