THE CHANGES IN THE INTERNAL STRUCTURE OF STARCH GRANULES CAUSED BY OXIDATION

Sławomir Pietrzyk
Department of Analysis and Evaluation of Food Quality, Agricultural University, Cracow, Poland

ABSTRACT

Native starches, such as potato, wheat, maize, and waxy maize, were subject to oxidation with sodium chlorate(I), hydrogen peroxide in the presence of Cu²⁺ ions, and sodium chlorate(III) in the presence of formaldehyde. Native starches and their modified preparations were determined for the content of carboxyl and aldehyde groups. In order to determine changes in the internal structure of starch granules, thermodynamic characterization of phase transitions (DSC) was taken and the degree of crystallinity with X-ray diffractions was determined. Potato starch, after oxidation with the applied reagents, became slightly more crystalline, while cereal starches were affected very little. The thermodynamic gelatinization indices obtained for oxidized starches were only changed a small amount in comparison to native ones, which means that the internal structure of the granules is almost unaffected by oxidation.

Key words: starch, oxidation and internal structure.

INTRODUCTION

Due to its structure, starch can react with a wide variety of chemicals. For example, the glucose residues building starch chains have free hydroxyl groups which could be oxidized.

Under oxidation, carbonyl and carboxyl groups are formed in the amounts that depend on the oxidative agent, reaction conditions and botanical origin of starch. Oxidation is frequently accompanied by depolimerisation of starch glucans, stretching of intermolecular bonds and changes in the surface structure of starch granules [14, 23, 24].

The structural changes of starch can be followed by various techniques including X-ray diffraction. X-ray diffraction estimates the crystallinity and differential scanning calorimetry (DSC) used to quantify the phase transitions occurring in starch by such parameters as gelatinisation enthalpy (DH) and onset/peak temperatures (T_o, T_p). Wang et al. [21] points out that the gelatinisation enthalpy as measured by DSC depends on energy changes caused by:

• disruption of crystalline network

• destruction of double helices

• disturbances in amorphous phase of starch granules

• changes occuring during swelling

According to Woolton and Bamunuarachi [22], the formation of new chemical groups in starch granules and the depolimerisation of glucans result in lowering of transition temperatures and gelatinisation enthalpy. Veelaert et al. [20] noticed the drop in gelatinisation enthalpy accompanying the increasing degree of oxidation in potato starch modified with sodium metaiodate(VII). They attribute this relation to a loss of crystallinity in dialdehyde starch.

Muhrbeck et al. [11] found the decrease of enthalpy in potato starch oxidized with bromium to correlate with rising reductive properties. They also observed that oxidation with bromium extends the temperature range of phase transitions.

Eliasson and Karlsson [1] noticed that large granules of starch reveal a higher onset gelatinisation temperature and lower peak temperature in comparison to small ones. However, they did not observe changes in the gelatinisation enthalpy.

The differences in gelatinisation parameters D, H, T_o, T_p) between starch isolated from different varieties of rice and waxy rice were reported by Jane et al.[7]. According to these authors the differences are caused by genome and amylose content in starch.

Noda et al. [12] demonstrated relatively high correlation between the amount of chains with DP 13-17 in amylopectin and the gelatinisation parameters (D, H, T_o, T_p) in sweet potato and buckwheat.

The influence of genetics on the above parameters have been studied by Singh and Singh [17] who observed changes in these parameters for different potato cultivars. They point out that such changes are more connected with the presence of double helices in starch granules than with crystallinity. According to them, the rise in gelatinisation enthalpy occurs during maturation of plant, from which starch is isolated.

The studies on starch oxidation focus on physico-chemical properties and almost ignore the changes that occur in the internal structure of starch granules. The aim of this study was to describe such changes following the oxidation process.

MATERIALS AND METHODS

The starches used in the experiment included wheat starch and potato starch "Superior" produced by ZPZ Niechlow and waxy and normal corn starch from National Starch and Chemical. The starch samples have been oxidized by three different methods.

