Journal of Food Engineering

Transcription

1 Journl of Food Engineering 97 (1) Contents lists vilble t ScienceDirect Journl of Food Engineering journl homepge: Noodle qulity ffected by different cerel strches Yu-Chn Hung, Hsi-Mei Li * Deprtment of Agriculturl Chemistry, Ntionl Tiwn University, No. 1, Roosevelt Rd., Sec. 4, Tipei 1617, Tiwn rticle info bstrct Article history: Received 23 Mrch 9 Received in revised form 28 September 9 Accepted 2 October 9 Avilble online 12 October 9 Keywords: Reconstituted flour Wxy strch White slted noodle Confocl lser scnning microscopy (CLSM) Stress-relxtion test To investigte the effects of strch chrcteristics on the qulity of noodle mking, white slted noodles (WSN) mde from reconstituted flours, in which the whet strch ws substituted by different cerel strches, including wxy nd non-wxy rice strches, wxy whet strch nd wxy corn strch, were prepred. The rheologicl properties of rw WSN were minly influenced by the size of strch grnules, where the smll strch grnules, such s for rice strches, exhibited high mounts of wter bsorption during dough preprtion nd dense pcking of strch grnules inside thin gluten-strnd network. The rheologicl properties of cooked WSN were minly dominted by the mylose content nd fine structure of the mylopectin, which resulted in the differences in wter bsorption nd cooking time required for cooked WSN. Ó 9 Elsevier Ltd. All rights reserved. 1. Introduction Noodles re the mjor whet products in the Asin diet consuming lmost % of whet in Asi (Jnto et l., 1998). Asin noodles re different from pst products in ingredients used, the processes involved nd their consumption ptterns. There is no systemtic clssifiction or nomenclture for Asin noodles; wide differences exist between countries. White slted noodles (WSN), mde using only flour, wter nd slt, cn hve mrkedly different textures bsed on the vribles, including the thickness, shpe, rw or cooked. In generl, good qulity WSN hs bright nd cremy ppernce nd smooth, soft nd elstic texture (Crosbie nd Brcly, 1998). Epstein et l. (2) reported tht the mylose content of strch is mjor contributor to the texture of WSN. The preferred texture of Jpnese WSN, Udon, is obtined by using high proportion of prtil wxy whet flour, which provides soft nd elstic eting texture becuse of its low mylose content, high pek pste viscosity, nd high geltiniztion temperture nd enthlpy (Bik nd Lee, 3; Konik et l., 1992; Miur nd Tnii, 1994; Morris, 1998; Toyokw et l., 1989; Zho et l., 1998). In the cse of Chinese WSN, consumers prefer firmer texture thn Jpnese Udon so tht the greter swelling ssocited with prtil wxy whet is undesirble (Epstein et l., 2). Strches nd their derivtives from different sources hve lso been dded to the flour to improve the cooking nd eting qulities of noodles (Chen et l., 3; Colldo nd Corke, 1996). The reconstituted flour method is often used to evlute the role of strch on the noodle qulity. By * Corresponding uthor. Tel.: ; fx: E-mil ddress: hmli@ntu.edu.tw (H.-M. Li). reconstituted flour from different strches to hve vrious mylose contents, the hrdness of pst nd instnt noodle ws positively correlted to the mylose content (Ginibelli et l., 5; Prk nd Bik, 4). Whet flour dough cn be considered s composite mteril in which gluten forms the continuous mtrix nd strch grnules ct s filler prticles within tht mtrix (Edwrds et l., 2). The rheologicl properties of the composite will depend on the mteril properties of the protein mtrix nd the strch filler, nd the degree of dhesion nd interction between the two. Stress-relxtion nd creep tests hve been performed on dough smples in which the whet strch hd been prtilly or fully replced with different strches nd glss powder to investigte the effects of strch grnule size on the dough rheology (Edwrds et l., 2). Rsper nd demn (198) reported tht grnule prticle size did hve n effect on the dough tensile properties but tht wter-binding cpcity of different strches my exert greter effect. Edwrds et l. (2) indicted tht incresed proportions of smller grnules incresed dough elstic chrcter nd lso confirmed tht strch grnule surfce chrcteristics nd protein strch interctions were responsible for the differences in liner viscoelstic behvior of durum whet flour dough. Although strch constitutes the lrgest volume frction of solids in whet flour dough, the contribution of strch to dough rheologicl properties nd eting qulity of WSN hve not been fully studied. The objectives of this work were to exmine the rheologicl properties nd cooking nd eting qulity of WSN tht were mde with reconstituted flours by substituting whet strch with vrious other cerel strches, including non-wxy nd wxy rice, wxy whet nd wxy corn. The viscoelstic property nd microstructure /$ - see front mtter Ó 9 Elsevier Ltd. All rights reserved. doi:1.116/j.jfoodeng.9.1.2

