Biomass concentration measurements

At any biomass concentration the culture was ammonium-limited (data not shown). The increase in incident light intensity (Table 2) affected neither the hydrogen production by the photobioreactor nor the specific activity of the cells. Thus, the culture was not limited by light even at high biomass concentrations. Measurements of residual lactate concentration showed that only at the highest biomass content it was quite low (100 pM), possibly limiting the culture. 

Other appHcations of firefly hioluminescence include measurement of the activity of bacteria in secondary sewage treatment activated sludge (296,297), detection of bacteria in clean rooms and operating rooms, measurement of bacteria in bottled foods, beverages (298), and pharmaceuticals (299), determination of the antimicrobial activity of potential dmgs (300), determination of the viabiHty of seeds (301), and measuring marine biomass concentrations as a function of ocean depth or geographical location (302). 

It is assumed that both state variables x, and x2 are measured with respect to time and that the standard experimental error (oe) is 0.1 (g/L) for both variables. The independent variables that determine a particular experiment are (i) the inoculation density (initial biomass concentration in the bioreactor), Xq i, with range 1 to 10 g/L, (ii) the dilution factor, D, with range 0.05 to 0.20 h 1 and (iii) the substrate concentration in the feed, cF, with range 5 to 35 g/L. 

Most of the studies05,20 271 show that a correlation between culture fluorescence and biomass concentration can be obtained mainly in the exponential growth phase. In addition, in order to obtain reproducible correlations, all of the fermentation conditions such as initial substrate concentration, pH, dissolved oxygen level, temperature, and agitation rate have to be the same. However, once the culture is past exponential growth, biomass measurement by following culture fluorescence is no longer accurate. 

D. W. Zabriskie and A.E. Humphrey, Estimation of fermentation biomass concentration by measuring culture fluorescence, Appl. Environ. Microbiol. 35, 337-343 (1978). 

Without appropriate cleanup measures, BTEX often persist in subsurface environments, endangering groundwater resources and public health. Bioremediation, in conjunction with free product recovery, is one of the most cost-effective approaches to clean up BTEX-contaminated sites [326]. However, while all BTEX compounds are biodegradable, there are several factors that can limit the success of BTEX bioremediation, such as pollutant concentration, active biomass concentration, temperature, pH, presence of other substrates or toxicants, availability of nutrients and electron acceptors, mass transfer limitations, and microbial adaptation. These factors have been recognized in various attempts to optimize clean-up operations. Yet, limited attention has been given to the exploitation of favorable substrate interactions to enhance in situ BTEX biodegradation. 

Now, assume that it is desired to estimate the biomass concentration X by using the measurements of S —presumably continuous—, without the knowledge of the growth rate. With this aim, let us introduce an auxiliary variable C defined by ... 

Due to the complexity of bioprocesses, and the lack of direct in-process measurements of critical process variables, much work is being done on development of soft sensors and model predictive control of such systems. Soft sensors have long been used to estimate biomass concentration in fed-batch cultivations. The soft sensors can be integrated into automated control structures to control the biomass growth in the fermentation. 

This shows that a plot of In X against t will have a slope equal to nm. The experiment, therefore, simply involves making measurements of the biomass concentration X at a series of times during the growth phase. 

Substrate concentrations were determined in centrifuged sample aliquots by standard COD analysis, and gravimetric method was used to measure the biomass concentrations [24], Analyses were repeated at least twice with the samples under the same experimental conditions and the average values were obtained. 

And using a new variable defined for biomass concentration as a function of the on-line measured biomass sensor, one obtains ... 

Figure 3 shows the experimental results of tests 1 and 2 obtained for the substrate concentration calculated using the software sensor and the filtered sensor measurements for ethanol and biomass. The differences between the two experiments are the initial biomass concentration and concentration of substrate in the feed, as shown in Table 1. 

Changes in biomass concentration throughout the fermentation process were followed by optical density (OD) measurement at 580 nm using an Ultrospec 2000 Spectrophotometer. Quantitative biomass concentration was assayed applying microbiuret cell protein determination (16). Biomass concentration was expressed in grams of dry matter per liter of fermentation broth by assuming a twofold multiplication constant for microbial protein to cell mass. For carbon balance calculations, the elemental composition of C. saccharolyticus was assumed to be CH1 8O0 5N0 2 (24.6 mg/mmol). 

The biomass concentration was determined from dry wt measurements. Three-milliliter samples were taken in duplicate at the beginning and the end of the fermentations. The samples were then centrifuged (1500y, 5 min), and the pellets were washed once with distilled water and dried at 103°C for 24 h. 

