The color of Undaria pinnatifida after boiling is an important factor determining its marketable value. Our previous study showed that decreased nutrients and elevated irradiance resulted in increases of lightness and yellowness (i.e., discoloration) of this alga. However, little is known about the optimal levels of nutrients and irradiance required to decrease these color values and the combined effects of these factors and boiling. We conducted two culture experiments to test the effects of nutrients (non-enriched and 1.25, 5, and 25% PESI enriched treatments), irradiance (0, 10, 30, and 180 µmol m−2 s−1), and boiling on lightness L*, redness a*, and yellowness b* of this alga. L* and b* did not differ between non-enriched and 1.25–5% PESI treatments, but were lower in the 25% PESI treatment. L* and b* were lowest at 0–10 µmol m−2 s−1, although negative growth occurred at 0 µmol m−2 s−1. Decreased irradiance had a positive or little effect on a* before boiling, but had a negative effect after boiling. These results suggest that around 25% PESI and 10 µmol m−2 s−1 were the optimal levels to decrease the three color values of this species after boiling.
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