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Effect of salts on the gelatinization process of Chinese yam (Dioscorea opposita) starch with digital image analysis method
Food Hydrocolloids 51 (2015) 468e475
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Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd
Effect of salts on the gelatinization process of Chinese yam (Dioscorea opposita) starch with digital image analysis method Qian Li, Li Zhang, Ying Ye, Qunyu Gao* Carbohydrate Lab, College of Light Industry and Food Sciences, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2015 Received in revised form 26 May 2015 Accepted 27 May 2015 Available online 12 June 2015
A comparative study was undertaken to examine the effect of different salts on the gelatinization of Ctype Chinese yam starch. The digital image analysis, integral optical density (IOD) method and the model of response difference of crystallite change (MRDCC) were employed to dynamically detect the gelatinization process in our research. Different concentrations and various salts had different degree of improvement and inhibition effect on the gelatinization process when heated in water. With the increase of NaCl concentration from 0 to 4 mol/L (M) the gelatinization degree (DG) of B-type allomorph increased at lower concentration to a maximum value and then decreased with increasing concentration, however all of the concentrations (1e4 M) had inhibition effect on A-type polymorph. The inhibition effect of low NaCl concentration contributed to the dominated water structure-making effect of Naþ, while in high concentration the electrostatic interaction between starch eOH groups and Naþ ions was significant. The influence of various neutral anions was in accordance with the Hofmeister series while the situation of cations was far more complicated. Anions with higher charge density had water structure making effect to reduce water activity, and repeled starch eOH groups to stabilize starch granules at the same time; however, the higher charge density of cations increased the water structure on the one hand, while attracted starch eOH groups and destabilize starch granules with generated heat on the other hand. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Chinese yam starch Gelatinization IOD method Salts
1. Introduction Chinese yam (Dioscorea opposita) has been used as an important invigorants in traditional Chinese medicine (TCM) for many years (Zuo & Tang, 2003). They benefit the lung, promote production of the body fluids and invigorate the spleen, stomach and kidney. Chinese yam starch is the most abundant carbohydrate in the rhizoma of D. opposita tubers (Ni & Song, 2002). Surprisingly, as far as we aware there are few international reports on the research of Chinese yam starch until 21 century. Shujun Wang (Wang et al., 2006) conformed that the starches of Chinese yam from four different cultivars exhibit a C-type X-ray diffraction pattern, similar to that of pea starch rather than the A-type patterns found for cereal starches. Two types of crystallite A and B, have been identified in starch, which can be distinguished by the packing density on, of double helices and the A polymorphs are denser than B (Bule Colonna, Planchot, & Ball, 1998; Imberty & Perez, 1988; Wang, Bogracheva, & Hedley, 1998). If C-type starches consist of these
* Corresponding author. Tel.: þ86 20 87113845; fax: þ86 20 87113848. E-mail address: [email protected] (Q. Gao). http://dx.doi.org/10.1016/j.foodhyd.2015.05.045 0268-005X/© 2015 Elsevier Ltd. All rights reserved.
two type polymorphs, then they will have unique properties depending upon the arrangement and percentage of these two polymorphs in the granule. Wang et al. (Wang, Yu, Zhu, Yu, & Jin, 2009) further proved that B-type allomorph basically existed at the center part of the Chinese yam starch granules which was surrounded by the A-type allomorph in the outer part of the granules. Addition of different solutes, such as sugars, salts, acids and alkalis have been used to modify the gelatinization temperature (Ahmad & Williams, 1999; Chiotelli, Pilosio, & Le Meste, 2002; Gough & Pybus, 1973; Jyothi, Sasikiran, Sajeev, Revamma, & Moorthy, 2005; Zhou, Wang, Li, Fang, & Sun, 2011). Salts have been shown to have a significant effect on the gelatinization properties of starches generally, and it has been found that they could cause an elevation or depression of the gelatinization temperature, Tp and gelatinization enthalpy, DH (Evans & Haisman, 1982; Jane, 1993; Rumpold & Knorr, 2005; Wootton & Bamunuarachchi, 1980). The majority of techniques that have been applied to study the influence of salts were differential scanning calorimetry (DSC) (Cheow & Yu, 1997; Chungcharoen & Lund, 1987; Maaurf, Che Man, Asbi, Junainah, & Kennedy, 2001),
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rheological measurements (Ahmed, 2012), light microscopy (Jane, 1993) et al. IOD method as the latest technique, is able to characterize the partially gelatinized starch granules, in the mean time, it is a realtime monitor without pre- and after-treatment of various starch samples. Thus real situation of the gelatinization process can be obtained. Our previous work has confirmed that this method is of advantage compared with the previous traditional method based on polarizing microscopy (Li, Li, & Gao, 2015; Li, Xie, Yu, & Gao, 2013). The model of response difference of crystallite change (MRDCC) (Li et al., 2013) is a characterization of the starch gelatinization speed changes with the temperature. Compared with DSC, MRDCC is more sensitive and accurate, even the subtle expansion in the pre-gelatinized stage can be detected (Li, Xie, Yu, & Gao, 2014). Various explanations have been given for the effect of neutral salts on starch gelatinization. What is worth mentioning is that Gough (Gough & Pybus, 1973) first classified the gelatinization phenomena into three major types (swelling from hilum, swelling from periphery, and gelatinized from both hilum and periphery), and he suggested that the relationship between temperature and salt concentration was attributed to the interplay of water and partially hydrated salts. In the further elegant experiment of Jane (Jane, 1993), the mechanism of starch gelatinization in salt solutions was summarized into two aspect: (I) structure-making and structure-breaking effects on water and (II) electrostatic interactions between salts and hydroxyl groups of starch. It has also been argued that there are two factors that might contribute to the influence of gelatinization: the effect of salts on polymeresolvent interactions, which are influenced to a greater extent by the anions; the interaction between cations and starch chains hydroxyl groups forms complexes and thus disrupting polymer chain aggregation (Ahmad & Williams, 1999). The purpose of this present investigation is to undertake a comprehensive study of those salt effects on starch gelatinization with a digital image analysis technology. Meanwhile, the MRDCC combined with integral optical density (IOD) method was involved in this work.
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slurries were prepared at starch concentrations of 3% (starch:distilled water ¼ 3:97, dry basis) with different salt solutions separately to evaluate the effects of salt concentration and type on gelatinization properties. Before detected, the measured starch-salt slurries were equilibrated for 2 h and then sealed between two glass cover slip using Dow Corning 732 Sealant before replaced in the hot stage (model THMS600, Linkam Scientific Instrument Ltd., Britain). Each measurement was carried out in triplicate. 2.3. Hot stage-light microscopy Each specimen in the hot stage was observed under a polarization microscope (Vanox BHS-2, Olympus Corp., Japan) equipped with a digital camera, which can display live video of birefringence granules in a real time. A temperature programmer was connected with the hot stage to control the heating progress from 50 C to 98 C at a rate of 2 C/min. Live pictures were captured every 2 C and stopped if the gelatinization process was finished (the pictures were black or the granules were disappeared completely). The digital pictures quantity of each sample varied according to their gelatinization situation. Each image (2048 1536, 12 bits) was saved as TIFF image file, without data compression. All of the samples were observed under the same aperture (maximum), light intensity (fixed at 9), and exposure time (40 ms). The combination of eyepiece and objective lens were selected with a magnification of 200 times, as described in our early research (Li et al., 2013). 2.4. Wide-angle X-ray diffraction patterns X-ray diffractogram was obtained by running a D/Max-2200 Xray diffracttometer (Rigaku Denki Co., Tokyo, Japan). Cu Ka radiation at 44 kV and 26 mA were used. The measured Chinese yam starches were equilibrated in a sealed desiccator with water at room temperature for 12 h. The diffractogram scanning was run between 4 and 35 (2q) at a rate of 5 /min (Shi, Chen, Yu, & Gao, 2013).
