A review of the factors affecting bark quality of black wattle (Acacia mearnsii De Wild.) in South Africa

Abstract

Black wattle (Acacia mearnsii De Wild.) stands as a pivotal commercial tree species, spanning 82 000 ha of land in South Africa. The revenue stream, sourced the from timber (85%) and bark (15%) sales, is estimated at US$200 million annually. Bark harvested from these trees is processed at three factories in South Africa. The resultant products, namely ‘Mimosa Extract Powder’ and ‘Mimosa Solid’, serve vital roles in the global leather tanning sector, facilitating an annual export of roughly 45 000 tons of bark extract, valued at around $60 million. Typically, these trees reach their optimal rotation age at 10 years, ensuring peak wood and tannin yields. Historical research primarily concentrated on timber and tannin yields, with scant attention being paid to other factors affecting bark and extract quality, notably colour. Various environmental, genetic and physical factors, alongside logistical and extraction processes, influence bark tannin content and colour. Post-harvest procedures involve quality grading and transportation to factories, where wattle bark undergoes industrial autoclave extraction to yield an extract liquor. Subsequently, this liquor is concentrated and either spray-dried to produce extract powder or evaporated into a solid, toffee-like product. This review delved into the mechanisms and factors affecting tannin composition and colour degradation in both the raw material (wattle bark) and the final extract powder. The focus lies on establishing a foundation for developing mitigation strategies to bolster the quality of wattle bark and its extract. Such efforts are imperative for maintaining the market presence of wattle extracts in the tanning industry and ensuring the sustainability of South Africa’s wattle industry.