Characteristics of acetylated starches prepared using starches separated from different rice
Characteristics of acetylated starches prepared using starches separated from di?erent rice cultivars
Navdeep Singh Sodhi,Narpinder Singh
*
Department of Food Science and Technology,Guru Nanak Dev University,Amritsar 143005,India
Received 24May 2004;accepted 17September 2004
Available online 18November 2004
Abstract
The physico-chemical properties of acetylated starches from di?erent rice cultivars were evaluated.The acetylation of starches from di?erent cultivars showed di?erent acetyl (%)and degrees of substitution (DS).The FT-IR spectral analysis con?rmed the introduction of acetyl moiety in the acetylated starches through a band at 1730.8cm à1.The scanning electron microscopy did not show any signi?cant di?erence between the external morphology of native and acetylated starches.The acetylated rice starches showed slightly higher amylose contents than their native counterparts.Acetylation increased swelling power and solubility and decreased T o ,T p ,T c and D H gel of all starches.In all starches,acetylation decreased pasting temperatures,but increased peak vis-cosities.Acetylation brought greatest change in T o of the starch showing highest DS and least in the starch showing lowest DS.Acetylation caused weakening of the starch gels,which was observed to be dependent on degrees of substitution.Starch gels stored at 4°C showed a signi?cant decrease in syneresis (%)upon acetylation.The e?ectiveness of acetylation in reducing retrogradation was more evident from syneresis data than from setback viscosities observed by RVA.ó2004Elsevier Ltd.All rights reserved.
Keywords:Rice starch;Acetylation;Physico-chemical;Morphological;Thermal;Pasting;Gel texture;Retrogradation
1.Introduction
Starch,nature ?s most abundant polysaccharide,is a major food reserve providing energy often at low cost in the human diet.Starches have many diverse applica-tions both in food production and industrial applica-tions.But the native starches present limitations that reduce their use at the industrial level.They have low shear stress resistance,thermal decomposition,high ret-rogradation,and syneresis (Betancur &Chel,1997).These facts motivated the employment of modi?ed starches (Fleche,1985)as important functional ingredi-
ents in processed foods in recent years because of their improved functional properties over unmodi?ed starches (Wurzburg &Szymanski,1970).Numerous chemical modi?cations may be applied to starch to im-part properties that are useful for particular applica-tions.One common starch modi?cation is acetylation,which is the esteri?cation of starch polymers with acetyl groups to form starch acetates (Jarowenko,1986).Acet-ylation of starches has been applied to starches to im-part the thickening needed in food applications.Acetylated starches have improved properties over their native counterparts and have been used for their stabil-ity and resistance to retrogradation (Wurzburg,1986).Rutenberg and Solarek (1984)reported that the intro-duction of acetyl groups upon acetylation reduces the bond strength between starch molecules and thereby in-creases the swelling power and solubility of the starch granule,decreases the coagulation of the starch,and
0260-8774/$-see front matter ó2004Elsevier Ltd.All rights reserved.doi:10.1016/j.jfoodeng.2004.09.018
*
Corresponding author.Tel.:+911832258802x3216;fax:+911832258820.
E-mail address:narpinders@https://www.sodocs.net/doc/997234350.html, (N.Singh).
https://www.sodocs.net/doc/997234350.html,/locate/jfoodeng
Journal of Food Engineering 70(2005)
117–127
provides improved freeze-thaw stability.The extent of physico-chemical property changes in the acetylated starch compared to the native starch is proportional to the degree of acetylation or degree of C@O substitution incorporated into the starch molecules(Phillips, Huijum,Duohai,&Harold,1999).Acetylated starches are extensively used in a large variety of foods including baked goods,canned pie?llings,sauces,retorted soups, frozen foods,baby foods,salad dressings,and snack foods(Wurzburg,1995).
