EFFECT OF GUM ARABIC AND GUM GHATTI ON THE STABILITY OF BEVERAGE EMULSIONS

Abstract

The aim of this study was to evaluate the effect of gums: arabic and ghatti on the stability of beverage emulsions. The stability was determined based on the characteristics of particle size of dispersed phase and by measuring changes in the intensity of backscattered light. Oil droplet distributions were measured by laser-light scattering. The highest stability was found in the emulsions stabilised, by 10% of gum arabic or by 5% of gum ghatti. The addition of the emulsifier may be reduced to 3% of emulsion mass, without decreasing emulsion stability, by applying a mixture (1:1 v/v) of these gums.

Headnote

ABSTRACT

The aim of this study was to evaluate the effect of gums: arabic and ghatti on the stability of beverage emulsions. The stability was determined based on the characteristics of particle size of dispersed phase and by measuring changes in the intensity of backscattered light. Oil droplet distributions were measured by laser-light scattering. The highest stability was found in the emulsions stabilised by 10% of gum arabic or by 5% of gum ghatti. The addition of the emulsifier may be reduced to 3% of emulsion mass, without decreasing emulsion stability, by applying a mixture (1:1 v/v) of these gums.

- Keywords: beverage emulsion, gum arabic, gum ghatti -

INTRODUCTION

Beverage emulsions are oil-in-water emulsions that are normally prepared as a concentrate and then diluted in a sugar solution in order to produce the finished beverage (BUFFO et al, 2002). The emulsion in its both concentrated and diluted form ought to be characterised by high stability (TAN and WU HOLMES, 1988).

Emulsion instability results from physical processes, le. flocculation, coalescence, Ostwald ripening and gravity separation. The rate of these changes can be measured by determining the size and distribution of oil droplets in the emulsion (McCLEMENTS and COUPLAND, 1996; MIRH OSSEINI et al., 2008). The Stokes' law states that the velocity at which a droplet moves is proportional to the square of its radius. The stability of emulsion to gravity separation can therefore be enhanced by reducing the size of the droplets (CHANAMAI and McCLEMENTS, 2000; HUANG et al., 2001).

In soft drinks the beverage emulsion may provide flavour, colour and suitable cloudy appearance (REINECCIUS, 1994). A typical composition includes flavour oils (often essential oils) and weighting agents in the oil phase as well as water, hydrocolloid, citric acid, preservatives, colorants and a sweetener in the water phase (BUFFO and REINECCIUS, 2000; CHANAMAI and McCLEMENTS, 2001). In beverage emulsions hydrocolloids serve as emulsifiers and stabilizers. They stabilised emulsions through viscosity effects, steric hindrance and electrostatic interactions. The most common hydrocolloid stabilizers include xanthan, gum arabic, modified starches, pectin and carrageenan (BUFFO et al, 2001).

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 deemed exquisite in many of its properties including the ability to form stable emulsions over a wide pH range and in the presence of electrolytes (DICKINSON, 2003; JAYME et al, 1999; SANCHEZ et al, 2002; TIPVARAKARNKOON et al, 2010; DJORDJEVIC et al, 2008). Gum arabic consists of at least 3 high molecular weight biopolymer fractions. The surface-active fraction of branched arabinogalactan blocks attached to a polypeptide backbone (RANDELL et al, 1988; CHANAMAI and McCLEMENTS, 2001; AOKI et al, 2007). The hydrophobic polypeptide chain adsorbs on the oil/water interface, while the hydrophilic arabinogalactan blocks extend into the solution, thus assuring stability against droplet aggregation through steric and electrostatic repulsion (JAYME et al. 1999; CHANAMAI and McCLEMENTS, 2001). Ghatti gum is also a plant exudates, the main species is Anogeissus latifolia (DESHMUKH et al, 2012; MIRH OSSEINI and AMID, 2012). Ghatti gum is a non-starch polysaccharide. The polysaccharide of gum ghatti has an extremely complex structure, its hydrolysis results in the production of: Ara, Gal, Man, Xyl, and GlcA (ratio: 48:29:10:5:10 M) and less than 1% Rha. Recent investigations have revealed its complete molecular structure, which proved that it has two fractions - gelling and soluble (KANG et al., 201 la, KANG et al 201 lb, KANG et al 201 le). As a result of its two fractions ghatti gum exhibits gelling surface-active as well as emulsifying properties, even better than these of gum Arabic (DESHMUKH et al, 2012). Ghatti gum is not digested in the stomach and small intestine of humans and is fermented in the large intestine by Bacteroides longwn (MARONPOT, 2013). It is classified as generally recognized as safe (GRAS) (DESHMUKH et al, 2012).

The purpose of this study was to investigate the effect of gum arabic, gum ghatti and thenmixture on the stability of model beverage emulsions.

