Maillard reaction-based routes for stable food emulsions

Oil-in-water (O/W) emulsions, where oil droplets are dispersed in an aqueous phase, often experience various physical and chemical destabilization phenomena, which compromise the quality and shelf life of the final products. To achieve acceptable shelf life, emulsifiers (e.g., surfactants and animal proteins) and synthetic antioxidants (e.g., ethylenediaminetetraacetic acid and butylated hydroxyanisole) are currently used. However, over the past decade, sustainable and natural food ingredients have become preferred options. This has been a strong incentive for the present work, which has aimed at identifying the potential of biobased Maillard reaction products (MRPs) to both physically and chemically stabilize emulsions.

In this thesis, we investigate the potential of MRPs, either prepared in model systems or inherently present in foods, to act as dual-function ingredients (i.e., physical stabilizers and antioxidants) in food emulsions.

The first part of the thesis was designed to study the MRPs prepared in model systems: In Chapter 3, we characterized the chemical and structural features of various Maillard reaction fractions prepared from soy protein isolate (SPI) and carbohydrates (dextran or glucose). We found that the Maillard reaction progressed further with glucose than dextran, with SPI-dextran conjugates showing higher activity than SPI-glucose conjugates, and water-soluble fractions showing higher activity than insoluble fractions. Therefore, a comprehensive understanding of the physicochemical properties in each fraction could be obtained. In Chapter 4, we further assessed the ability of initial stage MRPs (SPI-dextran conjugates) to prevent lipid oxidation, when added to the continuous phase of pre-formed stock emulsions. We found that the addition of protein-based compounds increased the oxidative stability of emulsions without significantly affecting the physical stability of emulsions. Therefore, it is possible to engineer oxidatively stable emulsions by adding protein-based compounds in the continuous phase, which could therefore reduce the need for synthetic antioxidants.

The second part of the thesis was designed to study MRPs extracted from food (dark roasted coffee): Chapter 5 aimed to explore the potential of melanoidins (high molecular weight fractions of the coffee brew) to physically stabilize emulsions. Therefore, coffee melanoidins were used to prepare emulsions that have a light brown, opaque, and homogeneous appearance with a nearly monomodal size distribution. It was shown that the polysaccharide-rich fractions from the melanoidins were present at the interface, whereas the protein-rich fractions remained in the continuous phase. This study shows the promising potential of coffee melanoidins as sustainable physical stabilizers of emulsions. In Chapter 6, we investigated the dual functionality of coffee melanoidin fractions with different molecular weights. When used as emulsifiers, coffee brew and (non-defatted) high molecular weight fraction (HMWF) of coffee brew were able to form emulsions that are both physically and oxidatively stable. When added to the continuous phase of the stock WPI-stabilized emulsions, all coffee fractions (coffee brew, its HMWF and low molecular weight fraction (LMWF)) were able to slow down lipid oxidation considerably without significantly changing the physical stability of the stock emulsions. This chapter shows that coffee ingredients can serve as dual-functional stabilizers in emulsions.

Overall, this work has shown that MRPs are able to contribute both to the physical and oxidative stability of emulsions, and to both effects, they may contribute when present at the interface (e.g., layer formation, metal chelation, and radical binding activity) and in the continuous phase (network formation, metal chelation, and radical binding activity).