Cold Brew is a wonderful way to drink coffee. It tends to have a delicate flavour that is sweet and toasty with only gentle acidity and bitterness – which makes it light and refreshing.

“How to make your own Cold Brew” is at the end of this page if you just want the juice, I’ll start with some background info.

Background: The Extraction of Coffee

Coffee is the most complex food or drink in the world. This is due in part to the many variables that exist when extracting coffee.

This extraction, or the dissolving of soluble compounds into water, is what we, the drinkers do at home or the barista does for us in a shop.

Water acts as a solvent and coffee is the substance that we are trying to dissolve. To alter our final product we can change what we dissolve by tweaking these variables.

Consider the difference between espresso and plunger coffee:

Espresso:

Very finely ground coffee

93°C water

High coffee: water ratio

30 second contact time

Pressurised extraction

Plunger:

Coarsely ground coffee

87-90°C water (just off the boil)

Lower coffee: water ratio (about 1:16)

3 minute contact time

No pressure

We have all tasted the difference between coffee from a plunger and coffee from espresso. Changing any of these variables – whether they are to do with the water or to do with the coffee will have an affect on the final brew.

Coffee variables:

Origin or blend of green beans including climate, processing, age etc.

Origin or blend of green beans including climate, processing, age etc. Roast profile

Grind

Time between each of these steps

Water variables:

Temperature

Ratio

Contact time

Turbulence

Pressure

Many of these variables will be controlled or limited by what brewing method you use i.e. espresso will (ideally) have the same amount of pressure, turbulence, temperature whereas in an Aeropress you can easily change all of these variables – which is finicky for beginners but wonderful for coffee geeks.

Tweaking the temperature

Water temperature has a positive affect on coffees absorption into water. That is, we can dissolve some very tasty compounds from the coffee using hot water.

But with those tasty compounds is a readiness to oxidise – have you ever left your hot coffee to go cold? After a few hours it tastes metallic and bitter and if you leave it longer it will taste grassy and even longer still it will taste of vinegar or soy sauce because of fermentation. In cold brew, you still get degradation, but not as readily.

So in a cold extraction, the water is kept cold. But just tweaking the temperature won’t do much because cold water simply won’t absorb as much as hot water. We need to tweak other variables to get a brew worth drinking. These tend to be:

Contact time (much longer than plunger, like 3-24 hours)

Ratio (higher coffee: water ratio, like between 1:5 and 1:10)

Since cold water absorbs different compounds than hot coffee we don’t get oxidation as readily – it does happen over a couple of weeks but not to the same extent as from a hot extraction.

We also get different flavours from the coffee. These tend to be sweeter, toastier notes with less acidity and bitterness. The flavours still depend greatly on the coffee variables mentioned above.

Methods of Cold Brewing Coffee

There are two main methods to make cold brew. Cold Dripping and Full Immersion Cold Brew. In both cases, we use cold water and a relatively long contact time.

Cold Drip

Cold Dripping requires special equipment, most commonly this is a ‘Hario Cold Drip‘, pictured below:

Water drips into the central chamber containing ground coffee, absorbs the tasty compounds and then drips through a metal filter and into the collection vessel. While it looks great it should also be observed that the coffee is constantly exposed to air, and defining the exact drip rate can be finicky. You may also notice an uneven extraction from the coffee – there’s only a small paper filter resting on the top which spreads the water out around the coffee.

But it looks fantastic and can produce a satisfactory brew with very little effort. You just set it up and leave it!

Full Immersion Cold Brew

In full immersion cold brew, coffee is steeped in water for an extended period (3-24 hours) and then filtered through paper. This can simply be done in a large air tight jar, or the Toddy system is a manufactured device that can do this for you:

The benefits of full immersion cold brew are that the coffee is evenly saturated with the water (and is therefor more consistent and controllable) but the downsides are the time it takes to filter through paper and if left alone there is less turbulence than cold dripping.

How to make your own Cold Brew

This is an example of full immersion cold brew, start it in the afternoon or evening and finish it the next the morning.

Equipment:

1x airtight jar

Funnel

Paper filter (large)

Collection vessel i.e. another jar

200g ground coffee (‘filter grind’)

1.8L water

Method:

Put ground coffee into jar, add water, stir and swizzle till all coffee is soaked. Leave for 30 mins before sealing.

Store for 12-18 hours in a dark, cool place

Open jar and swizzle around before filtering all through paper filter into collection vessel. Let gravity do its thing – don’t be tempted to squeeze the grinds

Expected yield: 1.4L

Expected strength (Total Dissolved Solids): 2500ppm

Enjoy your brew! Store sealed in the fridge, it is best enjoyed within two weeks.

Variables:

The blend or origin of your coffee will have the greatest affect on flavour

As does the roast degree – lighter roasts are sweeter, more acidic and showcase more of the inherent attributes of the beans

Grinding finer results in more bitterness, coarser grind results in more acidity – the reaches of this are over- and under- extraction

Ratio of coffee:water affects total dissolved solids (TDS). Maximum concentration is 1:4 but higher concentrations risk over-extraction.

Don’t be tempted to squeeze the grinds to increase your yield. Every 1g of coffee absorbs 2ml water and everything else runs through. Squeezing the grinds results in undesirable flavours coming out.

You could filter after 3 hours and you’ll find it quite similar flavour wise to 18 hours but caffeine absorbs quickly initially and then slowly over time – 18 hours is the time needed for maximum absorption.

Happy to answer any questions!

My product Harpoon Cold Brew Coffee uses coffee that’s specially blended and roasted for cold extraction, it’s then cold brewed at maximum concentration and filtered through a 40 micron stainless steel mesh under low pressure.