The availability of corn to the Indian starch industry is decreasing,day by day,because of increased demand by industries involved in the production of breakfast cereals and snacks.The broken rice,which is cheaper than corn and is available in abundance,can be used in the production and modi?cation of starch.No studies have been reported that characterize the acetylated starches from Indian rice cultivars.So,the present study was undertaken to evaluate the e?ect of acetylation on the physico-chemical properties of rice starches isolated from di?erent Indian cultivars.
2.Materials and methods
2.1.Materials
Five indica paddy cultivars(cv.)i.e.PR-106,PR-114, IR-8,PR-103and PR-113,commonly grown in Punjab State,were procured from Punjab Agricultural Univer-sity,Ludhiana,India.PR-106,PR-114,IR-8and PR-113had a maturity period of144days while PR-103 had125days.All the varieties have30days of grain?ll-ing period.Analytical grade acetic anhydride,sodium hydroxide and hydrochloric acid were procured from Glaxo India Limited,Mumbai.Ethanol was purchased from Hayman Ltd.,Essex(UK).
2.2.Dehusking and milling
The paddy samples were dehusked and milled to re-move6%bran as described earlier(Singh,Singh,Kaur, &Bakshi,2000).The paddy samples were dehusked on McGill sample sheller(Rapsco,Brookshire,TX,USA). The brown rice samples obtained were polished in McG-ill mill No.2(Rapsco,Brookshire,TX,USA)to obtain 6%degree of milling.
2.3.Starch isolation
Starch was isolated from various rice cultivars by al-kali extraction of the https://www.sodocs.net/doc/997234350.html,led rice was steeped in 5–6volumes of sodium hydroxide(0.2–0.3%)solution at 25°C for24h to soften the endosperms.The steep liquor was drained o?and the endosperms were ground lightly in successive small fractions with a mortar and pestle.The slurry was then diluted to the original volume with sodium hydroxide(0.2–0.3%).The mixture was stirred for10min.and allowed to settle overnight.The cloudy supernatant was drained o?,and the sediment was di-luted to the original volume with sodium hydroxide solution.The process was repeated until the supernatant became clear and gave a negative reaction to the biuret test for protein.Starch was suspended in distilled water, passed through a100–200mesh nylon cloth,and repeat-edly washed with water until the supernatant no longer showed any pink colour with the phenolphthalein.The starch was collected by sedimentation,and the white middle portion was collected and dried in cabinet drier at40°C(Nikuni&Hizukuri,1958).
2.4.Acetylation of isolated rice starches
The method described by Phillips et al.(1999)was used to prepare acetylated starches.Starch(100g)was dispersed in distilled water(225ml)and stirred for1h at25°C.NaOH(3%)solution was used to adjust the suspension pH to8.0.Acetic anhydride(6g)was added drop-wise to the stirred slurry,while maintaining the pH within the range of8.0–8.4using3%NaOH solution. The reaction was allowed to proceed for10min after the completion of acetic anhydride addition.The slurry was then adjusted to pH4.5with0.5N HCl.After sed-imentation,it was washed free of acid,twice with dis-tilled water and once with95%ethanol,and then oven-dried at40°C.
2.5.Acetyl(%)and degree of substitution
The percent acetylation(%acetyl)and degree of sub-stitution(DS)were determined titrimetrically following the method of Wurzburg(1978).Acetylated starch (1.0g)was placed in a250ml?ask and50ml of75%eth-anol in distilled water was added.The loosely stoppered ?ask was agitated,warmed to50°C for30min,cooled and40ml of0.5M KOH was added.The excess alkali was back-titrated with0.5M HCl using phenolphthalein as an indicator.The solution was stood for2h,and then any additional alkali,which may have leached from the sample,was titrated.A blank,using the original unmodi-?ed starch,was also used.
Acetyl%?
?eBlankàSampleT?Molarity of HCl?0:043?100
Sample weight
Blank and sample were titration volumes in millilitre, sample weight was in gram.DS is de?ned as the average number of sites per glucose unit that possess a substitu-ent group(Whistler&Daniel,1995).
