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Food Hydrocolloids 40 (2014) 64e70

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Food Hydrocolloids

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Effect of sucrose fatty acid esters on pasting, rheological propertiesand freezeethaw stability of rice flour

Yue-Cheng Meng*, Ming-Hui Sun, Sheng Fang, Jie Chen, Yan-Hua LiCollege of Food Science and Biotechnology Engineering, Zhejiang Gongshang University, Hangzhou 310035, China

a r t i c l e i n f o

Article history:Received 1 December 2013Accepted 14 February 2014

Keywords:Sucrose fatty acid estersRice flourGelatinizationRheological propertiesFreezeethaw stability

* Corresponding author. Tel.: þ86 571 88071024.E-mail address: myczjgsu@gmail.com (Y.-C. Meng

http://dx.doi.org/10.1016/j.foodhyd.2014.02.0040268-005X/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The present research is aimed to evaluate the effects of sucrose fatty acid esters (SEs) on the pasting,rheological properties and the freezeethaw stability of rice flour. Four types of SEs including S-570, S-970, S-1170 and S-1570 with hydrophilicelipophilic balance (HLB) values varying from 5 to 15 have beencompared. Viscosity behavior of rice flour with different SEs was first measured with the Rapid ViscoAnalyzer (RVA). The RVA profile showed that pasting temperature, peak and final viscosities of rice flourgels increased with SEs addition except for S-570. Rheological properties including the steady shearcharacteristics, the viscoelastic parameters (storage and loss moduli) and the creep-recovery responsewere determined. The obtained steady shear and creep data were fitted by power law (R2 > 0.983) andBurger’s models (R2 > 0.993) respectively. The results revealed that the addition of SEs except for S-570increased the storage and loss moduli, apparent viscosity, flow behavior index and the retardation time,and decreased loss tangent. Finally, the freezeethaw measurement demonstrated that all types of SEsenhanced the freezeethaw stability of rice flour with the order of S-1570 > S-1170 > S-970 > S-570.These results could have important theoretical and practical implications in choosing suitable SEs for theparticular requirements of final food products based on rice flour.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Rice is the primary food grain consumed in China and otherAsian countries (Sun & Yoo, 2011). There are many kinds of com-mercial rice products, such as baby foods, puffed grain, noodles, ricecakes and snack foods. Recently, rice was also studied as a wheatsubstitute in gluten-free food products (Lucisano, Cappa, Fongaro, &Mariotti, 2012; Sivaramakrishnan, Senge, & Chattopadhyay, 2004).However, the use of rice flour and starch as a wheat substituteexhibits some disadvantages, such as the weak ability to formviscoelastic dough (Lucisano et al., 2012), the tendency for exten-sive retrogradation and syneresis after freezing and thawing(Katekhong & Charoenrein, 2012).

In the food industry today, the application of various emulsifiersoffers a good way to achieve suitable properties of final foodproducts, optimize the production process, and guarantee constantquality (Stampfli & Nersten, 1995). Many researchers havedemonstrated that the addition of commercial emulsifiers such asdiacetyl tartaric acid esters of monoglyceride, glycerol

).

monostearate, lecithin, sucrose esters, and milk proteins to rawstarch can strongly improve the rheological and quality character-istics (Ashwini, Jyotsna, & Indrani, 2009; Ding & Yang, 2013;Stampfli & Nersten, 1995; Turabi, Sumnu, & Sahin, 2008). Howev-er, compared with other starches, much less effort has beendevoted to studying the influence of emulsifiers on rice starch orflour (Banchathanakij & Suphantharika, 2009; Huang, Kennedy, Li,Xu, & Xie, 2007). Recently, Banchathanakij and Suphantharika(2009) studied the effect of b-glucans on the gelatinization andretrogradation properties of rice starch, while Correa, Ferrero,Puppo, and Brites (2013) investigated the influence of locust beangum on rice flour gels focusing on the rheological properties.

