THE INFLUENCE OF PH AND FE(II) IONS ON PHYSICOCHEMICAL PROPERTIES OF OXIDISED POTATO STARCH

Sławomir Pietrzyk¹, Teresa Fortuna², Łukasz Raś^1
1 Department of Analysis and Evaluation of Food Quality, Agricultural University, Cracow, Poland
² Department of Analysis and Food Quality Evaluation, Agricultural University, Cracow, Poland

ABSTRACT

Potato starch was oxidised with hydrogen peroxide under acidic or alkaline conditions in the presence or without Fe(II) ions as a catalyst. Native starch and the modified products obtained were examined for the carboxyl and aldehyde groups, total phosphorus. Amylose contents and water-binding capacity, water solubility and pasting characteristics were also determined. Oxidation in acidic conditions resulted in a larger increase in the amounts of carboxyl and aldehyde groups, a greater decrease in the amylose and phosphorus levels in modified starch products. The presence of Fe(II) ions during starch oxidation in acidic conditions caused considerable increase of starch solubility at 60°C and whole solubilisation in water at 80°C. Starch oxidation without catalyst both (acidic and alkaline) conditions caused decrease of water-binding capacity at 80°C. The presence of the catalyst in the modification process in alkaline conditions increased thermal stability of the oxidised starch paste.

Key words: oxidised starch, catalyst Fe(II) ions, pH, physicochemical properties.

INTRODUCTION

Due to its specific structure starch can be modified in various physical, chemical or enzymatic ways. Glucose residues included in the starch chains possess free hydroxyl groups, that are susceptible to modification by different agents. Through such reactions, modified starches of required physicochemical and organoleptic properties are obtained.

Oxidation is one of the chemical methods of starch modification producing carbonyl/aldehyde and carboxyl groups in the starch granule. The amount of the created groups depends on the nature of oxidant and the conditions of the process. The botanical origin of starch and thus the structure of granules, has an important role in susceptibility to modification. The most common oxidating reagents are chlorates, peroxides, iodates and air [1,8,17,19,25,28]. Since catalytical oxidation increases the yield of the process, it becomes more and more widespread. The most common catalysts include metal ions, e.g. Cu(II), Fe(II), V(V) and Zn(II) [1,2,5,17,19].

Among various starches, potato starch displays the greatest susceptibility to chemical modification. This is due to the specific inner and outer structure of its granule, which makes it more easily accessible to the reagents [6].

Oxidised starches are used in the food, paper, textile, drilling and founding industries [25]. Oxidation of starch in the food industry is performed mainly in order to improve its rheological properties. Oxidised starches have a number of food applications, e.g. as thickeners and gelating agents for sauces, jellies, blancmanges, etc. [15].

The aim of present work was to determine the effect of Fe(II) ions and the reaction conditions on the effectiveness of potato starch modification by hydrogen peroxide.

MATERIAL AND METHODS

Potato starch “Superior”, produced by ZPZ Niechlów (Poland), was oxidised in an acidic or alkaline conditions, with or without Fe(II) ions (from FeSO₄ · 7H₂O) as a catalyst, as described by Parovuori et al. [17]. 100 g of starch were weighed and dispersed in distilled water in the amount needed to give a 42% starch suspension. The concentration of the metal ions was 0.1% w/w (on dry starch basis). Metal salt was dissolved in water before preparing the suspension of starch. The reaction mixture was heated in a 40°C water bath for 15 min while stirring it mechanically, a 30% solution of H₂O₂ (analytical grade, POCh) was added dropwise to obtain the final concentration of 2 g·100 g^-1 d.m. starch. Then, the mixture was thermostated for 60 min. The modified starches were washed, dried, pulverised and sieved.

The alkaline conditions of the reaction were created by adding dropwise 2M NaOH (before oxidation) to reach pH = 10.

The original starch and the modified products were subjected to the following determinations:

• Carboxyl groups content, according to ISO 11214 standard [13];

• Aldehyde groups content, according to Potze and Hiemistra [21];

• Amylose content, by spectrophotometric method with iodine [14]. Absorbance was measured at λ = 635 nm using a Specord M42 spectrophotometer (Carl Zeiss, Germany);

• Total phosphorus content, by spectrophotometric method according to ISO 3946 standard [23];

• Water-binding capacity and water solubility at a temperature of 60 and 80°C, by the modified method of Leach [22];

• Pasting characteristics of the water dispersions of starch, in a rotational viscometer Rheotest 2 at a concentration of 7.2 g·100 g^-1. Each sample was heated with continuous stirring at a rate of 27 rpm from 50 to 96°C, then held at this temperature for 20 min, cooled back to 50°C, and held at this temperature for 10 min. The rate of temperature increase and decrease was set at 1.5°C·min^-1. The following characteristic values of viscosity and temperature were recorded: pasting temperature (T_p) [°C], maximum viscosity (η_max) [mPa s], temperature at maximum viscosity (T_max) [°C], viscosity at 96°C (η₉₆) [mPa s], viscosity after 20 min of heating at 96°C (η_96/20) [mPa s], viscosity at 50°C (η₅₀) [mPa s], and viscosity after 10 min of cooling at 50°C (η_50/10) [mPa s] [27].

