Food Structure

Volume 5 Number 1 Article 11

1986

Texture and Microstructure of () as Affected by Different Coagulants

J. M. deMan

L. deMan

S. Gupta

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Recommended Citation deMan, J. M.; deMan, L.; and Gupta, S. (1986) "Texture and Microstructure of Soybean Curd (Tofu) as Affected by Different Coagulants," Food Structure: Vol. 5 : No. 1 , Article 11. Available at: https://digitalcommons.usu.edu/foodmicrostructure/vol5/iss1/11

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TEXTURE AND MICROSTRUCTURE OF SOYBEAN CURD (TOFU) AS AFFECrED IIY DIFFERENT COAGULANTS

J.M . dcMan. L. deMan and S. Gupta

Department of Food Science University of Guelph Guelph , Ont. , Ca nada NIG 2W I

Abstract Introduction

The coagulating properties of five coagulams and the Tofu. a soybean derived curd. is a l ow~cost. high of the curd obtai ned from soymilk was investigated . Viscosity product which has been widely used in the Orient. In a stud y changes during coagu lation were studied using a Namctrc Vibr..lt­ by Muto et al. (1963), tofu was judged to be ritionally equi ­ ing Sphere Viscometer and texture measurements were made va lent to the protein derived from a mixture of eggs. fi sh and by compression and computer ass isted analysis. pH and amount liver. Depending on the kind and concentnttion of coagulant of solids in the were determined. The microstructure of used. as well as stirring during coagulation and pressure ap­ the tofu was examined by sca nning electron microscopy. It was plied to the curd, tofu ranges in hardness from soft to firm with observed th at CaCI2.2 H20 and MgCI2.6H 20 coagulaled the a moi sture content of70 to 90 % and protei n content of 5 to 16 %. in stantl y while CaS04 .1/2H20. gl ucono delta l a~tonc Making tofu is a relatively simpl e process but due to its bland (GOL) and MgS04 .7 H20 acted comparatively slowl y. The t ex~ n~1 turc. its textural propenies play a big role in inlluencing quality ture of th e curd was greatl y influenced by type and concerHra­ and consumer acceptabi lit y. Shurtleff and Aoyagi (1984) presen t ~ ti on of coagulant. Curd obtained with CaCI2.2H20 and ed a good review on the 1hanufacturing of tofu . The va riety of MgCI2.6 H20 was coarse, granular and hard , whereas CaS04. ~oybea n s used may affect the quality of the tofu (Kamel and I/2H 20 and GDL (fresh solution) gave a very smooth . soft and deMan. 1982: Skurray et al. . 1980) and thi s is considered to uniform curd . Amo ng the fi ve coagulants stud ied, 0.75 % be due to differences in protein content of the and the CaSO_. and 0.4 % GD L (fresh solution) appeared to be most ratio of 7S and \IS protein s. Sa io (1979) reported that higher suitable for mak ing tofu of high bulk weight and smooth tex tu re. !-olid s in correla ted wi th harder tofu and increasing CO

Materials and Methods

Key \Vords: Soybean curd . tofu , textu re, viscos ity, mi c ros tru c~ Preparation of lofu ture. coagulation. sca nning electron mic roscopy. sa lts. The ye ll ow hilum Ontario soybeans used in thi s study con­ sa lt ~. g lu co n o ~ O~I acto nc. ta ined 10% moisture. 16.8 % and 36.6% protein . The soy­ bea ns we re made into milk by the following procedure: 300 g of we re soaked ove rnight at 20°C. The soaked beans we re drained, rinsed and blended for 4 min at high speed in a Waring ble nde r with 750 ml of water. The resultant slurry was mi xed with 800 rnl of boiling water and strained through a filt er cloth. The soy milk conmined 10 % total so lid s with 4.7 % protei n and 2.5% fat. Fresh soy milk was used to make tofu.

