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Investigation of glycerol concentration on corn starch morphologies and gelatinization behaviours during heat treatment
Accepted Manuscript Title: Investigation of glycerol concentration on corn starch morphologies and gelatinization behaviours during heat treatment Authors: Xu Chen, Li Guo, Xianfeng Du, Peirong Chen, Yishun Ji, Huili Hao, Xiaonan Xu PII: DOI: Reference:
S0144-8617(17)30938-4 http://dx.doi.org/10.1016/j.carbpol.2017.08.062 CARP 12669
To appear in: Received date: Revised date: Accepted date:
18-3-2017 3-7-2017 11-8-2017
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Investigation of glycerol concentration on corn starch morphologies and gelatinization behaviours during heat treatment Xu Chena, Li Guoa, Xianfeng Dua,*, Peirong Chenb,* Yishun Jic, Huili Haoa, Xiaonan Xua a School b
of Tea & Food Science and Technology, Anhui Agricultural University, Hefei 230036, China
Department of Applied Chemistry, School of Science, Anhui Agricultural University, Hefei 230036, China
c Anhui
province grain and oil products quality supervision and inspection station, Hefei 230031, China
Highlights
When corn starch granules with no added glycerol were treated at 65 °C, the characteristic birefringence of the starch granules disappeared.
Various microscopic techniques revealed that starch gelatinization was delayed to higher temperatures as the glycerol concentration increased.
In the presence of glycerol-water systems (5%, 10%, 20%, and 50%, w/w), the peak temperature (Tp), were 67.2 °C, 73.0 °C, 76.3 °C, and 85.3 °C, respectively.
The RVA pasting profiles showed that the gelatinization temperature increased as the glycerol concentration increased.
Abstract The effects of various glycerol concentrations (0%, 5%, 10%, 20%, and 50%, w/w) on the morphologies and gelatinization behaviours of corn starch were evaluated by confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and rapid visco-analyzer (RVA). When corn starch granules with no added glycerol were treated at 65 °C, the granules of corn starch were almost completely broken and tightly connected, and the characteristic birefringence of the starch granules disappeared. Various microscopic techniques revealed that starch gelatinization was delayed to higher temperatures as the glycerol concentration increased. In the presence of glycerol-water systems (5%, 10%, 20%, and 50%, w/w), the peak temperatures of corn starch increased by 1.6 °C, 7.4 °C, 10.7 °C, and 19.7 °C, respectively, compared to corn starch in water. The RVA pasting profiles showed that the gelatinization temperature increased as the glycerol concentration increased, which was consistent with polarized light microscope observations and DSC tests. Keywords: Corn starch; Gelatinization behaviours; Glycerol; Maltese crosses; Viscosity
1. Introduction Starch is recognized as the dominant carbohydrate reserve material of higher plants, with
excellent physicochemical properties in addition to being biodegradable, non-toxic and produced at low cost, thus allowing sustainable development in different industries in which it is used as a raw material (Koch et al., 2010; Seligra et al., 2016; Valencia, Henao, & Zapata, 2013; Zhao et al., 2015). Corn starch is a widely available ingredient in the food industry, where it is used as a thickener, gelling agent, bulking agent and water retention agent (Acosta et al., 2016; Singh et al., 2003). Although starch is used in many fields of the food industry, in almost all applications, starch must be gelatinized before use (Schirmer et al., 2013). * Corresponding author at: NO.130, Western Changjiang Road, Anhui Agricultural University, Hefei City, Anhui Province 230036, China. Tel.: +86 551 65786965; fax: +86 551 65786982. E-mail addresses: [email protected]; [email protected].
The swelling of the starch granules begins after completion of the first stage of gelatinization. Further heating leads to the helix–coil transition associated with the unwinding of amylopectin double helices and loss of the starch granular birefringence (Liu et al., 2011). The viscosity parameters during pasting are cooperatively controlled by the properties of the swollen granules and the soluble materials leached from the granules (Guo et al., 2016; Sandhu, Singh, & Malhi, 2005). The increase in viscosity has been ascribed to the swelling of the starch granules as they absorb water until they gradually burst (Sun et al., 2014). The gelatinization process is essential in that it determines the proper conversion of starch in the processing of foods and emerging biodegradable starch-based materials (Liu et al., 2011; Luo, Li, & Lin, 2012). For starch with excess water, a single gelatinization endotherm can be usually observed in the low temperature range (54–73 °C) (Liu et al., 2011; Perry & Donald, 2000). The crystalline starch structure is lost when it is subjected to heat at temperatures greater than 70– 90 °C in the presence of plasticizers such as glycerol (Taghizadeh & Favis, 2013). Glycerol is an additive used in the production of biodegradable packaging films which can generate other desirable properties in hybrid materials (Farahnaky, Saberi, & Majzoobi, 2013; Valencia, Henao, & Zapata, 2013). Taghizadeh and Favis (2013) visualized the loss of birefringence with glycerol and used an optical microscope technique to provide evidence for the occurrence of starch gelatinization under static conditions. A large number of studies have been carried out to investigate the effect of glycerol on starch gelatinization using differential scanning calorimetry (DSC) (Liu et al., 2011; Tan et al., 2004). To the best of our knowledge, the literature contains few reports of the visualization of gelatinized starch with various glycerol concentrations
(0-50%) and little analysis of thermal changes and pasting properties. Accordingly, the aim of this study was to investigate the effect of various glycerol concentrations on the gelatinization behaviours of corn starch. Multiple methods, including confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and rapid visco-analyzer (RVA) were applied, and the independent evidence derived from those methods provide a comprehensive understanding of the effect of various glycerol concentrations on the gelatinization behaviours of corn starch. It can be highly instructive in the exploration of biodegradable films and the applications of glycerol, especially in plasticizers (Fishman et al., 2000; Liu et al., 2013). 2. Materials and methods 2.1. Materials Commercial native corn starch with an amylose content of 26.3% was purchased from Zhucheng Xingmao Corn Development Co., Ltd., (Shandong, China). HPLC-grade fluorescein 5-isothiocyanate (FITC) and analytical-grade glycerol were purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). 2.2. Starch paste preparation Starch paste was prepared for CLSM and SEM observations as follows. Glycerol was mixed with distilled water in various ratios: 5/95, 10/90, 20/80, and 50/50 (glycerol/water, w/w). Then, dried starch samples were mixed with the glycerol–water mixture at a specific percentage of 10.0% (w/w) in a conical flask. One hundred grams of the 10.0% (w/w) starch suspensions was stirred at a paddle speed of 160 rpm. The starch suspension was heated from room temperature to a preset temperature (65 °C, 70 °C, 75 °C, 80 °C, 85 °C, and 90 °C) in an oil bath. The suspension was maintained at the preset temperature for 30 min. After this duration, the starch paste was poured into a beaker for further testing. 2.3. Confocal laser scanning microscopy The changing trends of starch granules during heat treatment was observed using an Olympus FV10 (Tokyo, Japan) confocal laser scanning microscope equipped with crossed polarizers. A stock solution of fluorescein 5-isothiocyanate (FITC) was prepared by dissolving 0.2 g of FITC in