2. Materials and methods
2.5. IOD method
2.1. Materials
It is a method to measure the degree of gelatinization (DG). The IOD value of each digital picture was calculated by the Image-pro plus 5.0 software (Li et al., 2013). The DG based on the IOD value (DGI) was calculated as defined in our early research (Li et al., 2013).
Tubers of Chinese yam were obtained from farmers of Taigu, Henan province, China. NaCl, KCl, CaCl2, MgCl2, FeCl3, NaNO3, Na2CO3, Na2SO4, CuSO4 and solvents used in this work were of analytical grade.
Background correction : C ¼ A B
(1)
2.2. Preparation of sample
DGI % ¼ ð1 C=C0 Þ 100%
(2)
Chinese yam tubers (1000 g) were washed, hand peeled and trimmed to remove defective parts. Then sliced and ground with an equal volume of water using a Jiuyang blender for 2 min at full speed. The slurry was filtered through a 200-mesh screen. The material remaining on the sieve was rinsed twice with deionized water, and the permeating was deposited for 2 h. Then, the supernatant was removed and the settled starch layer was resuspended in distilled water. After eight cycles of depositing and resuspending, the starch was then collected by suction filtrating and dried for 24 h at 45 C. Chinese yam starch slurries were prepared at starch concentration of 3% (starch: distilled water ¼ 3:97, dry basis), and the moisture content (12.56%) in starch should be taken into consideration during calculating. 1, 2, 3 and 4 M of NaCl solutions, 1 M KCl, CaCl2, MgCl2, FeCl3, NaNO3, Na2CO3, Na2SO4, and CuSO4 solutions were configured under 25 C with distilled water. Starch-salt
where A is the original IOD value (IOD value calculated from the original digital image when all of the birefringence remain unchanged), B is the background IOD value (IOD value calculated from the original digital image when all of the birefringence disappeared), and C0 is the initial real IOD value (IOD value of birefringence light derived from the specific crystal structure of starch in the initial digital image). In this study, the IOD value of 50 C was set as initial IOD value. 2.6. Model of response difference of crystallite change (MRDCC) The MRDCC used in this paper were obtained according to our previous method (Li et al., 2013). Response difference of crystallite change (RDCC) is the variation of crystallization degree in a certain temperature range which characterizes gelatinization speed, %/DT for units (%: Gelatinization
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degree difference; DT: the range of temperature corresponding to the crystallite change). For the drawing method, the MRDCC at a certain temperature point (anyone of 50, 52, 54 … 86, 88 … C) is half the degree of gelatinization (DGI) of this temperature measured by the IOD method minus that of the previous temperature. The temperature is chosen for the horizontal axis, while the RDCC is chosen for the vertical axis, fitting the scatter with the tension spline function, and the MRDCC is obtained.
Pn ¼
1 ðDGn DGn2 Þ 2
ð50 < n < 98Þ
(3)
Pn is the RDCC of a certain temperature point (n), DGn is the gelatinization degree of a certain temperature point (n) measured by IOD method. In this experiment, between 50 and 98 C, the RDCC at a certain temperature, such as 70 C, is the gelatinization degree at 70 C, minus the gelatinization degree of the previous temperature at 68 C, and multiplied by 1/2. 2.7. Statistical analysis All the tests were carried out in triplicate and the result was calculated and analyzed by Excel software, the significant difference among the means was estimated at 95% confidence level (p < 0.05). MRDCC curves were obtained by drawing and fitting the Curve Expert-Pro, while the other curves were performed using OriginPro 8. 3. Results and discussion 3.1. The gelatinization process of Chinese yam starch Focus on the DGI-temperature relationship of Chinese yam starch when heated in pure water as showed in Fig. 1 (the black curve). The DGI increased from 0 to 100% with the temperature climbing up from 50 to 84 C. According to our previous definition (Li et al., 2015), the temperature point when DGI reaches 10% is the start temperature Ts. Thus the Ts of Chinese yam starch in water was 62.8 C. When the DGI reached 50%, the corresponding temperature was 67.7 C, and the conclusion temperature (TC) was 80.1 C.
As could be seen from Fig. 2, only four representative photographs were selected to show the different gelatinization state of Chinese yam starch. Under 64 C, we could not see any apparent/ detectable change under the birefringence light, it was the ungelatinized state; under 68 C, the photograph showed obvious decrease of light intensity and/or area of birefringence light, which represented the partially gelatinized state; and the starch granules were in the totally gelatinized stage under 84 C for the birefringence light totally disappeared. The X-ray diffraction pattern of our Chinese yam starch was showed in Fig. 3. Relative strong diffraction peaks were showed at 5.7, 11.4 , 15.1, 17.1, 22.3 and 23.8 2q. Unlike other starches which also came from the tubers, namely B-type, potato starch (Van Soest, Hulleman, De Wit, & Vliegenthart, 1996), A-type sweet potato and cassava starch (Da Cruz Francisco, Silverio, Eliasson, & Larsson, 1996; Rocha, Carneiro, & Franco, 2010), our Chinese yam starch sample could be classified as C-type, a mixture of A-type and B-type. Our results were in agreement with the results reported by Shujun Wang (Wang et al., 2006, 2009). According to their research, the crystal type of starches separated from different D. Opposita cultivar was a typical CB-type pattern. In the diffraction spectra, all of the starches separated from different D. Opposita Thunb. Cultivars showed strong diffraction peaks at 6.5 , 6.2 , 17.8 , 20.1, 27.4 2q. The A- and B-type allomorph coexisted in the individual C-type starch granule. The outer semicrystalline and amorphous growth rings were mostly composed of A-type allomorph whereas the amorphous or less crystalline areas were predominantly composed of B-type allomorph. Focus on the MRDCC of Chinese yam starch heated in pure water as showed in Fig. 4 (the black curve). Unlike corn starch which was a typical A-type starch, the MRDCC of Chinese yam starch did not showed an uni-modal curve as presented in our previous work (Li et al., 2015). The big platform shoulder peak in the right-side of the main peak showed the multi-stage gelatinization process of Chinese yam starch. Two obvious peak temperature point (Tp1 and Tp2) corresponding to peak B and A were appeared in the figure: Tp1 was 67.5 C and Tp2 was 79.9 C. Peak B mainly related with the gelatinization of B-type allomorph in Chinese yam starch while peak A mostly connected with that of A-type allomorph. Like the situation of pea starch (Bogracheva, Morris, Ring, & Hedley, 1998; Li et al., 2013), A allomorph is firmer than that of B for its less porous structure character, and B polymorphs namely the amorphous or less crystalline areas, gives a transition with a lower peak temperature than A. It was showed that the B polymorphs gelatinized first and then decreased the melting temperature of the neighboring crystallites (A polymorphs) resulting in the progressive disruption of crystalline areas. 13C CP/MAS nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) results also (Wang et al., 2009) revealed that B-type allomorph in Chinese yam starch was hydrolyzed more rapidly than A-type. The gelatinization of Chinese yam starch around 62e72 C had a great influence on the overall crystallization, which could also be reflected from the RDCC of Chinese yam starch. 3.2. The gelatinization process of Chinese yam starch in different concentration of NaCl solution
Fig. 1. Relationship between temperature and DGI of Chinese yam starch under different concentrations of NaCl.
