The plant-based food market has grown appreciably in recent years due to increasing demand for foods that are better for the environment, human health, and animal welfare [1, 2]. In 2021, the sales of plant-based foods designed to mimic animal-based ones (like meat, seafood, eggs, milk, and their products) was reported to have grown by around 54% in a three year period [3]. However, there are still several technical hurdles that are holding back the more widespread acceptance and adoption of plant-based foods, including the need to mimic the texture, taste, and appearance of animal-based foods more accurately. Appearance is the first sensory attribute consumers experience when they encounter a food and so it greatly affects their purchasing decisions [4]. Moreover, the sensory perception and desirability of foods depends on their appearance, which is important because it impacts repeat purchasing of products [5]. For plant-based foods, consumers expect their appearance to closely match that of the animal-based products they are designed to replace, e.g., a matt brown cooked meat, a shiny raw pink salmon fillet, or a matt yellowish cooked egg [6, 7]. Creating plant-based foods with similar appearances to animal-based ones could increase their appeal to a broader range of consumers, thereby reducing the adverse environmental and animal welfare issues associated with the livestock industry.
To simulate the appearances of animal-based products more accurately it is important to understand how color characteristics and food matrix effects impact the optical properties of plant-based foods. For this type of product, it is important to use natural pigments that have been isolated from plants [8, 9]. However, formulating food products using natural pigments is usually more challenging than using synthetic colorants due to their poorer solubility and stability characteristics [8]. Researchers have therefore explored different strategies for improving the solubility and stability characteristics of natural colors, including co-pigmentation, encapsulation, and adding natural preservatives [10–12]. However, more studies are needed to better understand the factors impacting the optical properties of plant-based foods containing natural pigments.
The purpose of this study was therefore to systematically investigate the main factors influencing the optical properties of plant-based foods using oil-in-water emulsions containing natural plant-derived pigments as model food matrices. Emulsions are particularly useful as model systems because their compositions and structures can be systematically varied. Three plant-derived colorants were selected to represent the three primary colors: turmeric (yellow), red beet (red), and butterfly pea flower (blue) [13]. The main pigments in these colorants are anthocyanins (butterfly pea flower), curcumin (turmeric), and betalains (red beet), which have also been claimed to exhibit several health benefits and can therefore also be included as nutraceuticals in plant-based foods [14–16]. Both lipid-soluble (turmeric) and water-soluble (red beet and BP flower) colorants were used because plant-based foods are multiphasic systems, in which pigments can be located at either or both the dispersed and continuous phases [17].
Another reason we used oil-in-water emulsions as model systems is because they are inherently multiphasic systems that have compositions and structures that can be designed to represent a broad range of plant-based foods [18–21]. Previous researchers have already highlighted the potential of nanoemulsion technology to improve the dispersibility and stability of natural colorants [22–24].
An improved fundamental understanding of the factors impacting the appearance of plant-based foods depends on improving our knowledge of the physicochemical phenomena that contribute to their optical properties [6]. The color of animal-based products, like meat, eggs, and milk, is the result of a combination of selective absorption of light by pigments and scattering of light by particles [6, 25]. For instance, the fibers in meat, the lipoproteins in egg, and the fat droplets in milk scatter light, which causes these products to appear opaque. Similarly, the myoglobin in meat and the carotenoids in egg yolk selectively absorb light, which causes these products to have specific colors. The utilization of pigment-loaded emulsions as model systems can therefore simulate the scattering and absorption effects found in plant-based foods.
Most animal- and plant-based foods are opaque because of the strong scattering by the particles they contain. Consequently, their appearance is dominated by light scattering and absorption events that occur at their surfaces [26]. The optical properties of these kinds of systems can be modeled using the Kubelka-Munk theory, which relates light reflection from an object to its absorption and scattering characteristics:
$$R=1+\frac{K}{S}-\sqrt{\frac{K}{S}\left[\frac{K}{S}+2\right]}$$
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Here, R is the spectral reflectance, and K and S are the absorption and scattering coefficients of the material, respectively. The predicted change in the reflectance with K/S is shown in Fig. 1. The reflectance decreases as the K/S value increases, which means that it should decrease as either the absorption becomes stronger, or the scattering becomes weaker. Measurements of the spectral reflectance of an emulsion versus wavelength in the visible region (380–780 nm) can be used to calculate its X, Y, Z tristimulus coordinates, which can be converted into L*a*b* tristimulus coordinates [27]. These coordinates are related to the appearance of materials experienced by humans, such as their opacity and color.
For oil-in-water emulsions, the absorption coefficient depends on the type and concentration of chromophores present, while the scattering coefficient depends on the size, concentration, and refractive index contrast of the droplets [28]. The absorption and scattering coefficients can be calculated using the following equations:
$$S=\frac{3}{4}\pi {r}^{2}{Q}_{s}\left[1-g\right]-\frac{1}{4}{\alpha }_{E}$$
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Here, \({\alpha }_{E}\)is absorption coefficient, \({Q}_{s}\) is the scattering efficiency, r is the droplet radius, and g is the asymmetry factor, which depends on the scattering pattern of the light waves by a droplet. The absorption coefficient of an emulsion can be calculated from experimental measurements of the absorption of the oil and aqueous phases at different wavelengths: \({\alpha }_{E}\left(\lambda \right)=\varphi {\alpha }_{D}\left(\lambda \right)+(1-\varphi ){\alpha }_{C}\left(\lambda \right)\) where the E, D, and C subscripts refer to the emulsion, dispersed phase, and continuous phase, respectively, \(\varphi\) is the volume fraction of the dispersed phase, and l is the wavelength of light [27]. These equations show that the optical properties of emulsions should depend on the absorption spectra of the pigments, as well as the characteristics of the droplets.
In this study, we systematically investigated the impacts of natural plant-based colorant characteristics (type and concentration) and oil droplet characteristics (size and concentration) on the optical properties of model emulsions. The model oil-in-water emulsions were prepared using a plant-based oil (corn oil) and emulsifier (quillaja saponin). According to the Kubelka-Munk theory, we hypothesized that the optical properties of these emulsions would depend on the colorant and droplet characteristics used. However, the magnitude and nature of these effects is currently unknown. The results of this study should therefore provide a better understanding of the factors impacting the optical properties of multiphasic foods, which can be used to formulate plant-based food with closer appearances to the animal-based products they are designed to replace.