The fermentation kinetics of citric acid by Aspergillus niger were studied in a batch system. A simple model was proposed using the logistic equation for growth, the Luedeking-Piret equation for citric acid production and Luedeking-Piret-like equation for glucose consumption. The model appeared to provide a reasonable description for each parameter during the growth phase. The production of citric acid by A. niger is growth-associated

Baei, MS

Mahmoudi, M

Yunesi, H

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African Journal of Biotechnology Vol. 7 (19), pp. 3487-3489, 6 October, 2008     Available online at http://www.academicjournals.org/AJB ISSN 1684–5315 © 2008 Academic Journals     Short Communication  A kinetic model for citric acid productionfrom apple pomac by Aspergillus niger  M. Sharifzadeh Baei1*, M. Mahmoudi1 and H. Yunesi2  1Islamic Azad University, Ayatollah Amoli Branch, Amol, IRAN .P.O.Box 675. 2Department of Environmental Science, Faculty of Natural Resources and Marine Sciences, Tarbiat modares  University, Imam Reza Street, Noor, Iran B.O. Box: 46414-356.  Accepted 8 August, 2008  The fermentation kinetics of citric acid by Aspergillus niger were studied in a batch system. A simple model was proposed using the logistic equation for growth, the Luedeking-Piret equation for citric acid production and Luedeking-Piret-like equation for glucose consumption. The model appeared to provide a reasonable description for each parameter during the growth phase. The production of citric acid by A. niger is growth-associated.  Key words: Kinetic model, citric acid, Aspergillus niger, Luedeking-Piret equation, apple pomace, growth-associated.   INTRODUCTION  Citric acid, a tricarboxylic acid, is one of the world’s largest tonnages of fermentation products. It is used in the food and beverage industries as an acidifying and flavour-enhancing agent and also in other industries including the production of pharmaceuticals. Consider-able interest has been shown in using fruit pomace and other agricultural wastes for citric acid production. Apple pomace consists of the press cake resulting from press-ing apples for juice, as well as the press cake obtained in pressing peel and core wastes generated during the preparation of apples for canning, drying and freezing (Shojaosadati and Babaeipour, 2002).  A model describes relationships between principal state variables and explains quantitatively the behavior of a system. The model can provide useful suggestions for the analysis, design and operation of a fermenter. Fer-mentation models are normally divided into two classes: structured models where intracellular metabolic pathways are considered and unstructured models where the   biomass is described by one variable.  Structured model seems complicated for normal use (Jian-Zhong et al., 2002). Unstructured models are much easier to use,  and  have  proven  to  accurately  describe     *Corresponding author. E-mail: mazyar.sharifzadeh@gmail.com, maziarsharif@yahoo.com. Fax: +981212552789. several fermentation. Yet, to our knowledge, no investi-gations have been carried out on the unstructured model for citric acid production. In this study, experimental data from batch fermentations of citric acid by Aspergillus niger were examined in order to form the basis of kinetic model of the process.   MATERIALS AND METHODS  Preparation of apple  Apple refuse was obtained and was analyzed ash, moisture, reducing sugar, citric acid, etc.   Organism and inoculum  A. niger PTCC 5010 was supplied by biotechnology department (Iran science and industrial research organ). The organism was maintained on potato dextrose agar (PDA) slants at 4.8oC and renewed once at month intervals. A. niger spores for inoculation were produced on PDA containing 50 ml in a 250 ml Erlenmeyer flask, incubated at 28.8oC for 5 days. A spore suspension was prepared by adding 50 ml distilled water containing Tween-80 (2%) and was stored at 4.8oC for a maximum of 3 weeks. It contained 108 spores/ml   Prepreation of main media (batch cultures)  The apple refuse was first diluted with distillation water to 14% con- 3488         Afr. J. Biotechnol.    Time, hours0 50 100 150 200Residual sugar & citric acid production, g/Kg0306090120150180cell dry weight, g/l0510152025Residual sugarCitric acidCell dry weightEq. (1)Eq. (2)Eq. (3)  Figure 1. The comparison of experimental data in this work and calculated values of biomass, sugar and concentration of citric acid fermentation: () experimental data of biomass; () experimental data of sugar; and () experimental data of citric acid.    concentration. Then the diluted apple was centrifuged at 5000 rpm and 100 ml of the obtained solution transferred to 500 ml Erlen-meyer flask. Chemical treatments included potassium ferrocyanide (El-Abyad et al., 1992), sulfuric acid (Mayilvahanan et al., 1996), calcium phosphate (Mayilvahanan et al., 1996), sodium phytate (Wang, 1998) and methanol (El-Abyadet al., 1992; Botella et al., 2005). Pre treatment carried out by adding 0.1 g/liter potassium ferrocyanide, 1.5 percent (v/w) calcium triphosphate, 2.5 g/liter ammonium sulfate and 3 volume of methanol. The pH regulation (at 4.5) carried out by HCl and normal NAOH before sterilization.   Analyses  Samples were withdrawn twice at defined time. Fermentations were performed in duplicate culture, and analyses were carried out in duplicate. The data given here are the average of the measure-ments. The dry weights of mycelium were obtained after filtration of broth samples through preweighed filter discs. The harvested biomass was then washed with deionized water, dried for 8 h at 105°C, cooled in a desiccator and weighed. Sugar was estimated by the phenol sulphuric acid method of Dubois et al. (1956) and citric acid by the acetic anhydride and pyridine method of Marier and Boulet (1958).   Kinetic model  The model employs rate equations for biomass (X), citric acid (P) and total sugar (S) to describe the fermentation process.   Microbial growth  The most widely used unstructured models for describing cell growth are the Monod kinetic model, the logistic equation and the Haldane model. The logistic equation is a substrate independent model. It can finely describe the inhibition of biomass on growth, which exists in many batch fermentations (Gong and Lun, 1996; Habibollah et al., 2006). The logistic equation can be described as follows:                                                                    (1)                                                                     (2)   Product formation  The second-order rate of reaction with respect to citric acid production and sugar consumption for citric acid production is shown in Eq. 3 (Najafpour and Younesi, 2007).                                                                   (3)  The citric acid concentration profile is obtained by solving the differential equation in order to maximize citric acid production, as shown in Eq. 4:                                                                  (4)   Sugar uptake  Carbon substrate is used to form cell material and metabolic products as well as the maintenance of cells. The  sugar  consump-     tion equation given below is a Luedeking-Piret-like equation (Panilaitis et al., 2007):                            (5)  In this study, Equations (2), (4) and (5) are used to simulate the experimental results.   RESULTS AND DISCUSSION  Microbial growth  Citric acid fermentation by A. niger showed a classical growth trend. After a lag phase (about 5 – 10 h), the cells entered the exponential growth phase. The strain started to form citric acid when the cells entered the exponential phase and therefore cell growth and citric acid production took place simultaneously. Taking Xm = 22 g l−1 from the experimental data, fitting the experimental data to Eq. (2) yields the values of parameters as follows: X0 and m are 0.04 g l−1 and 0.015 h, respectively. A comparison of calculated value of Eq. (2) with the experimental data is given in Figure 1. The fitting of results was satisfactory.     Product formation  After fitting the experimental data to Eq. (4), the following equation was used to describe citric acid. From the result, it can be seen that citric acid formation is strongly linearly related to cell growth. The result shows that the biosynthesis of citric acid can be attributed to a growth-associated type. In the model, Po (0.652 g Kg−1), Pm (120 g Kg−1) and Pr (0.044 g Kg−1) are the growth-associated product formations   Substrate uptake  By fitting the experimental data to Eq. (5), the values of parameters of sugar uptake model were as follows: YX/S, YP/S and S0 is 0.62 g g−1, 0.38 g g−1 and 170 g Kg−1, respectively. The fitting of results was satisfactory (Figure 1).           Baei et al.        3489    ACKNOWLEDGEMENT  The authors wish to acknowledge the Islamic Azad University-Ayatollah Amoli Branch for financial support to complete this research.   REFERENCES  Botella C, de Ory I, Webb C, Cantero D, Blandino A (2005). Hydrolytic enzyme production by Aspergillus awamori on grape pomace. Biochem. Eng. J. 26: 100-106. Dubois M, Gilles KA, Hamilton JK, Rebers PA, Smith F (1956). Colorimetric method for determination of sugars and related substrates. Anal. Chem. 28: 350-356. Gong H, Lun S (1996). The kinetics of lysine batch fermentation, Chin. J. Biotechnol. 12: 219-225. Habibollah Y, Ghasem N, Abdul RM (2006). Ethanol and acetate production from synthesis gas via fermentation processes using anaerobic bacterium, Clostridium ljungdahlii. Biochem. Eng.  27: 110-119. Jian-Zhong L, Li-Ping W, Qian-Ling Z, Hong X, Liang-Nian J (2002). A mathematical model for gluconic acid fermentation by Aspergillus niger, Biochem. Eng. 3669: 1-5. Marier JR, Boulet M (1958). Direct determination of citric acid in milk with an improved pyridine acetic anhydride method. J. Dairy Sci. 41: 1683-1692. Najafpour G, Younesi H (2007). bioconversion of synthesis gas to hydrogen using a light – dependent photo synthesis bactium rhodospirillum rubrum. World J. Microbial. Biotechnol. 23: 275-284. Panilaitis B, Castro G, Solaiman D, Kaplan DL (2007). Biosynthesis of emulsan biopolymers from agro-based feedstocks. J. Appl. Microbiol. 102: 531-537 Shojaosadati SA, Babaeipour V (2002). Citric acid production from apple pomace in multi-layer packed solid-state bioreactor. Process Biochem. 37: 909-914.                  

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A kinetic model for citric acid production from apple pomac by Aspergillus niger

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A kinetic model for citric acid production from apple pomac by Aspergillus niger

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