Abstract
Fermentation parameters for surfactin production and sporulation in a submerged culture of Bacillus subtilis Y9 with various oxygen transfer rates in 5 L jar fermenters were investigated. The oxygen-uptake rate (OUR) was positively correlated with volumetric surfactin productivity. When OUR value increased from 0 to 250 s−1, productivity increased up to 45 mg/L·h; however, no further increase was observed at OUR values above 255 s−1. The volumetric mass transfer coefficient KLa increased with increasing agitation speed. However, a reduction in surfactin production was observed at the highest agitation speed of 500 rpm. Productivity sharply decreased after spore appearance, and remained low until the end of the culture. A mesh-type sparger was installed to generate microsized air bubbles. When the system was operated at 400 rpm with the mesh-type sparger, KLa was higher than that at 500 rpm with an original sparger. Under agitation at 400 rpm with the mesh-type sparger, productivity was maintained above 42.3 mg/L·h until 24 h, resulting in the highest surfactin concentration of 875 mg/L. Thus, a mesh-type sparger promotes KLa, leading to an increase in productivity.
Highlights
Biosurfactants are surface-active compounds that are produced by various micro-organisms, e.g., bacteria, yeast, and fungi [1,2]
Variability in surfactin production under different agitation speeds differed from the patterns in the dissolved oxygen tension (DOT) levels
Both the concentration and volumetric productivity of surfactin were determined by agitation speed in the first half of the culture
Summary
Biosurfactants are surface-active compounds that are produced by various micro-organisms, e.g., bacteria, yeast, and fungi [1,2]. Surfactin is a lipopeptide biosurfactant that consists of four isomers, i.e., surfactin A–D, with different chain length and branches of its hydroxyl fatty acid components. It is mainly produced by a rod-shaped Gram-positive bacterium, Bacillus subtilis, and shows biological potential for antimicrobial, antiviral, and insecticidal activities [5,6]. It has not been industrialized because of the high cost of production and purification [7]. The cost competitiveness of surfactin is lower than that of chemical surfactant, it should be used in some high-value products such as cosmetics, medicine, and biocontrol agents, due to increasing demand for safety
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