Stabilization of emulsions by OSA starches

Abstract

In continuous mechanical emulsification, droplets of a coarse premix are first deformed and disrupted. Secondly, the newly formed interface has to be stabilized. Emulsions are thermodynamically unstable, as droplets tend to coalesce. To avoid coalescence of droplets, different effects have to be taken into account. The objective of this paper is to investigate the possibility of producing finely dispersed emulsions by using OSA starches as additive. Experiments were carried out using two commercially available OSA starches that differ mainly in viscosity of their aqueous solutions. In the experiments, the dispersed phase content, starch concentration as well as pH value and the influence of dissolved salts of different ion strength have been investigated. By droplet size analysis directly after production of oil-in-water emulsions the emulsifying properties of such starches were determined. Measurements of interfacial tension prove that OSA starches are surface active substances. It was shown by emulsification experiments that the stabilization of droplets by OSA starches is very efficient. The emulsification results achieved with OSA starches have been found to be independent from the type of starch and the starch concentration above the lower limit of starch needed for stabilization. The emulsification results are independent of pH value and ion valence. Therefore it can be concluded that steric hindrance is the main stabilizing mechanism of OSA starches. As proteins stabilize emulsion droplets mainly by electrostatic repulsive forces, the emulsification result depends strongly on pH, which leads to poor emulsification at pH values near the iso-electric point. The results of the work presented here suggest that whey proteins can be substituted by OSA starches with a specific advantage at low pH values near the iso-electric point of the protein.

Introduction

The emulsification process includes two steps. First, deformation and disruption of droplets, and secondly, stabilization of the newly formed interfaces. The quality of an emulsion depends on the droplet size of the dispersed phase. Usually, the aim of emulsification is to produce as small droplets as possible. Emulsions have to be stable for a certain period of time, i.e., droplet size must not increase during storage. Emulsions are thermodynamically unstable, as droplets tend to coalesce. Thus, during the continuous mechanical emulsification process, there is the possibility of coalescence immediately after droplet disruption. To stabilize the disruption result, i.e., to improve the emulsion's short term stability, the coalescence frequency has to be controlled (Tesch & Schubert, 2001). Regarding avoidance of coalescence of droplets, different effects have to be taken into account. Hydrodynamic stabilization by slowing down the drainage of the liquid between two colliding droplets is one of these effects. Furthermore, steric hindrance or electrostatic repulsion are stabilizing mechanisms caused by surfactants adsorbed at the interface of the droplets.

Starches have always been essential to human nutrition. They provide a major part of the overall caloric intake worldwide. Besides their nutritional value starches have been used for their functional properties such as thickening in soups and sauces (Murphy, 2000). By chemical modification of starches, process and storage stability of starch containing foods may be increased. Furthermore, it is possible to add hydrophobic side chains to the originally mere hydrophilic starch molecule. By this means starch may adsorb to the interface of water and oil, thus stabilizing an emulsion.

Such emulsifying starch is octenyl-succinate starch, so-called OSA starch. It is made by esterification of starch and anhydrous octenylsuccinic acid under alkaline conditions (Trubiano, 1986). This modification has been FDA approved for the use in food. OSA starches have also been approved as food additive in the EU (E 1450). By use of various starch base (native starch, acid fluidities, dextrin) the desired functional properties, namely viscosity and solubility, are determined. Most useful are cold-water soluble OSA starches, as the heating step for the gelatinization – otherwise necessary – may be omitted.

OSA starches have been used successfully for many years for encapsulation and for beverage emulsions. It is a special advantage that these starches are almost colourless and tasteless in solution. When used in encapsulation processes valuable ingredients are most favourably protected against oxidation. In the production of beverage emulsions the high consistency of quality is remarkable, quite opposite to a complete natural ingredient like Gum Arabic. Using OSA starches in culinary products like low fat spreads provide some advantages; e.g., starches are cholesterol free unlike egg yolk. Because starches increase the viscosity of the continuous phase, a cost reduction for the final product is possible because emulsifying starches may act as a combination of surfactant and stabilizer.

It is the objective of the present study to investigate the stabilizing properties of OSA starches in the production of emulsions. Experiments were carried out using two commercially available OSA starches that differ mainly in viscosity of their aqueous solutions. In the experiments, the dispersed phase content, starch concentration as well as pH value and the influence of dissolved salts of different ion strength have been investigated. By droplet size analysis directly after production of oil-in-water emulsions the emulsifying properties of such starches were determined. Storage stability of such emulsions are part of further studies to be carried out.