The dynamic gelatinization process of Chinese yam starch heated in different concentration of NaCl solutions could be seen from Fig. 5. The influence of different concentration of NaCl solutions on the gelatinization process of Chinese yam starch was different. The relationship between temperature and DGI of Chinese yam starch when heated in different concentrations of NaCl also could be observed from Fig. 1. During heated in excess salt solution, the A
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Fig. 2. Micrographs of Chinese yam starch at different temperature (50, 64, 68, 82 C) when heated in water under polarized light (200).
and B polymorphs within C-type granules melted independently. The influence of different NaCl concentration on the A- and B-type allomorph of Chinese yam starch was different. For B-type allomorph (before about 72 C), 1 M and 2 M of NaCl inhibited the gelatinization with 1 > 2 M while 3 M and 4 M promoted gelatinization with 4 > 3 M significantly. When the DG reached 50%, the gelatinization temperature corresponding to heated in pure water, 1, 2, 3 and 4 M of NaCl were 67.7, 69.6, 68.3, 66.6 and 61.3 C respectively. As to the A-type allomorph, both of the NaCl concentration (1e4 M) showed inhibition effect (after about 72 C) on
the gelatinization. This result was the same as the situation of pea starch (Li et al., 2014). Fig. 4 also showed the MRDCC of Chinese yam starch heated in different concentration of NaCl solution. Like the condition of corn starch heated in same solutions (Li et al., 2014), the addition of NaCl also had no impact on the crystalline change peak shape (the curve shape of MRDCC). But the influence of NaCl on the A-type and Btype allomorph was different. The Tp1 corresponded with pure water, 1 M, 2 M, 3 M, 4 M of NaCl were 67.5, 68.6, 68.1, 65.3 and 59.7 C respectively. Tp1 increased slightly to a maximum value (~1 M) and then decreased with increasing concentration.
Fig. 3. X-ray diffraction patterns of Chinese yam starch.
Fig. 4. The MRDCC of Chinese yam starch heated with different concentrations of NaCl.
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Fig. 5. Micrographs of Chinese yam starch at different temperatures and different concentrations of NaCl solutions at different temperature during heating process under polarized light (200).
Combined with the results in Figs. 1 and 3, we made a conclusion that B-type allomorph accounted for about 80% of the whole crystalline structure in Chinese yam starch because the DGI corresponding to the BeA turning point (72 C) was about 80%. The Atype allomorph just accounted about 20% of the whole crystalline structure and its melting process were also influenced by the gelatinization of B-type allomorph, as a result, it seemed that it was hard to analyze it separately.
Many studies showed that salts like NaCl affected starch gelatinization in different way as KSCN (Jane, 1993). In the research of Jane with DSC, for example, To and DH of starch gelatinization were consistently decreased with the increase of KSCN and KI concentration, but the gelatinization of starches were changed in a complex mode with the increase of NaCl and CaCl2 concentration. The gelatinization pattern of corn starch in KSCN and KI solutions began at the hilum and the pattern remained unchanged with salt
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concentration. In contrast, the gelatinization pattern in NaCl and CaCl2 solutions varied with the salt concentration. The increase of NaCl concentration also caused the loss of allomorph to occur from the granule surface (Gough & Pybus, 1973). When the concentration of NaCl was less than 2 M, the electrostatic interaction between starch eOH groups and Naþ ions was not significant, the inhibition effect of low NaCl concentration contributed to the dominated water structure-making effect of Naþ; however, when the concentration of NaCl exceeded 2 M, the dipoleemetal interaction between starch and Naþ became dominant. As a result, 4 M showed such a great improvement effect on the gelatinization of Chinese yam starch. 3.3. The effect of different chlorine salts on the gelatinization of Chinese yam starch The DGI-temperature relationship of Chinese yam starch heated in different chlorine salts could be seen from Fig. 6. All of the chlorine salts beside FeCl3 inhibited the gelatinization of Chinese yam starch. The inhibition effect was MgCl2 > NaCl > KCl CaCl2, while the FeCl3 showed significant promotion effect. When the DG reached 50%, the gelatinization temperature heated in pure water, 1 M of NaCl, KCl, CaCl2, MgCl2 and FeCl3 were 67.7, 69.6, 69.1, 68.6, 72.7 and 64.8 C. From Fig. 7, we also knew that their Tp1 were 67.5, 68.6, 68.3, 68.3, 71.5, 64.2 (p < 0.05) C respectively. It has been reported that the chloride ion was located in the center of lyotropic series and thus was the least effective anion for modifying gelatinization behavior (Gough & Pybus, 1973). According to the theory proposed by Frank and Wen (1957) and later confirmed by many researchers (Ahmad & Williams, 1999; Jane, 1993): ions of high charge density, such as (Mg2þ) had strong electrostatic interactions with water molecules, they increased the viscosity of the aqueous solution by hydrogen bonds between water molecule and thus decreased the fraction of free water. Infrared spectroscopy (Paquette & Jolicoeur, 1977) showed that these ions promoted water structure, reduced the fraction of free water and increased the viscosity of the solution, liquid water was a mixture of hydrogen-bonded clusters and unbounded free-water molecules. As monovalent salts, the size of the K ion was bigger than Na, which means that the charge densities of Naþ was bigger than Kþ and thus the inhibition effect on gelatinization of NaCl was greater than KCl. For the same reason, the inhibition effect of MgCl2 was stronger than CaCl2. However, Ca2þ and Fe3þ showed opposite trend with higher charge densities than Naþ and Kþ under the
Fig. 6. Relationship between temperature and DGI of Chinese yam starch under different chlorine salts (NaCl, KCl, CaCl2, MgCl2, FeCl3) effect.
Fig. 7. The MRDCC of Chinese yam starch heated with different chlorine salts.
concentration of 1 M. The result of Ca2þ was in agreement with that of Rumpold (Rumpold & Knorr, 2005), the impact of salts on three kinds of starch (wheat, tapioca and potato) followed the order Ca2þ >Naþ and Kþ appeared only in higher concentrations (>2 M); under 1 M, the gelatinization inhibition effect of Ca2þ was smaller than that of Naþ and Kþ. In the research of Ahmad (Ahmad & Williams, 1999), the To of CaCl2 at 1 M (64.2 C) was lower than that in NaCl (66.5 C) and KCl (64.4 C). The Granule disruption mechanism of FeCl3 might be completely different from other chlorides in our study which was also confirmed by Gough and Pybus (1973) previously. With high charge density, cations like Ca2þ and Fe3þ increased the water structure on the one hand, while attracted starch eOH groups and destabilized starch granules with generated heat on the other hand. Under the concentration of 1 M, the electrostatic interaction between starch eOH groups and Fe3þ ions was so strong and significant that it induced the melting of the granules quickly.
3.4. The effect of different sodium salts on the gelatinization of Chinese yam starch Fig. 8 showed the DGI-temperature relationship of Chinese yam starch heated in different sodium salts. Unlike the influence of different concentrations of NaCl which had different effect on A and B-type allomorph, under the same concentration (1 M), the effect of different sodium salts on these two type allomorph were approximate consistent. Different sodium salts showed almost the same DGI -temperature curve trend. NaCl and Na2SO4 inhibited the gelatinization with Na2SO4 > NaCl dramatically while NaNO3 and Na2CO3 accelerated the disruption process of granule with NaNO3 > Na2CO3. When the DG reached 50%, the gelatinization temperature heated in pure water, 1 M of NaCl, NaNO3, Na2CO3 and Na2SO4 were 67.7, 69.6, 63.9, 65.7 and 79.6 C separately. From Fig. 9, we also knew that their Tp1 were 67.5, 68.6, 63.9, 65.1, 79.5 C respectively. The influence of various neutral anions was in accordance with the Hofmeister series (Evans & Haisman, 1982; Jane, 1993). The gelatinization temperature decreased in the order 2 SO2 4 > Cl > NO3 . With high charge density, anions like SO4 , had strong electrostatic interactions with water molecular reduced water activity, at the same time, they repeled starch eOH groups and stabilized starch granules, and the repulsion was proportional
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Fig. 8. Relationship between temperature and DGI of Chinese yam starch under different sodium salts (NaCl, NaNO3, Na2CO3, Na2SO4) effect.