The significance of differences in the parameter values between the samples was determined using a one-factor analysis of variance and the Tukey test.

RESULTS AND DISCUSSIONS

The influence of the catalyst, Fe(II) ions, on the effectiveness of starch oxidation process depended on the pH value of the reaction environment. Oxidation in the presence of Fe(II) ions was the most effective in acidic conditions, which was due to the formation of the largest amount of both carboxyl and aldehyde groups in starch (Table 1). In alkaline conditions, Fe(II) ions caused an increase in the amount of aldehyde groups and a decrease in the carboxyl groups, the changes, however, were less apparent than in the acidic environment. Oxidation of starch by hydrogen peroxide in the presence of Fe(II) ions followed the mechanism of Fenton’s reaction which is known to be more effective in an acidic than alkaline environment [3,9]. Moreover in acidic conditions Fe(II) ions probably formed Fe(OH)₂ which is not well soluble in water. Applied catalyst did not participate in oxidation, because it was precipitated from solution and could be absorbed on grains surface.

The ratio of COOH to CHO contents in starches oxidised by hydrogen peroxide is usually less than 1 [17, 29], which was also confirmed in the present study (only in starch oxidised by hydrogen peroxide in alkaline conditions it exceeded 1). Fe(II) ions in the process of oxidation did not cause an increase in the amount of carboxyl groups versus aldehyde groups (or vice versa as is the case with vanadium(V)) [2]. More carboxyl than aldehyde groups were formed when starch was oxidised in alkaline conditions without catalyst.

Differences in way of oxidation with means by hydrogen peroxide in acidic or alkaline conditions were mostly related to with presence of HOO^- ions, which subsequently were responsible hydroxyl radical and hydroxyperoxide radical creation [9]. In alkaline conditions hydrogen peroxide dissociates and large amount of HOO^- ions is created [7]. For that reason oxidised starch obtained in alkaline conditions was characterised by higer number of carboxyl groups than aldehyde ones.

Oxidation of starch in alkaline condition by chlorate(I) sodium generation also greater number carboxyl than aldehyde groups [28].

Different results were obtained by Hebeish et al. [8] who oxidised corn starch by hydrogen peroxide at pH = 4 and pH = 11. In that research starch oxidised in acidic conditions was characteristic by higher amount of carboxyl groups and lower aldehyde groups than in alkaline conditions.

Phosphorus contened in potato starch influences it physicochemical properties, especially rheological ones [10]. Phosphorus content must be measurmen in order to evaluate content of carboxyl groups in oxidised potato starch [13].

The native potato starch contained 49.7 mg of total phosphorus per 100 g d.m. (Table 1), which is less than the values reported in the literature [24]. It could be caused by growing number of ecologie hausehold in Poland, where synthetic fertilizer containg phosphorus are used on limited scale. Due to oxidation, the total phosphorus content of all the modified starches decreased, compared to native starch, which is in agreement with the results obtained earlier [4, 5]. Oxidation in alkaline conditions without catalyst caused greater reduction of phosphorus content in investigated starches than acidic ones.

Phosphorus in potato starch occurs in the form of orthophosphoric acid(V). Its washout during the washing of starch is supposed to be more intensive under alkaline than acidic conditions. Starch with highest level of oxidation had the smallest phosphorus content among all obtained starches (30%) less than in native starch) (Table 2). It is probably connected to higher solubility of obtained oxidised starch and for that reason phosphorus was easier washed out during purification of obtained preparation.

Compared to native starch, the amylose content of all the modified products was decreased (Table 1). The largest decrease (by ca 60%) occurred as a result of oxidation under acidic conditions in the presence of the catalyst. This was due to the depolymerisation of amylose taking place simultaneously with the oxidation. The greater was the aldehyde groups content (indicating a higher degree of starch depolymerisation), the smaller was the amylose content. Such results agree with those obtained in other studies [5, 20].

The water-binding capacity and water solubility at 60°C of modified starches were not much different from those of native starch. Starch modified in acidic conditions in the presence of the catalyst, had solubility increased to 56.11 g·g^-1 d.m. (Table 2). When heated to a temperature of 80°C, the latter starch completely dissolved. At that temperature, the water-binding capacity and water solubility of starches modified under acidic conditions without the catalyst decreased. The presence of the catalyst in the basic environment of reaction had no effect on the water-binding capacity of starch but caused its water solubility to increase, compared to native starch.

