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Fracture properties of konjac mannan gel: effect of gel temperature
Food Hydrocollolds YoU, no .2 pp . 147-1 5-1 . 1<)9-1
Fracture properties of konjac mannan gel: effect of gel temperature* Suzann e E.Case l and D .D .H am ann 2
Food Science Department, North Carolina State University. Raleigh, NC 276957624 and I National Starch and Chem ical Co., 10 Finderne A ve., Bridgewater, NJ 08807, USA l To who m correspo ndence sho uld be addressed . Abstract. Frac ture properties of 2% konj ac mann an (KM) gels were deter mined by to rsio n testing at seve ral temper atures. Two treatme nts were eva luated: 3: I and 2: I rati os of alka li:KM (v/w). Ca psta n geo me try gels were eq uilibrat ed to 5 . 20. 35, SO, 65 or 80°C. Shear stress at fracture was invariant with test temperature . Th is respon se suggests a permanently bo nde d network . However , there were no covalent links believed to be pre sent in stabilization of KM ge ls. Shea r strain at failure decrea sed linearly with increasing temperat ure , a typical response of entro py elastic gels. It is obse rved that the 'total' strain at fracture, the sum of mechanically pro duced str ain and internal ther mal moti on strain . remain s con stant thro ughout the rang e of test tempe rat ures . Tot al strain was estima ted at - 3.4 by extrap olation to absolute zer o. KM gels, altho ugh pro bably not covalent in nat ure . res po nd to mechanical deformation as pe rmanently bon ded syste ms during the short stre ss times involve d in the presen t study .
Introduction
Kon jac mann an (KM) , a polysacch aride pol ymer of 13- 1A-linke d D-mannose and o-glucose, ratio 1.6 :1 (1) , is a major component of A morphophallus konjak C. koch . Acetyl groups attached to the saccha ride units are scatte red randomly along the mol ecule , with an occ urre nce of - 1 per 19 glucose or mannose units (2) . A heat sta ble gel is formed upon addition of mild alka li and heat. It is sugges ted th at de acylat ing the polymer removes steric hindrance and the polymer s become free to assoc iate (3) . Molecular weight has been determined by Clegg et al, to be 438 000-490 000 g/mol using light scattering, intrinsic viscosity a nd ge l perm eat ion chroma tography. Thi s tr an slates into a degree of po lyme riza tio n (O P) ran ging fro m - 2400 to -2700. Case et al. (5) studied th e respo nse of KM gel to test temperature using small amplitude oscillatory deformati on . The phase angl e betw een stress and strain d ropped below 1° during ge latio n and remained there th roughout heating and cooling so only the storage modulus, G', component of th e complex modulus, G *, need ed to be considered . G ' increased with temper ature over the 25-85°C heat ing and cooling rang e , demon strating that the gel was rubber (entropy) el astic (6) . Mark (7) defin ed rubber elasticity as a very lar ge deform ability with esse ntia lly complete reco ver ab ility. Mull er (8) suggested th at a material should have the fo llowing cha racteris tics in orde r to exhibit rubber elastic cha rac ter: (i) the polymer sho uld consist of lon g-chain molecules with freely rota ting links, (ii) * Paper No . FS R-93-44 of the Jou rnal Se ries of th e Nor th Caro lina Agr icultur al Research Service , Raleigh, NC 27695-7643. T he use of trad e nam es does not imply e ndo rsement by the North Carolina A gricu ltural Rese ar ch Service of the pro du cts named , nor criticis ms of similar ones not menti on ed .
147
S.E.Case and D.D.Hamann
the secondary forces between molecules must be weak, and (iii) at a few points in the polymer chain there must be a firm cross-link with other molecule s. The third requirement , necessar y to induce the elastomeric reco verability, is obta ined through cross-linking of pairs of segments to pre vent the stretched polymer chains from irreversibly sliding past one another. Cros s-linking may be by covalent links found in proteins, vulcanized rubber and acr ylamide. Other possibilitie s include physical entanglement reinforced by a combination of hydrogen bonds and hydroph obic int eractions so the gel acts as though 'permane nt' cross -links are pre sent. Such structures fall into a group sometimes called physical networks having junction zones rather than point cross-links (9) . Foegeding et af . (10) prop osed that polyacrylamide gels are good reference gels to aid in understanding food gel chemistry and structure . Polyacrylamide gel structure makes it a good choice for representing what we could expect of a food gel formed by permanent point cross-links. Polyacryl amide gels incorporate large quantities of water and are entropy elastic in response to test temperature under both small strain and fracture conditions (10). The se authors show that the shear modulus increas es linearly with test temperature and values at fracture and small strain are not significantly different. The fracture modulus is influenced by fracture shear strain decreasing with increasing temperature with the fracture shear stress being temperature invariant. Temperature influence on fracture properties has been used to help understand stabilization of gel network s. Niwa et al. (11) noted that spherical probe breaking deformation of a fish muscle gel mad e from Alaska pollock incre ased with temperature up to 60°C , but it decreased above this temperature . Fracture force was maximal from -20 to 40°C. The y att ribute the se observations to the combined effect of the increase in the thermally indu ced random motion of the segments at higher temperatures (increased entropy) and hydrophobic interactions which increase in strength up to 60°e. Niwa and co-workers suggest that it is difficult to know which of the two contributes most to the changes in the fracture resistance of the mater ial. Howe et af . (12) used torsion fracture testing to study Ala ska pollock gels and concluded that test temperature influence on fracture shear stress and fracture shear strain of fish gels can be used to understand better the superimposed effects of covalent bonds, hydrophobic associations and hydrogen bonds. One aspect was that increased covalent bonding made the fracture str ain less dependent on test temperature. Fracture strain exhibited a quadratic response to temperature with a maximum in the 4555°C range where hydrophobic associations would be expected to be strong (13). This peak was less evident with increased covalent bonding. Oakenfull and Scott (14) studied the temperature response of hydrophobic interactions in high methoxyl pectin by testing rupture strength of cylindrical specimens compressed axially at temperatures between 0 and 50°e. The curve for the pectin showed a general decrease in strength with increased temperature , but was sigmoidal , with a small peak at -30°e. About half the strength was lost between 0 and 50°e. They compared this with «-carrageenan , which is stabilized through hydrogen bonds, and showed a quadratic decrease in rupture strength with increasing temperature with no strength rem aining at 50°e. 148
Fracture properties of konjac gel
The present study examines temperature response of the fracture properties of KM gels using torsion testing.