2 136 Y.-C. Hung, H.-M. Li / Journl of Food Engineering 97 (1) of dough sheet were investigted by stress-relxtion mesurements nd the mtrix structure by confocl lser scnning microscopy (CLSM). The psting properties nd fine structure of the strch molecules were lso nlyzed to relte them to the cooking nd eting qulity of WSN. 2. Mterils nd methods 2.1. Mterils Cerel strches were isolted from two whet vrieties, L488 BR flour (from Austrli) nd full wxy whet flour (from Northern Crops Institute, ND, USA), nd two rice vrieties, Tichung wxy 7 (TCW 7) nd Tichung Sen 1 (TCS 1). A commercil wxy corn strch (Vedn Enterprise Corp., Sh Lu, Tichung, Tiwn) ws lso used in this study. Amylose contents of the wxy whet, TCW 7 nd wxy corn strches determined by colorimetric method (Julino et l., 1981) were less thn 1%. L488 BR whet flour, contining 13.5% moisture, 1.5% crude protein,.7% crude ft, 88% strch nd.5% sh (on dry bsis), ws one of the flours imported from Austrli for Chinese noodle mking. The mylose contents of L488 BR whet strch nd TCS 1 rice strch were 25% nd 13%, respectively. The chemicls used were ll nlyticl grde Isoltion of cerel strches Whet flours (L488 BR nd wxy whet) were frctionted into prime strch, tiling strch, gluten nd wter-soluble substnces ccording to the method of Czuchjowsk nd Pomernz (1993). The prime strch frction, contining 67% strch, ws individully collected nd dried t C in n oven, then ground to pss through 1-mesh screen. The other frctions were combined, dried using freeze-dryer (Pnchum FD-12, Pnchum, Tipei, Tiwn), then ground to pss through 1-mesh screen. All frctions, except the prime strch, were ssigned s the crude gluten portion used for mking the reconstituted flours. The rice strch ws isolted by steeping rice flour (1 g) with 1 L of.5% NOH solution for h t room temperture nd centrifugtion t 3g for 1 min. The sediment ws wshed with distilled wter until neutrl, then ground with blender (Osterizer, Sunbem, Boc Roton, FL) t full speed for 4 min. The slurry ws djusted to ph 7.5 by dding.1 M HCl. The rice slurry ws treted with protese (.29 g) from Bcillus polymyx (Sigm Co., USA) t n enzyme ctivity of 1.2 U/mg t 37 C for 2 h. After the protese tretment, the slurry ws wshed nd centrifuged three times t 6g for min. After removing the top yellowish lyer, the residue ws dried t C, then ground. The prticle size distribution of the strch grnules ws determined using lser scttering prticle size nlyzer (Beckmnn Coulter LS23, CA, USA). The verge prticle sizes (d 5 ) of lrge nd smll grnules of whet strch (L488 BR) were 18.7 nd 4.7 lm, respectively. The d 5 of wxy corn nd rice strches were 12.4 nd 5.35 lm, respectively Preprtion of reconstituted flour The crude gluten portion frctionted from the L488 BR whet flour contined 5.38% moisture, 25.53% crude protein,.3% crude ft, 7.6% strch nd 1.17% sh. The reconstituted flour contined fixed mount of crude gluten nd strch isolted from other cerels so tht the reconstituted flour contined the sme protein content s the L488BR whet flour. The mximum substitution of strch from other cerels ws 67% of the totl strch in the reconstituted flour becuse the crude gluten portion contined some whet strch. Both ntive (L488 BR) nd reconstituted L488 BR (rec-l488 BR) whet flours were prepred s control smples Properties of strch The swelling power of strches ws mesured ccording to the method of Christine et l. (1) with some modifictions. The swelling power of strch ws determined by dding distilled wter (1.5 ml) into 2. ml Eppendorf tube contining mg strch, heting t specific temperture (25, 5, 6, 7, 8 or 95 C) in wter bth for 3 min, centrifuging t 12, rpm (Joun SA, Frnce) for 1 min, discrding the superntnt nd weighing the gel. The Eppendorf tube ws mixed by vortexing before incubtion, then mixed by inverting the tubes twice fter incubting for 5, 15,, 22.5, 25, 26, 27, 28, nd 28.5 min. The swelling power of strch ws clculted by dividing the gel weight by the dry smple weight. The determintions were done triplictes. The psting property of strch nd reconstituted flour ws determined by using Rpid Visco Anlyzer (RVA-4, Newport Scientific, Wrriewood, Austrli). Strch (2 g) or reconstituted flour (3 g) ws dded into n luminum cnister contining 25 g distilled wter. The heting profile ws set s heting from 5 to 95 C, holding t 95 C for 2.5 min, then cooling down to 5 C. The heting nd cooling rtes were C/min. The mesurements were done duplicte. The chin-length distribution of mylopectin ws