Enhanced acarbose production by Streptomyces M37 using a two-stage fermentation strategy.

In this work, we investigated the effect of pH on Streptomyces M37 growth and its acarbose biosynthesis ability. We observed that low pH was beneficial for cell growth, whereas high pH favored acarbose synthesis. Moreover, addition of glucose and maltose to the fermentation medium after 72 h of cultivation promoted acarbose production. Based on these results, a two-stage fermentation strategy was developed to improve acarbose production. Accordingly, pH was kept at 7.0 during the first 72 h and switched to 8.0 after that. At the same time, glucose and maltose were fed to increase acarbose accumulation. With this strategy, we achieved an acarbose titer of 6210 mg/L, representing an 85.7% increase over traditional batch fermentation without pH control. Finally, we determined that the increased acarbose production was related to the high activity of glutamate dehydrogenase and glucose 6-phosphate dehydrogenase

Thermal and crystalline properties of slowly digestible starch prepared from the starches physically modified by β-cyclodextrins

Slowly digestible starch (SDS) was prepared from rice starches physically modified by β-cyclodextrin (β-CD), maltosyl-β-CD, and hydroxypropyl-β-CD. Its thermal stability and crystalline properties were further explored, since such properties were important for SDS during heat processing. The results showed that the maximum percentage of remaining SDS was 78.2% for the starch physically modified by β-CD and subjected to dry heating for 15 min, and 73.7% for the starch subjected to moisture heating for 15 min. The starches physically modified by β-CD exhibited a higher peak melting temperature, conclusion temperature, and enthalpy change of melting than the control sample. The crystalline data indicated that the higher yield of SDS in the starch physically modified by β-CD could be attributed to a greater difference of partial V-type crystallites. More perfect crystallites and imperfect crystallites were formed, and designated as the major components of the SDS product. These findings suggest that the starch physically modified by β-CD had higher thermal stability because of the formation of VII-type crystallites, and was suitable for potential application during heat processing

Effect of pH on <i>Streptomyces</i> M37 fermentation.

<p>(A) DCW, (B) specific acarbose content, (C) specific growth rate (μ), (D) specific acarbose production rate (<i>q</i>_p), and (E) reducing sugars (glucose and maltose). Experiments were carried out in a 5-L fermentor. NaOH (2 M) and H₂SO₄ (2 M) were used to buffer the pH. ■ curve 1, pH 6.5; ● curve 2, pH 7.0; ▲ curve 3, pH 7.5; ▼ curve 4, pH 8.0; and ◆ curve 5, pH 8.5. Data correspond to mean ± S.D. (<i>n</i> = 3). Error bars were omitted for clarity.</p

Effect of culture medium initial pH on cell growth and acarbose yield during flask fermentation.

<p>Effect of culture medium initial pH on cell growth and acarbose yield during flask fermentation.</p

Effect of the addition of reducing sugars (glucose and maltose) on <i>Streptomyces</i> M37 DCW and acarbose production at pH 8.0 in a 5-L fermentor.

<p>Effect of the addition of reducing sugars (glucose and maltose) on <i>Streptomyces</i> M37 DCW and acarbose production at pH 8.0 in a 5-L fermentor.</p

Postulated acarbose biosynthetic pathway.

<p>Glucose and maltose are the carbon sources for acarbose biosynthesis.</p

Specific activities of glutamate dehydrogenase (GDH) and glucose 6-phosphate dehydrogenase (G6DPH) during batch fermentation and the two-stage fermentation strategy.

<p>Each value represents the mean of three separate determinations ± standard deviation.</p

Effect of the maltose-to-glucose mass ratio in the feeding medium on acarbose production by <i>Streptomyces</i> M37 at pH 8.0 in a 5-L fermentor ^a.

<p>Effect of the maltose-to-glucose mass ratio in the feeding medium on acarbose production by <i>Streptomyces</i> M37 at pH 8.0 in a 5-L fermentor <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0166985#t003fn001" target="_blank">^a</a>.</p

Effect of a two-stage fermentation strategy on acarbose production by <i>Streptomyces</i> M37.

<p>DCW (★), reducing sugars (■), and acarbose production (▲). Fermentations were carried out at 28°C and 200 rpm agitation speed. Glucose and maltose were fed at a 1:2 mass ratio and pH was adjusted with 2 M NaOH and 2 M H₂SO₄. Each value represents the mean of three separate determinations ± standard deviation (arrows indicate feeding times).</p

Effect of culture method on <i>Streptomyces</i> M37 acarbose fermentation in a 5-L fermentor.

<p>Effect of culture method on <i>Streptomyces</i> M37 acarbose fermentation in a 5-L fermentor.</p