AIR OXIDATION OF POTATO STARCH OVER ZINC (II) CATALYST

Dorota Gumul, Halina Gambuś, Marek Gibiński
Department of Carbohydrates Technology, Agricultural University of Cracow, Poland

ABSTRACT

Oxidation is one of the most frequently applied methods of chemical modification of starch.The process of starch oxidation can take place in different ways depending on the applied oxidation agent such as: hypochlorite sodium, hypochlorite potassium, hydrogen peroxide, potassium permanganate, periodate. An attempt to oxidize starch was made using: air, oxygen, ozone and the transition metals from VIII series.

The aim of our work was to obtained potato starch oxidized by air over Zn (II) catalyst and to analyze the selected physico-chemical properties of the obtained oxidized starches.

The oxidation process leads to a decrease in the amount of phosphorus and linear fraction–amylose in the starch. The result of the oxidation process is an increase in the number of carboxylic and carbonyl groups with a clear preference of forming those latter. The oxidized starches over catalyst are characterized by lower max viscosity in comparison to the native starch, which results from starch depolymerization during the reaction of oxidation. It is evident by a big number of the carbonyl groups. Paste stability at the temperature of 96ºC prepared from oxidized starches, in comparison to paste made from native starch is many times bigger thanks to the presence of the carboxylic and carbonyl groups in these starches.

Key words: starch; oxidation; DSC; SEM; carboxylic and carbonyl groups.

INTRODUCTION

Oxidation is one of the most frequently applied methods of chemical modification of starch. It relies on producing carbonyl and carboxylic groups in its granules depending on the applied oxidant and the conditions of oxidation. The reaction of starch oxidation is usually accompanied by depolymerization of big molecules and by loosening intermolecular bindings [13].

In the works of Torneport et al. [24] and Muhrbeck et al. [16] potato starch underwent the process of brome oxidation. It was stated that at a lower level of brome oxidation, amylopectin was degradated probably mainly in amorphous areas as well as amylose. Moreover, it was pointed out that in same causes starch depolimerization is unavoidable which is indicated by the decrease in molecular mass of the samples of the starch modified with brome when the degree of oxidation increases resulting at the same time in the decrease of viscosity of water – starch suspensions.

Starch pastes prepared from oxidized starch are characterized by high degree of stability of viscosity and they have a lower tendency to retrogradation and syneresis [13].

Granular or pasted starch is subjected to modification but in the case of granular starch the surface of granules undergoes a reaction and a reaction inside the granule is exceptional and take place only when reagent is big capable of penetrating starch [23].

Many factors influence the course and results of oxidation process. They are as follows: a botanical origin of starch, the size of starch granules and pH of the reaction medium as well as the kind of oxidant. Kuakpetoon and Wang [12] proved a different ability to take in oxygen by starches of various botanical origins oxidizing them with hypochlorite sodium. The biggest increase carboxylic groups were for potato starch and the smallest for maize starch, which is explained by a different content of protein in starch. According to the authors in the first stage hypochlorite sodium oxidizes proteins and latter starch. However Forsell et al. [6] used the same oxidant and obtained a higher degree of oxidation of potato starch compared to barley one. Big starch granules undergo oxidation more easierly than small starch granules [23]. The rate and degree of starch oxidation and the proportions of the content of carbonyl and carboxylic groups depends on the value of pH [13]. Both Abdel–Hafiz [1] who was oxidizing maize starch with sodium chlorite and Parovuori et al. [18] who were oxidizing potato starch with hydrogen peroxide stated that the process of oxidizing is more effected in the acidic medium than alkaline.

The process of starch oxidation can take place in different ways depending on the applied oxidation agent such as: hypochlorite sodium, hypochlorite potassium, hydrogen peroxide, potassium permanganate, periodate [6, 12, 13]. An attempt to oxidize starch was made using: air, oxygen, ozone and the transition metals from VIII series. Conca and Brussani [5] oxidizing starch in the alkaline medium using the elements from the VIII transition series such as copper and silver stated that oxidation did not cause a total destruction of starch and oxidized starch products can be used as: ink thickeners, coatings and detergent builders.