Materials and methods Konja c mannan gel preparation KM obtained from Ajinomot o, Co ., Inc. (Tokyo , Japan ) , was eth anol extracted and spray dried prior to shipme nt. The KM was used without further purification. Composition was - 90 .6% glucomanna n, 8.50% moisture , 0.09 % nitrogen and 0.81% ash . A trace amount of starch was identified by polarizing light microscope and by iod ine staining, but not quantified . Tw o percent KM was hydr ated by mixing with deionized water in a Stephan vertical cutter/mixer (model U MC 5-1, Stephan Machine Co rp. , Columbus , OH) at 600 r.p.m. for 2 min followed by 300 r.p.m, for 3 min . After hydrating for 1 hat 5°C, the material was sheared by mixing for 3 min at 1200 r.p.m. and held another 30 min at room temperature. Two treatments were tested: K 2CO:; (0 .16 N) added at 2:1 or 3:1 (v/w) with the KM. The alk ali was added in three portions and mixed for 5 min each time. This was critical for adequate dispersion of the alk ali due to the high viscosit y. All mixing took place under a vacuum to redu ce incorporation of air int o th e mixture . The mixture was vacuum packed in a plastic bag for extrusion , with a hand ope rated saus age stuffe r (Vogt series 9 , German y) , into stainless steel tubes (1.87 cm diameter , 17.75 cm length) . Tubes were capped and placed in a wat er bath at 75°C for 1 h. The tubes we re cooled in an ice-ba th for 30 min and gels held 18-24 h in the tubes at 5°C before te stin g.
Torsion testing Tor sion fracture analysis was used to determine fracture properties of the KM gels at 5 , 20, 35 , 50, 65 and 80°C. E ach treatment was tested in replicate (but from th e same lot of konj ac m ann an) using a Torsion Gelometer (Ge l Co nsulta nts , Raleigh , NC). Te sting shear strain rate was 0.125 S- I . A minimum of 10 sa mples from each repli cate were tested at each temperature . The gels were prepared for torsion analysis by cutting the cylindrical specime ns into 2.87 ern lengths and gluing (with cyanoacrylate glue) onto not ched styrene disposable disks . Specimens were ground into capstan shapes (10) with a minimum diameter of 1.0 ern using a specimen sha ping machine (Gel Consultants). Each test temperature was obtained by lowering the gelometer, with the specimen, into a water bath at the appropriate temperature . Specimens were held for 3-5 min prior to twisting for temperature equilibration . In related work with protein gels having protein contents > 10% (12) three min was found to be ad equate to bring the temperature of the test sectio n of the specimen to within l aC of the wat er bath temperature at the high est test temperature . Thermal conductivity of 2% konj ac gels is greater because of th e higher water content. The shear stress at fra cture and shear strain were calculated by the Gelometer software using the appro priate equations for the specimen geometry
(15). 149
S.E .Case and D.D.Hamann
Results and discussion Fracture stress of KM ge l is indepe ndent of temperatur e as see n in Figure 1A. T he frac ture shea r stress invariance wit h cha nge in test tempe rat ure is th e sa me as is see n for po lyacryla mide gel (10). T his finding was unexpected for KM gel, which is believed not to have a ny covalent bonds prese nt for ge l sta bilization . Cova lent bo ndi ng is discounted base d on Maekaji's (16) find ings using 'pe ptizing agents ' . The KM gel could be disso lved int o a solutio n in the presence of man y peptizing reagen ts, including urea and KSCN. This was reversible whe n th e reagent was dia lyzed away. T hese res ults sugges t tha t stabi lization of the KM gel occurs through hydrogen bo nd ing or hydrophobic interactions. Previous work on non -covale ntly link ed ma terial has shown fracture stress and strai n depende nce on temperatu re as discussed in the introduc tio n. It is be lieved that the combine d effects of hydr ogen bonds, weaken ed with incr easing temperature (17), an d hydro phobic interactions, strengthe ned with increasin g temperature (11,18 ,19) , could ge ne ra te a complex res po nse to test temperature . Howe et al. (12) working with a fish protein system, were ab le to prepare gels 80
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Fig. I. Fracture response of 2% KM gels to test temperat ure: (A) shear stress . (B) shear strain; • • 3: 1 (alkali/KM) ; 0 , 2: I (alkalilKM) . Respective regression equation s for strai n are : y = 2.43 O.00373x : R 2 = 0.92, and y = 2.26 - O.OO39 1x: R 2 = 0.94.
