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Weekly UpdatesExtractive yields
SYP bark is a rich source of both hydrophilic extractives (e.g., proanthocyanidin polymers) and lipophilic extractives (e.g., resin acids, fatty acids). A mixture of acetone:water was selected as the extraction solvent for this study since it had previously been used to remove proanthocyanidin polymers for characterization studies (Foo and Porter 1980, Eberhardt and Young 1994), and although not targeted for such studies, also removes lipophilic extractives. Steeping at room temperature, as usually done with this solvent mixture, was preferred to avoid temperature-related changes that may occur during Soxhlet extraction. Steeping the bark blocks over a 3-day period afforded a total extractives yield of 3.9 percent which was very close to the weight change (Table 1) measured for the blocks receiving this treatment. A rapid treatment with the same solvent mixture afforded a total extractives yield of less than 0.1 percent; a loss in weight greater than this amount was attributed to small changes in MC during processing. Extraction of a sample of bark meal afforded a total extractives yield of 10.8 percent. Assuming that the grinding process only improved the extraction efficiency, steeping the bark in the form of blocks, as described above, allowed the removal of less than one half of the potentially available bark extractives.
Mechanical testing of solvent-treated bark blocks
The SGs of the bark blocks both before and after the solvent treatments were essentially identical (Table 1). Thus, for the solvent-treated bark blocks, a reduction in volume accompanied the loss in weight from the removal of some of the extractives. Bark blocks subjected to the rapid and extended solvent treatments were also subjected to mechanical testing and generally showed values similar to those for the untreated bark blocks, especially in the tangential and radial directions. The values for the stress at proportional limit and maximum crushing strength collected in the longitudinal direction appeared to decrease with the solvent treatments, however, the differences between the treatments were not statistically significant when compared by analysis of variance. Analogous to wood, the bark extractives that are readily removed may only provide a small, if any, influence over the mechanical properties. Several pine barks have been shown to contain proanthocyanidins (condensed tannins) that are not readily removed by extraction (Matthews et al. 1997). Since these extractives may become bound to the cell wall matrix (Matthews et al. 1997), it is possible that certain extractives may ultimately contribute to the mechanical properties of bark as such modifications occur.
Conclusions
The compressive strength of SYP bark, like wood, is greatest in the longitudinal direction. It is likely that contradictory findings appearing in an earlier report resulted from an inability to detect the mechanical failure of the very fragile obliterated phloem tissues in pine bark, as well as challenges associated with the testing of small blocks of different dimensions. Results from the mechanical testing of solvent-treated bark blocks suggest that although extractives are present in significant amounts, their contribution to the compressive strength properties are minimal. However, it remains to be determined if the conversion of certain extractives to insoluble forms, as suggested for the proanthocyanidins, ultimately results in a significant influence over the mechanical properties.
Literature cited
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The author is a Research Scientist, USDA Forest Serv., Southern Research Sta., Pineville, Louisiana (teberhardt@fs.fed.us). Assistance with sample preparation and testing were provided by Karen Reed and Donna Edwards, respectively. This paper was received for publication in May 2006. Article No. 10204.
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