Effects of Flour Particle Size on the Textural Properties of Flour Tortillas

Journal of Cereal Science 31 (2000) 263–272 doi:10.1006/jcrs.2000.0302, available online at http://www.idealibrary.com on

Effects of Flour Particle Size on the Textural Properties of Flour Tortillas L. Wang and R. A. Flores Kansas State University, Department of Grain Science and Industry, Manhattan, KS 66506, U.S.A. Received 4 January 1999

ABSTRACT Three wheat flours, hard red winter (HRW), hard white winter (HWW) and soft red winter (SRW), were fractionated by sieving into four different particle size fractions, <38, 38–53, 53–75, and >75 lm. The medium fractions, 38–53 and 53–75 lm, of the HRW and HWW wheat had higher protein contents than the finest and coarsest fractions. The finest fractions had the highest levels of damaged starch. Tortillas made from the medium fractions of HRW and HWW, especially the 53–75 lm fraction, had longer rupture distance and better foldability. The finest fraction yielded tortillas with shorter rupture distance and worse foldability. Fractionation by flour particle size slightly affected protein composition of the flour. Size exclusion-high performance liquid chromatography (SE-HPLC) analyses showed that the compositions of SRW wheat proteins were significantly different from those of HRW and HWW wheat proteins. SRW had more low molecular weight (LMW) and less high molecular weight (HMW) protein than HRW and HWW had. The <38 lm fractions of HRW and HWW had more LMW proteins than other fractions. The amount of LMW proteins of flours correlated negatively with the rupture distance and foldability of tortillas. Particle size of the flours was the major factor affecting the tortilla texture.  2000 Academic Press

Keywords: wheat, tortilla, particle size, texture, protein, HPLC.

INTRODUCTION The wheat flour milling process involves a series of breaking, middling reduction, and sifting operations. Particle size is a very important concept in flour milling. The resulting flours vary in particle size and differ in chemical and physical properties. To study the effects of flour particle size on products, wheat flour has been separated by sieving or by air-classification according to differences in particle size and density. Contribution No. 99-216-J from the Kansas Agricultural Experimental Station  : HRW=hard red winter, HWW= hard white winter, SRW=soft red winter, SDS=sodium dodecyl sulphate, SE-HPLC=size exclusion-high performance liquid chromatography, LMW=low molecular weight, HMW=high molecular weight. Corresponding author: R. A. Flores 0733–5210/00/050263+10 $35.00/0

Some studies have shown that medium particlesize flours fractionated by sieving had better baking potential1–4. Others have shown that medium-fine particle-size flours fractionated by air-classification had better baking potential because of the higher protein content in those fractions5,6. Flours with the same levels of protein but different particle sizes demonstrated different baking results7; however, the levels of damaged starch in these two fractions were different. Fractionated flours also have been studied in cookies, cakes, and noodles6,8–10 but not in tortillas. However, no research has considered the protein quality of different fractions. Many researchers have tried to understand the protein in wheat kernels. Using stains and electron microscopy showed that the entire area between starch granules is filled with protein, and the water-soluble proteins surrounded the starch granules11. The protein bodies of hard and soft wheat in the mature  2000 Academic Press

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wheat kernels were very different12. The bonds between protein and starch granules are stronger in hard wheat than in soft wheat. During milling, the soft wheats fracture readily into storage protein and free starch granules, and the resulting flour had fewer damaged starch granules. However, some of the starch granules of the hard wheat were damaged by milling because of their strong binding with the protein11–13. Protein and starch from different air-classified fractions of bread flour can affect bread-making quality. A previous study14 indicated that wheat starch and gluten isolated from the coarse fractions were superior to those from the fine fractions in baking quality. However, another study15 showed that different air-classified fractions had similar gliadin and gluten ratios. The authors did not find any study concerning the effect of flour particle size on tortilla texture. Conflicting results also were found concerning the protein quality of different fractions of air-classified flours and flours with different particle sizes. This study was conducted to investigate the effect of particle size of wheat flours on tortilla texture and to study the effects of flour classification by particle size on the protein quality of different fractions. EXPERIMENTAL Wheat flours Straight-grade wheat flours were milled from hard red winter (HRW) wheat, Jagger, hard white winter (HWW) wheat, 4AT-9900, and soft red winter (SRW) wheat, Ernie, on the experimental MIAG Multomat Mill with extraction rates of 68·9%, 68·3%, and 64·7%, respectively16. The protein contents of the three flours were 11·25%, 10·90%, and 7·44%, respectively. The particle size distribution was determined using 50 g of flour with the Alpine Air Jet Sieve (model A 200 LS, Alpine Ag., Augsburg, Germany) on sieves with openings of 38, 53, 75, and 106 lm10. Four fractions, <38 lm, 38–53 lm, 53–75 lm, >75 lm, were obtained by separating the HRW, HWW, and SRW straight-grade flours on sieves with openings of 38, 53, and 75 lm on the Alpine Air Jet Sieve. The flour fractions were stored in a refrigerator (4 °C). The contents of moisture, protein, ash, and damaged starch of the flour samples were determined using the AACC Methods 44-15A, 4630, 08-03, and 76-31, respectively17. The colour

