The effect of sugars on the clarity of low acyl gellan gels V. Evageliou a, A. Zikas a, A. Basios a, A. Gerolemou a, M. Komaitis a

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1 The effect of sugars on the clarity of low acyl gellan gels V. Evageliou a, A. Zikas a, A. Basios a, A. Gerolemou a, M. Komaitis a a Department of Food Science and Technology, Agricultural University of Athens, 75 Iera Odos, 11855, Athens, Greece (evageliou@aua.gr) ABSTRACT The present work focused on the effect of glucose, fructose and sucrose (0-15 wt%) on the clarity of low acyl gellan gels (0.5 wt%) in the presence of increasing concentrations of a potassium chloride ( mm) and calcium chloride (10-80 mm). The characterisation of clarity was based on absorbance readings at the visible region (490 nm). The effect of glucose on the clarity in the presence of mixtures of the two salts, at a total molar concentration of 80 mm, was also investigated. Furthermore, seven mixtures of the above mentioned sugars, at a concentration of 15 wt% and in the presence of 100 mm KCl, were added and their effect on clarity was also evaluated. All gels with potassium chloride were less turbid than the ones with calcium. In the case of salt mixtures, increasing concentration of calcium in the mixtures resulted in decreased clarity. In addition, turbidity increased with increasing salt concentration. Moreover, and at the higher salt concentrations, the presence of sugars resulted in clearer gels. When mixtures of sugars were added, all combinations resulted in increased clarity compared to the individual sugars. Keywords: Gellan; Clarity; Salts; Sugars INTRODUCTION Biopolymers share certain properties (e.g. viscosity, gelation), all of which are exploited in food product applications. They are added in a food product in order to improve or impart a new property or to create a new product. Over the last years, the content of sucrose in food products is a matter of great importance. Consumers are interested in healthier new products. The desired properties in sweetness and texture are achieved by the addition of several sugars or their mixtures. The most commonly used sugars are monosaccharides, disaccharides, starch hydrolysates and inulin. Their presence not only affects the mechanical properties of biopolymer gels but also their organoleptic properties like clarity. As organoleptic properties are very important for the acceptability of a new food product, clarity becomes an important parameter and a study on the effect of the three most commonly used sugars and their mixtures on the clarity of gellan gels seems worth investigating. Thus, the present work focused on the effect of glucose, fructose and sucrose on the clarity of low acyl gellan gels in the presence of increasing concentrations of a monovalent (K + ) and a divalent (Ca 2+ ) cation. Sugar concentration varied from 0-15 wt%. The effect of glucose on clarity in the presence of mixtures of the two salts, at a total molar concentration of 80 mm, was also investigated. Furthermore, seven mixtures of the above mentioned sugars, at a concentration of 15 wt% and in the presence of 100 mm KCl, were added and their effect on clarity was evaluated. The characterisation of clarity was based on absorbance readings at the visible region (490 nm). MATERIALS & METHODS Low acyl gellan gum was provided by Sigma (Phytogel, P8169). KCl was from Merck and CaCl 2 from Panreac Quimica SA. Sucrose was normal food grade and purchased locally. Fructose and glucose were reagent grade from Fluka. Glucose was in its monohydrated form, which was taken into account in our calculations. Distilled water was used throughout. Initially, gellan was dissolved in distilled water at 90 C under gentle agitation. Sugars and salts were then added in the appropriate amounts. The polysaccharide concentration was kept constant at 0.5 wt% whereas that of the salts varied from mm for KCl and mm for CaCl 2. Mixtures of the two salts, at a total concentration of 80 mm and at KCl:CaCl 2 molar ratios of 20:60, 40:40 and 60:20, were also used to induce gelation. Sugars were added at concentrations of 0-15 wt%, at 5 wt% intervals. Moreover, the effect of seven mixtures of sugars, at a total concentration of 15 wt%, in the presence of 100 mm KCl was also explored. Those mixtures were Glucose (wt%): Fructose (wt%) at 5:10 and 10:5, Glucose(wt%): Sucrose (wt%) at 5:10 and 10:5, Fructose (wt%): Sucrose (wt%) at 5:10 and 10:5 and Glucose (wt%): Fructose (wt%): Sucrose (wt%) at 5:5:5 ratios. Samples were poured in polysterene cuvettes (4.5 ml, 1cm x 1 cm) and kept at room temperature overnight. Absorbance was measured at 490 nm with a double-beam UV-Vis spectrophotometer

