## 3 Results and Discussion
### Zeta Potential and pH of Spray-Dried Anthraquinone-Loaded CMs  
Figure 2 shows the zeta\(-\)potential of spray-dried casein micelles controlled, CM loaded with AQ powder (CMAQP), freeze-dried powder (CMFDP) and whole leaf Aloe vera gel (CMWLAG). The spray-dried casein micelles were negatively charged with a zeta potential of \(-\)17.2 mV. These results were consistent with previous studies [11] where casein micelles' \(\xi\)-potential was \(-\)16.77 mV after spray-drying [34]. The \(\xi\)-potential of CMAQP (\(-\)15.4 mV) significantly decreased compared to controlled CMs (\(-\)17.2 mV). The difference in zeta potential corresponds to a drop in the pH of controlled casein micelles from 7.4 to 6 after making complexes with anthraquinones, as shown in Table 1. This decrease in \(\xi-\)potential of CMAQP due to the shift in pH (from 7.4-5.5) is most probably caused by two pathways. Firstly, the disruption of hairy \(\kappa-\) casein layer and dissociation of some of the calcium phosphate results in rapid aggregation of casein micelles while encapsulating anthraquinones inside the hydrophobic core of the CMs while in the aqueous state [35]. Secondly, during spray-drying, hydrophobic layers of casein micelles are exposed to interact with aloin, aloe-emodin and rhein and form casein-anthraquinone through a well-known protein-phenolic interaction [36] during spray-drying. As reported previously, calcium ions act as bridges between casein micelles [37]; hence, the casein-calcium complex formation through ionic interaction makes it challenging for glucose to bind amine groups of casein, ultimately limiting glycation. The less negative zeta potential limits the formation of negatively charged intermediates during the Maillard reaction; hence a reduction in the glycation of casein micelles was observed. These results aligned with encapsulation efficiency and color parameters, confirming the early stage of the Maillard reaction. However, a decrease in \(\xi\)-potential indicates a decrease in casein micelles stability. These results fall between the previously reported \(\xi\)-potential of curcumin load nanocapsules where pectin is coated on the surface of casein micelles [12]. In contrast to zeta potential changes of CMAQP, spray-dried CMFDP and CMWLAG showed the highest decreases in zeta potential. CMFDP and CMWLAG resulted in \(\xi\)-potential values of \(-\)11.2 and \(-\)7.5 mV, respectively. As previously reported, the magnitude of \(\xi\)-potential increased as the pH decreased [33]. In our previous studies, the pH of casein micelles (CMFDP) decreased from 7.4 to 4.9 [14]. This decrease in pH in CMFDP caused a destabilization of \(\kappa\)-casein, causing the precipitation of casein micelles by releasing some calcium and phosphate ions into the solution. These precipitated CM was unable to hold anthraquinones efficiently while in solution. However, during spray-drying, the anthraquinones attached to the CMs via hydrogen bonding, while in an aqueous solution state, started to attach to the hydrophobic core of the casein micelles through folding and unfolding phenomena causing a reduction in negative charge. Moreover, some an thraquinones bound electrostatically to the hydrophilic terminal, causing a decrease in zeta potential. This could be confirmed by comparing the encapsulation efficiencies and surface attachment percentages after spray-drying. The \(\xi\)-potential of casein micelles in the case of CMWLAG was \(-7.5\) mV (Figure 2). This is primarily due to the decrease in pH. The decrease in zeta potential corresponds to a decrease in pH (7.4-4.6), where coagulation and flocculation of casein micelles occur by decreasing the net charge and solubilization of calcium phosphate. As the pH is close to the isoelectric point, the CM is capable of self-assembling and aggregating due to the loss of electrostatic repulsion among them [38]. These aggregated particles start to precipitate when the pH drops to 4.6.