From Wikipedia, the free encyclopedia:
 
Named after the English biochemist Herbert Grace Crabtree, the Crabtree effect describes the phenomenon whereby the yeast, Saccharomyces cerevisiae, produces ethanol (alcohol) aerobically in the presence of high external glucose concentrations rather than producing biomass via the tricarboxylic acid cycle, the usual process occurring anaerobically in most yeasts eg. Kluyveromyces spp. Increasing concentrations of glucose accelerates glycolysis (the breakdown of glucose) which results in the production of appreciable amounts of ATP through substrate-level phosphorylation. This reduces the need of oxidative phosphorylation done by the TCA cycle via the electron transport chain and therefore decreases oxygen consumption. The phenomenon is believed to have evolved as a competition mechanism (due to the antiseptic nature of ethanol) around the time when the first fruits on Earth fell from the trees [1].
 
Yeah, congratulations to those of you that grasped all of that. Now for the rest of us in plain English the Crabtree effect is that yeast will always ferment and produce alcohol when the sugar concentration of must is higher than 9 g/l, despite the vast amounts of air/oxygen you might introduce into propagation or fermentation. Uh… okay, so how does knowing this improve my life? Well some winemakers believe that by giving the fermentation (or propagation) “lots of air” it will allow the yeast to respire and not form alcohol. Why don’t you want alcohol formation when you try to build biomass? Because, yeasts grown in the presence of alcohol (as with winery propagation and fermentation due to higher than 9 g/l sugar concentrations) have weaker cell membranes than yeasts grown in the absence of alcohol, as is the case of wine yeast production at yeast factories. Weaker cell membranes equal lower alcohol tolerance equal more stuck fermentations. I think you are starting to get my point why knowing what the Crabtree effect is might improve your quality of life by sparing you some stuck fermentations. It could also help you explaining the yeast bill to your boss. And hey, if you are already convinced that inoculating each tank at the recommended yeast dosage of factory grown yeast is better than propagating, then boasting that you know what the Crabtree effect is to your mates is a great conversation starter at cocktail parties – or maybe not…