Aeration of hot wort and the use of copper in brewing equipment

Over the years, I have often emphasised the importance of limiting the oxidation of hot wort. The same applies to the use of copper and other reactive metals in the parts of the brewing equipment that come into contact with the wort.

Both can contribute to oxidation processes, which later affect the beer's freshness, aroma and shelf life. The damage can be difficult to detect during the brewing process itself, because the effect often only becomes apparent after the beer has been stored for some time.

Why is oxygenation a problem?

Oxygen is not always undesirable during beer brewing. Once the wort has cooled down to the correct fermentation temperature, the yeast needs oxygen to form cell membranes and multiply. Therefore, controlled aeration immediately before pitching the yeast can be an advantage.

On the hot side of the brewing process, the situation is different. During mash conversion, lautering, transfer, boiling and cooling, contact between wort and air should be limited. Oxygen can trigger or contribute to chemical reactions that degrade the flavours of the malt and hops.

• Cardboard-like taste
• Loss of fresh malt character
• Weakened hop aroma
• Darker colour
• Shorter shelf life
• A general taste of stale or flat beer

Reactions take place more quickly at high temperatures. Therefore, hot herbs should be handled gently and without unnecessary splashing.

Raise the insert gradually

In brewing systems with an insert strainer, the strainer should not be raised directly to its highest position whilst it still contains a significant amount of hot wort. The base of the insert strainer should, as far as possible, follow the liquid level in the kettle. The strainer should therefore be raised slowly and in small increments as the wort drains off. The distance between the bottom of the strainer and the liquid surface should be kept as small as is practically possible.

If the sieve hangs high above the liquid, the hot wort will drip or run through the air before it hits the wort in the kettle. The wort is distributed in droplets and thin streams, which increases the contact surface with the air. At the same time, the fall creates splashing and turbulence, which can draw air down beneath the liquid surface. The higher the fall and the stronger the splash, the greater the possibility of contact between the hot wort and the oxygen in the air.

1. Lift the insert ever so slightly.
2. Wait whilst some of the liquid drains off.
3. Lift the sieve further as the liquid level falls.
4. Keep the bottom of the container close to the surface of the liquid for as long as possible.
5. Avoid free-falling urine, powerful jets and unnecessary splashing.

It is not necessary for the bottom of the sieve to touch the liquid all the time. The purpose is to limit the drop height and ensure a smooth run-off. Only when most of the wort has drained can the sieve be lifted all the way up. The sparge water should likewise be gently distributed over the surface of the mash and not poured from a great height.

Copper can accelerate oxidation

Copper has traditionally been used in breweries and is still found in, amongst other things, cooling coils, pipes, fittings, boilers and copper-soldered plate coolers. The problem is not merely that the wort can absorb copper. Copper ions can act as catalysts for oxidation reactions. A catalyst is not necessarily consumed itself, but can cause the reactions to proceed more rapidly.

This means that even relatively low concentrations of copper can affect the oxidative stability of beer. Recent research confirms that copper, iron and manganese play a significant role in the oxidation of wort and beer.

A recent review article describes how these transition metals can accelerate the formation of reactive oxygen compounds and thereby contribute to the ageing of beer and the loss of its fresh flavour: Mertens, Kunz and Gibson: Transition metals in brewing and their role in the oxidative stability of wort and beer.

Another study found that adding as little as 10 ppb – equivalent to 0.01 mg per litre – of certain transition metals could result in a measurable change in the beer’s oxidative stability: Jenkins et al.: The impacts of copper, iron and manganese metal ions on the EPR assessment of beer oxidative stability.

Where does the copper come from?

Copper may already be present in water, malt and hops. Some is removed along with the wort, spent grain and yeast, but the brewing equipment may introduce further copper. The risk depends, amongst other things, on the acidity of the wort, the temperature, the contact time, the size and condition of the copper surface, the amount of dissolved oxygen and the cleaning agents used.

