Potential of Dairy Coproducts in Wood Modification: Dimensional Stability and Mechanical Properties

Abstract Chemical modification of wood using citric acid and polyols is a promising biosourced route to improve dimensional stability. However, conventional polyols can be costly and are derived from food-grade resources. Dairy coproducts such as whey and whey ultrafiltration permeate represent abundant, low-value side streams rich in carbohydrates and nitrogenous compounds that could act as sustainable polyol substitutes. Whey ultrafiltration permeate was already investigated for wood modification. However, sweet whey was not yet investigated. In addition, a direct comparison was not yet made with wood modified with only citric acid. Such a comparison would help to better understand the mode of action of these dairy coproducts and their advantages compared to citric acid, which is already well known in wood modification. Therefore, this study aimed to evaluate the potential of these coproducts as a source of polyol, alone or combined with citric acid, to improve the physical and mechanical performance of Scots pine (Pinus sylvestris L.) sapwood. Full-cell impregnation followed by curing at 160 °C was used to promote an in situ polycondensation reaction between citric acid and components of the dairy coproducts. Dairy coproduct-based formulations containing citric acid achieved high fixation after water leaching, positive cell wall bulking, and stable anti-swelling efficiency, confirming the formation of an ester-rich polymer in the wood matrix. Moisture content at 95% RH decreased by ∼32% relative to untreated wood. Surface hardness increased by 30%, while bending strength was reduced less compared with citric acid modification alone. Scanning electron microscopy and Fourier transform infrared spectroscopy confirmed extensive polymer deposition and ester formation within the wood matrix. These results demonstrate the promising potential of dairy coproducts as circular, low-cost reactants for biosourced wood modification.