Flavour formation and degradation in Cold Brew Coffee

Hot water is a wonderful thing: it draws many delicious compounds from coffee. It does so because water is a solvent and coffee has soluble compounds.

Tweaking extraction variables to get the best brew is all in a normal days work for a coffee geek, so using water at ambient temperature seems counter-intuitive – it is simply a worse solvent. So why then do we brew cold brew?

Firstly, it’s important to note that in coffee ‘more’ is never the best: In any extraction there’s a possibility of over-or under-extraction and that’s to do with how much soluble coffee has been absorbed into the water.

hcb bottleWhen hot water is used to brew coffee (say 93°C), soluble compounds quickly absorb into the water. If we only changed the temperature we would simply get less, so we need tweak other variables – time, turbulence and grind to get our extraction yield back up (and roast and blend for flavour).

Contact time goes from .5 – 4 minutes in a hot extraction to 8 – 20 hours in a cold extraction. Much more turbulence and movement can happen in this time and the grind will generally be coarser, or at least you’ll be “erring on the side of coars-ing”. These variables are a balance with what you’re trying to achieve, the main one is contact time.

Hot and Cold

A hot extraction readily draws out acids, so in a cold brew, with the absence of acids other compounds have the opportunity to be drawn out. Given there are 900 volatile compounds in coffee it’s hard to know exactly what they are.

What we tend to get in cold brew coffee is lighter, more delicate notes. To describe exact flavours wouldn’t be appropriate since these can be controlled through the other variables i.e. it’s largely dependent on the coffee that goes in.

The difference that all cold brew shares is the greater application – cold brew can be consumed more as a refreshment. It’s still complex, tasty and has zero calories but it’s a drink that you could happily quaff. It doesn’t have to replace a hot morning cup of milky coffee, it’s just used in a different time – perhaps as a mid afternoon pick me up.

There tends to be a lack of bitterness, so you are less likely to want to douse it with sugar and milk, it’s more friendly to being enjoyed black.

In addition to acids, cold water also doesn’t draw out compounds that readily oxidise. If you leave a hot coffee sit for 6 hours, it will start to taste metallic and bitter. This doesn’t happen with cold brew, cold brew will take a couple of weeks to degrade even when exposed to air and this is the major benefit – longevity. Which gives potential for greater application.

Caffeine Content of Cold Brew vs Espresso

Cold brew coffee has a fair whack more caffeine than espresso. The lab tests I had completed through Eurofins showed Harpoon Cold Brew Concentrate came back with 44% more caffeine than espresso.

Lab tests showed Harpoon Cold Brew Concentrate had 249mg caffeine per 100ml.
Espresso has approx. 173.5mg per 100ml.

Caffeine absorbs into water quickly initially and then slowly over time. There’s only so much caffeine to be absorbed but since cold brew has considerably higher contact time with water it draws out more, possibly all of the caffeine.

It’s important to note that this is one test. There are many extraction variables that affect caffeine absorption in both cold brew and espresso.

People often feel more caffeinated after drinking cold brew because also it’s rather easy to drink and to overpour: 30ml can easily become 50ml and then that’s twice as much caffeine as what you normally drink.

Flavour Degradation of Cold Brew

We know that cold brew coffee lasts significantly longer than hot brewed coffee. We’re talking around the two week mark but that depends on ambient temperature, light exposure and turbulence / actual exposure to air.

Replacing air with inert gas (eg nitrogen), limiting the amount of dissolved oxygen in the liquid, and ensuring all equipment is super clean, cold brew coffee can keep fresh for well over 3 months sealed.

The main cause for flavour degradation is oxidation, that is, exposure to air. Light encourages oxidation so it’s important to be kept in a non-clear container. Secondarily, cold brew coffee is susceptible to degradation from the presence of yeasts.

The first signs of flavour degradation come from oxidation. Oxidised cold brew coffee has a bright, gently unpleasant acidity that comes up front in the palate. It’s a little metallic. This can happen after a couple of days of exposure, exacerbated by light. It’s often confused with naturally occurring acidity from coffee. Many times have I tried cold brew coffee and got this oxidised flavour being presented to me as naturally occurring acidity.

Many lighter roasts and naturally processed coffees will have delightful acidic notes, and this is not that. This is unpleasant, and it is caused by oxidation and light damage. This is why cold drippers should be used for display purposes only – they are always exposed to light and air.

Furthermore, if left even longer:

  • 3 weeks exposed: grassy, mouldy flavours become present. In addition to the gently pungent acidity is a dull grassiness.
  • 3 months exposed: flavours and smells indicating fermentation and vinegary flavours. This is due to yeasts.

Oxidation and yeasts contribute to this slow demise. To avoid this it is imperative to clean and sanitise all equipment used in the making of cold brew coffee. One must be as pedantic as a brewer.

It’s also imperative to limit the amount of dissolved oxygen going into the liquid, and limiting exposure to air and light.

Summary

It’s important not to leave with the taste of soy sauce in the mouth so one must be reminded that cold brew lasts significantly longer than hot brewed coffee in the same conditions, with the potential for lasting well over three months.

Cold brew is still a relatively new phenomenon. It’s not better or worse than hot brewed coffee, it’s simply different. By removing a key drawer of flavour that is hot water, and tweaking other extraction variables we have a new realm of coffee to explore, and a different way to enjoy this wonderfully complex drink.

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