Effects of Cooking Methods and Starch Structures on Starch Hydrolysis Rates of Rice

H:Health,Nutrition&Food

Effects of Cooking Methods and Starch Structureson Starch Hydrolysis Rates of RiceMichael O. Reed, Yongfeng Ai, Josh L. Leutcher, and Jay-lin Jane

Abstract: This study aimed to understand effects of different cooking methods, including steamed, pilaf, and traditionalstir-fried, on starch hydrolysis rates of rice. Rice grains of 3 varieties, japonica, indica, and waxy, were used for thestudy. Rice starch was isolated from the grain and characterized. Amylose contents of starches from japonica, indica, andwaxy rice were 13.5%, 18.0%, and 0.9%, respectively. The onset gelatinization temperature of indica starch (71.6 ◦C)was higher than that of the japonica and waxy starch (56.0 and 56.8 ◦C, respectively). The difference was attributed tolonger amylopectin branch chains of the indica starch. Starch hydrolysis rates and resistant starch (RS) contents of therice varieties differed after they were cooked using different methods. Stir-fried rice displayed the least starch hydrolysisrate followed by pilaf rice and steamed rice for each rice variety. RS contents of freshly steamed japonica, indica, andwaxy rice were 0.7%, 6.6%, and 1.3%, respectively; those of rice pilaf were 12.1%, 13.2%, and 3.4%, respectively; andthe stir-fried rice displayed the largest RS contents of 15.8%, 16.6%, and 12.1%, respectively. Mechanisms of the largeRS contents of the stir-fried rice were studied. With the least starch hydrolysis rate and the largest RS content, stir-friedrice would be a desirable way of preparing rice for food to reduce postprandial blood glucose and insulin responses andto improve colon health of humans.

Keywords: cooking method, resistant starch, rice, starch hydrolysis, starch structure

Practical Application: After rice was cooked using different methods, including steamed, pilaf, and stir-fried, the stir-fried indica rice displayed the least starch hydrolysis rate and the largest resistant starch (RS) content. These results showedthat cold storage of steamed normal rice at 4 ◦C for 24 h followed by stir-frying with corn oil (10%) reduced therate of starch hydrolysis and increased the RS content. Ingesting stir-fried rice therefore can reduce the postprandialblood–glucose concentration and insulin response, which benefits the health of diabetics and prediabetics. The large RScontent of the stir-fried normal rice could also provide health benefits to the colon.

IntroductionRice is among the most important staples worldwide, and food

preparations of rice primarily involve steaming and sometimes fol-lowed by stir-frying or preparing rice pilaf. Rice starch is knownto be readily hydrolyzed by amylases after ingesting and is consid-ered as a high glycemic index (GI) food (Frei and others 2003).With increasing concerns over diabetes development worldwide,there is a pressing need to find methods of cooking rice withreduced starch hydrolysis rates. Starch in steamed rice is readilyhydrolyzed by amylolytic enzymes in the digestive tract, resultingin a high postprandial blood–glucose concentration (Juliano andGoddard 1986). This is detrimental to populations who need tocontrol the blood–glucose level, such as diabetics and prediabetics.Human feeding studies have been conducted using foods of thesame carbohydrate load but prepared by different cooking meth-ods, such as French fries and boiled potatoes. Glycemic responsesof human subjects that consumed French fries were significantlylower than that of the subjects consuming boiled potatoes, andthe results could not be explained by the fat content of the foods(Leeman and others 2008).

MS 20121727 Submitted 12/15/2012, Accepted 4/22/2013. Authors are withDept. of Food Science and Human Nutrition, Iowa State Univ., Ames, IA50011,U.S.A. Direct inquiries to author Jane (E-mail: [email protected]).

Amylose and amylopectin are the 2 major polysaccharides ofstarch, and their proportions and structures determine the gela-tinization, pasting, and enzymatic hydrolysis properties of thestarch (Lu and others 1997; Kim and others 2004; Jane 2006).Amylose is a primarily linear molecule that comprises α-1, 4 linkedglucose chains and a few α-1, 6 branch linkages. Amylopectin isa highly branched molecule consisting of about 5% α-1, 6 branchlinkages. Enzymatic hydrolysis of starch is affected by the amylosecontent, branch chain length (BCL) of amylopectin, and the in-teractions of starch with lipids, proteins, and cellulosic material infood (Goni and others 1997; Okuda and others 2005; Jane 2006;Ai and others 2013). Slowly digestible starch (SDS) in food canprovide a steady blood–glucose concentration after ingesting thefood without causing hyper- and hypoglycemic and insulinemicresponses (O’Dea and others 1981; Hasjim and others 2010).

