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Key Conditions for Optimum Fermentation

As any wise man and Beatles fan will tell you, a good thing will happen if you let it be – may it be love, a tender rack of barbecued ribs or, in the case at hand, fermentation.

Enter yeasts, living microorganisms that readily grow in sugary solutions, produce enzymes (sucrose and zymase) that break up sugar or starch, and convert it into ethyl alcohol and carbon dioxide gas. (Saccharomyces cerevisiae is the superstar species of yeasts, instrumental to baking, brewing, winemaking, and other such vital enterprises since ancient times.)

The process of turning sugar into ethyl alcohol and carbon dioxide uses up almost 95% of the sugar, making these the chief products of fermentation. The remaining 5% of sugar contributes to the simultaneous formation of several by-products: impurities such as glycerol, volatile acids, fusel oils, ethers, aldehydes, esters. These substances not only make for great band names, but also give character to ethyl alcohol with ever fascinating flavors and colors. The downside, alas, is that they’re also responsible for hangovers.

This is why, with fermentation, you have to let it be – but in the right conditions. Here are the key factors you should keep an eye on:

  1. Temperature: high temperatures kill the yeast plants, low temperatures decreases their activity. The higher the temperature, the faster the rate of fermentation, but the lower the alcoholic yield. The optimum temperature is 78º F. Never exceed 90º F.
  2. Proportion: the optimum sugar to water ratio is 2 pounds to 1 gallon.
  3. Yeast and time: the usual proportion is 1 cup yeast to 5 gallons of water. At this ratio, in the right conditions, the yeast will produce enough ethyl alcohol to stop fermentation in 14 days. Yeast reproduces rapidly in sweet solutions, so less is better, but it will take a little longer for active fermentation to get going. Stand by your mash, and let experience guide you.
  4. Vinegar inhibition: when exposed to oxygen, the mash or wine will tend to promote the growth of another fungi that will manufacture vinegar. No oxygen, no vinegar.
  5. Settling time: when fermentation is complete, the mash or wine will be turbid and must settle. Settling will take several days or a week, even months in the case of wine. Chilling the fermented mash and/or filtering it will speed the process. Siphon or decant the clear solution and discard sediment. Try not to aerate the mash or wine unnecessarily, thereby risking the formation of vinegar.

After fermentation, the mash will be no more than 16% and usually not less than 3% ethyl alcohol by volume. It’s a dilute alcohol solution, so now’s the time to crank up your whiskey still and distill in high spirits.

Posted by Jason Stone on

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  • 1 cup of yeast? That seems extremely high.

    Most yeasts recommend 10g per gallon max, which ends up being a few tablespoons. A cup of yeast would easily be 150-200g

    Cam Alfred on

  • This article refers to sucrose as an enzyme, incorrect. Sucrose is a disaccharide sugar composed of one molecule of glucose and one molecule of fructose.

    Chris on

  • Muchas gracias. ?Como puedo iniciar sesion?

    xhtahzkhtl on

  • 4 cups makes it 2 pounds… I remember that a kilo was 4 cups.

    Almino Malaza on

  • Hey Jim, I totally undstand as started making Shaoxing with vinting yeast; but soon found out Fleischmans traditional is robust! As well I started with the same suger/water ratio. With every new batch I would incrementaly increase the volume of sugar used and after few years it stalled, I believe that I have found the diminishing return point. I’m going to test the content with my work place digital alcohol meter, but I feel/taste like this batch is absurdly powerful stuff….

    Francesco Silveri on

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