Fermentation management

[blond.chap]

Hi guys and girls,

Just thought I'd post a bit of what I've been learning lately on fermentation control. This is mostly from discussions and research on winemaking rules of thumb. It's obviously less vital in fermenting for distillation, but might help some people out. For beginners, don't worry about this stuff too much, keep it simple while you're starting. This is strictly for people who have the time and interest. If you're happy throwing the ingredients in and letting it go, that works too.

For those who'd like the quick version, my advice is:
- Don't use too much sugar, 1kg per 5 Litres is about right
- Check the pH, and adjust to 3.5 or below
- Keep the fermentation at 15-30degC, especially near the end
- Use the temperature to try to get roughly a 0.01 drop on your hydrometer per day
- Use your wash ASAP following completion of fermentation, if you can't, then rack it off of the lees and keep in a sealed container (as full as possible), if you have access to CO2, then maintain a gas blanket on the top as soon as fermentation has completed.

The reason that fermentation control has some relevance to distillation is that poor control will result in more generation of ethyl aceteate (heads), proponols and butanols (tails). The more heads and tails you have in your drink, the more booze you have to cut out. You'll also form more acetaldehyde and acetic acid if fermentation isn't controlled, reducing your yield of ethanol.

Stepping through the dot points above one by one:
Sugar concentration: Too much sugar puts pressure on the yeast, can cause them to metabolise through alternative pathways, producing components like hydrogen sulphide (rotten egg smell), acetic acid (vinegar), ethyl acetate (nail polish), if you go to a high enough concentration you'll cause the yeast cells to lyse. Too little sugar and you'll get a low yield.

pH: pH is a measure of acidity, more specifically the free hydrogen ions (H+) floating about in your liquid. It influences the chemical reactions which can occur and how fast they occur. If you keep the pH at or below 3.5 the yeast will be comfortable to ferment, but other moulds, bacteria and fungi are inhibited. If allowed to develop, these other cultures will compete with the yeast for the sugar and may produce off aromas/flavours. Note however that in some cases, development of certain bacteria may be desirable for complexity of flavour, in this case you may need a higher pH. One example is rum, where Lactic Acid Bacteria contribute to the flavour, mainly where you’re using a dunder pit.

Temperature and fermentation rate: Similarly to the paragraph above on sugar concentration, too high a fermentation temperature may stress the yeast and/or make the fermentation progress too quickly. This can impart undesirable characters and decrease yield. If the temperature is too cold, the yeast will go dormant and you’ll get a delayed fermentation, which also increases the risk of bacterial infection. For the same reason, keep the temperature high toward the end of fermentation, otherwise the ferment can get “stuck”. And you’ll have trouble restating it, potentially leaving residual sugar, which can be a food source for spoilage bacteria.
Using the wash quickly: The longer you leave the wash, the more chance of bacterial infection there is (as discussed above). You also give the alcohol a greater chance to oxidise, converting ethanol to acetaldehyde (ethanal) and acetic acid (ethanoic acid). Acetic acid will also react with ethanol, giving ethyl acetate, the compound responsible for heads aroma/flavour. For the same reason, avoid splashing fermented wash around the place too much.

Please feel free to add to this, I definitely haven’t covered all components of fermentation management and I’m not by any means an expert.
If I get the time, I’ll do some pH measurements on common washes and post them to let you guys know whether pH adjustment is necessary (as accurate pH meters are pricey).