determined by using high-performnce nion-exchnge chromtogrph equipped with pulsed mperometric detector (HPAEC-PAD) (Dionex DS6, Sunnyvle, CA, USA). The mylopectin ws purified ccording to the method of Tked et l. (1987). Purified mylopectin (2 mg) ws dissolved in 9% dimethylsulphoxide (DMSO) (.5 ml) in boiling wter bth for 1 min, nd then t room temperture for 12 h with stirring. After precipitting the mylopectin by dding 1 ml of 95% ethnol nd setting for overnight, the DMSO ws removed by centrifuging t 5g for 5 min. The purified mylopectin ws then dissolved with 1 ml of 1 mm cette buffer (ph 3.8) nd 8 ll isomylse from Pseudomons sp. (Megzyme, Hyshibr Biochemicl Lbortories, Inc., Okym, Jpn) with the ctivity of.1 U/lL ws dded nd incubted t 5 C in wter bth for 2 h. The enzymtic debrnching rection ws stopped by heting the rectnts in boiling wter bth for 1 min. The debrnched mylopectin solution ws filtered through PVDF membrne filter (.45 lm) before injection into the HPAEC-PAD system. Known degree of polymeriztion (DP) stndrds including glucose (ChemService, PA, USA), mltotriose, mltotetrose, mltopentose, nd mltohexose (Fluk, Buchs, Switzerlnd)) were used to clibrte the elution time versus the DP of the sugrs. HPAEC determintions were done triplictes Dough sheet nd noodle mking The WSN ws mde of 6 g reconstituted flour, 1 ml slt solution (contining 1.2 g NCl) nd vrious mounts of distilled wter bsed on the flour wter bsorption of 35 45% (on flour bsis). The dough ws mixed by hnd then pressed through noodle-mking mchine (ATLAS Model 15, Itly) with roller-gp of 3 mm. The dough sheet ws folded nd turned 9 o before the next sheeting. The folding, turning nd sheeting procedure ws repeted six times. The dough sheet ws pcked with polyethylene bg nd kept in closed continer to prevent wter loss during resting. After resting for 3 min, the folding, turning nd the sheeting procedure ws repeted six dditionl times. After resting for n dditionl min, the thickness of the dough sheet ws reduced by pressing it through rollers with gp of 2.5 mm. For reconstituted rice flour (rec-tcw 7 nd rec-tcs 1), the roller-gp ws mintined t 3 mm for the lst sheeting. A dough piece with dimeter

3 Y.-C. Hung, H.-M. Li / Journl of Food Engineering 97 (1) of 2 cm cut by cookie cutter ws used for the stress-relxtion nd creep tests determintions. The dough sheet ws cut into 1 7 cm (width length) strnds for the evlutions of noodle cooking nd eting qulity. Rw noodles were cooked in boiling distilled wter (1:, w/w) then quenched in 4 C wter (1:1, w/ w) for 3 s. Optiml cooking time ws determined s described in Section 2.8. Surfce wter ws removed by ptting cooked noodle strnds with pper towel. The cooked noodle strnds were covered by plstic wrp then stored in plstic selble bgs. The cooked noodles were stored t 4 C up to 5 dys nd the texture of cooked noodles during storge ws determined s described in Section Microstructure of dough sheet observed by CLSM The microstructure of the dough sheet ws observed with Leic TCS SP2 confocl lser scnning microscopy (CLSM) mounted on n upright DM RXE fluorescence light microscope (Leic Lsertechnik GmbH, Heidelberg, Germny). The strches isolted from different cerels were first stined with 1.% (w/v) Rhodmine B solution (Fluk AG, Buchs, Switzerlnd). The stined strch ws combined with the crude gluten to prepre the reconstituted flour for noodle dough sheet mking. The thickness of the dough sheet ws further reduced to 1 mm for better resolution, nd then dyed with.5% (w/v) FITC (flurorescein isothiocynte, Sigm, USA) solution. After stining for min, the dough sheet ws wshed with dimethylformmide until no more free dye ws relesed. The double dyed dough sheet ws then illuminted vi krypton lser t the excittion wvelengths of Rodmine B nd FITC t 543 nd 488 nm, respectively; emission wvelength regions were 6 65 nd 5 53 nm, respectively. Both imges, tken t the sme focl length, were digitlly superimposed by using Leic Confocl softwre (LCS version 2.61, Wetzlr, Germny) Rheologicl properties of dough sheet The stress-relxtion of dough pieces ws determined by using texture nlyzer with 5 kg cell lod (TA-XT2i, Stble Micro Systems, Surrey, UK) nd cylinder probe (TA-P25, Stble Micro Systems, Surrey, UK). Both rw nd cooked dough pieces, which were cooked following the instruction in Section 2.8, were compressed to 1% strin level for 1 s nd the chnges in force with time were mesured. The test speed ws.1 mm/s for rw dough pieces nd.5 mm/s for cooked dough pieces. Force versus time plots were recorded for ll further dt processing. Stress-relxtion dt were converted to liner form using the following formul (Peleg, 