83 J.M . dcMan. L. de Man and S. Gupta

The following coagul ants were used : CaSO.J. I/2H20 or three rinses with 100% et ha nol. The sample:-. were then rinsed

plaster of Pa ris: CaCI2.2 H20 : MgCh.6H20: MgS0 4 J H20 and three times with chl oroform . C rili c<~ l poi nt dry ing (CPD) was glucono-0-lactonc (GDL). To make tofu . 300 ml of fresh soy milk conducted using C0 2. w.ts heatc<.l to near boiling and the required amount of coagul ant For freeze dryi ng. the samples were dehyd rated using the d i s~o l ved or su ~pc nd ed in 7.5 ml of \vater. The hot soymilk and ethanol se ries, froze n in liqu id nitrogen a nd tran ~fc rred to a coagul ant were poured simultaneously into a g l a ~ s container en­ Polaron E5300 frcete <.Jr ia and dried fo r 24 h. suring good mi xing without stirring. The c urd was left to set All of the samples we re mounted on s tu b~ a n<.J ~ putt e r coat ed for 15 min and the n transferred to a pe rforated plastic container with 20-30 nm of goi<.J palladium (60:40) using a Technics Hum­ with a diamete r of 9 e m and lined with a filter cloth. The curd mer V Sputter Coater. The obse rvations were made at 10 kV. was pressed by applying weights (31.4 g/cm2) for 15 min . After Making of commercial tofu pressing , the curd wa s left in running water for 1 h and then Tofu was made in a commercial tofu pl ant (Vi ctor Food Prod­ stored in a refri ge rator. ucts Ltd . . Toronto. Ont.) by a scmiau !Omatic process using the Viscosity optimum con('cll\rat ion of coagulants ha:-,ed on laboratory A Na mctrc vibrating sphe re viscometer (Nametre Co .. Edi­ experie nce. son. N. J. ) was used to follow changes in viscosity. The hot soy­ Results and Discussion milk and coagul ant were poured simultaneously into a glass con­ tainer ensuring good mix ing, and the vibrating sphere was im­ Results a rc reported o nl y lO r th o!:le co n ce ntra ti o n ~ of coagu­ mediately imme rsed to the mark. The kinematic viscosity was lants which gave with clear or nearly clear whey. The measured as a function of time. This prov ides a non-destructive minimum coagulant concentration required was 0.5 % method of measuring changes in viscosity. CaS04.1/2H, O. 0.15% CaCI2.2H20. 0.3% MgS0 4.7H 20 . 0.2% Texture MgCI 2.6H2 0 or 0.3% GDL. For tex ture evaluation. the mechanical pan or an ln!o,tron Uni­ Viscosil )' ve rsal Te~ t i n g Machine was used. The origin al load !:le nsi ng The coagul ation rates a !-. mea!-tured with the Namctrc vi brating mechanism was replaced with a Daytroni c load cell (cap. 12 ~ ph e rc viscometer are shown in Table I. The coagulation rate kg) and a Daytro nic 9000 strain gage amplific r-in<.J ic ator (Day­ was ve ry rapid with CaCI2.2 H20 and MgCI2.6H 10 a nd visi­ Ironic Corp., Miamisburg. OH ) . The signal voltage wa s tl:.d tO ble whey separation occurred at an early ~ tage . A more gradual an A-D conve ne r (AII3 Interactive Structures. Bala Cy nwyd . in c rease in viscome te r readings was obtained with PA ) and from the re to an Apple li e compute r. The in strument CaSO.l. I/2H10. MgS04 .7H 20 and GDL. With these coagul<:111ts output was stored on !loppy disks and analyzed using a pro­ up to 10 min were required lOr curd formation and no Whey gram developed by the Statistical and Engineering Research In­ se paration was ob ~e rve d . The recordings of viscosit y when using stitute. Agri culture Canada. Ottawa. Ont. (Buckl ey et al .. 1984). diffe rent conce ntratio ns of Cn$0 4 arc presented in Fig. I. The inforrnmion obtai ned by this system ind uded : peak force Sa io (1979) reported that G DL onl y coagulates :-.oym ilk whe n (N). time to peak (~). deformation to peak (mm). fi r mne~~ heated . GDL acti vi ty is influence <.J by tempe rature and time of (N/nun). and force at different points (N). Cylindrical !:!

