Changes in Temperature, Texture, and Structure of Konnyaku (Konjac Glucomannan Gel) During High-Pressure-Freezing A

JFS: Food Engineering and Physical Properties

Changes in Temperature, Texture, and Structure of Konnyaku (Konjac Glucomannan Gel) During High-pressure-freezing A. TERAMOTO AND M. FUCHIGAMI

ABSTRACT: To determine the effects of high-pressure-freezing, changes in temperature, texture, and structure of konnyaku (a gel with high water content) were measured during freezing for 60 min at 0.1 to 700 MPa and -20 °C. During freezing at 0.1, 100, 500, 600, and 700 MPa, exothermic peaks were detected (konnyaku froze). However, at 200 to 400 MPa, exothermic peak was not detected and temperature rose when pressure was released at -20 °C; the supercooled konnyaku froze by pressure-shift-freezing. The coarse gel network observed in unfrozen konnyaku was compressed by freezing due to formation of ice crystals. The rupture stress increased and strain decreased in all frozen konnyaku. High-pressure-freezing was ineffective in improving the texture of frozen-then-thawed konnyaku. Key Words: konnyaku, high pressure, freezing, thawing, texture, electron microscope

Introduction proximately 97% water). In other words, its zen but supercooled during pressurization ONNYAKU (KONJAC GLUCOMANNAN unique texture changes after freezing and at 200 to 400 MPa and –20 °C. Thus, we Kgel) is a food gel that is produced from thawing, and damage of the structure is measured the temperature of konnyaku a plant cultivar of Amorphophallus Konjac extensive. during high-pressure-freezing in order to K. koch. It has been a common dish for When water is frozen at atmospheric examine freezing under these pressures. more than 600 years in Japan. It was pressure (0.1 MPa), volume increases (ice Although konnyaku was not frozen during brought from China to Japan as a medi- I). This causes tissue damage during pressurization at 200 MPa at Ϫ20 °C (liq- cine along with Buddhism in the 6th cen- freezing at 0.1 MPa. However, under high uid phase), in this report the freezing tury. However, Europeans called it the pressure, several kinds of ices (ice II to IX) method of 100 to 700 MPa at Ϫ20 °C is re- Devil’s Tongue because of the somewhat with different chemical structures and ferred to as high-pressure-freezing. ugly appearance of the tubers (Ozu and physical properties are formed. The densi- Analysis and evaluation of food texture others 1992). ties of high pressure ices (ice II to IX), ex- can be classified into tests outside of, or The konjac glucomannan is a polysac- ceptice I, are higher than that of water, so within a linear viscoelastic range. Rheolog- charide, made up of D-glucose units alter- high pressure ices do not expand in vol- ical tests outside the linear range, that is, nating with D-mannose units in the ap- ume during phase transition from water to “large deformation” measurement, are re- Food Engineering and Physical Properties proximate 2:3 or 1:1.6 molar ratios, which ices (Fletcher 1970; Franks 1989; Hobbs lated to the sensory texture evaluations are linked by ␤-(1,4) bonds. The gluco- 1974; Maeno 1981). In previous studies, (Bourne 1982). However, testing within a mannan also has branches attached to a we indicated that even in foods with a linear range can provide important data main chain at the C-3 position through ␤- high water content, such as tofu (soybean relating to structure (Chanyongvorakul (1,3) bonds with length of branches as curd) (Fuchigami and Teramoto 1996, and others 1995). In this report, both large short as 2 to 3 units. The branching degree 1997), carrot (Fuchigami and others 1996, deformation and creep compliance tests may range from 1 to 19 units. The molecu- 1997a, 1997b), Chinese cabbage (Fuchi- were performed. Microstructures, deter- lar weight of glucomannan is about 100 ϫ gami and others 1998a), and agar gel mined by cryo-scanning electron micro- 104 Ϫ 120 ϫ 104 (Ozu and others 1992). The (Fuchigami and Teramoto 1998), the dam- scope, were also compared. glucomannan gel (konnyaku) is formed by age of texture and structure was reduced a glucomannan (konjac mannan) sol heat- by high-pressure-freezing at 200 to 400 Results and Discussion ed with alkaline compounds (for example, MPa. Furthermore, we indicated that sodium carbonate or calcium hydroxide). when high-pressure-frozen tofu was Changes in temperature of The molecules of konjac mannan, which thawed at the same pressure (high-pres- konnyaku during high-pressure- lose their acidic moieties with the aid of al- sure-thawing), tofu kept the quality (tex- freezing kalis, aggregate in part with one another ture and structure) the same as unfrozen Changes in pressure and temperatures through a linkage (for example, the hydro- tofu (Fuchigami and others 1998b; Tera- of konnyaku and pressure medium during gen bond) and form a network structure; moto and others 1999). However, the ef- high-pressure-freezing were compared thus a gel is formed (Maekaji 1974). fect of high-pressure-freezing depended (Fig. 1). After the defined pressure was Konnyaku is elastic and insoluble in on the type of food. Therefore, we investi- reached within 1 min, pressure was main- water. Probably, the most important factor gated whether high-pressure-freezing tained for 60 min, then reduced to atmo- of konnyaku is its texture. However, it is and high-pressure-thawing were effective spheric pressure for approximately 20 s. very difficult to produce frozen konnyaku for improvement in quality of frozen kon- The temperature of a pre-chilled thermo- with good textural quality, because konny- nyaku (a gel different from tofu). More- couple was approximately Ϫ20 °C, but the aku is a food with high water content (ap- over, it was assumed that foods are not fro- temperature rose when the thermocouple