100 mL of distilled water. Starch paste (100 µL) was stained by mixing with 20 µL of FITC stock solution (Chen et al., 2017; Zhou et al., 2015). A drop of stained starch paste (approximately 10 µL) was deposited onto a concave slide and observed within 15 min. The excitation wavelength was 488 nm, and the emission maxima were within 500–525 nm (Nagano, Tamaki, & Funami, 2008; Zhou et al., 2014). The same field was then viewed under polarized light microscopy. Each line was scanned four times and averaged to reduce noise. 2.4. Scanning electron microscopy (SEM) The morphologies of dried paste were observed and photographed using a Hitachi S-4800 scanning electron microscope (Tokyo, Japan). The prepared starch paste was stored in an air oven at 25 °C to be dried into a solid state, followed by drying at 105 °C for 10 h. The dried starch paste was affixed to a specimen holder using an aluminum plate and was subsequently coated with gold in a vacuum evaporator. The cross-sections were then observed by SEM operated at an accelerating voltage of 1.0 kV. 2.5. Differential scanning calorimetry Thermal properties of starch–glycerol-water mixtures were conducted using a PerkinElmer DSC Diamond-8000 (Shanghai, China) equipped with a refrigerated cooling system. Native corn starch was mixed with the prepared glycerol-water mixture (0/100, 5/95, 10/90, 20/80, and 50/50, glycerol/water, w/w) at the specific percentage of 10% in a glass vial. Thereafter, the components (approximately 8 mg) were transferred to the aluminum pan, then sealed and stored for 24 h for the water to reach equilibrium. The DSC samples were heated from 40 °C to 110 °C at a rate of 10 °C/min with an empty aluminum pan as a reference. Each test was performed three times. 2.6. Rapid Visco-Analyzer Pasting viscosity characteristics of starch-glycerol-water mixtures were determined using a Rapid Viscosity Analyzer (RVA-TecMaster, Perten, Sweden), and according to the method described by Qiu et al. (2016) with some modification. Three gram dried corn starches were added to an aluminum canister containing different glycerol-water ratios to make a total weight of 28.0 g. The suspensions were kept at room temperature for 10 min to reach equilibrium. The heating and cooling cycles of RVA were controlled using “Standard method 1” thermal program offered by the supplier. The suspensions were held at 50 °C for 50 s, and then raised to 95 °C within 225 s. They
were maintained at 95 °C for 150 s, cooled to 50 °C at the same rate, and held at 50 °C for 120 s to develop the final paste viscosity. The rotation speed of the plastic paddle was at 960 rpm for the first 10 s and then maintained at 160 rpm. All measurements were done in triplicate, and the pasting viscosity parameters were obtained. 2.7. Statistical analysis All experiments were conducted at least in triplicate, for which mean values and standard errors were determined. Analysis of variance (ANOVA) was used to determine significant differences between the results, and Duncan’s test was used to separate the mean with a significance level of 0.05. 3. Results and discussion 3.1. Morphological properties of corn starch CLSM was used to study the effects of glycerol concentrations (0%, 5%, 10%, 20%, and 50%, w/w) on granule morphologies and gelatinization behaviours of corn starch (Figs. 1-4). The native corn starch granules (Fig. 1A) appeared to be crannied in an elliptical or circular shape, except for some irregular larger granules. Before being heated, the corn starch granules with typical Maltese cross shapes were observed under polarized light microscopy, as shown in Fig. 1A2. Starch is composed of crystalline parts of linear amylose and highly branched amylopectin. Under polarized light microscopy, the birefringence due to this crystallinity results in a Maltese cross pattern for virgin corn starch. As seen in Fig. 1B, when corn starch granules were treated at 65 °C, the granules were almost completely broken and tightly connected and the characteristic birefringence of the starch granules disappeared. The phenomena observed here are in agreement with previous reports that normal corn starch starts to crack at approximately 60 °C and completely cracked at approximately 75 °C (Nagano, Tamaki, & Funami, 2008). However, an obvious difference in the morphologies and profiles between Fig. 1B and Fig. 1C was observed, illustrating the states when the starch granules were heated in water or in 5% glycerol concentration at 65 °C, respectively. The typical Maltese crosses disappeared and significant swelling occurred when the temperature was increased from 65 °C to 70 °C (Fig. 1D). The presence of glycerol appears to delay the granule swelling during the temperature increase, likely competing for the available water and
thus hampering the granule-to-granule interactions and requiring higher temperature input to dissociate completely (Liu et al., 2011; Tan et al., 2014). Subsequently, when the glycerol concentration was increased to 10%, all of the observed granules (Fig. 2A) exhibited typical Maltese crosses under polarized light microscopy, implying that this kind of starch maintained the original semi-crystalline structure at 65 °C. In addition, when compared with native corn starch, granules started to swell immediately before the loss of crystallinity. Nevertheless, Figs. 2B-D indicate that the starch granules were gradually destroyed, accompanied by the absence of Maltese crosses as the temperature was increased, indicating that the gelatinization occurred in the temperature range from 70 to 75 °C. A consistent phenomenon is observed with a glycerol concentration of 20%, exactly as shown in Fig. 3 where the starch Polarized light microscopy
CLSM
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Fig. 1. CLSM images of native corn starch and corn starch with a glycerol concentration of 0% or 5% after being heated to specific temperatures.
Polarized light microscopy
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Fig. 2. CLSM images of corn starch with a glycerol concentration of 10% after being heated to specific temperatures.
Polarized light microscopy
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Fig. 3. CLSM images of corn starch with glycerol concentration of 20% after being heated to specific temperatures.
CLSM
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Fig. 4. CLSM images of corn starch with glycerol concentration of 50% after being heated to specific temperatures.
granules were observed to swell slightly at 65 °C, to swell remarkably at 70 °C and to gelatinize completely at 80 °C. Similarly, the starch exhibits strong resistance to being hydrolyzed or disintegrated when the glycerol concentration was further increased to 50% (Fig. 4). Figs. 4A-B show no discernible changes of the Maltese crosses for the starch paste, which indicates that corn starch maintained its original structure above 70 °C, consistent with the previous study involving the presence of glycerol delaying granule swelling. Notably, the starch granules swelled remarkably at approximately 80 °C and broke down at 90 °C. The aforementioned phenomena revealed that starch gelatinization was displaced to higher temperatures as the glycerol concentration increased. These results are quite similar to both the conventional DSC (Section 3.2) and the RVA (Section 3.3) results presented in this study.
SEM images of the surfaces and cross-sections of unheated native corn starch and heated starch paste loaded with corn starch at a concentration of 10% are displayed in Fig. 5. Native corn starch granules with diameters ranging from 1 to 15 µm appear to be smooth in an angular or irregular shape, consistent with previous reports by Teng, Chin, and Yusof (2013). As shown in Fig. 5A2, water molecules were interspersed to form a uniform paste in which the starch granules were embedded, indicating that the native starch granules without glycerol were completely gelatinized at 65 °C. Fig. 5A3 exhibits a relatively coarse surface with many starch granules protruding on the surface, whereas Fig. 5A4 shows a smooth surface. The aforementioned phenomena reveal that the glycerol concentration of 5% increased the gelatinization temperature slightly. When the glycerol concentration was increased to 10%, similar coarse surfaces were observed in the SEM images shown in Fig. 5B1 (65 °C) and Fig. 5B2 (70 °C), which corresponds well with previous CLSM observations that the gelatinization of corn starch occurs at a temperature range of 70 to 75 °C. Indeed, the granules of native corn starch with a 10% glycerol concentration are almost completely broken following the heat treatment at 75 and 80 °C. Similarly, as evident in Fig. 5C1 and Fig. 5C2 for 20% glycerol concentration, the size and morphology of starches observed here were in agreement with the previously described native corn starches, suggesting no significant changes in the starch granules at temperatures under 70 °C. When the temperature was further increased to 75 °C (Fig. 5C), the residual starch granules demonstrated a significant degree of gelatinization in the glycerol-water matrix attributed to a large number of granules being gelatinized into paste. Similarly, with 20% glycerol concentration, the granules were completely broken following the heating, consistent with the previous polarized light microscopy observation that the typical Maltese crosses disappeared completely. Further analysis of the glycerol concentration at 50% is illustrated in Fig. 5D. As shown in Figs. 5D1-D2, the granules of corn starch maintained their original structure at 65 °C and exhibited a degraded semi-crystalline structure beginning at 75 °C. Compared with starch paste loaded with 20% glycerol (Fig. 5C), that loaded with 50% glycerol (Fig. 5D2) showed an increased presence of starch granules in its cross-section, with the granules arranged in close proximity at 75 °C. Instead, the swollen starch granules in Fig. 5D3 demonstrate a certain degree of integration with the starch paste without a clear identifiable boundary, which was attributed to partial gelatinization. Finally, when the temperature was further increased to 90 °C, starch
granules could barely be discerned in the SEM field. Combined with polarized light microscopy observations, SEM observations of the starch granules treated at 90 °C confirmed that the granules were substantially destroyed. These morphological changes are likely related to their pasting temperatures and the additive content of glycerol. In brief, starch gelatinization was delayed to
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Fig. 5. SEM images of corn starches with various glycerol concentrations after the samples were heated to specific temperatures: A1-A2 (0% glycerol): Native corn starch, 65 °C; A3-A4 (5% glycerol): 65 °C, 70 °C; B1-B4 (10% glycerol): 65 °C, 70 °C, 75 °C, 80 °C; C1-C4 (20% glycerol): 65 °C, 70 °C, 75 °C, 80 °C; and D1-D4 (50% glycerol): 65 °C, 75 °C, 85 °C, 90 °C.
higher temperatures with increasing glycerol concentration in these glycerol–water systems. This phenomenon can be explained by the fact that the most important structural element building the starch crystalline structure is the starch–starch hydrogen bonding, and glycerol shows strong hydrogen bonding with water which results in its higher water uptake level compared with pure starch (Liu et al., 2011; Nashed et al., 2003). Consequently, more thermal energy would be required to mobilize water molecules to penetrate amongst starch chains for disruption of the existing crystalline structure (Taghizadeh & Favis, 2013). 3.2. Thermal properties of starch–glycerol mixtures
DSC has been considered as an extremely valuable method to quantify transition temperatures and it is possible to provide the parameters of the order-disorder transition occurring in semi-crystalline starch granules during gelatinization. In addition to a slight increase in the peak and conclusion temperatures for the 5% glycerol concentration, thermal analysis revealed substantial differences in the behavior of starch between water and glycerol–water systems. Fig. 6 shows a single endothermic peak for corn starch in glycerol–water systems; this peak was attributed to the gelatinization of amylopectin, and its position shifted toward higher temperatures with increasing glycerol concentration (Liu et al., 2006). The transition temperatures corresponding to the peak temperature (Tp), as determined by DSC, were 65.6 °C, 67.2 °C, 73.0 °C, 76.3 °C, and 85.3 °C, respectively. In the presence of glycerol–water systems (5/95, 10/90, 20/80, and 50/50, glycerol/water, w/w), the peak temperatures of the corn starches increased by 1.6 °C, 7.4 °C, 10.7 °C, and 19.7 °C, respectively, compared to those in solely water. Not surprisingly, glycerol is commonly considered less effective than water for the phase transition of starch because of its higher molecular weight and viscosity (Liu et al., 2011). Our results are consistent with previous polarized light microscopy observations, effectively confirming the previous microscopic observations that starch gelatinization is delayed to higher temperatures with increasing glycerol concentration. It is proposed that glycerol acts as an anti-plasticizer in the gelatinization process and increases the water content required for complete gelatinization because it actually decreases the amount of water molecules available to penetrate the starch chains (Perry & Donald, 2000; Taghizadeh & Favis, 2013). Hence, more thermal energy would be required to mobilize water molecules to dissociate starch chains, and consequently gelatinization will proceed at higher temperature (Perry & Donald, 2000; Perry & Donald, 2002). Notably, however, in view of a glycerol concentration of 50%, a higher temperature (90 °C) can also enable water molecules to facilitate the dissociation of amylopectin double helices and the unwinding helix to coil transitions, as evidenced by the absence of Maltese crosses in the image in Fig. 4D2 (Liu et al., 2011; Tan et al., 2004).
Heat flow endo down
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Fig. 6. DSC thermograms of corn starches with various glycerol concentrations.
3.3. Pasting properties The pasting curves of native corn starch with different glycerol concentrations, as determined using an RVA, are shown in Fig. 7. During the period of heating to and holding at 95 °C, substantial differences were observed in the pasting profiles of corn starch with glycerol concentrations ranging from 0% to 50%. All of the starch–glycerol mixtures exhibited an increase in viscosity during the heating process, which was mainly attributed to the swelling of starch granules, as explained by Karim et al. (2008). 7500
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Time (s) Fig. 7. Pasting profiles of native corn starch with different glycerol concentrations.
The results in Fig. 7 reveal that increasing the glycerol concentration resulted in similar trends as those observed in polarized light microscopic observations (Figs. 1-4) and DSC experiments (Fig. 6); i.e., the gelatinization temperature increases with increasing glycerol concentration.
