Factors affecting the quality characteristics of black wattle (acacia mearnsii de wild.) bark.

Abstract

Black wattle (Acacia mearnsii De Wild) stands as a pivotal commercial tree species, spanning 82,000 hectares of land in South Africa. The revenue stream, sourced the from timber (85 %) and bark (1 5 %) sales, is estimated at $200 million annually. Bark harvested from these trees undergoes processing 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. Though historical research primarily concentrated on timber and tannin yields, scant attention was 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 aims to delve 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 wattle extracts' market presence in the tanning industry and ensuring the sustainability of South Africa's wattle industry. Tannins are the major extractable component of black wattle (Acacia Mearnsii de Wild) bark. In a wattle factory, extracted tannin levels are one of several quality parameters (Extraction efficiency and insoluble content are some of the other parameters) that are monitored, typically using a modified Soxhlet/ reflux extraction method. However, this method is slow and laborious, and the extracted product is unsuitable for tannin color analysis, which is another important product quality parameter. A new method was then developed, aiming to measure both the conventional bark parameters as well as the color of the extract solution, and increase sample throughput. The effect of oven-drying versus freeze-drying was compared as a method of moisture removal for tannin color preservation prior to extraction. Sample filtration methods, extraction times, and chemical additives were also tested, compared, and optimised as necessary. Suitably processed bark samples were subjected to four extraction methods, i.e., Soxhlet, cold water, pressure cooker, and autoclave extractions. Extract properties were analysed using the standard Society for Leather Technologists and Chemists (SLTC) methods. Freeze-drying was shown to arrest color development in the bark. Deionised water was the best extractant tested, although the addition of ethylenediaminetetraacetic acid (EDTA) enhanced the color of the extracts. The autoclave extraction method, involving two extractions and centrifugation, was found to be the most practical and effective for wattle bark. It minimizes color change, reduces sample analysis variability, and allows for high sample throughput. This method is valuable for future research on wattle bark properties and for routine quality analysis in wattle extract factories. Since the early 1900s, the Society of Leather Technologists and Chemists (SLTC) has used the hide powder method to evaluate tannin content and quality traits in wattle bark and Mimosa products. This approach has become the global standard in both the wattle bark and leather industries. Alternative methods, such as the Stiasny method, are predominantly employed for estimating adhesive tannin content, and ultraviolet (UV) spectroscopy is applied in both adhesive and tanning processes. However, Near-Infrared Reflectance Spectroscopy (NIRS) has emerged as a highly efficient, cost-effective, and non-destructive analytical tool. NIRS is widely used across various industries, including food and agriculture, and has proven its effectiveness for tannin analysis. This study explored the application of NIRS using different sample preparations, such as milled freeze-dried bark and bark extract solutions. By integrating NIRS scans with traditional SLTC methods and elemental analysis, models were developed to predict key properties of wattle bark and its extracts. UV spectroscopy was also tested for tannin determination but showed limited accuracy compared to NIRS. Notably, the NIRS analysis of milled freezedried bark demonstrated strong predictive performance for all critical quality parameters. These findings underscore the potential of NIRS as a viable alternative to traditional wattle bark analysis techniques. Its accuracy, speed, and noniii destructive nature make it a promising replacement for existing methodologies, offering enhanced efficiency for tannin content and quality analysis in the industry. Black wattle (Acacia mearnsii De Wild) is a key tree crop in South Africa, valued for its bark and timber, both significant contributors to export revenue. Wattle bark harvesting begins with the rainy season in September and extends to May. During this time, harvested bark is transported to three processing facilities. The journey, which includes stripping and transit lasting several days, exposes the bark to varying environmental conditions, such as temperature, rainfall, humidity, and light, all of which can influence the quality of the bark. The decrease in extractives, darkening of the bark, loss of tannins are all examples of bark quality degradation. To replicate post-harvest conditions, experiments were conducted using fresh bark samples collected bi-monthly from September 2020 to July 2021. These samples were subjected to varying temperature, light, and moisture conditions to simulate real-life scenarios and quantify the extent of bark quality degradation. Quality parameters, including total extractives, tannin content, and Lovibond color (red), were analysed. Advanced statistical techniques, such as principal component analysis (PCA) and redundancy analysis (RDA), were used to identify patterns and relationships among variables. The findings revealed that seasonal changes and site-specific conditions significantly influenced bark quality, particularly affecting Lovibond colour, a key quality indicator. This study underscores the impact of pre-extraction environmental conditions on the quality of bark extractives. Developing strategies to mitigate these effects is essential to minimize variability and ensure consistent production of highquality products. The study also highlights the need for more in-depth and future work. In South Africa black wattle (Acacia mearnsii De Wild) and green wattle (Acacia decurrens var. dealbata (Link) F. Muell) are the only two species of wattle that are cultivated commercially. Green wattle has superior quality as a timber crop whereas black wattle produces superior tannin extracts for leather tanning applications. The cultivation of green wattle was actively discouraged by the wattle industry, until 2013 the widespread death of black wattle trees as a result of infection by wattle rust (Uromycladium acaciae (Cooke) P. Syd. & Syd.) inadvertently stimulated the increased production of green wattle in mixed plantations. This study aimed to determine the maximum quantity of green wattle bark that can be mixed with black wattle bark without compromising the quality of the wattle extract products. Using mixtures of green and black wattle bark ranging from 0 % to 100% of each, the quality of extracts from bark mixtures were tested for the standard SLTC quality parameters. The addition of up to 40 % green wattle bark to black wattle bark had negligible effects on the quality of bark extraction liquor. Two Near Infrared Reflectance Spectroscopy (NIRS) models were developed for wattle bark mixtures. A Qualitative (IDENT) model was developed to detect the presence or absence of green wattle bark in consignments of largely black wattle bark. A Quantitative model was developed to quantify the levels of green wattle in mixtures of green and black wattle bark. Both models were successful, allowing for the detection of green wattle bark, and the accurate prediction of the level of green wattle bark in green and black wattle bark mixtures. Using NIRS to monitor green wattle bark levels, wattle extract factories will be able to use up to 40 % green wattle bark in mixed bark consignments without compromising the quality of the tannin extracts. This will benefit wattle farmers, who will be able to increase their timber revenue without reducing their bark revenue by increasing the percentage of green wattle to black wattle up to 40 % of each plantation. Black wattle (Acacia mearnsii de Wild.) is an important and high-value South African forestry species for bark and timber production. One of the objectives of the wattle breeding programme at the Institute for Commercial Forestry Research (ICFR) is to improve bark quality. Evaluating bark quality in breeding trials requires sampling of large numbers of trees, without causing any damage to the valuable wattle trees. Therefore, traditional wet-chemistry lab methods are impractical because of they are slow, expensive and require destructive sampling. Thus, the ICFR breeding programme would benefit from a rapid method to assess bark quality nondestructively. This study employed a novel Near Infrared Spectroscopy (NIRS) rapid and semi-destructive screening protocol to assess the quality of bark in field trials of black wattle. The protocol was used to compare wattle bark quality parameters of trees in block trials, established using various seed sources and clonal material. The predictive models developed showed a high degree of accuracy across the various parental trees and clonal varieties. The study showed that bark quality parameters across six different seed sources at a block trial in Luneburg showed very little variation, whereas bark from the same seed sources in another block trial in Iswepe