2 (Jasco V-530, Tokyo, Japan) using water as reference, according to the procedure of Tang et al. [1]. Each experiment was performed in triplicate. One way analysis of variance (ANOVA) and least significant difference tests (LSD) were carried out on the data in order to determine significant differences between the samples. The significant level was P<0.05 throughout the study. Analysis of data was carried out with Statistica (Stat-Soft, Inc., Tulsa, OK, USA). RESULTS & DISCUSSION Gellan is a water soluble biopolymer that gels in the presence of cations. For a given gellan concentration, the textural properties of the gels depend on both the concentration and the valency of the cations used to induce gelation [2]. The presence and type of sugars added to the gels also found to be significantly important for the observed mechanical properties [3,4]. However, the clarity of these gels in the presence of various sugars is not extensively investigated. In the present work, the clarity of gellan gels in the presence of various sugars and salts was studied (Figs 1-4). Fig. 1 shows the effect of the concentration of KCl on the clarity of 0.5 wt% gellan gels in the presence of glucose (Fig. 1a), fructose (Fig. 1b) and sucrose (Fig. 1c) at concentrations 0-15 wt%. For all glucose concentrations, clarity was decreased as the concentration of potassium chloride increased. According to the performed statistics, the concentration of glucose was an important parameter for samples incorporating the same amount of salt. In the presence of increased concentrations of fructose (Fig. 1b), once again, increasing concentrations of potassium chloride, led to decreased clarity. However, for 40, 60 and 80 mm salt concentration, statistical analysis showed that the concentration of fructose, present in the gel, did not affect the observed absorbance readings significantly. Increased turbidity with increasing salt concentration was also observed when sucrose, in three different concentrations, was added in the samples (Fig. 1c). Clarity for gellan gels in the presence of glucose (Fig. 2a), fructose (Fig. 2b) and sucrose (Fig. 2c) in the presence of mm calcium ions is presented in Fig. 2. As already seen in Fig. 1, clarity decreased as salt concentration increased, for all concentrations of sugars studied. The presence of glucose (Fig. 2a) resulted in significantly different readings, for all calcium concentrations used. Moreover, addition of increasing amounts of glucose led to increased clarity, as lower absorbance readings were observed. This was more obvious at the higher salt concentrations used. The same behaviour was also seen when fructose or sucrose were added in the gels (Figs 2b and c, respectively). Fig. 3 presents clarity for 0.5 wt% gellan gels as a function of glucose in the presence of mixtures of the two salts at a total molar concentration of 80 mm. Increased turbidity was observed as the concentration of the calcium in the mixtures increased. Furthermore, increasing glucose concentration resulted in significant differences in the observed clarity for the same salt mixture Furthermore, seven combinations of the three sugars, used in the present investigation, were added to gellan gels in the presence of 100 mm KCl. The corresponding clarity is illustrated in Fig. 4. For comparison reasons, the corresponding values for the samples incorporating 15 wt% sucrose, glucose and fructose are included. The basic conclusion is that for the same salt concentration all mixtures gave similar absorbance readings but significantly different with the samples incorporating only glucose, fructose and sucrose. Moreover, the gels in the presence of mixtures of sugars were more clear than the ones with only one sugar present in the same concentration. By comparing Figs 1 and 2, becomes obvious that regardless the type of added sugar, gels with potassium were less turbid than the ones with calcium. This was further manifested, in the case of mixtures (Fig. 3) where increasing concentration of calcium in the mixtures resulted in decreased clarity. Furthermore, and for both salts and their mixtures, turbidity increased with increasing salt concentration (Figs 1-3).This behaviour can be attributed to the different pattern of aggregation process that monovalent and divalent cations induce. Monovalent cations (e.g. K + ) promote the aggregation process by site-binding to the helices whereas divalent cations (e.g. Ca + ) by much stronger site-binding between pairs of carboxyl groups on neighbouring helices. Moreover, salts at high concentration result to excessive helix helix association into very large aggregates [5]. This can explain the decreased clarity of the gels with increasing salt concentration. Tang et al. [1] investigated the effect of sucrose and fructose (15, 25 and 35 wt%) on the clarity of gellan gels. They reported that both sugars resulted to less turbid gels. In addition, for the same sugar concentration clarity decreased as salt concentration increased. Our findings are in full agreement with these observations.