Sucrose fatty acid esters (SEs), also known as sugar esters, arenon-ionic type emulsifiers. Because sucrose has eight free hydroxylgroups, it can be esterifiedwith up to eight fatty acids to form esterswhich consist of a hydrophilic sugar head and one or more lipo-philic fatty acid tails. As a result, SEs can provide various hydro-philicelipophilic properties with hydrophilicelipophilic balance(HLB) values ranging from 1 to 16 (Chansanroj & Betz, 2010) whichcanmeet specific needs for food products. Because of this variety, aswell as other advantages such as tasteless and biodegradableproperties, SEs have beenwidely used in food, pharmaceuticals andcosmetics industry (Sz}uts & Szabó-Révész, 2012). According to

Y.-C. Meng et al. / Food Hydrocolloids 40 (2014) 64e70 65

previous studies (Addo, Slepak, & Akoh, 1995; Ebeler & Walker,1984; Selomulyo & Zhou, 2007), the addition of SEs couldimprove the physicochemical and functional properties of wheatstarch. For example, it was found that the alveograph rheologicalcharacteristics of wheat flour doughs improved as HLB value of thesucrose ester increased (Addo et al., 1995). However, there is noinformation available in the literature on the effect of SEs onrheological and functional properties of rice flour, especially on theuse of SEs with different HLB values for particular productrequirements.

The objective of this study is therefore to examine the influenceof SEs on the pasting, rheological properties and freezeethaw sta-bility of rice flour. Physical properties, especially the rheologicalbehavior, play an important role in controlling process conditionsand producing rice flour based products with desirable textures(Sun & Yoo, 2011). Rheological properties including the steadyshear characteristics, the viscoelastic parameters (storage and lossmoduli) and the creep-recovery response were determined. Theobtained steady shear and creep data were fitted by power law andBurger’s models respectively. On the other hand, since the structureand physical properties of the amylose-emulsifier molecule com-plex mostly depend on the type of the emulsifier molecules(Bemiller, 2011), the effect of four SEs with different HLB valuesvarying from 5 to 15 were compared in this study. The results couldhave important theoretical and practical implications in choosingsuitable SEs for the particular requirements of final food productsbased on rice flour.

2. Materials and methods

2.1. Materials

Japonica rice provided by Zhejiang Wufangzhai Co., Ltd (Jiaxing,China) was cultivated in Wuchang, China. The rice was first groundto flour by using an electric mill, and then sieved through a 177 mmmesh size sieve. The composition of rice flour was analyzed by thestandard AACC (2000) procedures. The composition of rice flourwas as follows (w/w, wet basis): moisture content, 12.04%; crudestarch, 80.03%; protein, 6.71%; and lipid, 0.84%. All SEs used in thisstudy were kindly supplied by Mitsubishi-Kagaku Foods Corpora-tion (Tokyo, Japan). The series of “S” is short for the sucrose stearate.The details of SEs used in this study are provided by the supplierMitsubishi-Kagaku Foods Corporation (2003) and shown in Table 1.The ultra-pure water with resistivity of 18.2 MU cm was used forsample preparation.

2.2. Rapid Visco Analyzer (RVA) measurements

The pasting properties of rice flour (RF) or RFeSEs mixturessuspended in ultra-pure water were determined by a RVA (ModelRVA-4, Newport Scientific Pty. Ltd, Warriewood, Australia). RFeSEmixtures were prepared by replacing 0.7% (w/w) of RF with S-570,S-970, S-1170 or S-1570 respectively. RF or RFeSE mixture (3 g at12% moisture basis) was added with distilled water to reach a total

Table 1Details of different types of sucrose fatty acid ester used in this study.

Type HLB Ester composition (%) Melting temperature

Monoester Di-, Tri-,Polyester

Start point (�C) Peak point (�C)

S-570 5 30 70 50 57e65S-970 9 50 50 49 56S-1170 11 55 45 49 55S-1570 15 70 30 49 55

weight of 28 g in the aluminum RVA canisters (Abiodun & Akinoso,2014). The test temperature was first held at 50 �C for 1 min, thenincreased to 95 �C in 3.75 min at a constant rate, held at 95 �C for2.5 min, decreased to 50 �C in 3.75 min at a constant rate, and thenheld at 50 �C for 1.5 min. The rotation speed of the plastic paddlewas set at 960 rpm in the first 10 s and thenmaintained at 160 rpm.