According to Habitante et al. (2008), although trends in the published results are sometimes contradictory, increasing the glycerol concentration increases the gelatinization temperature. The pasting properties of corn starch with various glycerol concentrations are summarized in Table 1. A gradual increase in peak viscosity (PV) and breakdown viscosity (BV) but a slow decrease in setback viscosity (SV) with an increase in glycerol concentrations occurred. PV for corn starch with various glycerol concentrations ranged between 2870 cP and 6319 cP, corresponding to 0% for the lowest and 50% for the highest. Similarly, the BV was found to be 0% for the lowest and 50% for the highest, respectively. The SV for corn starch with various glycerol concentrations ranged from 1270 cP to 1083 cP, where the highest value corresponds 0% glycerol concentration and the lowest corresponds to 50% glycerol concentration. Huang et al. (2015) proposed that corn starch with a low amylose content should be assumed to exhibit a high pasting viscosity and a low pasting temperature. The hydrogen bonds between the starch chains are disrupted, resulting in weaker granules, increasing breakdown and leading to the rapid loss of PV (Woggum, Sirivongpaisal, & Wittaya, 2015). In addition, the hydrogen bonding of glycerol molecules hindered the formation of crystallites, leading to a decrease of setback (Luo, Li, & Lin, 2012). Table 1 Pasting properties of native corn starch with various glycerol concentrations (w/w): 0%, 5%, 10%, 20%, and 50%. Glycerol
concentration
Viscosity (cP)
( w/w)
Peak
Breakdown
Setback
0%
2870±15
1140±23
1270±28
5%
3402±7
1479±9
1252±14
10%
3691±4
1638±11
1214±13
20%
4788±19
2158±24
1180±20
50%
6319±16
2230±25
1083±33
Data are expressed as the means ± standard deviations in triplicate; means were significantly different at the 5% level.
4. Conclusion The effects of various glycerol concentrations (0%, 5%, 10%, 20%, and 50%, w/w) on the morphologies and gelatinization behaviours of corn starches were investigated. When corn starch granules were treated at 65 °C, the granules of corn starch were almost completely broken and the
characteristic birefringence of the starch granules disappeared. Various microscopic techniques revealed that starch gelatinization was delayed to higher temperatures with increasing concentration of glycerol. DSC results showed that the peak temperature of corn starches increased by 1.6 °C, 7.4 °C, 10.7 °C, and 19.7 °C in samples with glycerol concentrations of 0%, 5%, 10%, 20%, and 50%, w/w, respectively, compared to systems with water as the sole solvent. The RVA pasting profiles revealed that increasing the glycerol concentration resulted in similar trends as those observed in polarized light microscopic observations and DSC experiments; i.e., the gelatinization temperature increases with increasing glycerol concentration. In brief, glycerol acts as an anti-plasticizer in the gelatinization process and increases the water content required for complete gelatinization because it actually decreases the number of water molecules available to penetrate among the starch chains. Acknowledgement The authors would like to thank the National Natural Science Foundation of China (grant no. 31471700 and 31371735) for financial support. References Acosta, S., Chiralt, A., Santamarina, P., Rosello, J. M., González-Martínez, C., Cháfer, M. (2016). Antifungal films based on starch-gelatin blend, containing essential oils. Food Hydrocolloids, 61, 233-240. Chen, X., Du, X. F., Chen, P. R., Guo, L., Xu, Y., Zhou X. H. (2017). Morphologies and gelatinization behaviours of high-amylose maize starches during heat treatment. Carbohydrate Polymers, 157, 637-642. Farahnaky, A., Saberi, B., Majzoobi, M. (2013). Effect of glycerol on physical and mechanical properties of wheat starch edible films. Journal of Texture Studies, 44, 176-186. Fishman, M. L., Coffin, D. R., Konstance, R. P., Onwulata, C. I. (2000). Extrusion of pectin/starch blends plasticized with glycerol. Carbohydrate Polymers, 41, 317-325. Guo, L., Hu, J., Zhang, J. J., Du, X. F. (2016). The role of entanglement concentration on the hydrodynamic properties of potato and sweet potato starches. International Journal of Biological Macromolecules, 93, 1-8. Habitante, A. M. B. Q., Sobral, P. J. A., Carvalho, R. A., Solorza-Feria, J. & Bergo, P. V. A. (2008). Phase transitions of cassava starch dispersions prepared with glycerol solutions. Journal of Thermal Analysis and Calorimetry, 93, 599-604. Huang, J., Shang, Z. Q., Man, J. M., Liu, Q. Q., Zhu, C. J., Wei, C. X. (2015). Comparison of molecular structures and functional properties of high-amylose starches from rice transgenic line and commercial maize. Food Hydrocolloids, 46, 172-179. Karim, A. A., Nadiha, M. Z., Chen, F. K., Phuah, Y. P., Chui, Y. M., & Fazilah, A. (2008). Pasting and retrogradation properties of alkali-treated sago (Metroxylon sagu) starch. Food Hydrocolloids, 22(6), 1044-1053. Koch, K., Gillgren, T., Stading, M., Andersson, R. (2010). Mechanical and structural properties of solution-cast high-amylose maize starch films. International Journal of Biological Macromolecules, 46, 13-19.
Liu, H. H., Adhikari, R., Guo, Q. P., Adhikari, B. (2013). Preparation and characterization of glycerol plasticized (high-amylose) starch-chitosan films. Journal of Food Engineering, 116, 588-597. Liu, H. S., Yu, L., Xie, F. W., Chen, L. (2006). Gelatinization of cornstarch with different amylose/amylopectin content. Carbohydrate Polymers, 65, 357-363. Liu, P., Xie, F., Li, M., Liu, X., Yu, L., Halley, P. J., et al. (2011). Phase transitions of maize starches with different amylose contents in glycerol-water systems. Carbohydrate Polymers, 85,180-187. Luo, X. G., Li, J. W., Lin, X. Y. (2012). Effect of gelatinization and additives on morphology and thermal behavior of corn starch/PVA blend films. Carbohydrate Polymers, 90, 1595-1600. Nagano, T., Tamaki, E., & Funami, T. (2008). Influence of guar gum on granule morphologies and rheological properties of maize starch. Carbohydrate Polymers, 72, 95-101. Nashed, G., Rutgers, P. P. G., & Sopade, P. A. (2003). The plasticisation effect of glycerol and water on the gelatinisation of wheat starch. Starch-Starke, 55(3-4), 131-137. Perry, P. A., & Donald, A. M. (2000). The role of plasticization in starch granule assembly. Biomacromolecules, 1(3), 424-432. Perry, P. A., & Donald, A. M. (2002). The effect of sugars on the gelatinisation of starch. Carbohydrate