was inserted into the sample. The initial 500 to 700 MPa. neither exothermic nor endothermic temperatures of samples were about 15 to Thus, the phase transition of water peaks were detected (not shown). There- 20 °C. However, when konnyaku was fro- from liquid to ice during high-pressure- fore, the phase transitions from liquid to zen in a pressure vessel at atmospheric freezing was detectable in the konnyaku ice and from ice to liquid did not occur pressure, temperature of a sample de- samples as changes in temperature. At during pressurization. This indicated that creased to 1 °C quickly and took about 10 0.1, 100, 500, 600, and 700 MPa, phase the konnyaku was supercooling during min to decrease from 1 °C to Ϫ5 °C. When transition occurred during pressurization. pressurization at 200 to 400 MPa and Ϫ20 pressurized at 100 MPa, konnyaku was su- On the other hand, at 200 to 400 MPa, the °C. percooled to Ϫ8 °C, and it took approxi- konnyaku samples kept supercooling at mately 22 min from Ϫ8 °C to Ϫ10 °C due to Ϫ20 °C, and only when pressure was re- Texture of high-pressure-frozen the release of latent heat. The onset tem- moved did the phase transition occur. konnyaku by large deformation perature of freezing at 100 MPa was Ϫ8 °C, The temperature of the pressure medi- tests while final temperature of konnyaku was um rose slightly at the beginning of pres- Typical stress-strain curves of high- approximately Ϫ20 °C. When pressure was surization due to the heat that evolved pressure-frozen konnyaku with various released, the temperature decreased rap- from pressurization and freezing, but it thawing methods were compared (Fig. 3). idly due to heat absorbed by decompres- decreased quickly and maintained at ap- The averages of rupture stress and strain sion. proximately Ϫ20 °C. No great change of were also compared (Table 1). When kon- When konnyaku was frozen at 200, 340, temperature in the pressure medium lo- nyaku was frozen at 0.1 to 700 MPa and and 400 MPa then supercooled to Ϫ20 °C, cated in the lower part of the pressure ves- thawed at atmospheric pressure (HPF + an exothermic peak was not detected dur- sel was detected during pressurization. CT, HPF + S + CT), the slope of stress- ing pressurization. However, when de- strain curves and the rupture stress of pressurized, the temperature rose quickly Change in temperature of konnyaku increased significantly (p < then decreased to Ϫ20 °C. Thus, the kon- konnyaku during freezing-thawing 0.001), while the rupture strain decreased nyaku supercooled at Ϫ20 °C froze for 8 under high pressure significantly (p < 0.001) compared to un- min when pressure was removed. In other Changes in temperature of konnyaku frozen konnyaku. In other words, the kon- words, the konnyaku froze through pres- during freezing-thawing at high pressures nyaku lost elastic properties and became