Moisture sim ilar to curd made with Mg$04 • The a~.: ti ve t:oagulant in For moisture determination about IOOg of tofu was homo­ GDL is glu coni c acid and when a fre!-t hl y prepared GDL solu­ ge nized in u ble nd er and 3-5g dried on a steam bath fOr 15 111 111 tion is aged more g\u('o ni c ac id is fOrmed . The pH of a 0.4 % foll owed by forced air oven drying at 98°- IOO oc ove rni ght. Total GDL solution dropped from 3.0 afte r 10 min to 1.7 aft er 4 h. solids in the whey wa s determined by drying !Or 15 min o n~~ GDL hn s been reponed to recove r more prote in in the 10fu steam bath and 3 h in the oven at 98°-100°C. (Shurtleff and Aoyagi. 1984) and is used for rna king silken tofu. pH Texture pH of the whey was measured using a Fi she r Accumet pH Results of textural evaluation arc prese nted in Table 2 . Re ­ meter model 825 M P. ported values arc me•·ms of six r c p l i ~o: at cs. Peak force values fo r Scanning Electron Microscope Obscrnttions CaS04 .1 /2H20 produced curd ;u e not re ported be<.· ause the A scanning electron mic roscope (ETEC Autoscan) w;b used samples fe ll apart befOre reaching 50 % compression. The hard­ to examine the fin e structure of tofu coagulated wi th diffc rem est curd was obtained wi th 0.57o MgCb.6H20. It appears from coagulants. The procedure used tOr sample prepara tion was that these data that curd firmness can be affec ted by using various of Saio (1981) with some modifications. Small pieces of ( < 2 coagul ants at d iffe re nt concentrations. 111111 cube) we re fi xed at room temperature with 5 % glutaralde­ pH hyde in O.IM phosphate buffer (pH 6.7) lOr 90 min . Afte r li ve According to Lu et a!. (1 980) pH . not the calcium ion concen­ \.v-.ts hes in O.IM phosphate buffer (pH 6.7) at 10 min inte rvals. tration. is by far the most important factor in the precipitation they we re post fixed in I% osm iu m te troxide in the same buf­ or soy prote in . T hese authors reported that protei n starts to fer fo r 90 min at room te mperature. The fi xed samples were coagulate when the pH drops to about 6.0. therefore. according rinsed fi ve times with phosphate buffer at 10 min inte rva ls. to Lu et al. (1980). the addition of should be stopped when Dehydrati on wa s done using a 10 % incremental eth anol series. the pH approaches 6.0. In thi s ~ lllcl y, with all the coagulants leav ing samples at each concentration for 15 min fo ll owc<.J by except GDL. the pH of whey wa s in th e range of 5.89 to 6.25

84 Tt.\x ture and Microstructure of Soybean Curd

T:tble l. Effect of coagulant type and concentration on texture of curd. 0.75% ,.. Force at 25% Coagulant Cone. Peak force compression Firmness % ( N) (Nl (N/mm)

CaS04.1/2H20 0.50 0.47 0. 11 0.75 0.54 0.14 1.00 0.87 0.2 1 CuCI 2.2H20 0.15 1.09 0.54 0. 14 0.20 2 66 1.34 0.30

5 100 200 MgS0,.7H,O 0.30 0.84 0.44 0. 11 KINEMATIC VISCOSITY ,p · Ghm3 0.40 1. 97 0.96 0.25

Fig. 1. Ot'vrlopmcnt of kinematic viscosit y with time of so~ · - MgCI , .6Ho0 0.20 0.80 0.44 0.09 milk wit h 0.5%, 0.75 % and 1.0% added CaS04 .U2H20. 0.30 2.88 1.43 0.36 0.50 3.25 2.37 0.62 Table I. EfiC("t of coagulant type a nd concentration on GDL (hea ted) 0.30 1.13 0.58 0. 15 viscosity of soymilk. 0.40 2.52 1.26 0.32 GDL 0.40 1.24 0.76 0.19 Cn:.tgulant Concentration Kinernatk viscosih (frc ~ h .-,olu tion) (cP.g / nn ·~) at time (1~1i n ) * Samplr.::~ dJ.-,integrated before 50% compression was reached. % ~I 0 10 15 T:.tble 3. Effect of coagulantl)'pe and ("Onccntration on pH CaS04 .1/2H 20 0.50 0.034 7.3 29 .8 72.8 106 .3 0.75 0.052 8.5 45.9 98 .3 IJ5 .3 uf whey. solids in whey, amount of whey and % moisture 1.00 0.069 9.3 70.5 153.0 2 13.5 of tofu.

