Characteristics and oil absorption of deep-fat fried dough prepared from ball-milled wheat flour

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Research ArticleReceived: 26 March 2008 Revised: 5 September 2008 Accepted: 14 September 2008 Published online in Wiley Interscience: 2 December 2008

(www.interscience.wiley.com) DOI 10.1002/jsfa.3442

Characteristics and oil absorption of deep-fatfried dough prepared from ball-milled wheatflourPariya Thanatuksorn,a∗ Kazuhito Kajiwaraa and Toru Suzukib

Abstract

BACKGROUND: High levels of oil in fried products has been recognized as causing health problems. The formation ofmicrostructure during frying is one factor that influences oil absorption. Above the glass transition temperature (Tg), thephysical properties of a polymer influences the formation of structure. The ball-milling process changes the physicochemicalproperties of wheat flour constituents. The present study investigated the effects of physicochemical changes in wheat flour bythe ball-milling process on structure formation and oil absorption in wheat flour dough model.

RESULTS: Dough samples were made from wheat flour that had been ball-milled for 0 to 10 h and then fried in frying oil at 150 ◦Cfor 1–7 min. Thermal properties of wheat flour, structure alteration, and textural properties of fried samples were evaluated. Ascompared with samples made of non-milled flour, samples made from milled flour had smaller pores and higher oil absorption.The fracture force of a fried sample prepared from non-milled flour was lower than that of a sample prepared from milled flour.

CONCLUSION: Ball-milling affected the microstructure formation in fried wheat flour dough, and subsequently oil absorption.The crispness of a sample prepared from non-milled wheat flour is higher than that of a sample prepared from ball-milled wheatflour. This may be due not only to a plasticization effect, but may also be dependent on microstructure.c© 2008 Society of Chemical Industry

Keywords: ball-milled wheat flour; glass transition; dough; deep-fat frying; crispiness; oil absorption

INTRODUCTIONDeep-fat frying is one of the most popular methods of foodpreparation, because this process provides a product of uniqueflavor and crispy texture.1 Nevertheless, the high level of oilcontent in fried products, especially saturated fat, from friedfood has been recognized as a cause of health problems suchas heart disease, cancer, diabetes, and hypertension. Measuresfor reducing oil content in fried food have been extensivelyinvestigated. Saguy et al.2 proposed that microstructure formationduring frying is one of the predominant factors affecting oilabsorption.

In the frying process, the morphology of developed porosity(e.g., pore distribution and pore size) due to water evaporationallows oil to penetrate into the voids. The microstructure influencesthe oil absorption mechanism both in the frying process and thepost-frying process.3 In addition, the microstructure also influencesdesirable textural characteristics. The formation of microstructureis believed to be governed by the glass transition of polymerfoods in fried products.4 The porous structure is brought out byalteration of physical state and the water vapor expansion insidethe food material, by heating in hot oil. Subsequently, formationof porous structure ceases when the moisture content of friedfood during frying decreases and the food exists in a glassy state.The physical properties of food polymers, i.e., molecular mobility,viscosity, and elasticity, above the glass transition temperature(Tg) influence the structural formation and consequently foodquality. Alterations in the physical properties of polymer foods

are thought to affect structure formation in foods of porousstructure.

Currently, the ball-milling process is applied to modify starchmechanically. Published studies have reported the influenceof ball-milling starch at room temperature on the change insemi-crystalline structure in starch granules to the amorphouscondition.5 – 7 Most ball-milling research has focused on thechanges in physicochemical properties in various kinds of starch;few reports have addressed use of ball-milled starch in foodapplications. Schlesinger8 reported that the alteration of wheatflour by ball-milling has an effect on bread loaf volume. Donelsonand Gaines9 used starch that had been separated from ball-milled wheat flour to produce completely damaged starch, whichwas then mixed with native wheat flour in a sugar-snap cookieformulation. They found that the high amount of ball-milledstarch resulted in smaller cookie diameter. However, no studyhas reported on using ball-milled wheat flour in the frying

∗ Correspondence to: Pariya Thanatuksorn, School of Bionics, Tokyo Universityof Technology, 1404-1 Katakura, Hachioji, Tokyo, Japan.E-mail: [email protected]

a School of Bionics, Tokyo University of Technology, 1404-1 Katakura, Hachioji,Tokyo, Japan

b Department of Food Science and Technology, Tokyo University of MarineScience and Technology, 4-5-7 Konan, Minato-ku, Tokyo, Japan