of the flours was measured using an Agtron Color Meter (Filper Industries, Inc., Reno, NV, U.S.A.) calibrated to zero by an Agtron disk 68 and to 100 by disk 97 (Magnuson Engineers Inc., San Jose, CA, U.S.A.). Experimental design and statistical analysis Flours were numbered randomly from 1 to 15 including the straight-grade, <38 lm, 38–53 lm, 53–75 lm, and >75 lm flours of HRW, HWW, and SRW wheat. Tortillas were made from these flours according to a partially balanced design with treatments (t)=15, replicates (r)=4, and block (b)=1518. Flour and tortilla data were analysed statistically using the SAS program19. The least significant difference (LSD) was conducted at the 95% confidence interval. Tortilla making The procedure for tortilla making was based on the one described by Bello et al.20 with the following modifications. The water absorption of the tortillas was 15% less of the Farinograph absorption determined on a Farinograph (C. W. Brabender Instruments, Inc., South Hackensack, NJ, U.S.A.) and the Farinograph curve was centred at 500 B.U. Dry ingredients and shortening were mixed at slow speed (110 rev/min) with a paddle (flat beater) for 4 min in a mixer (KitchenAid K45SS, St. Joseph, MI, U.S.A.); distilled water (25±2 °C) was added, and the dough was mixed at slow speed for 1 min and then mixed at medium (176 rev/min) for 5 min. The dough was put in a hermetically sealed plastic food container (Rubbermaid), allowed to rest for 5 min, divided into 45 g pieces, and rounded by hand. The rounded dough balls were proofed at room temperature (25±2 °C) for at least 30 min. Each dough ball was put between two pieces of parchment paper and flattened using a hot press (DOUGHPRO, DUAL/HEAT, Propress Corporation, Paramont, CA, U.S.A.) for 15 s. Both top and bottom temperatures of the hot press were set at 71 °C. The surface temperature of the top was 75±1 °C, and that of the bottom was 66±1 °C. The gap between the hot plates was set at thin (1·5–2·0 mm). The pressed dough was baked on a griddle (‘SPEEDSTER’, Walter & Carrell MFG Co., Denver, CO, U.S.A.) set at a surface temperature of 173±5 °C. The raw tortilla was baked on one side for 20 s, turned over

Textural properties of flour tortillas

and baked for another 30 s, and then turned over again and baked for 10 s. To avoid shrinkage during baking, the parchment paper was kept on the top of the tortilla for 10 s when the other side was baking. The baked tortilla was cooled on a wire rack to room temperature (25±2 °C) for about 4 min; placed in a reclosable polyethylene bag (8×10 in, Minigrip, ITW Company, Seguin, TX, U.S.A.); and kept at room temperature for 17 to 19 h for texture evaluation.

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through a subjective test by the researcher using a modification of the method of Twillman and White21. One tortilla was folded firmly, then unfolded, and the cracks on the surface of the tortilla were noted. A hedonic rating scale of 1 to 10 was used. Ten points were given to tortillas without any cracks on the surface after they were folded, and 1 point to tortillas showing a broken surface along the folding line. Protein extraction

