Fumaric acid production byRhizopus arrhizusfrom potato flour was studied at different initial substrate concentrations (S), C/N ratios and fermentation times (tf) in a composite design experiment. By using response surface methodology and canonical analysis, the experimental values of fumaric acid and mycelial biomass yields and productivity were fitted to the only statistically significant factors with mean percentage errors of 11, 26 and 13, respectively. Owing to the limited statistical significance of the C/N ratio, it was possible to determine the values of S (25–50 g/l of glucose equivalent) andtf(44–100 h) associated with fumaric acid yields ranging from 60 to 75 and productivities varying from 6 to 8.4 g/l per day. Since such results were in quite good agreement with previous experiments carried out on other starchy materials, the above operating conditions might be used to minimize fumaric acid production costs as a function of the feed-stock u
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