Kinetic fermentation

Figure 19.4 Typical Chardonnay fermentation profile. Soluble solids (Brix) measurement follows complex kinetics. Fermentation lag is due to the growth phase and buildup of new cells. Once maximal cell density is attained, fermentation proceeds at its fastest rate. At some point the accumulation of ethanol and other stressors leads to a reduction in fermentation rate, often accompanied by a dramatic transition in fermentation rate as shown. The fermentation then continues at the stress-adapted rate to completion.

Biochemical pathways consist of networks of individual reactions that have many feedback mechanisms. This makes their study and the elucidation of kinetics of individual reaction steps and their regulation so difficult. Nevertheless, important inroads have already been achieved. Much of this has been done by studying the metabolism of microorganisms in fermentation reactors. 

Except as an index of respiration, carbon dioxide is seldom considered in fermentations but plays important roles. Its participation in carbonate equilibria affects pH removal of carbon dioxide by photosynthesis can force the pH above 10 in dense, well-illuminated algal cultures. Several biochemical reactions involve carbon dioxide, so their kinetics and equilibrium concentrations are dependent on gas concentrations, and metabolic rates of associated reactions may also change. Attempts to increase oxygen transfer rates by elevating pressure to get more driving force sometimes encounter poor process performance that might oe attributed to excessive dissolved carbon dioxide. 

Figure 3.1 illustrates the main patterns for batch fermentation process kinetics for type 1,2 and 3 processes. 

In a chemostat and biostat or turbidostat, even with differences in the supply of nutrients and/or fresh media, constant cell density is obtained. The utilisation of substrate and the kinetic expressions for all the fermentation vessels are quite similar. It is possibile to have slight differences in the kinetic constants and the specific rate constants.3,4 Figure 5.9 shows a turbidostat with light sources. The system can be adapted for photosynthetic bacteria. 

Since enzyme is not shown in the reaction we assume an elementary rate equation may explain the above reactions. The simple kinetics are discussed in most fermentation technology and chemical reaction engineering textbooks.8-10... 

A feed concentration of 15 g glucose and 15 g xylose per litre was used over a feed rate of 20-200 ml/hr. Samples were taken at successive points along the reactor length, and the usual analysis for glucose and xylose consumption, organic acid production and cell density were done. A kinetic model for the growth and fermentation of P. acidipropionici was obtained from these data. 

Fig. 8.7. Kinetic model for batch fermentation, Langmuir-Hanes plot. Reprinted from Najafpour et al. (2004).18 Copyright with permission from Elsevier.

The use of stirred fermenters with automatic control of the culture environment is the most suitable technique to evaluate bacterial or fungal kinetics. Cultures can be operated in discontinuous mode (batch cultures). 

In a batch fermentation of ethanol, kinetic data were collected as product formed. The data are shown in Table E.5.1. The data will be used to design a continuous bioreactor (CSTR) with a 1001 working volume. 

A kinetic resolution was also observed in the reduction of racemic a-ketosulphoxides 277 by fermenting yeast337 (equation 153). Both the starting ketones 277 and the corresponding /1-hydroxysulphoxides 278 formed have been recovered in almost enan-tiomerically pure form. 

Many semibatch reactions involve more than one phase and are thus classified as heterogeneous. Examples are aerobic fermentations, where oxygen is supplied continuously to a liquid substrate, and chemical vapor deposition reactors, where gaseous reactants are supplied continuously to a solid substrate. Typically, the overall reaction rate wiU be limited by the rate of interphase mass transfer. Such systems are treated using the methods of Chapters 10 and 11. Occasionally, the reaction will be kinetically limited so that the transferred component saturates the reaction phase. The system can then be treated as a batch reaction, with the concentration of the transferred component being dictated by its solubility. The early stages of a batch fermentation will behave in this fashion, but will shift to a mass transfer limitation as the cell mass and thus the oxygen demand increase. 

Neither method will achieve a bumpless startup for complex kinetic schemes such as fermentations. There is a general method, known as constant RTD control, that can minimize the amount of off-specification material produced during the startup of a complex reaction (e.g., a fermentation or polymerization) in a CSTR. It does not require a process model or even a realtime analyzer. We first analyze shutdown strategies, to which it is also applicable. 

Despite their flaws, batch processes have stood the test of time for a number of reasons, the most important of which is the flexibihty it brings to the manufacturer in terms of the range of products that the plant can produce, the feedstocks used to produce them, and the speed at which they can be brought to market with very limited information on physical properties, reaction kinetics, and so on (very few, if any, Michelin-starred chefs have ever measured the rheology or kinetics of their latest culinary creation). This flexibility, however, has a price which comes in the form of lower efficiencies in terms of production, energy, labor, and so on, and ultimately efficiency equates to cost However, one should never underestimate the pull of flexibility particularly, as discussed earlier in the examples of fermentation, where control of important parameters is difficult to achieve. 

As a third example let us consider the growth kinetics in a chemostat used by Kalogerakis (1984) to evaluate sequential design procedures for model discrimination in dynamic systems. We consider the following four kinetic models for biomass growth and substrate utilization in the continuous baker s yeast fermentation. 

Theodorou MK, Williams BA, Dhanoa MS, McAllan AB, France J. A simple gas production method using a pressure transducer to determine the fermentation kinetics of mminant feeds. Animal Feed Science and Technology 1994 48 185-197. 

