Fermentation energy relationships

Energy relationships. If we disregard the synthesis of ATP, the equations for the lactic acid and ethanol fermentations are given by Eqs. 17-19 and 17-20. 

The relationships used in predicting microbial protein from fermentable energy have high standard errors of estimate and should be used with caution. In addition, such relationships may be used to calculate yields of microbial protein only when the supply of energy is limiting. When the supply of protein to the microorganisms is limiting, this will determine the yield of microbial protein. 

Fiber components are the principal energy source for colonic bacteria with a further contribution from digestive tract mucosal polysaccharides. Rate of fermentation varies with the chemical nature of the fiber components. Short-chain fatty acids generated by bacterial action are partiaUy absorbed through the colon waU and provide a supplementary energy source to the host. Therefore, dietary fiber is partiaUy caloric. The short-chain fatty acids also promote reabsorption of sodium and water from the colon and stimulate colonic blood flow and pancreatic secretions. Butyrate has added health benefits. Butyric acid is the preferred energy source for the colonocytes and has been shown to promote normal colonic epitheUal ceU differentiation. Butyric acid may inhibit colonic polyps and tumors. The relationships of intestinal microflora to health and disease have been reviewed (10). 

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). 

Figure 8 shows the curves at different superficial linear velocities and the relationship ofhorsepower to height of liquid in a fermenter. These curves are the mixing energy (power per unit volume) released by rising bubbles to the liquid. 

Sophisticated models attempt to relate microbial yield to the rate of carbohydrate fermentation and rate of passage, the theoretical growth rate, the energy cost of bacterial maintenance and the form of nitrogen available to the rumen microorganisms. Many of the relationships involved in such calculations are based on laboratory characterisation of the food, and the value of the model will depend on the validity of the relationships between the laboratory determinations and the values used in the models. 

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