EFFECT OF SACCHARIDES ON GELATINIZATION AND RETROGRADATION OF MODIFIED POTATO STARCH

Dorota Gałkowska
Department of Analysis and Evaluation of Food Quality, Agricultural Univeristy of Cracow, Poland

ABSTRACT

The gelatinization and retrogradation of native potato starch and modified potato starches (oxidized starch, distarch phosphate, and acetylated distarch adipate) in the presence of saccharides (glucose, fructose, sucrose, and lactose) were studied by differential scanning calorimetry (DSC) and by turbidimetric analysis, respectively. The obvious effects arising from the presence of saccharides in the starch systems were a shift in the transition temperatures of the gelatinization endotherm to higher temperatures and increase in gelatinization enthalpy. The extent of temperature shift and enthalpy change was dependent on both the kind of starch preparation and the kind of saccharide added to the starch-water system. However, the greatest elevating effect of saccharide on the gelatinization temperatures of the oxidized starch and the distarch phosphate was observed in the case of fructose. The addition of the saccharides, except for lactose, led to the reduction in the rate of retrogradation of the oxidized starch and the distarch phosphate starch.

Key words: modified starches, saccharides, calorimetric analysis, retrogradation.

INTRODUCTION

The influence of saccharides on starch gelatinization has been studied by many investigators, and it is well known that gelatinization characteristics are markedly altered by the addition of sugars. In general, gelatinization of starch of different origin in the presence of saccharides proceeds in higher temperatures and requires more energy input (ΔH) than without solutes [2,3,4,5,8,19,27,28,32]. However, the opposite effect of saccharides on the values of enthalpy of starch gelatinization can also occur [7, 18]. Moreover, in the works of Evans and Haisman [11] and Maaurf et al. [20] the enthalpy of gelatinization (ΔH) has been reported to be unaffected by the addition of saccharides.

The effects of different sugars on starch gelatinization vary: in general, monosaccharides delay gelatinisation less than disaccharides, except maltose, which acts like a monosaccharide [18,26,28]. A number of possible explanations for this effect have been proposed. One of them suggests that there is a competition between the sugars and starch for available water [9,15]. In opinion of Biliaderis et al. [6] a decrease in moisture content results in an increase in the glass transition temperature of the bulk amorphous phase of starch resulting in an increase in the temperature of the overall endothermic process of gelatinization. When solutes such as saccharide are added to the aqueous phase, less water is available to act as plasticizer, and consequently the temperatures associated with gelatinization increase. Sugar solution acts as an antiplasticising cosolvent compared with the action of water as a diluent, and thus greater energy is required to overcome the antiplasticising effect and melt the starch crystals. The elevating effect of saccharides on gelatinization temperatures has also been attributed to inhibition of swelling of the starch granules [4,32]. Spies and Hoseney [28] suggested that the delay in gelatinization is caused by the decreased water activity of the sugar solution compared with water. They explained that when sugars are added to water, the ability of water to interact with other components in the system decreases, resulting in higher energy requirements.

Starch retrogradation process is conditioned by the botanical source of starch as well as by the presence of other substances in the starch paste [13,16,23]. In the aspect of the effect of saccharides on the retrogradation of starches of different botanical sources, several researchers reported inconsistent results. Some of them have revealed that saccharides enhanced starch retrogradation [3,17,19,25], while the others reported that sugars prevented starch retrogradation [1,10,17,19,25]. There are several mechanisms through which saccharides may affect starch retrogradation. It is considered that the main factors affecting starch crystallization are the interactions of starch and saccharide, as well as the interactions of saccharide and water [2,3,10,25].

Because most of the studies on gelatinization and retrogradation of starch in the presence of saccharides are on native starches, and there is lack of information concerning the properties of commercial modified starch-saccharide systems, the objective of this work was to study gelatinization and retrogradation of the modified potato starches in the presence of monosaccharides and disaccharides.

MATERIALS AND METHODS

The starches used in the experiments were native potato starch (Superior Standard) and modified potato starches, i.e. oxidized starch (pudding starch E 1404), distarch phosphate (Lubostat E 1412), and acetylated distarch adipate (starch thickener AD E 1422), produced by WPPZ S.A. (Luboń, Poland). Sucrose and glucose anhydrous were purchased from Chempur (Piekary Śląskie, Poland), D-(–)-fructose was purchased from Riedel-de Haën Sigma-Aldrich (Seelze, Germany), and lactose monohydrate was obtained from POCh S.A. (Gliwice, Poland).