1. Modification with sodium chlorate(I) was performed according to Forsell et al. [2]. First, 400 g of starch was weighed and dispersed in water. Then, the water solution of NaOCI (commercial, activity 100 gCI/dm³, POCh) was slowly added to obtain a 40% suspension. Modification was then performed at room temperature by mixing the starch suspension in an alkaline medium (pH = 10.0) for 50 minutes and subsequently neutralizing the reaction mixture with a 1 M H₂SO₄ solution to reach a pH of 7.0. The amount of the NaOCI reagent used in the oxidation was equivalent to 40 gCI/kg starch.

2. Modification with hydrogen peroxide (with the addition of 0.1% CuSO₄·5H₂O) was performed according to Parovuori et al. [13]. First, 400 g of starch was dispersed in water to give a 42% suspension. Then, the reaction mixture was thermostated at a temperature of 40°C. Next, a 30% H₂O₂ (pure p.a., POCh) solution was added drop-wise to the reaction mixture to reach its final concentration of 2%. The reaction mixture was then thermostated once more for 60 min.

3. Modification of starch with sodium chlorate(III) in the presence of formaldehyde was performed according to the methodology of Hebeish et al. [6]. First, 400 g of starch was dispersed in water to give a 28% suspension. Then, after the addition of 4g of sodium chlorate (III) (pure p.a., Fluka) and 1.2 g of formaldehyde (pure p.a., Chempur), the suspension was mixed at a temperature of 50°C for 90 minutes.

Following all three oxidation methods, the modified starches were washed, dried and ground.

The following analyses were performed on the native and modified starches:

1. The amount of carboxyl groups according to ISO 11214 [10].

2. The amount of aldehyde groups according to Potze and Hiemstra [15].

3. The degree of crystallinity (DoC) with X-ray diffractometer XRD 7 (Seifert, Germany) based on the equation [21]

where Doc is the degree of crystallinity, I_semi is the intensity of diffraction on crystalline and amorphous phase and I_amorph is the intensity of diffraction on the amorphous phase.

4. The thermodynamic description of starch pasting (DSC) was obtained with a differential scanning calorimeter Perkin-Elmer DSC - 7 (Connecticut, USA). Starch samples (5-9 mg) were inserted into the aluminum pans (20 µl), weighed, combined with a double weight of water, hermetically closed and left for 1 h to equilibrate moisture. The pans were then heated in DSC in the range 20-90°C/min. An empty pan was used as a reference. From the thermograms the onset and peak temperatures were read and the phase transition enthalpy was calculated taking into account the mass of starch. Before the measurement, the DSC was calibrated with an indium standard. The accuracy of the temperature measurement was +/-0.5°C and the error in the calculation of the enthalpy was +/-0.05 J/g.

To determine the significance of the differences between the contents of carboxyl and aldehyde groups, use was made of one factorial analysis of variance and Duncan´s test.

RESULTS AND DISCUSSION

Most of the samples oxidized with sodium chlorate (I) revealed higher amounts of carboxyl groups in comparison to the starch modified with other methods. Only in the case of the wheat starch were higher amounts of carboxyl groups found after treatment with hydrogen peroxide. The lowest efficiency was observed in the method using sodium chlorate (III) and formaldehyde. It was reported before by others [3, 4, 6, 13] that potato starch was most prone to the action of sodium chlorate(I) and hydrogen peroxide, while corn starch was best oxidized with sodium (III) chlorate.

The results on aldehyde groups present in modified starches (Table 1) provide evidence to conclude that potato starch has the highest tendency to be hydrolyzed by the applied agents of all the studied starch samples, as in its case the most significant rise in aldehyde groups was observed. It is in agreement with the results of Kuakpetoon and Wang [9]. Among the oxidizing agents, hydrogen peroxide caused maximal increase of aldehyde groups, which is in accordance with our previous reports [4].

The degree of crystallinity in native starches and its changes were presented in Figure 1. It can be observed that among native starches, potato starch reveals the lowest degree of crystallinity (6%). This is due to the fact that its granules represent B type crystallinity, while cereal starch is of a type A [8, 21]. The highest degree of crystallinity was observed in the case of waxy corn starch (21%). Shi et al. [16] explain the fact that waxy corn starch exhibits higher degree of crystallinity by higher density of double helices in the crystalline phase of amylopectin.