RVA characteristic parameters were determined from the RVAcurves, including pasting temperature (PT), peak viscosity (PV),trough viscosity (TV, viscosity at the end of 95 �C), final viscosity(FV, viscosity at the end of test), breakdown value (BDV) andsetback value (SBV). Results of each RVA characteristics were pre-sented as means � SD of triplicate determinations.

2.3. Rheological properties

The gelatinized gel of RF or RFeSEmixturewas prepared by RVAwith the same program as described above, andwas cooled to 25 �Cand held for 5 min (Zhang, Tong, Zhu, & Ren, 2013). Air bubblesformed and trapped in gels were removed by centrifuging at1000 rpm for 1 min before all rheological measurements. Thesefreshly prepared gels were used for rheological measurement by acontrolled-stress rheometer (ARG2, TA Instruments, New Castle,USA) using a 40-mm parallel plate. All the steady, viscoelastic andcreep-recovery properties were determined at 25 �C. An equili-bration time of 2 min was applied to all samples beforemeasurement.

2.3.1. Steady shear viscosity measurementsThe plate was programmed to increase the shear rate from 0.01

to 100 s�1 with 5 points per decade and the gap was set at 1 mm.Shear stress and viscosity values were obtained as a function ofshear rate. The flow behaviors of RF or RFeSE gels were analyzed byusing a power law model as

s ¼ K$gn (1)

where s is the shear stress (Pa), g is the shear rate (s�1), K is theconsistency coefficient (Pa sn), and n is the flow behavior index(dimensionless). Eq. (1) is often used to describe the behavior ofnon-Newtonian fluids (Samutsri & Suphantharika, 2012).

2.3.2. Dynamic viscoelastic measurementsThe gelatinized gel mixtures obtained from RVA tests were used

for the dynamic oscillatory measurement freshly and then kept at4 �C for 30 days for testing again at the same conditions. A fre-quency sweep was conducted over the range of 0.1e10 Hz at 1%strain (within the linear viscoelastic region). The gap was set at1 mm. Storage modulus (G0), loss modulus (G00) and loss tangent(tand ¼ G00/G0) were obtained from TA rheometer Data Analysissoftware (version 5.7.1).

2.3.3. Creep-recovery measurementsThe fresh gel was rested between the plates for 3 min before

testing to allow residual stresses to relax and the gap was set at3mm. In order tominimize thewater loss duringmeasurements, theouter edge of samplewas coatedwith siliconeoil (Zhanget al., 2013).Creep-recovery test was carried out applying on the gel a constantstress (10 Pa) for 120 s and allowing strain recovery for 180 s afterremoval of stress. The strain values were collected as a function oftime. All measurements were performed in triplicate at 25 �C.

2.4. Freezeethaw stability measurements

Samples were prepared following the method of Muadklay andCharoenrein (2008). RF or RFeSE mixtures (0.7% SE on basis of RF,

Y.-C. Meng et al. / Food Hydrocolloids 40 (2014) 64e7066

8% total solidw/wwet basis) was gelatinized by heating in a boilingwater bath at 95 �C with continuous stirring at 250 rpm for 30 minand then cooled in water bath at 30 �C. After cooling, the gel wasthen loaded into a 10 mL centrifuge tube (20 mm in diameter) forthe syneresis measurement.

The tube was frozen in a freezer chest at �18 �C (Electroluxrefrigerator, model BCD-212M) for 24 h and then thawed in a 35 �Cwater bath for 1 h. This freezeethaw cycle was repeated for up tofive cycles. Three samples of each condition were centrifuged at4000 rpm in a centrifuge for 15 min. The supernatant was decantedand then the residue was weighed. The percentage of syneresis wasthen calculated as the ratio of the weight of liquid separated fromthe gel to the total weight of the gel before centrifugation.

2.5. Statistical analysis

Experimental data were analyzed using analysis of variance(ANOVA), and expressed as mean value � standard deviation.Duncan’s multiple range test was performed for post hoc multiplecomparisons with the level of significance set at P < 0.05. All sta-tistical analyses were performed using SPSS17.0 for Windows.