DS?
e162?Acetyl%T
?4300àe42?Acetyl%T
118N.S.Sodhi,N.Singh/Journal of Food Engineering70(2005)117–127
2.6.Fourier transform infrared(FT-IR)spectra
FT-IR spectra of native and acetylated starches were acquired on a Shimadzu FT-IR instrument(Shimadzu Corporation,Kyoto,Japan)using a potassium bromide (KBr)disc containing1%?nely ground sample.
2.7.Physico-chemical properties
2.7.1.Amylose content
Amylose content of the isolated starch was deter-mined by using the method of Williams,Kuzina,& Hlynka(1970).The principle of the test lies in the blue colour developed by the addition of an iodine reagent to a solution containing the amylose under standardized conditions.Starch sample(20mg)was taken and10ml of0.5N KOH was added into it.The suspension was mixed thoroughly.The dispersed sample was transferred to100ml volumetric?ask and diluted to the mark with distilled water.An aliquot(10ml)of test starch solution was pipetted into the50ml volumetric?ask and5ml of 0.1N HCl was added followed by the0.5ml of iodine re-agent.The volume was diluted to50ml and the absor-bance was measured at625nm.The measurement of amylose was determined from a standard curve devel-oped using amylose and amylopectin blends.
2.7.2.Swelling power and solubility
Swelling power and solubility were determined using 2%aqueous suspension of the starch by the method of Leach,McCowen,and Schoch(1959).
2.8.Morphological properties
Scanning electron micrographs were obtained with a scanning microscope(Jeol JSM-6100,Jeol Ltd.,Tokyo, Japan).Starch samples were suspended in ethanol to ob-tain1%suspension.One drop of the starch-ethanol solution was applied on an aluminum stub,and the starch was coated with gold–palladium(60:40).An acceleration potential of10kV was used during micrography.
2.9.Thermal properties
Thermal properties of isolated starches were ana-lyzed using a Di?erential Scanning Calorimeter(DSC-821e,Mettler Toledo,Switzerland)equipped with a thermal analysis data station.Starch(3.5mg,dwb) was weighed in an aluminum pan(Mettler,ME-27331)of40l l capacity.Distilled water was added with the help of a Hamilton microsyringe to achieve a starch-water suspension containing70%water.Samples were hermetically sealed and allowed to stand for1h at room temperature before DSC measurements.Indium and an empty aluminum pan were used as reference to calibrate the DSC.Sample pans were heated at a rate of10°C/min from30to100°C.Onset temperature (T o),peak temperature(T p),conclusion temperature (T c)and heat of gelatinization(D H gel)were calculated automatically.The gelatinization range was computed as T càT o because the peaks were symmetrical (Vasanthan&Bhatty,1996).D H gel were calculated on starch dry basis.
2.10.Pasting properties
The pasting properties of starches were evaluated with a Rapid Visco Analyser(RVA-4,Newport Scien-ti?c,Warriewood,Australia)using RVA TM Crosslinked and Substituted Method,No.9,Version4(Newport Scienti?c,1998).Starch(3g,14%mb)was weighed di-rectly in the aluminum RVA sample canister,and dis-tilled water was added to a total constant sample weight of28g.A programmed heating and cooling cycle was used where the samples were held at50°C for1min, heated to95°C in3.7min,held at95°C for2.5min be-fore cooling to50°C in3.8min,and holding at50°C for2min.Parameters recorded were pasting tempera-ture(P temp);peak viscosity(PV);hot paste viscosity (HPV)(minimum viscosity at95°C);cool paste viscosity (CPV)(?nal viscosity at50°C);breakdown(BD) (=PVàHPV);and set back(SB)(=CPVàHPV).All measurements were replicated thrice.
2.11.Textural properties of gels
Textural properties of RVA gels(Bhattacharya,Zee, &Corke,1999)were evaluated on an Instron universal testing machine(Model4464,Instron,Buckingham-shire,England).The starch paste formed in the canister after RVA testing was covered and kept at room tem-perature(%25°C)overnight and allowed to gel.The gel formed in the can(37mm dia.,20mm height)was used directly for texture analysis.The gel was com-pressed to a distance of10mm using a?at cylindrical probe(6mm dia.)at a cross-head speed of30mm/min. twice in two cycle.The textural parameters of hardness, cohesiveness,springiness,gumminess and chewiness were computed using textural pro?le analysis(TPA)as described by Bourne(1978).Twelve repeated measure-ments were performed for each sample.