198) Y t ¼ðF F t Þ=F where Y t is decying prmeter, F is the initil force nd F t is the force t ny time t. The intercept k 1 nd the slope k 2 were clculted by plotting t/y t versus t from the normlized function (t/ Y t = k 1 + k 2 t). Percent stress-relxtion (% SR) ws clculted from (F t /F ) 1. The cooked dough pieces were lso exmined by the creep test using texture nlyzer with cylinder probe (TA-P25, Stble Micro Systems, Surrey, UK). The smple ws compressed with constnt force to mintin the strin of the test smple less thn 1% for 1 s. To chieve strin level less thn 1%, the constnt force pplied for the cooked dough pieces of rec-l488 BR, rec-tcw 7, rec-tcs1, rec-wxy whet nd rec-wxy corn ws 1.47, 1.76, 3.43,.74 nd.78 N, respectively. The complince of cooked dough pieces ws clculted by the following eqution (Steffe, 1992) Complince ðp 1 Þ¼e t =r ð1þ ð2þ where r (N/m 2 ) is the stress produced when the compress force (F, N) ws pplied to the smple with specific cross-sectionl re (m 2 ). e t is the degree of deformtion (the height of smple fter compression compred to its initil height) t time t. To exmine the viscoelsticity of cooked noodles, the complince curve dt obtined from the creep-test mesurements were fitted by the Mxwell Voigt model using the following equtions (Steffe, 1992) JðtÞ ¼J þ J 1 ð1 expð t=k 1 ÞÞ þ J 2 ð1 expð t=k 2 ÞÞ þ t=g ð3þ where J is the instntneous complince, J 1 nd J 2 re the viscoelstic complinces, k is the men retrdtion time, g is the viscosity of Mxwell dshpot J mx ¼ e 1 =r where J mx is the mximum viscoelstic complince t the 1 s of creep test, e 1 is the degree of deformtion t 1 s Qulity of noodle The cooking qulity of WSN ws evluted by the optiml cooking time, cooking loss, nd texture of cooked noodles. The optiml cooking time ws defined s the time t which the white core in the centrl portion of the noodle strnd disppered when crushed between two pieces of glss. The cooking loss ws determined by the rtio of residue weight in the cooking wter to the weight of the noodle on dry weight bsis. The texture of freshly cooked nd ged cooked noodles ws evluted by mesuring the cutting force nd stickiness using texture nlyzer with the light knife blde ttchment (A/LKB-F, Stble Micro Systems, Surrey, UK). After storge, the degree of retrogrdtion of cooked noodles ws determined using differentil scnning clorimeter (DSC 291, TA Instruments Inc., New Cstle, DE, USA). Approximtely 3 ±.5 mg of freeze-dried cooked noodle powder ws weighed in n luminum pn nd distilled wter ws dded to obtin wter/strch rtio of 3. The smple pn ws seled nd kept t 4 C overnight. The thermogrms were cquired between 5 nd 1 C t heting rte of 1 C/min. The enthlpy under the retrogrdtion pek of test smple ws clculted by using the Universl Anlysis progrm Version 2.6D (TA Instruments, New Cstle, DE, USA). The degree of retrogrdtion (DR, %) ws clculted by dividing the enthlpy of n ged, cooked noodle by tht of the rw noodle. The geltiniztion enthlpies of rw noodle mde from L488 BR, rec-l488 BR, rec-wxy whet, rec-wxy corn, rec-tcw7 nd rec-tcs1 were 7., 6.6, 14.8, 11., 7.8 nd 9.1 J/g, respectively Sttisticl nlysis All mesurements were t lest preformed in duplicte. Onewy nlysis of vrince ws used to nlyze the dt nd the significnt differences mong mens were compred by the LSD test t level of p <.5. The creep model ws used to fit the dt using the nonliner regression routine in SAS softwre (version 9.1, Institute Inc., Cry, NC, USA) to obtin the corresponding coefficients of creep. 3. Results nd discussion 3.1. Properties of different cerel strches The swelling power (SP) of cerel strches in the temperture rnge of C is shown in Fig. 1. Among wxy strches, wxy whet nd TCW 7 strches took up wter quickly bove 5 C, becuse of lower psting nd geltiniztion tempertures thn wxy corn strch, determined by RVA (Fig. 2) nd DSC (dt not shown). ð4þ

4 138 Y.-C. Hung, H.-M. Li / Journl of Food Engineering 97 (1) Swelling power (g/g) 3 1 L488 BR wxy whet wxy corn TCW 7 TCS 1 Tble 1 Comprison of chin-length distribution of mylopectin of different cerel strches. Strch Proportion (%) DP 6 12 DP DP DP > 37 Whet strch (L488 BR) b bc e Wxy whet strch b c b 1.92 d Wxy corn strch c b b TCW 7 strch bc d 11.3 c TCS 1 strch 27.3 b b c Men vlues in ech column shring the sme letter were not significntly different (p <.5) Temperture ( o C) Fig. 1. Swelling power of cerel strches t different tempertures. There ws no significnt wter uptke of wxy corn strch until 7 C, ttributed to its high onset temperture (68.2 C) determined by DSC. A reltively high SP of wxy strches compred to non-wxy strches ws ttributed to the low psting nd geltiniztion temperture nd low rigidity of strch grnules. It hs been reported tht the low grnulr rigidity of wxy strches ws prtilly ttributed to the lck of mylose (Tsi et l., 1997). Non-wxy strches, L488 BR nd TCS 1, my contin mylose lipid complexes, either inherent or formed during heting tht might prtilly enhnce the rigidity of grnules (Yoo nd Jne, 2). The high rigidity of grnules resulted in the inhibition of wter diffusion into them nd the decrese of wter migrtion rte within Viscosity (cp) Viscosity (cp) L488 BR wxy whet wxy corn TCW7 TCS1 Temp Time (min) 7 b L488 BR rec-l488 BR rec-wxy whet rec-wxy corn rec-tcw7 rec-tcs1 Temp Time (min) Fig. 2. Viscogrms of () strch nd (b) reconstituted flour contining different cerel strches Temperture ( o C) Temperture ( o C) grnules t low temperture rnge (Ghisi et l., 1982; Elisson, 1985). The higher SP of wxy strches t 95 C thn tht of nonwxy strches indicted tht mylopecitn hd higher wter holding cpcity compred to mylose (Sski nd Mtsuki, 1998; Vndeputte et l., 3). The fine structure of mylopectin might ffect the crystllinity of strch grnules nd be relted to the psting temperture nd SP of strches. The fine structure of mylopecitn reveled tht TCW 7 contined reltively high proportion of short chins (DP = 6 12), but low proportion of long chins (DP = 25 36) compred to other cerel strches (Tble 1). High proportion of short chins in mylopectin my cuse crystlline defects of strch grnule (McPherson nd Jne, 1999). This defective crystlline structure, resulting in loss pck of strch grnule, will result in fst wter diffusion in strch grnule. Therefore, the low psting temperture nd high SP t 7 C of TCW 7 were observed. The fine structure of wxy corn strch, contining reltively low proportion of DP = 6 12, but high proportion of DP = 13 24, might be the reson why it hd high psting temperture nd low SP t 7 C. For non-wxy strches (L488 BR nd TCS 1), there re similr proportions of short chins (DP < 24), but L488 BR hd higher proportion of DP nd lower proportion of DP > 37 thn TCS 1. The psting properties of L488 BR nd TCS 1 were more complex thn wxy strches, which might lso be ffected by the mylose content, grnulr rigidity nd the fine structure of mylopectin Psting property of reconstituted flour Psting behvior of strches nd reconstituted flour mde of different cerel strches mesured by using RVA re shown in Fig. 2. The L488 BR, rec-l488 BR nd rec-tcs 1 hd similr psting behviors, except tht shoulder before the pek viscosity (PV) ws observed in L488 BR. The shoulder in the psting curve of ntive L488 BR ws ttributed to the formtion of n mylose lipid complex, which both exists in ntive whet strch nd forms during cooking. About 95% of lipids in L488 BR whet flour were wshed out nd only.3% lipids remined in isolted L488 BR strches. This is the reson why no shoulder ws observed in psting curve of reconstituted nd wxy whet flour (Guo et l., 3). The psting profiles of reconstituted flour were dominted by the type of strch replced, which hd its own specific psting behvior (Fig. 2). The psting temperture of reconstituted flour contining wxy strch ws in the order rec-wxy whet < rec-tcw 7 < rec-wxy corn strch nd the pek viscosity ws in the order rec-tcw 7 < rec-wxy whet < rec-wxy corn (Fig. 2b). The viscogrms of reconstituted flours contining non-wxy strches hd similr ptterns, exhibiting higher psting temperture nd lrger setbck thn those contining wxy strches Microstructure of dough sheet mde of reconstituted flour CLSM ws used to visulize the microstructure of dough sheets mde of reconstituted flours. Strch grnules were seen in red by

5 Y.-C. Hung, H.-M. Li / Journl of Food Engineering 97 (1) Fig. 3. Confocl lser scnning microgrphs of dough sheet mde of reconstituted flour: () L488 BR whet strch dyed with Rhodmine B in dough sheet; (b) L488 BR gluten network dyed with FITC in dough sheet; (c) superposed imges of nd b; (d) gluten network of rec-l488 BR dough sheet with lrge mgnifiction of imge b; (e) gluten network of rec-tcs 1 dough sheet; (f) gluten network of rec-tcw 7 dough sheet. dying with Rhodmine B (Fig. 3), nd the gluten network were seen in green by dying with flurorescein isothiocynte (Fig. 3b). The composite chrcter of dough in which strch grnules were embedded in the continuous gluten-strnd network ws observed by superposing Fig. 3 nd b (Fig. 3c). Since the weight rtio of gluten to strch ws the sme in ll reconstituted flours, the smller the grnule size is, the more strch grnules there could be nd the more strch grnulr surfce needed to be covered by the gluten film. Therefore, the thick gluten strnds (1 1 lm) round whet strch grnules were observed in the dough sheet