Influence of composition and structure on optical properties of food emulsions
Appearance plays an important role in determining the quality of food products, but little work has been done on the factors that influence the appearance of food emulsions. Appearance depends on the scattering and absorption efficiency of the emulsions. The scattering efficiency is determined mainly by droplet characteristics (size, concentration, aggregation and relative refractive index), while the absorption efficiency is determined by dye characteristics (absorption spectra and concentration). The objective of this study is to investigate the influence of droplet, TiO2 and dye characteristics on the optical properties of oil-in-water emulsions, and apply the knowledge gained from model systems to understand the factors determining the optical properties of a real food system, milk. Droplet and dye characteristics had a pronounced influence on emulsion appearance. The lightness of an emulsion was correlated to the scattering efficiency of the droplets. The color of emulsions (a- and b-values) was mainly determined by the type (red, green, blue) and concentration (0–0.1 wt%) of chromophores present. The lightness of emulsions increased steeply between 0 and 5 wt% oil, and increased slightly at higher concentrations (5–20 wt%). It also increased with decreasing droplet diameter (30–0.2 μm) and increasing refractive index difference between the two phases. The influence of droplet characteristics on the lightness and color of emulsions containing different types of dye was fairly similar. Droplet flocculation did have an impact on emulsion appearance, but the effect was relatively small compared to the effect of droplet size. Small amounts of TiO2 could be used to obtain the same emulsion appearance at lower oil contents. The optical properties of dairy emulsions showed the same trends as those of model ones. The effect was most pronounced below 5% milk fat. There was good correlation between sensory analysis and instrument measurement of emulsion color. Food emulsion appearance therefore can be optimized by carefully controlling droplet and dye characteristics. TiO2 has good potential to improve the appearance of low-fat foods. A mathematical model based on light scattering theory was shown to have good potential for predicting the influence of composition and microstructure on the optical properties of emulsions.