MATERIALS AND METHODS

Materials

Gum arabic samples (Valgum and Valspray A) and rosin esters (Valrosin) were provided by the Valmar, France. Ghatti gum was obtained from Hortimex, Poland. Essential citrus oil was purchased from JAR, Poland. Sodium benzoate and citric acid food grade were from Orffa Food Eastern Europe. Distilled water was used to prepare solutions and emulsions.

Emulsion preparation

Emulsion concentrates were prepared according to the following formula: essential oil - 10% (w/w), weighting agent (rosin esters) - 10% (w/w), emulsifier (gums) - 10 or 5 or 3 or 1.5% (w/w), sodium benzoate - 0.1% (w/w), citric acid and distilled water up to 100% (w/w). The mixture of Valgum and Valspray was added to the emulsions at a ratio of 1 : lw/w. The emulsifiers were dispersed for half an hour with an RW 20 DZW mixer by Janke & Kunkle, Germany, in water at 40°C, in which the sodium benzoate had been previously dissolved. The water phase was stored for 24 hours to hydrate the emulsifier. Pre-emulsion was prepared by adding together the water and oil phases (i.e., the hydrocolloid solution and the essential oil with a weighting agent) and stirring with an RW 20 DZM mixer for 15 min with the velocity-1700 rpm. At this point, the pH value of the premixes was adjusted to 4 with 2 M citric acid. A fine emulsion was achieved by subjecting the premixes to a two-stage homogenization with an APV-1000 homogenizer by APV, Denmark, at 55 MPa at the first stage and 18 MPa at the second stage.

Particle size determination

Mean particle size and particle size distribution of beverage emulsions were determined in the range of 0.05 - 1000 pm by the laser light scattering method using a Mastersizer (Malvern Instruments Ltd., Malvern, UK), equipped with an He-Ne laser (A = 633 nm). The volume size distribution is calculated from the intensity of light diffracted at each angle using the Mie theory. A refractive index ratio of 1.529 was used by the instrument to calculate the particle size distributions. The samples of emulsion were diluted at 1:200 with distilled water in a diffractometer cell, under stirring. The emulsion was measured the next day after being prepared. Each sample was analysed three times and data are presented as average values.

The average droplet size was characterised by mean diameters related to the volume D|43, defined respectively by:

(ProQuest: ... denotes formulae omitted.)

where n, is the number of droplets with diameter dr

Emulsion stability evaluation by turbidity measurement

Turbidity measurement was applied to determine emulsion stability (KAUFMAN and GARTI, 1984). It consisted in the measurement of absorbance of emulsion samples diluted at 1 to 1.000. The absorbance was measured at 400 and 800 nm, using a Helios ß spectrophotometer (Unicam). The size index (R) was determined from the ratio of absorbance values at 800 and 400 nm.

Emulsion stability measurement by the backscattering light method

The stability of emulsions was determined using Turbiscan (Turbiscan Lab., Formulaction) by measuring the backscattering of monochromatic light (A = 880 nm) from the emulsion as a function of its height. Emulsions were placed into flat-bottomed cylindrical glass tubes (40 mm height, 16 mm internal diameter) and stored at 37°±0.5°C for two weeks. The backscattering of light from emulsions was then measured as a function of height every other day for 2 weeks. The results are presented as backscattering versus height.

Statistical analysis

Data were analyzed using Statgraphics Plus 5.1. software (STSC Inc., Rockville, MD, USA). One-way analysis of variance (ANOVA) was performed. Significant differences between features were verified on the basis of Tukey HSD test at a significance level of p<0.05.

RESULTS

Effect of the type and amount of hydrocolloid on the dispersion degree of beverage emulsions

The destabilisation processes in beverage emulsions may be slowed down by among other things, obtaining a proper dispersion degree (HORNE and HERMAR, 1998). Emulsion stability is expected to be higher when the droplet size is smaller. An emulsion containing weighting agents and an acceptable emulsifying constituent will typically not separate if the average particle size of the emulsion is below 1 pm (BUFFO and REINECCIUS, 2000).

Fig. la presents cumulative distribution of particles in the emulsions stabilised by gum arabic. Most of particles (over 93%) with diameters below 1 pm were found in the samples of emulsion stabilised by 10% addition of gum arabic. The lower gum concentration (5% and 3%) resulted in a reduced number of particles with diameters below 1 pm to 84 and 50%, respectively.

Reducing the concentration of gum arabic caused an increase in the mean size of oil droplets. In the emulsion with 10% addition of gum arabic, the value of D[43] diameter was 0.57 pm, whereas in the emulsion with 3% of gum arabic it was almost twofold higher and reached 1.08 pm. Emulsifier concentration, which ensures a stable emulsion, should provide a complete coverage of the oil surface (ONSAARAD etoL, 2006). Gum arabic is used typically in high concentrations, i.e. 15-25% of the emulsion (LEROUX et oí, 2003).