3. Results and discussion

3.1. Pasting properties

Table 2 shows the RVA characteristic parameters of RF, with orwithout SE. It could be found that the pasting properties of RF weregreatly affected by the addition of different types of SEs. Comparedto the control sample, the addition of the SEs with HLB of 9, 11 and15 increased PT significantly, while the addition of S-570 showed nosignificant effect (P > 0.05). Azizi and Rao (2005) have found thatsome commonly used emulsifiers could increase the gelatinizationtemperature of wheat, corn and potato starches. Mira, Persson, andVillwock (2007) also found that the effect of emulsifiers on the PTofwheat starch was determined to a great extent by the surfactantchain length. Gelatinization is a process involving the irreversibleswelling of the starch granules with a destruction of structuralorder and followed by leaching of amylose (Morris, 1990). The valueof PT increasing with the increase of HLB of SEs can be attributed tothe formation of emulsifiereamylose complexes. These insolublecomplexes can cover the surface of starch granules and hinder theexudation of amylose from the granule (Gunaratne, Ranaweera, &Corke, 2007). On the other hand, due to multiple hydrophilic hy-droxyl groups in SE molecules, they could form extensive hydrogenbonding network with water molecules that around starch gran-ules (Sharma & Gujral, 2014). The less amount of water available tothe starch granules may restrict and delay the swelling and disin-tegration process of starch granules, which consequently leads tothe increase of PT values.

There was a significant increase in both values of PV and SBV bythe addition of SEs. On the other hand, the addition of S-1170 and S-1570 enhanced the value of TV and FV, whereas the addition of S-

Table 2Effect of sucrose fatty acid esters with different HLB on the pasting properties of rice flo

Sample PV (cP) TV (cP) BDV (cP)

Control 2353 � 30d 1646 � 36b 708 � 34S-570 2552 � 16c 1210 � 14d 1341 � 20S-970 2553 � 14c 1531 � 9c 1022 � 18S-1170 2632 � 17b 1676 � 13b 956 � 6c

S-1570 2705 � 42a 2069 � 46a 636 � 9e

Values represent the mean � standard deviation of triplicate tests. Columns with differerange test.

570 decreased TV and FV. It is well known that the pasting propertyof starch is primarily related to the swelling and rupture of starchgranules. During the RVA measurement, viscosity increases withcontinuous heating until the rate of granule swelling equals the rateof granule collapse. PV shows the maximum swelling of the starchgranule prior to disintegration and also is described as the equi-librium point between swelling and disintegration of starchgranule. PV often correlates with the quality of final products, sincethe swollen and collapsed granules relate to the texture of cookedstarchy food (Wani et al., 2012). Morris (1990) suggested that starchgel properties relate to the characteristics of gel matrix network,the deformable fillers (swollen granules), volume fraction of thefiller, and the fillerematrix interaction. SEs were embedded in thecontinuous medium consisting of soluble amylose and low mo-lecular weight amylopectin, and consequently more junction zoneswere created through conformational adjustments and intermo-lecular association (Ashwini et al., 2009). As a result, viscosity ofthis phase substantially increases.

An increase in SBV suggested that the recrystallization ofamylose molecules was promoted at a very early stage of storage bythe addition of SEs. From Table 2, we could also find that only theaddition of S-1570 exhibited a significant decrease in BDV(P< 0.05). The breakdown value BDV (PV minus TV after holding at95 �C) is a result of the disintegration of the swollen granules andthe leaching out of amylose molecules during continuous stirringand heating. Lower breakdown viscosity value means greaterresistance to disintegration in response to heat and shear, whichwill be beneficial in industrial processing for rice flour products.

It is known that the capability of sucrose stearate to form in-clusion complexes with the helical amylose molecule is determinedby its chemical and geometrical factors (Addo et al., 1995). Sincesucrose monostearate (with high HLB value) has better linearstructure and smaller steric hindrance, a more robust gel networkcan be formed. Based on the pasting properties shown in Table 2and above discussion, for SEs used in this study, it could concludethat the ability of sucrose stearate to form inclusion complexeswiththe helical amylose molecule is in the following order:monostearate > distearate > tristearate > polystearate.

