Biomass fermentation substrate

These rather unwieldy equations can be used to generate a graph showing the changes in biomass and substrate concentrations during the course of a batch fermentation (see Fig. 5.55). Their main disadvantage is that they are not explicit in X and S so that a trial and error technique has to be used to determine their values at a particular value of l. 

The experiment is then continued by adding feed to the fermentation broth, at a known flowrate which declines in a preset manner (usually linearly). This will cause the system to come to a net zero growth rate at some stage. Figure 5.70 shows the variation of the total biomass and substrate in the fermenter with time, noting that in this case the feed has been started at the beginning of the fermentation. 

KNIGHTS163 devised a procedure for determining the kinetic constants of a batch fermentation system which involves monitoring both the biomass and substrate concentrations, but without assuming a constant yield coefficient. Exponential growth in a batch fermenter is represented by equation 5.55 written as ... 

In fermentation reactors, cell growth is promoted or maintained to produce metabolite, biomass, transformed substrate, or purified solvent. Systems based on macro-organism cultures are usually referred as tissue cultures. Those based on dispersed non-tissue forming cultures of micro-organisms are loosely referred as microbial reactors. In enzyme reactors, substrate transformation is promoted without the life-support system of whole cells. Frequently, these reactors employ immobilized enzymes, where an enzyme is supported on inert solids so that it can be reused in the process. Virtually all bioreactors of technological importance deal with a heterogeneous system involving more than two phases. 

The OMBRE approach is applied to a biomass fermentation model [1], which, assuming Monod-type kinetics for biomass growth and substrate consumption, is described by the following DAEs set ... 

Lactic acid is a major end product from fermentation of a carbohydrate by lactic acid bacteria (Tormo and Izco, 2004). However, lactic acid can be produced commercially by either chemical synthesis or fermentation. The chemical synthesis results in a racemic mixture of the two isomers whereas during fermentation an optically pure form of lactic acid is produced. However, this may depend on the microorganisms, fermentation substrates, and fermentation conditions. Lactic acid can be produced from renewable materials by various species of the fungus Rhizopus. This has many advantages as opposed to bacterial production because of amylolytic characteristics, low nutrient requirements, and the fungal biomass, which is a valuable fermentation by-product (Zhan, Jin, and Kelly, 2007). 

Online analysis Online sample processing techniques such as flow injection provide advantages such as reliability, sample economy, ease of automation, measurement standardization, high speed, optional sample dilution, and the ability to derivatize the analyte so as to suit the analyzer/detector. These procedures facilitate the online monitoring of fermentation substrate materials, respiratory gases, and biomass. The modifications to flow injection analysis for accurate discontinuous flow operation include sequential injection analysis and bead injection spectroscopy. The most recent invention in online techniques is the introduction of the Lab-on-a-Valve, which opens the way to development of a novel type of microflow analytical system monitored by UV-visible spectrophotometry using fiber optics. This system is an ideal tool for fermentation monitoring. 

It is estimated that some 10 tons of carbohydrate polymers, mostly cellulose, are produced annually i.e., 25 tons per person on earth. Fortunately, carbohydrate-rich materials including urban refuse, agricultural residues and lignocellulosics are good fermentation substrates, hence microorganisms assure that biosynthesis and decay are in balance. That a major product of nature s biomass transformation should be a high molecular weight linear polyester is perhaps unexpected. 

Qureshi N, Blaschek H (2005) Butanol production from agricultural biomass Qureshi N, Lolas A, Blaschek HP (2001) Soy molasses as fermentation substrate for production of butanol using Clostridium beijerinckii BAIOI. J Ind Microbiol Biotechnol 26(5) 290-5 Qureshi N et al (2006a) Butanol production from com fiber xylan using Clostridium acetobutylicum. Biotechnol Prog 22(3) 673—80... 

Temperature, pH, and feed rate are often measured and controlled. Dissolved oxygen (DO) can be controlled using aeration, agitation, pressure, and/or feed rate. Oxygen consumption and carbon dioxide formation can be measured in the outgoing air to provide insight into the metaboHc status of the microorganism. No rehable on-line measurement exists for biomass, substrate, or products. Most optimization is based on empirical methods simulation of quantitative models may provide more efficient optimization of fermentation. 

Starting from an inoeulum, at t = 0, and an initial quantity of limiting substrate at t = 0, the biomass will grow after a short lag phase and will eonsume substrate. The growth rate slows as the substrate eoneentration deereases, and beeomes zero when all the substrate has been eonsumed. Simultaneously, the biomass eoneentration initially inereases slowly, then faster until it levels off when the substrate beeomes depleted. Figure 11-21 shows a sketeh of a bateh fermenter. 

The antibiotie Tylosin was produeed in a CSTR using Streptomyees fradiae in a 5 liter laboratory fermenter. For different substrate flow-rates the eoneentrations of produet and biomass were measured [23]. 

When excess substrate interferes with growth and/or product formation. One example is the production of baker s yeast. It is known that relatively low concentrations of certain sugars repress respiration and this will make the yeast cells switch to fermentative metabolism, even under aerobic conditions. This, of course, has a negative effect on biomass yield. When maximum biomass production is aimed at, fed batch cultures are the best choice, since the concentration of limiting sugar remains low enough to avoid repression of respiration. 

Where yield coefficients are constant for a particular cell cultivation system, knowledge of how one variable changes can be used to determine changes in the other. Such stoichiometric relationships can be useful in monitoring fermentations. For example, some product concentrations, such as CO2 leaving an aerobic bioreactor, are often the most convenient to measure in practice and give information on substrate consumption rates, biomass formation rates and product formation rates. 

We can see that for type 1 processes, high growth rate is obligately linked to a high rate of product formation. Indeed, this is the case for all products produced by a fermentative mode of metabolism, eg ethanol, lactic add, acetone. Chemostat studies have shown that for most aerobic processes when growth is limited by some nutrient other than the carbon source, the yield of product decreases with increase in spedfic growth rate (p or D p = dilution rate (D) in chemostat culture). Conversely, both the biomass yield and the spedfic rate of substrate utilisation (qs g substrate g biomass-1 h-1) increase with spedfic growth rate. 

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. 

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