Food Engineering and Physical Properties sure-shift-freezing (Kanda and others (HPF + HPT) were compared (Fig. 2). The hard. Thus, the texture of frozen konny- 1992, 1993). defined pressure was maintained below 0 aku differed from that of unfrozen konny- When konnyaku was frozen at 500, 600, °C, but pressure increased as the temper- aku greatly. There were no great differenc- and 700 MPa, konnyaku was supercooled ature rose from 0 to 20 °C. At 0.1, 100, 500, es in either rupture stress or strain of kon- to about Ϫ15, Ϫ6, and Ϫ3 °C, respectively, and 600 MPa, an exothermic phenomenon nyaku frozen at 0.1 to 400 and 700 MPa. and the temperature rose quickly then de- was detected during freezing, while an en- However, when frozen at 500 MPa, the ini- creased gradually. An exothermic peak dothermic reaction occurred during thaw- tial angle of curve was smaller, and the was detected in these samples during ing. The onset temperatures for melting rupture stress was greater than other fro- high-pressure-freezing. When pressure the konnyaku were lower than the solid zen konnyaku, but the decrease in rup- was released at Ϫ20 °C, temperature de- liquid equilibrium line of water experi- ture strain was inhibited. In comparing creased quickly, and the endothermic mentally determined (Fletcher 1970; HPF + CT to HPF + S + CT, the rupture peak was detected. This indicated that the Franks 1989; Hobbs 1974; Maeno 1981). stress and strain of konnyaku did not konnyaku froze during pressurization at On the other hand, at 200 to 400 MPa, change greatly during storage for 1 d in

Fig. 1—Changes in temperature of samples during high-pressure-freezing

492 JOURNAL OF FOOD SCIENCE—Vol. 65, No. 3, 2000 Table 1—Effect of high-pressure-freezing on rupture stress and strain of konnyaku Stress (× 104N/m2) Strain (%) Pressure (MPa) HPF+CT HPF+S+CT HPF+HPT HPF+CT HPF+S+CT HPF+HPT Control 31.0 ± 3.7 33.4 ± 4.6 32.7 ± 4.5 82.4 ± 2.1 81.7 ± 2.4 83.4 ± 1.9 0.1 44.6 ± 7.1 48.6 ± 3.6 41.9 ± 3.8 55.6 ± 4.2 58.6 ± 3.2 55.2 ± 3.2 100 46.4 ± 2.3 49.0 ± 7.9 46.3 ± 5.5 50.7 ± 3.2 52.2 ± 4.7 56.4 ± 1.5 200 42.1 ± 5.3 46.0 ± 3.2 21.7 ± 4.8 59.5 ± 2.7 55.6 ± 3.7 80.4 ± 2.6 340 41.4 ± 4.1 58.1 ± 4.3 24.3 ± 5.1 62.5 ± 3.3 53.8 ± 2.0 82.0 ± 3.0* 400 41.2 ± 4.3 63.2 ± 8.3 37.7 ± 7.2 64.6 ± 2.2 56.7 ± 3.0 83.9 ± 2.5* 500 73.9 ± 11.8 64.3 ± 9.4 39.1 ± 6.6 78.2 ± 3.7 73.2 ± 5.3 58.7 ± 4.4 600 67.1 ± 3.9 58.5 ± 6.2 47.7 ± 4.4 68.1 ± 1.8 65.4 ± 4.4 62.1 ± 2.0 700 50.1 ± 4.3 49.1 ± 5.0 57.0 ± 3.5 62.2 ± 5.5 “HPF + CT: High-pressure-frozen for 60 min, then immediately thawed at atmospheric pressure;” “HPF + S + CT: High-pressure-frozen for 60 min, stored for 1 d at –30 °C, then thawed at atmospheric pressure;” “HPF + HPT: High pressure-frozen for 60 min, then immediately thawed at high pressure for approximately 70 min.” *No significant difference compared to control.

