## 4 Discussion
Since the higher transmission of visible and infrared light by treating with the clear filter instead of the bronze-tinted filter in BA allowed some 3-MBT formation, it is also elucidated that some other visible wavelengths, especially those in the 400-500 nm range, can induce 3-MBT formation. This observation aligns with the observation of [PERSON] et al. [18], who quantified high 3-MBT formation in beer exposed to 500 nm light [20; 21]. For the control and all filter-treated beer samples except one (clear filter at maximum 8.9 J/cm\({}^{2}\) fluence), 3-MBT was detected, but the concentrations were <LOQ. Therefore, the clear and bronze-tinted filters worked to prevent 3-MBT formation during PL processing. This observation indicated that the reaction to form 3-MBT was highly sensitive to UV rays below 400 nm. Color analysis showed that CA beers were darker than BA beers due to the addition of malts. The data displayed in Table 3 showed that BAs were turned lighter when exposed to higher fluences, especially observable after treatment without filters for 2.5 and 5 min,suggesting that PL treatment may have affected the light degradation properties of the colored compounds in beer, including melanoidins and polyphenols [30]. This pattern was supported by the UV-induced discoloration and degradation of melanoidins observed by [PERSON] and [PERSON] (2016), where longer exposure to UV rays increased discoloration. Discoloration may have also been achieved through other mechanisms working simultaneously to alter the color of beer without UV exposure, which requires more specific research. For CA beers, it was observed that UV exposure increased the SRM value, as UV could also trigger the polymerization of phenol compounds contributing to increased darkness [37; 38]. The bitterness of CA beers was seen to increase with fluence, while that of BA beers was seen to decrease with fluence. The decrease in BA bitterness from PL treatment is indicative of the photo-oxidation and degradation of iso-\(\alpha\)-acids influenced by UV, which supports the formation of 3-MBT [19; 39]. However, the opposite trend for CA beers is interesting. Since the manual isooctane extraction method also measures non-isomerized-\(\alpha\)-acids and oxidative polar compounds, some of these might have been formed due to the malt-derived polar compounds in CA [40]. The corresponding change in bitterness was lower with filters, with generally no significant differences between the two filters, suggesting that the application of most of the bitterness change was mostly UV-derived. While instrumental bitterness is known to differ from perceived bitterness, higher bitterness is generally associated with a negative involuntary perception [41], which would require sensory trials to further investigate the effect of PL on perceived bitterness in different beer types. There were overall small reductions in the pH after PL treatment in both beers. Overall, the pH decreased as fluence increased and the filter settings changed from bronze-tinted filter to clear filter to no filter. The mixing with oxygen in the coil chamber and the PL treatment may have caused photooxidation in compounds such as terpen, majorly identified in hops essential oils carried into beer [26]. The oxidation of terpen leads to carbonyl production such as organic acids, thus slightly lowering the pH [42]. The bulk of TSS measured in finished beer accounts for residual sugar untermented by yeast. Differences in residual sugars could potentially alter the perceived bitterness of beer [43]. Although there were significant but minor differences (\(p<0.05\)) between TSS, the impact on the perceived bitterness was not tested. TSS measured in each beer did not have an observable trend that correlated with fluence or wavelength exclusions. As the beers tested herein were unfiltered and unpasteurized before canning, unequal amounts of suspended yeast may still have been present [44].