I am carrying out solid state fermentation using aspergillus niger to produce citric acid.But, I am not able to maintain the pH throughout the process.I want to set my pH between 3--4. What can be done in this case? Can I use a buffer solution and how much quantity of it?
Please,provide me with appropriate references.
Dear Markad,
What kind of SS fermentation are you following (flask level or SSF reactor)? Maintanence of pH is an ordeal in SSF, unless you have a specially designed reactor. I would recommend you to use a Koji bed height of 1-3 cm. You cannot continuously monitor the pH, like we do in a SMF. HOwever, my personal advise to you would be to measure it every 24 hours using pH strips (You Koji would be moist containing the minimal media). Maintain the moisture in a Humidity chamber, using the minimal media (pH adjusted). Alternatively, you may prefer using a sensitive load cell to monitor moisture or a dipped in moisture sensor.
Sir, I am carrying out in a tray bioreactor with adjusted moisture and temperature. Will it be preferable to use acetate buffer for it? And if yes how much concentration?
Hello Markad, first you are dealing with a fungus and of course you should know their optimum pH for growth. The pH of 1 M solution of citric acid is 3.24 and the pH range for fungal growth is 4-5.8. Usually, during fermentation of carbohydrate substrates especially, acidic products are released and the pH of medium drops. During solid state fermentation, there is great difficulty in measuring in-process pH changes, like Navnit Kumar Ramamoorthy said. Take a look at this reference, please: document: http://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_7/b_fdi_57-58/010024250.pdf. Your tray bioreactor is inherent with problems like mixing with bulky solid substrate, changes in moisture content of the medium and contamination. The suggestion of acetate buffer too could be very expensive. Besides acetate is an alternative carbon source for citric acid production. Alternatively incorporate 1 M-phthalic acid/triethanolamine pH 7.5 to the standard medium. Citric acid gets accumulated in mold cultures at pH 4-7.
Dear Markad,
I have worked in ssfs for cellulase production. I try my best to maintain the pH of the minimal media (inclusive of the koji bed). There is no harm in using a buffer to maintain pH. However, you would be diluting the nutrient availability to fungus. Since, we are not exactly aware of the nature of the substrate you are using and how significantly the production pH affects your end product, I would prefer not to recommend you exact conditions. Instead, you may do 2-3 trials to come up with the best possible way and productivity. Good luck !!
Maurice's take on the acetate buffer is spot on. You may consider his calculation and the buffer suggestion as well. the nature of your substrate's absorptivity will influence the pH measurement. Else, you would be superficially measuring the pH.
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