2.12.Syneresis(%)
Starch suspension(5%,w/w)was heated at90°C for 30min in a temperature controlled water bath,followed by rapid cooling in an ice water bath to room tempera-ture.The starch sample was stored for48,72,96and 168h at4°C.Syneresis was measured as the amount of water(%)released after centrifugation at3200·g for15min.
N.S.Sodhi,N.Singh/Journal of Food Engineering70(2005)117–127119
2.1
3.Statistical analysis
The data were subjected to statistical analysis using Minitab Statistical Software (Minitab Inc.,USA).The data reported in Tables 1–4and 6are average of tripli-cate measurements.The standard deviation of less than ±5%was observed in all the measurements.
3.Results and discussion
3.1.Acetyl (%)and degree of substitution
The addition of same quantity of acetic anhydride re-sulted in di?erent acetyl (%)and degrees of substitution (Table 1).The acetyl (%)and DS of acetylated starches for di?erent cultivars varied between 2.26–3.68%and
0.087–0.144,respectively.PR-113acetylated starch showed the highest acetyl (%)and DS while PR-103showed lowest values for the same.The acetyl (%)and DS were observed to be lower in this study as reported earlier for potato and corn starches treated with acetic anhydride under similar conditions (Singh,Chawla,&Singh,2004).This may be attributed to di?erence in amylose content and granular structure of rice,potato and corn starches.The variation in degree of substitu-tion among di?erent starches may also be due to di?er-ence in intragranule packing of rice starches.Because the way in which the amylose chain are packed in amor-phous regions as well as arrangement of amylose and amylopectin chains could a?ect the chemical substitu-tion reaction in the glucose units of starch macromole-cules (Islam,Rutledge,&James,1974).The rice starch granules have been reported to be rigid as compared to potato and corn starches (Singh,Singh,Kaur,Sodhi,&Gill,2003).Liu,Ramsden,and Corke (1997)observed acetyl group content between 2.71and 4.22%and DS between 0.105and 0.165in acetylated maize starches using 10%of acetic anhydride in starch slurry of 31%.Vasanthan,Sosulski,and Hoover (1995)reported an acetyl group content between 1.01and 2.80%in several acetylated starches (potato,waxy corn,corn,wheat,?eld pea and lentil)using 5and 10%of acetic anhydride.The di?erences in acetyl content reported in various studies may be due to di?erence in reaction conditions and starch sources used.The acetylated starches with acetyl content up to 2.5%are permitted for use in food formulation (Code of Federal Regulations,1994).
Table 1
Acetyl (%)and DS of acetylated rice starches from di?erent rice cultivars Cultivar %Acetyl DS PR-106 2.77b 0.107b PR-114 2.46a 0.095a IR-8 3.03b 0.118b PR-103 2.26a 0.087a PR-113
3.68c
0.144c
DS =degree of substitution.
Values with similar superscripts in column do not di?er signi?cantly (p <0.05).
Table 3
Thermal properties of native and acetylated starches from di?erent rice cultivars Cultivar Native Acetylated T o (°C)T p (°C)T c (°C)D H gel (J/g)T c àT o T o (°C)T p (°C)T c (°C)D H gel (J/g)T c àT o PR-10666.84b 70.07a 74.27ab 8.16a 7.43a 60.59b 65.09b 70.10b 8.15a 9.51b PR-11466.38a,b 70.09a 74.75b,c 8.97b 8.37b 60.08a 63.90a 69.10a 8.95b 9.02a IR-866.33a 69.74a 74.08a 8.55a 7.75a 59.95a 64.20a 69.52a 8.50a 9.57b PR-10367.26c 71.94b 78.04d 11.88d 10.78d 62.80c 67.69c 72.88c 11.43d 10.08c PR-113
66.00a
69.75a
75.08c
9.49c
9.08c
59.48a
63.89a
69.15a
9.26c
9.67b
T o =onset temperature,T p =peak temperature,T c =conclusion temperature,T c àT o =gelatinization range,D H gel =Enthalpy of gelatinization (dwb,based on starch weight).
