Semi-continuous culture

In some circumstances, most of a run is harvested while some broth is left in the fermenter to seed the next run. There are only a few practical examples of this technique because it provides too much opportunity for contamination, and high-yielding mutant cultures would probably revert over long times to less productive forms. This "back-seeding" can work for ethanol because yeast are hardy organisms that thrive at the low pH and alcohol concentrations that discourage contaminants. The applet lets you experiment with backseeding:

This simulation exercise tries for the briefest possible periods of low productivity, but you should not start a new cycle before the sugar is pretty well expended. Analyze what is happening and explain why the first harvest should be longer than the others (hint: size of inoculum). Note that in a fixed time period, you lose one or more harvests if you try a long harvest time. You may be able to get more product with very short harvest times, but these are impractical because very high concentrations of sugar are wasted and discharged from the process. Waste disposal would be very costly for so much sugar.

One important feature of back-seeding is shortening of the bioprocess cycle through inoculation with a relatively large amount of cells. The product left behind with this inoculum is not really lost because it contributes to the yield of the next batch. The organic byproducts and unused substrate that are left in the tank would have required waste treatment, so reusing substrate and concentrating byproducts before waste treatment are savings.


  • Concept of the limiting nutrient
  • Fedbatch culture
  • Relating product to growth
  • Synchronous culture
  • Teaching games
  • References