The results of fed-batch operation with the pH-stat mode offeeding are shown in Fig. 3. The agitation speed remained fixedat 600 rpm. Although both the peak biomass concentration andthe lipase activity were high (Fig. 3) compared with the batch cul-tures (Fig. 1), the pH-stat operation proved difficult to manage andcould not be relied on for consistent results. After the first feed-ing demarcated by the arrow in Fig. 3, the expected increase in pHwas not observed to guide the next feeding. The pH-stat operationhas previously been used for some other lipase fermentations [9],but the specific nature of the substrate (palm oil in this case) may