A copper immersion chiller in a small home brewery can have a relatively large surface area compared to the volume of wort. The same applies to copper-brazed plate chillers, where the wort passes through many narrow channels and comes into contact with the brazed joints.

It is difficult to set a single specific limit that applies to all brewing systems. However, the principle is simple: if we limit both oxygen uptake and contact with reactive metals, we reduce two key factors contributing to oxidation.

What about the beneficial properties of copper?

Copper is not necessarily harmful in any concentration. Yeast needs very small amounts of copper as a nutrient, and raw materials normally already contain the required amount. Copper can also react with certain sulphur compounds. That is one of the reasons why the metal has historically been valued in breweries and distilleries.

However, that does not mean it is appropriate to add uncontrolled amounts of copper via kettles, coolers, pipes or solder joints. The aim should be to ensure the yeast has the necessary trace elements – not to use the brewing equipment as a random source of copper.

The problem with copper-brazed plate heat exchangers

Many plate coolers at the lower end of the price range are made from stainless steel plates that have been soldered together with copper. Although the majority of the structure is made of stainless steel, the liquid may still come into contact with the copper solder joints.

The plate cooler also has a hygiene-related drawback: the narrow internal channels cannot normally be opened or inspected. Hop residues, proteins and trub can become lodged there, and it is difficult to check whether all the channels have been thoroughly cleaned. Backflushing and circulation with cleaning fluid can help, but do not provide the same level of control as a single, continuous tube.

The combination of copper brazing and limited cleaning options means that we do not recommend this type of plate chiller for homebrewing.

The stainless steel counter-flow cooler

We prefer a counterflow chiller where the wort runs through a continuous stainless steel tube, while the cooling water moves in the opposite direction around the tube.

• The herb should not come into contact with copper solder.
• The continuous pipe is easier to flush and clean.
• The cooling is effective and can be regulated with the flow rate.
• The cooler can operate using gravity without a pump.
• The wort can be piped directly to the fermenter at the desired temperature.
• The construction is relatively simple and inexpensive.

Effective cooling also shortens the time the wort spends at high temperatures. This reduces the opportunities for further hot-side oxidation and limits the formation of other undesirable substances.

Practical advice for reduced oxidation

• Avoid pouring or spraying hot water and hot herb through the air.
• Gradually lift the strainer insert so that its base roughly follows the liquid surface.
• Let the sparge water fall gently so that the surface of the mash is not whipped up.
• Keep hose ends submerged during transfer.
• Avoid unnecessary stirring and splashing with hot wort.
• Ensure that pipe and hose connections are secure and free from leaks.
• Simmer gently; a vigorous boil is not necessarily better.
• Cool the wort efficiently after boiling.
• Use stainless steel for the parts that come into contact with the wort.
• First check the wort once it has cooled to fermentation temperature.
• Once fermentation has begun, avoid exposing the mixture to oxygen.
• Limit oxygen uptake during racking, storage and bottling.

Brewers who wish to take things a step further can draw inspiration from the principles of Low Dissolved Oxygen, often abbreviated to LODO: On Brewing Bavarian Helles: Adapting to Inert Brewing.

It is not necessary to follow all the LODO methods to benefit from the basic principle. Careful handling of liquids, short transfer distances, suitable hoses and stainless-steel equipment are already good steps in the right direction.

Conclusion

Research suggests that oxygen uptake during the brewing process can impair the flavour stability of beer. It also shows that copper and other transition metals can catalyse the oxidation reactions that cause the beer to age more quickly.

It is difficult to set a single universal limit for how much copper is harmful. The effect depends on the entire brewing process, the composition of the beer and the simultaneous presence of oxygen. It therefore makes more sense to apply the precautionary principle than to debate exactly where the limit lies.

Our recommendation therefore remains: Minimise contact between air and hot herbs, avoid unnecessary use of copper and other reactive metals, and, where possible, choose stainless steel for the parts of the brewing equipment that come into contact with liquids.

Not only does this result in equipment that is easier to clean and more durable, it also creates better conditions for preserving the fresh malt and hop flavours in the finished beer.