Model yeast mass

The simple and structured model simulations for yeast mass and soluble protein, peptides and carbohydrates are compared in Figure 6 for the yeast and enzyme concentration shown in Figures 3 and 4, and in Figure 7 for a concentrated yeast cell slurry. The simple model fits the data fairly well at both yeast concentrations, in every variable except the peptides. The fit for all variables at longer reaction times is directly related to use of the extent-of-reaction term Y , in the yeast lysis equation. 

The structured model provides a distinct improvement over the simple model, in the initial stages of the reaction. The initial lags in the hydrolysis of total yeast mass and carbohydrate in figures 6 and 7 are very well represented. The possibility remains that the initial lags relate partly to adsorption of lytic enzymes to the cell wall. On the time scale of our experiments, however, adsorption appeared to be instantaneous (35). 

Example 4.8 Develop a fermentor model which consists of two mass balances, one for the cell mass (or yeast), Cl5 and the other for glucose (or substrate), C2. We have to forget about the... 

Beck and Bauer (1989) used an empirical kinetic model (derived by Rottenbacher et al. (1985a) for the commercial baker s yeast strain DHW DZ in submerged culture) to define the maximum possible rate of ethanol production in the bed as a function of the dry mass of yeast in the bed and the mole fraction of ethanol in the liquid phase of the bed, thus... 

Mr< 10,000 Da Yeast autolysed in a model Relative molecular masses HPLC profiles Martinez-Rodriguez and... 

The yeast cell cycle has also been analyzed at this high level of chemical detail [17]. The molecular mechanism of the cycle in the form of a series of chemical equations was described by a set of ten nonlinear ordinary differential kinetic rate equations for the concentrations of the cyclins and associated proteins and the cell mass, derived using the standard principles of biochemical kinetics. Numerical solution of these equations 3uelded the concentrations of molecules such as the cyclin, Cln2, which is required to activate the cell cycle, or the inhibitor, Sid, which helps to retain the cell in the resting Gi phase. The rate constants and concentrations ( 50 parameters) were estimated from published measurements and adjusted so that the solutions of the equations yielded appropriate variations, i.e., similar to those experimentally measured, of the concentrations of the constituents of the system and the cell mass. The model also provides a rationalization of the behavior of cells with mutant forms of various system constituents. 

Model reaction trials and modem analytical methods (gas chromatography/mass spectrometry (GC/MS), gas chromatography/olfactometry (GC/0)) permitted the identification of key mechanisms responsible for flavour generation in process flavourings and some of the most important ones are detailed below. Often chemically complex precursor raw materials (vegetables such as onions, spices, yeast extracts, animal products) are used. Research work on these complex reactions is rare but necessary and allows the discovery of new key odorants and formation pathways. For example, Widder and co-workers [13] discovered a new powerful aroma compound, 3-mer-capto-2-methylpentan-l-ol in a complex process flavour based on onion. 

In general, the concentrations of esters increase during aging (Section 2.5.3). Ethyl acetates of fatty acids are formed by yeast, under anaerobic conditions, in quantities greater than those predicted by the mass action law. Consequently, they are hydrolyzed during aging and concentrations tend to decrease (Table 2.5). Garofolo and Piracci (1994) determined the kinetics equations for the hydrolysis of esters of fatty acids and isoamyl acetate in model media and in wines, at various pH values, over a period of 29 months. 

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