Tortilla characteristics The moisture content of tortillas was measured using AACC method 44-15A17. The diameter of tortillas was the average of two diagonal measurements, and the weight was the average of three tortillas. The lightness of tortillas, avoiding burned areas, was measured in triplicate using a Chroma Meter (Minolta CR-300, Minolta Corporation, Ramsey, NJ, U.S.A.) with a standard calibration plate CR-A44 with values of L=93·2, a=0·3139, b=0·3204. L is the lightness of the flour, negative value of a shows greenness, where as positive value of a indicates redness. Positive value of b shows yellowness of the flour and negative value indicates blueness. The L values were recorded and three measurements were averaged. Firmness, stretchability and foldability The firmness of flour tortillas was determined with a TA-XT2 texture analyser (Texture Technologies Corp., Scarsdale, NY, U.S.A.) equipped with a 1·9 cm-diameter rounded-end probe (TA-108). The firmness was measured as the force (kg) required for a compression of 30% strain of the tortillas. The peak force was recorded. Three measurements were made for each tortilla on different spots. Based on the maximum distance (30% strain), the thickness of tortillas was determined at ten-thirds of the maximum compression distance. The stretchability of flour tortillas was determined according to the manufacturer’s instructions with the TA-XT2 texture analyser equipped with a TA-108 tortilla film fixture and the 1·90 cm rounded-end probe. The distance was 25·0 mm at a speed of 2·0 mm/s. For each test, measurements were taken of the maximum peak force value and the rupture distance. The foldability of tortillas was determined

Protein was extracted for size exclusion-high performance liquid chromatograph (SE-HPLC) analysis with 70% ethanol and 2% (w/v) sodium dodecyl sulphate (SDS) in 0·05  sodium phosphate buffer pH 6·922. The flour sample (0·250 g) was extracted with 1·0 mL of 70% ethanol or 2% SDS phosphate buffer for 1 h at room temperature (25±2 °C). The extract was separated by centrifuging for 7 min at 7000 rev/min and transferred to a vial and sealed for HPLC analysis. Size exclusion-high performance liquid chromatography (SE-HPLC) A HPLC system (HP1100 series, Hewlett-Packard GmbH, Hewlett-Packard-Strasse 8, Waldbronn, Germany) including an automatic sample injector and equipped with a diode array detector (DAD) with wavelength 280 nm was used. The column was a Biosep-Sec-S4000 (300×7·80 mm, Phenomenex). The elution fluids were (A) 0·1% trifluoracetic acid in water and (B) 0·1% trifluoracetic acid in acetonitrile in a ratio of 50:50 at a flow rate of 0·5 mL/min. Peak areas were regulated, and the relative area percentages were analysed. RESULTS AND DISCUSSION Wheat flour characteristics The particle size distribution of HRW and HWW wheat flours was different from SRW wheat flours (Table I). HRW and HWW wheat flours showed a lower percentage in the 38–53 lm fractions ranged from 15·55% to 16·61%, others ranged from 22·98% to 32·96%. The particle size distribution of SRW wheat flour showed a highest percentage in the <38 lm fraction with 66·33%, and other fractions ranged from 9·70% to 15·75%. The proximate analyses of the classified flours are shown in Table II. The protein contents of

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Table I

L. Wang and R. A. Flores

Particle size distribution of straight-grade flours Percent (weight %) Straight-Grade Flour

Particle size

HRW

HWW

SRW

<38 lm 38–53 lm 53–75 lm >75 lm

26·71 15·55 24·78 32·96

31·68 16·61 22·98 28·73

66·33 15·75 8·22 9·70

As is moisture basis. Mean values of duplicates with variance less than 3%.

medium particle sizes of HRW and HWW wheat flour, 38–53 lm and 53–75 lm fractions, ranged from 12·15% to 12·61% were higher than those of the <38 lm and >75 lm ranged from 9·87% to 11·83%. The <38 lm fractions had lower protein contents than other fractions, which indicated that the starch granules were released from the protein matrix during grinding. However, this could not explain why the >75 lm fractions of HRW and HWW wheat flours also had lower protein content. The protein content of SRW wheat flours increased with the increase in flour particle size from 6·08% to 10·34%. The protein matrix in SRW wheat was not so strong; it released starch granules much easier during grinding, so more clean starch granules were obtained.