mde of rec-l488 BR (Fig. 3d), while thin gluten strnds (.5 2 lm) round the rice strch grnules were observed in the dough sheet mde of rec-tcw 7 (Fig. 3e) nd rec-tcs 1 (Fig. 3f) Viscoelsticity of dough sheets mde of reconstituted flour In prctice, dough with consistent consistency is one of importnt fctors for controlling the qulity of noodle dough sheet btch by btch. The dough consistency is usully djusted by dding optiml wter into flour nd the mount of wter ddition hs to depend on the chrcteristics of the flour. The wter content of the dough ws thus djusted to obtin similr consistency of reconstituted flour dough sheets. The dough consistency ws indicted by the initil stress (F ), which hd similr F vlues re shown in the results of stress-relxtion test (Tble 2). High wter bsorption required by reconstituted flours consisting of wxy corn strch nd rice strches ws ttributed to the smller strch grnules tht resulted in more strch grnules nd more strch grnulr surfce needed to be moistened nd hydrted during dough mixing nd dough development s compred to whet strches. The differences in strch grnule size resulted in the different wter-binding cpcities (Crosbie nd Brcly, 1998; Epstein et l., 2; Soh et l., 6). The roller-gp of the finl sheeting process ws not further reduced when prepring rec-tcw 7 nd rec-tcs 1 dough sheets to prevent strong compression of the dough sheet. However, the thickness of dough sheet mde of reconstituted flour consisting of smll strch grnules, such s rec-tcw 7 nd rec-tcs 1, were still thinner thn tht from rec-l488 BR (Tble 2). Similr percent stress relxtion (% SR) vlues (53 56%) of dough sheets mde from reconstituted flours were Tble 2 Rheologicl prmeters of stress-relxtion test of dough sheets mde of reconstituted flour contining different cerel strches. Flour/reconstituted flour Wter bsorption (%) Thickness of dough sheet (mm) Prmeters of stress-relxtion test F (N) F t (N) % SR k 1 (s) k 2 L488 BR rec-l488 BR rec-wxy whet rec-wxy corn rec-tcw rec-tcs LSD (p =.5) F, initil compression force; F t, equilibrted force t s fter compression; % SR, percent stress relxtion, k 1 nd k 2, the intercept nd slope clculted by plotting t/y t versus t from the normlized function.

6 1 Y.-C. Hung, H.-M. Li / Journl of Food Engineering 97 (1) Tble 3 Rheologicl prmeters of stress-relxtion test of cooked white slt noodle (WSN) mde of reconstituted flour contining different cerel strches. Reconstituted flour Optiml cooking time (min) Prmeters of stress-relxtion test F (N) F t (N) % SR k 1 (s) k 2 rec-l488 BR rec-wxy whet rec-wxy corn rec-tcw rec-tcs LSD (p =.5) F, initil compression force; F t, equilibrted force t s fter compression; % SR, percent stress relxtion, k 1 nd k 2, the intercept nd slope clculted by plotting t/y t versus t from the normlized function. Tble 4 Rheologicl prmeters of cooked white slt noodle (WSN) derived from the mesurements of creep test bsed on the Mxwell Voigt model. Reconstituted flour Instntneous complince Viscoelstic complince Men retrdtion time Viscosity Mximum creep complince J 1 5 (P 1 ) J (P 1 ) J (P 1 ) k 1 (s) k 2 (s) g 1 5 (P s) J mx 1 5 (P 1 ) rec-l488 BR rec-wxy whet rec-wxy corn rec-tcw rec-tcs LSD (p =.5) obtined, except those mde of reconstituted flour consisting of rice strches. The dough sheets mde from reconstituted rice strch flours hd low % SR, indicting tht the dough sheets were less elstic. The dense pcking of rice strch grnules inside the thin gluten-strnd network resulted in less open ir spce in the sheet (Fig. 3e nd f), which might cuse the dough sheet to loose its bility to bounce bck fter relesing the compressing force on the dough sheet. k 1, the intercept clculted by plotting t/y t versus t from the normlized function of stress-relxtion dt, is the initil rte of relxtion nd cn be interpreted s the viscous component of the smples (Peleg, 198; Singh et l., 6). All the dough sheets (except for rec-wxy corn) hd similr k 1 vlues, indicting tht the viscous property of the doughs ws not ffected by the type of strch. k 2 represents the portion of the stress tht would hve remined unrelxed t equilibrium (Peleg, 198). There were differences in k 2 vlues, indicting tht there were differences in the elstic ntures of dough sheets. The different viscoelsticities of dough sheets mde of reconstituted flour substituted with different cerel strches would substntilly ffect the cooking nd eting qulity of WSN Viscoelsticity of cooked WSN Both stress-relxtion nd creep tests were crried out on cooked dough pieces to evlute the viscoelsticity of the cooked WSN. F, % SR nd k 2 vlues derived from the stress-relxtion test showed tht the cooked noodles