The weight of soil used in the measurement depends on the initial soil microbial biomass concentration and the flask volume. Normally, 20-40 g moist soil is suitable for this measurement. Lin and Brookes (1999a) showed that SIR can give reliable estimates of microbial biomass in unamended soils, soils that contain actively decomposing plant residues, or soils recently treated with pesticides or fumigants. The final results are usually expressed as the SIR rate (tl C02 evolved g soil h Biomass carbon (Bc) is estimated by Bc (gig C g l soil) = 15 SIR (Lin and Brookes 1999a). 

The antibiotic Tylosin was produced in a CSTR using Streptomyces fradiae in a 5 liter laboratory fermenter. For different substrate flow-rates the concentrations of product and biomass were measured [23]. 

Therefore, if no impact can be found in a plant community in terms of species composition, frequency, presence, total biomass, or quality of product after long term exposures to specific pollutants or pollutant complexes, the pollutant or pollutant complex is of no importance to the plant community at the ambient concentrations measured. Thus our final concern over air pollutants and their impact on vegetation should involve their effects on plant communities, whether simple or complex, domestic or natural. 

Biomass concentration is of paramount importance to scientists as well as engineers. It is a simple measure of the available quantity of a biocatalyst and is definitely an important key variable because it determines - simplifying - the rates of growth and/or product formation. Almost all mathematical models used to describe growth or product formation contain biomass as a most important state variable. Many control strategies involve the objective of maximizing biomass concentration it remains to be decided whether this is always wise. 

The measure of mass is important with respect to calculating mass balance. However, the elemental composition of biomass is normally ill defined. Another reason for determining biomass is the need for a reference when calculating specific rates (q ) q = r /x. An ideal measure for the biocatalysts in a bioreaction system of interest would be their activity, physiological state, morphology or other classification rather than just their mass. Unfortunately, these are even more difficult to quantify objectively and this is obviously why the biomass concentration is still of the greatest interest. 

Since an on-line generated signal for biomass concentration is decisive for control purposes a series of sensors and methods that can be automated have appeared in recent decades. Many of them rely on optical measuring principles, others exploit filtration characteristics, density changes of the suspension as a consequence of cells, or (di)electrical properties of suspended cells. Some of the... 

Geppert and Thielemann [125] and Geppert et al. [126] have used a similar method but a different instrument to measure a suspension aliquot outside the bioreactor and reported a fairly good linear correlation between OD and biomass concentration for some bacteria and yeasts. 

Bioreactions are exothermic. The net heat released during growth represents the sum of the many enzymatic reactions involved. Reasonably, this measure depends on both the biomass concentration and the metabolic state of the cells. Its general use in biotechnology has been reviewed by von Stockar and Marison [415]. A theoretical thermodynamic derivation for aerobic growth gives a prediction for the heat yield coefficient YQ/0 of 460 kj (mol 02) 1 and it was ex-... 

The latter requirement and the concomitant improvement of comfort led to the exploitation of alternative methods to estimate the biomass concentration. All of them have in common (1) that they are indirect measures, and (2) that models are mandatory to relate these measures to biomass concentration. The models are of the descriptive rather than the mechanistic type which means... 

The determination of the biomass concentration in complex media by weight is not possible. Therefore, indirect methods were tested. The OTR and CPR are not suitable for the biomass evaluation, because RQ considerably varies during the cultivation. In the first 60 h, it increases, between 60 and 80 h it drops sharply and only after 100 h does it attain a constant value. However, after 100 h, the biomass concentration can be measured by weight in any case. The best agreements were obtained between RNA and biomass concentrations in semisynthetic media. The biomass concentration increases with cultivation time up to 140 h. 

NAD(P)H concentration in biomass and its ratio to NAD(P) gives a measure for the culture reduction status [35]. NAD(P)H can, in principle, be detected directly in vivo by in situ fluorescence measurement (see the chapter by Sonnleitner in this volume - the section on culture fluorescence). This measure serves as a biomass concentration sensor if the specific NAD(P)H concentration stays constant. If not, the ratio of culture fluorescence and biomass concentration, the specific fluorescence, can be a measure for the culture reduction state, or indicate other more complex events like metabolic pathway shifts [35] or even the formation of a variant in a culture [36] (see also Figs. 2 and 3). 

Dielectric spectroscopy or culture capacitance measurement is used as an on-line, non-invasive method for biomass estimation (see the chapter by Sonnleitner in this issue - the section on electrical properties) and responds mainly to living cells [43,44]. Observed difficulties in using the signal as a pure biomass concentration sensor, i.e. deviations from the simple correlation with cell density, were attributed to dependencies on the physiological state [43], and could be used to discriminate different populations in yeast cultures [45]. Connections with morphological features could be found for budding yeast... 

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