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Kraft pulp, the raw material in paper manufacturing, is obtained by exposing pieces of wood to a cooking process. The parameters of this process (temperatures, times, chemicals, etc.) depend strongly on the density of the different woods in the cooker and have an influence on the quality of the pulp that is obtained. In order to optimize both the production process and the pulp quality, one wishes to cook together woods having similar densities. However, given that the harvest areas contain limited quantities of trees and that wood densities vary significantly from one area to another, deciding how harvested wood from different areas should be mixed for processing is a significant operational decision in pulp production planning. This situation is modelled as a scheduling problem where one has to decide which harvested area goes to each available processing units in order to minimize the variance of wood densities within each cooker for each period of the planning horizon. We also present an approximated solution approach based on the formulation proposed. Some results are reported and the efficiency of the method is discussed. [ABSTRACT FROM AUTHOR]