CaCI2.2 H20 0. 15 0.010 20 .9 19 .6 24.4 29 .8 0.20 0.014 23.5 22.0 25 . 1 30.1 Co a~.:u l a nt Cone. pH of Solids in \\'h{'}' Moisture whey whey MgS0 .7HoO 0.30 0.012 27 .8 41.8 55 .0 68.4 of tofu 4 % % % % 0.40 0.016 45 .0 83 .5 105 .0 128 .0

MgCI2 .6 H20 0.20 0.010 12 .2 24.5 39 .0 )2. J CaSO, . I/2H20 0.50 6.04 3.1 16 .17 9 1. 0 0.30 0.0 15 31 .2 27 .8 .1 5.8 44 .6 0. 75 5.97 3.2 17.50 89.4 0.50 0.025 40.3 33 .7 42 .1 51 .8 1. 00 5.94 3.0 18 .93 89 .1 GDL (heated) 0.30 0.01 7 12.0 28 .3 52.0 75.0 CaCI,.2Ho0 0.15 5.98 3.3 36.57 87.4 0..10 0.022 13.0 53.5 91.0 11 7.5 0.20 5.94 3.3 45 .33 86.4 GDL 0.40 0.022 3.9 81.7 172.0 244 .0 MgS0,.7Ho0 0.30 6.09 3.2 33. 11 88.2 (fresh !)Oi lHi on) 0.40 6.07 3.2 47 .83 86.4 MgCI 2. 6H10 0.20 6.25 3.2 40.00 88.4 (Table 3) and even with the ~ame pH of whey. the t·oagulant .':> 0.30 6.03 3. 1 56.23 85.0 behaved differentl y. For cx;unple, the curd obtained with 0.3 % 0.50 5.89 2.7 59.67 82 .8 MgCI 2.6 H20 wa.-, thrl!c times harde r than the cu rd obwi ned GDL (heated) 0.30 5.52 3.5 42. 10 89.2 with 0.5 % CaS04.1/2H 20 although the pH of whey was 6.0J 0.40 5.27 3.5 49.33 84.8 in both cases. GD L 0.40 5.4 1 3.3 19.43 88.5 Moisture content of tofu a nd solids in whey (fres h solutio n) Results in Table 3 ~ h ow that the coagulant used affects the amount of whey liberatetl and, therefOre. the weight and moi.~oturc (CPD) and freeze-dried (FD) tofu coagulated with 0.4 % fresh content of the lin a I pro<.luct. Wi th increase in coagulant concen­ GDL. The network structure appeared to be similar in the two tration. there wa~ a decrease in moisture contenl of the tofu . picll1res. although the CPD sampl e seemed to have shrunke n With increase in coagul an t concent ration. the struc ture of tofu considerably. CPD has been shown to cause shrinkage (Cohen, became more porou:-. separat ing more whey a nd leaving l e~s 197 7). Freeze drying appeared to be more appropriate for ob­ moislllre in the tofu. serving tofu structu re. In Figures 3 and 4. SEM micrographs There was a ral trend towa rds decrease in sol ids in the of tofu coagulated with diffe rent coagula nt.:; show clearly differ­ whey wi th increase in ('Oagulant concentration. However. the ent fine structures. The mic rostructures as indicated in these difference was not ~ i gnificant. The solids coment of the whey pictures can be easily related w the visually observed texture. increased dramaticall y when the coagulant concentration used Tofu obtained with GDL (fresh solution) W'JS judged best in tex­ was lowe r than the mininlllm concentratio n listed in Table 3. ture on the basis of smoothness. and the micrograph showed Scanning electron microscopy a fine and uniform honeycomblike strucmre (Fig. 3a). The struc­ Figures 2a and 2b are mic rogruph s of the uitical-point dried ture was very unifo rm with smaller holes than those prepared

85 J.M. dcMan. L. dcMan and S. Gupta

Table 4. Texture and moisture cont ent of some co mmercial tofu samples. Sam pi• Coagulant Peak force forte at 20% ··irm nl'SS Moisture compression (N) (N) (Nimm) %

