EFFECT OF GUM ARABIC AND MODIFIED STARCH ON STABILITY OF BEVERAGE EMULSIONS

Elżbieta Dłużewska, Małgorzata Panasiewicz, Krzysztof Leszczyński

ABSTRACT

The aim of this study was to evaluate the effect of various emulsifiers on stability of flavour emulsion being applied to production of soft drinks. Two preparations of gum arabic and hydrophobically modified starch have been applied to emulsify and stabilize the emulsion. The stability was determined by: the ringing test, centrifugal method and by determining the particle size of dispersed phase. In comparison with emulsions stabilized by single preparations, the improvement of emulsion stability was achieved by using together two preparations of gum arabic or modified starch and Valspray A in proportion 2:1. Higher degree of emulsion opacity was achieved by using mixture of gums arabic than by using the modified starch. Such higher opacity allows to obtain beverages with a typical cloudiness for natural juices.

Key words: beverage emulsion, stability, gum arabic, modified starch..

INTRODUCTION

Beverage emulsions are oil-in-water emulsions that are normally prepared as a concentrate, then diluted in sugar solution in order to produce the finished beverage [5]. The emulsion in both its concentrated and diluted form must have a high degree of stability. In soft drinks the beverage emulsion may provide flavour, colour and suitable cloudy appearance [12].

A typical composition includes flavour oils (often essential oils) and weighing agents in the oil phase, and water, hydrocolloid, citric acid, preservatives, colourings and sweetener in the water phase [2, 6]. Hydrocolloids serve as emulsifiers and stabilizers in beverage emulsions. Hydrocolloids stabilise emulsions through viscosity effects, steric hindrance and electrostatic interactions. Among the most common stabilizers are such hydrocolloids as xanthan, gum arabic, modified starches, pectin and carrageenan [4].

Gum arabic, the dried exude from certain species of the acacia tree, is one of most widely used biopolymer on an industrial scale. It is considered to be unequalled in many of its properties including the ability to form stable emulsions over a wide pH range and in the presence of electrolytes. Gum arabic is a complex mixture of polysaccharide, protein and arabino galacto protein species [8, 11].

Hydrophobically modified starch (octenyl succinate derivative of waxy-maize) has been identified as one of the most promising replacements for gum arabic. It primarily consists of amylopectin that has been chemically modified to contain nonpolar side-grups. These side-grups anchor the molecule to the droplet surface, while the hydrohilic starch chains protrude into the water phase and protect droplets against aggregation through steric repulsion [6].

The purpose of this study was to investigate the effect of gum arabic, modified starch and their mixture on stability of model beverage emulsions.

MATERIALS AND METHODS

Materials
Valgum (combination of different species of gum acacia), Valspray A (spray dried gum acacia cleaned Kordofan) and Valdamar (deodorised and purified damar gum) were obtained from the Valmar, France. Modified starch (Purity Gum 2000) was obtained from the National Starch and Chemical. Essential oil was purchased from JAR – Jaskulski Aromaty, sodium benzoate and citric acid food grade from Orffa Food Eastern Europe, liquid, water soluble beta carotene from Chr. Hansen Poland. Distilled water was used to prepare solutions and emulsions.

Emulsion preparation
Emulsion concentrates were prepared to the following formula: essential oil, 10%; weighing agent (Valdamar), 8%; emulsifier 8% or 12% or for Valspray A 25%; preservative (sodium benzoate), 0.1%; citric acid, 0.3%; β-karoten, 2%; distilled water up to 100%.

The emulsifiers were dispersed for half an hour by means of the mixer RW 20 DZM by IKA-WERK, in a water at 20°C, in which the sodium benzoate had been dissolved. Next, citric acid was added and such obtained solution was stored for three hours in order to hydrate gum arabic. A coarsely emulsified premix was prepared by adding together the water and oil phases (i.e., the gum solution and the essential oil with weighing agent) and homogenizing with the mixer RW 20 DZM for 10 min with the velocity 1700 rpm. A fine emulsion was achieved by running the premixes through a high-pressure homogenizer (ALM-2) at 200/70 bar using two passes. The soft drinks were prepared by diluting beverage emulsion to 0.2% (w/w) in a 5% sugar solution.

Measurements
Micrometric measurement of emulsion stability
Mean particle size of emulsion was determined using the optic microscope Studar Lab coupled with a video camera Oscar-35 by Mintron.

A drop of emulsion was placed on a microscope slide, covered by a cover-slip, and observed at a magnification of 1000×. An image of the emulsion was acquired using digital image processing software (Muliscan v 6.08 by Scanning Systems and Mocha v 1.2.10 by Jadel Scientific).

Turbidity measurement of emulsion stability [7]
Every emulsion was diluted 1 part to 1000, prior to the absorbance measurements. The absorbance was taken at 400, 660 and 800 nm, using a Helios β spectrophotometer (Unicam). From the absorbance values at 660 nm the opacity was determined, and from the ratio of the absorbance at 800 to 400 nm the size index (R) and stability were predicted.

Centrifugal method of evaluating emulsion stability [9]
The samples of emulsion were placed in the test-tube for centrifuge which were next stored in thermostat at 37°C for 24 h. After that the samples were centrifuged by using Centrifuge MPW-340 for 10 min with 3500 rpm. The emulsion stability S was determined from the formula: S = [(V_o – V)/V_o] × 100%,

where: S – emulsion stability %, V_o – volume of emulsion undergone centrifugation cm³, V – volume of the phase given off cm³.

The ringing test [12]
Samples of emulsion, as well as soft drinks (bottles 1 liter), made from the beverage flavor emulsions, were stored for 12 weeks at room temperature. Visible ringing indicates an inadequate emulsion.

