Aerated Coffee – Sweeter with Greater Clarity of Flavours

Earlier this year, James Hoffmann wrote a post on his blog about his experiments with a wine aerator. He found by pouring brewed coffee through such a device made the coffee taste sweeter and gave the flavours greater clarity.

How does an aerator work?

Water (which makes up a large percentage of both coffee and wine) naturally contains dissolved oxygen molecules. It’s these molecules that support marine life in rivers and seas. Water gains most of its oxygen from the atmosphere and can only store a limited amount. There are two factors which affect the capacity of water to hold oxygen:

  1. Pressure – higher the pressure the higher the saturation point
  2. Temperature – higher the temperature the lower the saturation point

An aerator works by increasing pressure on the wine and then exposing it to air to increase the saturation of dissolved oxygen. This is only a temporary measure as when the pressure on the wine returns to normal it will slowly release the extra oxygen molecules until it reaches the original saturation point. But in this short time, the increased oxygen has already sped up the oxidation of the wine.
Oxidation of wine either by an aerator or by decanting is a controversial subject among the wine community. It can have both positive and negative affects. Daniel Sogg wrote a good article about this in the Wine Spectator.

Oxidation is usually considered a bad thing in coffee as it leaches away flavour. But this is concerned more with storage rather than actual brewing. Oxygen is said to be the worst enemy of roasted coffee beans as it shortens their shelf-life. However, the general consensus is that oxygen in the water used to brew coffee is a good thing as it helps extract the flavour. This has scientific grounding as oxygen atoms are very reactive and aren’t particularly choosey about what they react with. So it’s reasonable to expect oxygen to assist in breaking down the compounds in the coffee grounds into some that can be more easily dissolved in water.

Why did aeration make the coffee taste sweeter and give the flavours greater clarity?

When James wrote his post, he wasn’t sure what chemical changes occurred in his coffee to make it taste better. To be perfectly honest I’m not either. I’ve been searching around the internet for a while and haven’t come up with any answers. I guess that as oxygen is highly reactive it could have any number of affects on brewed coffee’s chemical make-up. I’ve come up with two theories which may (or may not) have something to do with it.

  1. Aeration could alter the pH of brewed coffee. High levels of CO2 are trapped inside the coffee bean when it’s roasted. While most of this disperses into the atmosphere a couple of days after roasting, a small amount is still contained. When we brew coffee we see CO2 escaping, bubbling out of the bloom. Some CO2 won’t escape though. It will be dissolved in water and form carbonic acid. By increasing the dissolved oxygen level in water, it should dispel more CO2 and will reduce the presence of carbonic acid and therefore raise the pH (slightly).

    Other acids may also be broken down by oxidation; and many of the flavours in coffee come from acids. So fewer acids, equals fewer flavours, which in turn gives greater clarity to those flavours remaining.

    James also had an interesting discussion on his blog about why coffee changes taste as it cools. Oxidation of acids may be another piece in this jigsaw.

  2. Oxidation could break down complex sugars. Have you every noticed how a stale biscuit tastes sweeter. This is because oxidation breaks down starch into simpler sugars and it’s these sugars we can taste. Coffee doesn’t contain starch, but it does contain other polysaccharides which aren’t soluble in water. It’s possible that these complex sugars could oxidise and break down into simple sugars which are then soluble.

    In Scott Rao’s new book ‘Everything but Espresso’ he talks about a scale in coffee with clarity on one end and body on the other. Greater clarity equals less body and vice versa. Now much of the sensation of body in the cup is caused by insolubles. So one thing is for certain; for James to have achieved greater clarity, some of these insolubles must have been transformed by aeration into solubles.

Simple steps to aerate coffee

Wine aerators aren’t the cheapest of items to purchase and some of you might think that pouring your coffee through an aerator before drinking is a bit of a faff. So here are some simple things you can do to increase the dissolved oxygen content of the water you use to brew:

  1. Use freshly drawn water – use water freshly drawn from the tap. Re-boiling water is a big NO NO!
  2. Boil the water as quickly as possible – as I mentioned earlier the higher the temperature of water the lower the saturation point of dissolved oxygen. However, this isn’t an instantaneous reaction. The loss of oxygen occurs slower than the rise in temperature. So the quicker we boil water, the greater the level of dissolved oxygen, as it takes time for oxygen levels to fall and reach equilibrium with the higher temperature of water.


To boil water quicker we can do the following:

  1. Keep the kettle clean – limescale slows the efficiency of the heating element.
  2. Use filtered water – the more impurities in water the higher the boiling point. Filters with an ion exchange, like a Brita one, are quite effective.
  3. Only boiling the amount of water you need doesn’t just help save the environment!
  4. Manually turn the kettle off yourself as kettle’s have the reaction of a sloth! They don’t really, but rather then use a thermostat to measure the temperature of water, kettles typically rely on a steam sensor to tell when water is boiling. As the old saying goes: there’s no steam without boiling water (or was that smoke without fire?). So there’s a 10-20 second delay between water reaching boiling point and the kettle turning itself off.

kettle vs sloth
Of course boiling a kettle quickly only gives you a temporary advantage and if you then have to wait for it to cool before using (you don’t want to scorch those precious grounds), any advantage quickly diminishes. It would be much better if we could only heat the water to the desired temperature.

More and more kettles are now coming on to the market with built in thermostats. Some you can heat to anywhere between 60-100°C in 5°C increments. This would be ideal for making coffee. But are these kettles actually any good or are they a bit gimmicky? Speaking to someone actually in the industry (household appliance design that is) he said that the first kettles that came onto the market weren’t very accurate at all. The temperature could be as much as 5°C out. But I would love to hear the experiences of anyone with a more recent model. See if it tempts me to take the plunge and buy a new kettle.