J Sci Food Agric 2009; 89: 363–371 www.soci.org c© 2008 Society of Chemical Industry

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www.soci.org P Thanatuksorn, K Kajiwara, T Suzuki

process. Therefore, the present study aims to investigate oilabsorption in the frying process and the quality of the friedwheat flour dough model prepared from ball-milled flour. Thephysical state of ball-milled wheat flour was investigated usingthermal analysis as fundamental information. In addition, theinfluence of the physical alteration of wheat flour on structureformation was observed in order to understand the cellularformation related to oil absorption in the frying process andtextural quality.

MATERIALS AND METHODSWheat flour preparation and thermal propertiesBall-milling process carried out using a rolling-type ball-mill(Irie Shokai V-I, Kanagawa, Japan). Twenty gram of wheat flour(75 g kg−1 gluten, 120 g kg−1 moisture content; Pioneer, Co. Ltd,Kanagawa, Japan) was placed in a stainless cylindrical containerwith a diameter of 9 cm and a height of 9 cm. Each sample wasmilled with 50 stainless balls of 1 cm diameter. The cylindrical millwas tumbled at room temperature for 0, 5, or 10 h at 90 rpm.

Differential scanning calorimetry (DSC) (Shimadzu DSC-50,Kyoto, Japan) was used to determine Tg and the meltingtemperature (Tm) of wheat flour as reported in Thanatuksornet al.4 The wheat flour was defatted by 750 g kg−1 n-propanol, andmoisture content was adjusted by exposure to the saturated saltsK2SO4 (0.973 aw), KNO3 (0.924 aw), KCl (0.842 aw), Mg(NO3)2.6H2O(0.528 aw), CH3COOK (0.224 aw), and LiCl (0.11 aw). Samples ofhigher moisture content were prepared by mixing with distilledwater. The 20 mg samples were weighed into an aluminium pan.Scanning temperature was 0–200 ◦C with a heating rate of 5 ◦Cmin−1. Average values of two replicate samples were taken as Tg

and Tm. Moisture content of samples was determined by differencein weight before and following the pitted DSC pans were driedafter the second scanning. Tg was determined from the onset ofan endothermic shift of baseline using TA-60 software interfacedwith the calorimeter.

Extension of doughThe extension of dough made of 0, 5, and 10 h ball-milled wheatflour was assessed. Dough was sheeted until a final thickness of1.8 mm was achieved. The sheeted dough was cut into 20 mmwide strips. The strips of dough were folded around a holder andstretched by Tensipresser (TTP-50BX, Taketomo, Tokyo, Japan)at 5 mm s−1. Two main parameters were measured: maximumresistance force (Rmax) and extension.

Sample preparation and fryingSample preparation was performed as reported in Thanatuksornet al.4 The wheat flour model samples were prepared from 0,5, and 10 h ball-milled wheat flour at initial moisture levelsof 400 g kg−1 (wet basis). This level was achieved by mixingthe flour with distilled water at room temperature (25 ◦C)for 15 min. Each sample was re-formed into a sheet with afinal thickness of 0.9 mm. The sample was later cut usinga circular stainless steel mold into a 5 cm diameter circulardisk.

Samples were then deep-fat fried in an oil bath at 150 ◦C for1, 3, 5, and 7 min. After frying, the samples were immediatelydipped in a beaker containing 100 mL petroleum ether for 2 sto remove the oil adhering to the surface. The oil absorbed byeach sample was determined by Soxhlet extraction according to

AOAC.10 Residual moisture content in the samples was determinedin a hot-air oven at 105 ◦C for 12 h or until the sample weightbecame constant.10 The residual moisture and the oil content ofsamples were based on the dried sample weight. Two replicateswere used for all experiments. The SPSS software program wasused to perform analysis of variance and Duncan’s new multiplerange tests11 on the resulting data. In state diagrams, the residualmoisture content of the fried sample was expressed on a wetbasis.

Physical properties of fried samplesScanning electron microscopyAfter frying, samples were immediately dipped in petroleum etherfor 24 h for extraction of oil.12 The defatted samples were dried,mounted on stubs, and coated with platinum gold. The cross-sectional surface of each coated sample was observed under ascanning electron microscope (Hitachi S-4000, Tokyo, Japan) withan accelerating voltage of 15 kV.