Sensory evaluation [1]
Using the scaling method the sensory evaluation of beverage samples was carried out by a group of 10 persons.

Statistical analysis
The statistical analysis has been done with the use of the computer program Statgraphics produced by Statistical Graphics Corp., using the one-variable analysis of variance, with the confidence level of 5%.

RESULTS AND DISCUSSION

Taking into account the effect of particle size of phase dispersed (oil) on the emulsion stability and made of it beverages the dependence between the oil droplet size, the size index (R), opacity (O) and the kind of emulsifier was analysed. The emulsion stability was greater when the droplet size was smaller. Reineccius [9] noted that an emulsion containing weighing agents and an acceptable emulsifying constituent will typically not separate if the average particle size of the emulsion is in the range of 1–3 micrometers.

The superfluous increase of oil droplet size resulting from joining the oil droplets into larger aggregates during emulsion storage can be the reason of appearing a characteristic ring or even the breakdown of emulsion [2].

Immediately after obtaing, emulsions which among all investigated contained the smallest oil droplet size were those obtained with additive of modified starch and also with mixture of Valgum and Valspray A. What was identify when emulsifier in amount of 8% as well as 12% was applied (Fig. 1a and 1b). During 12 weeks of storage of emulsion samples increase of droplets size in all investigated samples was observed. Much higher growth of oil droplets was found in the case of emulsions with additive of starch (increase of mean size of 170%) than in mixture of gums arabic (increase of 109%, respectively). The attempts to increase emulsion stability by using mixture of gum arabic and modified starch as stabiliser did not give expected results. Mean particle size in majority of stabilised in such way emulsions after 12 weeks was considerably larger than in the case of emulsions stabilised only with Purity Gum 2000 or Valgum. Only when Purity Gum 2000 and Valspray A were used in proportion 2:1 no significant difference was found as far as particle size of emulsions stabilised by mixture or single preparations was concerned (Fig. 1b).

According to the results of evaluation of emulsion stability by centrifugal method, presented in Figure 2, the poorer stability was found among emulsions with additive of mixture of modified starch and gum arabic (Valspray A) in proportion 1:2 and mixture of starch and Valgum in proportion 2:1. These results are compatible with Buffo and Reineccius [3], who stated that emulsion stability was slightly poorer on incorporating modified starch into the gum acacia system. However, this does not mean that the modified starch is a poor emulsifier but that its use in combination with gum acacia does not give improved emulsion stability.

Emulsion samples with additive of single preparations of gum arabic as well as with mixture of these preparations had 100% stability (Fig.2). This was not in complete accordance with evaluation of stability based on particle size measurement and ringing test. Sample with additive of Valspray A after 12 weeks of storage had the largest particle size of dispersed phase (2.73 ľm), and after 9 weeks creaming was observed. Centrifugation is very rapid method of determining beverage emulsion stability, but it does not give consideration to coalescence proceed with time [13].

Results of determination of size index (R) are presented in Tables 1a and b. Increase of size index value shows forming larger aggregates of oil droplets [7].

Emulsion samples which were consider to be stable according to determination of particle size by micrometric method had also the lowest value of size index R. In the case of emulsions with additive of modified starch and Valspray A in different proportion large differences in size index R were found. Immediately after obtaining emulsion sample with additive of Purity Gum 2000 and Valspray A in proportion 1:2 size index R reached 0.621, and for sample with additive of these emulsifiers in proportion 2:1 it reached 0.401, such differences remained for 12 weeks. Measurements of size index R confirmed high stability of emulsion sample with starch and Valspray A, which was identify only when emulsifiers in proportion 2:1 were joined.

Mean particle size as well as size index R only slightly decreased when the amount of emulsifier changed from 8% to 12%. After increasing the amount of emulsifier the change of emulsion stability was found only in the case of emulsion with Valgum on the base of the ringing test.

The measure of oil droplets size and thus emulsion stability can be also the degree of opacity. The increase in the opacity indicates undesirable changes in emulsion, but on the other hand it is more beneficial as beverage obtained from such emulsion is more similar to natural fruit juice [10]. Immediately after obtaining the opacity degree was very diversified for each sample. It ranged from 0.266 to 0.824 (Tables 2a and 2b). The lowest opacity degree was found in case of emulsion with additive of gum arabic Valspray A (0.266) and modified starch (0.334). High degree of opacity (0.606), which ensure obtaining beverages with similar clarity to natural juices, was received when mixture of gums arabic was used in amount of 12%. In the same time there were not any significant differences in the opacity of emulsion sample stabilised with this amount of gum (12%) during 12 weeks of storage. In the rest of samples the significant increase of opacit y was found. This confirms their lower stability, stated on the base of determination of particle size, in comparison with emulsion obtained with additive of mixture of gums arabic.

The results of sensory evaluation of beverages obtained from discussed emulsion samples (Fig. 3) were correlated with results of indication of opacity by turbidity method. Beverages obtained from emulsions stabilised by Valspray A as well as by modified starch had the highest clarity. Desired opacity degree, according to people who evaluated, had beverages obtained with additive of mixture preparations of gum arabic.

CONCLUSIONS

1. Mean particle size of emulsion below 1 ľm, immediately after homogenisation was obtained when additive of modified starch or Valgum and Valspray A was used. This indicates good emulsifying properties of this preparations.

2. Among investigated mixtures of modified starch and gum arabic the best stabilising and emulsifying properties had the mixture of Purity Gum 2000 and Valspray A
   in proportion 2:1.

3. Mixture of gum arabic preparations had the ability to form higher emulsion and beverage opacity in comparison with modified starch.

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