The Return of the Smell: The Instability of Lignin’s Odor

Abstract

Lignin is a major constituent of renewable plant resources. Its complete utilization is therefore a key element in a biobased economy. Lignin products can fail to be accepted commercially if they release volatile organic compounds, since they must conform to emission standards, and consumers must not reject them due to a repulsive smell. We therefore investigated low-odor kraft lignins for the stability of their volatile profiles, both on the long and the short term. With an increase in temperature, the emitted amount of volatile compounds increased, while the types of volatiles remained unchanged. The activation energy for the release was found to be in the area of physical desorption. It is therefore unlikely that chemical degradation processes contribute to the release of volatiles up to a temperature of 100 °C, and lignin can in principle be deodorized without degradation. Humidity had a stronger effect on the release of volatiles than temperature. When the relative humidity exceeded 60% or liquid water was added to a lignin powder, the detected volatiles multiplied. While we were unable to elucidate the underlying mechanism, our observations indicate a physical release, which is probably caused by the displacement of adsorbed organic molecules by water. Based on these findings, the influence of high-humidity conditions and elevated temperature must be taken into account during the development of lignin-based products to guarantee their stability.