Modern Industrial Microbiology and Biotechnology

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Fermentors and Fermentor Operation 193rather alkalinity is the situation to be combated. Such acidity usually arises frommicrobial activity. It is therefore usual to arrange for the introduction of anhydrousammonia as acidity increases.9.2.5.2 Carbon dioxide measurementWater and carbon dioxide are two of the most common end-products of aerobicfermentations. The measurement of CO 2 therefore helps determine the course of thefermentation as well as the carbon balance. At least three principles are employed incurrent equipment for CO 2 determination. The first method, which is the most widelyused, depends on the ability of CO 2 to absorb infrared rays. A sensitive sensor translatesthis absorption to a gauge or record, from which it can be read off. In another principle,the effluent gas emerging from the broth is bubbled through a dilute solution of NaOHcontaining phenol red. The change in color of the phenol red is reflected in a photocelland the amount of CO 2 may be calculated from a standard curve. The third methoddepends on the thermal conductivities of the various gases in a mixture.9.2.5.3 Oxygen determination and controlA number of methods are available for determining the oxygen concentration in afermentation broth.Of the chemical methods, the best known is that of Winkler which is routinely used todetermine the biochemical oxygen demand (B.O.D) of water (Chapter 29). This methodrelies on the back-titration, using iodine and starch, of unoxidized manganous saltadded to the liquid to be analyzed. Interfering substances are usually present infermentation broths. Furthermore, the method is cumbersome. Modern sensing methodsare not, however, based on this method. They rather sample the dissolved oxygen (DO) inthe medium. Modern dissolved oxygen probes are autoclavable and are based on one orthe other of two principles: the polarographic or the galvanic method.In the polarographic method, a negative electric current 0.6-0.8 in voltage is passedthrough an electrode immersed in an electrolyte made of neutral potassium chloride. Thisnegative electrode (cathode) is made of a noble metal such as platinum or gold. The anodeis calomel or Ag/Ag CI. Under this condition the dissolved oxygen is reduced at thesurface of the cathode according to the following reactions:Cathode: O 2 + 2H 2 O + 2e ® H 2 O 2 + 2OH–H 2 O 2 + 2e– ® 2OH–Anode: Ag + Cl– ® Ag Cl + e–Overall: 4 Ag + O 2 + 2H 2 + 4 Cl ® 4 Ag Cl + 4OH–The current which is measured after it has passed through the electrolyte isproportional to the dissolved oxygen reacting at the cathode. A plastic membranepermeable to gases but not ions separates the cathode, anode, and the electrolyte from theliquid to be studied. The dissolved oxygen diffuses through the membrane and itsreaction at the cathode is measured at the current meter (Fig. 9.5). The electrolyte soonbecomes depleted by the constant replacement of Cl - by OH - (see equations above) andthe electrolyte has to be replaced.
194 Modern Industrial Microbiology and BiotechnologyFig. 9.5Structure of Oxygen Electrodes: (A) Polarographic (B) GalvanicIn the galvanic method, no external source of electricity is applied. Instead theelectricity generated between a base metal anode (zinc, lead, or cadmium) and noblemetal cathode (silver or gold) is sufficient to cause the reduction of oxygen at the cathode.The reactions are thus:Cathode:Anode:O 2 + 2H 2 + 4e ® 4 OHPb ® Pb 2+ + 2eOverall: O 2 + 2Pb + 2H 2 O ® 2Pb(OH) 2The principle remains the same otherwise. The electric current generated in the systemis proportional to the quantity of oxygen reacting at the cathode. The electrolyte does nothowever participate but the anode surface is gradually oxidized.9.2.5.4 PressureIt is important to know the pressure of gases in order to ensure that a positive pressure ismaintained. A positive pressure helps eliminate contamination and contributes to themaintenance of proper aeration. Pressure may be determined with aid of a manometer.9.2.5.5 Computer controlThe fermentation industry, especially the antibiotic manufacturing aspect, usuallycompares its operations with those in the chemical industry. Leaders in the fermentationindustry usually point to the fact that the fermentation industry in the early 1970s laggedbehind chemical industries in applying computers in regulating and managingfermentations. The situation today is different and fermentation procedures are nowhighly automated. Automation is an engineering problem and the expected advantagesof computerization have been given as follows:(i) It should reduce labor by eliminating manual intervention.
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DedicationThis book is dedicated to
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PrefaceThe field of industrial micr
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ContentsixContentsPrefaceviiSECTION
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Contentsxi5.6 Carbon Pathways for t
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Contentsxiii10.3 Purification 21710
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Contentsxv16.5 Yeast Products 314Su
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Contentsxvii21.2 Methods for the Ma
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Contentsxix26.3.2 Fermentation cond
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