Fig. 10. Relationship between temperature and DGI of Chinese yam starch under two different sulphates (Na2SO4, CuSO4) effect.
to the charge density of the anions (Jane, 1993). Anions like NO 3 had low charge density, broke water structure and formed helical complexes with starch chains, resulting in the degradation of the granule. The ability of aqueous alkali in inducing starch disruption was common knowledge (Ragheb, Abdel-Thalouth, & Tawfik, 1995; Wootton & Ho, 1989). The result of Na2CO3 was consistent with those which had been reported earlier (Zhou et al., 2011). Besides, the increase of gelatinization transition and peak temperature were also observed in starches like wheat, rice and corn in the presence of excess water and low Na2CO3 concentrations (1e4 g/100 g starch) (Lai, Karim, Norziah, & Seow, 2002). This effect was attributed to the structure making effect of Naþ.
great inhibition effect on starch gelatinization because of the existence of SO2 4 while the effect of Na2SO4 was bigger than that of CuSO4. When the DG reached 50%, the gelatinization temperature corresponding to heated in pure water, 1 M of Na2SO4 and CuSO4 were 67.7, 79.6 and 77.7 C. From Fig. 11, we also knew that their Tp1 were 67.5, 79.5 and 77.8 C respectively. Back to Figs. 1 and 4, we realized that the influence of 2 M Naþ on the gelatinization process was small. The gap of Tp1 and temperature corresponded to 50% DGI between heated in pure water and 2 M NaCl were both 0.6 C which could be neglected, then comparison could be made by acting Na2SO4 (2 M of Naþ) as a reference. Under 1 M, the water structure making effect of Cu2þ was smaller than the destabilization effect of the heat generated by the Cu2þ and starch eOH groups attraction.
3.5. The effect of two different sulphates on the gelatinization of Chinese yam starch
4. Conclusion
The DGI -temperature relationship of Chinese yam starch heated in two sulphates were compared in Fig. 10. Both of them showed
Salts have been found to have a complex effect on the gelatinization process of Chinese yam starch (D. opposita) when heated in water medium. The Chinese yam starch involved in our study was a
Fig. 9. The MRDCC of Chinese yam starch heated with different sodium salts.
Fig. 11. The MRDCC of Chinese yam starch heated with two different sulphates.
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C-type starch which could be confirmed both from X-ray diffraction and the MRDCC figure. In MRDCC, the first peak corresponded to the B-type polymorphs within the C-type Chinese yam starch while the second peak corresponded to the A type polymorphs outer moiety of the C-type crystal. We could deduce that the B-type allomorph accounted about 80% of the whole C-type starch crystalline. The influence of different NaCl concentrations on the A- and B-type allomorph in Chinese yam starch were different. With the increase of NaCl concentration from 0 to 4 M, the DG of B-type allomorph increased at lower concentration to a maximum value (at about 1 M) then decreased with increasing concentration. The influence of various neutral anions was in accordance with the Hofmeister series: DG decreased in the order SO42 > Cl > NO 3; however the situation of cations was far more complicated. As aqueous alkali Na2CO3 induced starch disruption under 1 M, and under the same concentration, the destabilization effect induced by the electronic interaction between Cu2þ and starch eOH groups was bigger than the water structure making effect of Cu2þ. Abbreviations used IOD DG DGI MRDCC RDCC DSC NMR XRD
integral optical density the degree of gelatinization the degree of gelatinization measured by IOD method the model of response difference of crystalline change response difference of crystallite change differential scanning calorimeter nuclear magnetic resonance X-ray diffraction.
Acknowledgments The study was carried out with financially support of the State Key Program of National Natural Science of China (Grant No. 31230057) and Guangdong Province Program of China (Grant No. 2012B091100291). References Ahmad, F. B., & Williams, P. A. (1999). Effect of salts on the gelatinization and rheological properties of sago starch. Journal of Agricultural and Food Chemistry, 47(8), 3359e3366. Ahmed, J. (2012). Rheometric non-isothermal gelatinization kinetics of mung bean starch slurry: effect of salt and sugar e part 1. Journal of Food Engineering, 109(2), 321e328. Bogracheva, T. Y., Morris, V., Ring, S., & Hedley, C. (1998). The granular structure of C-type pea starch and its role in gelatinization. Biopolymers, 45(4), 323e332. on, A., Colonna, P., Planchot, V., & Ball, S. (1998). Starch granules: structure and Bule biosynthesis. International Journal of Biological Macromolecules, 23(2), 85e112. Cheow, C. S., & Yu, S. Y. (1997). Effect of fish protein, salt, sugar, and monosodium glutamate on the gelatinization of starch in fishestarch mixtures. Journal of Food Processing and Preservation, 21(2), 161e177. Chiotelli, E., Pilosio, G., & Le Meste, M. (2002). Effect of sodium chloride on the gelatinization of starch: a multimeasurement study. Biopolymers, 63(1), 41e58. Chungcharoen, A., & Lund, D. (1987). Influence of solutes and water on rice starch gelatinization. Cereal Chemistry, 64(4), 240e243.
