Effects of Heat Treatment and Moisture Contents on Interactions

Jul 16, 2014 - treatment at 80 °C, and order of adding lauric acid (LA) to starch before or after the heat treatment on the physicochemical propertie...
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Effects of Heat Treatment and Moisture Contents on Interactions Between Lauric Acid and Starch Granules Fengdan Chang,† Xiaowei He,† Xiong Fu,† Qiang Huang,*,† and Jay-lin Jane‡ †

College of Light Industry and Food Sciences, South China University of Technology, 381 Wushan Road, Guangzhou, Guangdong 510640, People’s Republic of China ‡ Department of Food Science and Human Nutrition, Iowa State University, Ames, Iowa 50011, United States ABSTRACT: This study aimed to understand the effects of the moisture content of granular normal cornstarch (NC), heat treatment at 80 °C, and order of adding lauric acid (LA) to starch before or after the heat treatment on the physicochemical properties and digestibility of the starch. LA was added to NC priority heated with different moisture contents (10, 20, 30, 40, and 50%) or added to dried NC and then heated with different moisture contents. The hydrothermal/LA treatments increased the pasting temperature but decreased the peak viscosity of the NC. Light and scanning electron microscopy revealed that the addition of LA retarded gelatinization. The hydrothermal/LA treatments changed the X-ray pattern of the NC to a mixture of Aand V-type patterns. The thermal property and digestibility analysis showed that 40% was the optimum moisture content for the formation of the amylose−LA complex and adding LA prior to heating the NC favored the formation of slowly digestible starch. KEYWORDS: starch−lipid complex, hydrothermal treatment, lauric acid, normal cornstarch

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incubated at 50 °C and revealed that the teff starch complexed with stearic acid improved mouthfeel in foods. Cui et al.16 reported that the starch−lipid complex prepared at 60 °C using native sago starch decreased the susceptibility of the starch enzyme hydrolysis and also retarded the retrogradation of the foods made from the lipid-complexed starch. In our previous studies, we observed that higher incubation temperatures and larger amylose contents of the starch improved amylose−lauric acid complex formation. We also observed that the order of adding lauric acid to starch suspensions affected the location of the amylose−lauric acid complex present in starch granules.14,17 Impacts of amylose chain length and lipid structures, including fatty acid chain length and the degree of saturation, as well as lipid concentration and solubility, on the physicochemical properties of the amylose−lipid complex have been reported.18 Most of studies reported that the amylose−lipid complex was prepared in excess or intermediate water content to increase the accessibility of amylose to interact and complex with lipids. Most lipids, however, are not dispersed in the aqueous phase and cannot penetrate into starch granules to complex with amylose.14 The objective of this study was to understand the mechanism of granular starch−lipid complex formation using normal cornstarch heated at 80 °C with different moisture contents (10, 20, 30, 40, and 50%) and mixing the starch with lauric acid (LA) before and after the hydrothermal treatment. The study helped us understand effects of mixing LA with starch before or after heating the starch with different moisture contents on the physicochemical properties of the starch.

INTRODUCTION Starch is a major component of staple foods and plays an important role in food texture and glycemic response. Frequent consumption of rapidly digestible starchy food is associated with the development of insulin resistance and metabolic syndrome.1 Resistant starch and slowly digestible starch have been recommended for diets to prevent metabolic syndrome. The digestive rate of starch is related to the amylose content, branch chain length of amylopectin, and interactions of starch with lipids and proteins in foods.2−5 The amylose−lipid complex is resistant to enzyme hydrolysis because of its collapsed helical conformation and crystalline structure, resulting from the hydrophobic interaction between amylose and fatty acids.1,6,7 The starch−lipid complex is mostly formed between amylose and lipids. Only a small portion of amylopectin can form a complex with lipids because of its steric hindrance and short branch chain lengths.8 Therefore, most of studies of the starch−lipid complex formation used extracted amylose and lipids. Amylose from potato with an average degree of polymerization (DP) of 900 has been used for this purpose.9−11 Hasjim et al.1 and Zhang et al.12 used debranching enzyme (isoamylase and pullulanase) to pretreat starches to remove the α-1,6 branch linkages to enhance the complex formation with lipids. Zakazawa and Wang13 pretreated starches using annealing to improve the formation of the amylose−lipid complex. Gelatinized starch has also been used in the preparation of the starch−lipid complex because leached amylose is readily available for complexation. The starch− lipid complex prepared with extracted amylose and pretreated or gelatinized starch, however, is costly and not practical for use in the food industry.14 Complex formation between granular starch and lipids has attracted much attention in recent years. D’Silva et al.15 studied the pasting properties of starch−stearic acid complexes © 2014 American Chemical Society