Table II

Ash contents of HRW and HWW wheat flours did not correlate with flour particle size. The >75 lm fraction of HRW and SRW wheat flour had higher ash content, 0·43% and 0·57% respectively, and the 38–53 lm fraction of HWW wheat flour had higher ash content (0·43%). Others ranged from 0·33% to 0·42%. The damaged starch of flour fractions decreased with the increase in flour particle size, from 10·3% to 5·0% for HRW flours, from 7·9% to 3·6% for HWW wheat flours, and from 3·2% to 2·5% for SRW wheat flours. The Agtron colour of the flours increased with the decrease in particle size. Farinograph characteristics of flours The results of the Farinograph for the wheat flours are shown in Table III. The straight-grade flours of HRW and HWW wheat had higher water absorption and longer peak time and mixing stability than that of soft wheat. For different ranges of particle size, the protein content was correlated positively with water absorption, peak time and mixing stability (r=0·90, 0·84, and 0·80 with p<0·001, respectively). The water absorption of HRW and HWW wheat flour fractions increased with the reduction of particle size, except for the <38 lm fractions. The peak times of the <38 lm fractions of HRW and HWW wheat flours were 2·1 and 2·3 min, respectively, and the mixing

Properties of flour fractionsa Protein content (%)

Ash content (%)

Damaged starch (%)

Agtron colour

HRW

Straight-grade <38 lm 38–53 lm 53–75 lm 75 lm

11·25 9·87 12·61 12·41 11·53

0·35 0·35 0·34 0·34 0·43

7·6 10·3 7·8 5·8 5·0

79 87 81 77 73

HWW

Straight-grade <38 lm 38–53 lm 53–75 lm 75 lm

10·90 10·33 12·15 12·36 11·83

0·35 0·38 0·43 0·35 0·36

5·2 7·9 5·8 4·3 3·6

80 82 80 78 74

SRW

Straight-grade <38 lm 38–53 lm 53–75 lm 75 lm

7·44 6·08 6·37 9·94 10·34

0·39 0·33 0·35 0·42 0·57

3·0 3·2 3·1 2·5 2·5

89 89 89 85 79

Flours

a

Content of protein, ash, and damaged starch were on 14% m.b. (moisture basis). Mean values of duplicates with variance less than 3%.

Textural properties of flour tortillas

Table III

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Farinograph characteristics of flours with different particle size Water absorption (%, 14% m.b.)

Peak time (min)

Mixing stability (min)

HRW

Straight-grade <38 lm 38–53 lm 53–75 lm 75 lm

67·3 63·9 65·2 63·4 61·3

15·6 2·1 15·4 14·5 11·5

17·8 2·5 16·1 14·4 16·4

HWW

Straight-grade <38 lm 38–53 lm 53–75 lm 75 lm

63·7 58·3 62·8 61·8 60·7

16·2 2·3 19·8 17·4 14·3

17·9 2·5 15·2 14·9 16·4

SRW

Straight-grade <38 lm 38–53 lm 53–75 lm 75 lm

52·0 45·6 46·2 55·5 58·3

1·5 1·0 1·1 8·4 7·4

5·1 1·2 2·5 13·4 12·4

Flours

Mean values of duplicates with variance less than 3%.

stability for both was 2·5 min. The Farinograph curves of the <38 lm fractions of HRW and HWW wheat flours showed a very weak gluten network. For other fractions, the peak time increased with the reduction of flour particle size. The water absorption of SRW wheat flour fractions increased with the increase of particle size. The peak time and mixing stability of the <38 lm and 38–53 lm fraction were shorter than those of other fractions; however, the peak time and mixing stability were longer for the 53–75 lm and >75 lm fractions of SRW wheat flour than for straight-grade flour. Flour particle size affected the Farinograph characteristics. The Farinograph curves of the <38 lm fractions of HRW and HWW wheat flours were like the curve for SRW wheat flour. These results agreed with those of Wichser and Shellenberger6. They explained that this phenomenon was caused by the lower protein content in the <38 lm fractions and also probably by their finely divided state, which prevented the protein from forming a satisfactory matrix during dough development. However, the protein contents in <38 lm fractions of HRW and HWW wheat flours were 9·87% and 10·33%, which should not display those Farinograph patterns. The Farinograph curves of other fractions of HRW and HWW wheat flours were close to those of their straight-grade flours. The Farinograph curves of the <38 lm and 38–53 lm fractions of SRW wheat flour were similar to that of straight-

grade flour, but that of the <38 lm fraction was even lower. The 53–75 lm and >75 lm fractions of SRW flour showed a stronger gluten network compared to the SRW straight-grade flour, but had shorter peak time and mixing stability compared to the straight-grade flours of HRW or HWW wheats. The higher protein content probably could explain that.