the freezer. compared to conventionally thawed kon- ery followed by further recovery. Finally, ir- As a result, when high-pressure-frozen nyaku. Due to unfreezing (supercooling), reversible deformation was detected due konnyaku was thawed at atmospheric the rupture stress and strain of konnyaku to viscoelasticity. pressure, great textural changes occurred. frozen then thawed at 200 to 400 MPa The instantaneous creep compliance Therefore, even a high-pressure-freezing were similar to unfrozen konnyaku. of the unfrozen control was larger than method had no recognizable effect. This that of the frozen-thawed konnyaku (HPF result differed from the results of tofu Texture of high-pressure-frozen + S + CT): control > 500 MPa > 700 MPa > (Fuchigami and Teramoto 1996, 1997; konnyaku by creep compliance 0.1 to 400 MPa, respectively. This implied Fuchigami and others 1998b; Teramoto tests that gels frozen at 0.1 to 400 MPa were and others 1999) and agar gel (Fuchigami Creep and recovery creep compliance stiffer. Furthermore, the difference in and Teramoto 1998). According to the curves for frozen konnyaku were com- creep-compliance curves was noted in the kinds of gels, the effect of high-pressure- pared (Fig. 4). When a 10 g weight was retarded deformations. When konnyaku freezing on improving the texture varied placed on konnyaku, there was an imme- was frozen below 400 MPa, the compliance greatly. diate deformation called “instantaneous first increased rapidly followed by a slower On the other hand, when high-pres- elastic deformation.” This was followed by increase at the same rate as unfrozen kon- sure-frozen konnyaku was thawed at the deformation that continuously increased nyaku, but creep and recovery creep com- same high pressure (HPF + HPT), changes over time. When the weight was removed, pliances decreased. On the other hand, in rupture stress reduced only slightly there was an instantaneous elastic recov- when konnyaku was frozen above 500 Food Engineering and Physical Properties

Fig. 2—Changes in temperature of samples during freezing-thawing under high pressure

Vol. 65, No. 3, 2000—JOURNAL OF FOOD SCIENCE 493 High-pressure-frozen Konnyaku . . .

Table 2—Viscoelastic parameters of high-pressure-frozen konnyaku E (×105N/m2) ␩ (×106Pa · s) Pressure 0 N (MPa) HPF+CT HPF+S+CT HPF+HPT HPF+CT HPF+S+CT HPF+HPT Control 1.4 ± 0.3 1.3 ± 0.2 1.4 ± 0.2 27.7 ± 4.3 26.9 ± 2.9 29.4 ± 2.5 0.1 4.0 ± 0.2 3.9 ± 0.3 4.0 ± 0.2 27.0 ± 5.8 26.3 ± 6.0 27.0 ± 5.8 100 4.3 ± 0.5 3.8 ± 0.3 2.7 ± 0.6 30.9 ± 4.2 24.7 ± 3.8 30.9 ± 8.4 200 3.3 ± 0.7 3.8 ± 0.2 1.1 ± 0.1 46.3 ± 7.5 45.6 ± 5.1 56.9 ± 6.8 340 4.0 ± 0.3 3.5 ± 0.3 1.6 ± 0.3 38.6 ± 4.0 34.5 ± 3.1 66.6 ± 7.1 400 4.0 ± 0.1 3.2 ± 0.4 1.5 ± 0.3 34.6 ± 2.8 27.2 ± 2.8 57.3 ± 4.7 500 3.1 ± 0.2 2.0 ± 0.5 3.0 ± 0.6 63.6 ± 0.7 10.3 ± 1.4 23.5 ± 2.5 600 3.2 ± 0.2 2.7 ± 0.8 3.1 ± 0.7 18.3 ± 3.2 8.5 ± 1.3 22.3 ± 3.0 700 3.5 ± 0.3 3.1 ± 0.3 14.0 ± 2.2 12.6 ± 1.4 HPF + CT: High-pressure-frozen for 60 min, then immediately thawed at atmospheric pressure;” “HPF + S + CT: High-pressure-frozen for 60 min, stored for 1 d at –30 °C, then thawed at atmospheric pressure;” “HPF + HPT: High pressure-frozen for 60 min, then immediately thawed at high pressure for approximately 70 min.” “E0: instantaneous elastic modulus, ␩N:final Newtonian viscosity.”

