Understanding the relationship between acid precipitation and lignin odor in Kraft Black liquor

• Kraft lignin is valued for high-end products; acidity affects odor profiles. • Study explores acidity's impact on lignin odor, crucial for quality improvement. • Different acid levels influence odorous compounds and perception. Kraft black liquor, utilized as a lignin feedstock, is increasingly acknowledged as a valuable resource for the production of high-value products. Acid precipitation has proved to be one prominent method for separating this lignin, but its impact on lignin odor remains understudied. This study addresses this gap, aiming to elucidate the relationship between precipitation acidity and lignin odor profiles, offering insights for improving lignin quality and its industrial applications. To this purpose, lignin from Kraft black liquor was precipitated under different acidity levels compared one to another in terms of release of odorous compounds, as mass concentrations and olfactory perception. To establish a comprehensive odor profile representative of lignin, the investigation focuses on the odor activity values (OAVs) of compounds contributing significantly to the overall odor mixture. Results revealed that acidity significantly influences the generation of volatile organic compounds and the olfactory profile of lignin. Lignins extracted under acidic conditions exhibit fewer aromatic characteristics, whereas those under neutral conditions display more intense and diverse odors. The presence of the guaiacol aroma, characteristic of lignin, is notably pungent and predominant in all samples. Odor profile analysis provides insights into lignin structural changes during precipitation. Decomposition reduces molecular weight, removing salts and sulfur impurities, decreasing sulfur-derived VOCs. Changes in odor profile, like increased acetic acid odor and more methoxyphenol emission, reflect ester bond degradation and greater methoxy group exposure.