3 0.075 a Glucose (wt%) b Fructose (wt%) c Sucrose (wt%) Figure 1. Clarity of 0.5 wt% gellan gels with increasing concentrations of glucose (Fig. 1a), fructose (Fig. 1b) and sucrose (Fig. 1c) in the presence of 40 mm ( ), 60 mm ( ), 80 mm ( ) and 100 mm (Δ) KCl.

4 0.60 a Glucose (wt%) 0.60 b Fructose (%wt) 0.60 c Sucrose (wt%) Figure 2. Clarity of 0.5 wt% gellan gels with increasing concentrations of glucose (Fig. 2a), fructose (Fig. 2b) and sucrose (Fig. 2c) ) in the presence of 10 mm ( ), 20 mm ( ), 40 mm ( ), 60 mm (Δ) and 80 mm ( ) CaCl 2.

5 When incorporating mixtures of sugars at a total concentration of 15 wt%, the resultant gels were more clear than the ones with only each sugar at this concentration, and had a similar turbidity with each other. Analogous results were observed when studying the mechanical properties of these gels [6]. According to these studies, strength and firmness were, for all sugar combinations, lower than for the individual sugars. Taking both studies into consideration, we can safely conclude that this behaviour is indicative of non synergistic interactions among the sugars a Glucose (wt%) Figure 3. Clarity of 0.5 wt% gellan gels with increasing concentrations of glucose in the presence of mixtures of the two salts, at a total concentration of 80 mm and at KCl:CaCl 2 molar ratios of 80:0 ( ), 60:20 ( ), 40:40 ( ), 20:60 (Δ) and 0:80 ( ) F 5, G 5, S 5 G 5, S 10 G 10, S 5 G 5, F 10 G 10, F 5 F 5, S 10 F 10, S 5 S G F Figure 4. Clarity of 0.5 wt% gellan gels in the presence of 100 mm KCl for seven mixtures of sugars at a total concentration of 15 wt%. The corresponding values for the samples incorporating 15 wt% sucrose, glucose and fructose are also included. G stands for glucose, F for fructose and S for sucrose.

6 CONCLUSION The clarity of gellan gels in the presence of various sugars and salts was investigated. A major conclusion was that all gels with potassium were less turbid than the ones with calcium. This became more evident in the case of salt mixtures, where increasing concentration of calcium in the mixtures resulted in decreased clarity. Another important parameter in the observed behaviour is salt concentration, as for increasing salt concentration turbidity increased. In addition the sugar concentration became important only at the higher salt concentrations used, leading to less turbid gels. When investigating the effect of the type of sugars in the presence of 100 mm potassium chloride, it was interesting to report that all combinations of sugars resulted in increased clarity comparing to use of individual sugars at the same concentration. REFERENCES [1] Tang, J., Mao, R., Tung, M.A. & Swanson, B.G Gelling temperature, gel clarity and texture of gellan gels containing fructose or sucrose. Carbohydrate Polymers, 44, [2] Moritaka, H., Fukuba, H., Kumeno, K., Nakahama, N. & Nishinari, K Effect of monovalent and divalent cations on the rheological properties of gellan gels. Food Hydrocolloids, 4, [3] Tang, J., Lelievre, J., Tung, M.A. & Zeng, Y Polymer and ion concentration effects on gellan gel strength and strain at failure. Journal of Food Science, 59, [4] Tang, J., Tung, M.A. & Zeng, Y Mechanical properties of gellan gels in relation to divalent cations. Journal of Food Science, 60, [5] Morris, E.R., Richardson, R.K. & Whittaker, L.E Rheology and gelation of deacylated gellan polysaccharide with Na + as the sole counterion. Progress in Colloid and Polymer Science, 114, [6] Evageliou, V., Mazioti, M., Mandala, I. & Komaitis, M Compression of gellan gels. Part II: effect of sugars. Food Hydrocolloids, 24,