150
Fracture properties of konjac gel
which differed in the extent of covalent cross-linking due to transglutaminase ('set' surimi gels and less covalently linked 'unset' surimi gels). Fracture stress of the unset gels showed a quadratic response to test temperature increasing from 5 to 30°C then decreasing at an increasing rate <65°C. Fracture stresses for the set gels were higher and showed a linear decrease in stress with increasing temperature. It is believed that in the system with greater transglutaminase activity, the hydrogen bonding superimposed on greater covalent bonding was observed in the temperature response. The stress data in the present paper suggests the uniqueness of KM gel. The observed invariance with change in test temperature is what one would expect from a system stabilized purely by covalent cross-links with no hydrogen bonds or hydrophobic interactions superimposed on the covalent links. The fracture shear strains of konjac gels show a linear decrease in strain as the specimen temperature increases (Figure 1B). This also is the response to temperature one might expect of a permanently bonded system. Polyacrylamide gels show a similar linear response to temperature (10). Both fracture stress and fracture strain of konjac gel respond qualitatively to test temperature in the same manner as polyacrylamide gel, so it is necessary to examine quantitative differences to discern between them on the basis of fracture. The observed temperature dependence of KM gel fracture strain may be understood by theorizing that there is a 'total' strain made up of a combination of internal thermal motion strain plus externally imposed deformation strain. Since fracture stress was independent of temperature, the 'total' strain at fracture might also be expected to remain constant throughout the test temperature range. What is believed to happen is that the internal entropy driven motion increases with an increase in temperature causing a decrease in external deformation strain at fracture. Fracture still occurs at the same total strain, but a larger portion is in the form of strain due to internal thermal motion, which cannot be measured by deformation testing. When strain is plotted against temperature (Kelvin), assuming a linear response throughout the temperature range, it is possible to extrapolate to a temperature of absolute zero where only strain due to external deformation would be observed and the 'total' strain would be measured by deformation testing. In the present case, total strain required for fracture was estimated to be 3.5 and 3.4 for 3:1 and 2:1 ratios of alkali:KM respectively. It is noteworthy that this is about the same value obtained for a polyacrylamide gel (10). It would seem that crosslin king by junction zones in konjac gel produces fracture characteristics nearly identical to those produced by point cross-links in polyacrylamide gel. From Figure 1B we estimate that at 5°C about two thirds of the total strain would be due to external deformation. Foegeding et al. (10) show that relaxation does not occur in a polyacrylamide gel. In contrast, Case (20) shows that stress relaxation occurs in KM gels. In the work of Case (20) a generalized Maxwell model composed of three Maxwell units in parallel was developed and described as representing entanglements stabilized through hydrogen bonds and hydrophobic interactions. The two smallest value relaxation times (0.02 and 0.97 h) were test temperature 151
S.E.Case and D.D.Hamann
invariant, but the largest value relaxation time became larger as temperature increased, changing from ~ 10 to > 100 h as temperature increased from 10 to 50°C. This was considered due to the high viscosity of the dashpots in the generalized Maxwell model (long relaxation times). Fracture testing can be considered a short duration test since failure occurs at :::;30 s. Therefore, the entanglement system reacts to temperature as one would expect a covalently linked or permanently bonded system would with no change in fracture stress at different test temperatures. Shear modulus (at fracture) response to temperature shows an increase with increasing test temperature (Figure 2). This was also observed in small strain oscillatory testing during heating/cooling cycles (5) and the response of the relaxation test initial modulus to temperature (20). Fracture shear modulus, being the ratio [shear stress]/[shear strain], G = aJE s , is controlled by the change in fracture strain with temperature. Rubber elasticity theory yields (21): (1) where Me is the mean molecular weight between crosslinks, c is the concentration of the polymer, R is the universal gas constant and T is the absolute temperature (K). In our work the only variable on the right side of equation (1) is T so G and T are directly linearly related if KM gel is truly rubber elastic. Figure 2 supports KM being rubber elastic on this basis. Equation 1 also predicts that G will be zero at absolute zero temperature. Experimentally this is not the case. Both lines in Figure 2 extrapolated to zero K have positive intercepts. Averaging the two alkali treatments, the intercept value is ~ 12 kPa. Extrapolation of polyacrylamide gel regressions for either fracture or small strain moduli to absolute zero produces intercepts near zero as the theory predicts (10). This suggests that KM gel does not follow rubber elastic 30