Amylose contents, as determined by the method described by Morrison and Laignelet [22], of the starch samples were 30.3, 18.1, 12.4, 14.3% (dry basis) for the native potato starch, the oxidized starch, the distarch phosphate, and the acetylated distarch adipate respectively.

The DSC measurements were carried out in a Perkin-Elmer DSC 7 (Connecticut, USA). Starch samples (approximately 10 mg dry basis) were weighted into the tarred stainless steel pans and saccharide solutions were added with microsyringe to give a precaltulated starch-to-water-to-saccharide ratio of 1 : 3 : 1. The control samples were prepared by adding water to the sample pans instead of saccharide solution. After sealing, the pans were equilibrated for 1 h at room temperature and scanned at a rate of 10°C·min^-1 from 30 to 120°C. An empty pan was used as a reference to balance the heat capacity of the sample pan. The calorimeter was calibrated using indium metal. The characteristic onset (T_o), peak maximum (T_p) and end (T_e) temperatures, as well as overall enthalpy (ΔH, expressed as J·g^-1 dry starch) associated with starch gelatinization, were determined. The accuracy of the temperature measurement was ±0.5°C and the error in the calculation of the enthalpy was ±0.05 J·g^-1.

Retrogradation of starches was examined by turbidimetric analysis of Jacobson et al. [16]. The starch pastes containing saccharides were prepared as follows: the appropriate amounts of sacharide and water were added to the starch so that the weight ratio of starch (dry basis) to saccharide was 1 : 1 and the starch solids concentration of the starch dispersion was 2% (w/w). The control samples were prepared in the same manner, except that the samples did not contained saccharide. Each sample was stirred for 5 min at 300 r.p.m. at ambient temperature before being heated to 95°C±1°C in water bath for 1 h. After heating the sample was cooled by placing the beaker containing the sample under continuous stirring in a water bath heated to 25°C±1°C. All samples were stored in a refrigerator at 8°C. On the day of sample preparation and after 1, 3, 5, 7, 14, and 21 days at 8°C the turbidance value of each sample were determined using the Jasco V-530 spectrophotometer (Tokyo, Japan) at wavelength of 640 nm. Normalized turbidance was calculated as:

,

where S₀, S_x, S₂₁ are turbidances of the fresh paste, paste after X days, and paste after 21 days, respectively.

In this work, each value of turbidance represents the average of triplicate determinations. The maximum standard deviation of any measurement was <3.5% of the average value.

RESULTS

Differential scanning calorimetry was used to study the effect of saccharides on the gelatinization of native and modified potato starches. The gelatinization temperature changes as a result of this effect are given in Table 1. Starch samples containing saccharide exhibited higher onset (T_o), peak (T_p), and end (T_e) temperatures and involved greater enthalpy changes (ΔH) than the corresponding control samples (without saccharide).

Among the investigated starch samples, the acetylated distarch adipate was characterized by the lowest onset, peak, and end temperatures. The highest gelatinization temperatures were recorded in turn for the distarch phosphate sample (see Table 1). This phenomena can be explained as being due to presence of phosphate cross-links that stabilize and strengthen starch granules and thus make them more resistant to swelling and pasting [29]. The acetylated distarch adipate also had the lowest gelatinization enthalpy of all examined starch preparations. This observation can confirm a greater ability of the acetylated distarch adipate to swell than other modified starches. As the gelatinization temperatures such as T_o and T_p reflect the degree of ordering of molecular structure in the starch granules [14], it also may be assumed that starch crystallites of the acetylated distarch adipate had less ordered structure than other starch preparations. Both gelatinization temperatures and enthalpy of the oxidized starch were only a little bit higher than respective parameters stated for the native potato starch (see Table 1), even though these starches differed in amylose content (30.3 and 18.1%, respectively). This phenomenon could result from the presence of carbonyl groups in the oxidised starch [31]. The transition temperatures (T_o, T_p and T_e) of the native potato starch (60.3, 65.2, and 72.4°C, respectively) were quite similar to the temperatures found by McPherson and Jane [21] who studied starch-water systems of the same starch to water ratio (1 : 3) as in the present study. It is worth to add that the results of DSC analysis presented in this work are in a good agreement with the results of viscosimetry experiments obtained by Fortuna et al. [12].