Resistant starch (RS) refers to a portion of starch that is resistantto enzymatic hydrolysis in the small intestine but is fermentableby microflora in the colon (Englyst and Macfarlane 1986; Englystand Cummings 1987). RS has been reported for its health benefitsof preventing colon cancer, hyperglycemia, hyperinsulinemia, di-abetes, and obesity (Topping and Clifton 2001; Behall and others2006; Hasjim and others 2012). There are 5 types of RS: physicallyinaccessible starch (type 1 resistant starch, RS1), the B- or C-typecrystalline starch with native or uncooked starch-granules (RS2),retrograded amylose (RS3), chemically modified starch (RS4), and

C© 2013 Institute of Food Technologists R©

H1076 Journal of Food Science � Vol. 78, Nr. 7, 2013 doi: 10.1111/1750-3841.12165Further reproduction without permission is prohibited

H:He

alth,

Nutrit

ion&

Food

Effects of cooking on starch hydrolysis . . .

amylose–lipid complex (RS5) (Englyst and others 1992; Eerlingenand others 1994; Hasjim and others 2010).

Japonica, indica, and waxy rice are 3 common rice varietiesused for food. These varieties have different chemical structures,including amylose content and amylopectin BCLs, and physicalproperties, including gelatinization and pasting properties. Thejaponica rice has lower gelatinization temperatures than the indicarice, resulting from its shorter amylopectin BCLs (Okuda and oth-ers 2005). The activity of starch synthase IIa (SSIIa), responsible forelongation of amylopectin branch chains from DP ≤ 11 to DP 12-25, is lower in the japonica rice than in indica rice (Umemoto andothers 1999, 2002). Consequently, the indica rice displays longerBCL and higher gelatinization temperatures (Umemoto and oth-ers 2002). Waxy rice is missing the granule-bound starch synthase I(GBSSI) gene, which is responsible for the biosynthesis of amylose(Sano 1984). Thus, waxy rice starch does not have amylose.

Steamed, pilaf, and stir-fried rice are the 3 most common waysof preparing rice for human consumption. While much researchhas been conducted on starch hydrolysis rates of boiled rice flourand steamed rice, effects of different cooking methods on starchhydrolysis rates of cooked rice and their mechanisms have not beenthoroughly studied and reported. Objectives of this study were tounderstand effects of the cooking methods, steamed, pilaf, andstir-fried, and starch structures of japonica, indica, and waxy riceon starch hydrolysis rates of cooked rice. Results of this study willenable us to understand effects of the cooking methods of foodson the enzymatic hydrolysis of starch and, in turn, the glycemicresponses of humans after ingesting the foods.

Materials and Methods

MaterialsTwo normal rice varieties, japonica (Nomura and Com-

pany, Burlingame, Calif., U.S.A.) and indica (Riceland, Stuttgart,Alaska, U.S.A.), 1 waxy rice variety (Oriental Mascot Brand, Sum-mit Import Corporation, Jersey City, N.J., U.S.A.), and corn oilwere purchased from a local grocery store. Amyloglucosidase fromAspergillus niger (200 U/mL), porcine pancreatic α-amylase (PPA),and porcine pancreatin were purchased from Sigma-Aldrich (St.Louis, Mo., U.S.A.). The Total Starch Assay Kit (AA/AMG) andD-Glucose Assay Kit (glucose oxidase/peroxide, GOPOD) werepurchased from Megazyme International Ireland Ltd. (Wicklow,Ireland).

Grinding and composition of rice grainsRice grains were ground using a cyclone mill (UDY Corp., Fort

Collins, Colo., U.S.A.) with a sieve of 0.5 mm opening, and theground rice was used for compositional analysis and starch hydrol-ysis of ground rice powder studies. The starch content was deter-mined using the Total Starch Assay Kit following the procedureprovided by the manufacturer. The lipid content was determinedusing the Goldfisch Fat Extractors (Labconco Corp., Kansas City,Mo.) with hexanes following the AACC Method 30-25. The pro-tein content was determined using a CN Analyzer (Vario MAX,Elementar 107 Analysen systeme, Hanau, Germany) and calcu-lated by multiplying the nitrogen content with a conversion factorof 5.95 following the AACC Method 46-13.01 (2000). The aboveanalyses were performed in duplicate.

Starch isolationRice starch was isolated from rice grains by wet milling, fol-

lowing the method of Yang and others (1984) with modifications.Rice grains (approximately 50 g) were soaked in a sodium hydrox-

ide solution (0.05%, w/w) at room temperature for 24 h. Ricegrains were then blended using a blender (Osterizer 14 speedblender, Sunbeam Products Inc., Boca Raton, Fla., U.S.A.), andthe slurry was filtered through a nylon cloth with openings of 53μm. Rice starch precipitated was collected and resuspended in asodium chloride solution (0.1 M, 450 mL) with 50 mL tolueneand stirred for 1 h to remove proteins and lipids (Li and others2008). This treatment was repeated until the toluene layer be-came clear and contained no protein. The purified starch waswashed 3 times with water, twice with absolute ethanol, and driedat 37 ◦C.

Amylose content of rice starchThe amylose content of rice starch was determined using an io-

dine potentiometric-titration method (Yoo and Jane 2002). Amy-lose content was calculated by dividing the iodine affinity of thestarch by 0.20 (Takeda and others 1987). The analysis was done induplicate.