Received: Revised: Accepted: Published: 7862

April 5, 2014 July 15, 2014 July 16, 2014 July 16, 2014 dx.doi.org/10.1021/jf501606w | J. Agric. Food Chem. 2014, 62, 7862−7868

Journal of Agricultural and Food Chemistry

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Article

Wide-Angle X-ray Diffraction (XRD). The native NC and LAcomplexed NC were equilibrated in a chamber with 100% relative humidity at 25 °C for 24 h prior to X-ray analysis. The X-ray diffractometer (D8 Advance, Bruker, Germany) was operated at 40 kV and 40 mA with Cu Kα radiation (λ = 0.154 nm). The starch powder was packed tightly in a rectangular glass cell and scanned over the range of 4−35° 2θ angles at a rate of 2°/min at 25 °C. The percentage crystallinity of the starch was calculated using the following equation:22

MATERIALS AND METHODS

Materials. Normal cornstarch (NC) was purchased from COFOO Biochemical Energy Co. (Yushu, China). Pancreatin from porcine pancreas (catalog number P7545) and amyloglucosidase (catalog number A7095) were purchased from Sigma-Aldrich (St. Louis, MO). Glucose oxidase−peroxidase (GOPOD) assay kits were purchased from Megazyme International Ireland, Ltd. (Wicklow, Ireland). Lauric acid (LA) was purchased from Nanjing Chemical Co. (Nanjing, China). All other chemicals used in this study were of analytical grade and used without further treatments. Preparation of LA-Complexed Starch. Heating Starch Prior to the Addition of LA (HM). NC was dried at 105 °C for 3 h, and the moisture content of the starch was 2.05%. Distilled water was added to the dried NC [40 g, dry basis (db)] and stirred vigorously to obtain starches with 10, 20, 30, 40, and 50% moisture contents. The moisturized starches were heated in a sealed stainless-steel reaction vessel at 80 °C for 12 h. LA [6 g, 15%, w/w, dry starch basis (dsb)] dissolved in ethanol (15 g, 35%, w/w, dsb) was then added to the prior heated starch at different moisture contents, stirred vigorously in the reaction vessel for 5 min, and then uncovered to evaporate ethanol at 80 °C. The final mixture was heated at 80 °C for another 2 h. Adding LA to the Starch Prior to Heating (MH). LA (6 g, 15%, w/ w, dsb) dissolved in ethanol (15 g, 35%, w/w, dsb) was added to the dried NC (40 g, db) and mixed vigorously. The mixture was heated without cover at 80 °C for 5 min to evaporate ethanol before being heated in a sealed stainless-steel reaction vessel at 80 °C for 2 h. Distilled water was then added to the mixture and stirred vigorously to obtain a mixture with 10, 20, 30, 40, and 50% moisture contents and then heated at 80 °C for 12 h. All LA-complexed NC prepared using HM and MH were cooled to 25 °C, washed 3 times with 50% distilled water−ethanol mixture, and recovered by centrifugation (1500g for 10 min). The resulting precipitates, HM-NC represented LA-complexed NC prepared using HM and MH-NC represented LA-complexed NC prepared using MH, were dried at 40 °C overnight and subsequently ground to a fine powder. Control samples were prepared with the same procedure without adding LA. Lipid Content. The lipid contents in the native NC and LAcomplexed NC were determined according to previous reports.19,20 Free lipid extraction was Soxhlet extracted with petroleum ether at 50 °C for 10 h. The total lipid content was determined after acid hydrolysis. Approximately 1.00 g of starch was accurately weighed and suspended in 10 mL of water. HCl (15 mL, 8.0 N) was added, and the mixture was heated in a boiling water bath for 25 min. After the addition of 50.0 mL of distilled water, the mixture was filtered through paper and washed with distilled water until the filtrate was neutral. The filter paper with the residue was dried at 40 °C overnight and then transferred to a defatted extraction thimble. The dried samples were Soxhlet-extracted with petroleum ether at 50 °C for 8 h. The recipients with the extracted fat were then dried at 110 °C to a constant weight. All samples were performed in duplicates. Pasting Property. Each starch−LA complex sample (2.24 g, db) was suspended in deionized water to make a total weight of 28.0 g (8%, w/w) and then analyzed using a rapid visco analyzer (RVA, Newport Scientific, Sydney, Australia) following the method reported by Ai et al.21 The pasting temperature and peak, breakdown, and setback viscosity were recorded. Light Microscopy. Light micrographs were obtained using an Olympus BX-51 light microscope (Tokyo, Japan) with brightfield and polarized light. Starch powder was sprinkled on a glass slide and covered with a single drop of an aqueous glycerol solution (1:1 water/ glycerol). The starch suspension was covered with a coverslip. Images were recorded at 500× magnification. Scanning Electron Microscopy (SEM). Morphology of starch granules was studied using SEM (ZEISS EVO18, Germany) operated at 10 kV accelerating voltage. The native NC, control NC, and LAcomplexed NC were mounted on aluminum stubs with double-sided sticky carbon tape and sputter-coated with a thin film of gold.

