rpt said:
Wort from grains also contains simple sugars.
Bingo! The only sugar that yeast can use directly is glucose.
Truth be told, dried brewers yeast is propagated in a bioreactor using a low gravity wort that is composed primarily of water and molasses. The gravity is kept very low in order to avoid the Crabtree effect.
Contrary to what many home brewing books claim, brewer's yeast does not respire in normal gravity beer wort. Dissolved oxygen plays no role in the decision to switch from respiration to fermentation in the presence of glucose concentrations greater than 0.3%. The choice of fermentative growth over respirative growth is known as the Crabtree effect. Brewer's yeast consume oxygen while undergoing fermentative growth (which is an anaerobic process) because their metabolic pathways are leaky. The oxygen that goes through the respirative metabolic pathway is used to synthesize ergosterol, which, in turn, improves yeast health.
In an industrial bioreactor, low gravity wort is continuously fed to a stirred well-aerated culture. As the glucose in solution remains below the Crabtree threshold, the yeast culture remains in respiration mode. During respiration the carbon source (a.k.a. sugar) is metabolized to water and carbon dioxide gas (yeast metabolizes the carbon source to ethanol and carbon dioxide gas during fermentation). Respirative growth is significantly more efficient than fermentative growth, which is why it is the preferred way to propagate yeast.
With that said, if propagating brewer's yeast outside of normal brewery wort conditions led to poor wort fermentation, then dried brewer's yeast would never work. Bioreactor grown dried yeast is the poster child for how far from the target environment brewer's yeast can be grown. Dried brewer's yeast cells have never experienced fermentative growth, the toxicity of alcohol, or the osmotic pressure that normal, let alone high-gravity worts place upon yeast cells. Yet, most fermentations reach normal terminal gravity.
In closing, anyone who doubts what I have claimed, should perform a Google search using the terms "Lallemand" and "bioreactor" or "Lesaffre" and "bioreactor" (Lesaffre is the parent company of Fermentis).
