Iced Coffee, Part 1: Hypotheses

In preparation for further brewing of coffee stouts, I've decided to again try to master iced coffee. In this post, I will summarize the hypotheses I've come up with to explain my previous experiences with making iced coffee, and lay out future plans for testing these hypotheses.

The challenges of iced coffee emerge chiefly from two sources: oxidation and under-extraction. Oxidation results in the acrid taste of stale coffee, and is a function of oxygen exposure, time, and temperature. Oxidation is a concern for cold-brewed coffee because of the long brewing times necessary to achieve proper levels of extraction. Oxidation is also a concern for hot-brewed iced coffee if it isn't chilled immediately, because oxidation occurs much more rapidly at high temperatures. Oxidation is especially worrying for me, because the effects of oxidation increase over time, and I want my coffee stouts to last much longer than iced coffee is expected to.

Extraction refers to the percentage of solids dissolved into the water during brewing. One intuitive way to think about extraction is as brewing efficiency—but higher efficiency is not always better. According to the SCAA, ideal extraction is about 20% (interestingly, this is true for both brewed coffee and espresso). Coffee with much less than 20% extraction is said to be under-extracted; coffee with much more than 20% extraction is said to be over-extracted, which is even worse.

Extraction is a function of a number of variables, including: brew ratio (coffee-to-water ratio), grind, water temperature, brew time, and agitation. Crucially, brew strength is distinct from amount of extraction: strength equals brew ratio multiplied by amount of extraction. Therefore, if a high enough brew ratio is used, it is quite easy to make very strong but under-extracted coffee, as in the top left corner of this chart:



Under-extraction in brewed coffee often yields a musty or tea-like aroma that I find particularly unpleasant at cold temperatures.

One method of brewing iced coffee that has become popular recently—sometimes under the moniker 'Japanese iced coffee'—is to brew hot coffee directly onto ice, compensating for the dilution the ice produces by using half the usual amount of brewing water. I've tried this method many times over the past few months, using both Hario V60 brewers and press pots, with consistently disappointing results. Specifically, every attempt yielded coffee with the musty aroma that I now (tentatively) identify as under-extraction. Varying the amount of coffee I used didn't seem to help.

So why did the brew-onto-ice method fail? The problem, I think, is this: by reducing the amount of brewing water so dramatically, the percentage of solids extracted from the coffee grounds dropped precipitously. Less hot water passing through the coffee, less solids extracted from the coffee.

There is, therefore, a trade-off inherent in the brew-onto-ice method. Keeping the total coffee-to-water ratio constant, one can vary the ratio of hot water to ice. (Note that ice should always be measured in weight, since the amount of ice in a given volume will vary depending on the size of the ice cubes.) Adding more hot water and less ice will result in better extraction, but also slower chilling of the coffee, possibly leading to oxidation. Adding more ice and less hot water will result in faster chilling, and hence less risk of oxidation, but also lower extraction. I believe that there is a ratio (or range of ratios) at which neither under-extraction nor oxidation are major concerns, but I have yet to determine where this sweet spot lies.

One strategy to compensate for the decrease in extraction caused by the reduction in hot water is to use a finer grind. This is a technique already in use by my former employer, but until now I did not fully understand the reasoning behind it. Using a finer grind increases the strength of the coffee by raising extraction, while reducing the amount of water (or increasing the amount of coffee) increases the strength of the coffee but decreases extraction. In order to maintain optimal extraction, therefore, an increase in the brew ratio should always be accompanied by a finer grind and/or an increase in brewing time. (In drip methods of brewing, a finer grind also increases the brewing time.) This is a bit counter-intuitive, but I'm pretty sure it's true!

In order to test these ideas and try to make better iced coffee (and hence better coffee stouts) I plan to brew iced coffee using several different methods.

1. Traditional cold-brew. 80 grams of coffee per liter of room temperature water. Medium grind. 18 hour brew time (room temperature).

2. Barismo-style cold-brew. 80 grams of coffee per liter of 40˚F water. Medium grind. 18 hour brew time (refrigerated).

3. Hario V60 brew-onto-ice. 50 grams of coffee per liter of total water. (Variable hot water/ice ratio). Medium-fine/fine grind.

4. Full-immersion hot brew poured onto ice. 50 grams of coffee per liter of total water. (Variable hot water/ice ratio). Medium-fine grind (or medium-fine/fine grind for a direct comparison with method 3).

In order to filter the cold-brewed and full-immersion coffee I will probably just use the Hario. Any oxygen introduced shouldn't affect the flavor in the short-term. In any case, I will taste the coffee before and after filtration. In the long run I would like to find a way to filter without exposure to air, and without having to use a coarse grind (as a press would require). Hario makes an iced coffee maker called the Mizudashi that might work. The Eva Cafe Solo is also an option. Or a cotton sack, perhaps.

Results to come.