Values with similar superscripts in column do not di?er signi?cantly (p <0.05).
Table 2
Amylose content,swelling power and solubility of native and acetylated rice starches Cultivar Native
Acetylated
Amylose content (%)Solubility (%)Swelling power (g/g)Amylose content (%)Solubility (%)Swelling power (g/g)PR-10616.05b,c 15.95bc 14.40b 16.15b 17.01b 15.51b PR-11416.13c 18.00d 14.30a,b 16.23b 18.98c 15.47b IR-815.62b 15.35a,b 15.03c 15.90b 17.00b 16.02b PR-1037.83a 14.35a 16.60d 7.88a 14.78a 19.31c PR-113
18.86d
17.30cd
13.03a
18.94c
18.21c
14.28a
Values with similar superscripts in column do not di?er signi?cantly (p <0.05).
120N.S.Sodhi,N.Singh /Journal of Food Engineering 70(2005)117–127
The introduction of acetyl moiety in the acetylated starches was also con?rmed by the FT-IR spectral anal-ysis.A comparison of the spectrum of native starch with acetylated starch clearly indicated the introduction of acetyl moiety through a band at1730.8cmà1(Figs.1 and2).3.2.Physico-chemical properties
Amylose content of rice starches increased after acet-ylation,however,increase was not statistically signi?-cant(Table2).Similar e?ects of acetylation on the amylose content of the starches have been
observed Fig.2.FT-IR spectra of acetylated starch
(PR-114).
Fig.1.FT-IR spectra of native starch(PR-114).
N.S.Sodhi,N.Singh/Journal of Food Engineering70(2005)117–127121
earlier(Betancur,Chel,&Canizares,1997).The pres-ence of acetyl groups has been reported to interfere with the functioning of amylose and amylopectin fractions of starch and it a?ects the absorption of iodine during amylose testing(Whistler&Daniel,1995;Betancur et al.,1997).The acetyl groups introduced in rice starch chains impeded the formation of the helical structure of amylose in some areas,by sterical hindrance,and in consequence,formation of amylose–iodine complex, which resulted in underestimation(Gonzalez&Perez, 2002).
Acetylation treatment increased the swelling and sol-ubility of starches from all rice cultivars(Table2).The changes observed in swelling and solubility by acet-ylation may be attributed to the introduction of hydrophilic substituting groups that retained water mol-ecules to form hydrogen bonds in the starch granules (Betancur et al.,1997).The introduction of acetyl groups in starch could have also facilitated the access of water to amorphous areas.The acetylated starches with higher acetyl content showed higher solubilities. The acetylation of starches with low amylose content showed limited change in solubility.Similar trends of in-crease in swelling power and solubility upon acetylation has been reported earlier for potato and corn starches (Singh et al.,2004;Liu et al.,1997).
3.3.Morphological properties
The scanning electron micrographs in Fig.3showed the presence of mainly polyhedral granules having size in the range of 2.4–5.4l m in di?erent native rice starches.PR-103starch showed the presence of smallest size granules(2.5–3.5l m)whereas the PR-113starch showed the presence of largest size granules(3.1–5.4l m).IR-8starch granule size ranged between2.6 and4.8l m.IR-8starch showed the presence of round starch granules in addition to polyhedral granules. PR-106and PR-114starches showed the granule size of3.1–4.8l m and2.4–4.2l m,respectively and presence of more uniform size granules as compared to starches from other rice cultivars.PR-106starch also showed the presence of hexagonal granules in addition to nor-mally present pentagonal granules.Li and Yeh(2001) reported an average granule size of6.4l m for Taiwan rice starch.SEM revealed no signi?cant di?erences be-tween external morphology of native and acetylated starches.However,the acetylation brought about
slight Fig.3.Scanning electron micrographs(SEM)of native starches from di?erent rice cultivars(A.PR-106;B.PR-114;C.IR-8;D.PR-103;E.PR-113). 122N.S.Sodhi,N.Singh/Journal of Food Engineering70(2005)117–127
aggregation of granules (Fig.4).Similar observations have been reported earlier regarding the morphology of acetylated corn,potato (Singh et al.,2004)and rice starches (Jeong,Bae,&Oh,1993;Gonzalez &Perez,2002).
