Scaled-Up Preparation of Stems for Extrusion Tests

The larger-scale columns, while necessary to produce treated straw stems for use in the preparation of composite formulations for extrusion testing, were also a good test of the sensitivity of the system to scale up to larger columns and to changes in inoculum source and inoculation method. After 6 wk of treatment in the small columns used to inoculate the drums, 28-30% xylan was degraded while about 18.5% of glucan was degraded (Table 6). This gave ЛХ/AG ratios of 1.52-1.64. For comparison, at 40.0 mg of P. ostreatus/g of stems and 1.60 g of H2O/g of stems, the regression models predict 27.2% xylan degradation and 21.1% glucan deg­radation at 6 wk, for a AX/AG of 1.29. Clearly, P. ostreatus was dominant in the small columns used to prepare enrichment inoculum for the barrels. However, the AX/AG values were well outside the range predicted by the regression models. Apparently, the nitrogen-limited inoculum produced by Utah State University and shipped to INEEL for these columns was either more active or better acclimated to the nitrogen-limited conditions in the straw stems. This effect was repeatable, indicating that the history of the inoculum used may have a significant effect on AX/AG, although the actual glucan and xylan conversions were not far from the predicted values. The inoculum produced for the small columns used to inoculate the drums was produced differently than the inoculum used to inoculate the small columns in the moisture and inoculum tests—it was better accli­mated to nitrogen-limited conditions. This is because the mycelia were transferred directly into the nitrogen-limited medium (C/N of 32.6) with­out first being grown in the carbon-limited YM broth (C/N of 7.7). Since both enrichment steps in the production of the mycelial inoculum were carried out in the nitrogen-limited medium, this likely resulted in a myce­lial inoculum that was better acclimated to low-nitrogen conditions when it was added to the straw stems, which have a C/N of about 80 (17). Thus, system performance is sensitive to inoculum source and history.

The altered inoculation method also resulted in a different degrada­tion pattern than that observed in the small-column tests. After 6 and 12 wk of treatment in the drums, only 11.9 and 24.2% of the xylan was degraded, respectively (Table 6). Glucan degradation was similarly reduced, with only 13.4 and 26.8% of the glucan degraded at 6 and 12 wk, respectively. This equates to AX/AG values of 0.89 and 0.90 at 6 and 12 wk, respectively. Thus, selective degradation did not occur in the larger-scale columns, which indicates that P. ostreatus was not dominant. In fact, less degradation occurred in the barrels after 6 wk of degradation than was either observed or predicted in the small columns. It is likely that the low levels of degra­dation observed in the drums were owing to slower colonization of the fresh stems by the P. ostreatus growing in the solid enrichment inoculum. In the small-column tests, the indigenous microbes were shown to degrade about 15-20% of the polysaccharides in 12 wk in the absence of P. ostreatus (4,15), which likely represents the most easily accessible glucan and xylan fractions. If P. ostreatus were to colonize the straw more slowly from the solid enrichment inoculum, the primary effect on the degradation system would be to extend the treatment time necessary to reach the desired levels of xylan and/or glucan degradation in the final product. Thus, inoculating the fresh stems with partially degraded stems before introduction to the drums was an ineffective inoculation method when compared with inocu­lating by spraying homogenized mycelia onto the stem surfaces. The non­selective degradation pattern in the drums may or may not be a detriment to the physical properties of straw-thermoplastic composites produced from Degrade1 and Degrade2 straw stems, since selectively degraded stems have not yet been compared with nonselectively degraded stems.

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