The results of particle size index R measurement confirmed the significant effect of gum arabic concentration on the dispersion degree of emulsions. The index R was increasing with a decreasing content of gum arabic in the beverage emulsions (Table 1).

Different observations were made in the case of oil droplets size distribution in the emulsions stabilised by gum ghatti. In the emulsion with 10% addition of gum ghatti, only 68% of the particles had diameters under 1 pm. The decrease in emulsifier concentration to 5%, and further to 1.5%, resulted in 99% of the particles having diameters below 1 pm (Fig. lb). It indicates that the lower dose of gum ghatti improves the dispersion degree in beverage emulsions. This effect was probably related to reduced viscosity of the water phase. The viscosity of the water phase of the emulsion containing 10% of gum ghatti was undoubtedly too high and prevented the formation of a proper dispersion of the emulsion at the adopted parameters of homogenisation. This effect was not observed when us- ing gum arabic because this hydrocolloid forms a solution with much lower viscosity than gum ghatti does in the same concentration.

The course run of curves in Fig. lc was almost similar. Irrespective of the dose of a gum mixture addition, over 99% of particles of the dispersed phase of the examined emulsions had diameters lesser than 1 pm. Likewise, the mean size of oil droplets, D|4 3], was contained in a very narrow interval of 0.43-0.48 pm. A decrease in the dose of the arabic and ghatti gums (1:1, w/w) mixture from 5 to 3% resulted in an increased value of the size index R. However, the value of this index in the emulsion with 3% addition of the gum mixture (0.34 immediately after obtaining) was so small that it did not indicate the possibility of emulsion stability deterioration as a result of decreasing the amount of the emulsifier.

Effect of the type and amount of hydrocolloid on emulsion stability monitored by measuring the backscattering light

The examination of emulsion stability by backscattering light method was based on exposing the samples of emulsions to the action of infrared light with a wavelength of 880 nm. As a result, curves were obtained that showed transmission and backscattering light level in the function of height of the test tube with the emulsion. The study was carried out for 2 weeks, during which samples of emulsions were stored at 37°±0.5°C, in order to accelerate possible processes leading to emulsion break down. The curves of successive measurements showing the percentage distribution of transmission and backscattering light for stable products should overlap, while the curves of unstable products have a diverse course.

Fig. 2a shows the course run of the curves of backscattering light level for an emulsion with a 3% addition of gum arabic. The analysis of the data in Fig. 2a showed that the curves on the left side of the graph, between 0 and 10 mm of the height of the tube, did not meet. This was caused by a decrease in the backscattering level at the bottom of the emulsion, resulting from a decreased droplet concentration. This was characteristic of the beginning of the creaming process. Lesser changes were observed in the emulsion with a 5% addition of gum arabic. On the other hand, the curves plotted for the emulsion with 10% of gum arabic covered each other perfectly. Similarly, no changes were noted in the course of the curves plotted for the emulsion stabilised by 5% of gum ghatti. For this emulsion the backscattering of light was fairly constant. For the emulsion containing 3% of gum ghatti (Fig. 2b) slight deflections of the curves were observed in the first part of the graph. This could indicate that the processes leading to emulsion destabilization, i.e. gravity separation and/or flocculation, had already begun.

For emulsions with the addition of a mixture of arabic and ghatti gums (1:1, w/w) no noticeable disturbances were noted in the course of curves illustrating the stability of the examined sample (Fig. 2c). However, analysing the course of the curves showing changes of backscattering light level (Fig. 2d), a relatively low stability was determined for the emulsion with arabic and ghatti gums addition in 1:2 (w/w) proportion. The curves showing the percentage distribution of backscattering light clearly differed from each other at the top, which was certainly caused by the flocculation and/or sedimentation phenomenon.

Amongst the examined samples of beverage emulsions the highest stability was found in emulsions stabilised by 10% of gum arabic or by 5% of gum ghatti. The addition of the emulsifier can be reduced to 3% of emulsion mass, without decreasing its stability, by applying a mixture of these gums (1:1, w/w).

CONCLUSIONS

Both gum arabic as well as gum ghatti form stable emulsions. In order to obtain emulsions with the same stability it is necessary to use double amount of gum arabic in relation to gum ghatti. Emulsions with high stability can be produced by applying a mixture of these gums. Emulsions with higher stability were formed using a mixture of gum arabic and gum ghatti in the ratio of 1:1 (w/w), as opposed to a mixture prepared in the ratio of 1:2 (w/w). The synergetic effect of gum arabic and gum ghatti enables reducing emulsifier addition without decreasing emulsion stability.

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