In the case of native potato starch the differences in the effects of monosaccharides, i.e. glucose and fructose on the phase transition temperatures (T_o, T_p and T_e) as well as on the enthalpy of gelatinization (ΔH) were observed. The addition of fructose raised the gelatinization temperatures more than the addition of glucose. These results are not in accordance with results published by other authors, who studied the effect of monosaccharides on the gelatinization of sweet potato starch [19], wheat starch [27], or sago starch [2]. The results of the viscosimetric measurements of the maize starch dispersions by Torley and van der Molen [30] also confirmed the higher pasting temperatures of starch in the presence of glucose, but not fructose. It seems, therefore, that an essential factor affecting the greater effectiveness of fructose than glucose on the starch transitions was the chemical structure of the native potato starch. It could not be excluded that phosphate groups of the potato starch contributed to occurrence of stronger interactions between fructose and starch compared to glucose-starch interactions.

The gelatinization temperatures of the native potato starch-disaccharide systems occurred to be higher in comparison to these of starch-glucose system. This phenomenon of increasing gelatinization temperature of starch to a greater extent by disaccharides compared with monosaccharides was also observed by other authors, both in the case of potato starch [18,19], and starch of other botanical origin [2,3,5,24,28,30]. This trend can be explained by the context of plasticising effect of sugars on an amorphous phase of starch. In opinion of Perry and Donald [24] the greater the molecular weight of the chemical compound introduced to the starch system the less effective it would be expected to be at plasticising the starch granule.

On the basis of results presented in Table 1, it can be stated that the effect of an individual saccharide on the DSC thermal profiles of the modified starches depended on the type of starch preparation used. Relatively the greatest increase of onset, peak and end temperatures caused by the addition of saccharides was observed in the case of the oxidised starch. The saccharide that inhibited swelling of that starch the most readily was fructose, and the less readily – glucose. The effect of disaccharides, i.e. sucrose and lactose on the gelatinization parameters did not differ statistically significantly.

The onset gelatinization temperatures of all of the distarch phosphate systems containing saccharides were qualitatively similar. The same trend was found in the case of the acetylated distarch adipate. It can be therefore stated that the temperature at which each of these above starch began to gelatinize was governed more by the presence of saccharide in the system than by the chemical structure of a given saccharide.

The effectiveness of sucrose and lactose in the context of increase in gelatinization peak and conclusion temperatures of the modified starches were not statistically different (see Table 1). It was observed that both the oxidised starch and the distarch phosphate exhibited the highest peak and conclusion temperatures in the presence of fructose as compared to other saccharides. On the other hand, among all saccharides fructose showed the least pronounced delaying effect on gelatinization of the acetylated distarch adipate. The above observations suggest that chemical structure of modified starches appears to be a determining factor affecting the degree of influence of the saccharides on the thermodynamic transitions of starch granules. Moreover, the differences in the chemical structure of the monosaccharides seemed to be more important factor than these of the disaccharides.

Addition of saccharides to the modified starch-water systems caused an increase in the enthalpy of starch gelatinization. The effects of the same saccharide on the changes in gelatinization enthalpy of each of investigated starches differed from each other (see Table. 1). It can be therefore stated that the chemical structure of the starch preparation determined the starch-saccharide interactions. It deserves to pay attention to the fact that in the starch systems containing sucrose the enthalpy values were increased the most among all the tested systems. On the basis of these results, it can be said that sucrose stabilized structure of the modified starches to the least extent in comparison with other examined saccharides. The reason for such effect may be the chemical structure of sucrose. In the case of the oxidized starch and the distarch phosphate the saccharide in presence of which the enthalpy of gelatinization was found to be affected the most was glucose. As for the acetylated distarch adipate, sample containing fructose showed a higher enthalpy value than other acetylated distarch adipate-saccharide systems.