Tortilla characteristics The characteristics of the tortillas are listed in Table IV. The moisture content of baked tortillas made with SRW wheat flours was much lower than that of tortillas made with HRW and HWW wheat flours. Moisture contents ranged between 27·99% and 29·23% for HRW wheat flours, 26·10% and 28·43% for HWW wheat flours, and 20·81% and 26·74% for SRW wheat flours. The moisture content of tortillas was correlated positively with the protein contents of wheat flours (r=0·97, p<0·001). The diameters of tortillas ranged from 15·9 cm to 16·9 cm for those made from HRW and HWW wheat flours and from 17·4 cm to 19·2 cm for those made from SRW wheat flours. Tortilla diameters were correlated negatively with flour protein contents (r=−0·95, p<0·001), and tortilla moisture contents (r= −0·94, p<0·001). The thickness of tortillas was affected by the

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Table IV

Characteristics of tortillas made from flours with different particle sizes Moisture content (%)

Diameter (cm)

Thickness (mm)

L-Value

HRW

Straight-grade <38 lm 38–53 lm 53–75 lm 75 lm

28·36 27·99 28·74 29·23 28·00

16·7 16·9 16·0 15·9 16·1

1·67 1·74 1·83 1·93 1·91

81·86 82·57 82·02 82·07 83·79

HWW

Straight-grade <38 lm 38–53 lm 53–75 lm 75 lm

27·59 26·10 28·43 28·17 28·33

16·9 16·7 16·4 16·4 16·7

1·68 1·64 1·70 1·75 1·75

81·78 83·13 80·95 82·41 83·10

SRW

Straight-grade <38 lm 38–53 lm 53–75 lm 75 lm

22·58 20·81 21·75 26·51 26·74

18·1 19·2 18·2 17·4 16·6

1·75 1·56 1·82 1·59 1·65

84·48 88·33 87·85 83·98 80·99

Flours

Mean values of duplicates with variance less than 3%.

diameter, contents of protein and moisture, and cooking. Thickness was correlated negatively with the diameter (r=−0·51, p<0·05). Tortilla dough made with higher protein flours tended to shrink back after pressing, and produced thicker tortillas. The protein level of SRW wheat flours partially affected the diameter of tortillas but not the thickness. The moisture content had a tendency to increase the tortilla thickness. Areas that puffed during cooking were thicker than areas did not puff. The L-values of baked tortillas ranged from 80·99 to 88·33 (Table IV). The L-values of tortillas made with all particle sizes of SRW wheat flours except the coarsest were higher than those of tortillas made with corresponding particle sizes of HRW and HWW wheat flours. Protein contents of flour and tortilla moisture were the major factors contributing to the lightness of baked tortillas and had negative correlation with lightness (r=−0·85 and −0·87, respectively, p<0·001). The ash content of flours also affected tortilla lightness. The ash content of SRW wheat flours was highly negatively correlated with lightness (r=−0·97, p<0·001). Textural properties of baked tortillas Table V shows the textural properties of baked tortillas. When firmness was measured, the peak forces of tortillas made from different particle sizes of flours ranged from 5·07 kg to 11·78 kg. The peak force showed that the tortillas made from

SRW wheat flours were significantly less firm than those made from HRW and HWW wheat flours. Tortillas made from the >75 lm fractions of HRW and HWW wheat flours were the firmest, but the protein content of that fraction was not the highest. Firmness of tortillas was correlated positively with protein content of flours (r=0·89, p<0·001). The moisture content of tortillas was correlated positively with the peak force (r=0·93, p<0·001). When the stretchability of tortillas was measured, peak forces ranged from 0·83 kg to 1·45 kg (Table V). The 53–75 lm and >75 lm fractions of HRW wheat flour showed higher values than other fractions. The rupture distance determined with the Texture Analyzer was expressed as the distance that the probe traveled to tear a tortilla (Table V). The values varied from 11·2 mm to 17·0 mm. Tortillas made from 53–75 lm fractions of HRW and HWW wheat flours had the highest values and those made from the <38 lm fraction had the lowest. The rupture distances of tortillas made from HRW and HWW wheat flours were correlated positively with flour protein content (r=0·88, p<0·001). Moisture content of HRW and HWW wheat tortillas were also correlated positively with the rupture distance (r=0·76, p<0·01). Tortillas made from SRW wheat flours were different. Tortillas made from the <38 lm and 38–53 lm fractions had longer rupture distances than those from other fractions, although the >75 lm fraction had high protein content. Also, the values were correlated negatively with