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Da Cruz Francisco, J., Silverio, J., Eliasson, A.-C., & Larsson, K. (1996). A comparative study of gelatinization of cassava and potato starch in an aqueous lipid phase (L2) compared to water. Food Hydrocolloids, 10(3), 317e322. Evans, I. D., & Haisman, D. R. (1982). The effect of solutes on the gelatinization €rke, 34(7), 224e231. temperature range of potato starch. Starch e Sta Frank, H. S., & Wen, W.-Y. (1957). Ionesolvent interaction. Structural aspects of ionesolvent interaction in aqueous solutions: a suggested picture of water structure. Discussions of the Faraday Society, 24, 133e140. Gough, B. M., & Pybus, J. N. (1973). Effect of metal cations on the swelling and €rke, 25(4), gelatinization behaviour of large wheat starch granules. Starch e Sta 123e130. Imberty, A., & Perez, S. (1988). A revisit to the three-dimensional structure of B-type starch. Biopolymers, 27(8), 1205e1221. Jane, J.-L. (1993). Mechanism of starch gelatinization in neutral salt solutions. Starch €rke, 45(5), 161e166. e Sta Jyothi, A. N., Sasikiran, K., Sajeev, M. S., Revamma, R., & Moorthy, S. N. (2005). Gelatinisation properties of cassava starch in the presence of salts, acids and €rke, 57(11), 547e555. oxidising agents. Starch e Sta Lai, L. N., Karim, A. A., Norziah, M. H., & Seow, C. C. (2002). Effects of Na2CO3 and NaOH on DSC thermal profiles of selected native cereal starches. Food Chemistry, 78(3), 355e362. Li, Q., Li, H., & Gao, Q. (2015). The influence of different sugars on corn starch gelatinization process with digital image analysis method. Food Hydrocolloids, 43, 803e811. Li, Q., Xie, Q., Yu, S., & Gao, Q. (2013). New approach to study starch gelatinization applying a combination of hot-stage light microscopy and differential scanning calorimetry. Journal of Agricultural and Food Chemistry, 61(6), 1212e1218. Li, Q., Xie, Q., Yu, S., & Gao, Q. (2014). Application of digital image analysis method to study the gelatinization process of starch/sodium chloride solution systems. Food Hydrocolloids, 35, 392e402. Maaurf, A. G., Che Man, Y. B., Asbi, B. A., Junainah, A. H., & Kennedy, J. F. (2001). Gelatinisation of sago starch in the presence of sucrose and sodium chloride as assessed by differential scanning calorimetry. Carbohydrate Polymers, 45(4), 335e345. Ni, S., & Song, X. (2002). Nutritional components analysis of Chinese yam. Jiangsu Pharmaceutical and Clinical Research, 10, 26e27. Paquette, J., & Jolicoeur, C. (1977). A near-infrared study of the hydration of various ions and nonelectrolytes. Journal of Solution Chemistry, 6(6), 403e428. Ragheb, A., Abdel-Thalouth, I., & Tawfik, S. (1995). Gelatinization of starch in €rke, 47(9), 338e345. aqueous alkaline solutions. Starch e Sta Rocha, T.d. S., Carneiro, A. P.d. A., & Franco, C. M. L. (2010). Effect of enzymatic hydrolysis on some physicochemical properties of root and tuber granular starches. Food Science and Technology (Campinas), 30(2), 544e551. Rumpold, B. A., & Knorr, D. (2005). Effect of salts and sugars on pressure-induced €rke, 57(8), gelatinisation of wheat, tapioca, and potato starches. Starch e Sta 370e377. Shi, M., Chen, Y., Yu, S., & Gao, Q. (2013). Preparation and properties of RS III from waxy maize starch with pullulanase. Food Hydrocolloids, 33(1), 19e25. Van Soest, J. J. G., Hulleman, S. H. D., De Wit, D., & Vliegenthart, J. F. G. (1996). Changes in the mechanical properties of thermoplastic potato starch in relation with changes in B-type crystallinity. Carbohydrate Polymers, 29(3), 225e232. Wang, S., Liu, H., Gao, W., Chen, H., Yu, J., & Xiao, P. (2006). Characterization of new starches separated from different Chinese yam (Dioscorea opposita Thunb.) cultivars. Food Chemistry, 99(1), 30e37. Wang, S., Yu, J., Zhu, Q., Yu, J., & Jin, F. (2009). Granular structure and allomorph position in C-type Chinese yam starch granule revealed by SEM, 13C CP/MAS NMR and XRD. Food Hydrocolloids, 23(2), 426e433. Wang, T. L., Bogracheva, T. Y., & Hedley, C. L. (1998). Starch: as simple as A, B, C? Journal of Experimental Botany, 49(320), 481e502. Wootton, M., & Bamunuarachchi, A. (1980). Application of differential scanning calorimetry to starch gelatinization. III. Effect of sucrose and sodium chloride. €rke, 32(4), 126e129. Starch e Sta €rke, Wootton, M., & Ho, P. (1989). Alkali gelatinisation of wheat starch. Starch e Sta 41(7), 261e265. Zhou, H., Wang, J., Li, J., Fang, X., & Sun, Y. (2011). Pasting properties of Angelica dahurica starches in the presence of NaCl, Na2CO3, NaOH, glucose, fructose and €rke, 63(6), 323e332. sucrose. Starch e Sta Zuo, Y., & Tang, D. (2003). Science of Chinese materia medica (pp. 301e303). Shanghai: Shanghai University of Traditional Chinese Medicine Press.
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Effect of salts on the gelatinization process of Chinese yam (Dioscorea opposita) starch with digital image analysis method Qian Li, Li Zhang, Ying Ye, Qunyu Gao* Carbohydrate Lab, College of Light Industry and Food Sciences, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2015 Received in revised form 26 May 2015 Accepted 27 May 2015 Available online 12 June 2015
A comparative study was undertaken to examine the effect of different salts on the gelatinization of Ctype Chinese yam starch. The digital image analysis, integral optical density (IOD) method and the model of response difference of crystallite change (MRDCC) were employed to dynamically detect the gelatinization process in our research. Different concentrations and various salts had different degree of improvement and inhibition effect on the gelatinization process when heated in water. With the increase of NaCl concentration from 0 to 4 mol/L (M) the gelatinization degree (DG) of B-type allomorph increased at lower concentration to a maximum value and then decreased with increasing concentration, however all of the concentrations (1e4 M) had inhibition effect on A-type polymorph. The inhibition effect of low NaCl concentration contributed to the dominated water structure-making effect of Naþ, while in high concentration the electrostatic interaction between starch eOH groups and Naþ ions was significant. The influence of various neutral anions was in accordance with the Hofmeister series while the situation of cations was far more complicated. Anions with higher charge density had water structure making effect to reduce water activity, and repeled starch eOH groups to stabilize starch granules at the same time; however, the higher charge density of cations increased the water structure on the one hand, while attracted starch eOH groups and destabilize starch granules with generated heat on the other hand. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Chinese yam starch Gelatinization IOD method Salts
1. Introduction Chinese yam (Dioscorea opposita) has been used as an important invigorants in traditional Chinese medicine (TCM) for many years (Zuo & Tang, 2003). They benefit the lung, promote production of the body fluids and invigorate the spleen, stomach and kidney. Chinese yam starch is the most abundant carbohydrate in the rhizoma of D. opposita tubers (Ni & Song, 2002). Surprisingly, as far as we aware there are few international reports on the research of Chinese yam starch until 21 century. Shujun Wang (Wang et al., 2006) conformed that the starches of Chinese yam from four different cultivars exhibit a C-type X-ray diffraction pattern, similar to that of pea starch rather than the A-type patterns found for cereal starches. Two types of crystallite A and B, have been identified in starch, which can be distinguished by the packing density on, of double helices and the A polymorphs are denser than B (Bule Colonna, Planchot, & Ball, 1998; Imberty & Perez, 1988; Wang, Bogracheva, & Hedley, 1998). If C-type starches consist of these
* Corresponding author. Tel.: þ86 20 87113845; fax: þ86 20 87113848. E-mail address: [email protected] (Q. Gao). http://dx.doi.org/10.1016/j.foodhyd.2015.05.045 0268-005X/© 2015 Elsevier Ltd. All rights reserved.