3.4.Thermal properties
DSC results of native and acetylated starches from di?erent rice cultivars are summarized in Table 3.Native starches from di?erent rice cultivars showed transition temperatures between 66.0and 78.0°C.PR-103starch (with the lowest amylose content)exhibited higher tran-sition temperatures and D H gel .This may be attributed to the compact nature of small starch granules and higher degree of molecular order of the granules (Krueger,Knutson,Inglett,&Walker,1987).Because amylopectin plays a major role in starch granule crystallinity,the presence of amylose lowers the melting point of crystal-line regions and the energy for starting gelatinization (Flipse,Keetels,Jacobson,&Visser,1996).More energy was needed to initiate melting in the absence of amylose-rich amorphous regions (Krueger et al.,1987).Acetyla-tion of starches decreased the transition temperatures and D H gel ,however,the decrease was not signi?cant for D H gel .Acetylation caused highest decrease in T o from 66.0to 59.5°C in PR-113starch having maximum degree of substitution.While it caused lowest decrease in PR-103starch with minimum degree of substitution.Acetylation broadened the melting temperature range (T o –T c )in all cultivars except for PR-103,however,PR-106starch showed highest change in the same.These changes may be attributed to the presence of hydrophilic substituting groups and increase in hydrogen bonding in starch molecules,which lowered gelatinization tempera-tures and broadened the melting temperature range.Similar results have been reported earlier (Singh et al.,2004;Eliasson,Finstand,&Ljunger,1988;Hoover &Sosulski,1985;Liu et al.,1997).These results are also in agreement with those reported by Wootton
and
Fig.4.Scanning electron micrographs (SEM)of acetylated starches from di?erent rice cultivars (A.PR-106;B.PR-114;C.IR-8;D.PR-103;E.PR-113).
N.S.Sodhi,N.Singh /Journal of Food Engineering 70(2005)117–127123
Bamunuarachchi(1979).They suggested that introduc-tion of acetyl groups into polymer chains resulted in destabilization of granular structure,thus causing in-crease in swelling and decrease in gelatinization temperature.
3.5.Pasting properties
The native rice starches displayed a signi?cant varia-tion in all their pasting parameters(Table4and Fig.5). Pasting temperature of rice starches varied between 71.80°C for PR-106and74.15°C for PR-103.Pasting temperatures were consistent with the gelatinization temperatures measured using DSC measurements.The peak viscosities were observed in the range of2530–3116cP,lowest for PR-113and highest for PR-103. The hot paste viscosity ranged between1125cP and 2021cP,while the cold paste viscosity varied from 1781cP to3359cP,lowest for PR-103and highest for PR-113.PR-103native and acetylated starch in compar-ison to their counterpart starches from other cultivars reached the peak viscosity at lower temperature(Figs.5and6).PR-103cultivar behaved more like waxy rice. Liu et al.(1997)also observed that the native and acet-ylated starches from waxy rice reach their peak viscosity at lower temperature than normal rice starches.Hot paste viscosity has been reported to be in?uenced by the extent of amylose leaching,amylose–lipid complex formation,friction between swollen granules,granule swelling,and competition between leached amylose and remaining granules for free water,while cold paste viscosity is largely determined by the retrogradation ten-dency of the leached amylose upon cooling(Olkku& Rha,1978).
