Why does it bubble when mash ferments?

The days are already getting a bit cooler, the leaves on the trees are changing color, our mash tanks are bubbling peacefully, and the supermarkets are already stocking gingerbread and speculatius biscuits. Yes, autumn has arrived.

But first, a little bit about our current, daily work: For us, autumn means that the fruit harvest is now in full swing. The stone fruit (i.e., mirabelle plums, plums, etc.) is almost finished, and now we're moving on to apples and pears. The most well-known pear is probably the Williams pear. Did you know that the Williams pear is a rather tricky pear for distilling? The perfect time to mash the Williams pear is the moment after its color turns from green to yellow.

If it's still grass-green, it's simply not quite ripe yet, and its typical varietal flavor hasn't fully developed yet. But it shouldn't be a bright dark yellow either, let alone buttery soft. That would be overripe and oily. The time window between these two ripeness levels is relatively small; depending on the weather, it can be one or two days, sometimes just a few hours.

On August 26, we harvested our Williams Christ pears and placed them in a compact container in the cold storage room to ripen. Pears release a ripening gas during ripening, which inflames the neighboring pears. You get the same effect if you place bananas next to apples and pears: The banana immediately turns brown. The goal is for the pears to ripen evenly and together.

By the way: There's a simple trick to determine whether stone fruit is ripe: For apples and pears, the seeds should be brown. White seeds mean the fruit isn't yet ripe.

A reduced equation shows

There are different types of yeast that secrete different types of alcohol. In a spirit, you want the good ethyl alcohol (ethanol), which is produced by a specific type of yeast. For this reason, you should "inoculate" a mash with a pure yeast to ensure that the good alcohol is produced, not the bad.

Of course, it's essential to process only clean and healthy fruit. Furthermore, all equipment must be clean. Otherwise, other substances can form that negatively affect the flavor of the distillate.

The following, very reduced, equation shows what happens during the fermentation of a fruit mash: