World Applied Sciences Journl 4 (2): 183-187, 2008 ISSN 1818-4952 IDOSI Publictions, 2008 The Effect of Acetic nd Lctic Acid on the Oil Uptke, Texture nd Color of Rice (Sng Trom) During Cooking 1 1 2 E. Ghsemi, M.T. Hmed Mosvin nd M.H. Hddd Khodprst 1 Deprtment of Chemicl Engineering, Ferdowsi University of Mshhd, Mshhd, Irn 2 Deprtment of Food Science nd Technology, Ferdowsi University of Mshhd, Mshhd, Irn Abstrct: The effects of edible cids ddition to rice during cooking t three levels (distilled wter (control smple), 0.05M cetic cid nd 0.05 M lctic cid) on the texturl nd morphologicl properties, geltiniztion temperture (GT) nd color of cooked Sng Trom rice were investigted. The results show tht cid ddition to rice during cooking, cuse decrese in hrdness nd GT, but hd no significnt effect on dhesiveness nd whiteness index of smples. Using scnning electron microscopy technique, the microstructure reveled tht the outer lyer of cooked rice with 0.05M lctic cid nd 0.05M cetic cid becme less porous compred to the control smple. Since Irnins use oil during or fter cooking rice to provide better ppernce, glossiness nd kernelness, the effect of edible cids ddition on oil content of the end-product, hs been investigted, too. By ttention to mylogrphy nd SEM results, we found tht cidic tretments hd less oil in inner structures of rice kernels. Key words: Rice cooking cetic cid lctic cid geltiniztion texture oil color INTRODUCTION There re some methods to chnge nd improve the rice texture, flvour, color nd other properties. It hs Rice (Oryz stiv L.) is one of the importnt stple been reported tht the trnsprency, glossiness nd foods of lmost hlf of the world popultion. There is stickiness of rice cooked with cetic cid were incresed bout 90% of its production nd consumption in Asi. It nd hrdness ws decresed with respect to control is consumed s cooked milled kernels. Cooking of milled smple depending on molr concentrtions of cetic cid rice is one of the most importnt processing steps tht is rnging from 0.05 to 0.2 M [8]. Also, the effects of cetic done to provide desirble texture in the kernels for direct cid on the rice geltiniztion nd psting properties of eting. It involves het nd mss (wter) trnsfer. The rice rice strch during cooking hve been investigted. It hs kernels re boiled in limited or excess mount of wter for been reported tht cetic cid promoted wter bsorption cooking. About 90% of the dry mss of milled rice is of strch nd the structure of rice strch becme more strch [1]. The strch content of the milled rice kernel fluid by ddition of cetic cid [9]. bsorbs moisture nd swells during cooking due to its Irnins dd oil to rice during or fter cooking to geltiniztion [2]. provide the better flvour, more glossiness nd less In generl, the rice qulities re evluted bsed on stickiness of kernels together. So, the lrge prt of oil its color, flvor, texture, eting qulities nd other consumption in Irn is ttributed to oil which is consumed properties like glossiness, looseness nd kernelness [3]. with rice dily. Oil consumption, especilly sturted ft For the color of rice, the white is usully preferred. is recognized s one of the mjor fctors tht hs n Eting qulities of cooked rice cn be evluted by dverse effect on helth nd leds to some diseses instrumentl tests [4]. The texture of cooked rice is such s coronry hert disese (CHD), cncer, obesity, not single ttribute, but it is composite of severl dibetes, etc. ttributes such s hrdness or firmness (or its opposite, There is no informtion bout oil uptke of cooked tenderness), stickiness or dhesiveness [5], chewness rice nd especilly the Irnin rice vrieties. As edible nd cohesiveness [6], s well s moistureness to cids such s cetic cid, lctic cid nd citric cid ffect touch [7]. on the geltiniztion behviour of rice during heting nd Corresponding Author: Dr. M.T. Hmed Mosvin, Deprtment of Chemicl Engineering, Ferdowsi University of Mshhd, Mshhd, Irn 183
cooking, it is expected tht dding them to rice during cooking, will be effective in its oil bsorption behviour, too. The objective of this work ws to study the effect of cetic nd lctic cids ddition to rice during cooking on oil uptke, texture nd color of kernels. MATERIALS AND METHODS 30 s before conducting the cylindricl test cell ws compressed by sphericl plte plunger of 35 mm dimeter. Pre-test speed, test speed nd post-test speed of plunger were set t 1.0, 1.0 nd 10 mm/s, respectively. Compression distnce ws 50% strin [12]. A force-time curve ws obtined from the test nd the following texturl prmeters were determined: Rice smples nd mterils: Sng Trom vriety of milled Hrdness-the mximum compressive force during rice grin tht is one of the most importnt vrieties is extrusion (N). used