crystallinity (%) = 100Ac /(Ac + A a ) where Ac is the area of the crystalline peak and Aa is the area of the amorphous peak. Differential Scanning Calorimetry (DSC). Thermal properties of starches were analyzed using a differential scanning calorimeter (Diamond DSC, PerkinElmer, Norwalk, CT). The native NC or the LA-complexed NC (2−5 mg, db) with deionized water (3×, w/w) was scanned from 10 to 120 °C at a rate of 10 °C/min in a sealed aluminum pan. The thermal properties were calculated using Pyris software (PerkinElmer). The analysis was performed in duplicate. In Vitro Starch Digestibility. In vitro digestibility of the native NC and the LA-complexed NC was analyzed using the Englyst method as described by Englyst et al.,23 with slight modification. Starch samples (1.00 g, db) were suspended in a sodium acetate buffer solution (20 mL, 0.1 M, pH 5.2), and five glass beads were added to 50 mL centrifuge tubes. The tubes were capped and mixed for 5 min. Samples were placed in a boiling water bath for 30 min and vortexed at 5 min intervals to prevent agglomeration of starch. The dispersion was equilibrated in a shaker water bath (160 strokes/min) at 37 °C for 30 min. The enzyme mixture containing porcine pancreatin and amyloglucosidase was added to each tube. The enzyme mixture was prepared by stirring 3 g of porcine pancreatin in 20 mL of deionized water in each of four tubes for 10 min and then centrifuging at 1500g for 10 min. The supernatant (54 mL) mixed with 6 mL of diluted amyloglucosidase solution and 4 mL of deionized water instead of invertase. Aliquots (1.00 mL) were taken at the end of 20 and 120 min reaction and mixed with 20 mL of ethanol (66%, v/v) to stop the enzyme reaction. Starch digestibility was determined using the GOPOD assay. Each sample was analyzed in duplicate. Statistical Analysis. The mean values and differences were analyzed using Duncan’s multiple-range test. Treatment means were tested separately for least significant difference (LSD) with SPSS 18.0 statistical software for Windows (SPSS, Inc., Chicago, IL). The significance level was set at p < 0.05.