Texture measurementsFried samples were kept in Ziploc bags after the temperatureof samples reached room temperature. Texture determinationwas performed within 1 h after frying. Texture assessment wasperformed using a Tensipresser (TTP-50BX, Taketomo) as reportedby Thanatuksorn et al.4 Puncture tests were performed (at 25 ◦C)by means of a 3 mm diameter flat-ended cylindrical probe.Cross-head speed was 1 mm s−1. The plunger was pressedthrough the sample to a depth of 1.5 cm, for 15 repetitionsper treatment.

DiameterSample diameter was measured by a digital caliper (Mitutoyo,no. 500-111, Kanagawa, Japan). The accuracy of the device was±0.02 mm. About 20 readings were made for five samples of eachtreatment.

RESULTSThermal properties of ball-milled wheat flourThe thermal properties of defatted wheat flour, as assessedby DSC, are shown in Fig. 1. Large relaxation peaks wereobserved on the first scan, whereas in the second run thetransitions were less clear and broader. A high-temperatureendotherm appearing in the first scan corresponds to the meltingof crystalline regions of wheat flour. Two different Tg valueswere identified in the second scan of ball-milled wheat flour.These are in agreement with our previous work,4 in whichtwo different values of Tg were observed in the second-scanthermogram of native wheat flour. The Tg values of starchand gluten have indicated that these two transitions probablycorrespond to the starch-rich phase for the higher-temperaturetransition and to the gluten-rich phase for the lower-temperaturetransition.13 – 15

Figure 2 illustrates the state diagram for ball-milled wheatflour. Tm has a tendency to shift to a lower temperaturewith increasing ball-milling time. The apparent Tg values ofgluten and starch obtained from ball-milled wheat flour havea tendency to decrease with longer ball-milling time. Theseresults coincide with the lower Tg value of starch and crudegluten separated from ball-milled wheat flour16 and Tg of ball-milled potato starch.7 The Tg depression of the gluten-rich

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Characteristics and oil absorption of deep-fat fried dough www.soci.org

Figure 1. Typical DSC thermograms of ball-milled wheat flour.

Figure 2. State diagram of wheat flour with the various ball-milling times.

phase could be considered to be due to depolymerization bycleavage of the covalent bonds and conformational change ofgluten subunits.17,18 These lead to reduced interaction betweenthe chains of gluten macropolymer during thermal treatment.Meanwhile, Tg depression in the starch-rich phase may be aresult of the decrease in crystallinity of starch granules during theball-milling process. One glass transition temperature at maximalfreeze concentration (Tg

′) was observed when moisture content

Figure 3. Typical extensigraph of wheat flour dough.

was more than 250 g kg−1. Tg′ of non-milled and ball-milled

wheat flour are in the range of −5.5 to −8 ◦C. No notabledifferences were observed in Tg

′ of wheat flour with variousball-milling times. Chung et al.19 reported that Tg

′ is slightly higherfor native starch than for gelatinized starch. Tg

′ for starch andgluten have been reported as −5 and −7.5 ◦C, respectively.20 Ball-milling within the observation period probably slightly changedthe crystallinity in starch granules to a partially amorphousstate. Water-binding ability in the amorphous region may beinadequate to change Tg

′. Moreover, the complication of wheatflour components could also be considered to affect this obtainedresult.

Extensibility of wheat flour doughIn the present study, samples having a moisture content of 400 gkg−1 have a dough-like consistency. Because gluten governsstructural formation in a dough,21 the extension of dough isconsidered to reflect the gluten functionality of wheat flour,and to consequently influence the characteristic of the final

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Table 1. Extension and maximum resistance force of dough madefrom wheat flour using various ball-milling times

Ball milling time (h) Rmax (kg) Extension (mm)

0 0.0544 ± 0.017b 56.88 ± 2.07a

5 0.1173 ± 0.036a 42.48 ± 9.89b

10 0.1557 ± 0.023a 34.85 ± 3.21b

Means followed by the same letter are not significantly different atP ≤ 0.05.

product. As shown in a typical extensigraph (Fig. 3), the mainparameters characterizing the qualities of gluten in dough are

the maximum resistance to extension (Rmax), which relates tomaximum tensile strength, and extension, corresponding to thelongest distance that dough elongates before breaking. Rmax andextension are shown in Table 1. Dough prepared from non-milledflour exhibits the greatest extension. On the other hand, the Rmax

of dough prepared from non-milled flour is lower than that ofthe other samples. That is, the dough prepared from non-milledflour could be stretched the longest before breaking, with lowerforce.

