Thoughts and Theories on Iced Coffee


As this summer’s crazy swelter continues into September, many lovers of great coffee have been turning their attention to iced coffee and espresso beverages. Articles are popping up everywhere, even by the New York Times on the subject. While there are plenty of how tos online (we’ve covered this subject on CoffeeGeek in the past too), there’s not much actual investigation or explanations of the process of making iced coffee and how it is different from brewing coffee and espresso the more traditional way.

Heat vs. Cold

In brewing and tasting coffee or espresso, there are some important fundamental differences to understand when it comes to temperature extremes. Two specifics come to mind: how coffee extracts with heated water vs. cold water, and how coffee tastes when hot vs. cold.

Brewing temperatures
Literally hundreds of years of scientific examination of the extraction of an elixir from the mixing of water with ground coffee has concluded there is an ideal temperature range required to yield a well balanced, good tasting cup of coffee (or espresso). We even featured an article recently from 1918 that talked about the importance of pretty precise brewing temperatures close to (but not exceeding) water’s boiling point.

The actual temperature of water used to brew coffee depends on the type of beans, the roast of the beans, the age of the roasted coffee and the brewing environment, but the common held belief (backed up by a lot of science including home tyros using devices like the Extract Mojo refractometer and software) is that coffee’s ideal brewing water temperatures range from 195F to 205F. Above this temperature and you tend to extract too many bitter-tasting components that can ruin the cup and create unpleasant aromas. Below this temperature and you can extract too many perceived sours, limiting the minuscule sugars the ground coffee has to give.

So, knowing that lower water temperatures produce sours, what’s up with brewing with cold water? Wouldn’t that just amplify sour tastes? The short and simple answer is, not necessarily, because of how traditional cold brewing is done. We’re talking about things like dwell time, how heat (or lack thereof) causes different reactions in the bed of coffee, and more.

Unlike hot brews, which take place over a range of 60 seconds (small siphons, aeropress, other devices) to upwards of 5-8 minutes (larger volume pourover / drip coffees) cold brew coffees are usually produced using extremely long extraction times. The Toddy method (essentially a large vessel, filled with water and lots of ground coffee that sits in your refrigerator for a day) does an extremely slow extraction that manages to coax out residual sugars and mute acidic content (which like hotter temperatures to fully extract). Ice drip brews also run a long time – upwards of 4 hours or longer and are categorized by controllable drips of water, sometimes as slow as one drip per second or longer. While this brewing style is a flow through method and not a full immersion method, again you’re dealing with an extraction time measured in hours and not minutes.

Click for larger image
Ice at the ready
The ice in a slow-drip ice coffee brewing tower.

I don’t know of any indepth scientific studies into the effects and details on extraction using these cold brew methods (anyone know of any? leave a comment in our forums); almost everything I’ve read on the subject is based on empirical evidence, tasting evidence and the like. The general concensus about ice brewed or cold brewed coffee is the following:

  • results in a low-acid cup. Some people refer to this as a soft cup or mellow cup, with virtually no bright acidity present in the taste, even when using notoriously high-acid coffees like Kenyas or brighter Centrals.
  • cold or ice-brewed coffee has a bit of a shelf life, unlike traditional hot-brewed coffee does. Some very experienced palates in the world of specialty coffee claim that cold or ice-brewed coffee can be kept in the refrigerator for up to 72 hours or more. My opinion, based on taste, is it has a 24 hour shelf life.
  • the coffee can be produced in a more concentrated format since the extended brewing times can lead to greater overall extraction and form a more concentrated brew. We’re not talking espresso concentration levels here, but as much as a 2:1 ratio or more compared to the normal viscosity and taste of a hot brewed cup. This allows for more dilution with ice while still retaining a good viscosity and depth to the beverage.
  • body-rich coffees (heavy on spices, chocolate notes, etc) tend to do the best in cold and ice-brew coffees; that said, fruity-acidic coffees also do well because the fruits can sometimes carry through to the cup while leaving behind the acidity. Some people refer to these brews as “juicy tasting” cups.

So getting back to the question of sour coffees. If brewing slightly under temperature (say 180F vs 195F) results in perceived sourness and weird acidity in a cup of coffee, why isn’t this amplified the colder you brew?