Cone JW, Gelder AH, Visscher GJW, Oudshoom L. Influence of rumen fluid and substrate concentration on fermentation kinetics measured with fully automated time related gas production apparatus. Animal Feed Science and Technology. 1996 61 113-128. 

An autocatalytic reaction is one in which the reaction rate is proportional to a product concentration raised to a positive exponent. Some of the first articles in the literature of chemical kinetics deal with reactions of this type. For example, in 1857, Baeyer (12) reported that the reaction of bromine with lactose was autocatalytic. The hydrolyses of several esters also fit into the autocatalytic category, since the acids formed by reaction give rise to hydrogen ions that serve as catalysts for subsequent reaction. Among the most significant autocatalytic reactions are the fermentation reactions that involve the action of a microorganism on an organic feedstock. 

Jin, Q., 2007, Control of hydrogen partial pressures on the rates of syntrophic microbial metabolisms a kinetic model for butyrate fermentation. Geobiology 5, 35-48. 

A continuous fermenter is operated at a series of dilution rates though at constant, sterile, feed concentration, pH, aeration rate and temperature. The following data were obtained when the limiting substrate concentration was 1200 mg/1 and the working volume of the fermenter was 9.8 1. Estimate the kinetic constants Km, //, and kd as used in the modified Monod equation ... 

When a pilot-scale fermenter is run in continuous mode with a fresh feed flowrate of 65 1/h, the effluent from the fermenter contains 12 mg/1 of the original substrate. The same fermenter is then connected to a settler-thickener which has the ability to concentrate the biomass in the effluent from the tank by a factor of 3.2, and from this a recycle stream of concentrated biomass is set up. The flowrate of this stream is 40 1/h and the fresh feed flowrate is at the same time increased to 100 1/h. Assuming that the microbial system follows Monod kinetics, calculate the concentration of the final clarified liquid effluent from the system. /x, = 0.15 h-1 and Ks = 95 mg/1. 

Two continuous stirred-tank fermenters are arranged in series such that the effluent of one forms the feed stream of the other. The first fermenter has a working volume of 100 1 and the other has a working volume of 50 1. The volumetric flowrate through the fermenters is 18 h-1 and the substrate concentration in the fresh feed is 5 g/1. If the microbial growth follows Monod kinetics with //, = 0.25 h-1, Ks = 0.12 g/1, and the yield coefficient is 0.42, calculate the substrate and biomass concentrations in the effluent from the second vessel. What would happen if the flow were from the 50 1 fermenter to the 100 1 fermenter ... 

Growth/production kinetics, in fermentation, 11 29-31 Growth-promoting genes, transgenic animals with, 12 463-464 Growth regulators, insect, 14 343-345. 

Kinetic profiles, fermentation, 11 29 Kinetic pumps, 21 54—56 types of, 21 63-70 Kinetic rates, 14 607 Kinetics. See also Adsorption kinetics batteries, 3 421-423 chemical vapor deposition, 5 810-812 colloids, 7 291-292... 

Below the results of Sensitivity Runs with MADONNA are given from the BIOREACT example that is run as a batch fermenter system. This example involves Monod growth kinetics, as explained in Section 1.4. In this example, the sensitivity of biomass concentration X, substrate concentration S and product concentration to changes in the Monod kinetic parameter, Ks, was investigated. Qualitatively, it can be deduced that the sensitivity of the concentrations to Ks should increase as the concentration of S becomes low at the end of the batch. This is verified by the results in Fig. 2.30. The results in Fig. 2.31 give the sensitivity of biomass concentration X and substrate concentration S to another biological kinetic parameter, the yield coefficient Y, as defined in Section 1.4. 

Fermentation systems obey the same fundamental mass and energy balance relationships as do chemical reaction systems, but special difficulties arise in biological reactor modelling, owing to uncertainties in the kinetic rate expression and the reaction stoichiometry. In what follows, material balance equations are derived for the total mass, the mass of substrate and the cell mass for the case of the stirred tank bioreactor system (Dunn et ah, 2003). 

J. L. Galazzo and J. E. Bailey, Fermentation pathway kinetics and metabolic flux control in suspended and immobilized Saccharomyces cerevisiae. Enzyme Microb. Technol. 12(3), 162 172 (1990). 

Applications of chemical kinetics to enzyme-catalyzed reactions soon followed. Because of the ease with which its progress could be monitored polarimetrically, enzyme hydrolysis of sucrose by invertase was a popular system for study. O Sullivan and Tompson (1890) concluded that the reaction obeyed the Law of Mass Action and in a paper entitled, Invertase A Contribution to the History of an Enzyme or Unorganized Ferment , they wrote [Enzymes] possess a life function without life. Is there anything [in their actions] which can be distinguished from ordinary chemical action ... 

CPDMO is a new bioreagent for the synthesis of optically pure lactones with excellent enantioselectivity. CPDMO is not only effective in desymmetrization of meso and prochiral compounds (Procedure 2, Section 11.8.2), but excellent in carrying out the kinetic resolution of racemates (Procedure 3, Section 11.8.3). Additional examples of optically pure lactones that can be obtained are summarized in Table 11.4. In the fermenter work (Procedure 4, Section 11.8.4), (R)-2-methyl cyclohexanone was not converted, but evaporated under aeration condition (1 wm). This led to the expected product (5)-7-methyl oxepanone at the end of the experiment. The optically pure lactone could be recovered without sdica-gel chromatography separation. However, the production yield may be improved by using a better condenser. 

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