A caso, 1.47 0.93 0.22 87.57 B MgSO, 3.08 1.76 0.45 83.41

c CaS0 4 1.1 2 0.82 0. 17 88.68 D MgSO, 3.68 2.06 0.45 83 .60 E CaCi r 11 . 19 6.23 1.63 70.05 CaSO, F CaCI,- 10 .63 5.44 1.64 73.79 CaS04 during the transfer to the press. Due to this difference. the mois­ ture content of tofu made in the was sligh tly lower than tofu made in the laboratory and turned out to be harder (Table 2 and Table ~ ) . For good curd production and clear whey for mation. the con­ centration of coagulants on a molarity basis ranged from 0.01 to0.02M except for CaS04 .tn H20 which required a minimum of 0.03M. his not feasible to dectdc on a common optimum concentration for all of the coagul ants a~ has been pointed out in some other studies (Wang and Hesseltine, 1982 ; Tsai ct al., 1981) . Tsai eta\. (1981) noticed a dramutic cha nge in the texture of tofu when increasing the concentra tion of coagulant s above 0.03N (0.015M ). The present swdy reemphasizes the different behav iour of va riou s coagul ants. Tab le 4 lists re sults of tex ture and moisture analyses of some commercial tofu sampl es. Tex ture and moisture content of ~am pl es A. B, C and 0 fall in the range obta ined with the laboratory made tofu . It has been suggested that the coagulation of wymilk is due to the crosslinking between protein molecules by divalent cations (Saio ct al .. 1967). HQ\\'CVCr. the !;itc of crru,!-tlinking i' still under debate. Saio et al. (1967) s uggc~tcd that the free carboxyl group of soybean protein is the major site of calcium binding and also acts as a binding !-tile. Accordmg to Appurno and ­ singa Rao (1975) a probable binding !;itc on the protein molecules is the imidazole group. In addition to unccnainties about the binding sites of the soy protei ns, there i ~ a lack of understanding of the mechanism of coagulation with GDL.

Acknowledgements Fi '· 2. Comparison of critical point drying {•1) and frct:LC drying (b) of tofu coagulated with 0.40% GilL (fres h solu­ Financial support was provided by the Natura l Sc iences a nd tion.) (Bar = 5 ~ m ) . Engi neering Research Council of Canada and the Ontario Minis­ try of and Food . Technical support for the SEM with CaCI .2H 0. MgCI .6H 0 and MgS0,.7H ,O. Coagula­ 2 2 2 2 work was supplied by A. Smith. Software for the computer as­ tion with CaS0 .1/2H10 gave a structure si milar to that ob­ 4 sisted texture analys is w·.ts supplied by Statistical and Engineer­ wincd wilh GOL but less unifom1. The SEM picture~ taken at ing Research Service. Agriculture Canada, OttawJ. Ontario. Mr. h1gher mag nification (Fig. 4) of curd made with M gC I :z. 6H~O Stephen Yu of Victor Food Products Ltd .. Scarborough. Ont. and CaCI:z.2 H:zO appear to show si milar structures. The net­ gave helpful advice and provided usc of manufacturing facilities. works of these ~m pl es were not as fine and continuous as those obtained with GDL and CaS04 .112 H20 . MgS04 .7H,O gave a more cont inuous and uniform structure than curd prepared wah References CaCI,.2Hz0 "nd MgCI,.6H, O. Commercial tofu Appu rao AG. Narnsinga Rao MS . (1975 ). Binding of Ca(ll) The process used in the ~!ant was basically the same a~ c~d by the liS fra::: tion of soybca;, protein:;. Chem. 52 . 21-33. in the laboratory. However. in the plant the curd i!i. broken up Buckley DJ , Timbe rs GE, Klock M, Lalande MJ L. (1984).

86 Text ure and M i cro~1ruct ure of Soybean Cu rd

Fi . 3. SEM-images of freeze dri ed tofu prepared with dif­ ferent coagulants. (Bar = 20 ~-t m ). a) - 0.40% GDL (fresh solution) b) - 0.75 % CaS0 4.112H20 <) - 0.30% MgCI,.6H,O d) - 0.15% CaCI,.2H,O e) - 0.30% MgS04 .7H 20