MPa, the 2 phases (rapid and slow in- + Voigt model + Voigt model + a dashpot is that supercooled (not frozen) konnyaku ␩ crease) were not clearly distinguishable, at the bottom. The creep parameters (E0, was difficult to deform by flow. The N of ␩ ␩ ␩ ␶ ␶ and compliance increased almost at an E1, E2, 1, 2, N, 1, and 2) according to a konnyaku frozen above 500 MPa de- equal speed from beginning to end. 6-element model were calculated. The increased. This suggested that deformation Therefore, compliance of konnyaku frozen stantaneous elastic modulus (E0) and by flow became greater. ␩ at 500 MPa was lower in the beginning, Newtonian viscosity ( N) of frozen-thawed but after 60 s it was higher than unfrozen konnyaku were compared (Table 2). The Structure of high-pressure-frozen ␩ ␶ konnyaku. other parameters (E1, 1, 1: retarded elas- konnyaku When konnyaku was frozen at 200 to 400 tic modulus, retarded viscosity, and retar- Cryo-scanning electron micrographs of Food Engineering and Physical Properties MPa then thawed at the same high pressure dation time of the first retarded compli- konnyaku high-pressure-frozen then ␩ ␶ (HPT), the creep and recovery creep compliance; E2, 2, 2: retarded elastic modulus, thawed under various conditions were ances were the same as unfrozen konnyaku, retarded viscosity, and retardation time of compared in Fig. 5. When konnyaku was while the creep compliance of konnyaku fro- the 2nd retarded compliance) were not frozen at 0.1 to 700 MPa then thawed at at- zen-thawed at 0.1, 100, 500, and 600 MPa shown, due to no significant differences mospheric pressure (HPF + CT, HPF + S + decreased. This implied that they became among all freezing-thawing methods. The CT), ice crystal traces (pores) were ob- stiffer. However, the pattern of curves was E0 of conventionally thawed konnyaku served (low magnification). The size of ice similar to that of unfrozen konnyaku; the was greater to least: frozen at 0.1 to 400 crystals varied with a location of specimen compliance increased at the same rate as MPa > 500 to 700 MPa > control, respec- (1 mm2) for cryo-SEM. There was no nota- unfrozen konnyaku, even above 500 MPa. tively. These results showed the same ten- ble size difference of ice crystals in konny- The changes in the recovery creep compli- dency as the initial angle of stress-strain aku frozen at 0.1 to 700 MPa. It is difficult ance of high-pressure-thawed konnyaku curves (Fig. 3) and the reciprocal of com- to produce homogeneous konnyaku gel (HPT) were smaller compared to those con- pliance at the beginning (Fig. 4). The elas- even for a maker. It appears that not only ventionally thawed (CT). tic modulus shows springiness. The freezing-thawing methods but also the The rheological behavior of viscoelastic springiness of konnyaku became stronger presence of air in konnyaku and coarse or materials can be described by several me- by freezing-thawing. However, E0 of kon- fine structures of gel network affected the chanical models consisting of springs (ex- nyaku frozen-thawed at 200 to 400 MPa size of ice crystals. However, the size of ice hibit elastic property: E) and dashpots (HPT) was the same as the E0 of the unfro- crystals in the frozen konnyaku was con- ␩ ␩ (viscous property: ). A 6-element model zen control, while N was greater than that siderably smaller than that of frozen tofu is expressed as follows: The top is a spring of the unfrozen control. This indicated (Fuchigami and Teramoto 1996, 1997;

