Microbial Growth Kinetics

Lecture 2

Tahir

Fermenters

Fermentation Process

Fermentation Technology-> Fermentation: from latin -> ”fervere” -> to boil (describing the

anaerobic process of yeast producing CO2 on fruit extracts)

-> Nowadays: more broad meaning!!!!

The five major groups of commercially important fermentations:

-> Process that produces microbial cells (Biomass) as a product-> Process that produces microbial enzymes as a product-> Process that produces microbial metabolites (primary or secondary) as a

product-> Process that produces recombinant products (enzymes or metabolite) as a

product -> Process that modifies a compound that is added to the fermentation –

transformation process

Growth: basic concepts

Anabolism = biosynthesis

Catabolism = reactions to recover energy (often ATP)

Precursors

Regeneration of NAD+

FermentationRespiration

No added terminal e--acceptor Oxidant = terminal e--acceptor

ATP: substrate level phosphorylationATP: (e--transport) oxidative phosphoryl.

Glucose

2 Glyceraldehyde-3-P 2 ATP2 NADH

2 Pyruvate

2 Lactate+ 2 H+

Acetaldehyde+2 CO2

2 Ethanol

Acetate+ Formate

H2 + CO2

Glucose2 ATP2 NADH

2 Pyruvate

2 Acetyl-CoACO2

Citric acidcycle

CO2

GTPNADH, FADH

Cytoplasmic membrane

out

inATP

H+H+H+H+H+H+

O2H2O

1 Glucose 2 ATP 1 Glucose 38 ATPSlow growth/low biomass yield Fast growth/high biomass yield

Major functions of a fermentor

1) Provide operation free from contamination;

2) Maintain a specific temperature;

3) Provide adequate mixing and aeration;

4) Control the pH of the culture;

5) Allow monitoring and/or control of dissolved oxygen;

6) Allow feeding of nutrient solutions and reagents;

7) Provide access points for inoculation and sampling;

8) Minimize liquid loss from the vessel;

9) Facilitate the growth of a wide range of organisms.

(Allman A.R., 1999: Fermentation Microbiology and Biotechnology)

Microbial Growth Kinetics

• Fermentation can be carried out as:1. Batch2. Continuous3. Fed-batch processesMode of operation is to a large extent dictated

by the type of product being produced.

Biotechnological processes of growing microorganisms in a bioreactor

Batch Culture

Growth cycle of yeast during beer

fermentation

From: Papazian C (1991), The New Complete Joy of Home Brewing.

Alternate modes of energy generation

(H2S, H2, NH3)(in autotrophs)

FermentationFermentation

Fermentation Technology

-> Process that produces microbial cells (Biomass) as a product mainly for -> baking industry (yeast) -> human or animal food (microbial cells)

Fermentation Technology

Fermentation Technology

-> Process that produces microbial enzymes as a product

mainly for -> food industry

Fermentation Technology

-> Process that produces microbial metabolites (primary or secondary) as a product

Fermentation Technology

-> Process that produces microbial metabolites (primary or secondary) as a product

Fermentation Technology

-> Process that produces microbial metabolites (primary or secondary) as a product

Fermentation Technology

-> Process that produces microbial metabolites (primary or secondary) as a product

Continuous cultures

Fed-Batch CultureA fed-batch is a biotechnological batch process which is based on feeding of a growth limiting nutrient substrate to a culture. The fed-batch strategy is typically used in bio-industrial processes to reach a high cell density in the bioreactor. Mostly the feed solution is highly concentrated to avoid dilution of the bioreactor. The controlled addition of the nutrient directly affects the growth rate of the culture and allows to avoid overflow metabolism (formation of side metabolites, such as acetate for Escherichia coli, lactic acid in cell cultures, ethanol in Saccharomyces cerevisiae), oxygen limitation (anaerobiosis).

Batch culture versus continuous culture

Continuous systems: limited to single cell protein, ethanol

productions, and some forms of waste-water treatment

processes.

Batch cultivation: the dominant form of industrial usage due to its

many advantages.

(Smith J.E, 1998: Biotechnology)

Advantages of batch culture versus continuous culture

1) Products may be required only in a small quantities at any given time.

2) Market needs may be intermittent.3) Shelf-life of certain products is short.4) High product concentration is required in broth for optimizing

downstream processes.5) Some metabolic products are produced only during the stationary

phase of the growth cycle.6) Instability of some production strains require their regular

renewal.7) Compared to continuous processes, the technical requirements

for batch culture is much easier.