Thermal properties of starchThermal properties of isolated starch were determined in du-

plicate following the procedure reported by Ai and others (2011).Starch (approximately 2.5 mg, dry starch basis, dsb, preciselyweighed) was sealed in an aluminum pan with water (3X, w/w,dsb), equilibrated at 25 ◦C for 2 h, and heated at 10 ◦C/min from10 to 110 ◦C. The thermal transition parameters were measuredusing a Pyris Software (Perkin-Elmer, Norwalk, Conn., U.S.A.).Retrograded starch was prepared by storing the gelatinized starchin the differential scanning calorimetry pan at 4 ◦C for 7 d.Thermal properties of the retrograded starch were analyzed usingthe same method, and the percentage retrogradation (R%) wascalculated using the equation: R% = 100 × �H of dissociationof retrograded starch/�H of starch gelatinization.

Pasting properties of starchPasting properties of isolated starch were analyzed in duplicate

using a Rapid Visco-Analyzer (RVA, Newport Scientific, Sidney,Australia). A starch suspension (8%, w/w, dsb; 28 g of total weight)was equilibrated at 50 ◦C for 1 min, heated at a rate of 6 ◦C/minto 95 ◦C, maintained at 95 ◦C for 5 min, and then cooled to 50 ◦Cat a rate of 6 ◦C/min. The paddle-rotating speed was at 960 rpmfor the first 10 s, followed by 160 rpm for the remainder of theanalysis (Ao and Jane 2007).

BCL distribution of amylopectinAmylopectin was separated from amylose using Sepharose CL-

2B gel-permeation chromatography (Li and others 2008). Pu-rified amylopectin was collected and then debranched usingisoamylase. The amylopectin branch chains were labeled with 8-amino-1, 3, 6-pyrenetrisulfonic acid, and the BCL distributionwas analyzed using a fluorophore-assisted capillary electrophore-sis (P/ACEMDQ, Beckman Courter, Fullerton, Calif., U.S.A.)(Hasjim and others 2009).

Cooking methods of rice grainsSteamed rice was prepared by cooking rice grains (100 g) with

water (250 g for japonica and indica, and 200 g for waxy rice)using a rice cooker (Aroma Rice Cooker, model: ARC-914SB,San Diego, Calif., U.S.A.). Because of excessive swelling of waxyrice starch, reduced water was used for cooking steamed waxyrice to prevent it from turning to mash. Rice was boiled for20 min using the cooking setting of the rice cooker and held atthe warming setting for 12 min. Stir-fried rice was prepared using

Vol. 78, Nr. 7, 2013 � Journal of Food Science H1077



Table 1–Rice grain compositions and amylose contents of rice starches.a

Rice grain Rice starch BCL distribution of amylopectin

Amylose AverageVariety Starch (%) Lipid (%) Protein (%) content (%) DP 6-12 (%) DP 13-24 (%) DP 25-36 (%) DP ≥ 37 (%) CL (DP)b

Japonica 83.7 ± 0.8a 0.6 ± 0.0b 5.7 ± 0.1b 13.5 ± 0.5b 37.7 ± 0.8a 42.4 ± 1.5b 6.7 ± 0.2a 13.1 ± 2.5a 19.4 ± 1.0aIndica 82.7 ± 0.3ab 0.6 ± 0.0b 6.9 ± 0.0a 18.0 ± 0.6a 20.9 ± 2.4b 58.4 ± 2.7a 8.1 ± 1.8a 12.7 ± 2.0a 21.3 ± 0.1aWaxy 80.1 ± 0.8b 0.9 ± 0.0a 5.4 ± 0.0c 0.9 ± 0.2c 39.7 ± 1.9a 43.7 ± 0.6b 6.0 ± 0.0a 10.8 ± 2.2a 18.3 ± 1.1a

aMean ± standard deviation. Values with the same letter in a column are not significantly different at P < 0.05.bCL = chain length; DP = degree of polymerization.

the steamed rice after being held at 4 ◦C for 24 h (cold-stored).The cold-stored steamed rice was then stir-fried in a pan (93 ◦C)with corn oil (10%, dry basis, db) for 3 min.

Pilaf rice was prepared by precooking rice grains (100 g) withcorn oil (10%, db) at 88 ◦C for 2 min to coat the surface of the ricegrains with the oil. Water (200 g) was added to the oil precookedrice. The mixture in the pan was covered, boiled on a stove, andthen placed in an oven at 177 ◦C for 18 min or until water wasabsorbed. After removing from the oven, the rice was allowed tosit, with lid covered, for 10 min (Conway 1991).

Stepwise cooking methods were used to study the mechanismof changing starch hydrolysis of the stir-fried rice. To test theeffect of cold storage on the starch hydrolysis, the streamed riceafter being cold stored at 4 ◦C for 24 h was stir-cooked in a panfor 3 min without adding corn oil. To test the effect of addingoil on starch hydrolysis, freshly steamed rice, without prior coldstorage, was stir-fried in a pan with corn oil (10%, dry basis, db)for 3 min.