The breakdown and setback viscosities di?ered signif-icantly in the starches from di?erent rice cultivars and ranged between509–1991cP and656–1338cP,respec-tively.After acetylation all starches began to paste at a lower temperature and showed higher subsequent vis-cosities than their native counterparts(Table4and Fig.6).Liu et al.(1997)reported similar results for acet-ylated maize starch.They attributed the results to the three possible mechanisms by which acetylation can in?uence the interactions between the starch
chains; Fig.5.Rapid Visco Analyser pasting pro?les of native starches from
di?erent rice cultivars(A.PR-106;B.PR-114;C.IR-8;D.PR-103;E.
PR-113).
Fig.6.Rapid Visco Analyser pasting pro?les of acetylated starches
from di?erent rice cultivars(A.PR-106; B.PR-114; C.IR-8; D.
PR-103;E.PR-113).
Table4
Pasting properties of native and acetylated starches from di?erent rice cultivars
Cultivar Type P temp(°C)PV(cP)HPV(cP)CPV(cP)BD(cP)SB(cP)
PR-106Native71.80b2583a1901d3190b682a1289b
PR-106Acetylated69.45a3363c2396e5322e967b2926e
PR-114Native73.40cd2593a1763c2990b830a1227b
PR-114Acetylated69.35a3255bc1817c4007d1438d2190d
IR-8Native72.65b2621a2005d3326c616a1321b
IR-8Acetylated69.35a2991b1945d3992d1046b2047cd
PR-103Native74.15e3116b1125a1781a1991e656a
PR-103Acetylated71.00b3527d1576b3289c1951e1713c
PR-113Native73.50d2530a2021d3359c509a1338b
PR-113Acetylated69.30a3086b1934d4013d1152c2079cb
P temp=pasting temperature,PV=peak viscosity,HPV=hot paste viscosity(minimum viscosity at95°C),CPV=cool paste viscosity(?nal viscosity at50°C at the end of13min.cycle),BD=break down(=PVàHPV),SB=setback(=CPVàHPV).
Values with similar superscripts in column do not di?er signi?cantly(p<0.05).
124N.S.Sodhi,N.Singh/Journal of Food Engineering70(2005)117–127
(a)simple steric hindrance preventing close association of chains to allow formation of hydrogen bonds,(b) altering the hydrophilicity of the starch and thus a?ect-ing bonding with water molecules,or(c)participation of the acetyl groups in improved hydrogen bonding with other starch chains.The higher peak viscosity of acety-lated starches than their native starches could also be ex-plained by the increased swelling power and solubility. Jeong et al.(1993)and Hoover and Sosulski(1985)also observed increase in viscosity at90°C of acetylated starches,which could be due to weakening of the associ-ated forces in the amorphous areas of the granules.HPV of the acetylated starches from PR-106,PR-114and PR-103showed an increase in comparison to native starches,however,the increase was not signi?cant.This can be attributed to the fact that the thermal and shear forces reduce the starch-starch interactions and hence HPV would not be expected to show much change. Acetylated starches showed higher setback viscosities than their native counterparts.The introduction of ace-tyl groups in starch chains were expected to prevent close parallel alignment of amylose chains and thus lower setback viscosities,however such an e?ect was not observed during the cooling period.Hoover and Sosulski(1985)suggested that the increased amylose exudation and the shearing action of the blades might have nulli?ed any steric e?ects of acetyl groups in lower-ing setback viscosities.3.6.Gel texture properties
The textural parameters of native starch gels from all the rice cultivars varied signi?cantly(Table5).Hardness values varied between6.72g for PR-103and79.54g for PR-106starch gels.Gumminess and chewiness from all starch gels varied between 4.18–29.79g and8.071–31.91g mm,respectively,highest for IR-8and lowest for PR-103starch gels.The variations in mechanical properties of starch gels may be attributed to di?erences in the rheological characteristics of the amylose matrix, the volume fraction and rigidity of gelatinized starch granules,as well as the interactions between dispersed and continuous phases of the gel(Biliaderis,1998). These factors,in turn,have been reported to be depen-dent on the amylose content and the structure of amylo-pectin(Yamin,Lee,Pollak,&White,1999).Acetylated starches from all rice cultivars showed weaker gels than their native counterparts(Table5).Starches with higher degrees of substitution showed higher percent decrease in hardness of their gels.This can be attributed to the inhibition of amylose chain interactions reducing the formation of junction zones leading to the formation of a weaker gel.The weakening of gel structure was con-?rmed from a decrease in cohesiveness,gumminess and chewiness of starches.Similar changes on corn starch gel texture upon acetylation have been reported earlier by Liu et al.(1997).
