My questions revolve around mead fermentation. Q's for the panel: What is the ideal timing for nutrient additions for mead fermentations?

- Ken Schramm


RESPONSE:

Yeast prefer that the nutrients be added in increments over the first 1/3 of the fermentation. When all of the nutrients are added only at the beginning a large cell mass is produced with each cell having a low protein content. This low protein content makes it difficult to complete the fermentation and withstand the alcohol toxicity near the end. Adding the nutrients, primarily Nitrogen, in increments results in less, but adequate, yeast growth with each cell having a larger amount of protein. High protein yeast ferment faster and withstand alcohol toxicity better than low protein yeast.

Yeast sugar transport systems that bring the sugar into the cell at a prescribed rate, contain nitrogen. Some of their half life span is about two hours, so new transport enzymes must be generated constantly. This requires a fresh source of available nitrogen.

Yeast require a steady source of nitrogen through out the growth phase to produce DNA, RNA, amino acids, proteins and other cell components. If the sulfur containing amino acid skeleton is not available to receive the H2S as the yeast produces it, the yeast will expel it out of the cell resulting in rotten egg odor.

Just as a point of interest, yeast produce 30+ times as much alcohol in he growth phase as it does in the stationary phase. So be nice to the yeast while it is growing and keep it growing as long as possible.

Clayton


How do deficiencies of specific amino acids of wine and mead musts contribute to the production of higher alcohols? The two approaches to their production I have seen in print are oxidative deamination and the carbohydrate route. Could you provide some input on which one is correct, and how it proceeds?


RESPONSE:

Both routes are used by the yeast.

1. In a amino acid rich or sufficient environment, the yeast takes the catabolic or Ehrlich route to strip the amino acid of its NH3 to use for its own building blocks. The remainder of the amino acid produces a one carbon less higher alcohol.

2. In an amino acid deficiency environment, the yeast will use the anabolic biosynthesis route and produce the alpha-keto acid from glucose then on to a higher alcohol.

Example: Catabolic or Ehrlich route -- Valine to alpha-keto acid to Isobutanol. Anabolic route --- glucose to alpha keto acid to Isobutanol.

The above are over simplifications of what really takes place and brings into question the exact Ehrlich pathway.

Clayton


The four higher alcohols that I have seen presented that account for off flavors in beer and wine are n-propanol, isoamyl alcohol, active amyl alcohol, and isobutanol. Are these the culprits in mead?


RESPONSE:

The four higher alcohols that you mentioned are present in all fermented beverages and are an important part of their flavor and aroma profile. A full bodied ale can handle more than a lager. A rich red wine requires and perhaps masks more than a light white wine. A metheglin requires more of these alcohols than traditional mead before they become an off flavor. The secret is having them there in the right amount.

Clayton


Do you feel it is possible to conduct a .100 point SG fermentation of mead must with a FAN content of 20-40 ppm and no nutrient addition without the production of off flavors and higher alcohols? Which yeast strains might favor nutrient free fermentations, if any? (I am not a believer in this option, but I am asking on behalf of some I know who are.)


RESPONSE:

I wish that I had a good answer for you from actual experimental data but I don't. .100 SG is about 26% sugar. This alone would require additional yeast and nutrients to complete the fermentation. Before nutrients requirements were understood and added, mead makers were satisfied to have fermentations that lasted 6 to 18 months. If I had to make a guess as to what strain of yeast might work under those conditions I would try Uvaferm 43 or Lalvin EC-1118. Both tend to be neutral in ester formation and have low Nitrogen requirements and are strong fermenters. I would try it on a very small amount. Double the amount of yeast inoculum and rehydrate the yeast in the presence of Go-Ferm. Give it plenty of air the first 72 hours and watch the pH during the first 12 to 24 hours. Do not let the pH drop below 3.0. Add potassium carbonate if necessary. Let me know your results.

Clayton