## 3 Results and Discussion
### Fucoxanthin Content and Stability by UVA and Heat Exposure  
The fucoxanthin contents in the ethanol extract and solvent-partitioned fractions were determined using UPLC with a PDA detector at 450 nm (Figure 1). The limit of detection and the limit of quantification of fucoxanthin were 0.004 and 0.013 ug/L, respectively. One prominent peak with a retention time of 7.17 min was temporarily identified as all-trans-fucoxanthin by comparison with the fucoxanthin standard. The relative abundances of all-trans-fucoxanthin in the ethanol extract, hexane, chloroform, and ethyl acetate fractions were 5.52 \(\pm\) 0.02, 7.66 \(\pm\) 0.02, 84.24 \(\pm\) 0.11, and 1.28 \(\pm\) 0.02 mg/g, respectively (Table 1). Noteworthy in our study is that the liquid-liquid extraction methodology intensely concentrated fucoxanthin in the chloroform fraction. Fucoxanthin has a highly unsaturated structure, including an allelic bond, 5,6-monoepoxide, and nine conjugated doubled bonds, which can be easily degraded via oxidation and isomerization in the presence of heat, air, light, strong acids and bases, and other pro-oxidant molecules [19; 20]. During the kelp drying process, drying at a high temperature after air drying is usually conducted to reduce processing costs [21]. Consistent with previous studies [20; 22], our findings also showed that the fucoxanthin in _S. japonica_ was unstable to UV and heat. However, the different degradation patterns of fucoxanthin in response to UVA radiation and heat suggested that the degradation mechanisms induced by UVA and heat may be distinct. As shown in Figure 2, during the 24-h UVA treatment course, fucoxanthin gradually degraded in the ethanol extract and all solvent fractions. The degradation pace of fucoxanthin in the chloroform fraction was slower than other fractions, depending on different coexisting compounds in different fractions. Several groups studied the protective effect of fucoxanthin against UVA-induced cellular photoaging. It was reported that specific components, such as maltodextrin, gum arabic, and whey protein isolates, could improve the thermal stability of encapsulated fucoxan \begin{table}
\begin{tabular}{c c c c c c} \hline
**Solvent** & **Yield (\%) 1** & **Fucoxanthin** & **TPC** & **ABTS** & **FRAP** \\  & & **(mg/g)** & **(mg PGE/g dw)** & **(mg VCE/g dw)** & **(mM FeSO/g dw)** \\ \hline Ethanol extract & & 5.25 \(\pm\) 0.02 c & 5.05 \(\pm\) 0.00 d & 2.96 \(\pm\) 0.56 c & 0.04 \(\pm\) 0.00 d \\ \hline Hexane fraction & 14.39 & 7.66 \(\pm\) 0.02 b & 11.60 \(\pm\) 0.26 c & 4.19 \(\pm\) 0.37 b & 0.10 \(\pm\) 0.00 b \\ \hline Chloroform fraction & 1.23 & 84.24 \(\pm\) 0.11 a & 58.85 \(\pm\) 1.29 a & 20.43 \(\pm\) 0.81 a & 0.22 \(\pm\) 0.00 a \\ \hline Ethyl acetate fraction & 0.32 & 1.28 \(\pm\) 0.02 d & 20.42 \(\pm\) 0.92 b & 10.75 \(\pm\) 0.47 b & 0.07 \(\pm\) 0.00 c \\ \hline Butanol fraction & 8.62 & ND 3 & ND & ND & ND \\ \hline Aqueous fraction & 73.78 & ND & ND & ND & ND \\ \hline \end{tabular}  
* Yield of fractions was expressed as % of 50 g crude ethanolic extract. 2 TPC and TAC stand for total phlorotannin content and total antioxidant capacity, respectively. ABTS radical scavenging capacity and ferric-reducing antioxidant power were measured for the analysis of TAC. 3 ND stands for not detected. 4 Different letters a-d in the same column indicate significant differences (\(p<\) 0.05).  
\end{table} 
Table 1: Fucoxanthin content, total phlorotannin content, and total antioxidant capacity in crude ethanol extract and fractions 2, 4.  
thin [23]. [PERSON] et al. also reported that fucoxanthin from _Undaria pinnatifida_, another kelp species, extracted by different extraction methods showed a different photosensitivity [22]. Thermal treatment showed a different degradation pattern. When heated at 55 \({}^{\circ}\)C and 75 \({}^{\circ}\)C (Figures 3 and 4), the fucoxanthin content in the ethanol extract and hexane fraction decreased slightly and reached a plateau after 1 h, while the fucoxanthin in the chloroform fraction showed rapid degradation.