Culturing microorganisms on an industrial scale

Primary and secondary metabolites

• sometimes what is wanted are what's formed as an essential part of the microorganisms' natural function e.g. ethanol, ethanoic acid or range of amino acids aka primary metabolites
• sometimes the organisms produce substances that aren't required for natural growth but are still used in the cells e.g. pigments, toxic chemicals used for protection aka secondary metabolites

Types of bioprocess

• Batch fermentation
  • microogranisms are inoculated into a fixed volume of medium
  • as growth takes place, nutrients are used up and new biomass and waste build up
  • as the culture reaches the stationary phase, overall growth ceases but they often carry out biochemical changes that form the desired end products
  • process is stopped before the death phase and the products are harvested
  • whole system is cleaned and sterilised before a new batch is introduced
• Continuous fermentation
  • microorganisms are inoculated into sterile nutrient medium and start to grow
  • sterile nutrient medium is added continuously once it reaches the exponential phase
  • culture broth is continually removed (medium + waste + microorganisms + products) to keep the volume of bioreactor constant
  • enables continuous balanced growth, with levels of nutrients, pH, temperature and metabolic products kept constant
• both can be adjusted to ensure maximum production of biomass or product --> most are adapted for this
• majority of industry uses batch or semi-continuous processes
• continuous fermentation mostly used for the production of single celled proteins and in some waste water treatment
• useful parts of each mixture has to be separated by downstream processing --> most difficult part of the process

Controlling bioreactors

• Temperature 
  • too low and the mircoorganisms won't grow enough
  • too high and their enzymes will denature
  • bioreactors often have a heating/cooling system attached to temperature sensors to maintain optimum conditions
• Nutrients and oxygen
  • they can be added in controlled amounts to the broth when probes indicate levels are dropping
• Mixing things up
  • diffusion isn't enough for the microorganisms to have access to everything they need so bioreactors have big mixing mechanisms to make sure the broth is always circulating 
• Asepsis
  • if a bioprocess is contaminated it can drastically affect the yield so bioreactors are aspetic units