Microstructure of fried samplesInitial moisture content in this sample was not sufficient forgelatinization, as reported by Thanatuksorn et al.4 Therefore, thegluten was considered to be a dominant factor for structure

1 min 3 min

5 min 7 min

(b)

3 min

7 min

1 min

(a)

5 min

Figure 4. SEM photographs of sample prepared from (a) 0, (b) 5 and (c) 10 h ball-milled wheat flour (×60).


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1 min 3 min

5 min 7 min

(c)

Figure 4. (Continued).

formation in wheat flour dough. Figure 4 shows the microstructureof fried sample prepared from ball-milled flour as observed byscanning electron microscopy. The structure of fried sample hasmany small pores, due to the rapid evaporation of the waterinside. Water that has not been bound with starch molecules canrapidly become vapor. The pore size in the fried sample madeof non-milled flour was larger than that of samples preparedfrom 5 and 10 h ball-milled flour. In addition, as compared withnon-milled flour samples, the fried samples made from ball-milledwheat flour have denser structures. This provides evidence thatthe ball-milled process alters the physical properties of wheatflour, and subsequently affects the microstructural formation ofdough during frying.

Figure 5. Diameter of fried dough prepared from ball-milled wheat flourwith various ball-milling times.

DiameterFigure 5 shows the diameters of all fried samples. In the firstminute of frying time, the sample prepared from 10 h milledflour became smaller in diameter, whereas samples preparedfrom 0 and 5 h milled flour expanded in the first 1 min, afterwhich they gradually decreased in diameter. However, thediameter of sample prepared from non-milled flour remainedlarger than the original diameter, whereas the sample preparedfrom 5 h milled flour shrunk to the original diameter at thefinal frying time. The diameter of sample made of 10 h milledflour remained fairly constant after the extreme decrease in thefirst minute. These results coincide with the work of Donelsonand Gaines,9 who studied the physical characteristics of sugar-snap cookie using amorphous starch. They found that cookiediameter decreased with addition of ball-milled or pre-gelatinizedstarch.

Figure 6. Absorbed oil content of fried samples prepared from 0, 5, and10 h ball-milled wheat flour.


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Figure 7. Residual moisture content of fried samples prepared from 0, 5,and 10 h ball-milled wheat flour.

Oil absorption and residual moisture contentPrevious studies12,22 suggested that oil absorption occurs duringboth the frying process and the post-frying process. Oil absorptionin the sample containing 400 g kg−1 moisture mainly takesplace during frying; therefore, the present study assessed onlyabsorbed oil content during the frying process. The absorbed oilcontent of the fried sample is shown in Fig. 6. For all samples,absorbed oil increases with frying time. The absorbed oil contentin the sample prepared from non-milled flour is lower thanthat in the sample prepared from 10 h ball-milled wheat flour.Figure 7 shows residual moisture content of all samples duringthe frying process. Moisture loss occurred rapidly in the first3 min of frying, and gradually declined thereafter. Throughoutthe process, the moisture loss of the samples prepared from 10 hball-milled wheat flour was obviously lower than that of the othersamples.

Texture analysisCrispness is a desirable characteristic of fried food. The crispnessof snack foods is often evaluated by determining fracture forceand the number of peaks.23 Fracture force is the force at thefirst significant break in the first peak in one bite area in thetexture profile.24 A lower force to fracture the sample and ahigh number of peaks correspond to higher fracturability. That is,the sample is easily broken. The texture profile of fried samplesand fracture force are shown in Figs 8 and 9, respectively. Thefracture force appeared at 3 min of frying. At 3 min of frying,the force required to break the fried sample made of non-milledflour is lower than that prepared from 5 and 10 h milled flour.The results also reveal that the sample prepared from non-milled flour exhibits the lowest force at point as the puncturepasses through, throughout the frying process. This suggests thatthe sample prepared from non-milled wheat flour has highercrispness than the samples prepared from ball-milled wheatflour.