Enzymatic hydrolysis rate of starch in ground rice powdersand in cooked rice grains

Starch hydrolysis rates of cooked ground rice powder and ricegrains cooked using different cooking methods were analyzed induplicate following the method of Ai and others (2013) withmodifications. Ground rice powder (containing 300 mg starch,dsb) was suspended in deionized water (15.0 mL) and cooked ina boiling water bath with mechanical stirring for 10 min. Thesamples were equilibrated in a water bath at 37 ◦C with shaking(80 rpm) for 30 min. PPA (32 units), in a phosphate buffer so-lution (5.0 mL, 0.4 M, pH 6.9, containing 1.0 mM CaCl2, and0.08% NaN3), was added to the cooked rice dispersion to hy-drolyze starch for 30, 60, and 120 min. An aliquot (0.4 mL) of thehydrolyzate was collected at each time interval and mixed withethanol (0.6 mL) to stop the enzyme activity. After centrifugingat 5200 × g for 5 min, the supernatant was collected, and sol-uble sugars in the supernatant were hydrolyzed to glucose andquantified using a GOPOD method (Setiawan and others 2010).

Cooked rice grains (containing 300 mg starch, dsb) weretransferred to a 50 mL tube with a phosphate buffer solution(15.0 mL, 0.1 M, pH 6.9, containing 0.25 mM CaCl2 and 0.02%NaN3) and homogenized using a homogenizer (T25 Digital Ultra-TurrexR©, IKAR© Works Inc., Wilmington, N.C., U.S.A.) for 20 s.The same starch hydrolysis procedure described above was usedfor the cooked rice grains. The percentage starch hydrolysis wascalculated using the equation: % starch hydrolysis = 100 × totalmass of glucose released/initial dry mass of starch × (162/180).

Contents of rapidly digestible starch (RDS), SDS, and RSRDS, SDS, and RS contents of the cooked rice samples were

analyzed following the method of Englyst and others (1992) withmodifications as described by Li and others (2008). Cooked rice(containing 1.0 g starch, dsb) was homogenized for 20 s in a

sodium acetate buffer (20.0 mL, 0.1 M, pH 5.2) and preincubatedin a shaker water bath (37 ◦C and 80 rpm) for 30 min. The ricedispersion was then hydrolyzed using porcine pancreatin extractand amyloglucosidase at the same condition. Concentrations ofglucose released from the starch at 20 and 120 min were quantifiedusing a D-Glucose Assay Kit. RDS, SDS, and RS contents of thestarch samples were calculated on the dry starch basis (Englyst andothers 1992). The analysis was done in duplicate.

Statistical analysisStatistical significance was analyzed using one-way ANOVA and

multiple comparison test with Tukey’s adjustment. The statisticalanalyses were conducted in SAS (Version 9.2, SAS Inst., Inc., Cary,N.C.) at P value < 0.05.

Results and Discussion

Compositions of rice grains and structures of starchesRice grain compositions and starch structures of rice varieties

are summarized in Table 1. Starch contents of the rice grainsranged from 80.1% (waxy rice) to 83.7% (japonica rice) for thevarieties. Lipid contents of the grains varied between 0.6% and0.9%, and protein contents ranged from 5.4% to 6.9% (Table 1).For normal rice varieties, the indica rice starch had a larger amylosecontent (18.0%) than the japonica rice starch (13.5%). The waxystarch, however, had little amylose (0.9%).

BCL distributions of amylopectin are summarized in Table 1.The japonica rice and waxy rice showed larger proportions ofshort branch chains with DP 6-12 (37.7% and 39.7%, respectively)and smaller proportions of branch chains with DP 13-24 (42.4%and 43.7%, respectively) than indica rice (DP 6-12, 20.9% andDP 13-24, 58.4%). The japonica rice, however, consisted of more(13.1%) long branch chains (DP > 37) than the indica rice (12.7%)and waxy rice (10.8%). The average amylopectin BCL of theindica starch (DP 21.3) was longer than that of the japonica andwaxy starch (DP 19.4 and 18.3, respectively). The large proportion(39.7%) of the short branch chains (DP 6-12) of the waxy ricestarch indicated that the waxy starch had a japonica-rice geneticbackground. These results were in agreement with literature resultsshowing that amylopectin of indica rice displayed longer averageBCL than that of japonica rice (Lu and others 1997).

Thermal properties of isolated starchThermal properties of japonica, indica, and waxy rice starches

are shown in Table 2. The indica rice starch displayed a highergelatinization temperature (To = 71.6 ◦C) than the japonica andwaxy rice starch (56.0 and 56.8 ◦C, respectively). The gelatiniza-tion enthalpy change of the indica starch was 13.9 J/g, which waslarger than that of the japonica and waxy rice (12.6 and 13.3 J/g,respectively). The differences in the gelatinization temperatureand enthalpy change between the indica and japonica starch wereattributed to the larger proportion of branch chains with DP

H1078 Journal of Food Science � Vol. 78, Nr. 7, 2013

H:He

alth,

Nutrit

ion&

Food


Table 2–Thermal properties of isolated rice starches.a

Dissociation ofGelatinization of starch amylose–lipid complex Dissociation of retrograded starch