3.2. Rheological properties

3.2.1. Steady shear propertiesThe typical steady flow curves of RF gels in the presence or

absence of various SEs are presented in Fig. 1. All the samplesshowed shear-thinning behavior and pseudoplastic properties,which means that apparent viscosity decreases as the shear rateincreases. The power law model shown in Eq. (1) was used torepresent the steady flow properties. The consistency coefficients(K) and the flow behavior indices (n) along with the coefficients ofdetermination (R2) and apparent viscosities at a shear rate of100 s�1 for each flow curve are summarized in Table 3.

The results in Table 3 show that Eq. (1) can fit the flow curves ofRF gels satisfactorily with R2 between 0.983 and 0.998. It is known

ur.

FV (cP) SBV (cP) PT (�C)

d 2921 � 20e 1276 � 27e 85.6 � 0.3ca 2703 � 10d 1492 � 6d 85.4 � 0.9cb 3280 � 19c 1749 � 16c 87.4 � 0.2b

3514 � 59b 1839 � 46b 88.6 � 0.5a

4098 � 61a 2029 � 15a 89.6 � 1.0a

nt superscripts are significantly different at P < 0.05 according to Duncan’s multiple

Table 3Steady shear rheological properties of rice flour gels with and without sucrose fattyacid esters at 25 �C.

Sample Apparent viscosity,ha,100 (Pa s)

Consistency coefficient,K (Pa sn)

Flow behaviorindex, n (e)

R2

Control 3.40 � 0.03d 88.82 � 1.80b 0.295 � 0.002c 0.995S-570 3.21 � 0.01e 79.20 � 1.51c 0.298 � 0.001c 0.998S-970 3.53 � 0.01c 82.95 � 1.87c 0.303 � 0.003b 0.996S-1170 3.73 � 0.04b 91.98 � 1.14b 0.305 � 0.001b 0.998S-1570 3.92 � 0.03a 96.30 � 3.14a 0.317 � 0.004a 0.983

Columns with different superscripts are significantly different at P < 0.05 accordingto Duncan’s multiple range test.

Fig. 1. Typical flow curves of RF gels with and without SEs at 25 �C.

Y.-C. Meng et al. / Food Hydrocolloids 40 (2014) 64e70 67

that the flow behavior index (n) signifies sample deviation fromNewtonian flow (n ¼ 1) (Alamri, Mohamed, & Hussain, 2012). Inthis study, all the samples exhibited highly shear-thinning behaviorwith flow behavior index (n) as low as 0.295e0.317. Pseudoplas-ticity can be attributed to the progressive orientation and align-ment of the starch molecules, and to the breaking of hydrogenbonds formed among amylose molecules under the influence ofshear field (Sun & Yoo, 2011). The flow behavior index decreased inthe following order: S-1570, S-1170, S-970, S-570, and control,indicating that all the SEs reduce the pseudoplasticity of RF gel dueto starch molecule immobilization. The influence of SEs on ha,100and K values was in good agreement with the final viscosities of thecorresponding mixtures in RVA measurements.

Fig. 2. Typical frequency dependence of G0 (a) and G00 (

3.2.2. Dynamic viscoelastic propertiesSmall deformation oscillatory measurements are always used to

elucidate structural insight of viscoelastic materials (Wani et al.,2012). The typical frequency sweep oscillatory curves of fresh RFgels, with or without SE, are shown in Fig. 2.

Both storage modulus (G0) and the loss modulus (G00) values ofall samples were dependent on frequency, indicating a typicalviscoelastic nature of the food matrix. In Table 4, the G0, G00 and tandvalues of freshly prepared and stored samples at an oscillatoryfrequency of 1 Hz are summarized respectively. The addition of SEscaused a significant increase in the G0 and G00 for fresh RF gels exceptfor S-570. It can be further found that the higher HLB values of SEsadded, the larger extent of G0 and G00 increased. The overall dynamicviscoelastic properties of starch gels would be determined not onlyby the density of cross-links in the continuous phase but also by therigidity, entanglements between the amylose and amylopectin,spatial distribution, and effective contacts between the dispersedphase (Biliaderis & Tonogai, 1991). The largest increase in G0 by theaddition of S-1570 indicates that the junction zones of starch gelcan be strengthened by the interaction of monostearate in thecontinuous phase to form a stronger cross-linking network. Incontrast, polystearate weaken the network structure because of itslarger molecule structure and lower HLB as discussed above.