There is a reporting the literature describing the air oxidation of corn starch over vanadium (V) catalyst [8]. The authors claimed the oxidation of starch for carboxyl and carbonyl compounds with a preference to the latter. They also observed that the oxidation products gave more viscous gels than unprocessed starch.

It is essential that the oxidation products retained their granular structure. In such type reactions the penetration of air might be strongly dependent on specific structure of starch granules i.e. their ability to swell and be penetrated by the oxidant. This paper, as our previous two publications [2, 3] describes the course and result of air oxidation of potato starch following the method described by Harmon et al. [8].

The aim of our work was to obtained potato starch oxidized by air over Zn (II) catalyst and to analyze the selected physico-chemical properties of the obtained oxidized starches.

MATERIALS AND METHODS

Potato starch was isolated by Pila Potato Enterprise in Poland, ZnO was obtained from POCh Gliwice (Poland).

Oxidation

Suspension of air – dried potato starch containing 12% moisture (50 g) in distilled water (500 cm³) was supplemented with ZnO (either 2.5, 10 or 20 g) . The pH of each blend was adjusted to 9.0 by addition of 10% aq. NaOH. Each continuously stirred reaction mixture was maintained at 35-40°C and was aerated for 48 h with a slow air stream. Since in this period pH decreased to 7.0 a subsequent volume of 10% aq. NaOH was added to rise pH to 9.0. After 48 h reaction mixture was filtered and rinsed on the filter with re-distilled water (1000 cm³). The product was dried in air. The control samples were prepared analogously but without aeration.

Analyses

Content of amylose and phosphorus were determined according to Morrison and Laignelet [15] and Marsh [14] respectively.

Carboxylic and carbonyl groups were determined according to Norma ISO 11214 [17] and Potze and Hiemstra [20], respectively.

Aqueous solubility and water binding capacity was determined according to Richter, Augustat and Schierbaum [21].

Characteristic of gelation were carried out for 7.2% aqueous starch suspensions using the Brabender viscograph (Germany) and following the standard program i.e. rotor velocity was 75rpm, rate of temperature increase from 20 to 96°C was 1.5°C × min^-1. Then the gel was maintained on stirring for 20 min at 96°C followed by cooling to 50ºC with the 1.5°C rate of the temperature decrease.

Thermodynamic determination of gelation characteristics: This determination was carried out according to Fredriksson et al. [7] using Shimadzu 60 W differential scanning calorimeter (Columbia, MD, USA). Sample : water = 1:2 were sealed in aluminium pans and left for equilibration for 2 hours prior to analysis. Instrument was calibrated with indium. Heating in the range of 20–120°C at the rate of 10°C × min^-1 was done with precision of ± 0.5°C and precision of gelation enthalpy was ± 0.05 J × g^-1 of dry substance.

Electron micrographs were made in scanning electron microscopy JEOL ISM 5400.

One factional ANOVA was used to estimate the significance of differences between the obtained results. Calculations were made with a computer program Start Skierniewice 1998.

RESULTS AND DISCUSSION

Table 1 contains the results of the phosphorus content in a native potato starch and an oxidized and non-oxidized starches over Zn (II) catalyst. As one can conclude from Table 1 the content of phosphorus in the native starch is bigger than all the analyzed samples with the exception of three samples: the oxidized and non-oxidized starches without catalyst and non-oxidized starch with 2 g addition of Zn (II).

Comparing the oxidized starches using a different amount of zinc (II) with the non-oxidized starches one can conclude that the first one were characterized by a smaller total content of phosphorus than the other one (Table 1).

Along with the increase in the amount of the used catalyst the content of phosphorus was going down in all the modified samples of starch (Table 1) which is confirmed by the data from earlier papers, because many scientist emphasized that results of oxidation was a small amount of the phosphorus bound in the starch [9, 13].

The content of phosphorus in the native starch is within the limits determined for this non-carbohydrate component in the early works [13].

Analyzing the data presented in Table 2 it was claimed that generally the content of amylose in the oxidized starches (or not) with an addition (or without) zinc (II) as a catalyst was smaller in comparison with the native starch. In this regard the exception was the non–oxidized starch without the addition of catalyst which contain 27.75% of amylose.

