Comparative evaluation of various lignin determination methods on hemicellulose-rich fractions of spruce and birch obtained by pressurized hot-water extraction (PHWE) and subsequent ultrafiltration (UF)

Various lignin determination methods have been applied to hemicellulose-rich wood extracts obtained after pressurized hot-water extraction (PHWE) and membrane ultrafiltration (UF). In focus were the chlorine number (Cl no.) method, the acetyl bromide method, and the four modifications of the Klason lignin determination, such as the KCL, TAPPI, LAP, and Goldschmid methods. The furfural (F) and hydroxymethylfurfural (HMF) concentrations in the acid hydrolysates were also determined. The mass balances of the fraction were calculated with respect to the contents of dry solids and lignin, including the acid-soluble lignin. The reliability of the methods was evaluated based on the lignin mass balances and the gross chemical composition of the extracts. Although the results were dependent on the method applied, the lignin mass balance calculations yielded similar results in general.