Variety To (◦C) b Tp (◦C) Tc (◦C) �H (J/g) To (◦C) Tc (◦C) �H (J/g) To (◦C) Tp (◦C) Tc (◦C) �H (J/g) R(%)c

Japonica 56.0 ± 0.3c 63.2 ± 0.3 70.0 ± 0.0 12.6 ± 0.8a 96.0 ± 0.5 104.3 ± 0.0 0.3 ± 0.0 34.2 ± 1.5 48.3 ± 1.5 61.3 ± 0.8 3.5 ± 0.0 27.9bIndica 71.6 ± 0.1a 77.5 ± 0.2 83.3 ± 0.1 13.9 ± 0.4a 96.8 ± 0.7 105.0 ± 0.3 0.4 ± 0.0 39.0 ± 0.4 51.7 ± 0.7 61.9 ± 0.2 7.7 ± 0.1 55.3aWaxy 56.8 ± 0.1b 65.1 ± 0.0 74.6 ± 0.2 13.3 ± 1.0a – – – 32.4 ± 0.1 56.3 ± 0.0 77.2 ± 0.9 3.0 ± 0.1 22.8b

aMean ± standard deviation. Values with the same letter in a column are not significantly different at P < 0.05.bTo = onset temperature, Tp = peak temperature, Tc = conclusion temperature, and �H = enthalpy change.cR (%) = percentage of retrogradation.

13-24 of the indica rice starch, which formed a stable crystallinestructure (Vandeputte and others 2003). The indica rice starch alsodisplayed a substantially greater percentage retrogradation (55.3%)than the japonica (27.9%) and waxy (22.8%) rice starch afterbeing stored at 4 ◦C for 7 d. These differences could be explainedby the larger amylose content and fewer short branch chains (DP6-12) of the indica rice starch (Table 1). The thermograms ofjaponica and indica starches showed a dissociation peak of theamylose–lipid complex (96.0 to 105.0 ◦C), whereas that of thewaxy rice starch showed no such peak at the temperature rangebecause of lacking amylose. The enthalpy change (0.4 J/g) of theamylose–lipid complex dissociation peak of the indica starch waslarger than that of the japonica starch (0.3 J/g), which agreedwith the results of greater amylose content of the indica starchthan the japonica starch (Table 1).

Pasting properties of isolated starchPasting profiles of rice starches analyzed using an RVA are shown

in Figure 1. The waxy rice starch displayed the lowest pasting tem-perature (63.7 ◦C), the greatest peak viscosity (215.5 RVU), andbreakdown viscosity (150.3 RVU), but the least setback viscosity(22.4 RVU), which were attributed to the smallest amylose contentof the starch (0.9%) (Table 1). The japonica rice starch displayeda lower peak viscosity (130.6 RVU), smaller breakdown viscosity(47.5 RVU), but higher pasting temperature (83.1 ◦C) than theindica starch (159.3 RVU, 63.8 RVU, and 79.5 ◦C, respectively).The greatest peak viscosity and the least setback viscosity of thewaxy rice starch agreed with the facts that amylopectin was pri-marily responsible for the swelling power and the viscosity of the

starch (Jane and others 1999) and the setback viscosity was at-tributed to amylose network formation upon cooling to increasethe viscosity (Singh and others 2006).

Enzymatic hydrolysis rate of starch in cooked ground ricepowder

The percentage starch hydrolysis of cooked ground waxy ricepowder (58.1%) after incubation with PPA for 30 min was greaterthan that of the japonica and indica rice counterparts (48.3% and52.9%, respectively) (Figure 2). The greatest percentage starchhydrolysis of the cooked waxy rice was a result of the lack of amy-lose and the greatest swelling of the waxy rice starch (Figure 1),which made the starch most susceptible to the enzymatic hydrol-ysis. Although the indica rice starch had a larger amylose content(18.0%) than the japonica rice (13.5%) (Table 1), the indica ricedisplayed a greater percentage starch hydrolysis than the japonicarice (Figure 2). The greater starch hydrolysis rate of the indicarice could be attributed to the higher peak (159.3 RVU) andbreakdown (63.8 RVU) viscosity and lower pasting temperature(79.5 ◦C) of its starch than that of the japonica rice starch (130.6RVU, 47.5 RVU, and 83.1 ◦C, respectively). When the starch wasswollen to a greater extent, it was more susceptible to enzymatichydrolysis. This result was consistent with that reported previously(Hu and others 2004; Okuda and others 2005). The percentageof long branch chains (DP > 37) of the indica rice starch (12.7%)was smaller than that of the japonica rice (13.1%), which couldalso contribute to the greater starch hydrolysis rate of the indicarice as reported previously with other rice varieties (Okuda andothers 2005).

0

10

20

30

40

50

60

70

80

90

100

0

50

100

150

200

250

0 5 10 15 20

Tem

pera

ture

(ºC

)

Vis

cosi

ty (R

VU

)

Time (min)

Japonica Rice Indica Rice Waxy Rice Temperature

Figure 1–Pasting profiles of isolated ricestarches (8%, w/w, dsb; 28 g total weight)analyzed using a Rapid Visco-Analyzer (RVA).