Tand is a dimensionless parameter that gives a direct measure ofwhether the material behaves as solid-like or liquid-like. As shownin Table 4, tand of fresh RF gels was decreased by the addition of SEsexcept for S-570. For all fresh samples, the tand values at 1 Hz werein the range of 0.153e0.228, indicating that the elastic natureprevailed over the viscous nature. On the other hand, all values of G0

for stored samples significantly increased compared to the freshsamples, which indicated that the elastic structure can be furtherreinforced during storage. Meanwhile, the G0 values of stored gelsshowed different growth rates which are 68.44%, 24.26%, 18.12%,17.00%, and 11.55% for the control, S-570, S-970, S-1170 and S-1570samples respectively. It has been demonstrated that starch gels aremetastable and non-equilibrium systems and therefore undergostructural changes during storage (Gunaratne et al., 2007). Amyloseretrogradation was reported to be the dominating factor in theearly stage (several hours) development of gel structure, whileamylopectin retrogradation occurred several weeks or evenmonths later during storage for equilibrium (Biliaderis & Tonogai,1991). So the result presented in Table 4 indicates that the addi-tion of SEs accelerated recrystallization of amylose molecules andenhanced the network structure. Whereas it retarded the progressof recrystallized amylopectin by reducing mobility of the starchchains and increasing the number of entanglements or chemicalcross-links among the starch molecules. In other words, the addi-tion of SEs could reduce long-term retrogradation.

b) of fresh RF gels with and without SEs at 25 �C.

Table 4Storage (G0) and loss (G00) moduli, and tand at 1 Hz for fresh and stored rice flour gels.

Sample Fresh gels Stored gels

G0 (Pa) G00 (Pa) tand G0 (Pa) G00 (Pa) tand

Control 194.83 � 4.91d 44.03 � 0.52c 0.226 � 0.006a 336.60 � 7.83b 49.85 � 2.81b 0.148 � 0.008c

S-570 192.60 � 6.32d 43.97 � 0.28c 0.228 � 0.006a 240.00 � 12.94d 43.65 � 1.98c 0.182 � 0.009a

S-970 241.50 � 5.89c 47.97 � 0.63b 0.199 � 0.007b 285.25 � 10.63cd 45.89 � 1.42c 0.161 � 0.008b

S-1170 258.67 � 5.14b 46.29 � 3.45b 0.179 � 0.010c 302.64 � 9.86bc 49.37 � 2.37b 0.163 � 0.010b

S-1570 363.87 � 13.96a 55.65 � 3.47a 0.153 � 0.005d 405.90 � 16.36a 55.59 � 3.03a 0.140 � 0.015d

Columns with different superscripts are significantly different at P < 0.05 according to Duncan’s multiple range test.

Fig. 3. Typical creep-recovery curves of RF gels with and without SEs at 25 �C.

Y.-C. Meng et al. / Food Hydrocolloids 40 (2014) 64e7068

3.2.3. Creep-recoveryFig. 3 shows the creep-recovery curves of the control and RF

gels with different types of SEs. The creep behavior of samples inthe absence and presence of the SEs was quite different. It is foundthat, the increment of strain subjected to a constant stress (10 Pa)decreased at a constant time in the creep stage with the SEsaddition except for S-570. It meant that RF samples treated withSEs except for S-570, exhibited higher resistances to the stress andproduced a stronger network. The deformation of all tested gelssamples followed the order: S-570 > control > S-970 > S-1170 > S-1570. This observation is in good agreement with theresults obtained from RVA tests and oscillatory tests as describedabove.