two type polymorphs, then they will have unique properties depending upon the arrangement and percentage of these two polymorphs in the granule. Wang et al. (Wang, Yu, Zhu, Yu, & Jin, 2009) further proved that B-type allomorph basically existed at the center part of the Chinese yam starch granules which was surrounded by the A-type allomorph in the outer part of the granules. Addition of different solutes, such as sugars, salts, acids and alkalis have been used to modify the gelatinization temperature (Ahmad & Williams, 1999; Chiotelli, Pilosio, & Le Meste, 2002; Gough & Pybus, 1973; Jyothi, Sasikiran, Sajeev, Revamma, & Moorthy, 2005; Zhou, Wang, Li, Fang, & Sun, 2011). Salts have been shown to have a significant effect on the gelatinization properties of starches generally, and it has been found that they could cause an elevation or depression of the gelatinization temperature, Tp and gelatinization enthalpy, DH (Evans & Haisman, 1982; Jane, 1993; Rumpold & Knorr, 2005; Wootton & Bamunuarachchi, 1980). The majority of techniques that have been applied to study the influence of salts were differential scanning calorimetry (DSC) (Cheow & Yu, 1997; Chungcharoen & Lund, 1987; Maaurf, Che Man, Asbi, Junainah, & Kennedy, 2001),
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rheological measurements (Ahmed, 2012), light microscopy (Jane, 1993) et al. IOD method as the latest technique, is able to characterize the partially gelatinized starch granules, in the mean time, it is a realtime monitor without pre- and after-treatment of various starch samples. Thus real situation of the gelatinization process can be obtained. Our previous work has confirmed that this method is of advantage compared with the previous traditional method based on polarizing microscopy (Li, Li, & Gao, 2015; Li, Xie, Yu, & Gao, 2013). The model of response difference of crystallite change (MRDCC) (Li et al., 2013) is a characterization of the starch gelatinization speed changes with the temperature. Compared with DSC, MRDCC is more sensitive and accurate, even the subtle expansion in the pre-gelatinized stage can be detected (Li, Xie, Yu, & Gao, 2014). Various explanations have been given for the effect of neutral salts on starch gelatinization. What is worth mentioning is that Gough (Gough & Pybus, 1973) first classified the gelatinization phenomena into three major types (swelling from hilum, swelling from periphery, and gelatinized from both hilum and periphery), and he suggested that the relationship between temperature and salt concentration was attributed to the interplay of water and partially hydrated salts. In the further elegant experiment of Jane (Jane, 1993), the mechanism of starch gelatinization in salt solutions was summarized into two aspect: (I) structure-making and structure-breaking effects on water and (II) electrostatic interactions between salts and hydroxyl groups of starch. It has also been argued that there are two factors that might contribute to the influence of gelatinization: the effect of salts on polymeresolvent interactions, which are influenced to a greater extent by the anions; the interaction between cations and starch chains hydroxyl groups forms complexes and thus disrupting polymer chain aggregation (Ahmad & Williams, 1999). The purpose of this present investigation is to undertake a comprehensive study of those salt effects on starch gelatinization with a digital image analysis technology. Meanwhile, the MRDCC combined with integral optical density (IOD) method was involved in this work.
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slurries were prepared at starch concentrations of 3% (starch:distilled water ¼ 3:97, dry basis) with different salt solutions separately to evaluate the effects of salt concentration and type on gelatinization properties. Before detected, the measured starch-salt slurries were equilibrated for 2 h and then sealed between two glass cover slip using Dow Corning 732 Sealant before replaced in the hot stage (model THMS600, Linkam Scientific Instrument Ltd., Britain). Each measurement was carried out in triplicate. 2.3. Hot stage-light microscopy Each specimen in the hot stage was observed under a polarization microscope (Vanox BHS-2, Olympus Corp., Japan) equipped with a digital camera, which can display live video of birefringence granules in a real time. A temperature programmer was connected with the hot stage to control the heating progress from 50 C to 98 C at a rate of 2 C/min. Live pictures were captured every 2 C and stopped if the gelatinization process was finished (the pictures were black or the granules were disappeared completely). The digital pictures quantity of each sample varied according to their gelatinization situation. Each image (2048 1536, 12 bits) was saved as TIFF image file, without data compression. All of the samples were observed under the same aperture (maximum), light intensity (fixed at 9), and exposure time (40 ms). The combination of eyepiece and objective lens were selected with a magnification of 200 times, as described in our early research (Li et al., 2013). 2.4. Wide-angle X-ray diffraction patterns X-ray diffractogram was obtained by running a D/Max-2200 Xray diffracttometer (Rigaku Denki Co., Tokyo, Japan). Cu Ka radiation at 44 kV and 26 mA were used. The measured Chinese yam starches were equilibrated in a sealed desiccator with water at room temperature for 12 h. The diffractogram scanning was run between 4 and 35 (2q) at a rate of 5 /min (Shi, Chen, Yu, & Gao, 2013).
2. Materials and methods
2.5. IOD method
2.1. Materials
It is a method to measure the degree of gelatinization (DG). The IOD value of each digital picture was calculated by the Image-pro plus 5.0 software (Li et al., 2013). The DG based on the IOD value (DGI) was calculated as defined in our early research (Li et al., 2013).
Tubers of Chinese yam were obtained from farmers of Taigu, Henan province, China. NaCl, KCl, CaCl2, MgCl2, FeCl3, NaNO3, Na2CO3, Na2SO4, CuSO4 and solvents used in this work were of analytical grade.
Background correction : C ¼ A B
(1)
2.2. Preparation of sample
DGI % ¼ ð1 C=C0 Þ 100%
(2)
Chinese yam tubers (1000 g) were washed, hand peeled and trimmed to remove defective parts. Then sliced and ground with an equal volume of water using a Jiuyang blender for 2 min at full speed. The slurry was filtered through a 200-mesh screen. The material remaining on the sieve was rinsed twice with deionized water, and the permeating was deposited for 2 h. Then, the supernatant was removed and the settled starch layer was resuspended in distilled water. After eight cycles of depositing and resuspending, the starch was then collected by suction filtrating and dried for 24 h at 45 C. Chinese yam starch slurries were prepared at starch concentration of 3% (starch: distilled water ¼ 3:97, dry basis), and the moisture content (12.56%) in starch should be taken into consideration during calculating. 1, 2, 3 and 4 M of NaCl solutions, 1 M KCl, CaCl2, MgCl2, FeCl3, NaNO3, Na2CO3, Na2SO4, and CuSO4 solutions were configured under 25 C with distilled water. Starch-salt
where A is the original IOD value (IOD value calculated from the original digital image when all of the birefringence remain unchanged), B is the background IOD value (IOD value calculated from the original digital image when all of the birefringence disappeared), and C0 is the initial real IOD value (IOD value of birefringence light derived from the specific crystal structure of starch in the initial digital image). In this study, the IOD value of 50 C was set as initial IOD value. 2.6. Model of response difference of crystallite change (MRDCC) The MRDCC used in this paper were obtained according to our previous method (Li et al., 2013). Response difference of crystallite change (RDCC) is the variation of crystallization degree in a certain temperature range which characterizes gelatinization speed, %/DT for units (%: Gelatinization
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degree difference; DT: the range of temperature corresponding to the crystallite change). For the drawing method, the MRDCC at a certain temperature point (anyone of 50, 52, 54 … 86, 88 … C) is half the degree of gelatinization (DGI) of this temperature measured by the IOD method minus that of the previous temperature. The temperature is chosen for the horizontal axis, while the RDCC is chosen for the vertical axis, fitting the scatter with the tension spline function, and the MRDCC is obtained.
Pn ¼
1 ðDGn DGn2 Þ 2
ð50 < n < 98Þ
(3)
Pn is the RDCC of a certain temperature point (n), DGn is the gelatinization degree of a certain temperature point (n) measured by IOD method. In this experiment, between 50 and 98 C, the RDCC at a certain temperature, such as 70 C, is the gelatinization degree at 70 C, minus the gelatinization degree of the previous temperature at 68 C, and multiplied by 1/2. 2.7. Statistical analysis All the tests were carried out in triplicate and the result was calculated and analyzed by Excel software, the significant difference among the means was estimated at 95% confidence level (p < 0.05). MRDCC curves were obtained by drawing and fitting the Curve Expert-Pro, while the other curves were performed using OriginPro 8. 3. Results and discussion 3.1. The gelatinization process of Chinese yam starch Focus on the DGI-temperature relationship of Chinese yam starch when heated in pure water as showed in Fig. 1 (the black curve). The DGI increased from 0 to 100% with the temperature climbing up from 50 to 84 C. According to our previous definition (Li et al., 2015), the temperature point when DGI reaches 10% is the start temperature Ts. Thus the Ts of Chinese yam starch in water was 62.8 C. When the DGI reached 50%, the corresponding temperature was 67.7 C, and the conclusion temperature (TC) was 80.1 C.