Fig. 3—Stress-strain curves of high-pressure-frozen konnyaku. HPF + CT: High-pressure-frozen for 60 min, then immediately thawed at atmospheric pressure; HPF + S + CT: High-pressure-frozen for 60 min, stored for 1 d at -30 °C, then thawed at atmospheric pressure; HPF + HPT: High-pressure-frozen for 60 min, then immediately thawed at high pressure for approximately 70 min. Control: unfrozen.

494 JOURNAL OF FOOD SCIENCE—Vol. 65, No. 3, 2000 Food Engineering and Physical Properties

Fig. 5—Cryo-scanning electron micrographs of high-pressure-frozen konnyaku. HPF + CT: High-pressure-frozen for 60 min, then immediately thawed at atmospheric pressure; HPF + S + CT: High-pressure-frozen for 60 min, stored for 1 d at -30 °C, then thawed at atmospheric pressure; HPF + HPT: High-pressure-frozen for 60 min, then immediately thawed at high pressure for approximately 70 min. Control: unfrozen.

Vol. 65, No. 3, 2000—JOURNAL OF FOOD SCIENCE 495 High-pressure-frozen Konnyaku . . .

Fuchigami and others 1998b; Teramoto and others 1999) and frozen agar gel (Fuchigami and Teramoto 1998). A gel network was observed in unfrozen konnyaku (high magnification). With pressurization at 20 °C, there was no notable change in the minute structure of the gel network; therefore, micrographs are not shown. However, a gel network could not be observed in the any of frozen konnyaku (HPF + CT, HPF + S + CT). This occurred because the gel network was compressed due to the formation of ice crystals. The concentration of solute contain- ing a coagulating agent (calcium hydrox- Fig. 4—Creep-compliance curves of high-pressure-frozen konnyaku. HPF + S + CT: High- pressure-frozen for 60 min, stored for 1 d at -30 °C, then thawed at atmospheric pressure; ide) from the formation of ice crystals HPF + HPT: High-pressure-frozen for 60 min, then immediately thawed at high pressure for might have affected the dense gel net- approximately 70 min. Control: unfrozen. work. Perhaps this is one cause pertaining to the firm condition of frozen konnyaku. Consequently, high-pressure-freezing was ineffective when konnyaku was and dense gel network were observed. during pressurization. thawed at atmospheric pressure. This re- There was no notable difference be- sult differed from that of high-pressure- tween HPT and CT. Conversely, when Conclusions frozen tofu, which maintained a compara- konnyaku was frozen-then-thawed at The texture and structure of konnyaku tively coarse network. 200 to 400 MPa (HPF + HPT), no ice crys- frozen at 0.1 to 700 MPa then thawed at at- The structure of konnyaku frozen- tals were observed. This konnyaku had mospheric pressure changed greatly.

Food Engineering and Physical Properties then-thawed at 0.1, 100, 500, or 600 MPa the same gel network as the unfrozen High-pressure-freezing was ineffective in (HPF + HPT) changed greatly; ice pores konnyaku because they were not frozen improving the texture of frozen konnyaku.