Polymers, 49(2), 155-165. Qiu, S., Yadav, M. P., Liu, Y., Chen, H., Tatsumi, E., & Yin, L. J. (2016). Effects of corn fiber gum with different molecular weights on the gelatinization behaviours of corn and wheat starch. Food Hydrocolloids, 53, 180-186. Sandhu, K. S., Singh, N., & Malhi, N. S. (2005). Physicochemical and thermal properties of starches separated from corn produced from crosses of two germ pools. Food Chemistry, 89/4, 541-548. Schirmer, M., Höchstötter, A., Jekle, M., Arendt, E., Becker, T. (2013). Physicochemical and morphological characterization of different starches with variable amylose/amylopectin ratio. Food Hydrocolloids, 32, 52-63. Seligra, P. G., Jaramillo, C. M., Fama, L., Goyanes, S. (2016). Biodegradable and non-retrogradable eco-films based on starch-glycerol with citric acid as crosslinking agent. Carbohydrate Polymers, 138, 66-74. Singh, N., Singh, J., Kaur, L., Sodhi, N. S., & Gill, B. S. (2003). Morphological, thermal and rheological properties of starches from different botanical sources. Food Chemistry, 81, 219-231. Sun, Q. J., Xing, Y,. Qiu, C., Xiong, L. (2014). The pasting and gel textural properties of corn starch in glucose, fructose and maltose syrup. The Properties of Corn Starch in Syrup, 9, 1-6. Taghizadeh, A., Favis, B. D. (2013). Effect of high molecular weight plasticizers on the gelatinization of starch under static and shear conditions. Carbohydrate Polymers, 92, 1799-1808. Tan, I., Wee, C. C., Sopade, P. A., Halley, P. J. (2004). Investigation of the starch gelatinization phenomena in water-glycerol systems: application of modulated temperature differential scanning calorimetry. Carbohydrate Polymers, 58, 191-204. Teng, L.Y., Chin, N. L., Yusof, Y. A. (2013). Rheological and textural studies of fresh and freeze-thawed native sago starchsugar gels. II. Comparisons with other starch sources and reheating effects. Food Hydrocolloids, (31), 156-165. Valencia, G. A., Henao, A. C., Zapata, R. A. V. (2013). Effect of glycerol concentration and temperature on the rheological properties of cassava starch solutions. Journal of Polymer Engineering, 33(2), 141-148. Woggum, T., Sirivongpaisal, P., Wittaya, T. (2015). Characteristics and properties of hydroxypropylated rice starch based biodegradable films. Food Hydrocolloids, 50, 54-64. Zhao, D. D., Palaparthi, A. D., Huang, Q. R., Fu, X., Liu, H. S., Yu, L. (2015). Effects of ionic liquid 1-allyl-3-methylimidazolium chloride treatment on the microstructure and phase transition of cornstarch. Industrial Crops and Products, 77, 139-145.
Zhou, Y., Winkworth-Smith, C. G., Wang, Y., Liang, J. F., Foster, T. J., & Cheng, Y. Q. (2014). Effect of a small amount of sodium carbonate on konjac glucomannan-induced changes in thermal behavior of wheat starch. Carbohydrate Polymers, 114, 357-364. Zhou, Y., Zhao, D., Winkworth-Smith, C. G., Foster, T. J., Nirasawa, S., Tatsumi, E., et al. (2015). Effect of a small amount of sodium carbonate on konjac glucomannan-induced changes in wheat starch gel. Carbohydrate Polymers, 116, 182-188.
S0144-8617(17)30938-4 http://dx.doi.org/10.1016/j.carbpol.2017.08.062 CARP 12669
To appear in: Received date: Revised date: Accepted date:
18-3-2017 3-7-2017 11-8-2017
Please cite this article as: {http://dx.doi.org/ This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of glycerol concentration on corn starch morphologies and gelatinization behaviours during heat treatment Xu Chena, Li Guoa, Xianfeng Dua,*, Peirong Chenb,* Yishun Jic, Huili Haoa, Xiaonan Xua a School b
of Tea & Food Science and Technology, Anhui Agricultural University, Hefei 230036, China
Department of Applied Chemistry, School of Science, Anhui Agricultural University, Hefei 230036, China
c Anhui
province grain and oil products quality supervision and inspection station, Hefei 230031, China
Highlights
When corn starch granules with no added glycerol were treated at 65 °C, the characteristic birefringence of the starch granules disappeared.
Various microscopic techniques revealed that starch gelatinization was delayed to higher temperatures as the glycerol concentration increased.
In the presence of glycerol-water systems (5%, 10%, 20%, and 50%, w/w), the peak temperature (Tp), were 67.2 °C, 73.0 °C, 76.3 °C, and 85.3 °C, respectively.
The RVA pasting profiles showed that the gelatinization temperature increased as the glycerol concentration increased.
Abstract The effects of various glycerol concentrations (0%, 5%, 10%, 20%, and 50%, w/w) on the morphologies and gelatinization behaviours of corn starch were evaluated by confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and rapid visco-analyzer (RVA). When corn starch granules with no added glycerol were treated at 65 °C, the granules of corn starch were almost completely broken and tightly connected, and the characteristic birefringence of the starch granules disappeared. Various microscopic techniques revealed that starch gelatinization was delayed to higher temperatures as the glycerol concentration increased. In the presence of glycerol-water systems (5%, 10%, 20%, and 50%, w/w), the peak temperatures of corn starch increased by 1.6 °C, 7.4 °C, 10.7 °C, and 19.7 °C, respectively, compared to corn starch in water. The RVA pasting profiles showed that the gelatinization temperature increased as the glycerol concentration increased, which was consistent with polarized light microscope observations and DSC tests. Keywords: Corn starch; Gelatinization behaviours; Glycerol; Maltese crosses; Viscosity
1. Introduction Starch is recognized as the dominant carbohydrate reserve material of higher plants, with
excellent physicochemical properties in addition to being biodegradable, non-toxic and produced at low cost, thus allowing sustainable development in different industries in which it is used as a raw material (Koch et al., 2010; Seligra et al., 2016; Valencia, Henao, & Zapata, 2013; Zhao et al., 2015). Corn starch is a widely available ingredient in the food industry, where it is used as a thickener, gelling agent, bulking agent and water retention agent (Acosta et al., 2016; Singh et al., 2003). Although starch is used in many fields of the food industry, in almost all applications, starch must be gelatinized before use (Schirmer et al., 2013). * Corresponding author at: NO.130, Western Changjiang Road, Anhui Agricultural University, Hefei City, Anhui Province 230036, China. Tel.: +86 551 65786965; fax: +86 551 65786982. E-mail addresses: [email protected]; [email protected].
