Abstract
Extractive fermentation in a nonionic surfactant aqueous solution provides a promising and efficient method to produce Monascus pigments. The behaviour of pigment secretion during the extractive cultivation was investigated in the present work. The results revealed that the secretion of intracellular pigment was limited by its saturation concentration in the nonionic surfactant aqueous solution. The intracellular pigment was completely extracted to the outside of the cell at a low cell density and high concentration of Triton X-100 (TX) in fermentation broth; otherwise, a restriction for pigment extraction would occur. The decrement of the intracellular orange and yellow pigments was inconsistent with the increment of extracellular pigments with an increase in the TX concentration. It could be inferred that the intracellular orange pigment was converted to extracellular yellow pigment during the transmembrane secretion process, which might be attributed to the enzyme catalysis in the non-aqueous phase solution. This study helps explain the mechanism of variation of pigment characteristic and extraction capacity in extractive fermentation.
Highlights
Monascus pigments, which are the functional secondary metabolites synthesized by Monascus fungi, are a mixed group of azaphilones that contain yellow, orange and red compounds and have been widely used as promising pigment additives in food and pharmaceutical industries (Lin et al 2008; Patakova 2013)
Saturation of pigment in fermentation broth with nonionic surfactant By adding Triton X-100 (TX), the amount of intracellular pigments in the mycelia decreased for extraction of the pigments to the extracellular broth (Fig. 1b), and the spectrum of intracellular pigment remained unchanged with a peak absorbance of orange pigment at approximately 470 nm (Zheng et al 2009)
The results showed that the intracellular pigment could nearly be extracted completely at a high TX concentration of 40 or 160 g/L (Fig. 1d) and that the spectrum of the residual intracellular pigment kept a stable peak wavelength at approximately 460 nm with an increasing concentration of TX (Fig. 1d)
Summary
Monascus pigments, which are the functional secondary metabolites synthesized by Monascus fungi, are a mixed group of azaphilones that contain yellow, orange and red compounds and have been widely used as promising pigment additives in food and pharmaceutical industries (Lin et al 2008; Patakova 2013). Monascus pigments are mainly synthesized and accumulated in the mycelia (Chen and Johns 1993), in which two orange pigments (rubropunctatin and monascorubrin), two red pigments (rubropunctamine and monascorubramine) and two yellow pigments (monascin and ankaflavin) have been identified as the main intracellular pigments (Feng et al 2012). Accumulation of the intracellular pigments trapped in the mycelia became a significant challenge for the higher productivity of the Extractive fermentation with micelles of the nonionic surfactant in an aqueous solution is an efficient method for eliminating product inhibition and preventing product degradation, which promotes the productivity of Monascus pigments (Wang and Dai 2010; Kleinegris et al 2011). By using TX as the additive, an enrichment of the hydrophobic pigments occurred upon being “milked” in the artificial nonionic micelle aqueous solution to improve the pigment penetration across the cellular membrane (Hu et al 2012b; Kang et al 2013a). Further study indicated that the nonionic surfactant may modify the cell membrane lipid layer and improve the rate of secretion of intracellular hydrophobic pigments across the cell membrane (Wang et al 2013)
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