Friday, October 4, 2013

Iced Coffee, Part 2

Last year, I wrote a post detailing my attempts to brew good iced coffee at home. I intended to follow up on it once I'd done some more experiments. Then summer ended and I stopped drinking iced coffee. Pretty much the same thing happened this summer, but I've made some progress. Iced coffee, for me, will never compare to hot coffee, but I've grown to appreciate both cold brew and chilled iced coffee.

For cold brew, I like a ratio of 1 part coffee to 12 parts water (i.e. 83 grams per liter). If you cold brew in the refrigerator, รก la Barismo, use a medium (cupping) grind. If you cold brew at room temperature, use a medium-coarse (press) grind. (Room temperature coffee will melt the serving ice quicker, and hence become diluted faster.) Both methods produce good results in 12 - 24 hours.

Filtering cold brew can be challenging, especially if you make a lot of it. The first step is to remove the large particles of coffee with a metal chinois, china cap, or tea strainer. But even extremely fine metal filters will not provide an acceptable level of clarity by themselves. For a polishing filter, there are three options: paper, cloth, or synthetic. Paper filters are easy to find, but clog up very quickly. Organic cotton filters are a better option if you can find them, but they're still fairly slow and require careful cleaning. My preferred filtration medium is food grade monofilament nylon, so long as it's rated 20 microns or smaller.

In Peter Giuliano's influential post on iced coffee from last year (Japanese Iced Coffee), he argues against cold-brewed coffee, on the grounds that it is (allegedly) underextracted, oxidized, and lacking in aromatics. I'll discuss these objections individually, then look at what alternatives to cold brewing exist.

Giuliano claims that low brewing temperatures necessarily result in underextracted coffee. It's true that most cold brew is underextracted, if its extraction yield is calculated according to the traditional formula (Extraction[%] = BrewedCoffee[g] * TDS[%] / CoffeeGrounds[g]). But this is only true because cold brew is usually made as a full immersion brew. As Vince Fedele has argued, calculating extraction yield for immersion brewing requires a different calculation (Extraction[%] = TotalWater[g] * TDS[%] / CoffeeGrounds[g]). Making this adjustment puts cold brew back in the proper range of extraction, if performed properly.

Giuliano also claims that long brewing times result in oxidized (i.e., stale) coffee. In my experience, cold brew does certainly become oxidized, but not nearly as quickly as hot brewed coffee, because oxidation occurs more slowly at lower temperatures. Still, I prefer to drink cold brew within 12-24 hours of brewing. Even with refrigeration and nitrogen flushing, more than a couple of days is pushing it. Of course, many people enjoy the woody flavors that result from oxidized cold brew. (The entire success of coffee stouts is built on this fact.)

Giuliano's preferred iced coffee brewing method, which he calls the "Japanese iced coffee method", involves brewing hot coffee via pour-over directly onto ice. I've tried this technique dozens and dozens of times over the past couple of years. I've tried varying grind, dose, ice/water ratio, and water temperature. I've tried both V60s and Chemex, as well as immersion brews poured through a paper filter. Every single time, I get a unpleasant musty aroma.

Now that I'm using an espresso machine again, I've noticed a very similar aroma when making espresso over ice. I think, therefore, the most likely source of the unpleasant aroma is temperature shock. I've been reluctant to believe that temperature shock really exists, because no one I know of has given it an adequate scientific explanation. But I've found that by allowing the coffee to cool somewhat before adding ice, the unpleasant aroma can be minimized.

Upon re-reading Giuliano's post, I was struck by this passage on volatility:

"Cooling the coffee quickly, though, reduces volatility dramatically.  This effectively locks the ephemeral volatiles (like floral and fruit notes) into solution until the coffee is warmed again.  This happens on the coffee’s way down your throat (sorry to get graphic here), which sends a punch of beautiful volatile aromatics through your retronasal cavity to your olfactory receptors.  And that explains the olfactory-flavor punch of brewed-hot-quickly-cooled Japanese-style iced coffee."

Is it possible that the aromas that I perceived as unpleasant and musty are the same that Giuliano describes as 'floral and fruit notes'? My, how tastes vary! What it comes down to, I think, is this: Cooling coffee very rapidly results in a distinctive aromatic profile not found in cold brewed or slow chilled coffee. The degree to which this aromatic profile is desirable depends on both the coffee and consumer.

Since I have not yet found a coffee that I enjoy brewed directly over ice, I prefer to slow down the chilling process somewhat. My basic method is this: brew coffee at a 1:10 ratio (100 g/l). After brewing, cover and allow the coffee to cool slowly, until it's below 150˚F (66˚C). Then add ice and stir until the coffee is under 50˚F (10˚C). Strain the coffee over fresh ice and serve. The whole cooling process should take 5 - 10 minutes. For large batches, an ice bath may be necessary to hit this time frame.

There are two standard objections to this technique. The first objection is that the increased coffee/water ratio will decrease extraction yield, making the resulting coffee underextracted. The premise of the argument is true, however, this effect can be compensated for by using a slightly finer grind, as in bypass brewing, and/or by adding water at a slower rate.

The second, more serious, objection is that allowing the coffee to cool slowly allows an unacceptable amount of oxidation to occur. Oxidation is largely responsible for the staling of brewed coffee, and oxidation occurs much more rapidly at high temperatures. Therefore, the slow cooling to 150˚F involved in the above method means that more oxidation occurs than if the coffee were chilled immediately.

My response: When I brew hot coffee, I don't drink it when it's over 150˚F, because at that temperature it's impossible to taste all its subtleties. (Also, I don't like to burn my tongue.) I (gasp!) let it sit for a few minutes before tasting. If the above objection were valid, it would imply that all the hot coffee I'm drinking is stale. But the fact is, even at high temperatures it takes a little while for oxidation to reach a noticeable level—at least 20 minutes. Therefore, there's no reason to believe that allowing coffee to cool somewhat before adding ice will make it noticeably oxidized, provided the coffee is served soon after chilling.

Other iced coffee resources (not all of which I agree with):
http://coffeegeek.com/opinions/coffeeatthemoment/09-10-2011
http://beansandwater.tumblr.com/post/22588752858/iced-coffee
http://blog.barismo.com/2011/08/iced-coffee.html

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