2.4. Experimental design and optimizing procedureIn preliminary studies, strain THLC0409 grew well on variouslignocelluloses in medium 122 in an initially aerobic environmentat a neutral pH and temperatures of 30e31 _C (datanot presented). Those works also revealed that yeast extractsignificantly accelerates the utilization of lignocelluloses andthe production of ethanol by the strain THLC0409.Central composite design (CCD) was adopted to optimizethe production of ethanol. The fermentation time (X1, day),the initial pH of the liquid medium (X2), the cultivatingtemperature (X3, OC), and the yeast extract (YE) concentration(X4, g l_1) were chosen as the independent variables, aspresented in Table 1. The range of values of the parameterswas set based on the results of the preliminary studies and thecodes that correspond to these parameters were resolved.A factorial 24 design was determined from CCD with theethanol concentration (Y, ppm) as an output that depends onfour basic variables. The experiment comprised 30 runs,which were performed under the conditions presented inTable 1. Six-replicate runs were at the center point, while theaxial points were determined to be (16)1/4. The ethanolproducingresponse was estimated using the following equation(a second-order response surface model) [22].Y ผ b0Xkiผ1biXi Xkiผ1biiX2iXiXbijXiXj 3 (1)where Y is the predicted response (ethanol concentration,ppm), and the b terms are parameters whose values are to be
determined (regression coefficients); bij represents an interaction
between two individual factors; bii represents pure
second-order or quadratic effects; k is the number of experimentally
studied factors, and 3 is a random experimental
error. The significance of each variable (factor) was determined
using Student’s t-test. The results were analyzed using
a Statistica 6.0 software package (STATISTICA, Inc.). Threedimensional
plots and their corresponding contour plots were
obtained to study the interaction of one parameter with
another. The critical concentration was determined using an
established regression model and was based on the hump in
the three-dimensional plots.
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