Efficiency of growth and product formation 37 Yield coefficients may be defined for different substrates in the medium and are usually based upon substrate change in units of mass or mol of substrate eg yo2 is used to relate the amount of biomass formed to the amount of oxygen consumed, so 9 g glucose 684 oxygen 27.2 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 stbichiom tre stoichiometric relationships can be useful in monitoring fermentations. For example, some product concentrations, such as Co 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 In practice, variations in yield factors are often observed for a given organism in a given medium. For example, yield coefficients often vary with growth rate. An explanation for these variations comes from a consideration of the fate of substrate in the cell, which can be divided into three parts: assimilation into cell mass · energy for growth; energy for maintenance energy for Energy for maintenance is the energy required for survival, or non-growth related mantuan purposes. It includes activities such as active transport across membranes and turnover (replacement synthesis)of macromolecules. Where a single substrate serves both as carbon and energy source, which is the case for chemoheterotrophic organisms used for biomass production, we can write: ASassimilation+ ASgrowth energy ASmaintenance energy As= the total amount of substrate consumed ASassimilation amount of substrate assimilated Sgrowth energy=amount of substrate consumed to provide energy for grwoth Asmaintenance energy=amount of substrate consumed to provide energy for maintenance Or, expressed as yield coefficients Yx/s=△X/△ Assimilation+△X/△ Sgrowth energy+△X/△ maintenance energyEfficiency of growth and product formation 37 Yield coefficients may be defined for different substrates in the medium and are usually based upon substrate change in units of mass or mol of substrate eg, Ydo2 is used to relate the amount of biomass formed to the amount of oxygen consumed, so: Organism Pseudomonas fluorescens Substrate YXJ. 9 9-’ g mol-’ glucose 0.38 68.4 y*2 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 COZ 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. In practice, variations in yield factors are often observed for a given organism in a given medium. For example, yield Coefficients often vary with growth rate. An explanation for these variations comes from a consideration of the fate of substrate in the cell, which can be divided into three parts: assimilation into cell mass; energy for growth; energy for maintenance. Energy for maintenance is the energy required for survival, or non-growth related purposes- It includes activities such as active transport across membranes and turnover (replacement synthesis) of macromolecules. Where a single substrate serves both as carbon and energy source, which is the case for chemoheterotrophic organisms used for biomass production, we can write: AS = ASassimilation + &growth energy + &aintenance energy where AS = the total amount of substrate consumed Asassimilation = amount of substrate assimilated AS~OW~ energy = amount of substrate consumed to provide energy for grwoth &,&tenan- energy = amount of substrate consumed to provide energy for maintenance Or, expressed as yield coefficients: