Evaluation of current methods to estimate pulp yield of hemp

Large-scale evaluation of hemp stems from field trials requires a rapid method for the characterization of stem quality. The large differences between bark and woody core in anatomical and chemical properties, make a quantification of these two fractions of primary importance for quality assessment. This paper evaluates accuracy and power of discernment of current procedures for the analysis of the composition of hemp stems. To reduce the amount of plant material to be analysed it was investigated if the contents of bark, bark fibre and woody core in the stem could be estimated in a representative stem segment (according to Arnoux et al., 1969). Analysis of segments, however, gave less accurate information on stem composition than analysis of entire stems. The recovery of the bark and woody core in the pulping process is referred to as the pulp yield of the two fractions. Two procedures for the estimation of potential pulp yield, one common in fibre hemp breeding (Bredemann method) the other in paper pulp technology (Tappi method 212), were compared. The Bredemann method determines the content of the economically important bark fibres, and approximates the content of woody core. The Tappi method determines, separately, the solubility of ground bark and woody core as a measure for yield of these stem fractions in the pulping process. The two methods show similarity since a boiling NaOH solution is used for fibre extraction in the first method as well as for dissolving ground components in the latter. Results of the simple and less laborious Bredemann method were compared with those of the Tappi method, which is readily interpretable in pulp technology, and also with an intermediate method. The latter determines the initial content of woody core and subjects the separated bark tissue to the Bredemann fibre extraction procedure. The initial content of woody core of the stem (intermediate method) correlated well with the content of insoluble woody core (Tappi). The measurements of bark fibre content (Bredemann) were quite similar to those of the content of insoluble bark components (Tappi)