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152
Fracture properties of konjac gel
theory comparable to polyacrylamide. Departure from rubber elastic character is further substantiated by the fact that small strain G values are only a fraction of fracture values (5) and small strain G versus temperature regressions are much different with extrapolations to absolute zero yielding large magnitude negative intercepts. The initial (small strain) shear modulus of a 2.5% KM gel subjected to constant shear strain, i.e. stress relaxation testing (20), was -1/10 that of the shear modulus at fracture shown in the present paper, for 2.0% KM gels. This can be explained by the shape of the torque-angular rotation curve from the torsion test (Figure 3). Polyacrylamide gels have a linear torque-angular rotation relation (10). For the KM gel Figure 3, has a smaller slope in the small strain region (early in torsion testing), the slope increasing at higher strains. In relaxation testing, where the strain imposed was 0.25, the modulus was much lower than for fracture testing (strains -2-2.5). A mechanism for this may involve a weak structure based on low energy bond strength that is overcome at small deformation before the multiple-chain junction zones are fully stressed. Conclusions Under short time deformation to fracture, with test temperature as the independent variable, KM gel behaves like polyacrylamide gel, which is known to be stabilized by covalent bonds. KM gels do not behave like polyacrylamide gels in the following ways: (i) Small strain values of G are a small fraction of fracture G values whereas for polyacrylamide gels they are the same. (ii) KM gel fracture G versus temperature regressions have positive intercepts at absolute zero in contrast to polyacrylamide gel which intercepts near the origin. (iii) Based on other work cited, KM gels do relax in a stress relaxation test, whereas polyacrylamide gels do not. An overall conclusion is that KM gels are not as permanently stabilized as polyacrylamide gels.
Angular Rotation
Fig. 3. Qualitative torque-angular rotation curve from torsion testing of 2% KM gel at 25°C; strain rate = 0.125 S-l
153
S.E.Case and D.D.Hamann
References Kato,K. and Matsuda,K. (1969) 1. Agric. Bioi. Chem., 33,1446-1453. Maekaji,K. (1978) J. Agric. Bioi. Chem., 42,177-178. Maekaji.K. (1974) J. Agric. Bioi. Chem., 38, 315-321. Clegg,S.M., Phillips,G.O. and Williams,P.A. (1990) In Phillips,G.O., Wedlock,D.l. and Williams,P.A. (eds), Gums and Stabilizers for the Food Industry, Vol. 5. Oxford University Press, New York, pp. 463-474. 5. Case,S.E., Knopp,J.A., Hamann,D.D. and Schwartz,S.l. (1992) In Phillips,G.O., Williams, P.A. and Wedlock,D.l. (cds), Gums and Stabilizers for the Food Industry, Vol. 6. Oxford University Press, New York, pp. 489-500. 6. Flory,P.l. (1953) Principles of Polymer Chemistry. Cornell University Press, Ithaca, NY. 7. Mark,l.E. (1984) In Mark,J.E., Eisenberg.A.; Graessley,W.W., Mandeklern,L. and Konig,J.L. (cds), Physical Properties of Polymers. American Chemical Society, Washington, DC, pp. 1-95. 8. Muller,H.G. (1973) An Introduction to Food Rheology. Heinemann, London. 9. Clark,A.H., Ross-Murphy,S.B., Nishinari.K. and Watase.M. (1990) In Burchard,W. and RossMurphy,S.B. (eds), Physical Networks. Elsevier Applied Sci. Pub., London, pp. 209-229. 10. Foegeding,E.A., Gonzalez.C.; Hamann,D.D. and Case,S.E. (1994) Food Hydrocoll., in press. 11. Niwa,E., Chen,E., Wang,T., Kanoh.S. and Nakayama,T. (1988) Nippon Suisan Gakkaishi, 54, 1789-1793. 12. Howe,J.R., Hamann,D.D., Lanier,T.e. and Park,J.W. (1994) J. Food Sci., in press. 13. Scheraga,H.A., Nernethy.G. and Steinberg,I.Z. (1962) J. Bioi. Chem., 237, 2506-2508. 14. Oakenfull,D. and Scott.A. (1984) 1. Food Sci., 49, 1093-1098. 15. Hamann,D.D. (1991) In Parris.N. and Barford.R. (eds), Interactions of Food Proteins. American Chemical Society, Washington, DC, Symposium Series 454, pp. 212-227. 16. Maekaji.K. (1973) J. Agric. Bioi. Chem., 37, 2433-2434. 17. loesten,M.D. and Schaad,L.l. (1974) Hydrogen Bonding, Marcel Dekker, New York, p. 182. 18. Oakenfull,D. and Fenwick,D.E. (1977) Aust. J. Chem., 30, 741-752. 19. Ben-Nairn.A. (1980) Hydrophobic Interactions. Plenum Press, New York, p. 185. 20. Case,S.E. (1992) Rheology and Chemistry of Konjac Mannan. PhD Thesis, North Carolina State University, Raleigh, Ne. 21. Treloar,L.R.G. (1975) The Physics of Rubber Elasticity, 3rd edn, Clarendon, Oxford. I. 2. 3. 4.