Turbidimetric analysis were used to study the effect of glucose, fructose, sucrose, and lactose on retrogradation of native and modified potato starches. Results of this effect during storage of starch pastes for 21 days are displayed in two ways: as absolute changes in turbidance (see Fig. 1a-4a) and as normalized turbidance, i.e., as the relative rates of turbidity development between pastes over the 21-day period of analysis (see Fig. 1b-4b). Based on the initial values of turbidance (on zero day) it can be stated that the starch pastes containing the saccharide were characterised by lower turbidity than the adequate control samples. The observed changes of the values of turbidity were dependent both on the kind of starch preparation, and the kind of added saccharide. Only in the case of the distarch phosphate and the acetylated distarch adipate the addition of glucose to the dispersions of these starches did not affect significant changes in the values of turbidity. The most distinct differences in the initial values of turbidance caused by the addition of the saccharide was observed in the case of the oxidized starch, while the saccharide that influenced the most turbidity of all the examined starches was fructose.

Susceptibility of the native potato starch to retrogradation in the first 24 h of storage at 8°C was restricted by all the used saccharides (see Fig. 1b). On the following days of the experiment it was stated that the rate of the native potato starch retrogradation decreased in the presence of sucrose and glucose. Addition of sucrose, glucose or fructose to the starch systems led to a decreased rate of retrogradation of the oxidized starch and the distarch phosphate in the first 24 h of storage (see Fig. 2b, 3b). On the following days – up to seventh day of measurement – susceptibility of the oxidized starch and the distarch phosphate to the retrogradation was reduced in the presence of glucose and fructose and in the presence of glucose and sucrose, respectively. The oxidized starch and the distarch phosphate pastes with lactose showed increased rates of turbidity development compared to the control pastes, during all the experiment period (see Fig. 2b, 3b). It can be therefore stated that the retrogradation of the above starches was intensified in the presence of lactose. In the case of the acetylated distarch adipate pastes there were no significant changes in their turbidity produced by the addition of the saccharides, especially during the first three days of experiment (see Fig. 4b).

The above results allow to conclude that the modified potato starch which susceptibility to retrogradation was changed relatively the most in the presence of the saccharides was the oxidized starch. The effect of the saccharides on retrogradation of cross-linked starches, i.e. the distarch phosphate and the acetylated distarch adipate, appeared to have a less significance than of the other starches.

In the present work, starch retrogradation was examined by turbidimetric analysis, which required preparing starch pastes of a specified concentration. In an opinion of Jang et al. [17] the essential factor affecting starch retrogradation in the presence of saccharides is, besides storage temperature and time, water content in the starch-water-saccharide systems. It is also emphasised that suppressing effect of sugars on the extent of starch retrogradation increases with increasing concentration of the saccharide in the sample [3,10,19]. The above factors should be taken into account when comparing results of different studies on the effect of saccharides on starch retrogradation.

CONCLUSIONS

1. The presence of glucose, fructose, sucrose or lactose both in the native potato starch-water system and in the modified potato starch-water systems affected values of thermodynamic parameters of starch gelatinization. The phase transition onset (T_o), peak (T_p), and end (T_e) temperatures shifted to higher temperatures, and enthalpy associated with the endothermic process increased.

2. Gelatinization of native potato starch in the presence of disaccharide (sucrose or lactose) proceeded in higher temperatures than in the presence of monosaccharide (glucose or fructose). Among all examined saccharides addition of fructose to the native potato starch-water systems resulted in the highest increase of value of the gelatinization enthalpy, whereas addition of sucrose contributed to the lowest increase of value of this parameter.

3. The effects of examined saccharides on gelatinization of the modified starches varied depending on both the kind of starch preparation and the kind of saccharide added to the starch-water system. The greatest elevating effect of saccharide on the onset, peak, and end temperatures was observed in the case of the oxidised starch-fructose system.

4. The process of gelatinization of the acetylated distarch adipate in the presence of fructose showed the highest enthalpy value of all starch samples containing saccharide. The addition of sucrose to all examined starch preparations resulted in a smaller increase of values of gelatinization enthalpy in comparison to other saccharides.

5. The rate of retrogradation of native potato starch was restricted by the addition of each of the studied saccharides in the first 24 hours of storage. The addition of glucose, fructose, and sucrose both to the oxidized starch, and to the distarch phosphate led to reduction in the rate of starch retrogradation, while the addition of lactose had the opposite effect. The susceptibility of the acetylated distarch adipate to retrogradation did not change substantially in the presence of the saccharides.

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Accepted for print: 05.03.2008

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