fermentation lecture 11

8/13/2019 Fermentation Lecture 11

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FermentationKinetics of Yeast Growth

and Production


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Introduction

Fermentation can be defined as an energy yielding process where yeastconverts organic molecules (such as sugar) into energy, carbon dioxideor/and ethanol depending on the respiration pathway.

Yeast can respire in anaerobically and aerobically.

However, yeast gets more energy from aerobic respiration, but in the

absence of oxygen it can continue to respire anaerobically, though it doesnot get as much energy from the substrate. Yeast produces ethanol when itrespires anaerobically and ultimately the ethanol will kill the yeast (find outwhy is yeast continue to produce ethanol even the last is an inhibitor).

C6H1206 2 CH3CH2OH + 2 CO2+ 2 ATPC6H1206+ 6O26CO2+ 6H2O + 16-18 APT


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When the feed substrate to the reactor is notmonosaccharide e.g. sucrose (C12H22O11), yeastenzyme cause glycosidic bond to break in a processcalled hydrolysis


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Industrial and Commercial Applications

Food Industry

~ Beer

~ Bread

~ Cheese

~ Wine

~ Yogurt

Pharmaceutical Industry

~ Insulin

~ Vaccine Adjuvants

Energy

~ Fuel Ethanol


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Objective

To find the kinetics of the system by using

Nonlinear Regression (guess for ks and m) The Sum of the Least Squares and the

Lineweaver-Burk Plot methods in order todetermine the parameters mand ks

To determine the yield coefficient and to

project min. and max. amount yeast cell mass,carbon dioxide and ethanol produced


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Experimental Set Up

Apparatus

Bioreactor

pH meter

Sampling

device

Mixer

Temperature

sensor

YSI 2700

Biochemistry

Analyzer

pH probe

D-oxygen

probe


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Experimental: Procedure

Using Biochemistry Analyzer and Spectrophotometer

to measure and make calibration curves for sugar and

yeast cell concentrations

Reactant initial concentration dextrose/or sucrose 25 g/L

yeast 3 g/L

volume reactant solution 2 L


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Initial conditions & assumptions

Initial Conditions 2 L of solution

50 g sugar

pH around 5.0

Temperature around 28-30C

Assumptionsthe bioreactor content is

well mixed and has a constant medium volume at a certain

initial conditions Temperature is constant

pH maintained at optimal pH of 3.00

All reactants or nutrients present in excess except for sugarsubstrate.


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Theory

In ideal fermentation process in which the growing cells are

consuming the substrate (sugars), and producing more cells

according to the following scheme.

rsx = rate of substrate consumption

rx = rate of cell growth

s = substrate concentration

x = cell concentration

P = ethanol concentration (in anaerobic case)

rxCells (x)

P

Cells (x)

rsx


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Theory

The plot showing the trends for yeast cell growth over time

rx =

dCx

dt

rx =

dx

dt

xBiomass


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Lineweaver-Burk Rearrangement

+=

ss

s

ms

sk

s)(mm

s

m

s

s s

k

s

sk

111

)(

+=

+

=


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Nonlinear Regression

1. Define Model

2. Solve for Rpredicted(dx/dt)(calculate dx/dt from the polynomial equation fittedto the curve x(t)

3. Make initial guess for ks and m

(mis the max. specific growth rate can be achievedwhen S >> ks

ks is saturation constant or the value of limitingsubstrate conc. S at which sequal to the half of m

4. Minimize (R-Rpredicted)2 using solver function in

Excel by varying ksand m

+==

ss

s

mxsk

sx

dt

dxr


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Yield Coefficient Determination

Ratio of cell or Ethanol concentration to substrate concentration.

Knowing Yx/s will give you an idea for how much additionalyeast cell mass, on average, is produced for a given amount ofsugar substrate consumed.

As well allowed you to calculate a lower bound on theexperimental stoichiometric coefficient, , and therefore to

calculate ranges for ethanol and CO2production.(Yeast Cell) + C6H12O6 (CO2 + CH3CH2OH) + (Yeast Cells)

ss

xx

ds

dxY

o

o

s

x

==

Yps

=dP

ds=P P0

s0 s


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Error in Lineweaver-Burk

Parameters

Error in ksand mrelative to error in slope and y-intercept of

linear fit

Random Error in y values:

STDEV of slope:

STDEV of y-intercept:

( )

( )2

2

= nyy

s ii

xy

( )

=2

xx

s

s

i

xy

b

( )

=

2

2

xxn

xss

i

i

xya


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Lower Bound on

(stoichiometric coefficient)

(Yeast Cell) + C6H12O6(CO2+ CH3CH2OH) + (Yeast Cells)

Where, theoretically, = 2.

Assume all yeast generated is attributable only to sugarcomplete consumption

Conservation of mass requires that the remaining product be

equimolar amounts CO2and ethanol

fermentation lecture 11

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