154
Fracture properties of konjac mannan gel: effect of gel temperature* Suzann e E.Case l and D .D .H am ann 2
Food Science Department, North Carolina State University. Raleigh, NC 276957624 and I National Starch and Chem ical Co., 10 Finderne A ve., Bridgewater, NJ 08807, USA l To who m correspo ndence sho uld be addressed . Abstract. Frac ture properties of 2% konj ac mann an (KM) gels were deter mined by to rsio n testing at seve ral temper atures. Two treatme nts were eva luated: 3: I and 2: I rati os of alka li:KM (v/w). Ca psta n geo me try gels were eq uilibrat ed to 5 . 20. 35, SO, 65 or 80°C. Shear stress at fracture was invariant with test temperature . Th is respon se suggests a permanently bo nde d network . However , there were no covalent links believed to be pre sent in stabilization of KM ge ls. Shea r strain at failure decrea sed linearly with increasing temperat ure , a typical response of entro py elastic gels. It is obse rved that the 'total' strain at fracture, the sum of mechanically pro duced str ain and internal ther mal moti on strain . remain s con stant thro ughout the rang e of test tempe rat ures . Tot al strain was estima ted at - 3.4 by extrap olation to absolute zer o. KM gels, altho ugh pro bably not covalent in nat ure . res po nd to mechanical deformation as pe rmanently bon ded syste ms during the short stre ss times involve d in the presen t study .
Introduction
Kon jac mann an (KM) , a polysacch aride pol ymer of 13- 1A-linke d D-mannose and o-glucose, ratio 1.6 :1 (1) , is a major component of A morphophallus konjak C. koch . Acetyl groups attached to the saccha ride units are scatte red randomly along the mol ecule , with an occ urre nce of - 1 per 19 glucose or mannose units (2) . A heat sta ble gel is formed upon addition of mild alka li and heat. It is sugges ted th at de acylat ing the polymer removes steric hindrance and the polymer s become free to assoc iate (3) . Molecular weight has been determined by Clegg et al, to be 438 000-490 000 g/mol using light scattering, intrinsic viscosity a nd ge l perm eat ion chroma tography. Thi s tr an slates into a degree of po lyme riza tio n (O P) ran ging fro m - 2400 to -2700. Case et al. (5) studied th e respo nse of KM gel to test temperature using small amplitude oscillatory deformati on . The phase angl e betw een stress and strain d ropped below 1° during ge latio n and remained there th roughout heating and cooling so only the storage modulus, G', component of th e complex modulus, G *, need ed to be considered . G ' increased with temper ature over the 25-85°C heat ing and cooling rang e , demon strating that the gel was rubber (entropy) el astic (6) . Mark (7) defin ed rubber elasticity as a very lar ge deform ability with esse ntia lly complete reco ver ab ility. Mull er (8) suggested th at a material should have the fo llowing cha racteris tics in orde r to exhibit rubber elastic cha rac ter: (i) the polymer sho uld consist of lon g-chain molecules with freely rota ting links, (ii) * Paper No . FS R-93-44 of the Jou rnal Se ries of th e Nor th Caro lina Agr icultur al Research Service , Raleigh, NC 27695-7643. T he use of trad e nam es does not imply e ndo rsement by the North Carolina A gricu ltural Rese ar ch Service of the pro du cts named , nor criticis ms of similar ones not menti on ed .
147
S.E.Case and D.D.Hamann
the secondary forces between molecules must be weak, and (iii) at a few points in the polymer chain there must be a firm cross-link with other molecule s. The third requirement , necessar y to induce the elastomeric reco verability, is obta ined through cross-linking of pairs of segments to pre vent the stretched polymer chains from irreversibly sliding past one another. Cros s-linking may be by covalent links found in proteins, vulcanized rubber and acr ylamide. Other possibilitie s include physical entanglement reinforced by a combination of hydrogen bonds and hydroph obic int eractions so the gel acts as though 'permane nt' cross -links are pre sent. Such structures fall into a group sometimes called physical networks having junction zones rather than point cross-links (9) . Foegeding et af . (10) prop osed that polyacrylamide gels are good reference gels to aid in understanding food gel chemistry and structure . Polyacrylamide gel structure makes it a good choice for representing what we could expect of a food gel formed by permanent point cross-links. Polyacryl amide gels incorporate large quantities of water and are entropy elastic in response to test temperature under both small strain and fracture conditions (10). The se authors show that the shear modulus increas es linearly with test temperature and values at fracture and small strain are not significantly different. The fracture modulus is influenced by fracture shear strain decreasing with increasing temperature with the fracture shear stress being temperature invariant. Temperature influence on fracture properties has been used to help understand stabilization of gel network s. Niwa et al. (11) noted that spherical probe breaking deformation of a fish muscle gel mad e from Alaska pollock incre ased with temperature up to 60°C , but it decreased above this temperature . Fracture force was maximal from -20 to 40°C. The y att ribute the se observations to the combined effect of the increase in the thermally indu ced random motion of the segments at higher temperatures (increased entropy) and hydrophobic interactions which increase in strength up to 60°e. Niwa and co-workers suggest that it is difficult to know which of the two contributes most to the changes in the fracture resistance of the mater ial. Howe et af . (12) used torsion fracture testing to study Ala ska pollock gels and concluded that test temperature influence on fracture shear stress and fracture shear strain of fish gels can be used to understand better the superimposed effects of covalent bonds, hydrophobic associations and hydrogen bonds. One aspect was that increased covalent bonding made the fracture str ain less dependent on test temperature. Fracture strain exhibited a quadratic response to temperature with a maximum in the 4555°C range where hydrophobic associations would be expected to be strong (13). This peak was less evident with increased covalent bonding. Oakenfull and Scott (14) studied the temperature response of hydrophobic interactions in high methoxyl pectin by testing rupture strength of cylindrical specimens compressed axially at temperatures between 0 and 50°e. The curve for the pectin showed a general decrease in strength with increased temperature , but was sigmoidal , with a small peak at -30°e. About half the strength was lost between 0 and 50°e. They compared this with «-carrageenan , which is stabilized through hydrogen bonds, and showed a quadratic decrease in rupture strength with increasing temperature with no strength rem aining at 50°e. 148
Fracture properties of konjac gel
The present study examines temperature response of the fracture properties of KM gels using torsion testing.
