We can also envision a process operated intentionally at low concentration of nitrogenous nutrients to restrict growth and at low sugar concentration to favor a particular pathway.
There is a presentation that covers our attempts to develop simple equations for toxic and/or multiple growth-limiting nutrients. Use "BACK" repeatedly to return to here if you try it.
When confronted with a normal substrate such as glucose, most organisms will not waste energy for elaborating the enzymes for an uncommon substrate such as lactose. Only when the concentration of glucose falls to a low level will the enzymes for the other sugar be synthesized. There are actual data by Egli, et al., (1986).
There is a Java exercise that shows a likely situation in continuous culture for a transition from sugar limitation to oxygen limitation in continuous culture.