Lipids which are essential for yeast membrane growth. Sterols contribute to the flexibility of yeast membranes and require oxygen for their synthesis by the yeast cell. Although yeast may multiply extensively without oxygen during fermentation they must be exposed to oxygen before a further pitching so that they have an opportunity to produce sterols. This will typically happen during cropping but many brewers aerate their collected worts before fermentation to ensure that their yeast is fully prepared.