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RESULTS AND DISCUSSION Thermal Properties. Thermal properties of the native NC and LA-complexed NC are listed in Table 1. The onset gelatinization temperature (To) increased from 63.4 to 78.2 °C for the HM-NC and from 61.7 to 69.5 °C for the MH-NC with the moisture content increased from 10 to 50%. There were two peaks observed in the HM-NC thermograms: peak I was attributed to the melting of the double-helical crystallite of amylopectin side chains; peak II was attributed to the melting of the amylose−LA complex. The MH-NC displayed two peaks when the moisture content was 10, 20, and 30%. The MH-NC, however, displayed three separate peaks at 40 and 50% moisture content (Table 1), in which the peak (41−44 °C) was attributed to the melting of free LA12 and the other two peaks represented the same as those in the HM-NC. These results were consistent with higher free lipid contents in MHNC prepared with 40 and 50% moisture contents (Table 2). Except the starches treated with 10% moisture content, an increase in the onset gelatinization temperature in peak I suggested that the hydrothermal/LA treatments resulted in a more ordered crystalline structure. The HM-NC and MH-NC all exhibited decreases in the enthalpy change of peak I with the increase in the moisture content, which was attributed to the 7863

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2.1 ± 0.1 1.8 ± 0.9 43.7 ± 0.4 43.8 ± 0.1 42.4 ± 0.3 42.5 ± 0.1 41.4 ± 0.2 41.6 ± 0.0

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Table 2. Lipid Content of the Native NC and LA-Complexed NCa

a All data are averages of two measurements with standard deviation. Means in a column with different letters are significantly different (p < 0.05) by the least significant difference (LSD) test. bSample code: HM-NC, LA-complexed NC was prepared using heating starch prior to the addition of LA; MH-NC, LA-complexed NC was prepared using adding LA to the starch prior to heating; and 10, 20, 30, 40, and 50%, moisture contents of LA-complexed NC. cTo, Tp, Tc, and ΔH are onset temperature, peak temperature, conclusion temperature, and enthalpy change, respectively.

a a ac acd ac a ab ad c ac 0.7 0.2 0.2 0.1 0.0 0.2 0.1 0.2 0.0 0.9 ± ± ± ± ± ± ± ± ± ± 2.0 1.9 2.4 2.8 2.6 1.6 2.1 2.1 3.4 2.4 d abd abcd abc abc abcd abcd bd c c 0.5 0.4 0.1 0.3 0.3 0.9 1.3 0.6 0.0 0.4 ± ± ± ± ± ± ± ± ± ± 104.8 103.9 103.0 102.4 102.3 103.4 103.4 104.0 101.9 101.8 0.0 0.2 0.7 0.4 0.3 0.8 1.7 0.6 0.1 0.1 ± ± ± ± ± ± ± ± ± ± 98.6 98.3 95.6 95.3 95.2 97.8 97.1 98.1 94.2 94.5 ac ab bd bd bd ab ab b cd d 0.7 0.2 0.2 0.2 0.0 0.5 0.8 0.2 0.2 0.4 ± ± ± ± ± ± ± ± ± ± 88.1 88.9 88.8 88.3 88.5 88.9 88.8 89.2 87.2 86.8 a a a a d e a a ac cd e 0.7 0.3 0.3 0.1 0.2 0.6 1.2 1.3 0.3 0.2 1.2 ± ± ± ± ± ± ± ± ± ± ± 12.2 11.1 11.9 10.7 7.2 3.8 11.6 11.2 10.6 8.5 3.5 0.7 0.0 0.1 0.2 0.2 0.1 0.4 0.2 0.5 0.3 0.0 ± ± ± ± ± ± ± ± ± ± ± 74.7 73.7 74.9 81.6 83.2 83.9 72.5 72.5 74.4 73.6 75.3 0.6 0.0 0.1 0.1 0.2 0.1 0.0 0.4 0.7 0.2 0.1 ± ± ± ± ± ± ± ± ± ± ± 70.3 68.9 70.3 78.1 80.5 81.3 67.8 68.2 69.2 69.9 72.2 64.9 63.4 65.5 72.3 77.6 78.2 61.7 65.1 65.9 65.7 69.5

± ± ± ± ± ± ± ± ± ± ±

0.6 0.0 0.9 0.1 0.2 0.0 0.3 1.9 1.4 0.3 0.1

a ab a d e f b a a a c (°C) (°C) To (°C) sample

NC HM-NC10% HM-NC20% HM-NC30% HM-NC40% HM-NC50% MH-NC10% MH-NC20% MH-NC30% MH-NC40% MH-NC50%

(°C) ΔH (J/g)

