Impact of laccase on the colour stability of structured oil-in-water emulsions

- Structured emulsions were physically stable over storage.
- Addition of laccase induced a colour fading at low pH.
- Emulsions stored at neutral pH maintained colour properties.
- High temperatures accelerated decolorization of emulsions.

The optical properties of food emulsions play a key role in determining their perceived quality because they are the first sensory cue that many consumers receive. The purpose of the current study was to investigate the impact of a cross-linking enzyme (laccase) on the appearance of structured oil-in-water emulsions containing a lipophilic model colorant (Nile red). A layer-by-layer electrostatic deposition approach was used to prepare oil-in-water emulsions stabilized by interfacial protein-pectin complexes under acidic conditions (pH 3.5, 10 mM citrate buffer). Laccase (an oxidoreductase) was then added to the system, since this enzyme is often used to covalently cross-link interfacial biopolymer layers. The optical properties of the emulsions were monitored during storage using spectral reflectance to determine the L*a*b values, while the physical properties were monitored by measuring changes in droplet surface charge and particle size distribution. No changes in the size or charge of the droplets were observed during storage, indicating that the emulsions had good physical stability. In the absence of laccase, the emulsions were stable to colour fading, but in the presence of laccase rapid colour changes occurred (red to blue to white). These results have important implications for the formation of structured food emulsions containing certain types of food dyes.

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