in this study. This vriety is grown in the northern Adhesiveness-the negtive re under the curve prt of Irn. The milled rice ws plced in ir-tight (Ns). polyethylene bgs nd kept in cold room. The verge moisture content of rw rice ws equl to 8% (w.b.). Lctic Color ssy: Whiteness of cooked rice ws mesured 3 cid 95% (v/v) with density of 1.21 gr/cm, cetic cid using colorimeter (Model D25 opticl Hed, Hunter 5% (w/w) (Golchekn Zmni vinegr) nd sunflower ssocites lbortory INC., U.S.A). Mesurement ws liquid oil (Shdgol) which were purchsed t locl bsed on the Hunter system of color vlues of L mrket in Irn were used. (lightness), (redness), b (yellowness) [13]. The mesurement ws performed in triplicte. The L, nd b Cooking method: The rice grins were clened from vlues of the white color stndrd were 94.99, -0.7 nd foreign mterils nd wshed for five times using wter. 0.2, respectively. Furthermore the whiteness index (WI) Rice cooking ws performed s follow: 10 g of crude rice ws clculted s follows [12]. nd 100 ml of distilled wter or cid solution (0.05 M cetic cid nd lctic cid) were subjected to constnt boiling for 2 2 2 0.5 WI = 100-[(100-L) + +b ] 15 min in 250 ml beker. Cooking wter ws eliminted nd cooked rice ws llowed to cool [10]. Scnning electron microscopy (SEM): The morphologicl chnges of cooked rice kernels ws observed using Oil uptke: The cooked rice grins were immersed in scnning electron microscopy (LEO 1450VP, Germny) beker of liquid oil in room temperture for 15 min to t 10 KV. ensure tht the mximum of oil uptke to rice occurs. Then The cooked rice kernel ws freeze-dried by liquid the beker content ws trnsferred to lech for dripping Nitrogen. The outer surfce on the lterl side of the nd removing the excess oil for sufficient time (20 h). kernel ws observed. Smples were ttched to SEM Oil uptke ws mesured by the four hours soxhlet stub using double-bcked cellophne tpe. The stub extrction using petroleum ether s solvent. nd smple were coted with gold-plldium by sputter coter (SC 7620), exmined nd photogrphed. Amylogrphy: The rw rice ws clened nd ground in grinder for rice flour preprtion. The rice geltiniztion Sttisticl nlysis: The experiments were done in behviour ws studied with Brbender mylogrph completely rndomized (CR) technique with 3 tretment (type 800120, Germny) using rice flour slurry with nd 3 replictions. An nlysis of vrince ws used to concentrtion of 6% on dry bsis (d.b.) [11]. The mixture nlyze the dt nd significnt differences between ws mde up to 460 g with distilled wter or 0.1 M cetic the tretment mens were compred t significnt cid or0.1 M lctic cid nd heted from the room level of 95%. temperture to 90 C t rte of 1.5 C/min. RESULT AND DISCUSSION Texture mesurement: A texture nlyzer (QTS, CNS FARNELL) ws utilized to exmine the texture of cooked rice smples using the bck extrusion test. A 15 grm smple of cooked rice ws plced inside the test cylinder of 6 cm dimeter nd pressed with 100 grm weight for Generl: The mcroscopic chnges of cooked rice s result of cooking conditions (with distilled wter, 0.05 M cetic cid nd 0.05 M lctic cid) were described in terms of texturl nd color ltertion, s summrized in Tble 1. 184
Tble 1: Summry of texturl nd color chnges of cooked rice smples undergoing vrious cooking conditions A Cooking conditions Hrdness (N) Adhesiveness (NS) Whiteness With distilled wter 8.38-0.19 76.793 With cetic cid 0.05 M b 6.31-0.22 77.386 With lctic cid 0.05 M b 4.89-0.52 77.32 A Different letters within the sme column indicte significnt difference (P<0.05) using Duncn's multiple rnge test 300 250 200 150 Temperture ( C) 75 90 95 95 Distillted wter Acetic cid Lctic cid Texturl properties Hrdness: The hrdness of smples tht cooked in cidic conditions ws significntly lower thn control smple hrdness. This my be due to tht the cidic conditions promoted wter bsorption of mylopectin in rice strch. Also cetic nd lctic cid cuse esy to geltiniztion nd more fluidity of rice strch structure. Zhou et l. reported tht the hrdness nd dhesiveness re likely to be ssocited with the hydrtion process of strch grnules [14]. During cooking, the rice grnules bsorbed moisture nd swelled to gret extent compred to their originl size. The expnsion of grnule cused ruptures nd, hence, mylose leching. The leching components cn be responsible for decresing hrdness of cooked rice smples [12]. Adhesiveness: There were no significnt difference between the dhesiveness of cooked rice smples by ddition edible cids, but is seen little increse in dhesiveness of cidic smples especilly lctic cid tretment in comprison with control smple. This my be due to the more leching of mylose from the strch grnule during geltiniztion in these tretments. Also lctic cid my cuse to protein extrction from the surfce of the rice kernel. Proteins present in cooking wter my bind with leched mylose t the surfce of rice[15]. Mny reserches found tht stickiness of cooked rice depends on the mount of leched mylose. When mount of leched mylose ws higher, cooked rice ws stickier [16, 17]. This result correlted with the work by Nophrtn et l., which investigted the effect of soking chemicls on properties of cooked rice nd reported tht the stickiness of cooked rice soked in 0.1% lctic cid ws significntly higher thn of other soked smples. They lso found t 100% geltiniztion, the leched mylose content of cooked rice soked in 0.1% lctic cid ws significntly higher thn other smples [18]. In ddition, it ws demonstrted tht the dissolution nd degrdtion of proteins in the cidic condition might ccelerte the bsorption of strch. These mutul chnges 100 50 0 30 40 50 60 Time (min) Fig. 1: Effect of cidic conditions on the psting behviour of rice flour (6% d.b.) in strch nd protein cn ffect the texturl chnges such s the incresing stickiness nd the decresing hrdness of cidic tretment [9]. Whiteness: There ws no significnt difference between the degrees of whiteness of cooked rice smples by ddition edible cids. Amylogrphy: Figure 1 shows the Brbender Viscogrms of 6% rice flour (control nd cidic smples). The geltiniztion temperture (GT) of rice cooked by distilled wter, 0.05 M cetic cid nd 0.05 M lctic cid ws 90, 87 nd 84 C respectively. This obvious decrese in GT by cidic smples my be due to the strch grnules tht becme frgile nd broke down reltively rpidly fter cidifiction. Acidic rice flour pste showed lower tendency of retro grdtion. This my be scribed to the short-chin strch molecules tht pper due to cid hydrolysis, which were very ctive to form highly ordered crystlline structure[11]. Ohishi et l., showed by DSC mesurements tht the rice strch heted with cetic cid ws esy to geltinize compred to tht without cetic cid [9]. Scnning electron microscopy: The cidic cooking conditions, produced noticeble chnges of microstructure of cooked rice kernels t cidic tretments (Fig. 2(b) nd (c)), in comprison with the control smple (Fig. 2()). The SEM displyed more even structure with fine porosity. In cidic tretments, the porosity seemed to be grdully dispersed nd the surfce t high cidic cooking conditions ppered to be smooth nd solid. 185
() 25 20 15 10 5 0 with distillted wter with cetic cid with lctic cid Acidic conditions of cooking (b) Fig. 3: The percentge of oil uptke in smples cid rther to control smple (Fig. 3) my relte to only surfce oil, not to the oil which hs diffused to the inner structure of the rice kernel. It my be refers to the more dhesiveness in this tretment. These kernels sum the more oil in own round nd due to the closer wys to oil penetrtion on the surfce of kernels, the lower oil goes to the inner lyer of rice endosperm nd rest on the outer surfce. Thus, by consumption of less oil, is provided the desirble ppernce, glossiness nd other properties. (c) CONCLUSIONS Fig. 2: Scnning Electron Microgrphs of the outer surfce of cooked rice, ()with distilled wter (b) with 0.05M cetic cid (c) )with 0.05M lctic cid It seems tht the better geltiniztion nd swelling of rice strch due to cid ddition, cuse decrese in pore size. 3.6. Oil uptke: By considering the SEM nd mylogrphy results, we found tht the cid ddition during cooking rice, results the better geltiniztion nd the closer pores nd chnnels on the surfce of rice kernels. Therefore, this cts s brrier for oil penetrtion to the inner structure of the rice kernel. By ttention to the oil uptke mesurement method tht pplied in this work, increse in oil content in smple cooked with 0.05M lctic Acid ddition during the cooking of rice ffected the texturl ppernce, including color nd texture s well s the geltiniztion behviour of rice strch. The SEM demonstrted the different microstructures of cooked rice undergoing three cidic tretments. The use of 0.05M cetic nd lctic cid for cooking rice, compred to control smple, cuse decrese in geltiniztion temperture, hrdness nd porosity nd decrese in oil penetrted to the inner lyer of endosperm due to better geltiniztion nd closer pores in the outer surfce of the rice kernel. Also, it hd no significnt effect on dhesiveness nd whiteness index of smples. REFERENCES 1. Rmesh, M.N., 2001. An ppliction of imge nlysis for the study of kinetics of hydrtion of milled rice in hot wter. Interntionl Journl of Food Properties, 4(2): 271-284. 2. Ydv, B.K. nd V.K. Jindl, 2007. Dimensionl chnges in milled rice (Oryz stiv L.) kernel during cooking in reltion to its physicochemicl properties by imge nlysis. Journl of food engineering, 81: 710-720. 186
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