Enzymatic hydrolysis rate of starch in cooked rice grainsStarch hydrolysis rates of rice grains cooked using different

methods are shown in Figure 3. The stir-fried rice displayed theleast starch hydrolysis rates (39.9%, 38.3%, and 49.4% for japon-ica, indica, and waxy rice varieties, respectively) after incubationwith PPA for 30 min compared with the pilaf rice (45.7%, 41.1%,and 52.9%, respectively) and the steamed rice (62.2%, 51.6%, and57.8%, respectively). The lower starch hydrolysis rates of the stir-fried rice than that of the steamed rice were in agreement withthe results of Ai and others (2013). In a study to understand mech-anisms of lipid effects on starch physical properties and enzymatichydrolysis rates, Ai and others (2013) report that corn oil forms he-lical complexes with amylose as revealed by 13C-nmr spectra. Theamylose–lipid complex restricts the starch swelling and reducesthe starch hydrolysis rate. The difference in the starch hydroly-sis rate between the fried and steamed rice also agreed with theresults of a lower glycemic response after ingesting French fries(GI = 54) than boiled potatoes (GI = 78) (Leeman and others2008).

RDS, SDS, and RS contents of cooked rice grainsTo understand the mechanism of the least hydrolysis rate of

starch in the stir-fried rice, we conducted stepwise studies to re-veal effects of the cold storage of steamed rice and stir-frying ricewith oil on starch hydrolysis rates. The RDS, SDS, and RS con-tents of different rice varieties cooked using different methods areshown in Table 3. Among the steamed rice, RS contents of thejaponica, indica, and waxy rice were 0.7%, 6.6%, and 1.3%, respec-tively. The largest RS content of the indica rice (6.6%) was a resultof its largest amylose content (18.0%) (Table 1). Stir-frying thefreshly steamed rice without cold storage increased RS contentsto 4.6%, 7.1%, and 4.3%, respectively, resulting from amylose–lipid complex formation (RS5) (Ai and others 2013) and lipidcoating effects (RS1). After cold storage of the steamed rice at 4◦C for 24 h followed by stir-cooking for 3 min without corn oil,RS contents increased to 11.0%, 12.2%, and 7.9%, respectively.These results were in agreement with the percentages retrograda-tion of the isolated starch after cold storage (Table 2) and confirmedthat retrograded starch was more resistant to enzymatic hydrolysis(RS3) (Englyst and Cummings 1987; Eerlingen and others 1994).Stir-frying the cold-stored rice with corn oil further increased RScontents to 15.8%, 16.6%, and 12.1%, respectively. Stir-fried riceof all varieties displayed significantly larger RS contents (P < 0.05)

0102030405060708090

100

0 20 40 60 80 100 120

% h

ydro

lysi

s

Time (min)

JaponicaIndicaWaxy

Figure 2–Starch hydrolysis rates of cooked ground rice powder samplesusing PPA. The reactions were conducted in a shaker water bath at 37 ◦Cand 80 rpm.

than their steamed and pilaf rice counterparts. The largest RS con-tent of the stir-fried indica rice (16.6%) was a result of the greatestamylose content of the indica rice starch, which developed themost retrograded starch after cold storage (Table 2) and formedamylose–lipid complex after stir-frying with corn oil (Ai and oth-ers 2013).

RDS contents of the cooked rice varieties were the largestfor the steamed and stir-frying freshly steamed rice, followed bythe stir-cooking of cold-stored steamed rice without oil, and theleast RDS contents were found in the stir-fried rice using cold-stored steamed rice. Ingesting cooked rice with reduced RDScontents can prevent a postprandial hyperglycemic response and asubsequent hypoglycemic response, which is critical for diabeticpatients. There were no significant impacts of cooking methodson SDS contents of the cooked rice.

Rice varieties cooked using the pilaf rice method displayed RScontents of 12.1%, 13.2%, and 3.4% for japonica, indica, andwaxy rice, respectively. The smaller RS content of pilaf waxy rice

0102030405060708090

100

0 20 40 60 80 100 120

% h

ydro

lysi

s

Time (min)

Steamed

Pilaf

Stir-fried

A

0102030405060708090

100

0 20 40 60 80 100 120

% h

ydro

lysi

s

Time (min)

B

0102030405060708090

100

0 20 40 60 80 100 120

% h

ydro

lysi

s

Time (min)

C

Figure 3–Starch hydrolysis rates of rice cooked using different methods.The starch was hydrolyzed using PPA in a shaker water bath at 37 ◦C and80 rpm. Different rice varieties were used for the study: (A) japonica, (B)indica, and (C) waxy rice grains.