Toc c

Tcc

melting of free LA

Tpc c b

Table 1. Thermal Properties of the Native NC and LA-Complexed NCa

Tpc

(°C)

a cd a f g g b bc d ad e

peak I

Tcc

(°C)

a c a d e f b b ac c a

c

ΔH (J/g)

Toc

(°C)

Tpc

(°C)

a a bc bc bc a ab a c c

peak II

Tcc (°C)

ΔHc (J/g)

Journal of Agricultural and Food Chemistry

lipid content (g/100 g of dry starch sample)

sample

b

NC HM-NC10% HM-NC20% HM-NC30% HM-NC40% HM-NC50% MH-NC10% MH-NC20% MH-NC30% MH-NC40% MH-NC50%

acid hydrolyzed (total lipids, T) 0.98 1.98 1.91 2.50 2.80 2.64 1.55 1.88 1.97 3.76 2.87

± ± ± ± ± ± ± ± ± ± ±

0.03 0.25 0.41 0.04 0.20 0.36 0.04 0.01 0.72 0.23 0.01

a b b c d cd e b b f d

petroleum ether extracted (free lipids, F) 0.47 0.75 0.76 0.80 0.89 0.83 0.52 0.53 0.59 1.40 1.13

± ± ± ± ± ± ± ± ± ± ±

0.08 1.00 0.29 0.52 0.18 0.01 0.29 0.76 0.14 0.09 0.58

a b b b bc bc a a a d e

T−F (complexed lipids) 0.51 1.22 1.15 1.69 1.91 1.80 1.03 1.35 1.38 2.35 1.74

a

The data are averages of two measurements with standard deviation. Means in a column with different letters are significantly different (p < 0.05) by the LSD test. bSample code: HM-NC, LA-complexed NC prepared using heating starch prior to the addition of LA; MH-NC, LA-complexed NC prepared using adding LA to the starch prior to heating; and 10, 20, 30, 40, and 50%, moisture contents of LAcomplexed NC.

loss of the double helices of amylopectin. The enthalpy change of peak II showed an increasing trend when the moisture content increased from 10 to 40% (Table 1), indicating increases in the formation of the amylose−LA complex. However, when the moisture content increased to 50%, the enthalpy change decreased. This could be attributed to substantial starch gelatinized when the moisture content increased to 50%, resulting in a viscous texture, which retarded the amylose−LA complex formation, which is consistent with the results of the lipid content. The content of complexed lipids of LA-complexed NC showed an increase when the moisture content was increased from 10 to 40% but a decrease when the moisture content was 50% (Table 2). Morphology of NC and LA-Complexed NC. Morphology of the NC and the LA-complexed NC was studied using optical microscopy and scanning electron microscopy (Figures 1 and 2). Polarized light micrographs of the native NC showed a characteristic Maltese cross (image not show). LA-complexed NC, both prepared by HM or MH, showed that increasing numbers of granules lost the Maltese cross with the increase in the moisture content. HM-NC with a 50% moisture content was mostly gelatinized and showed few Maltese cross remaining, whereas MH-NC maintained more intact granules and Maltese cross even after heating with 50% moisture content. These results indicated that the starch gelatinized to a greater extent during heating without the presence of LA than that of LA-complexed NC.24 In contrast, with the addition of LA prior to heating in the MH-NC, LA coated on the granule surface, complexed with amylose, impaired water movement into the granules, and retarded the gelatinization of the starch. The structures of the control NC and LA-complexed NC studied using SEM are shown in Figure 2. The surface of native NC appeared smooth and exhibited pinholes on its surface (image not shown), which was consistent with the findings by Jane et al.25 and Rocha et al.26 Following the hydrothermal treatment, control NC with 40% moisture content was partially gelatinized and the granules were aggregated. When the moisture content increased to 50%, the control NC was 7864

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Figure 2. Morphology of the control NC and LA-complexed NC. Sample code: HM-con, control samples for HM-NC; HM-NC, LAcomplexed NC prepared using heating starch prior to the addition of LA; MH-NC, LA-complexed NC prepared using adding LA to the starch prior to heating; and 40 and 50%, moisture contents of LAcomplexed NC.