The swelling of the starch granules begins after completion of the first stage of gelatinization. Further heating leads to the helix–coil transition associated with the unwinding of amylopectin double helices and loss of the starch granular birefringence (Liu et al., 2011). The viscosity parameters during pasting are cooperatively controlled by the properties of the swollen granules and the soluble materials leached from the granules (Guo et al., 2016; Sandhu, Singh, & Malhi, 2005). The increase in viscosity has been ascribed to the swelling of the starch granules as they absorb water until they gradually burst (Sun et al., 2014). The gelatinization process is essential in that it determines the proper conversion of starch in the processing of foods and emerging biodegradable starch-based materials (Liu et al., 2011; Luo, Li, & Lin, 2012). For starch with excess water, a single gelatinization endotherm can be usually observed in the low temperature range (54–73 °C) (Liu et al., 2011; Perry & Donald, 2000). The crystalline starch structure is lost when it is subjected to heat at temperatures greater than 70– 90 °C in the presence of plasticizers such as glycerol (Taghizadeh & Favis, 2013). Glycerol is an additive used in the production of biodegradable packaging films which can generate other desirable properties in hybrid materials (Farahnaky, Saberi, & Majzoobi, 2013; Valencia, Henao, & Zapata, 2013). Taghizadeh and Favis (2013) visualized the loss of birefringence with glycerol and used an optical microscope technique to provide evidence for the occurrence of starch gelatinization under static conditions. A large number of studies have been carried out to investigate the effect of glycerol on starch gelatinization using differential scanning calorimetry (DSC) (Liu et al., 2011; Tan et al., 2004). To the best of our knowledge, the literature contains few reports of the visualization of gelatinized starch with various glycerol concentrations
(0-50%) and little analysis of thermal changes and pasting properties. Accordingly, the aim of this study was to investigate the effect of various glycerol concentrations on the gelatinization behaviours of corn starch. Multiple methods, including confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and rapid visco-analyzer (RVA) were applied, and the independent evidence derived from those methods provide a comprehensive understanding of the effect of various glycerol concentrations on the gelatinization behaviours of corn starch. It can be highly instructive in the exploration of biodegradable films and the applications of glycerol, especially in plasticizers (Fishman et al., 2000; Liu et al., 2013). 2. Materials and methods 2.1. Materials Commercial native corn starch with an amylose content of 26.3% was purchased from Zhucheng Xingmao Corn Development Co., Ltd., (Shandong, China). HPLC-grade fluorescein 5-isothiocyanate (FITC) and analytical-grade glycerol were purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). 2.2. Starch paste preparation Starch paste was prepared for CLSM and SEM observations as follows. Glycerol was mixed with distilled water in various ratios: 5/95, 10/90, 20/80, and 50/50 (glycerol/water, w/w). Then, dried starch samples were mixed with the glycerol–water mixture at a specific percentage of 10.0% (w/w) in a conical flask. One hundred grams of the 10.0% (w/w) starch suspensions was stirred at a paddle speed of 160 rpm. The starch suspension was heated from room temperature to a preset temperature (65 °C, 70 °C, 75 °C, 80 °C, 85 °C, and 90 °C) in an oil bath. The suspension was maintained at the preset temperature for 30 min. After this duration, the starch paste was poured into a beaker for further testing. 2.3. Confocal laser scanning microscopy The changing trends of starch granules during heat treatment was observed using an Olympus FV10 (Tokyo, Japan) confocal laser scanning microscope equipped with crossed polarizers. A stock solution of fluorescein 5-isothiocyanate (FITC) was prepared by dissolving 0.2 g of FITC in
100 mL of distilled water. Starch paste (100 µL) was stained by mixing with 20 µL of FITC stock solution (Chen et al., 2017; Zhou et al., 2015). A drop of stained starch paste (approximately 10 µL) was deposited onto a concave slide and observed within 15 min. The excitation wavelength was 488 nm, and the emission maxima were within 500–525 nm (Nagano, Tamaki, & Funami, 2008; Zhou et al., 2014). The same field was then viewed under polarized light microscopy. Each line was scanned four times and averaged to reduce noise. 2.4. Scanning electron microscopy (SEM) The morphologies of dried paste were observed and photographed using a Hitachi S-4800 scanning electron microscope (Tokyo, Japan). The prepared starch paste was stored in an air oven at 25 °C to be dried into a solid state, followed by drying at 105 °C for 10 h. The dried starch paste was affixed to a specimen holder using an aluminum plate and was subsequently coated with gold in a vacuum evaporator. The cross-sections were then observed by SEM operated at an accelerating voltage of 1.0 kV. 2.5. Differential scanning calorimetry Thermal properties of starch–glycerol-water mixtures were conducted using a PerkinElmer DSC Diamond-8000 (Shanghai, China) equipped with a refrigerated cooling system. Native corn starch was mixed with the prepared glycerol-water mixture (0/100, 5/95, 10/90, 20/80, and 50/50, glycerol/water, w/w) at the specific percentage of 10% in a glass vial. Thereafter, the components (approximately 8 mg) were transferred to the aluminum pan, then sealed and stored for 24 h for the water to reach equilibrium. The DSC samples were heated from 40 °C to 110 °C at a rate of 10 °C/min with an empty aluminum pan as a reference. Each test was performed three times. 2.6. Rapid Visco-Analyzer Pasting viscosity characteristics of starch-glycerol-water mixtures were determined using a Rapid Viscosity Analyzer (RVA-TecMaster, Perten, Sweden), and according to the method described by Qiu et al. (2016) with some modification. Three gram dried corn starches were added to an aluminum canister containing different glycerol-water ratios to make a total weight of 28.0 g. The suspensions were kept at room temperature for 10 min to reach equilibrium. The heating and cooling cycles of RVA were controlled using “Standard method 1” thermal program offered by the supplier. The suspensions were held at 50 °C for 50 s, and then raised to 95 °C within 225 s. They
were maintained at 95 °C for 150 s, cooled to 50 °C at the same rate, and held at 50 °C for 120 s to develop the final paste viscosity. The rotation speed of the plastic paddle was at 960 rpm for the first 10 s and then maintained at 160 rpm. All measurements were done in triplicate, and the pasting viscosity parameters were obtained. 2.7. Statistical analysis All experiments were conducted at least in triplicate, for which mean values and standard errors were determined. Analysis of variance (ANOVA) was used to determine significant differences between the results, and Duncan’s test was used to separate the mean with a significance level of 0.05. 3. Results and discussion 3.1. Morphological properties of corn starch CLSM was used to study the effects of glycerol concentrations (0%, 5%, 10%, 20%, and 50%, w/w) on granule morphologies and gelatinization behaviours of corn starch (Figs. 1-4). The native corn starch granules (Fig. 1A) appeared to be crannied in an elliptical or circular shape, except for some irregular larger granules. Before being heated, the corn starch granules with typical Maltese cross shapes were observed under polarized light microscopy, as shown in Fig. 1A2. Starch is composed of crystalline parts of linear amylose and highly branched amylopectin. Under polarized light microscopy, the birefringence due to this crystallinity results in a Maltese cross pattern for virgin corn starch. As seen in Fig. 1B, when corn starch granules were treated at 65 °C, the granules were almost completely broken and tightly connected and the characteristic birefringence of the starch granules disappeared. The phenomena observed here are in agreement with previous reports that normal corn starch starts to crack at approximately 60 °C and completely cracked at approximately 75 °C (Nagano, Tamaki, & Funami, 2008). However, an obvious difference in the morphologies and profiles between Fig. 1B and Fig. 1C was observed, illustrating the states when the starch granules were heated in water or in 5% glycerol concentration at 65 °C, respectively. The typical Maltese crosses disappeared and significant swelling occurred when the temperature was increased from 65 °C to 70 °C (Fig. 1D). The presence of glycerol appears to delay the granule swelling during the temperature increase, likely competing for the available water and
thus hampering the granule-to-granule interactions and requiring higher temperature input to dissociate completely (Liu et al., 2011; Tan et al., 2014). Subsequently, when the glycerol concentration was increased to 10%, all of the observed granules (Fig. 2A) exhibited typical Maltese crosses under polarized light microscopy, implying that this kind of starch maintained the original semi-crystalline structure at 65 °C. In addition, when compared with native corn starch, granules started to swell immediately before the loss of crystallinity. Nevertheless, Figs. 2B-D indicate that the starch granules were gradually destroyed, accompanied by the absence of Maltese crosses as the temperature was increased, indicating that the gelatinization occurred in the temperature range from 70 to 75 °C. A consistent phenomenon is observed with a glycerol concentration of 20%, exactly as shown in Fig. 3 where the starch Polarized light microscopy
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Fig. 1. CLSM images of native corn starch and corn starch with a glycerol concentration of 0% or 5% after being heated to specific temperatures.