The oxidation of potato starch with applied chemicals caused the crystallinity to almost double. It signifies that during this process the amorphous phase of potato starch is significantly changed, probably due to the formation of carboxyl groups. Such groups are involved in the formation of crystalline structures. The amount of phosphorus can also influence the crystallinity of potato starch, so its removal in modified starch preparations (by washing) could increase the crystallinity. Jane et al. [7] observed that the lowered content of starch phosphorus is correlated with increased crystallinity. Kuakpetoon and Wang [9] stated that there was no change in the crystallinity of potato starch oxidized with sodium chlorate (I). According to them, the high tendency for potato starch to oxidize is connected with the amorphous regions, as the oxidation agents act mainly in this part of the starch granules. This would explain the higher increment of carboxyl groups in comparison to cereal starches confirmed also by our results (Table 1).

Wheat starch crystallinity was decreased only after the action of hydrogen peroxide (from 14 to 12%), other oxidizing agents did not influence this parameter. The loss of crystallinity could be caused by depolimerisation (associated with oxidation) of crystalline clusters present in amylopectin.

The oxidation of corn starch with sodium chlorate (I) and sodium chlorate (III) caused an increase in crystallinity, while there was no change after hydrogen peroxide. It is possible that sodium chlorate (I) and (III) caused a formation of carboxyl groups in the amorphous part of corn starch granules, which resulted in partial reorganization of these regions. Kuakpetoon and Wang [9] did not observe significant changes in the degree of crystallinity of corn starch oxidized with sodium chlorate (I).

Waxy corn starch after oxidation with any of the applied chemicals revealed slightly lower crystallinity. It seems that carboxyl groups in amylopectin may disturb the normal structure of its clusters.

In summary, the oxidation of cereal starches influenced their crystallinity in a limited way, as the observed changes did not exceed 10% (in comparison to native starch).

Table 2 includes the thermodynamic characterization of the phase transitions observed for native and oxidized starches.

Gelatinisation enthalpy of native starches varied from 8.4 J/g d.m. for wheat starch to 17.1 J/g d.m. for potato starch. This data is in agreement with the earlier reports [5, 22]. According to Stevens et al. [19] cereal starches reveal lower gelatinisation enthalpy in comparison to potato.

The results show that wheat starch, which exhibits the lowest gelatinisation enthalpy (8.4 J/g d.m.), is also highly crystalline in contrast to potato starch which has the lowest degree of crystallinity (6%) and the highest gelatinisation enthalpy (17.1 J/g d.m.). This indicates that the crystalline structures have little impact on the gelatinisation enthalpy of potato starch.

All the applied methods of starch oxidation caused in most cases a slight lowering of gelatinisation enthalpy (DH), which is in agreement with the studies of Woolton and Bamunuarachi [22]. The exceptions were potato starch oxidized with hydrogen peroxide and waxy corn starch oxidized with sodium chlorate (I), where there was no change in this parameter and wheat starch oxidized with sodium chlorate (III), which after modification revealed higher gelatinisation.

The oxidation of starch resulted in a rise of phase transition onset and peak temperatures. A decrease in transition temperature was observed only in the case of corn starch oxidized with sodium chlorate (I) and (III) and potato and waxy corn starch oxidized with sodium chlorate(I).

Very limited changes were observed in the thermodynamic gelatinisation indices after the oxidation of starch with the applied agents. This proves that the internal structure of starch granules was little affected.

Based on the data from other researchers [17, 18, 21, 22] and our own lab results, it can be stated that the thermodynamic parameters of starch gelatinization depend mostly on the botanical origin of starch and much less on the applied chemical modifications.

CONCLUSIONS

1. Potato starch revealed the highest tendency to modification with sodium chlorate (I) and hydrogen peroxide, while corn starch was most prone to the action of sodium chlorate (III).

2. The oxidation of starch with sodium chlorate (I) and hydrogen peroxide was more effective and caused higher rise in carboxyl and aldehyde groups than the modification with sodium chlorate (III).

3. Potato starch after oxidation with the applied reagents was a little more crystalline, while cereal starches were little affected in this aspect.

4. The thermodynamic gelatinisation indices obtained for oxidized starches were only slightly changed in comparison to native ones, which means that the internal structure of the granules is almost unaffected by oxidation.

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