50% in the MH-NC process, it gave mobility for LA to segregate and merge to crystallize. At high moisture contents (40 and 50%) in the HM-NC process, however, starch was partially gelatinized and the viscosity of the gelatinized starch retarded the mobility of LA. Therefore, LA did not aggregate to crystallize. The crystallinity of LA-complexed NC was attributed to the contents of free lipids, double-helix amylopectin, and singlehelix amylose−LA complex and had a correlation with its corresponding enthalpy change in DSC thermograms. The crystallinity of HM-NC increased from 28.9 to 30.0% and then decreased to 28.2% (higher than native NC) when the moisture content increased from 10 to 30%. With a further increase in the moisture content (up to 50%), the crystallinity sharply decreased to 18.6%, which was consistent with the enthalpy change of gelatinization and the amylose−LA complex (Table 1). These results suggested that, when the moisture content increased to 10 and 30%, there was a heat−moisture treatment effect and amylose−LA complex formation (peak II in DSC thermograms) to improve the crystalline structure. When the moisture content was 40 and 50%, partial gelatinization took place, which was confirmed by the loss of Maltese cross (Figure 1). The crystallinity of MH-NC samples with the moisture content from 10 to 50%, however, decreased from 26.3 to 18.9%. It appears that heat−moisture treatment effect did not take place in starch with the presence of LA in MH-NC. Pasting Properties. Pasting viscosity profiles of the native NC and LA-complexed NC are shown in Figure 4. The hydrothermal/LA treatments increased the pasting temperature of the starch and decreased the peak viscosity, which was consistent with reports in previous studies.2,17,31,32 The pasting

Figure 1. Light microscopic images of LA-complexed NC with brightfield and polarized light. Sample code: HM-NC, LA-complexed NC prepared using heating starch prior to the addition of LA; MHNC, LA-complexed NC prepared using adding LA to the starch prior to heating; and 40 and 50%, moisture contents of LA-complexed NC.

gelatinized and lost the shape of granules. Because HM control and MH control displayed similar images, only HM controls are shown. Following the hydrothermal/LA treatments, LAcomplexed NC showed intact granular shape, except HM-NC with 50% moisture content, which was partially gelatinized and disrupted (Figure 2). The addition of LA retarded the gelatinization of starch granules, which was consistent with the results of DSC. Wide-Angle XRD. XRD patterns and percentage crystallinity of the native NC and LA-complexed NC are shown in Figure 3. The native NC showed an A-type diffraction pattern.27 The hydrothermal/LA treatments changed the Xray pattern to a mixture of A- and V-type patterns. This change in diffraction pattern was attributed to the fact that the increasing moisture content increased the mobility of amylose and facilitated the formation of the amylose−LA complex. The peaks shown at 8°, 13°, and 20° reflected the V pattern of the amylose−LA complex.12,28 The peak shown at 21.5° was the crystallites of LA,29,30 which was only observed in MH-NC prepared with the moisture contents of 40 and 50%. The result was consistent with a melting peak of crystalline LA appearing in DSC thermograms of MH-NC prepared with 40 and 50% moisture contents (Table 1). These results could be attributed to the fact that, when the moisture content increased to 40 and 7865

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Figure 3. XRD patterns of the native NC and LA-complexed NC. Sample code: HM, heating starch prior to the addition of LA; MH, adding LA to the starch prior to heating; and 10, 20, 30, 40, and 50%, moisture contents of LA-complexed NC.

