This week I was perusing Instagram and came upon a surprising coffee recommendation from author and consultant Scott Rao. According to Rao, when making coffee with manual-fill batch brewers, such as our favorite, the Bonavita 1901, filling the tank with cold water can help slow the brew cycles, increasing extraction (i.e. more delicious coffee flavor).

This tip is surprising because water temperature is one of the key variables that affects coffee extraction. Overwhelmingly, most manual-fill batch brewers struggle to get the water hot enough. (And don’t get us started with baristas brewing your $5 pour-over with too cool water.) In fact, we’ve long heard coffee experts recommend the opposite: preheat your brew water to help the machine get it to temperature. 

But there is counter-intuitive logic to Rao’s recommendation. Brewers with effective flash heaters don’t have a way of controlling the water pulses. More sophisticated plumbed brewers, which draw from a large water boiler, offer the ability to set prewet percentages and even control the number of pulses. But manual-fill brewers simply dispense the water as it reaches temperature. In some cases, this applies the water too quickly, resulting in shorter brew times, ergo lowering extractions.

We decided to test Rao’s hypothesis with our Marco Bru 45M. This manual-fill batch brewer is a great option for caterers or boutique cafés that do a small amount of filter coffee (i.e. European cafés– that would never fly in filter-centric America). Thanks to its patented spray head, it has very good water distribution. Though we have often lamented the lack of a prewet feature, for the most part we’ve been happy with the quality of coffee it’s capable of brewing.

In order to minimize any variance we weighed our doses to the tenth of a gram and our brew water to a gram, and ground each dose on the same setting immediately before brewing. Our refractometer was freshly calibrated with distilled water and we followed all of the best practices for measuring total dissolved solids (TDS). Our control brews used room temperature water to fill the brewer. For the compariston brews, we chilled the water by putting a carafe in a refrigerator for an hour (it came out with a nice layer of condensation).

Across the board, the brews with cold water in the tank yielded higher extractions. With identical brew ratios (60 g: 1 liter), the refrigerated water produced brews of anywhere between 0.02-0.1 higher TDS, which produced extractions up to an entire point higher.

It’s worth noting, a higher extraction doesn’t always mean a better extraction. A TDS reading can only be used to calculate an average. But overwhelmingly, the bad cups of coffee we’re served are under-extracted, especially with lighter roasts. And chilling your brew water when using a coffee maker might be the easiest brewing hack we’ve encountered yet.

Do you make coffee with a manual-fill brewer? Try our experiment for yourself and let us know the results.