Taking the content of amylose in the oxidized and non-oxidized starches into account one can observed some fluctuation in the value of this linear fraction. The content of amylose in oxidized starches without the addition of catalyst and with the use of 2 g zinc oxide was lower than in the case of the analogical samples of the non-oxidized starch, while using the other addition of catalyst the content of amylose in the oxidized starches is slight higher in comparison to the analogical samples of the non-oxidized starch (Table 2).

It was generally accepted that along with the increase in the amount of the used catalyst the content of amylose was gradually going down in all the modified samples of the starch (Table 2).

The content of the linear fraction in the native potato starch was in accordance with the result of the other authors [22].

Table 3 presents the content of carbonyl groups in the native potato starch and in the oxidized and non-oxidized starches over a different amount of zinc (II) catalyst.

The content of carbonyl groups in the oxidized and non-oxidized starches without applying a catalyst was 3.5 times higher than in the native starch. However, the content of the carbonyl groups in the starches oxidized with air (or not) over Zn (II) catalyst was many times (in some samples even 22-times) higher than the content of these groups in native starch.

The content of carbonyl groups was gradually decreasing along with the increase in the amount of the Zn catalyst in all the analyzed samples of the starch. The oxidized starches were characterized by a higher content of the carbonyl groups than the non-oxidized starches over catalyst (Table 3).

Such a big content of the carbonyl groups in the modified starches can probably be connected with reaction of hydrolysis taking place during oxidation and resulting in depolymerization of the starch. It was confirmed by determining viscosity of pastes made from the oxidized starches, using a Brabender viscometer, because the viscosity of these pastes is much lower in regard to the native starch. According to Boruch [4] an integral part of starch oxidation is a hydrolysis of this polysaccharide resulting in obtaining a big number of the carbonyl groups. Therefore, the content of the carboxylic groups rather than the carbonyl ones is a determiner of starch oxidation. It is so, because the carbonyl groups are the results of side effect of hydrolysis in the process of oxidation.

Table 4 presents the content of the carboxylic groups in the native starch and potato modified starches. The content of the carboxylic groups in oxidized starches subjected to a different amount of Zn, as catalyst was higher than in the non-oxidized starches over the same amount of catalyst. It suggests that air is an oxidant and zinc oxide is a catalyst accelerating this process.

It is worth noticing that along with the increase in the amount of the used catalyst the content of carboxylic groups was growing in the starches. The exception was a sample of the starch with an addition of 20 g of ZnO with which the content of carboxylic groups was rapidly going down, both for the oxidized and non-oxidized starches (Table 4). It can be firmly stated that the amount of the used catalyst has no linear impact on the results of starch oxidation.

Comparing the results of content of the carbonyl and carboxylic groups in starches it can be noticed that along with the increase of the amount of the used catalyst the content of carbonyl groups decreases (Table 3), but the carboxylic groups increase (Table 4) for two kinds of starch: the oxidized and non-oxidized. It confirms the fact that Zn catalyst is an intensifying factor in starch oxidation process.

According to Pfannemüller [19] potato starches are characterized by a rapid increase in their ability to bind water within the range 20-95°C and solubility within the temperatures from 50-95°C. Therefore, a swelling characteristic was determined at three temperatures: 25, 40 and 60°C (Tables 5a and 5b).

Generally solubility of starches, both the oxidized and non-oxidized over Zn catalyst was higher than of the native one within the above range of temperatures. The highest solubility was characterized for the oxidized and non-oxidized starches with the smallest addition of the catalyst, however, the lowest one with the highest amount of the catalyst, in regard to the native starch (Tables 5a and 5b). It was stated that the biggest amount of the catalyst was used, solubility was decreasing within the analyzed the range of temperatures. At the temperatures 25 and 40°C solubility of the modified starches was slightly increasing,but decisive increase was observed at the temperature 60°C, which is confirmed in the numerous paper [19].

Solubility of the oxidized starches over catalyst was lower than for the non-oxidized starches, particularly, at the temperatures of 25 and 40°C, but in both cases it was higher than the native starch.