Materials and Methods Table 3—Konnyaku mixture as S) and then thawed at 20 °C (HPF + S 270 g konjac flour + CT); pressure vessel was heated by Sample preparation and method 10 L water circulation-type heater (60 °C) for about of high-pressure-freezing 1 L 1.8 % calcium hydroxide solution 70 min at the same pressure as high- Sashimi konnyaku that was produced pressure-freezing (HPF + HPT). When from konjac flour and calcium hydroxide the temperature of samples reached 20 (Soryo Konnyaku Co. Ltd., Hiroshima- °C, pressure was released (designated ken, Japan) was used. Konjac flour was et), previously kept at approximately as high-pressure-thawing). These gels added to water, stirred slowly for 5 min, Ϫ20 °C by circulation-type cooler (Ϫ35 °C were compared with unfrozen original and allowed to stand 60 to 90 min at to 10 °C), was removed. The samples konnyaku (control) and konnyaku fro- room temperature. Calcium hydroxide were placed into a pressure vessel, a zen in a pressure medium at Ϫ20 °C for solution was added, and the mixture thermocouple was inserted in the middle 60 min at atmospheric pressure (0.1 (Table 3) was stirred vigorously for 1 min part of the sample, then the pressure MPa) then thawed at 20 °C. then poured into forms. The forms were medium (Ϫ20 °C) was poured into a heated for 30 min in 80 °C water. Konny- pressure vessel. Samples were immedi- Large deformation tests aku was decorticated with a borer of 18 ately pressurized at 100 to 700 MPa. The Large deformation tests were per- mm in dia to produce cylinders and cut operation was fully automated, and both formed using a creepmeter (Rheoner, into disks 10 mm thick using an ultrason- pressure and temperatures of the sam- RE-33005, Yamaden Co. Ltd., Tokyo, Ja- ic cutter (Yamaden Co. Ltd., Tokyo, Ja- ple (the upper part of the pressure ves- pan) by the method reported in a previ- pan). The dia of disks shortened approx- sel) and pressure medium (the lower ous paper (Fuchigami and Teramoto imately 15 mm due to shrinkage like rub- part) were recorded. 1997). Thickness of samples was mea- ber. The 8 pieces of konnyaku were vacu- sured using a sample-height counter um-packed in heat-sealed polyethylene Methods of thawing (HC-3305, Yamaden, Tokyo, Japan) then bags. After high-pressure-freezing for 60 punctured by using a plunger (cylindri- High hydrostatic pressure treatments min, samples were thawed as follows cal shape: 5 mm dia, 22 mm long) at 0.5 were carried out using a high pressure (Fuchigami and Teramoto 1998): Pres- mm/s stopping at 99% the thickness us- food processor Dr. Chef (Kobe Steel Ltd., sure was released, and konnyaku was ing a load cell of 2 kg. Because the sur- Kobe, Japan) as described by Kato and thawed immediately at 20 °C for 45 min face area of the specimen was changed others (1997), Fuchigami and others in a low temperature incubator (HPF + from freezing-thawing, a plunger about (1997a), and Fuchigami and Teramoto CT: high-pressure-freezing + conven- 1/3 the dia of the specimen was used for (1997). Propylene glycol (pressure medi- tional thawing); pressure was released, a puncture test. Several different speeds um) in a pressure vessel (6 cm inside dia and konnyaku was immediately stored were used (0.5 mm/s, 1 mm/s, 5 mm/s), and 20 cm high, surrounded with a jack- for 1 d in a freezer of Ϫ30 °C (designated but best results could be obtained at 0.5

496 JOURNAL OF FOOD SCIENCE—Vol. 65, No. 3, 2000 mm/s. Both rupture stress and strain mm/s, and a constant force of 10 g was Structure measurement were indicated. applied to the konnyaku. Creep and re- The structure of the central parts of covery were measured for 1-min inter- high-pressure-frozen konnyaku was ob- Creep compliance tests vals. Results were analyzed using com- served with a cryo-scanning electron mi- Creep behavior was analyzed using a puter software for creep analysis (CAS- croscope (S-4500, Hitachi Co. Ltd., To- creepmeter (Rheoner RE-33005, Ya- 3305-16, ver. 2, Yamaden Co. Ltd., To- kyo, Japan) (Fuchigami and others maden Co. Ltd., Tokyo, Japan). Konny- kyo, Japan). Creep and recovery curves 1995). The magnifications used to ob- aku was compressed by a plunger (5 are shown as compliance (strain Ϭ serve ice crystals and gel networks were mm dia) at a cross-head speed of 1.0 stress). ϫ1000 and ϫ30,000, respectively.

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