Product appearance is an important factor for consumers when determining the quality of a product, and colour is one of the most important factors which contribute to product appearance. Currently, the safety and consumer acceptance of some colorants used in food products, such as titanium dioxide and some synthetic colorants, are under discussion. Therefore, new ways to use natural colorants as alternatives to these suspect colorants for future applications are being investigated. A promising method for increasing the applicability of the often sensitive natural colorants is the encapsulation of these colorants in colloidal particles by natural polymers such as carbohydrates, lipids and proteins. In recent years, micro-and nano-encapsulation have increasingly been used for various purposes concerning several food properties such as colour, flavour and micronutrient content. This technique results in improved stability for the often sensitive natural colorants and presents the possibility of entrapping water-insoluble colorants for improved use in an aqueous system. This paper reviews the main methods that are used for encapsulation by natural polymers, discusses the different types thereof that are used for encapsulation of colorants, and provides a short overview of natural colorants successfully encapsulated in these natural polymers.

Growing interest in using natural, biodegradable ingredients for food products leads to an increase in research for alternative sources of functional ingredients. One alternative is zein, a water-insoluble protein from corn. Here, a method to investigate the optical properties of white zein colloidal particles is presented in both diluted and concentrated suspensions. The particles are synthesized, after purification of zein, by anti-solvent precipitation. Mean particle diameters ranged from 35 to 135 nm based on dynamic light scattering. The value of these particles as white colorant is examined by measuring their optical properties. Dilute suspensions are prepared to measure the extinction cross section of individual particles and this was combined with Mie theory to determine a refractive index (RI) of 1.49 ± 0.01 for zein particles dispersed in water. This value is used to further model the optical properties of concentrated suspensions. To obtain full opacity of the suspension, comparable to 0.1-0.2 wt% suspensions of TiO2, concentrations of 2 to 3.3 wt% of zein particles are sufficient. The optimal size for maximal scattering efficiency is explored by modeling dilute and concentrated samples with RI's matching those of zein and TiO2 particles in water. The transport mean free path of light was determined experimentally and theoretically and the agreement between the transport mean free path calculated from the model and the measured value is better than 30%. Such particles have the potential to be an all-natural edible alternative for TiO2 as white colorant in wet food products.

HighlightsLutein-zein nanoparticles were successfully synthesized.Photo-stability of lutein-zein particles was improved, compared to pure lutein.Ascorbic acid had a positive effect on the photo-stability of the dispersions.

In this research, we model the color of optically dense colloidal dispersions of dyed and undyed zein particles using results from multiple light scattering theory. These particles, as well as monodisperse silica colloids, were synthesized and characterized to obtain particle properties such as particle size, particle size distribution, refractive index, and absorption spectrum of the dye. This information was used to model the diffuse transmission of concentrated particle dispersions, which was measured using a specially designed variable path length quartz glass cuvette. For the nonabsorbing silica dispersions, a transport mean-free path throughout the visible range was obtained. Results showed a difference of less than 5% from the values calculated with a multiple scattering model using the single-particle properties as an input. For undyed zein particles, which are off-white, the deviation between the model and the experiment was about 30% because of slight absorption at wavelengths below 550 nm but <7% at higher wavelengths. From these results, it was concluded that the model correctly describes diffuse transmission and that the measurements are sensitive to absorption. Finally, this method was applied to dispersions of dyed zein particles. Here, the transport mean-free path was first determined for wavelengths at which there is no absorption, which agreed with the theory better than 4%. The modeled transport mean-free path was then used to extract the reciprocal absorption mean-free path in the remaining parts of the visible spectrum, and a reasonable agreement with the absorption spectrum of the dye solution was obtained.

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