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Fig. 2. CLSM images of corn starch with a glycerol concentration of 10% after being heated to specific temperatures.
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Fig. 3. CLSM images of corn starch with glycerol concentration of 20% after being heated to specific temperatures.
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Fig. 4. CLSM images of corn starch with glycerol concentration of 50% after being heated to specific temperatures.
granules were observed to swell slightly at 65 °C, to swell remarkably at 70 °C and to gelatinize completely at 80 °C. Similarly, the starch exhibits strong resistance to being hydrolyzed or disintegrated when the glycerol concentration was further increased to 50% (Fig. 4). Figs. 4A-B show no discernible changes of the Maltese crosses for the starch paste, which indicates that corn starch maintained its original structure above 70 °C, consistent with the previous study involving the presence of glycerol delaying granule swelling. Notably, the starch granules swelled remarkably at approximately 80 °C and broke down at 90 °C. The aforementioned phenomena revealed that starch gelatinization was displaced to higher temperatures as the glycerol concentration increased. These results are quite similar to both the conventional DSC (Section 3.2) and the RVA (Section 3.3) results presented in this study.
SEM images of the surfaces and cross-sections of unheated native corn starch and heated starch paste loaded with corn starch at a concentration of 10% are displayed in Fig. 5. Native corn starch granules with diameters ranging from 1 to 15 µm appear to be smooth in an angular or irregular shape, consistent with previous reports by Teng, Chin, and Yusof (2013). As shown in Fig. 5A2, water molecules were interspersed to form a uniform paste in which the starch granules were embedded, indicating that the native starch granules without glycerol were completely gelatinized at 65 °C. Fig. 5A3 exhibits a relatively coarse surface with many starch granules protruding on the surface, whereas Fig. 5A4 shows a smooth surface. The aforementioned phenomena reveal that the glycerol concentration of 5% increased the gelatinization temperature slightly. When the glycerol concentration was increased to 10%, similar coarse surfaces were observed in the SEM images shown in Fig. 5B1 (65 °C) and Fig. 5B2 (70 °C), which corresponds well with previous CLSM observations that the gelatinization of corn starch occurs at a temperature range of 70 to 75 °C. Indeed, the granules of native corn starch with a 10% glycerol concentration are almost completely broken following the heat treatment at 75 and 80 °C. Similarly, as evident in Fig. 5C1 and Fig. 5C2 for 20% glycerol concentration, the size and morphology of starches observed here were in agreement with the previously described native corn starches, suggesting no significant changes in the starch granules at temperatures under 70 °C. When the temperature was further increased to 75 °C (Fig. 5C), the residual starch granules demonstrated a significant degree of gelatinization in the glycerol-water matrix attributed to a large number of granules being gelatinized into paste. Similarly, with 20% glycerol concentration, the granules were completely broken following the heating, consistent with the previous polarized light microscopy observation that the typical Maltese crosses disappeared completely. Further analysis of the glycerol concentration at 50% is illustrated in Fig. 5D. As shown in Figs. 5D1-D2, the granules of corn starch maintained their original structure at 65 °C and exhibited a degraded semi-crystalline structure beginning at 75 °C. Compared with starch paste loaded with 20% glycerol (Fig. 5C), that loaded with 50% glycerol (Fig. 5D2) showed an increased presence of starch granules in its cross-section, with the granules arranged in close proximity at 75 °C. Instead, the swollen starch granules in Fig. 5D3 demonstrate a certain degree of integration with the starch paste without a clear identifiable boundary, which was attributed to partial gelatinization. Finally, when the temperature was further increased to 90 °C, starch
granules could barely be discerned in the SEM field. Combined with polarized light microscopy observations, SEM observations of the starch granules treated at 90 °C confirmed that the granules were substantially destroyed. These morphological changes are likely related to their pasting temperatures and the additive content of glycerol. In brief, starch gelatinization was delayed to
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Fig. 5. SEM images of corn starches with various glycerol concentrations after the samples were heated to specific temperatures: A1-A2 (0% glycerol): Native corn starch, 65 °C; A3-A4 (5% glycerol): 65 °C, 70 °C; B1-B4 (10% glycerol): 65 °C, 70 °C, 75 °C, 80 °C; C1-C4 (20% glycerol): 65 °C, 70 °C, 75 °C, 80 °C; and D1-D4 (50% glycerol): 65 °C, 75 °C, 85 °C, 90 °C.
higher temperatures with increasing glycerol concentration in these glycerol–water systems. This phenomenon can be explained by the fact that the most important structural element building the starch crystalline structure is the starch–starch hydrogen bonding, and glycerol shows strong hydrogen bonding with water which results in its higher water uptake level compared with pure starch (Liu et al., 2011; Nashed et al., 2003). Consequently, more thermal energy would be required to mobilize water molecules to penetrate amongst starch chains for disruption of the existing crystalline structure (Taghizadeh & Favis, 2013). 3.2. Thermal properties of starch–glycerol mixtures
DSC has been considered as an extremely valuable method to quantify transition temperatures and it is possible to provide the parameters of the order-disorder transition occurring in semi-crystalline starch granules during gelatinization. In addition to a slight increase in the peak and conclusion temperatures for the 5% glycerol concentration, thermal analysis revealed substantial differences in the behavior of starch between water and glycerol–water systems. Fig. 6 shows a single endothermic peak for corn starch in glycerol–water systems; this peak was attributed to the gelatinization of amylopectin, and its position shifted toward higher temperatures with increasing glycerol concentration (Liu et al., 2006). The transition temperatures corresponding to the peak temperature (Tp), as determined by DSC, were 65.6 °C, 67.2 °C, 73.0 °C, 76.3 °C, and 85.3 °C, respectively. In the presence of glycerol–water systems (5/95, 10/90, 20/80, and 50/50, glycerol/water, w/w), the peak temperatures of the corn starches increased by 1.6 °C, 7.4 °C, 10.7 °C, and 19.7 °C, respectively, compared to those in solely water. Not surprisingly, glycerol is commonly considered less effective than water for the phase transition of starch because of its higher molecular weight and viscosity (Liu et al., 2011). Our results are consistent with previous polarized light microscopy observations, effectively confirming the previous microscopic observations that starch gelatinization is delayed to higher temperatures with increasing glycerol concentration. It is proposed that glycerol acts as an anti-plasticizer in the gelatinization process and increases the water content required for complete gelatinization because it actually decreases the amount of water molecules available to penetrate the starch chains (Perry & Donald, 2000; Taghizadeh & Favis, 2013). Hence, more thermal energy would be required to mobilize water molecules to dissociate starch chains, and consequently gelatinization will proceed at higher temperature (Perry & Donald, 2000; Perry & Donald, 2002). Notably, however, in view of a glycerol concentration of 50%, a higher temperature (90 °C) can also enable water molecules to facilitate the dissociation of amylopectin double helices and the unwinding helix to coil transitions, as evidenced by the absence of Maltese crosses in the image in Fig. 4D2 (Liu et al., 2011; Tan et al., 2004).
