Improving agar properties of farmed Gracilaria gracilis by using filtered sunlight

The effects of different light spectra on agar yield, composition and gel properties, pigment content and antioxidant activity of Gracilaria gracilis cultivated in a land-based IMTA-system were investigated. Total phenol content, antiradical activity and lipid peroxidation inhibitory capacity in the residue obtained from G. gracilis agar extraction (agar waste, AW) were also investigated. Gracilaria gracilis was cultivated for 4 weeks applying neutral (NT), blue (BL) and red (RD) spectral filters in outdoor tanks in duplicates. BL light (13% agar extraction yield) increased the agar yield 1.3 and 2 times compared to both NT and RD conditions, respectively. BL light produced an agar gel strength (1037.9 g cm−2) similar to the NT condition (960.0 g cm−2). Light spectra had no significant effect on melting (~ 93.5 °C) nor gelling temperatures (~ 44.1 °C) of agar, also comparable to the agar obtained from G. gracilis before the growth trial (WD). However, the productivity of G. gracilis farmed under the BL condition (0.70% day−1) decreased when compared to the NT condition (1.14% day−1), while RD condition (0.93% day−1) growth was intermediate. BL condition increases the total carbohydrate content of G. gracilis compared to NT, RD and WD conditions (78.72, 68.48, 27.41, and 67.68 mg Glc Eq g−1 SW, respectively). The content of phycoerythrin, phycocyanin and chlorophyll-a in G. gracilis was similar among different light spectra. Total phenol (13.24–15.93 mg PE g−1 SW), as well as the antioxidant activity, remained similar in G. gracilis growing under the different light spectra. The total phenol content and the antioxidant activity of AW were higher than in the G. gracilis (31.76 mg vs. 7.52 mg PGE g−1 E and 1.67 vs. 2.92 mg mL−1, respectively). Results show that the agar yield and gel strength from farmed G. gracilis is improved when applying the BL spectrum filter despite a decrease in growth. This study also shows that AW could have potential applications due to its antioxidant activity (e.g., as a feed additive).