Water binding capacity of the oxidized and non-oxidized starches over Zn catalyst was also higher compared to native starch and was decreasing along with the increase of amount of the used catalyst (Tables 5a and 5b). Water binding capacity was growing along with the temperature but for the oxidized and non-oxidized starches with the addition of 5 g of ZnO water binding capacity at temperature of 40°C was characterized by a slight decrease compared to the temperatures of 25 and 60°C (Tables 5a and 5b).

It is worth emphasizing that starches containing a big amount of the carbonyl groups were characterized by higher water binding capacity and solubility (Tables 3 and 5a and 5b). It results from the hydrolysis reaction-taking place in the oxidation process, because solubility of starch increases proportionally to the decreasing length of starch chain [19].

Analyzing the temperature of the beginning of pasting of the oxidized and non-oxidized over ZnO can be observed that it oscillated around the temperature of pasting for the native starch, which is 64.5°C (Tables 6a and 6b).

All the modified starches were characterized by decisively lower max viscosity in regard to the native starch, as a result of hydrolysis, which was an integral part of the oxidation process (Tables 6a and 6b).

The difference between viscosity after 20 min at the temperature of 96°C and max viscosity at 96°C (in Brabenders’ units) determines paste stability at this temperature.

The result presented in Tables 6a and 6b points to the fact that in both the cases of the oxidized and non-oxidized starches an addition of catalyst was causing an increase in paste stability, but did not influence this property in a linear way. Paste stability of modified starches is many times bigger than the paste prepared from native starch. A higher paste stability at a given temperature which are prepared from the starch oxidized with air over catalyst at a given temperature seems to be an important achievement of this work. It should be stated that the highest stability was characteristic for the pastes prepared from starches with the lowest addition of catalyst because it showed the highest number of carbonyl groups (Tables 3 and 6a).

In Tables 7a and 7b gelation enthalpy and temperatures characterized for the gelation process are presented. The research conducted in this work proved that gelation enthalpy of the all the modified samples was lower than the native starch and was getting smaller along with the amount of the used catalyst (Tables 7a and 7b). As for two samples of the oxidized and non-oxidized starch with an addition of 10 g of ZnO gelation enthalpy was the lowest of all the analyzed samples (Tables 7a and 7b). According to Jankowski [11] an oxidation causes the reduction of enthalpy, which has been confirmed in this paper.

Comparing the parameters of thermodynamic characteristic of gelation of native starch we can see the accordance with the earlier works [7, 25].

The microphotographs made with SEM show a big differentiation in granulation starch granules. Confirming the publication about this subject by [10, 22] comparing the microphotography of the native starch with the modified one a number of differences have been observed starting with a loss of smoothness of the starch granules. The microphotographs of all samples of the oxidized starch allowed detecting the changes on the surface of starch granules, which cannot be avoided in any process of modification (Figures 1 and 2). These changes on the surface of the starch granule are desirable due to a bigger contact surface of the starch with a reagent. According to Tomasik [22] it was important that all the products of oxidation retain their granulation, which has been achieved in the present work.

CONCLUSIONS

1. The oxidation process leads to a decrease in the amount of phosphorus and linear fraction – amylose in the starch.

2. The result of the oxidation process is an increase in the number of carboxylic and carbonyl groups with a clear preference of forming those latter.

3. Solubility and water binding capacity of oxidized starches over catalyst is bigger than of the native starch especially at the temperature 60°C.

4. The oxidized starches over catalyst are characterized by lower max viscosity in comparison to the native starch, which results from starch depolymerization during the reaction of oxidation. It is evident by a big number of the carbonyl groups.

5. Paste stability at the temperature of 96°C prepared from oxidized starches, in comparison to paste made from native starch is many times bigger thanks to the presence of the carboxylic and carbonyl groups in these starches.

6. Gelatinization entalphy of all the modified starches determined by using DSC is smaller than the native starch.

7. Zink (II) in the air oxidation process of starch plays a role of a catalyst,but the amount of the used catalyst influence both the degree of oxidation, as well as the physico-chemical properties of so modified starches, so the concentration of ZnO should not exceed 10 g/50 g strach.

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