PEER-REVIEWED ARTICLE bioresources.com Kalliola et al. (2012). “VOCs of SW kraft lignin,” BioResources 7(3), 2871-2882. 2871 REDUCING THE CONTENT OF VOCS OF SOFTWOOD KRAFT LIGNINS FOR MATERIAL APPLICATIONS Anna Kalliola, a, * Anne Savolainen, b Taina Ohra-aho, a Greta Faccio, a and Tarja Tamminen a Three laccases, functioning in mild acidic, and one in slightly alkaline conditions, were evaluated in order to reduce low-molecular phenolic VOCs of kraft lignins, which could be used in lignin/natural fibers composites. The potential of a sulfhydryl oxidase to catalyze the oxidation of sulfur containing VOCs (thiols) was also tested in combination with the laccase-catalyzed oxidation. In addition, oxidation at alkaline pH at room temperature that may induce polymerization of phenolics in an analogous manner to the laccase-catalyzed reaction was investigated. Enzyme reactivity towards lignin was evaluated as consumption of oxygen in the reaction solution. The effect of treatments on VOC reduction was determined both by sensing (odorimetry) and chemical (TD-GC/MS, SEC) analyses. Laccases, Lcc2, and MaL from Thielavia arenaria and Melanocarpus albomyces, respectively, showed potential in reducing odors. The most promising results were obtained by oxidizing lignin with O 2 at alkaline pH. However, the odor threshold values of the main VOC compounds are extremely low, which poses a challenge to VOC reduction. Keywords: VOC; Kraft lignin; Composite; Laccase; Sulfhydryl oxidase; Oxygen; Oxidation Contact information: a: VTT Technical Research Centre of Finland, VTT, P.O. Box 1000, FI-02044 VTT, Finland; b: Trelleborg Industrial Products Finland Oy, Kikkerläntie 72, 38300 SASTAMALA, Finland. *Corresponding author: anna.kalliola@vtt.fi INTRODUCTION The potential of lignin to replace oil-based raw materials is being actively investigated for various material applications, such as composites. Based only on renewable resources, the Arboform ® composites consist of isolated lignin, natural fibers, and natural additives, which are used in conventional thermoplastic processes such as injection molding (Naegele et al. 2002). However, there are several difficulties in applying lignin. One of them is the volatile organic compounds (VOCs), either present in the isolated lignin, or formed as they are processed at high temperature in thermoplastic processes. This drawback reduces the applicability of lignin-based composites, e.g. in the inner parts of cars. VOCs in kraft lignin are typically lignin-originated phenolic structures, e.g. guaiacol (2-methoxyphenol), or reduced sulfur compounds formed in cooking. The odor threshold values of these VOCs are extremely low. For example, the odor threshold value for guaiacol in water solutions is reported to vary from 3 to 21 ppb (Fazzalari 1978; Buttery et al. 1988; Guth 1997). The great variation is due to different measurement techniques. For reduced sulfur compounds, dimethyl disulfide, and dimethyl trisulfide,