"Propagation" or "conditioning" of active dry yeast (ADY) for the production of fuel ethanol is thought to reduce lag times in fermentation and reduce overall fermentation times. The objectives of this study were to determine the optimal time that ADY should be conditioned prior to fermentation and to determine how well yeast and bacterial contaminants present in ADY are propagated. A new term, metabolic acclimatization (MAcc), was introduced to describe events occurring during conditioning. It was observed that MAcc is not necessary prior to alcoholic fermentation. Lag times were reduced but overall fermentation times were unaffected. In fact, fermentation rates were slower than control conditions where ADY was directly inoculated into fermentors. In addition, if MAcc systems were to be used, it is recommended that only batch MAcc systems be practiced. Continuous MAcc (continuous yeast propagation) leads to increased bacterial numbers and organic acid levels, which become stressful to yeast. (c) 2004 Elsevier Ltd. All rights reserved.