I have a theory. At different temperatures for the brewing water, different taste-contributing elements in the ground coffee are extracted at different levels. In addition, temperatures can cause (or mute, or not cause) chemical reactions in the complex genome that is roasted coffee. Perhaps at 180F brewing, acids can’t interact enough with fats and lipids to form  a pleasing acidity – instead they come out as perceived sours. At 198F, the balance might be perfect for fats, lipids, oils, sugars and the acidity to deliver balance. Take it down to 40F or colder, and different things happen – oils don’t boil; fats don’t liquify, but acids never move a micron or change to another state, resulting in a low (or no) acidic taste and the non-boiled oils result in a creamy, mellow taste that does get extracted.

Cooling Down Hot Coffee Quickly

All this covers ice brewed and slow, cold brewing methods. But what if you brew traditionally, and want the beverage cooled down quick? There is that option too, and some industry luminaries like George Howell and Peter Guiliano feel this is the best method to make cold coffee. It is important to note that some interesting chemical changes can take place with complex liquids like coffee when you essentially do a “force chill”. Personally, I really enjoy iced americanos during the summer, but I’ve developed a process over the years getting hot shots of espresso cooled down to 2-4C (around 35-38F) so as to minimally affect the cup taste.

When you brew hot, you get most of the benefits and tastes those brewing methods provide, and as Howell has said previously, brewing hot and chilling rapidly helps conserve the coffee’s terroir in the cup. But you can affect and possibly even damage the taste depending on how you cool down the beverage. When we did a lot of experimenting in the CoffeeGeek Lab last year on rapid cooling of coffee (and espresso) brews, we noticed espresso had different reactions to a fast chilling when compared to filter or press coffee. Crema probably plays a big role in this taste change (I notice a metallic twang from too-rapidly cooled espresso) which some have taken to calling “espresso shock”, though others in the industry disagree there’s any kind of shock to espresso when it is chilled too fast.

Again, I have a theory on this. “Phase Change” is when an element changes from one state (solid, liquid, gas) to another. Add to this the fact that different elements have different phase change states: various oils may have a lower or higher boiling point than water. Fats can liquify at lower temperatures than others. Then there are chemical reactions induced by temperature changes. Think about milk and the steaming of it. Did you know that at around 100F, there is a very quick conversion of the milks’ sugars (lactose) into other elements? Some of it converts to alcohol; some converts to carbon dioxide. The sugar “taste” goes away quite quickly in a very short temperature span.

Thinking about how milk can lose one of its key elements so quickly makes me think you can induce good (or bad, as is the case here) changes in taste with a rapid change in the temperature of espresso.

Because of this, when you brew hot espresso, I believe it is important to cool the shots down in controlled stages. I do it as part of my drink build: the espresso is brewed into cold espresso cups. the shot(s) are then transfered over to a large glass vessel (my iced americanos hold two doubles, or about 60-70g of brewed espresso). Then cold milk is added. Then perhaps 50ml of cold water. Next in is a sweetener (more on that below). Then ice is added and the beverage is stirred (some prefer to blend at this point in a machine). Taste, and usually top off the glass with more cold water to adjust overall taste, balance and viscosity of the beverage.

I find the iced americano, black or with milk, to be the best tasting iced coffee beverage I can make. It has many of the nuances I enjoy from traditional espresso, including a bit more zing and sharpness in things like brightness, specific flavour profiles and characteristics of the espresso’s body. Most ice-brew and cold brew coffees just taste like “coffee, cold” to me, but iced americanos can retain some of those great individualistic tasting notes present in the hot shot. That said, I do notice more flavour nuances making it into the cup when doing a hot concentrated brew of coffee onto ice than I do when brewing the toddy or ice-drip method.

This more traditional method certainly is a quick method – brewing with hot water, using more coffee grounds, ground a bit finer, to create a more concentrated brew. Ice usually fills the vessel the brewed elixir drips into. You get more of the coffee’s individual flavours (or terroir, as Howell puts it) in the cup, but the downside to this way of brewing is the coffee seems to have a much shorter shelf life – you brew it and drink it – forget about putting the concentrate in the fridge for extended periods.