The determination of rheological parameters provides a valu-able tool for deeper understanding of the delicate structure of food.It is known that a mechanical analog composed of springs anddashpots can be applied to conceptualize the viscoelastic behaviorsof starchy food. The Burger’s model has beenwell applied to starch-based systems (Shi, Wang, Li, & Adhikari, 2013). Thus, the experi-mental data in this study were fitted by a 4-element Burger’s model(Juszczak et al., 2012). This model is described by the equation as

Table 5The creep parameters from the Burger’s model for rice flour gels with and without sucro

Sample EM (Pa) EK (Pa) s (s)

Control 1.407 � 0.004d 4.108 � 0.045d 6.387 � 0.182b

S-570 1.277 � 0.005e 3.019 � 0.028e 5.539 � 0.133c

S-970 1.742 � 0.005c 5.167 � 0.048c 6.409 � 0.157b

S-1170 2.004 � 0.005b 6.674 � 0.076b 7.596 � 0.242a

S-1570 2.863 � 0.005a 11.598 � 0.112a 7.399 � 0.185a

Columns with different superscripts are significantly different at P < 0.05 according to D

εðtÞ ¼EM

þEK

1� e s þhM

t (2)

s0 s0

� �t� s0

s ¼ hKEK

(3)

where ε is the strain (%) of gels, s0 is the stress (Pa), t is the time (s)after loading, EM and hM represents the elastic modulus (Pa) and theviscous coefficient (Pa s) of the Maxwell spring and dashpot,respectively; EK and hK represents the elastic modulus (Pa) and theviscous coefficient (Pa s) of the KelvineVoigt spring and dashpotsimilarly. The retardation time (s) represents the time required todeform to (1 �1/e) or 63.21% of the total deformation in the Kelvinbody. The parameters above are obtained from fitting the experi-mental data to Eqs. (2) and (3).

The obtained parameters are summarized in Table 5. It is shownthat the Burger’s model can fit the experimental data of all sampleswell (R2> 0.993). The sample of RFwith the S-1570 addition showedthe highest EM and hM values, while other samples exhibited rela-tively lowervalues anddecreased in theorder: S-1170, S-970, controland S-570. The other creep parameters including EK and s showedthe same tendency. The creep and recovery behavior of viscoelasticmaterial is associated with the reorientation of bonds (Juszczaket al., 2012). The strain is related to the disruption and conversionof thebonds,while the retardation time (s) stands for the responseofa viscoelastic material subjected to a constant stress. The results inTable 5 show that the RF gel with S-570 addition can deform to acertain extent (63.21%) of the total deformation with the shortesttime,while the gelwith S-1570 addition exhibits the opposite trend.

Based on the Eq. (2), the creep rate ε0(t) can be expressed by

using Eq. (4) (Shi et al., 2013).

ε0ðtÞ ¼ dεðtÞ

dt¼ s0

hK$e

�ts þ s0

hM(4)

Thus, in the stable creep stage (t ¼ N), we can calculate the creeprate gradually reaching a constant value as described by Eq. (5).

ε0ðNÞ ¼ dεðtÞ

dt

����t¼N

¼ s0hM

(5)

The ε0(N) of all tested samples are presented in Table 5. A wide

variation in the ε0(N) indicates that the addition of different types

se fatty acid esters at 25 �C.

hM (Pa s) R2 ε0(N) (s�1) Recovery (%)

515.371 � 10.262d 0.993 0.019 85.58299.925 � 3.772e 0.996 0.033 78.33592.901 � 9.278c 0.995 0.017 82.62747.713 � 14.584b 0.995 0.013 83.19

1573.046 � 31.394a 0.995 0.006 88.43

uncan’s multiple range test.

Table 6Syneresis values of rice flour gels with and without sucrose fatty acid esters in eachcycle.

Sample Syneresis (%)

Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5

Control 25.4 � 2.2aD 40.3 � 3.4aC 46.4 � 0.7aB 49.6 � 0.9aAB 51.2 � 0.8aA

S-570 23.7 � 3.7abD 34.3 � 0.4bC 40.3 � 1.2bB 44.8 � 1.5bA 48.2 � 1.0bA

S-970 21.0 � 1.0bcE 35.3 � 1.1bD 39.8 � 0.9bcC 43.4 � 0.9bB 48.5 � 1.3bA

S-1170 20.3 � 0.8cE 29.1 � 1.3cD 36.9 � 1.7cC 40.5 � 1.7cB 46.9 � 1.0bcA

S-1570 17.0 � 1.9dD 28.1 � 2.0cC 31.4 � 2.7dC 40.1 � 1.3cB 45.4 � 1.0cA

Columns with different lower case letter (aed) are significantly different at P < 0.05according to Duncan’s multiple range test. Rows with different upper case letter (AeE) are significantly different at P < 0.05 according to Duncan’s multiple range test.