The water-binding capacity and water solubility of potato starch depend largely on its molecular weight, the inner and outer structure of granules, and the amounts of orthophosphoric acid(V) and amylose [6]. All the parameters mentioned are subject to change in the oxidation process through which modified starches are produced [1, 12, 18, 19, 29]. In addition, the carboxyl and aldehyde groups newly formed during modification facilitate the penetration of the water molecule into the starch granule [11]. The presence of metal ions (i.e. catalyst, alkaline reaction conditions) during oxidation is causing the possibility of oxidised starch-metal complex creation. Newly created complex may prevent oxidised starch to solubilize or bind water [1]. The lack of significance changes in water-binding capacity and solubility at 60°C is probably caused by low modification level and small changes in starch grain structure. Exception was starch oxidised in acidic conditions with the Fe(II), where changes (carboxyl, aldehyde groups, phosphorus and amylose content) were the greatest. Significant changes in the values of both parameters occurred only above the temperature of pasting, i.e. at 80°C.

Oxidising starch under acidic conditions in the presence of Fe(II) ions decreased the viscosity of the paste obtained so much that it fell below the sensitivity limit of the measuring instrument.

Modification with hydrogen peroxide in an acidic environment affected only some rheological properties of starch: the maximum viscosity (η_max) was decreased, the temperature at maximum viscosity (T_max) was raised, and the viscosity at 96°C (η₉₆) of the paste was increased, compared to native starch (Table 3). The other parameters changed insignificantly. The paste of starch modified in a basic environment had a higher pasting tempearture (T_p) and (T_max), and lower characteristic viscosities, such as (η_max), viscosity after 20 min at 96°C (η_96/20), viscosity at 50°C (η₅₀) and viscosity after 10 min at 50°C (η_50/10), than the paste of native starch, only its η₉₆ was not significantly different from the latter paste (Table 3). For starch modified in a basic environment in the presence of Fe(II) ions, the properties of the paste obtained differed most from those of the native starch paste (Table 3): the former had the highest T_p, the lowest η_max, η₅₀ and η_50/10, and the biggest η₉₆ and η_96/20 among all pastes.

Starch with low oxidation level were characterised by lower pasting temperature as weel as higher viscosity. It is caused by the electrostatic interaction of the newly formed carboxyl groups, facilitating the water‘s access to the interior of the starch granule. The aldehyde groups created as a result of oxidation may form cross-linked hemiacetals which also make it easier for water to penetrate the granule [11, 26]. Obtained in alkaline conditions oxidised starches had higher pasting temperature (T_p) in comparison to native starch. Increase of pasting temperature (T_p), temperature of maximum viscosity (T_max) influenced, among other, Na(I) ions which are contained in potato starch [16]. During starch oxidation in alkaline conditions (addition of Na(OH)) the ions probably were connected to ortophosphoric acid(V) and also to newly created carboxylic groups. It could elevate pasting temperature and temperature at maximum viscosity of the modified starches. The decreased viscosity of the pastes of modified starches is connected with depolymerisation that occurs simultaneously with the oxidation of polysaccharide chains. Habeish [8] suggested that Fe(II) ions used in the modification process strengthen the oxidant properties of hydrogen peroxide, thus contributing to a greater degradation of the starch granule. Such finding was confirmed in the present study in which starch modification in the presence of Fe(II) ions caused the highest decrease in the viscosity of starch pastes.

Fig. 1 shows the viscosity changes of starch pastes as dependent on time and temperature. The pasting curve for starch modified in basic conditions in the presence of Fe(II) ions differed most from that for native starch. The pastes of this modified starch exhibited the highest rheological stability over the whole range of the temperatures and viscosities determined. The applied temperature changes inconsiderably affected viscosity, suggesting a higher heat stability and resistance to mechanical forces of the paste. Small changes of oxidised starch paste during cooling to 50°C and after helding it at this temperature for 10 min indicates that the oxidised potato starch had a low susceptibility to retrogradation, which confirms our earlier results [20].

CONCLUSIONS

1. Potato starch oxidation in the presence of Fe(II) ions in an acidic environment of reaction caused the largest increase in the amount of carboxyl and aldehyde groups and the highest decrease in the amylose and phosphorus contents compared to native starch.

2. The presence of Fe(II) ions during starch oxidation in acidic conditions caused considerable increase starch solubility at 60°C and whole solubilisation in water at 80°C

3. Starch oxidation without catalyst both (acidic and alkaline) conditions caused decrease of water-binding capacity at 80°C.

4. The presence of the catalyst in the modification process in alkaline conditions increased thermal stability of the oxidised starch paste.

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Accepted for print: 26.09.2007

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