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3.3. Pasting properties The pasting curves of native corn starch with different glycerol concentrations, as determined using an RVA, are shown in Fig. 7. During the period of heating to and holding at 95 °C, substantial differences were observed in the pasting profiles of corn starch with glycerol concentrations ranging from 0% to 50%. All of the starch–glycerol mixtures exhibited an increase in viscosity during the heating process, which was mainly attributed to the swelling of starch granules, as explained by Karim et al. (2008). 7500
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Time (s) Fig. 7. Pasting profiles of native corn starch with different glycerol concentrations.
The results in Fig. 7 reveal that increasing the glycerol concentration resulted in similar trends as those observed in polarized light microscopic observations (Figs. 1-4) and DSC experiments (Fig. 6); i.e., the gelatinization temperature increases with increasing glycerol concentration.
According to Habitante et al. (2008), although trends in the published results are sometimes contradictory, increasing the glycerol concentration increases the gelatinization temperature. The pasting properties of corn starch with various glycerol concentrations are summarized in Table 1. A gradual increase in peak viscosity (PV) and breakdown viscosity (BV) but a slow decrease in setback viscosity (SV) with an increase in glycerol concentrations occurred. PV for corn starch with various glycerol concentrations ranged between 2870 cP and 6319 cP, corresponding to 0% for the lowest and 50% for the highest. Similarly, the BV was found to be 0% for the lowest and 50% for the highest, respectively. The SV for corn starch with various glycerol concentrations ranged from 1270 cP to 1083 cP, where the highest value corresponds 0% glycerol concentration and the lowest corresponds to 50% glycerol concentration. Huang et al. (2015) proposed that corn starch with a low amylose content should be assumed to exhibit a high pasting viscosity and a low pasting temperature. The hydrogen bonds between the starch chains are disrupted, resulting in weaker granules, increasing breakdown and leading to the rapid loss of PV (Woggum, Sirivongpaisal, & Wittaya, 2015). In addition, the hydrogen bonding of glycerol molecules hindered the formation of crystallites, leading to a decrease of setback (Luo, Li, & Lin, 2012). Table 1 Pasting properties of native corn starch with various glycerol concentrations (w/w): 0%, 5%, 10%, 20%, and 50%. Glycerol
concentration
Viscosity (cP)
( w/w)
Peak
Breakdown
Setback
0%
2870±15
1140±23
1270±28
5%
3402±7
1479±9
1252±14
10%
3691±4
1638±11
1214±13
20%
4788±19
2158±24
1180±20
50%
6319±16
2230±25
1083±33
Data are expressed as the means ± standard deviations in triplicate; means were significantly different at the 5% level.
4. Conclusion The effects of various glycerol concentrations (0%, 5%, 10%, 20%, and 50%, w/w) on the morphologies and gelatinization behaviours of corn starches were investigated. When corn starch granules were treated at 65 °C, the granules of corn starch were almost completely broken and the
characteristic birefringence of the starch granules disappeared. Various microscopic techniques revealed that starch gelatinization was delayed to higher temperatures with increasing concentration of glycerol. DSC results showed that the peak temperature of corn starches increased by 1.6 °C, 7.4 °C, 10.7 °C, and 19.7 °C in samples with glycerol concentrations of 0%, 5%, 10%, 20%, and 50%, w/w, respectively, compared to systems with water as the sole solvent. The RVA pasting profiles revealed that increasing the glycerol concentration resulted in similar trends as those observed in polarized light microscopic observations and DSC experiments; i.e., the gelatinization temperature increases with increasing glycerol concentration. In brief, glycerol acts as an anti-plasticizer in the gelatinization process and increases the water content required for complete gelatinization because it actually decreases the number of water molecules available to penetrate among the starch chains. Acknowledgement The authors would like to thank the National Natural Science Foundation of China (grant no. 31471700 and 31371735) for financial support. References Acosta, S., Chiralt, A., Santamarina, P., Rosello, J. M., González-Martínez, C., Cháfer, M. (2016). Antifungal films based on starch-gelatin blend, containing essential oils. Food Hydrocolloids, 61, 233-240. Chen, X., Du, X. F., Chen, P. R., Guo, L., Xu, Y., Zhou X. H. (2017). Morphologies and gelatinization behaviours of high-amylose maize starches during heat treatment. Carbohydrate Polymers, 157, 637-642. Farahnaky, A., Saberi, B., Majzoobi, M. (2013). Effect of glycerol on physical and mechanical properties of wheat starch edible films. Journal of Texture Studies, 44, 176-186. Fishman, M. L., Coffin, D. R., Konstance, R. P., Onwulata, C. I. (2000). Extrusion of pectin/starch blends plasticized with glycerol. Carbohydrate Polymers, 41, 317-325. Guo, L., Hu, J., Zhang, J. J., Du, X. F. (2016). The role of entanglement concentration on the hydrodynamic properties of potato and sweet potato starches. International Journal of Biological Macromolecules, 93, 1-8. Habitante, A. M. B. Q., Sobral, P. J. A., Carvalho, R. A., Solorza-Feria, J. & Bergo, P. V. A. (2008). Phase transitions of cassava starch dispersions prepared with glycerol solutions. Journal of Thermal Analysis and Calorimetry, 93, 599-604. Huang, J., Shang, Z. Q., Man, J. M., Liu, Q. Q., Zhu, C. J., Wei, C. X. (2015). Comparison of molecular structures and functional properties of high-amylose starches from rice transgenic line and commercial maize. Food Hydrocolloids, 46, 172-179. Karim, A. A., Nadiha, M. Z., Chen, F. K., Phuah, Y. P., Chui, Y. M., & Fazilah, A. (2008). Pasting and retrogradation properties of alkali-treated sago (Metroxylon sagu) starch. Food Hydrocolloids, 22(6), 1044-1053. Koch, K., Gillgren, T., Stading, M., Andersson, R. (2010). Mechanical and structural properties of solution-cast high-amylose maize starch films. International Journal of Biological Macromolecules, 46, 13-19.
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