Stabilization of oil-in-water emulsions using sodium caseinate complex and kathira gum

Abstract

Background and purpose: Emulsions are an important part of most food, pharmaceutical and cosmetic systems, which are very important in various industries due to their special rheological and physicochemical properties. But the main limitation of these systems is their thermodynamic instability, which causes phase separation over time. In this research, the effect of using Iranian ketira gum as a natural hydrocolloid and sodium caseinate as a natural emulsifier in stabilizing oil-in-water emulsions (10% by weight) and some physical and physicochemical characteristics of these systems were investigated. Materials and methods: In the first part of the study, the stability of emulsion systems was investigated by examining the effect of different concentrations of ketira obtained from two different species. Astragalus gossypinus (AG) and Astragalus flaccosus (AF) and considering the role and concentration of soluble and insoluble components during thirty-five days of monitoring were determined. Then, in the second phase of the study, the effect of different ratios of sodium caseinate to ketira (1.1, 1.2, 1.3, 1.5 and 1.9) in the total concentration of two fixed biopolymers and 0.9% by weight and different pH (7, 6, 5, 4.5 and 3.8) on the stability and physicochemical properties of emulsions was determined during 35 days. Then, in order to find the mechanisms related to stabilization, sugar analysis of gum with HPAEC-PAD, rheological characteristics of oil-in-water emulsion samples with rheometer device, particle size distribution using laser light diffraction technique, interfacial tension with tensiometer device were investigated.

Findings: The results showed that AF in a concentration of 0.5% by weight reduces surface and interfacial tension more than other samples, while AG It creates the most stable emulsions at a concentration of 0.5% by weight. It seems that the reason for the higher stability of AG is the presence of a soluble component with an emulsifying role and an insoluble component with an effect on the viscosity of the continuous phase.

Conclusion: a specific ratio of soluble to insoluble components, the chemical structure (degree of methylation, acetylation, uronic acid content and sugar composition) of the whole gum and its two constituent components have caused the appropriate ability of AG emulsification. The addition of sodium caseinate and katira had an antagonistic effect on the stability of the emulsions, and the emulsions containing these two biopolymers quickly became unstable.

Key words: oil-in-water emulsion, stability, katira gum, particle size, rheology and interfacial tension

Expression Problem

Emulsions are among the colloidal systems that are present in natural products as well as in the production process of a wide range of food, pharmaceutical and cosmetic products as a part or basically form the entire structure of the final product. According to one of the definitions provided in scientific sources, emulsions are colloidal and heterogeneous systems that contain two immiscible liquids (usually water and oil), so that one of the phases is dispersed in the form of droplets with a diameter of more than 0.1 micron (dispersed phase) in the other liquid (continuous phase). Emulsions are divided into single (straight, inverted and multi-layered) and multiple (multiple) categories according to the state of oil and water phase dispersions. Among the limitations of these systems is their thermodynamic instability. In fact, due to the positive Gibbs free energy, the presence of interfacial tension force and the density difference between the two phases of water and oil contact is unfavorable and the emulsion system breaks easily and becomes two phases with the passage of time. Among the major mechanisms involved in the physical instability of emulsions (breaking), we can mention such things as creaming, flocculation, coalescence, partial coalescence, and phase inversion (1-2). Despite this, it is usually possible to produce emulsions that are kinetically stable (metastable) by using emulsifiers and stabilizers (3-5). In the past years, the tendency to use emulsifiers and surface-active compounds, synthetic surfactants with low molecular weight and even phospholipids (as compounds that increase stability by absorbing oil droplets on the surface and reducing interfacial tension) has increased greatly.But today, due to the increase in the level of awareness of consumers and their unwillingness to use food containing unnatural additives, investigating the possibility of using natural biopolymers (proteins and polysaccharides) in stabilizing emulsions has occupied a major part of studies. Hydrocolloids are hydrophilic biopolymers with high molecular weight that are used in the food industry to control and improve texture, flavor and increase stability. They are a group of polysaccharides and proteins that are widely used in the industry (5-6). Usually, hydrocolloids control the behavior of water in a wide range of foods, improving texture, flow properties, controlling flavor release, adhesion, thickening, gelation of aqueous solutions, stability of foams, emulsions and dispersions, preventing the formation of ice and sugar crystals, and improving mouthfeel (7). In recent studies, usually the effect of adding protein-polysaccharide compounds or a mixture of two or more polysaccharides with specific ratios on the stability of emulsions has been investigated in the form of model or real systems (8-12). Gums usually have a hydrophilic structure and by increasing the viscosity of the continuous phase, trapping water in a three-dimensional network and finally reducing the mobility of the droplets of the dispersed phase, they increase stability, despite this property of surface activation (the ability to reduce interfacial tension), these compounds are not significant and do not play a major role in stabilization (13-14). Usually, a small number of hydrocolloids have emulsifying properties and reduce the interfacial tension and have the ability to be placed between the surface of water and oil due to the presence of auxiliary functional groups (esters and methyl or protein residues). Among these absorbent hydrocolloids are gum arabic, modified starch, modified cellulose polymers, some types of pectin, ketira, etc. (15-16). Katira gum is a natural dried extract obtained from some species of Astragalus and as a high-quality hydrocolloid resistant to acid and heat, it was included in the GRAS (Generally Recognized As Safe) list in 1961 (17-18). Reports show that the types of Katira gum obtained from different species have different and unique functional characteristics due to differences in structure and chemical composition and the amount and content of aronic acid and methyl functional groups. Catira gum is an anionic, branched and heterogeneous polysaccharide. This gum consists of two main components soluble in water (tragacantin) and insoluble in water (basorin). Previous studies have shown that these two components are different in terms of chemical structure, sugar content, and the amount of eronic acid and functional groups, including methyl, and they are also different in terms of functional characteristics (19-20). As it was said, the mechanism of stabilization of emulsions by most polysaccharides is limited to increasing the viscosity of the continuous phase, despite these studies have shown that Katira gum has a dual function (20). So that in addition to increasing the viscosity of the continuous phase up to about 1-10 pascal seconds (depending on the species and different parameters), it has a favorable emulsifying property for oil-in-water emulsions (HLB: 11.9) (7) and also creates steric repulsion (it prevents droplets from coming together by absorbing on the surface and creating electrostatic repulsion and increases the stability of the emulsion) (21). Determining the rheological characteristics of emulsions is not only important in calculations related to stability, but also plays a special role in cases such as the design of devices and equipment, including pumps and pipes, the determination and recognition of frauds, and the quality control and formulation of the production of practical new products (22). Meanwhile, the investigation of particle size distribution is very important due to the effect on the interaction between particles and the rheological characteristics of the samples, and then the effect on the stability and physicochemical characteristics of emulsions (23). The first aim of this study is to conduct an experimental research to investigate the stabilization of oil-in-water emulsion using the gum obtained from two Iranian species, A. gossypinus (soluble to insoluble ratio: 0.51) and A. floccosus (soluble to insoluble ratio: 3.51). Also, in order to determine the mechanisms of stabilization and two-phase formation, the investigation of rheological characteristics, indices describing the size of particles, measurement of interfacial tension was determined as the second goal of this research.