The two main requirements for consistent, predictable fermentations are healthy yeast and a nutrient balanced wort.
It is generally recognized that only wort produced from a well modified, all malt grist approaches these needs, and even this requires supplementation with oxygen and zinc.
The common practice of high gravity, adjunct brewing has introduced two fermentation problems:
nutrient deficiency and conditions of high stress for yeast.
Yeast generated under these difficult conditions typically show symptoms of reduced vigor, which can be compounded at each regeneration.
Problems associated with reduced vigor include slow or sticking fermentation, off-flavors associated with sulphur containing compounds, slow diacetyl reduction,