87 J.M . dcMan. L. dcMan and S. Gupta

Fi •. 4. SE~ 1 - im ages of freeze dried tofu prepa red with dif­ Sa io K . (1981 ). Mic rostructure of traditional Japa nese soy­ fe rent coagula nts. (Ba r = 5 fl m). . Scanning Electro n Mi ro!l.c. 1981: 111 :553-559. a) - 0.75 % Caso,.um,o Saio K. Koyama E . Wntanabc T. {1967). PrOiein-calcium­ b) - 0.30% MgCI1.6H 10 phytic acid relationships in soy bean. Pnrt I. Effect of calciu m c) - 0. 15 % CaCI1.2H10 and on the solubi lity chamctcri stics of soybean d) - 0.30% MgS04.7Hz0 prote in . Ag r. Bi oi. C hem . 31, 11 95- 1200. ShurtleffW, Aoyagi A. (1 984). Tofu a11 d soy milk production. Tex w re profi le a nalysis with curve smooth ing using ;;1 pe rsonal The Book of Tofu . Vol. II . Soy food s Ccnlcr. La faycuc, CA, compute r ~y~ t c m . J. Texture Studies. 15, 247- 26 1. U.S.A .. 115 - 131. Cohe n AL. (1 97 7). A critical look at crit ical point dry ing­ Skurray G, Cunich J, Carte r 0. (1 980). The effect of different theory. practice and artefa cts. Scanning El ectron Microsc. 1977: varieties of soy bean and calcium ion concent ration on the quality 1:525- 536. of 1ofu . Food Chcm. 6. 89- 95. Kamel BS. dcMan JM. (1982). Composition and pro perties Tsai SJ. l.an CY, Kao CS. Chen SC. (198 1). S1udies on !he of bean curd made from Ontario soybcans. Can. Jnst. Food Sci. yield and quality cha racte ri sti c!

88 Texture and Micmstructure of So) bean Curd

than for any of the ot her coagulants. Moreover. pH value!:~ of K. Saio: As ~ hown in Fig. 4 the hardnc::,::, of tofu i" mflucnccd whey obtained by GDL coagulation are all below 6 and arc ap­ by the concentration and kinds of coagulant\. J apane~ like tofu proaching the i~oc l ectric range for the . because of ib texture and bland flavor. Different coagulant!, are Authors: GDL acts by opening of the lactone ring to form U!)cd for va rious tofu types. e.g .. CaCI2 for kori tofu. CaS04 gluconic acid. Th1s occurs when GDL is dissolved in water even or MgS04 or Mg Cl1 (with phosphoric or ) for hard at room temperature a:-. is demonstrated in the paper by monitor­ tofu and GDL and CaS04 (alone or with GDL) for silken tofu. ing the pH of a GDL solution at room temperature. Saio (1979) It was mentioned in the paper that tofu coagui::Hcd wi th GDL reponed that GDL-solution should be added to cold soymilk had the best smooth. soft and uniform texture but do you think and then re heated. In industry this means cooling of the soy­ North Americans prefer such silke n tofu to the hard kind? milk and the n reheating wi th GDL solution. It is bette r to dis­ Authors: From our experience it appears that North Ameri cans solve the required amount or GDL in water just before addition pretCr the firmer styles of tofu . These seem to be more suitable to a bmch of hot soy milk coming from the production line as for weste rn style and arc prefe rred in ~a l ads. is done with the other coagulants. The release of gluconic acid K. Saio: Is it possible to di stinguish the difference~ of coagula­ at that temperature results in a very un iform curd as shown in rhc SEM photograph . When the GDL solution is left at room tion Mate among soybean va rieties with a Nametre vibrating sphere viscomete r? tempcralUre for a longer time the reaction with hot soymilk b like acid precipitation and produces a coarse curet. Aut hors: We have tested this on 17 d ifferent va rieties of soy­ beans grown in Ontario. and found no significant differences Reviewer Ill: HQY.• do you define texrure and what proof is there between them. However. these soybeans were harvested at the that -peak fo rce .~ -force at 25% compression·· and - r.nnncssR ~amc time and stored under identical co nd i t ion!~. We .. u~pcct measure texture? that sroragc conditions have a greater effect than v-Jrictal differ­ Authors: Texture can be defined as WThe way in which the ence~ and this is now being investigated. va rious constituents a nd structural elements are a rranged and combi ned into a micro- and macrostrucwre and the C)(tcrna l ma nife~ta ti o n s or this structure in te rms of now and deforma­ tion ." Instrume ntal anal ys1s of textu re involves measurement of mech;.tni cal properties such as resistance to deformation , in thi s case peak force and force at 25% compression and also the stress/strain ratio whic h is defin ed as firmness.

89