There are two considerations that are needed to ensure that the wort has been properly prepared to support a good fermentation. The first is oxygen supplied via aeration. The methods for aerating the wort were covered in Chapter 6- Yeast. The role of oxygen in yeast growth will be discussed further in the Adaptation Phase section later in this chapter.
The second consideration is the level of amino acid nutrients in the wort, specifically referred to as Free Amino Nitrogen or FAN. Malted barley normally supplies all of the FAN and nutrients that the yeast need to grow and adapt to the fermentation environment. However, if the recipe incorporates large amounts of adjuncts (e.g. corn, rice, unmalted wheat, unmalted barley), or refined sugars, then the wort may not have the minimum levels of nutrients necessary for the yeast to build strong cells. It is always advisable to add some yeast nutrient powder to worts that are made exclusively from light extracts because these extracts are typically thinned with corn sugar.
In addition, brewers should be aware that in a wort that contains a high percentage of refined sugar (~50%), the yeast will sometimes lose the ability to secrete the enzymes that allow them to ferment maltose. They will adapt themselves right out of a job!