A Druid's Handbook to the Spiritual Power of Plants: Spagyrics in Magical and Sexual Rituals - Jon G. Hughes 2014

Fermentation
The Process of Obtaining Suitable Alcoholic Spirits
The Druidic Workshop

The term fermentation describes the chemical changes in organic substances produced by the action of enzymes. More often today, and certainly in our case, the term refers to the action of specific enzymes, called ferments, produced by minute organisms found in yeasts. In the context of the Druidic tradition, the most important type of fermentation is alcoholic fermentation, in which the action of the enzyme zymase, secreted by yeast, converts the simple sugars found in fruits and berries into ethyl alcohol and carbon dioxide.

The exact process used in the Druidic tradition is described in great detail here, where the focus is on the fermentation techniques necessary to produce the alcohol bases needed for our workings.