Structural and textural changes in kinu-tofu due to high-pressure-freezing

To determine the effect of high-pressure-freezing on quality, kinu-tofu (soybean curd) was frozen at 100 MPa (ice I), 200 MPa (liquid phase), 340 MPa (ice III), 400, 500, 600 MPa (ice V) or 700 MPa (ice VI) at ca. -20 degrees C for 90 min. After reduction to atmospheric pressure, tofu was stored 2 days at -30 degrees C then thawed at 20 degrees C. Texture and structure were compared with kinu-tofu frozen (-20 degrees C, -30 degrees C or -80 degrees C) at atmospheric pressure (0.1 MPa). The rupture stress and strain of tofu frozen at 0.1 MPa and 100 MPa increased, but that of tofu frozen at 200 MPa and 340 MPa was similar to untreated tofu. As pressure rose above 500 MPa, rupture stress increased. The ice crystals in tofu frozen at 200 MPa similar to 400 MPa were smaller than in tofu frozen at 100 MPa or 700 MPa. Thus, high-pressure-freezing at 200 MPa similar to 400 MPa was effective in improving the texture of frozen tofu.