Y.-C. Meng et al. / Food Hydrocolloids 40 (2014) 64e70 69

of SEs exerted different effects on the internal structure and creepbehavior of RF gels. As shown in Table 5, the recovery rates of alltested samples followed order as: S-1570, control, S-1170, S-970and S-570. Polystearate makes samples more susceptible to stressand slower to recover because of their weaker interaction withstarch molecules and larger molecule structure resulting in sterichindrance. While sucrose monostearate with a more linear struc-ture exhibits stronger intermolecular force throughout the systemwhich is in agreement with the result discussed above.

3.3. Freezeethaw stability

Freezeethaw stability is often determined as an importantparameter to evaluate the ability of starch to withstand undesirablephysical changes induced by freezing and thawing (Muadklay &Charoenrein, 2008). Repeated freezeethaw cycles enforce thephase separation and ice crystals growth. Thus, the syneresis andsponge formation would be drastically accelerated by repeatingfreezeethaw cycles (Katekhong & Charoenrein, 2012).

As can be seen in Table 6, freezeethawed RF gels with theaddition of different SEs showed a lower percentage syneresis incomparison to the control sample in each cycle.When starch gels orstarch-containing foods are frozen, the formation of ice crystals andstarch-rich regions will lead to phase separation within food ma-trix. Upon thawing, water in RF is easily separated from the poly-meric network. Syneresis is caused by the enhancement ofassociation between starch chains, in particular the retrogradationof amylose (Morris, 1990). Thus, the amount of water separated dueto syneresis is a useful parameter to measure the retrogradation ofstarch (Karim, Norziah, & Seow, 2000). The percentage of syneresisof RF gels depended on the type of SEs and the number of freezeethaw cycles. The addition of S-1570 exhibited the best ability toenhance the freezeethaw stability of RF gels. The control samplethat without SEs addition showed 25.4% syneresis after the firstfreezeethaw cycle, and a rapid increase in percentage of syneresisto 40.3% after the second freezeethaw cycle. On the other hand, thesyneresis effects of control sample increased slightly through cycles3e5 in comparison to RF with SEs addition. As described earlier, thefreezeethaw stability can be regarded as an indicator of the ten-dency of starch to retrograde. Thus, addition of SEs could delay theretrogradation of RF gels. Results in this study also showed that theeffectiveness of SEs to delay the retrogradation of starch is ac-cording to the order of S-1570 > S-1170 > S-970 > S-570. Freezeethaw stability enhances potential use of RF in frozen food products.Overall, the results suggested that SEs can be effective additives forpreservation of the eating quality of starch-based frozen foods.

4. Conclusions

This study demonstrated that the different type of SEs additionexhibited various effect on the pasting, rheological properties and

freezeethaw stability of RF. Results showed that pasting tempera-ture, peak and final viscosities, storage and loss moduli, apparentviscosity, flow behavior index and the retardation time (s) of RF gelsincreased and loss tangent decreased with SEs addition except forS-570. Flow curves of all the samples showed shear-thinningbehavior and pseudoplastic properties. Power law and Burger’smodels were capable of fitting steady shear (R2 > 0.983) and creepdata (R2 > 0.993) respectively. Moreover, different types of SEsaddition all displayed a lower percentage syneresis subjected torepeated freezeethaw cycles in comparison to the control, indi-cating SEs could prevent the molecular associations between starchchains and hinder retrogradation process of RF gels. The order ofeffectiveness was S-1570, S-1170, S-970, S-570. These results couldhave important practical implications in choosing suitable SEs withdifferent HLB based on the specific requirements in rice-relatedfood products.

Acknowledgments

The authors acknowledge financial support from the NationalNatural Science Foundation of the People’s Republic of China(project-no: 21006094).

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