DISCUSSIONOil absorption and characteristics of fried dough made ofmechanically modified wheat flour were investigated. Throughoutthe process, the fried sample made from 10 h milled flourabsorbed slightly more oil than did the other samples. This isattributed to the pore structure affecting oil absorption.25 Several

other studies have pointed out that difference in microstructureleads to a difference in oil absorption.26 – 28 Capillary pressureis regularly used for explaining the infiltration of liquid intothe cellular structure. Wider pores result in weaker capillarypressures,29 which means lower oil absorption. A 7 min friedsample prepared from 10 h milled flour absorbs slightly moreoil than does a sample prepared from non-milled wheat flour,but the difference is not significant, possibly because of theshrinkage in diameter. The surface area of a product is thedominant factor in oil absorption.22,30,31 However, in this study,the decrease in surface area could be considered to exert a smallereffect on oil absorption than the high capillary pressures due tothe small pore size. This point of speculation should be studiedfurther.

Gluten of wheat flour dough plays a role in structural formation.Once the sample is immersed in hot oil, water evaporation startsand simultaneously the gluten shows sufficient mobility due tothermal and water plasticization, and forms a thermoset network,via disulfide crosslinking.32 Toufeili et al.14 reported that a glutennetwork started to develop at 93 ◦C when water was presentat levels of 300–400 g kg−1. Crust formation occurred primarilyat the sample surface. The gluten network and the hard crustaround the surface act to prevent movement of vapor, leadingto the formation and extension of pores in the structure. Thestrength of gluten plays a role in the entrapment of gas duringheating. Li and Lee33 reported that adding cysteine to wheat flourbreaks the interchain disulfide bond of gluten; consequently, thenetwork formation of gluten in the extrusion matrix is weakened,resulting in smaller pore size. The drop in Tg value could beconsidered to reduce the interaction of interchain segments inthe system. In addition, the lower extensibility of dough preparedfrom ball-milled wheat flour brings about the decrease in theability to entrap vapor. Therefore, as ball-milling time increase, theporous structure becomes smaller in the sample made from wheatflour.

In defining quality parameters of snack foods, the texture offried food is the most important mechanical property. In orderto observe the relationship between textural properties andglassy state, in the state diagram shown in Fig. 10 the moisturecontent of all fried samples from 1 to 7 min frying time wasreplotted against room temperature. The moisture content ofall samples fried for 1 min is such that the samples are in arubbery state; no brittleness is observed (Figs 8 and 9). As fryingtime increased, the moisture within each sample decreased, andconsequently the sample went through the glassy state. Forall treatments, the 3, 5, and 7 min fried samples exist in theglassy state, but the fracture force of fried sample prepared fromnon-milled flour is significantly lower (Fig. 9). We observed thatall 7 min fried samples differed in moisture content, but notsignificantly, and differed remarkably in fracture force. The highamount of absorbed oil of the fried sample prepared from ball-milled wheat flour probably acted as a plasticizer. However, thenoticeable dissimilarity in the generated microstructure couldbe considered to govern this occurrence. Higher force wasrequired to break the dense structure of the sample preparedfrom 5 and 10 h ball-milled flour (Fig. 9). Segnini et al.34 reportedthat chips with the same moisture content require more forceto break when the sample possesses higher specific gravity.Castro et al.35 reported that the glass transition temperature itselfis not a good predictor of the mechanical properties in theamorphous starch system. This suggests that the microstructureobviously also corresponds to fracturability in the fried food.


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1 min 3 min

5 min 7 min

(a)

1 min 3 min

5 min 7 min

(b)

Figure 8. Texture profile of fried samples prepared from (a) 0, (b) 5, and (c) 10 h ball milled wheat flour.

The relationship between glass transition and crispness in friedfood systems is still not fully understood, and further study isrequired.

CONCLUSIONThe ball-milling process depresses Tg in the starch-rich phase andgluten-rich phase of wheat flour, which plays an important role

in microstructure formation. This suggests that the Tg depression

in the gluten-rich phase affects the microstructural formation of

wheat flour–water dough during the frying process. The generated

porous structure governs the amount of oil absorption by capillary

pressure. For determination of texture, the sample prepared from

the non-milled wheat flour exhibits lower fracture force and lower

maximum force of break. This may have been due not only to the

plasticization effect, but may also depend on the microstructure.


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1 min 3 min

5 min 7 min

(c)

Figure 8. (Continued).

Figure 9. Average fracture force of fried sample prepared from 0, 5, and10 h ball milled wheat flour.

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Characteristics and oil absorption of deep-fat fried dough prepared from ball-milled wheat flour