Ai Teramoto

Trehalose and hydrostatic pressure effects on the structure and sensory properties of frozen tofu (soybean curd)

Tofu with 0, 2.5 or 5% trehalose was pressurized at 100–686 MPa and approximately −20 °C for 60 min to determine changes in temperature and sensory evaluation of high-pressure-frozen tofu as affected by trehalose. Tofu froze during pressurization at 100 or 686 MPa; conversely, tofu did not freeze between 200 and 600 MPa and −20 °C, but it froze rapidly when the pressure was released. It was found that tofu frozen at 0.1, 100 or 686 MPa had larger ice crystals and was firmer (less like unfrozen tofu) than tofu frozen at 200–600 MPa. In the sensory evaluation, results showed that mouth feel (texture) of tofu frozen at 400 MPa was more like the control when 2.5% trehalose was added.

Effect of pressure-shift freezing on texture, pectic composition and histological structure of carrots

Abstract Pressurizing enhanced de-esterification of pectin in carrots, but did not enhance pectic degradation by transelimination; therefore, pressurized carrots did not soften. When raw or blanched carrots were frozen (ca. 18°C) at 100MPa (ice-I) or 700MPa (ice-VI), firmness decreased and strain increased, while changes in texture, release of pectin and histological damage in carrots frozen at 200MPa (liquid), 340MPa (ice-III), 400MPa (ice-V) were less than those frozen at 100 and 700MPa. It was also found that carrots stored in a freezer (30°C), after freezing at 200MPa and 340MPa, decreased in firmness and increased strain, but the histological structure was better than those frozen at 30°C (0.1MPa).

Texture and structure of high-pressure-frozen gellan gum gel

Abstract To determine the effects of sucrose and high-pressure-freezing, unsubstituted form-gellan gum gels with 0, 5, 10 or 20% sucrose were frozen at 0.1–686 MPa and −20 °C. Gels were frozen during pressurization at 0.1, 100, 600–686 MPa. However, at 200–500 MPa, gels did not freeze but froze during pressure release (pressure-shift-freezing). On pressure release, a sharp rise in sample temperature was observed for the samples between 200 and 500 MPa. This was a consequence of the exothermic freezing event. Thus, appearance and structure of gels frozen at 200–500 MPa were better than other treated samples due to quick freezing. However, when gels were frozen at 0.1–686 MPa, rupture stress decreased remarkably and strain increased. Texture of pressure-shift-frozen gel was somewhat better than that of gels frozen in freezers (−20, −30 or −80 °C) at atmospheric pressure. Consequently pressure-shift-freezing was more effective. It was found that the addition of sucrose to gels was effective in improving the quality of frozen gellan gum gels.

Texture and cryo-scanning electron micrographs of pressure- shift-frozen tofu

Abstract The objective of this study was to research the effect of high pressure on improvement in texture of frozen tofu. While the firmness and strain of tofu frozen at 100MPa at ca. 18°C increased, that of tofu frozen at 200MPa and 340MPa was almost the same as untreated tofu. As pressure rose, firmness and strain increased, texture became worse. The pore size in tofu frozen at 200MPa400MPa was smaller than in tofu frozen at 100MPa and 700MPa. Freezing formed ice crystals in the gel network causing it to contract. Tofu with a tight network was firmer than tofu with a loose network by cryo-SEM observation. It appeared that the shrinkage affected the firmness, while the size of the ice crystals affected the strain of frozen tofu.

Effects of high pressure and salts on frozen egg custard gel

Our objective was to determine the effect of high pressure and/or the addition of salts on the improvement in texture of frozen egg custard gel. Stress and strain of gel frozen at 0.1, 100, 600 and 686 MPa increased, while those of gel frozen at 200–500 MPa changed only slightly. The pore size in gel frozen at 200–500 MPa was smaller than in gel frozen at 0.1, 100, 600 and 686 MPa. The 0% salt-gel, NaCl-gel and KCl-gel were supercooled at −20°C during pressurization at 200–400 MPa, 200–600 MPa and 200–500 MPa, respectively. When pressure was released, the supercooled gel froze quickly by pressure-shift-freezing and small ice crystals were dispersed throughout keeping the texture more suitable when it was thawed.

Texture and structure of high-pressure-frozen konjac

Publisher Summary 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.