Research Article
Batch Fermentation Kinetics of Pullulan from Aureobasidium pullulans Using Low Cost Substrates
Department of Technology, Annamalai University, Annamalainagar 608002, India
T.R. Manikkandan
Department of Technology, Annamalai University, Annamalainagar 608002, India
R. Dhanasekar
Department of Technology, Annamalai University, Annamalainagar 608002, India
Kuan-Chen Cheng Reply
Dear Editor,
The recently published paper by Thirumavalavan et al. (2008) described a new fermentation medium (cashew fruit juice) which can enhance the production of pullulan by Aureobasidium pullulans (MTCC 2195) to 92.5 g/L. Although the paper was well written in many respects including literature review and method development, omissions leave the readers with more questions than should be reasonably expected.
A critical omission was found from the “Microorganisms and Growth conditions” under “Materials and Methods”. The authors only mentioned the standard cultivation medium for seed culture but not exact medium composition used for pullulan production in flasks (p. 318). The reader is later referred to “Effect of initial pH” under “Results and Discussion”. The authors claimed that A. pullulans elaborated 59.0 g/L pullulan when pH was set at 6.5. The unclear part left for readers is that what is the carbon source employed in the medium. The authors claimed that A. pullulans can produce 59.0 g/L of pullulan out of 50 g/L of carbon source, however, the cultivation time was also unclear. Pullulan yield was later improved to 92.5 g/L with cashew fruit juice (initial carbon concentration was 50 g/L) after 168 h. The question is that how Aureobasidium pullulans produced 92.5 g/L pullulan with only 50 g/L initial carbon source since pullulan is a simple polysaccharide comprised with only glucose units. If the author did add any other supplemental carbon source such as sucrose, it is authors’ responsibility to mention this clear in the “Materials and Methods”.
Another issue comes from Figure 2 (p. 319). The authors presented the hydrolysis results of different substrates. The glucose yield from bakery waste was 1.16 g/g, which is not reasonable. One g of bakery waste included not only dietary fibers, which could be hydrolyzed into glucose units, but also other ingredients such as protein and fat. Is that possible for authors to recheck their procedure of total sugar estimation?
Moreover, it is commonly accepted that polysaccharides contribute around 10 to 15% to the cell weight (Luria, 1960). The biomass value of A. pullulans found in the cited references is from 2.0 to 15.0 g/L which is conflict with the results presented by the authors. The extreme biomass value in Figure 1 is around 70.0 g/L. If we add up biomass and pullulan values, A. pullulans will produce 130 g/L dry product with only 50 g/L carbon source input. How did they make it? Moreover, the concentration of acetone used to precipitate crude pullulan is also crucial for pullulan recovery. The authors did not mention it in the method section.
From Figure 1 to 4, the authors presented all polysaccharides produced by A. pullulans as pullulan. However, it is well known that A. pullulans can elaborate more than one type of polysaccharide during cultivation (Simon et al., 1993). The pure content of pullulan varies from stain to stain and the range is from 30 to 100%. If the authors did not perform any validation of pullulan purity, the readers would be confused with the exact pullulan yield.
In conclusion, although this paper provided much needed information about pullulan production using low cost substrates as medium, it falls short in providing sufficient background for medium composition, pullulan purity, and reasonable explanation of their high yield. Both pullulan and biomass yield are also not reasonable for mass balance calculation. The validation of raw substrate hydrolysis procedure is also needed.
Reference Cited
Luria SE (1960). The bacterial protoplasm: composition and organization. In I. C. Gunsalus and R. Y. Stainer (ed.), The bacteria, vol. 1. Academic Press, Inc., New York, pp. 1-34.
Simon L, Caye–Vaugien C, Bouchonneau M (1993). Relation between pullulan production, morphological state and growth conditions in Aureobasidium pullulans: new observations. J. Gen. Microbiol. 139: 979–985.
Thirumavalavan K, Manikkadan TR, Dhanasekar R (2008). Batch fermentation kinetics of pullulan from Aureobasidium pullulans using low cost substrates. Biotechnology. 7(2): 317-322.
JESSIE TENG Reply
HELLO,
I AM A STUDENT WHO PURSUING MY CHEMICAL ENGINEERING DEGREE PROGRAM. MY FINAL YEAR PROJECT IS REGARDING Aureobasidium pullulans. HOWEVER,I WISH TO KNOW THE BIOMASS COMPOSITION OF Aureobasidium pullulans. I WILL REALLY APPRECIATE IT IF U COULD PROVIDE ME THAT INFORMATION.THX U