Textural properties of flour tortillas

Table V

Textural properties of baked tortillas Firmness

Flours

269

Stretchability

Peak force (kg)

Peak force (kg)

Rupture distance (mm)

Foldability

HRW

Straight-grade <38 lm 38–53 lm 53–75 lm 75 lm

10·50cd 11·66ab 11·30abc 11·38abc 11·78a

1·09cde 1·01def 1·20bcd 1·45a 1·34ab

14·2bc 11·2e 14·8b 17·0a 13·5cd

6·0bc 2·5d 6·5bc 7·0a 5·5c

HWW

Straight-grade <38 lm 38–53 lm 53–75 lm 75 lm

10·75bcd 10·23de 9·15f 11·55ab 11·79a

1·09cde 0·94ef 1·19bcd 1·26abc 1·23bc

13·6bc 11·9d 14·0bc 14·8a 14·1ab

5·0c 2·5d 7·0a 6·5bc 5·5c

SRW

Straight-grade <38 lm 38–53 lm 53–75 lm 75 lm

6·82g 5·70h 5·07h 9·38ef 8·91f

1·01def 0·83f 1·14cde 1·14cde 1·07cde

12·9b 14·7a 14·6a 13·3b 12·2c

2·5d 3·5c 2·0d 4·5bc 5·5a

n=4, means with the same letter in the same column are not significantly different (p<0·05).

the moisture content of tortillas (r=−0·86, p<0·06), less water made the tortilla tougher to tear. Farinograph data were correlated with the peak forces and foldability of baked tortillas, but not the rupture distance. The Farinograph water absorption was correlated positively with the peak force of firmness and the foldability (r=0·89 and 0·66 with p<0·01, respectively). The peak time was correlated positively with foldability and the peak forces of firmness and stretchability (r=0·92, 0·62 and 0·66 with p<0·01, respectively). The mixing stability has the same correlation as the peak time. Tortillas made from flours with higher protein content showed higher moisture content and a longer rupture distance. Tortillas from SRW wheat flours were lower in moisture content, and the rupture distance was not affected by the protein content, which indicated a poor protein quality. Tortillas made from the <38 lm fractions of HRW and HWW wheat flours had the lowest foldability score compared with those made from other fractions (Table V). The scores of tortillas made with SRW wheat flours were lower than those tortillas made with hard wheat flours. The foldability of tortillas was affected by flour protein content (r=0·82, p<0·001) and the moisture content (r=0·73, p<0·002). Higher moisture content made tortillas softer, so they did not break when

they were folded. It also was correlated positively with the peak time of the Farinograph (r=0·92, p<0·001). Whether the poorer rupture distance and foldability of tortillas made with the <38 lm fractions of HRW and HWW wheat flours was caused by finer particle size or by poor protein quality was not clear. The particle size and protein content of the SRW wheat flours did not significantly affect the rupture distance of tortillas, however, protein content improved foldability by increasing the moisture content of tortillas. Size exclusion-high performance liquid chromatography Size-exclusion chromatography separates wheat proteins based on molecular weight. High-molecular-weight (HMW) proteins are eluted first, and low-molecular-weight (LMW) proteins are eluted later. Based on molecular size, proteins larger than 100 kDa were classified as glutenin, those between 100 and 25 kDa as gliadin, and those smaller than 25 kDa as albumin and globulin23,24. The typical HPLC patterns of extracted wheat proteins are shown in Figures 1(a) and 2(a) for SRW and HRW wheat flours and in Figures 1(b) and 2(b) for HWW wheat flours. Figure 1(a, b) shows four peaks with some degree of overlaps. Only a small amount of peak 1 was extracted with 70% ethanol.

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(a)

3

4

1

SRW-SG

2

(b)

2 3

4 HWW <38 µm

1

HRW <38 µm HWW 38–53 µm A-280 nm

A-280 nm

HRW 38–58 µm

HRW 53–75 µm

HWW 53–75 µm

HWW >75 µm HRW >75 µm

HRW-SG

0

5

HWW-SG 10 15 20 Retention time (min)

25

30

0

5

10 15 20 Retention time (min)

25

30

Figure 1 (a) SE-HPLC profiles of wheat proteins extracted from SRW and HRW wheat flours with 70% ethanol. (b) SEHPLC profiles of wheat proteins extracted from HWW wheat flours with 70% ethanol.