mde of reconstituted flour contining non-wxy strches (rec-l488 BR nd rec-tcs 1) were higher thn those contining wxy strches (rec-wxy whet, rec-wxy corn, nd rec-tcw 7) (Tble 3). The high k 1 nd k 2 of the cooked noodles mde of reconstituted flour contining nonwxy strch reflected the high viscous nture nd elsticity of cooked noodles. The high F nd % SR of cooked noodles contining non-wxy strches reflected the psting properties of the nonwxy strches. The mjor chrcteristics of non-wxy strches include high gel strength with high elsticity. Among the noodles mde of reconstituted flour contining wxy strch, the% SR of rec-tcw 7 ws the lowest, indicting tht the dense noodle structure nd low strength of the strch pste cused the cooked noodle to lose their bounce bck bility fter relesing the compression force from the cooked noodles. The creep test ws lso performed to provide informtion on the viscoelsticity of cooked noodles nd to relte the derived prmeters to cooked noodle eting qulity. For the creep test, initil compressing force ( N) ws pplied for 1s so tht the strin of the test smples would be in the liner viscoelstic region nd rech stedy-stte flow. The creep curves of cooked noodles exhibited typicl viscoelstic behvior, combining both viscous fluid nd elstic components (Fig. 4). The noodles mde of reconstituted flour contining non-wxy strches (rec-l488 BR nd rec-tcs 1) reched the equilibrium stte quicker with lower creep complince thn those mde of reconstituted flour Tble 5 Cooking property nd stickiness of WSN mde of reconstituted flour contining different cerel strches. Flour/reconstituted flour Optiml cooking time (min) Thickness (cm) Before cooking After cooking Moisture content of cooked noodle (%) Cooking loss (%) Stickiness (g) L488 BR ba b c 246 d rec-l488 BR b c 254 d rec-wxy whet b b 4.16 b 192 c 48 b rec-wxy corn d d 171 e rec-tcw d.42 b 53.8 d 2.3 d rec-tcs c.44 b 56.8 c 2.75 c 249 b 338 c A Vlues followed by different letters in the sme column re significntly different (p<.5). Fresh Stored t 4 C for 1 dy

7 Y.-C. Hung, H.-M. Li / Journl of Food Engineering 97 (1) contining wxy strches (rec-wxy whet, rec-wxy corn, nd rec-tcw 7) did. Negtive correltions of mylose content nd both mximum creep complince (J mx ) nd recovery complince of noodles hve previously been reported by Sski et l. (4). Among the reconstituted flours contining wxy strch, the creep complince over time of rec-wxy whet ws the lrgest, indicting the lrgest deformtion t the comprble time scles Compression force (N) rec-wxy whet rec-wxy corn rec-tcw 7 rec-l488 BR rec-tcs Time (min) Fig. 4. Creep complince of WSN mde of reconstituted flour contining different cerel strches. (Fig. 4). The high J mx of cooked noodles mde of rec-wxy whet nd rec-wxy corn ws prtilly due to their high moisture contents (65.7% nd 66.9%) of cooked noodles (Tble 5). The complince curve dt obtined from the mesurements of the creep test were ll well fitted by the Mxwell Voigt model with the coefficients of determintion (R 2 ) of The complinces (J) cn be the index of elstic components of mterils. The low complinces (J 1 nd J 2 ) of noodles mde of rec-l488 BR nd rec-tcs1 indicted the high elsticity of cooked noodles, which ws ttributed to the strong mylose gel (Tble 4). The short retrdtion time (k 1 ) of noodles mde of rec-l488 BR indicted tht the noodles were rigid gels. While, the long k 1 of noodles mde of rec-wxy whet indicted the wek mylopectin pste. The viscosity (g) t the stedy-stte of cooked noodles followed the order rec-l488 BR > rec-tcs 1 > rec-wxy corn > rec-tcw 7 > recwxy whet. The reltively viscous surfce of cooked noodles mde from reconstituted flour contining non-wxy strches were determined in both creep nd stress-relxtion tests. This resulted from the leches of mylose from grnules during strch geltiniztion. According to the results of rheologicl properties of dough sheets, it my be concluded tht the dough rheologicl properties of noodle sheets were minly influenced by the size of strch grnules tht resulted in vrious mounts of wter bsorption during dough preprtion nd different degree of interctions of strch grnules nd gluten-strnd network in dough. While the strch psting properties, influenced by the mylose content nd fine structure of mylopectin, dominted the wter bsorption nd 8 6 dy 1dy 5dy MCS (g 1) 6 5 dy 1 dy 5 dy b Retrogrdtion (%) 3 1 L488 BR rec-l488 BR rec-wxy whet rec-wxy corn Reconstituted flour rec-tcw 7 rec-tcs 1 Fig. 5. Cutting properties nd degree of retrogrdtion of cooked noodles mde of reconstituted flour contining different cerel strches: () mximum cutting stress, MCS; (b) degree of retrogrdtion, %.

