Characterization of crude xylanase produced by A. lentulus
The pH and thermal stabilities of cellulase-free xylanases are
important attributes for their potential industrial applications. The
crude xylanase enzyme obtained from A. lentulus was characterized
for its optimal pH and temperature range. The relative xylanase
activities obtained at different pH ranges (4e10) are presented in
Fig 3a. The pH was adjusted with 0.5 M citrate buffer (pH 4e6) or
0.5 M phosphate buffer (pH 7e10). The highest xylanase activity
was reported at pH 5 which reduced to 92% at pH 4. The decline in
enzyme activity was marginal till neutral pH range (91.6% residual
activity at pH 7) however; it declined sharply in alkaline pH ranges
with 38.5% activity at pH 10 where statistically significant differences
were observed at p 0.05. Although a sharp decline at
alkaline pH was observed in the xylanase activity, still 77.2% and
59.1% activity was retained at pH 8 and pH 9 which is the prevailing
pH during the bleaching process of the pulp. The pH stability profile
for crude xylanase is represented in Fig. 3b. The data shows that the
enzyme was more stable at acidic pH ranges. Good stabilities were
seen in the pH range of 4e7 even after 3 h of incubation. Enzyme
was most stable at pH 5, retaining more than 98% activity even after
3 h of incubation. Even at pH 8 and 9, more than 85% and 75%
enzyme stability, respectively, was observed after 3 h of incubation.
Fig. 4a represents the relative xylanase activity of the xylanase
produced by A. lentulus at different reaction temperatures ranging
from 50 to 90 C. The optimum temperature for xylanase activity
was found to be 60 C, with 94.1% activity obtained at 50 C and
86.4% at 70 C. The enzyme activity declined sharply with further
increase in temperature (19.4% activity at 90 C).