## 3 Result and Discussion
### Evaluation of the Skin Whitening Properties of the Optimized Red Pitaya Peel Extract
#### 3.7.2 Anti-Tyrosinase Activity  
The potential of the optimized red pitaya peel extract to aid in skin whitening was measured in terms of its ability to inhibit the tyrosinase enzyme. The anti-tyrosinase activity assay performed by taking kojic acid as a standard exhibited that the optimized red pitaya peel extract possessed a good tyrosinase inhibition percentage of 66.29%, whereas the standard solution of kojic acid showed an inhibition percentage of 76.20% (Figure 6). The results of anti-tyrosinase activity were classified into four groups: highly active (IC\({}_{50}\leq 15.00\) mg/mL), moderately active (IC\({}_{50}\): 15.01-50.00 mg/mL), weakly active (IC\({}_{50}\): 50.01-100.00 mg/mL), and inactive (IC\({}_{50}\) > 100.00 mg/mL). The optimized red pitaya peel extract showed moderately active tyrosinase inhibition with IC\({}_{50}\) of 24.06 \(\mu\)g/mL, whereas kojic acid showed a high activity against mushroom tyrosinase with IC\({}_{50}\) of 12.56 \(\mu\)g/mL. Kojic acid is evidently the stronger inhibitor of tyrosinase enzyme, taking into account it is a single molecule with a specific function whereas the optimized red pitaya peel extract is a mixture of various types of phytochemicals with different functionalities. The absorbance values recorded at 490 nm for20 min also showed a decrease as the melanin synthesis was disrupted by the presence of bioactive compounds with tyrosinase inhibiting properties. The good tyrosinase inhibition property shown by the optimized red pitaya peel extract might be contributed by the presence of L-(+)-ascorbic acid 2,6-dihexadecanoate, or also known as ascorbyl dipalmitate, which was found in the highest percentage in this optimized extract. Ascorbyl dipalmitate is produced from the condensation of the hydroxy groups of L-ascorbic acid with two molecules of palmitic acid and it is a fat-soluble form of vitamin C [38]. In fact, fat-soluble vitamin C such as ascorbyl palmitate and ascorbyl dipalmitate are often preferred to be incorporated in the formulation of products, targeting topical administrations since they exhibit a much-improved stability than certain water-soluble forms of vitamin C. The dual solubility of ascorbyl dipalmitate resulting from the water-soluble L-ascorbic acid and fat-soluble palmitic acid suggested that this form of vitamin C can penetrate the skin, as well as the thin cell membranes at a higher efficiency to deliver better results [39]. Depigmenting agents fall into two important categories, whereby one acts by imposing toxic effects to the melanocytes while the other will disrupt the pivotal steps of the melanogenesis process. As for vitamin C, it breaks the continuity of the melanin synthesis process by interacting with the copper ions situated at the tyrosinase-binding site. This impairment to the active site of tyrosinase eventually hinders its proper functioning. As a consequence, the formation of melanin will start to decline and this will boost skin lightening effects [9; 40; 41]. Apart from this, benzaldehyde identified from the optimized red pitaya peel extract also enhances its anti-tyrosine property. Several studies in the past decade confirmed that both benzaldehyde and benzoate derivatives isolated from plants have been identified as potential tyrosinase inhibitors, including benzaldehyde, benzoic acid, anisic acid, anisaldehyde, cinnamic acid, and methoxycinamic acid from the roots of Pulsatilla cernua [42], 4-substituted benzaldehyde from cumin [43], and 2-hydroxy-4-methoxyenzaldehyde from the roots of Mondia whitei [44]. Furthermore, linoleic acid and oleic acid detected in the extract also possess tyrosinase inhibiting properties as unsaturated fatty acids are capable of suppressing tyrosinase activity and melanogenesis [35]. While keeping the messenger ribonucleic acid (mRNA) at a constant level, linoleic acid decreases the tyrosinase activity in the melanocytes.