Nutrient oxygen and yeast management

Dr. Clayton Cone, of Lallemand (a major supplier of yeast for all varieties of fermentation) has spent years studying "difficult" fermentations (honey, corn syrup, rice syrup, cane sugar, ultra-filtered fruit juices, etc), and determined these all shared a number of common problems:

1. Very little available nutrients, B vitamins and particulate matter for the yeast.

Although yeast are largely self-sufficient, they need some vitamins (especially B vitamins), mineral nutrients (especially nitrogen, phosphorus and zinc) and particles for them to stick to, (which helps to keep them suspended in the fermenting fluid.)

Commercial yeast nutrient powders supply the nutrients and particles that yeast need to thrive. Examples are Lallemand's Fermaid K® and Wyeast Lab's yeast nutrient. These contain vitamins, minerals, amino acids, free amino nitrogen compounds, zinc, phosphates, dead yeast shells and other trace elements. Optimum yeast nutrition will also require the addition of diammonium phosphate (DAP) during the fermentation.

Little or no buffering material to stop the pH from dropping rapidly during the first few hours.

Yeast produce significant amounts of acid as they ferment, which can lower the fluid pH. Natural juices and worts contain buffering materials, which limit how low the pH can go. Ferments without buffering capacity can drop as low as 2.7, which can stress the yeast so badly it never recovers. A pH below 4.5 is good, since it dramatically slows the growth of organisms which might contribute unpleasant flavors. Ideally, the ferment should not be allowed to go below pH 3.4 nor above 4.0. Judicious use of potassium or calcium carbonate or bicarbonate will allow you to keep the pH in this range (potassium carbonate is more soluble and easier to work with, calcium carbonate is more widely available). Both sodium carbonate (washing soda) and bicarbonate (baking soda) will work, but are not as beneficial to the yeast as the potassium or calcium compounds

3. Not enough oxygen for the growth phase of the yeast

Yeast needs oxygen early in the fermentation process to grow and to produce lipids, which protect the yeast cells from high alcohol levels later in the fermentation. Frequent agitation or aeration of the fermenting fluid during the first 24-36 hours will supply enough, but oxygen should be excluded after this time by use of an airlock.

4. Too much carbon dioxide in solution in the fermenting liquid

Carbon dioxide is a waste product of fermentation, and at high concentration can slow down the fermentation, especially when the yeast is already stressed by other factors. Occasionally agitating the fermenting fluid will release the excess, as will the addition of powdered materials (nutrient or potassium carbonate additions). This can result in the production of considerable foam, so be careful!

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