As could be seen from Fig. 2, only four representative photographs were selected to show the different gelatinization state of Chinese yam starch. Under 64 C, we could not see any apparent/ detectable change under the birefringence light, it was the ungelatinized state; under 68 C, the photograph showed obvious decrease of light intensity and/or area of birefringence light, which represented the partially gelatinized state; and the starch granules were in the totally gelatinized stage under 84 C for the birefringence light totally disappeared. The X-ray diffraction pattern of our Chinese yam starch was showed in Fig. 3. Relative strong diffraction peaks were showed at 5.7, 11.4 , 15.1, 17.1, 22.3 and 23.8 2q. Unlike other starches which also came from the tubers, namely B-type, potato starch (Van Soest, Hulleman, De Wit, & Vliegenthart, 1996), A-type sweet potato and cassava starch (Da Cruz Francisco, Silverio, Eliasson, & Larsson, 1996; Rocha, Carneiro, & Franco, 2010), our Chinese yam starch sample could be classified as C-type, a mixture of A-type and B-type. Our results were in agreement with the results reported by Shujun Wang (Wang et al., 2006, 2009). According to their research, the crystal type of starches separated from different D. Opposita cultivar was a typical CB-type pattern. In the diffraction spectra, all of the starches separated from different D. Opposita Thunb. Cultivars showed strong diffraction peaks at 6.5 , 6.2 , 17.8 , 20.1, 27.4 2q. The A- and B-type allomorph coexisted in the individual C-type starch granule. The outer semicrystalline and amorphous growth rings were mostly composed of A-type allomorph whereas the amorphous or less crystalline areas were predominantly composed of B-type allomorph. Focus on the MRDCC of Chinese yam starch heated in pure water as showed in Fig. 4 (the black curve). Unlike corn starch which was a typical A-type starch, the MRDCC of Chinese yam starch did not showed an uni-modal curve as presented in our previous work (Li et al., 2015). The big platform shoulder peak in the right-side of the main peak showed the multi-stage gelatinization process of Chinese yam starch. Two obvious peak temperature point (Tp1 and Tp2) corresponding to peak B and A were appeared in the figure: Tp1 was 67.5 C and Tp2 was 79.9 C. Peak B mainly related with the gelatinization of B-type allomorph in Chinese yam starch while peak A mostly connected with that of A-type allomorph. Like the situation of pea starch (Bogracheva, Morris, Ring, & Hedley, 1998; Li et al., 2013), A allomorph is firmer than that of B for its less porous structure character, and B polymorphs namely the amorphous or less crystalline areas, gives a transition with a lower peak temperature than A. It was showed that the B polymorphs gelatinized first and then decreased the melting temperature of the neighboring crystallites (A polymorphs) resulting in the progressive disruption of crystalline areas. 13C CP/MAS nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) results also (Wang et al., 2009) revealed that B-type allomorph in Chinese yam starch was hydrolyzed more rapidly than A-type. The gelatinization of Chinese yam starch around 62e72 C had a great influence on the overall crystallization, which could also be reflected from the RDCC of Chinese yam starch. 3.2. The gelatinization process of Chinese yam starch in different concentration of NaCl solution
Fig. 1. Relationship between temperature and DGI of Chinese yam starch under different concentrations of NaCl.
The dynamic gelatinization process of Chinese yam starch heated in different concentration of NaCl solutions could be seen from Fig. 5. The influence of different concentration of NaCl solutions on the gelatinization process of Chinese yam starch was different. The relationship between temperature and DGI of Chinese yam starch when heated in different concentrations of NaCl also could be observed from Fig. 1. During heated in excess salt solution, the A
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Fig. 2. Micrographs of Chinese yam starch at different temperature (50, 64, 68, 82 C) when heated in water under polarized light (200).
and B polymorphs within C-type granules melted independently. The influence of different NaCl concentration on the A- and B-type allomorph of Chinese yam starch was different. For B-type allomorph (before about 72 C), 1 M and 2 M of NaCl inhibited the gelatinization with 1 > 2 M while 3 M and 4 M promoted gelatinization with 4 > 3 M significantly. When the DG reached 50%, the gelatinization temperature corresponding to heated in pure water, 1, 2, 3 and 4 M of NaCl were 67.7, 69.6, 68.3, 66.6 and 61.3 C respectively. As to the A-type allomorph, both of the NaCl concentration (1e4 M) showed inhibition effect (after about 72 C) on
the gelatinization. This result was the same as the situation of pea starch (Li et al., 2014). Fig. 4 also showed the MRDCC of Chinese yam starch heated in different concentration of NaCl solution. Like the condition of corn starch heated in same solutions (Li et al., 2014), the addition of NaCl also had no impact on the crystalline change peak shape (the curve shape of MRDCC). But the influence of NaCl on the A-type and Btype allomorph was different. The Tp1 corresponded with pure water, 1 M, 2 M, 3 M, 4 M of NaCl were 67.5, 68.6, 68.1, 65.3 and 59.7 C respectively. Tp1 increased slightly to a maximum value (~1 M) and then decreased with increasing concentration.
Fig. 3. X-ray diffraction patterns of Chinese yam starch.
Fig. 4. The MRDCC of Chinese yam starch heated with different concentrations of NaCl.
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Fig. 5. Micrographs of Chinese yam starch at different temperatures and different concentrations of NaCl solutions at different temperature during heating process under polarized light (200).
Combined with the results in Figs. 1 and 3, we made a conclusion that B-type allomorph accounted for about 80% of the whole crystalline structure in Chinese yam starch because the DGI corresponding to the BeA turning point (72 C) was about 80%. The Atype allomorph just accounted about 20% of the whole crystalline structure and its melting process were also influenced by the gelatinization of B-type allomorph, as a result, it seemed that it was hard to analyze it separately.
Many studies showed that salts like NaCl affected starch gelatinization in different way as KSCN (Jane, 1993). In the research of Jane with DSC, for example, To and DH of starch gelatinization were consistently decreased with the increase of KSCN and KI concentration, but the gelatinization of starches were changed in a complex mode with the increase of NaCl and CaCl2 concentration. The gelatinization pattern of corn starch in KSCN and KI solutions began at the hilum and the pattern remained unchanged with salt
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concentration. In contrast, the gelatinization pattern in NaCl and CaCl2 solutions varied with the salt concentration. The increase of NaCl concentration also caused the loss of allomorph to occur from the granule surface (Gough & Pybus, 1973). When the concentration of NaCl was less than 2 M, the electrostatic interaction between starch eOH groups and Naþ ions was not significant, the inhibition effect of low NaCl concentration contributed to the dominated water structure-making effect of Naþ; however, when the concentration of NaCl exceeded 2 M, the dipoleemetal interaction between starch and Naþ became dominant. As a result, 4 M showed such a great improvement effect on the gelatinization of Chinese yam starch. 3.3. The effect of different chlorine salts on the gelatinization of Chinese yam starch The DGI-temperature relationship of Chinese yam starch heated in different chlorine salts could be seen from Fig. 6. All of the chlorine salts beside FeCl3 inhibited the gelatinization of Chinese yam starch. The inhibition effect was MgCl2 > NaCl > KCl CaCl2, while the FeCl3 showed significant promotion effect. When the DG reached 50%, the gelatinization temperature heated in pure water, 1 M of NaCl, KCl, CaCl2, MgCl2 and FeCl3 were 67.7, 69.6, 69.1, 68.6, 72.7 and 64.8 C. From Fig. 7, we also knew that their Tp1 were 67.5, 68.6, 68.3, 68.3, 71.5, 64.2 (p < 0.05) C respectively. It has been reported that the chloride ion was located in the center of lyotropic series and thus was the least effective anion for modifying gelatinization behavior (Gough & Pybus, 1973). According to the theory proposed by Frank and Wen (1957) and later confirmed by many researchers (Ahmad & Williams, 1999; Jane, 1993): ions of high charge density, such as (Mg2þ) had strong electrostatic interactions with water molecules, they increased the viscosity of the aqueous solution by hydrogen bonds between water molecule and thus decreased the fraction of free water. Infrared spectroscopy (Paquette & Jolicoeur, 1977) showed that these ions promoted water structure, reduced the fraction of free water and increased the viscosity of the solution, liquid water was a mixture of hydrogen-bonded clusters and unbounded free-water molecules. As monovalent salts, the size of the K ion was bigger than Na, which means that the charge densities of Naþ was bigger than Kþ and thus the inhibition effect on gelatinization of NaCl was greater than KCl. For the same reason, the inhibition effect of MgCl2 was stronger than CaCl2. However, Ca2þ and Fe3þ showed opposite trend with higher charge densities than Naþ and Kþ under the
Fig. 6. Relationship between temperature and DGI of Chinese yam starch under different chlorine salts (NaCl, KCl, CaCl2, MgCl2, FeCl3) effect.
