New research at NIZO is focussing on ways to stabilise water-in-water emulsions for applications that include low-fat foods. To meet the demand for label-friendly food additives in such food products, these stabilisers are based on zein – a naturally occurring insoluble plant protein – that researchers have made dispersible in water using a new technique.
The health risks associated with fat consumption have created a demand for low-fat alternatives for many food products. However, the texture – or mouthfeel – of high-fat products such as mayonnaise, salad dressing and sauces relies on oil globules destabilising in the mouth, thereby coating the tongue with a film of oil. When it comes to consumer acceptance of reduced-fat products, this creamy or fatty mouthfeel is a key factor.
Food manufacturers therefore emulate this texture by replacing oil-in-water emulsions with water-in-oil-in-water emulsions (so called ‘double emulsions’) or even water-in-water emulsions. Emulsions that deviate from the classical oil-in-water or water-in-oil pattern demand new types of emulsifiers. Many classical emulsifiers are listed as E-numbers and are therefore label-unfriendly. This need for new types of emulsifiers has led us to explore natural, plant-based candidates that have no E-number.
NIZO is currently developing low-fat emulsions that not only create the same texture as oil-containing emulsions, but also make use of plant-derived emulsifiers rather than those derived from animal products. In this we are following a general trend within the food industry to become more sustainable. The use of vegetable source ingredients contributes to a greener food industry and takes account of consumers’ tendency to be wary of ingredients with E numbers. The move towards plant-derived food additives is also in line with the emerging concept of “waste stream” valorisation, i.e. extracting value from organic waste.
The demand for natural raw materials without E numbers that have a lower carbon footprint than traditional materials is increasing in all branches of food technology. After all, the amount of fossil fuel required to produce animal protein for human consumption is far greater than that required for the production of plant proteins.
Despite the attractions of plant proteins as food additives, they are often insoluble in water, and are therefore also challenging to process and include in formulations. The plant-derived proteins that NIZO has been working on include zein from maize kernels and gluten from wheat grains. The hydrophobicity of these cereal proteins means they cannot be hydrated and therefore cannot be mixed homogeneously with water or aqueous systems. This also has consequences for mouthfeel when they are added to food products. However, the solubility problem has recently been solved following a scientific collaboration between NIZO and the University of Utrecht.
This collaboration focused on fundamental research into the properties of colloidal particles at oil–water and water–water interfaces (e.g. interfaces between concentrated protein and polysaccharide solution, which often do not mix). Researchers on this project discovered a method of making sub-micron zein particles that adsorb at oil–water interfaces and which – at least in some cases – stabilise oil-in-water, water-in-oil and water-in-water emulsions. As the researchers discovered, the trick to making colloidal zein particles is to use a so-called anti-solvent precipitation method whereby zein protein is first dissolved in a food-grade organic solvent, in this case 80% ethanol (see also the article here). Given the differences in zein’s solubility in water and food-grade ethanol, subsequent mixing with water results in oversaturation of the protein and the formation of nuclei, which then grow into particles approximately 100–500 nm in size that are dispersible in water.
One of the experiments tested whether these sub-micron particles could be used as emulsifiers to stabilise water-in-water emulsions through adsorption at the water–water interface. To this end, the researchers first mixed two aqueous solutions, each containing a biopolymer – one with fish gelatin and the other with dextran. The resulting dispersed emulsion was similar to that seen in foods that contain a mixture of protein and polysaccharide. The two liquids were immiscible and were seen to separate out into two layers within hours.
The addition of colloidal zein particles to the mixture largely prevented this gravity-induced demixing. A closer look at the behaviour of the particles at the water–water interface revealed that they accumulated and formed aggregates. These aggregates could then stop the late stage of the demixing process by forming a stable particle-rich layer at the water–water interface of the emulsion. This foam-like layer was not affected by centrifugation of the mixture and was found to contain dispersed dextran droplets surrounded by a particle-stabilised layer of gelatin. Although the particle concentration was too low to provide full surface coverage of the emulsion droplets, these results suggest that adding a higher concentration of colloidal zein particles could provide full stability of such a water-in-water emulsion.
The mechanism by which these particles adsorb to the emulsion droplets and stabilise them is fundamentally different from that of traditional molecular stabilisers, which also adsorb at the interface, but do not impart the interface with stiffness. Zein particles are therefore potentially much better stabilisers than traditional molecular stabilisers such as globular proteins. The particle size (typically 10–100 times larger than protein molecules) also adds to their stability, since they have a higher absorption energy making it more difficult for them to be released from the surface of the droplet. While the exact adsorption dynamics of zein particles at the water–water interface have not been studied, they are likely to be similar to those of other colloidal particles such as latex, which have been studied in similar water-in-water emulsions.
While further studies are needed before these particles can be used as emulsifiers in foods, the research also opens up new possibilities for the use of zein particles as plant-derived stabilizers in non-food applications such as in paint or cosmetics. We speculate that the technology could be used to replace the traditional organic solvents in these products with water-based emulsions that are more natural and more environmentally friendly.