8 142 Y.-C. Hung, H.-M. Li / Journl of Food Engineering 97 (1) cooking time during noodle cooking nd the strch gel properties, which would be reflected by the rheologicl properties of cooked noodles Cooking nd storge qulity of WSN mde of reconstituted flour The cooking time of WSN ws significntly ffected by the thickness of noodle strnds nd the type of strch used (Tble 5). The noodle strnds mde of rec-tcw 7 nd rec-tcs 1 were thinner thn the others, lthough the finl sheeting roller-gp ws not further reduced. A thin noodle hving short distnce for wter diffusion resulted in short cooking time. As result, short cooking time ws required for the WSN mde of reconstituted flour consisting of rice strches nd resulted in the cooked noodles hving low moisture content nd low cooking loss (Tble 5). A significnt increse of thickness of noodle mde from reconstituted flour contining rec-wxy corn strch ws ttributed to its high SP t 95 C (Fig. 1). The higher SP combined with the smller grnulr size of wxy corn strch thn wxy whet strch might result in the lower stickiness of cooked noodles contining wxy corn strch thn those contining wxy whet strch. The stickiness of cooked noodles incresed significntly fter being stored t 4 C for 1 dy. It ws the most obvious in noodles mde from reconstituted flour contining wxy strch. This is typicl chrcteristic of wxy strch pstes which hve high stickiness nd poor film or gel forming bility. The low stickiness of freshly cooked WSN compred to ged noodles ws ttributed to the lubriction effect of free wter on the surfce re of noodles (Dexter et l., 1983). After storge, moisture migrtion from the surfce to the centrl region might decrese the moisture content on the surfce of WSN (Li nd Hwng, 4). Therefore, the lubriction effect decresed between surfce re of ged noodle nd the test probe. The hrdness of cooked WSN fter storge ws minly ffected by the degree of strch retrogrdtion, which ws influenced by the chrcteristics of the strch gel nd storge conditions. The mximl cutting stress (MCS) nd retrogrdtion enthlpy of cooked WSN mde of reconstituted flours during storge were compred (Fig. 5). Freshly cooked WSN mde of rec-wxy whet hd slightly low MCS compred to the freshly cooked WSN mde of L488 BR nd rec-l488 BR (Fig. 5). A similr trend of freshly cooked WSN mde of rec-tcw 7 nd rec-tcs 1 ws observed. These results were ttributed to the wek gel property of mylopectin in wxy strch nd the strong gel strength of mylose in non-wxy strch (Ginibelli et l., 5; Prk nd Bik, 4). The drmtic increses in the MCS of cooked noodles mde of rec-rice strch fter 5 dys storge ws ttributed to the moisture content of cooked noodles, which ws close to the optimum moisture conditions ( 5%) for ccelerting the retrogrdtion of strches (Zeleznk nd Hoseney, 1986). Therefore, the high degree of strch retrogrdtion of stored cooked noodles mde of rec-tcw 7 nd rec-tcs 1 were investigted (Fig. 5b). 4. Conclusions From the dt generted in this study, cerel strches with different grnulr sizes nd mylose content significntly ffected the rheologicl properties of rw nd cooked WSN. When the lrge strch grnules (such s whet strches) were substituted by the smll strch grnules, dense pcking or filling of strch grnules inside the gluten-strnd network ws expected. As result, the strch grnule size is criticl fctor ffecting the rheologicl properties of dough sheets nd rw noodles. Dough mde with reconstituted flour contining smll strch grnules, such s rec-tcw 7 nd rec-tcs1, hs low % SR, indicting low elsticity. While, the chrcteristics of strch, including the mylose content, fine structure of mylopectin, swelling power, nd psting properties, dominte the viscoelsticity of cooked noodles, such s rec-tcw 7, hving lower% SR nd higher J mx thn rec-tcs 1, nd indicte lrge deformtion of cooked noodle. Although blending strches or cerel flours re prcticlly performed in noodle mking, the noodles qulity ws hrd to be predicted, becuse of complex strch strch or strch flour interctions. However, commercil vitl gluten could be dded into cerel strch or flour to mke noodles for the purpose of improved noodle eting qulity. In the future, it could be expected tht the noodle qulity might be modified by selecting the whet strches hving different grnulr size nd psting properties by breeding method or genetic modifiction. Acknowledgments This reserch ws kindly supported by the grnts from the Council of Agriculture (94AS FD-Z1(4) nd 95AS FD-Z1(4)), Executive Yun, Tiwn. 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