H1080 Journal of Food Science � Vol. 78, Nr. 7, 2013

H:He

alth,

Nutrit

ion&

Food


Table 3–RDS, SDS, and RS contents of rice grains cooked using different methods.a

Variety Cooking method RDS (%) SDS (%) RS (%)

Japonica Steamed 84.8 ± 0.7bc 14.5 ± 0.3ab 0.7 ± 0.5hStir-frying (oil, no cold storage) 88.7 ± 1.3ab 6.8 ± 0.7de 4.6 ± 0.6defStir-cooking (cold-stored, no oil) 83.7 ± 0.4cd 5.3 ± 0.6de 11.0 ± 0.2bStir-fried (cold-stored, oil) 75.9 ± 2.2e 8.4 ± 1.9cd 15.8 ± 0.4aPilaf 80.0 ± 1.8de 7.9 ± 2.4cd 12.1 ± 0.6b

Indica Steamed 85.4 ± 0.7bc 8.1 ± 1.2cd 6.6 ± 0.4cdeStir-frying (oil, no cold storage) 84.3 ± 1.1bcd 8.5 ± 0.9cd 7.1 ± 0.2cdStir-cooking (cold-stored, no oil) 70.2 ± 0.6f 17.7 ± 0.7a 12.2 ± 0.1bStir-fried (cold-stored, oil) 66.8 ± 0.8f 16.6 ± 1.6a 16.6 ± 0.8aPilaf 85.1 ± 1.1bc 1.7 ± 0.2e 13.2 ± 1.0b

Waxy Steamed 86.0 ± 0.1bc 12.8 ± 0.0abc 1.3 ± 0.1ghStir-frying (oil, no cold storage) 90.7 ± 0.7a 4.9 ± 1.3de 4.3 ± 0.6efStir-cooking (cold-stored, no oil) 83.5 ± 2.3cd 8.6 ± 3.8bcd 7.9 ± 1.6cStir-fried (cold-stored, oil) 80.2 ± 0.4de 7.7 ± 1.1cd 12.1 ± 0.7bPilaf 86.1 ± 0.1bc 10.4 ± 0.2bcd 3.4 ± 0.1fg

aMean ± standard deviation. Values with the same letter in a column are not significantly different at P < 0.05.

(3.4%) was a result of lacking amylose to form helical complexwith corn oil. The larger RS contents of pilaf indica and japonicarice compared with that of the steamed rice counterparts indicatedthe formation of amylose–lipid complex during cooking (Ai andothers 2013). There was little or no reduction in the RDS contentof the pilaf rice compared with the steamed rice. This could be aresult of precooking rice with oil followed by boiling with water,which caused swelling of starch not complexed with oil and madeit susceptible to amylolysis.

ConclusionsResults of the present study clearly showed that stir-fried rice

displayed the least starch hydrolysis rates compared with steamedand pilaf rice. The differences were attributed to the formationof retrograded starch during cold storage, the development ofamylose–lipid complex, and lipid coating of the starch during stir-frying with corn oil. Indica rice displayed the greatest RS contentbecause of its largest amylose content. The least starch hydrolysisrate and the largest RS content of stir-fried rice make it a betterchoice of food to maintain a stable postprandial blood–glucoselevel and to prevent hyper-and hypoglycemic responses in humansafter ingesting the rice.

AcknowledgmentsThe authors thank Plant Science Inst. of Iowa State Univ. and

USDA-NRI/AFRI for funding supports, Ignacio Alvarez for sta-tistical analysis, and Jovin Hasjim for discussions.

ReferencesAACC. 2000. Approved methods of the AACC. 10th ed. St. Paul, MN: American Association

of Cereal Chemists.Ai Y, Hasjim J, Jane J. 2013. Effects of lipids on enzymatic hydrolysis and physical properties of

starch. Carbohydr Polym 92(1):120–7.Ai Y, Medic J, Jiang H, Wang D, Jane J. 2011. Starch characterization and ethanol production

of sorghum. J Agric Food Chem 59(13):7385–92.Ao Z, Jane J. 2007. Characterization and modeling of the A- and B-granule starches of wheat,

triticale, and barley. Carbohydr Polym 67(1):46–55.Behall KM, Scholfield DJ, Hallfrisch JG, Liljeberg-Elmstahl HGM. 2006. Consumption of both

resistant starch and beta-glucan improves postprandial plasma glucose and insulin in women.Diabetes Care 29(5):976–81.

Conway LG. 1991. The new professional chef. 5th ed. New York: Van Nostrand Reinhold.869 p.

Eerlingen RC, Jacobs H, Delcour JA. 1994. Enzyme-resistant starch .5. Effect of retrogradationof waxy maize starch on enzyme susceptibility. Cereal Chem 71(4):351–5.

Englyst HN, Cummings JH. 1987. Digestion of polysaccharides of potato in the small-intestineof man. Am J Clin Nutr 45(2):423–31.

Englyst HN, Kingman SM, Cummings JH. 1992. Classification and measurement of nutritionallyimportant starch fractions. Eur J Clin Nutr 46:S33–S50.

Englyst HN, Macfarlane GT. 1986. Breakdown of resistant and readily digestible starch by humangut bacteria. J Sci Food Agric 37(7):699–706.

Frei M, Siddhuraju P, Becker K. 2003. Studies on the in vitro starch digestibility and theglycemic index of six different indigenous rice cultivars from the Philippines. Food Chem83(3):395–402.

