by Kido Hirotaka

Beer is a beverage created when, through fermentation, yeast gobbles up sugars from the malt to produce alcohol. There are basically two types of yeast: ale and lager. Ale yeast works best roughly between 20 and 22C for three to four days; lager yeast, 10 to 15C for as much as ten days. Wort into which yeast has been pitched gradually loses gravity (density depending on sugar content) once fermentation begins. After a week to ten days, the decline slows and yeast activity comes to a rest. This period is called primary fermentation. Next, I’d like to talk about the technique of secondary fermentation.

With home brewing and professional equipment alike, after the primary fermentation, young beer is usually moved to another tank for removal of yeast so it doesn’t ruin the flavor. This is called racking. If the yeast comes into contact with oxygen or is exposed to heat, it may undergo autolysis where cells rupture and die, releasing off-flavors like sulfur and rubber into the beer. With wine, there are cases when autolysis imparts flavors considered good for the wine, but this just isn’t the case with beer. For this reason, it’s generally best to move the beer to a separate vessel after primary fermentation.

In young beer from which yeast has been removed, there are still trace amounts of yeast that can ferment. When brewers want gentler carbonation through natural means, they may add some kind of new sugars to this young beer in the secondary tanks, thus activating fermentation again. England’s legendary Real Ale is traditionally brewed this way.

In Japan, there are some breweries that use this technique. They enable secondary fermentation to occur in both their bottles and kegs, thus making the beer lively and very drinkable.

Along with the gentle carbonation that comes with secondary fermentation, you also get a rest period to handle off-flavors often present in young beer. Fermentation produces CO2, alcohol and esters, as well as some undesirables. As the beer matures, yeast reabsorbs these substances producing off-flavors, rendering them flavorless and odorless. One of these is acetaldehyde, which may be responsible for your hangovers.

When you have a hangover, your body has broken down the alcohol, releasing acetaldehyde, which remains in your body. This is poisonous stuff that may make you nauseous and give you headaches. Let the beer mature properly and after this key rest period, you’ll have some delicious beer. Dry-hopping where brewers add fresh aroma hop to the beer usually occurs in the secondary tank, too.

Secondary fermentation, then, can eliminate off flavors and give beer gentle carbonation. It’s one of the last steps brewers can take to make their beer even more delicious.

This article was published in Japan Beer Times # () and is among the limited content available online. Order your copy through our online shop or download the digital version from the iTunes store to access the full contents of this issue.