Texture and structure of high-pressure-frozen konjac

This chapter sheds light on the study conducted to investigate the texture and structure of high-pressure-frozen Konjac. To determine the effect of high-pressure-freezing and thawing on the quality of konjac, konjac arefrozen for 45 min at ca. -20°C at 100 MPa (ice I), 200 MPa (liquid phase), 340 MPa (ice III), 400, 500 or 600 MPa (ice V), 700 MPa (ice IV). Then they were thawed at atmospheric pressure (A: frozen 45 min; B: frozen 45 min then stored 1 day at -30°C; C: frozen 115 min) or thawed at the same pressure as high-pressure-freezing (D: frozen 45 min). Texture and structure were compared with frozen konjac (atmospheric pressure at 0.1 MPa, freezing at -20°C, -30°C, or -80°C) and unfrozen konjac. Texture (stress-strain curves) and the structure of all frozen konjac (thawed at 0.1 MPa) differed greatly from the original gel; final rupture stress increased and strain decreased. Conversely, texture and structure of konjac frozen-then-thawed at 200–400 MPa were the same as the original gel. This suggested that phase transitions (ice VI ® ice V ® ice III ® liquid ® ice I) occurred either during reduction of pressure at -20°C, or during storage in a freezer. Thus, high-pressure-freezing-thawing at 200–400 MPa was effective in improving the quality of frozen konjac. This explain the process of food gels of high-water content (e.g. konjac, tofu, agar), which damages to structure through freezing is extensive and the texture after thawing becomes unacceptable. When water is frozen at atmospheric pressure (0.1 MPa), volume increases.