Goni I, Bravo L, Larrauri JA, Calixto FS. 1997. Resistant starch in potatoes deep-fried in oliveoil. Food Chem 59(2):269–72.

Hasjim J, Lee SO, Hendrich S, Setiawan S, Ai Y, Jane J. 2010. Characterization of a novelresistant-starch and its effects on postprandial plasma-glucose and insulin responses. CerealChem 87(4):257–62.

Hasjim J, Li EP, Dhital S. 2012. Milling of rice grains: the roles of starch structures in thesolubility and swelling properties of rice flour. Starch-Starke 64(8):631–45.

Hasjim J, Srichuwong S, Scott MP, Jane J. 2009. Kernel composition, starch structure, andenzyme digestibility of opaque-2 maize and quality protein maize. J Agric Food Chem57(5):2049–55.

Hu P, Zhao H, Duan Z, Linlin Z, Wu D. 2004. Starch digestibility and the estimated glycemicscore of different types of rice differing in amylose contents. J Ceral Sci 40:231–7.

Jane J. 2006. Current understanding on starch granule structures. J Appl Glycosci 53:205–13.Jane J, Chen YY, Lee LF, McPherson AE, Wong KS, Radosavljevic M, Kasemsuwan T. 1999.

Effects of amylopectin branch chain length and amylose content on the gelatinization andpasting properties of starch. Cereal Chem 76(5):629–37.

Juliano BO, Goddard MS. 1986. Cause of varietal difference in insulin and glucose responses toingested rice. Qualitas Plantarum-Plant Foods Hum Nutr 36(1):35–41.

Kim JC, Kim JI, Kong YW, Kang MJ, Kim MJ, Cha IJ. 2004. Influence of the physical formof processed rice products on the enzymatic hydrolysis of rice starch in vitro and on thepostprandial glucose and insulin responses in patients with type 2 diabetes mellitus. BiosciBiotechnol Biochem 68(9):1831–6.

Leeman M, Ostman E, Bjock I. 2008. Glycaemic and satiating properties of potato products.Euro J Clin Nutr 62(1):87–95.

Li L, Jiang H, Campbell M, Blanco M, Jane J. 2008. Characterization of maize amylose-extender(ae) mutant starches. Part I: Relationship between resistant starch contents and molecularstructures. Carbohydr Polym 74(3):396–404.

Lu S, Chen LN, Lii CY. 1997. Correlations between the fine structure, physicochemical prop-erties, and retrogradation of amylopectins from Taiwan rice varieties. Cereal Chem 74(1):34–9.

O’Dea K, Snow P, Nestel P. 1981. Rate of starch hydrolysis in vitro as a predictor of metabolicresponses to complex carbohydrate in vivo. Am J Clin Nutr 34(10):1991–3.

Okuda M, Aramaki I, Koseki T, Satoh H, Hashizume K. 2005. Structural characteristics,properties, and in vitro digestibility of rice. Cereal Chem 82(4):361–8.

Sano Y. 1984. Differential regulation of waxy gene-expression in rice endosperm. Theor ApplGenet 68(5):467–73.

Setiawan S, Widjaja H, Rakphongphairoj V, Jane J 2010. Effects of drying conditions of cornkernels and storage at an elevated humidity on starch structures and properties. J Agric FoodChem 58(23):12260–7.

Singh N, Kaur L, Sandhu KS, Kaur J, Nishinari K. 2006. Relationships between physicochem-ical, morphological, thermal, rheological properties of rice starches. Food Hydrocolloids20(4):532–42.

Takeda Y, Hizukuri S, Juliano BO. 1987. Structures of rice amylopectins with low and highaffinities for iodine. Carbohydr Res 168(1):79–88.

Topping DL, Clifton PM. 2001. Short-chain fatty acids and human colonic function: roles ofresistant starch and nonstarch polysaccharides. Physiol Rev 81(3):1031–64.

Umemoto T, Nakamura Y, Satoh H, Terashima K. 1999. Differences in amylopectin structurebetween two rice varieties in relation to the effects of temperature during grain-filling. Starch-Starke 51(2–3):58–62.

Umemoto T, Yano M, Satoh H, Shomura A, Nakamura Y. 2002. Mapping of a gene respon-sible for the difference in amylopectin structure between japonica-type and indica-type ricevarieties. Theor Appl Genet 104(1):1–8.

Vandeputte GE, Vermeylen R, Geeroms J, Delcour JA. 2003. Rice starches. I. Structural aspectsprovide insight into crystallinity characteristics and gelatinisation behaviour of granular starch.J Cereal Sci 38(1):43–52.

Yang CC, Lai HM, Lii CY. 1984. The modified alkaline steeping method for the isolation ofrice starch. Food Sci 11:158–62.

Yoo SH, Jane J. 2002. Structural and physical characteristics of waxy and other wheat starches.Carbohydr Polym 49(3):297–305.


Documents

Effects of Cooking Methods and Starch Structures on Starch Hydrolysis Rates of Rice