Antioxidants play a crucial role in human health, as they can prevent oxidative stress related diseases like high blood pressure, neurodegenerative disorders, or cancer. In particular, natural antioxidants are getting increasing attention due to sustainability issues. One prominent example is lignin, a polyphenolic biopolymer that can be extracted from lignocellulosic biomasses. Due to their great number of phenolic hydroxyl groups, lignins are able to terminate oxidative chain reactions and quench free radicals. To determine the antioxidant activity (AOA), various assays are established. However, AOA studies are sophisticated, as no single method is capable to measure all naturally occurring reactions, and thus can depict the whole potential of any antioxidant. In this study, four different assays, namely the 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), ferric ion reducing antioxidant potential (FRAP) and Folin-Ciocalteu (F-C) assay, were used to investigate the AOA of 50 organosolv lignins from miscanthus, paulownia, silphium, beech, and wheat straw, and two kraft lignins. The coefficient of variation was determined for each method and was lowest for ABTS (0.25 %) and highest for DPPH (5.30 %). When comparing these two radical scavenging assays, ABTS seems to be favorable regarding reaction time, sensitivity, and reproducibility. The best correlation between two assays is found for FRAP and F-C (R2 = 0.944), which is not surprising as both rely on the same mechanism, the single electron transfer (SET). In contrast, the underlying reaction mechanisms for ABTS and DPPH cannot be assigned to either SET or hydrogen atom transfer (HAT) but are more complex as both occur in varying ratios. Nevertheless, a good correlation (R2 &gt; 0.75) between ABTS, FRAP and F-C, respectively, could be found, while DPPH merely correlates with the others (R2 &lt; 0.5), leading to the conclusion that the DPPH assay is closer to the HAT mechanism. Conversely, this does not mean that the DPPH assay is not feasible for the measurement of lignins, it rather reflects varying antioxidative attributes, most probably due to a different reaction mechanism.

Comparing the four spectrophotometric assays DPPH, ABTS, FRAP and Folin-Ciocalteu to evaluate the antioxidant activity of a set of lignins

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