Comment on Kraft Process─Formation of Secoisolariciresinol Structures and Incorporation of Fatty Acids in Kraft Lignin.

This paper presents comments on the recently published article by Lahtinen et al., showing results about study the chemical structure of insoluble residual lignin, the presence of fatty acids in kraft lignin, and the origin of the secoisolariciresinol fragment. The analytical procedures used in the study seem to be reliable. However, in my opinion, the authors made mistakes when interpreting the heteronuclear single-quantum correlation (HSQC) nuclear magnetic resonance data; therefore, their conclusions require discussion. Here, an alternative interpretation of some of the results is proposed. In my opinion, the identification of lignin structural fragments from the literature data is a critical step for many studies. Therefore, the introduction of criteria for evaluating the accuracy of identification of lignin structural fragments according to HSQC spectra will significantly affect the scientific context of the structural correctness of subsequent studies of the lignin structure.