Dear Mr. Wizard,

When do you need to worry about hot-side aeration (HSA)? I realize that after the boil one needs to be careful not to aerate the wort until it is cool, but do you need to be concerned while mashing your grains? If so, at what temperature does HSA become an issue?

Gregory Probst
Minooka, Illinois

Mr. Wizard replies:

Hot-side aeration is a loose term referring to oxygen pickup in the “hot side” operations of brewing. These include mashing, lautering, wort boiling and hop separation at temperatures ranging from about 120–212° F. Over about the last 15 years, researchers have presented evidence that hot-side aeration — especially prior to wort boiling — decreases the shelf-life of beer by increasing the concentration of oxidized fatty acids. These compounds are carried forward into the beer and impart classic stale flavors such as the infamous “wet cardboard” type of oxidation.

I am a skeptic by nature, but I find that the data demonstrating the negative affects of hot-side aeration during mashing and lautering are convincing. I am still skeptical that deleterious HSA occurs in the boil, however. The temperature range during mashing and lautering is low enough that oxygen can indeed dissolve into the liquid and cause oxidation. In contrast, boiling wort (and wort immediately after boiling) is so hot that very little oxygen can be dissolved in it.

Enzymes are present in lightly-kilned malts that oxidize lipids and form staling compounds when oxygen and lipids are present. (Lipoxygenase enzymes are one example of these enzymes.) There has also been data presented demonstrating the presence of non-enzymatic lipid oxidation during mashing. This oxidation forms staling compounds and free radicals that carry on this cycle of fat oxidation, commonly known as rancidity.

Hot-side aeration can be demonstrated in medium and large commercial breweries because the brewing equipment is so big that splashing is a really dramatic event. Think of liquid flowing through a six-inch pipe at 400 gallons per minute and cascading 12 feet through the air before hitting the bottom of a tank. This — not roughly stirring a five-gallon mash with a wooden spoon —is what commercial brewers are trying to minimize.

Some of the things commercial brewers are doing to minimize splashing include mixing malt and mash water in a small mixing vessel and then pumping the mash into the bottom of the mash mixer. This replaces the practice of blending the two ingredients together in a grist hydrator and allowing the hydrated malt to cascade into the mash mixer. Key pieces of equipment might also be introduced. New mixing paddles are low-shear, low-splash agitators that work well without baffles in the tank. These paddles minimize splashing during the stirred, heated steps of the mash. When the mash is transferred to the lauter tun, it enters right at the false bottom level instead of being dropped in from the top of the vessel, as was common some 25 years ago. Kettles are also bottom-filled for similar reasons.

For a homebrewer, the most likely time for aeration to occur during mashing is while you are mashing in. One procedure to address this concern is the following: Add 2.5 quarts of water out of 10 to the mash tun, then carefully add two pounds of malt out of eight and gently mix malt into water. Add 2.5 more quarts of water and three pounds of malt and gently mix, then 2.5 quarts water and three pounds malt and mix, and finally add the last 2.5 quarts of water. As this process may take longer than your usual mash-in, it is important to make sure that your water stays hot enough during this suggested procedure.

If you use a separate vessel for wort collection and transfer your mash from the mash tun to a lauter tun, develop a procedure to minimize splashing. This is not always easy and the solution to this concern may be to mash and lauter in the same vessel. (Sometimes the solution is found by subtracting steps and pieces of equipment rather than always adding that new tool.)

I think the thing to remember about hot-side aeration is that it is not that hard to avoid at home. The real question that will probably spark heated debates for years to come is this: How good is good enough when it comes to warding off potential risks of hot-side aeration? There are some commercial brewers who have addressed all the obvious issues and now are looking at purging their mills and brewhouse vessels with nitrogen. I am anxious to hear if these aggressive methods improve the beer.

It must be remembered that oxygen pick-up after fermentation (e.g., from racking, filtering and bottling) also causes oxidation and staling of beer. In fact, the effects of oxygen pick-up after fermentation are more apparent and severe than the effects of hot-side aeration. This is partly because there are a wider variety of compounds in beer susceptible to oxidation than there are in wort. The point here is that, if you are thinking about changing your brewing procedure to avoid oxidation, you should begin addressing oxygen pick-up from the end of the process and work your way forward toward mashing.

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