Materials and methods Konja c mannan gel preparation KM obtained from Ajinomot o, Co ., Inc. (Tokyo , Japan ) , was eth anol extracted and spray dried prior to shipme nt. The KM was used without further purification. Composition was - 90 .6% glucomanna n, 8.50% moisture , 0.09 % nitrogen and 0.81% ash . A trace amount of starch was identified by polarizing light microscope and by iod ine staining, but not quantified . Tw o percent KM was hydr ated by mixing with deionized water in a Stephan vertical cutter/mixer (model U MC 5-1, Stephan Machine Co rp. , Columbus , OH) at 600 r.p.m. for 2 min followed by 300 r.p.m, for 3 min . After hydrating for 1 hat 5°C, the material was sheared by mixing for 3 min at 1200 r.p.m. and held another 30 min at room temperature. Two treatments were tested: K 2CO:; (0 .16 N) added at 2:1 or 3:1 (v/w) with the KM. The alk ali was added in three portions and mixed for 5 min each time. This was critical for adequate dispersion of the alk ali due to the high viscosit y. All mixing took place under a vacuum to redu ce incorporation of air int o th e mixture . The mixture was vacuum packed in a plastic bag for extrusion , with a hand ope rated saus age stuffe r (Vogt series 9 , German y) , into stainless steel tubes (1.87 cm diameter , 17.75 cm length) . Tubes were capped and placed in a wat er bath at 75°C for 1 h. The tubes we re cooled in an ice-ba th for 30 min and gels held 18-24 h in the tubes at 5°C before te stin g.
Torsion testing Tor sion fracture analysis was used to determine fracture properties of the KM gels at 5 , 20, 35 , 50, 65 and 80°C. E ach treatment was tested in replicate (but from th e same lot of konj ac m ann an) using a Torsion Gelometer (Ge l Co nsulta nts , Raleigh , NC). Te sting shear strain rate was 0.125 S- I . A minimum of 10 sa mples from each repli cate were tested at each temperature . The gels were prepared for torsion analysis by cutting the cylindrical specime ns into 2.87 ern lengths and gluing (with cyanoacrylate glue) onto not ched styrene disposable disks . Specimens were ground into capstan shapes (10) with a minimum diameter of 1.0 ern using a specimen sha ping machine (Gel Consultants). Each test temperature was obtained by lowering the gelometer, with the specimen, into a water bath at the appropriate temperature . Specimens were held for 3-5 min prior to twisting for temperature equilibration . In related work with protein gels having protein contents > 10% (12) three min was found to be ad equate to bring the temperature of the test sectio n of the specimen to within l aC of the wat er bath temperature at the high est test temperature . Thermal conductivity of 2% konj ac gels is greater because of th e higher water content. The shear stress at fra cture and shear strain were calculated by the Gelometer software using the appro priate equations for the specimen geometry
(15). 149
S.E .Case and D.D.Hamann
Results and discussion Fracture stress of KM ge l is indepe ndent of temperatur e as see n in Figure 1A. T he frac ture shea r stress invariance wit h cha nge in test tempe rat ure is th e sa me as is see n for po lyacryla mide gel (10). T his finding was unexpected for KM gel, which is believed not to have a ny covalent bonds prese nt for ge l sta bilization . Cova lent bo ndi ng is discounted base d on Maekaji's (16) find ings using 'pe ptizing agents ' . The KM gel could be disso lved int o a solutio n in the presence of man y peptizing reagen ts, including urea and KSCN. This was reversible whe n th e reagent was dia lyzed away. T hese res ults sugges t tha t stabi lization of the KM gel occurs through hydrogen bo nd ing or hydrophobic interactions. Previous work on non -covale ntly link ed ma terial has shown fracture stress and strai n depende nce on temperatu re as discussed in the introduc tio n. It is be lieved that the combine d effects of hydr ogen bonds, weaken ed with incr easing temperature (17), an d hydro phobic interactions, strengthe ned with increasin g temperature (11,18 ,19) , could ge ne ra te a complex res po nse to test temperature . Howe et al. (12) working with a fish protein system, were ab le to prepare gels 80
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Fig. I. Fracture response of 2% KM gels to test temperat ure: (A) shear stress . (B) shear strain; • • 3: 1 (alkali/KM) ; 0 , 2: I (alkalilKM) . Respective regression equation s for strai n are : y = 2.43 O.00373x : R 2 = 0.92, and y = 2.26 - O.OO39 1x: R 2 = 0.94.
150
Fracture properties of konjac gel
which differed in the extent of covalent cross-linking due to transglutaminase ('set' surimi gels and less covalently linked 'unset' surimi gels). Fracture stress of the unset gels showed a quadratic response to test temperature increasing from 5 to 30°C then decreasing at an increasing rate <65°C. Fracture stresses for the set gels were higher and showed a linear decrease in stress with increasing temperature. It is believed that in the system with greater transglutaminase activity, the hydrogen bonding superimposed on greater covalent bonding was observed in the temperature response. The stress data in the present paper suggests the uniqueness of KM gel. The observed invariance with change in test temperature is what one would expect from a system stabilized purely by covalent cross-links with no hydrogen bonds or hydrophobic interactions superimposed on the covalent links. The fracture shear strains of konjac gels show a linear decrease in strain as the specimen temperature increases (Figure 1B). This also is the response to temperature one might expect of a permanently bonded system. Polyacrylamide gels show a similar linear response to temperature (10). Both fracture stress and fracture strain of konjac gel respond qualitatively to test temperature in the same manner as polyacrylamide gel, so it is necessary to examine quantitative differences to discern between them on the basis of fracture. The observed temperature dependence of KM gel fracture strain may be understood by theorizing that there is a 'total' strain made up of a combination of internal thermal motion strain plus externally imposed deformation strain. Since fracture stress was independent of temperature, the 'total' strain at fracture might also be expected to remain constant throughout the test temperature range. What is believed to happen is that the internal entropy driven motion increases with an increase in temperature causing a decrease in external deformation strain at fracture. Fracture still occurs at the same total strain, but a larger portion is in the form of strain due to internal thermal motion, which cannot be measured by deformation testing. When strain is plotted against temperature (Kelvin), assuming a linear response throughout the temperature range, it is possible to extrapolate to a temperature of absolute zero where only strain due to external deformation would be observed and the 'total' strain would be measured by deformation testing. In the present case, total strain required for fracture was estimated to be 3.5 and 3.4 for 3:1 and 2:1 ratios of alkali:KM respectively. It is noteworthy that this is about the same value obtained for a polyacrylamide gel (10). It would seem that crosslin king by junction zones in konjac gel produces fracture characteristics nearly identical to those produced by point cross-links in polyacrylamide gel. From Figure 1B we estimate that at 5°C about two thirds of the total strain would be due to external deformation. Foegeding et al. (10) show that relaxation does not occur in a polyacrylamide gel. In contrast, Case (20) shows that stress relaxation occurs in KM gels. In the work of Case (20) a generalized Maxwell model composed of three Maxwell units in parallel was developed and described as representing entanglements stabilized through hydrogen bonds and hydrophobic interactions. The two smallest value relaxation times (0.02 and 0.97 h) were test temperature 151
S.E.Case and D.D.Hamann
invariant, but the largest value relaxation time became larger as temperature increased, changing from ~ 10 to > 100 h as temperature increased from 10 to 50°C. This was considered due to the high viscosity of the dashpots in the generalized Maxwell model (long relaxation times). Fracture testing can be considered a short duration test since failure occurs at :::;30 s. Therefore, the entanglement system reacts to temperature as one would expect a covalently linked or permanently bonded system would with no change in fracture stress at different test temperatures. Shear modulus (at fracture) response to temperature shows an increase with increasing test temperature (Figure 2). This was also observed in small strain oscillatory testing during heating/cooling cycles (5) and the response of the relaxation test initial modulus to temperature (20). Fracture shear modulus, being the ratio [shear stress]/[shear strain], G = aJE s , is controlled by the change in fracture strain with temperature. Rubber elasticity theory yields (21): (1) where Me is the mean molecular weight between crosslinks, c is the concentration of the polymer, R is the universal gas constant and T is the absolute temperature (K). In our work the only variable on the right side of equation (1) is T so G and T are directly linearly related if KM gel is truly rubber elastic. Figure 2 supports KM being rubber elastic on this basis. Equation 1 also predicts that G will be zero at absolute zero temperature. Experimentally this is not the case. Both lines in Figure 2 extrapolated to zero K have positive intercepts. Averaging the two alkali treatments, the intercept value is ~ 12 kPa. Extrapolation of polyacrylamide gel regressions for either fracture or small strain moduli to absolute zero produces intercepts near zero as the theory predicts (10). This suggests that KM gel does not follow rubber elastic 30
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Fig. 2. Effect of test temperature on the fracture shear modulus of the 2% KM gels: ., 3:1 (alkali/ KM); 0, 2: 1 (alkali/KM). Respective regression equations for shear modulus are: y = 25.2 + 0.0368x: R 2 = 0.51 and y = 19.6 + 0.0450x: R 2 = 0.86.