Table5
Textural properties of native and acetylated starch gels from di?erent rice cultivars
Cultivar Type Hardness(g)Cohesiveness Springiness(mm)Gumminess(g)Chewiness(gmm)
PR-106Native79.54c0.312b 1.281a24.78e31.75g
PR-106Acetylated15.60c0.164a 1.582b 2.558b 4.046b
PR-114Native78.88c0.301b 1.100a23.75e26.12f
PR-114Acetylated16.66c0.200b 1.881c 3.332b 6.267c
IR-8Native78.45c0.380b 1.071a29.79f31.91g
IR-8Acetylated12.66b0.162a 1.533b 2.051b 3.144b
PR-103Native 6.72a0.626c 1.929b 4.18c8.071d
PR-103Acetylated 4.10a0.270b 2.007c 1.106a 2.219a
PR-113Native74.84c0.224b 1.030a16.67d17.19e
PR-113Acetylated11.17b0.141a 1.586b 1.575a 2.498a
Values with similar superscripts in column do not di?er signi?cantly(p<0.05).
Table6
Syneresis(%)of native and acetylated starches from di?erent rice cultivars
Cultivar Syneresis(%)
Native Acetylated
48h72h96h168h168h
PR-106 2.41d 3.22c 3.82d 6.60d0.15a
PR-114 1.54b 2.71b 2.79b 6.00c0.14a
IR-8 1.51b 2.57b 3.15c 4.61b0.09a
PR-1030.04a0.04a0.04a0.07a–
PR-113 1.79c 3.20c 6.38e8.74e0.11a
Values with similar superscripts in column do not di?er signi?cantly(p<0.05).
N.S.Sodhi,N.Singh/Journal of Food Engineering70(2005)117–127125
3.7.Syneresis
The syneresis of starches progressively increased with the increase in storage duration in all rice starches except PR-103(Table6).The starch separated from PR-103 showed lowest syneresis among all the rice starches. The starch gel from this cultivar did not show any signif-icant change in syneresis during storage.The increase in syneresis(%)during storage has been attributed to the interaction between leached out amylose and amylopec-tin chains which led to development of junction zones, which would re?ect or scatter a signi?cant amount of light(Perera&Hoover,1999).Amylose aggregation and crystallization has been reported to be complete within the?rst few hours of storage while amylopectin aggregation and crystallization occurred during later stages(Miles,Morris,Orford,&Ring,1985).Acetyla-tion decreased syneresis(release of water was not ob-served even after7days of storage at4°C).Changes in syneresis on acetylation may be attributed to the pres-ence of acetyl groups that increased the water retention capacity of the starch molecules in the refrigerated stored gels.The e?ectiveness of acetyl groups in reduc-ing retrogradation was more evident from syneresis data than from setback viscosities observed by RVA.
4.Conclusion
The acetylated starches from di?erent rice cultivars showed signi?cant di?erences in physico-chemical prop-erties.The rice starches showed variations in the intro-duction of acetyl groups during acetylation under similar conditions.The results revealed that acetylated rice starches had lower gelatinization temperatures,ret-rogradation,gel strength and higher swelling power,sol-ubility and peak viscosity than their native counterparts. The amylose content and degree of substitution seemed to be major factors responsible for di?erences in various properties in acetylated starches prepared from di?erent rice cultivars.
Acknowledgement
Financial support by ICAR,New Delhi is acknowledged.
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自我管理演讲稿英语翻译
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