Figure 4. Pasting profiles of the native NC and LA-complexed NC: (a) pasting profiles of the native NC and HM-NC and (b) pasting profiles of the native NC and MH-NC. Sample code: HM-NC, LAcomplexed NC prepared using heating starch prior to the addition of LA; MH-NC, LA-complexed NC prepared using adding LA to the starch prior to heating; and 10, 20, 30, 40, and 50%, moisture contents of LA-complexed NC.

temperature of HM-NC and MH-NC both showed increases when the moisture content increased from 10 to 50%. The peak viscosity of HM-NC decreased with an increase in the moisture content. For MH-NC, however, when the moisture content increased from 10 to 30%, the peak viscosity displayed the same trend as its counterparts in HM-NC. However, when the moisture content increased to 40 and 50%, the peak viscosity increased. The presence of the amylose−LA complex and ordered crystallites in LA-complexed NC restricted the swelling of starch granules and resulted in higher pasting temperature and lower peak viscosity.2,33 These results were consistent with a high enthalpy change of the amylose−LA complex and free lipid (Table 1). When the moisture content was 40%, MH-NC showed a higher content of complexed lipids and free lipids than other LA-complexed NC (Table 2), which resulted in a higher pasting temperature and lower peak viscosity. HM-NC displayed a decrease in the breakdown and setback viscosity when the moisture content increased from 10 to 50%. MH-NC showed the same trend of breakdown and setback viscosity as their counterparts in HM-NC when the moisture content increased from 10 to 30%. However, when the moisture content increased to 40 and 50%, MH-NC showed increases in the breakdown viscosity and drastic increases in the setback viscosity. The remarkable increase in the setback

viscosity could result from the formation of the helical complex between amylose and free LA present in the samples. During the heating procedure of RVA, most starch and part of the amylose−LA complex were dissociated and there was sufficient amylose to complex with LA in the amylose−LA complex and free LA. When the procedure temperature was cooled from 95 to 50 °C, amylose, LA, and amylopectin formed a new amylose−LA complex and an amylose−amylopectin network, which increased the viscosity of MH-NC with moisture contents at 40 and 50%. In Vitro Digestibility. The contents of RDS, SDS, and RS in the native NC and LA-complexed NC are shown in Table 3. LA-complexed NC showed decreases in the RDS content and increases in the SDS and RS contents with increases in the moisture content. MH-NC was more resistant to the enzyme than that of HM-NC. The SDS content in MH-NC was much higher than that in the counterparts of HM-NC with greater moisture contents (10−50%). The increase in SDS and RS contents in LA-complexed NC with greater moisture contents was attributed to the formation of the amylose−LA complex and more ordered crystallites, which had a positive relationship with enthalpy change, as shown in Table 1. The V−amylose complex is known to be resistant to enzyme hydrolysis.2,6 The presence of the 7866

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3). However, all HM-NC contained higher free lipids than that of NC in the lipid content (Table 2). For MH-NC, however, LA was added to the dried starch and heated in a sealed container at 80 °C for 2 h. LA coated on the surface of the granule, delayed water penetrate into starch granules, and retarded starch gelatinization. The mixture was then heated under different moisture contents for another 12 h. During this heating process, the increased moisture content facilitated mobility of amylose and LA to form a complex. The crystalline structure of the starch remained as shown in the polarized light micrographs of MH-NC (Figure 1). Some of the free LA aggregated and crystallized, as shown in the DSC thermograms and XRD patterns. The physicochemical properties of the amylose−LA complex were influenced by the moisture content of starch and the order of adding LA to starch before or after heating at 80 °C. The presence of LA retarded the gelatinization of starch during heating. A high moisture content enhanced the mobility of amylose, amylopectin, and LA and promoted the formation of the amylose−LA complex. The formation of the amylose−LA complex was the key factors in decreasing the hydrolysis of starch. With increasing the moisture content, LA had a greater chance to complex with amylose. MH favored the formation of slowly digestible starch and resistant starch, especially when the moisture content was 40%. The results of this study may have potential application in starch food processing applications that desire a higher content of SDS or RS.

