## 4 Discussion
The truncated penetration depth of UV rays in CA could have been due to higher levels of melanoidins in darker beers ([PERSON] et al., 2013). Melanoidins have enough absorbance in the visible spectrum for humans to perceive the darkness in dark beer, but they have especially higher absorbance in the UV spectrum ([PERSON] et al., 2013). [PERSON] (2013) observed that higher intensities of melanoidins had increased absorbance in the UV region, which is synonymous with the lower UV penetration depth and lower microbial reduction in CA. High levels of melanoidins in CA beers would have been sourced from malts added during its production. As CA has higher concentrations of iso-\(\alpha\)-acids than BA, indicated by higher IBU, the former theoretically would have a higher concentration of reactants to degrade and form more 3-MBT than the latter. However, the opposite trend was observed, possibly due to the penetration depths of both beers. High levels of melanoidins in the dark-colored CA, potentially reaching 1.49 g/L for dark beers, would have absorbed a substantial amount of UV rays compared to riboflavin and iso-\(\alpha\)-acids, usually present in 0.2-0.4 mg/L and 17-55 mg/L, respectively [3; 32; 33]. Furthermore, melanoidins are large, heterogeneous polymers with high molecular weights between 85 and 232 kDa [34]. Hence, melanoidins are bigger in size and could shadow riboflavin and iso-\(\alpha\)-acids from the UV incidence, preventing the \"lightstruck\" reaction. While only slight microbial deactivations were displayed in BA treated without filters for 0.25 and 0.5 min residence times, substantial concentrations of 3-MBT were still formed, suggesting that the energy threshold to form 3-MBT was lower than to deactivate _L. brevis._  
The high penetration depth characterized by PL in BA samples allowed the energy carried in electromagnetic radiation to be dispersed more evenly throughout the coil chamber's diameter. With low fluences, the energy absorbed by each thin radial layer of the fluid in the coil chamber was not sufficient to deactivate _L. brevis_, resulting in overall less microbial reduction. On the other hand, the low penetration depth of CA caused less energy dispersion but allowed energy to be absorbed and concentrated on the outermost fluid layer nearing the chamber coil's interior surface. As fluids were treated under laminar conditions with Reynolds numbers ranging from 154 to 2053, parabolic velocity profiles formed in the coil chamber during treatment. The parabolic velocity profile endorses maximum fluid velocity in the coil's centerline. In contrast, the fluid velocity is lower near the surface, further enhancing surface decontamination in CA as it concentrates the electromagnetic energy [17; 35]. Fluid entrance regions were between 0.083 and 1.1 m, which allowed fully developed fluid flow during a large portion of each treatment. Considering particles dispersed throughout the cross-section of the fluid, there is a possibility of unequal treatment between all radial distances from the centerline inside the fully developed flow. 3-MBT formation was quantified in beers treated without filters at all residence times and only at the highest residence time (8.9 J/cm\({}^{2}\) fluence) with a clear filter. As the residence time of beer treated without filters increased, there was a substantial increase in the final concentration of 3-MBT due to the increased fluence that drove the \"lightstruck\" reaction forward. This finding was aligned with [PERSON] et al. [36], who quantified more 3-MBT in beers exposed to more sunlight. BA and CA beers treated without a filter, even for the shortest residence time, resulted in concentrations of 3-MBT 11.77 and 3.693 times higher than the LOQ, respectively, while 3-MBT in beers treated with the filters even for the longest residence time (except BA PL-treated with a clear filter at a maximum of 5 min residence time) stayed below the LOQ. This indicated that UV rays removed by the clear and bronze-tinted filters were needed to substantially form 3-MBT in beers.