FE 411 FOOD BIOTECHNOLOGY

1. FE 411 FOOD BIOTECHNOLOGY 2-Microbial Kinetics

2. INTRODUCTION • Understanding the growth kinetics of microbial, animal, or plantcells is important for the design and operation of fermentationsystems employing them. Cell kinetics deals with the rate of cellgrowth and how it is affected by various chemical and physicalconditions. • Unlike enzyme kinetics, cell kinetics is theresult of numerous complicated networks of biochemical andchemical reactions and transport phenomena, which involvesmultiple phases and multicomponent systems. • The heterogeneous mixture of young and old cells iscontinuously changing and adapting itself in the media environmentwhich is also continuously changing in physical and chemicalconditions. As a result, accurate mathematical modeling of growthkinetics is impossible to achieve. Even with such a realistic model,this approach is usually useless because the model may contain manyparameters which are impossible to determine

3. Various models can be developed based onthe assumptions concerning cell components and population

4. GROWTH CYCLE FOR BATCH CULTIVATION If you inoculate unicellular microorganisms into a fresh sterilizedmedium and measure the cell number density with respect to timeand plot it, you may find that there are six phases ofgrowth anddeath. 1. Lag phase 2. Accelerated growth phase 3. Exponential growth phase 4. Decelerated growth phase 5. Stationary phase 6. Death phase

5. GROWTH CYCLE FOR BATCH CULTIVATION 1. Lag phase: A period of time when the change of cell number iszero. 2. Accelerated growth phase: The cell number starts to increaseand the division rate increases to reach a maximum. 3. Exponential growth phase: The cell number increasesexponentially as the cells start to divide. The growth rate isincreasing during this phase, is constant at its maximum value. 4. Decelerated growth phase: After the growth rate reaches amaximum, it is followed by the deceleration of both growth rateand the division rate. 5. Stationary phase: The cell population will reach a maximumvalue and will not increase any further. 6. Death phase: After nutrients available for the cells aredepleted, cells will start to die and the number of viable cellswill decrease.

6. GROWTH CYCLE FOR BATCH CULTIVATION 1. Lag phase: The lag phase (or initial stationary, or latent) is an initial period ofcultivation during which the change of cell number is zero ornegligible. Even though the cell number does not increase, the cellsmay grow in size during this period. The length of this lag period depends on many factors such as thetype and age of the microorganisms, the size of the inoculum, andculture conditions.

7. GROWTH CYCLE FOR BATCH CULTIVATION 2. Exponential phase: In unicellular organisms, the progressive doubling of cell numberresults in a continually increasing rate of growth in the population. Abacterial culture undergoing balanced growth mimics a first-orderautocatalytic chemical reaction.

8. GROWTH CYCLE FOR BATCH CULTIVATION 2. Exponential phase: The rate of the cell population increase at any particulartime is proportional to the number density of bacteria present atthat time: where the constantμ is known as the specific growth rate (hr-1). Thespecific growth rate should not be confused with the growth rate,which has different units and meaning. The growth rate is the changeof the cell number density with time, while the specific growth rate is If μ is constant with time during the exponential growth period, equation can be integrated from time to as

9. GROWTH CYCLE FOR BATCH CULTIVATION Factorsaffectingthespecificgrowth rate • Substrate Concentration: One of the most widely employedexpressions for the effect of substrate concentration on μis the Monodequation, which is an empirical expression based on the form ofequation normally associated with enzyme kinetics : where Cs is the concentration of the limiting substrate in themedium and Ks is a system coefficient. This equation has the same form as the rate equation for an enzymecatalyzed reaction,the Michaelis-Menten equation:

10. GROWTH CYCLE FOR BATCH CULTIVATION • The value of Ks is equal to the concentrationof nutrient when the specific growth rate is half of its maximum valueμmax. According to the Monod equation, further increase in the nutrientconcentration after μ reaches μmax does not affect the specific growthrate.

11. EVALUATION OF KINETIC PARAMETERS • The Michaelis-Menten equation can be rearranged to be expressed in linear form. This canbe achieved in three ways: The Lineweaver-Burk plot The Langmuir plot The Eadie-Hofstee plot

12. GROWTH CYCLE FOR BATCH CULTIVATION Factorsaffectingthespecificgrowth rate • Product Concentration: As cells grow they produce metabolic byproductswhich can accumulate in the medium. The growth ofmicroorganisms is usually inhibited by these products, whose effectcan be added to the Monod equation as follows: • Other conditions: The specific growth rate of microorganisms isalso affected by medium pH, temperature, and oxygen supply. Theoptimum pH and temperature differ from one microorganism toanother.

13. GROWTH CYCLE FOR BATCH CULTIVATION 3. Stationary phase: The growth of microbial populations is normally limited either by theexhaustion of available nutrients or by the accumulation of toxicproducts of metabolism. As a consequence, the rate of growthdeclines and growth eventually stops. At this point a culture is said tobe in the stationary phase.

14. GROWTH CYCLE FOR BATCH CULTIVATION 4. Death phase: The stationary phase is usually followed by a death phase in whichthe organisms in the population die. Death occurs either because ofthe depletion of the cellular reserves of energy, or the accumulation oftoxic products. Like growth, death is an exponential function. Insome cases, the organisms not only die but also disintegrate, aprocess called lysis.

15. Modeling of the Bacterial Growth Curve M. H. ZWIETERING,* I. JONGENBURGER, F. M. ROMBOUTS, AND K. VAN 'T RIET APPLIED AND ENVIRONMENTAL MICROBIOLOGY, June 1990, p. 1875-1881