# Clove Oil Protects \(\beta\)-Carotene in Oil-in-Water Emulsion against Photodegradation
## 3 Results and Discussion  
\(\beta\)-Car is lipophilic (log\(P\) = 12.2) and dissolves in the soy oil of the oil-in-water emulsion. Clove oil consists mainly of eugenol with log\(P\) = 2.49 [14,15] which distributes between the oil and the aqueous phase. As evident from Figure 1, the emulsion appeared homogeneously red. The concentration of \(\beta\)-Car in the emulsion oil phase was 7.4 mg/mL soy oil, while the phenols from clove oil were distributed between the two phases. Soy oil was selected for the oil phase of the emulsion as it is edible, with good nutritive value, and is available worldwide. Lecithin with 8.4 mg/mL emulsion was also added because it is commonly used as an emulsifier in foods. When stored in the dark, the color remained constant, as depicted in Figure 2. In Table 1, \(a\), i.e., the redness parameter of the LAB color system, is shown for 21 days of storage, while \(a_{0}\) is the redness parameter of day 0. The presence of clove oil did not affect the color during dark storage at any of the three concentrations. This finding was in agreement with the robustness toward uncatalyzed degradation of \(\beta\)-Car which was previously observed [16]. Upon exposure to light, the redness faded, as was evident from visual inspection; see Figure 1. Carotenoids are generally sensitive to radiation, including light and \(\gamma\)-irradiation [17]. The redness parameter \(a\) also showed a significant decrease during storage when exposed to light (Table 1 and Figure 2). The presence of clove oil clearly provided protection, as bleaching was reduced to approximately half of that in the emulsion without clove oil. Notably, the protection of color, and accordingly, of \(\beta\)-Car, was not dependent on either the amount of clove oil added or the concentration of the plant phenols in the concentration range studied (clove oil between 2.0 \(\upmu\)L/mL and 8.0 \(\upmu\)L/mL), due to the saturation of plant phenols at the emulsion interface. The decrease of the redness parameter \(a\) could be described by a mono-exponential model function for each of the three independent experiments for which the rate constant was 0.089 days\({}^{-1}\), and was not dependent on the clove oil concentration. This type of protection was similar to that of plant phenols toward the carotenoids involved in the visual function [18]. The light source used for the storage experiment had an intensity of 22,000 Lx, was mainly in the visible region, and had a minor UV-component. The glass of the jars further served as a UV-filter. As seen from the absorption spectra of Figure 3, the light was absorbed by \(\beta\)-Car rather than by the clove oil phenols. Excitation of \(\beta\)-Car to the singlet or triplet states generated radical cations, leading to bleaching:  
\[\text{Car}+\text{h}\
u\rightarrow{}^{1}\text{Car}* \tag{3}\]  
\[{}^{1}\text{Car}*\rightarrow{}^{3}\text{Car}* \tag{4}\]  
\[{}^{3}\text{Car}*/{}^{1}\text{Car}*\rightarrow\text{Car}^{\bullet+}+\text{ e}^{-}\text{ (solv.)} \tag{5}\]  
\[\text{Car}^{\bullet+}\rightarrow\text{Degradation products} \tag{6}\]According to Equations (1) and (6), the regeneration of the carotenoid from radical cations will compete with its degradation. As seen in Figure 2, the bleaching was independent of the concentration of eugenol in the concentration range studied. The regeneration rate previously found for homogeneous solution [7]:  
\[\frac{d[\text{Car}]}{dt}=k_{2}[Car^{+}][\varphi\text{-OH}] \tag{7}\]  
seems for the present conditions independent of the total phenol concentration in the emulsion. This apparent zero-order dependence on the plant phenol for the emulsion probably indicated: (i) a rapid electron transfer, and (ii) an apparent similar excess of phenol available for reduction under all conditions investigated. These observations pointed toward a mechanism occurring in the emulsion interface that was saturated with the plant phenols. The rate expression of Equation (7) was based on a series of more systematic kinetic studies in homogenous solutions [7; 8; 9; 11].