Table 3. RDS, SDS, and RS Contents in the Cooked Native NC and LA-Complexed NCa sampleb NC HM-NC10% HM-NC20% HM-NC30% HM-NC40% HM-NC50% MH-NC10% MH-NC20% MH-NC30% MH-NC40% MH-NC50%

RDS (%) 88.9 81.1 80.3 78.6 76.8 74.8 78.7 77.1 75.1 74.1 72.3

± ± ± ± ± ± ± ± ± ± ±

0.6 0.5 0.3 0.1 0.1 0.5 0.3 0.1 0.8 0.5 0.6

SDS (%) a b b c d e c d e e f

5.0 5.8 5.7 6.0 7.5 7.4 7.7 8.0 9.0 9.5 10.2

± ± ± ± ± ± ± ± ± ± ±

0.3 0.2 0.3 0.1 0.1 0.9 0.7 0.6 0.1 0.2 0.8

a a a ab c bc cd cef df ef f

RS (%) 6.6 13.0 14.0 15.4 16.2 17.8 13.6 14.9 14.9 16.0 17.6

± ± ± ± ± ± ± ± ± ± ±

0.2 0.3 0.0 0.0 0.3 0.4 0.3 0.7 0.2 0.2 0.2

a b bc de e f b cd cd e f

a

RDS, rapidly digestible starch; SDS, slowly digestible starch; and RS, resistant starch. The data are averages of two measurements with standard deviation. Means in a column with different letters are significantly different (p < 0.05) by the LSD test. bSample code: HMNC, LA-complexed NC prepared using heating starch prior to the addition of LA; MH-NC, LA-complexed NC prepared using adding LA to the starch prior to heating; and 10, 20, 30, 40, and 50%, moisture contents of LA-complexed NC.

amylose−LA complex restricted the swelling of starch granules, as shown in Figure 4, resulting in reduced starch hydrolysis.1,16 Hasjim et al.1 and Zhang et al.12 used partially gelatinized debranched high amylose cornstarch complexed with lipids to produce highly resistant starch, which was attributed to a combination of amylose double-helical crystallite and the amylose−stearic acid complex. The SDS and RS contents in LA-complexed NC prepared in this study both increased. MHNC with 40 and 50% moisture contents showed higher SDS content than that of other LA-complexed NC, indicating that more amylose−LA complex formed between amylose and free LA after cooking. The diagram of the difference between the hydrothermal treatments of the two methods is shown in Figure 5. For HMNC, starch granules swelled, partially gelatinized during the heating before adding LA, resulting in a viscous texture, and decreased the mobility of amylose. LA was then thoroughly mixed with the partially gelatinized starch and subjected to additional heating. When the moisture content was at 40 and 50%, most of the starch crystalline structure was lost. LA dispersed in the viscous starch could not move freely to form aggregate and crystallites, as shown by no melting peak of crystalline LA in DSC and XRD patterns (Table 1 and Figure

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-20-8711-3845. Fax: +86-20-8711-3848. Email: [email protected]. Funding

The authors thank the financial support received from the National Natural Science Foundation of China (NSFC) (31130042 and 31101378), the Fundamental Research Funds for the Central Universities (2013ZZ0065), the Science and Technology Planning Project of Guangdong Province, China (2012A020602005 and 2012NL016), and the Department of Science and Information Technology of Guangzhou, China (2013J4500036). Notes

The authors declare no competing financial interest.

Figure 5. Schematic diagram of LA-complexed NC. Sample code: HM, heating starch prior to the addition of LA; MH, adding LA to the starch prior to heating; HM-NC, LA-complexed NC prepared using heating starch prior to the addition of LA; and MH-NC, LA-complexed NC prepared using adding LA to the starch prior to heating. 7867

dx.doi.org/10.1021/jf501606w | J. Agric. Food Chem. 2014, 62, 7862−7868

Journal of Agricultural and Food Chemistry

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Article

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ACKNOWLEDGMENTS The authors acknowledge the discussion and technical assistance of Joshua Lyte, Department of Food Science and Human Nutrition, Iowa State University.

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ABBREVIATIONS USED NC, normal cornstarch; LA, lauric acid; HM, heating starch prior to the addition of LA; MH, adding LA to the starch prior to heating; HM-NC, LA-complexed NC prepared by HM; MHNC, LA-complexed NC prepared by MH; HM-con, control samples for HM-NC; RDS, rapidly digestible starch; RS, resistant starch; SDS, slowly digestible starch; SEM, scanning electron microscopy; XRD, X-ray diffraction; DSC, differential scanning calorimetry; To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; ΔH, enthalpy change

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dx.doi.org/10.1021/jf501606w | J. Agric. Food Chem. 2014, 62, 7862−7868