Fig. 7. The MRDCC of Chinese yam starch heated with different chlorine salts.
concentration of 1 M. The result of Ca2þ was in agreement with that of Rumpold (Rumpold & Knorr, 2005), the impact of salts on three kinds of starch (wheat, tapioca and potato) followed the order Ca2þ >Naþ and Kþ appeared only in higher concentrations (>2 M); under 1 M, the gelatinization inhibition effect of Ca2þ was smaller than that of Naþ and Kþ. In the research of Ahmad (Ahmad & Williams, 1999), the To of CaCl2 at 1 M (64.2 C) was lower than that in NaCl (66.5 C) and KCl (64.4 C). The Granule disruption mechanism of FeCl3 might be completely different from other chlorides in our study which was also confirmed by Gough and Pybus (1973) previously. With high charge density, cations like Ca2þ and Fe3þ increased the water structure on the one hand, while attracted starch eOH groups and destabilized starch granules with generated heat on the other hand. Under the concentration of 1 M, the electrostatic interaction between starch eOH groups and Fe3þ ions was so strong and significant that it induced the melting of the granules quickly.
3.4. The effect of different sodium salts on the gelatinization of Chinese yam starch Fig. 8 showed the DGI-temperature relationship of Chinese yam starch heated in different sodium salts. Unlike the influence of different concentrations of NaCl which had different effect on A and B-type allomorph, under the same concentration (1 M), the effect of different sodium salts on these two type allomorph were approximate consistent. Different sodium salts showed almost the same DGI -temperature curve trend. NaCl and Na2SO4 inhibited the gelatinization with Na2SO4 > NaCl dramatically while NaNO3 and Na2CO3 accelerated the disruption process of granule with NaNO3 > Na2CO3. When the DG reached 50%, the gelatinization temperature heated in pure water, 1 M of NaCl, NaNO3, Na2CO3 and Na2SO4 were 67.7, 69.6, 63.9, 65.7 and 79.6 C separately. From Fig. 9, we also knew that their Tp1 were 67.5, 68.6, 63.9, 65.1, 79.5 C respectively. The influence of various neutral anions was in accordance with the Hofmeister series (Evans & Haisman, 1982; Jane, 1993). The gelatinization temperature decreased in the order 2 SO2 4 > Cl > NO3 . With high charge density, anions like SO4 , had strong electrostatic interactions with water molecular reduced water activity, at the same time, they repeled starch eOH groups and stabilized starch granules, and the repulsion was proportional
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Fig. 8. Relationship between temperature and DGI of Chinese yam starch under different sodium salts (NaCl, NaNO3, Na2CO3, Na2SO4) effect.
Fig. 10. Relationship between temperature and DGI of Chinese yam starch under two different sulphates (Na2SO4, CuSO4) effect.
to the charge density of the anions (Jane, 1993). Anions like NO 3 had low charge density, broke water structure and formed helical complexes with starch chains, resulting in the degradation of the granule. The ability of aqueous alkali in inducing starch disruption was common knowledge (Ragheb, Abdel-Thalouth, & Tawfik, 1995; Wootton & Ho, 1989). The result of Na2CO3 was consistent with those which had been reported earlier (Zhou et al., 2011). Besides, the increase of gelatinization transition and peak temperature were also observed in starches like wheat, rice and corn in the presence of excess water and low Na2CO3 concentrations (1e4 g/100 g starch) (Lai, Karim, Norziah, & Seow, 2002). This effect was attributed to the structure making effect of Naþ.
great inhibition effect on starch gelatinization because of the existence of SO2 4 while the effect of Na2SO4 was bigger than that of CuSO4. When the DG reached 50%, the gelatinization temperature corresponding to heated in pure water, 1 M of Na2SO4 and CuSO4 were 67.7, 79.6 and 77.7 C. From Fig. 11, we also knew that their Tp1 were 67.5, 79.5 and 77.8 C respectively. Back to Figs. 1 and 4, we realized that the influence of 2 M Naþ on the gelatinization process was small. The gap of Tp1 and temperature corresponded to 50% DGI between heated in pure water and 2 M NaCl were both 0.6 C which could be neglected, then comparison could be made by acting Na2SO4 (2 M of Naþ) as a reference. Under 1 M, the water structure making effect of Cu2þ was smaller than the destabilization effect of the heat generated by the Cu2þ and starch eOH groups attraction.
3.5. The effect of two different sulphates on the gelatinization of Chinese yam starch
4. Conclusion
The DGI -temperature relationship of Chinese yam starch heated in two sulphates were compared in Fig. 10. Both of them showed
Salts have been found to have a complex effect on the gelatinization process of Chinese yam starch (D. opposita) when heated in water medium. The Chinese yam starch involved in our study was a
Fig. 9. The MRDCC of Chinese yam starch heated with different sodium salts.
Fig. 11. The MRDCC of Chinese yam starch heated with two different sulphates.
Q. Li et al. / Food Hydrocolloids 51 (2015) 468e475
C-type starch which could be confirmed both from X-ray diffraction and the MRDCC figure. In MRDCC, the first peak corresponded to the B-type polymorphs within the C-type Chinese yam starch while the second peak corresponded to the A type polymorphs outer moiety of the C-type crystal. We could deduce that the B-type allomorph accounted about 80% of the whole C-type starch crystalline. The influence of different NaCl concentrations on the A- and B-type allomorph in Chinese yam starch were different. With the increase of NaCl concentration from 0 to 4 M, the DG of B-type allomorph increased at lower concentration to a maximum value (at about 1 M) then decreased with increasing concentration. The influence of various neutral anions was in accordance with the Hofmeister series: DG decreased in the order SO42 > Cl > NO 3; however the situation of cations was far more complicated. As aqueous alkali Na2CO3 induced starch disruption under 1 M, and under the same concentration, the destabilization effect induced by the electronic interaction between Cu2þ and starch eOH groups was bigger than the water structure making effect of Cu2þ. Abbreviations used IOD DG DGI MRDCC RDCC DSC NMR XRD
integral optical density the degree of gelatinization the degree of gelatinization measured by IOD method the model of response difference of crystalline change response difference of crystallite change differential scanning calorimeter nuclear magnetic resonance X-ray diffraction.
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