152
Fracture properties of konjac gel
theory comparable to polyacrylamide. Departure from rubber elastic character is further substantiated by the fact that small strain G values are only a fraction of fracture values (5) and small strain G versus temperature regressions are much different with extrapolations to absolute zero yielding large magnitude negative intercepts. The initial (small strain) shear modulus of a 2.5% KM gel subjected to constant shear strain, i.e. stress relaxation testing (20), was -1/10 that of the shear modulus at fracture shown in the present paper, for 2.0% KM gels. This can be explained by the shape of the torque-angular rotation curve from the torsion test (Figure 3). Polyacrylamide gels have a linear torque-angular rotation relation (10). For the KM gel Figure 3, has a smaller slope in the small strain region (early in torsion testing), the slope increasing at higher strains. In relaxation testing, where the strain imposed was 0.25, the modulus was much lower than for fracture testing (strains -2-2.5). A mechanism for this may involve a weak structure based on low energy bond strength that is overcome at small deformation before the multiple-chain junction zones are fully stressed. Conclusions Under short time deformation to fracture, with test temperature as the independent variable, KM gel behaves like polyacrylamide gel, which is known to be stabilized by covalent bonds. KM gels do not behave like polyacrylamide gels in the following ways: (i) Small strain values of G are a small fraction of fracture G values whereas for polyacrylamide gels they are the same. (ii) KM gel fracture G versus temperature regressions have positive intercepts at absolute zero in contrast to polyacrylamide gel which intercepts near the origin. (iii) Based on other work cited, KM gels do relax in a stress relaxation test, whereas polyacrylamide gels do not. An overall conclusion is that KM gels are not as permanently stabilized as polyacrylamide gels.
Angular Rotation
Fig. 3. Qualitative torque-angular rotation curve from torsion testing of 2% KM gel at 25°C; strain rate = 0.125 S-l
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S.E.Case and D.D.Hamann
References Kato,K. and Matsuda,K. (1969) 1. Agric. Bioi. Chem., 33,1446-1453. Maekaji,K. (1978) J. Agric. Bioi. Chem., 42,177-178. Maekaji.K. (1974) J. Agric. Bioi. Chem., 38, 315-321. Clegg,S.M., Phillips,G.O. and Williams,P.A. (1990) In Phillips,G.O., Wedlock,D.l. and Williams,P.A. (eds), Gums and Stabilizers for the Food Industry, Vol. 5. Oxford University Press, New York, pp. 463-474. 5. Case,S.E., Knopp,J.A., Hamann,D.D. and Schwartz,S.l. (1992) In Phillips,G.O., Williams, P.A. and Wedlock,D.l. (cds), Gums and Stabilizers for the Food Industry, Vol. 6. Oxford University Press, New York, pp. 489-500. 6. Flory,P.l. (1953) Principles of Polymer Chemistry. Cornell University Press, Ithaca, NY. 7. Mark,l.E. (1984) In Mark,J.E., Eisenberg.A.; Graessley,W.W., Mandeklern,L. and Konig,J.L. (cds), Physical Properties of Polymers. American Chemical Society, Washington, DC, pp. 1-95. 8. Muller,H.G. (1973) An Introduction to Food Rheology. Heinemann, London. 9. Clark,A.H., Ross-Murphy,S.B., Nishinari.K. and Watase.M. (1990) In Burchard,W. and RossMurphy,S.B. (eds), Physical Networks. Elsevier Applied Sci. Pub., London, pp. 209-229. 10. Foegeding,E.A., Gonzalez.C.; Hamann,D.D. and Case,S.E. (1994) Food Hydrocoll., in press. 11. Niwa,E., Chen,E., Wang,T., Kanoh.S. and Nakayama,T. (1988) Nippon Suisan Gakkaishi, 54, 1789-1793. 12. Howe,J.R., Hamann,D.D., Lanier,T.e. and Park,J.W. (1994) J. Food Sci., in press. 13. Scheraga,H.A., Nernethy.G. and Steinberg,I.Z. (1962) J. Bioi. Chem., 237, 2506-2508. 14. Oakenfull,D. and Scott.A. (1984) 1. Food Sci., 49, 1093-1098. 15. Hamann,D.D. (1991) In Parris.N. and Barford.R. (eds), Interactions of Food Proteins. American Chemical Society, Washington, DC, Symposium Series 454, pp. 212-227. 16. Maekaji.K. (1973) J. Agric. Bioi. Chem., 37, 2433-2434. 17. loesten,M.D. and Schaad,L.l. (1974) Hydrogen Bonding, Marcel Dekker, New York, p. 182. 18. Oakenfull,D. and Fenwick,D.E. (1977) Aust. J. Chem., 30, 741-752. 19. Ben-Nairn.A. (1980) Hydrophobic Interactions. Plenum Press, New York, p. 185. 20. Case,S.E. (1992) Rheology and Chemistry of Konjac Mannan. PhD Thesis, North Carolina State University, Raleigh, Ne. 21. Treloar,L.R.G. (1975) The Physics of Rubber Elasticity, 3rd edn, Clarendon, Oxford. I. 2. 3. 4.
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