4

(a)

3 4

(b)

1

3

1 2

2

5

5

HWW <38 µm

SRW-SG

HWW 38–53 µm

A-280 nm

A-280 nm

HRW <38 µm

HRW 38–53 µm

HRW 53–75 µm

HWW >75 µm

HRW >75 µm

HRW-SG 0

5

HWW 53–75 µm

HWW-SG 10 15 20 Retention time (min)

25

30

0

5

10 15 20 Retention time (min)

25

30

Figure 2 (a) SE-HPLC profiles of wheat proteins extracted from SRW and HRW wheat flours with 2% SDS phosphate buffer. (b) SE-HPLC profiles of wheat proteins extracted from HWW wheat flours with 2% SDS phosphate buffer.

Textural properties of flour tortillas

In the ethanol-extractable protein fraction, the HRW and HWW wheat flours had more HMW proteins than SRW flour had. Among different particle sizes of HRW and HWW wheat flours, the 53–75 lm fraction had more HMW at peak 2 than other fractions, 58·64% and 56·81%, respectively. The <38 lm fraction had the lowest amounts at peak 2, 49·99% and 48·56% for HRW and HWW wheat, respectively. The area percentage of peak 2 was correlated positively with the rupture distance (r=0·63, p<0·05) and foldability (r=0·83, p<0·05) of baked tortillas. However the area percentage of peak 4 was correlated negatively with rupture distance (r=−0·73, p<0·05) and foldability (r=−0·90, p<0·05). Extraction total protein with 2% SDS25 produced five peaks that decreased in molecular weight from peak 1 to peak 5 [Fig. 2(a, b)]. The results showed significant differences (p<0·05) among HRW, HWW and SRW wheat proteins, but few differences among HRW wheat fractions. At peak 1, the proportions of eluted protein were 24·65% for protein of SRW wheat straight-grade flour and 25·00% for the <38 lm fraction of HWW wheat. Others ranged from 26·80 to 28·95% at peak 1. At peak 5, the proportions of eluted protein were 12·22% for SRW wheat straight-grade flour and 14·13% for the <38 lm fraction of HWW wheat, and others ranged from 7·79 to 11·38%. The HWW fractions had more LMW proteins compared with the corresponding factions of HRW wheat flours. The fine fractions had more LMW proteins than the coarse fractions. The area percentage of peak 5 was correlated negatively with rupture distance (r=−0·52, p<0·05) and foldability (r=−0·55, p<0·05) of baked tortillas. Isolation of flour by particle size would result in different protein content and slightly alter the protein composition of the fractions, especially for the <38 lm fractions. During milling, the weak protein bonds can be broken easily and small particle size flour can result. Strong protein bonds are not easy to break. Serious middling reduction would yield a high level of damaged starch.

CONCLUSIONS Flours of different particle sizes varied in protein, damaged starch, and ash content. The <38 lm fractions of HRW and HWW wheat flours, which were lower in protein content and higher in damaged starch, were the only samples that resulted in

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a significant difference (p<0·05) in tortilla texture. Although higher protein content fractions of SRW wheat flour slightly improved tortilla foldability, they did not improve the stretchability. Proteins extracted with different solvents and analysed with SE-HPLC showed that different particle size of HRW and HWW wheat flours resulted in slightly different protein compositions of the fractions, especially for the <38 lm fractions. They contained more LMW proteins and less HMW proteins than other fractions. The amount of LMW proteins was correlated negatively with tortilla textural properties. However, protein differences among HRW and HWW wheat flour fractions were not as big as the differences between hard wheat flours and SRW wheat flour. Thus, the effect of flour particle size was a major factor contributing to tortilla texture, especially for the <38 lm fractions of HRW wheat flour. According to the theory of Wichser and Shellenberger6, smaller particle size granules need more protein to bind. The observations in our study supported this theory. The protein content, particle size, and protein quality of flour affected tortilla texture, but the particle size was the major factor affecting the stretchability and foldability of tortillas made from fractionated flours. Finer particle size flours would produce poor textural tortillas. Acknowledgements We would like to thank Dr Kathy Tilley, Ms Rachel Benjamin and Mr Aaron Dare for their assistance in the protein analyses.

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