Heat vs. Cold, Taste Considerations

There’s another element at play when it comes to drinking cold coffee or espresso. Perceived sugars and sweetness.

Our ability to perceive sweetness in anything we ingest increases the warmer the food (or beverage) is – at least until the point where the beverage is so hot that sugars convert into other chemicals (like the aforementioned changes to lactose that occur when steaming and frothing milk). The same is true in reverse: the colder a food (or beverage) item is, the more difficult it is for our taste senses to perceive the sweet elements in that food or beverage item.

You can test this yourself with ice cream. When you eat ice cream at serving temperatures, it is quite sweet (to most people) but in a very pleasing way. Let that ice cream melt to a liquid state, and for most people it is too rich, too sweet to enjoy. The same holds true for soft drinks – at serving temperatures, the majority of people perceive the taste as being sweet, but appropriately so. Served at room temperatures, many people will find the sweetness levels excessive to their taste.

Now try another tact. Take fresh, locally picked strawberries. Leave some out on the counter to settle down to room temperature, and put others in the refrigerator to chill down to 2-4C. Taste both samples. The room temperature strawberries can have a very pleasing mix of acidity and fructose-driven sweetness. The chilled strawberries from the same batch can have more of a tart, less sweet-forward taste.

Now think about milk – specifically breast milk. At its normal serving temperature (which is only a a degree or two cooler than your body temperature) the lactose-driven sweetness is at its maximum zenith, and very appealing to newborn babies. The instinctual drive for that sweetness is what makes milk so appealing for babies as a form of nutrition and substance. But when milk is cold, it only contains a (perceived) fraction of that same sweet taste.

Coffee behaves the exact same way: at certain hot temperatures, the (miniscule) residual sugars can be perceived in taste for most palates, and play a nice balancing game with bitters and acidics. But when you cool a beverage down, those three primary taste influences change (nb- all of these assume the original beverage was prepared traditionally – ie, with hot water):

  • acidity is the wildcard – some acidic elements can become more pronounced in cooler temperatures, others can become more muted in cooler temperatures.
  • bitters become more pronounced with cooler temperatures
  • sugars become less pronounced with cooler temperatures

These three conditions are pretty much universal for all taste senses and experiences, but especially valid for coffee. They do not take into account how the bitter, acidic, and sweet elements are first created in the brew (ie, hot vs. cold vs. ice brews) but only relate to the sweet, bitter and acidic elements present in the cup.

This summer, we were witness to some folks in the specialty coffee community debating the addition of a sugar (sweetener) component to a cold brewed coffee or espresso. The knee jerk reaction is “how dare you add sugar!” stemming from the occasional derision about adding sugar to a hot brewed coffee or a shot of espresso. Others, especially those who have a more keen understanding on what temperature does to taste perception, have no problems with adding sweeteners to a cold coffee or espresso beverage, but with a caveat: Instead of always just adding outside sugars to the cold brewed drink, some are  seeking ways and methods to make the sugars existing in the coffee brew more pronounced, so when chilled, the the coffee’s built in sweetness still seems present.

I’m not sure if this is possible though, because of the role bitters play in serving temperatures: more pronounced bitters with less pronounced sugars is a big uphill battle to play when dealing with balanced taste in a cold beverage.

In short, if the goal is to re-establish the taste balance in the cup that you lose through different serving temperatures, the the addition of a sweetener is definitely acceptable. Keep these thoughts in mind while doing so:

  • different brewing methods require different levels of sweetener to bring the cup back to a similar taste experience when comparing to hot brewed and hot-served. For instance, the long extraction toddy method results in very low acid, low bitter (and, unfortunately, very minimal flavour nuances) coffee and wouldn’t require much sweetener at all to balance the cup. Espresso on the other hand can taste quite bitter when drunk at 2-4C, and may require a bit more sweetener than a toddy brew to balance the taste.
  • have a goal of achieving balance in the cup, and not producing a sweet, soft drink like experience when adding your sweetener. Instead of tablespoons of sugar, think 1/2 teaspoon or less. Seek balance, not “sweet”.
  • have a goal of tasting coffee first, everything else second in the cup. Taste the coffee brewed traditionally, and brewed meticulously well. Remember those tastes. Then experiment with various cold brewing methods and try to mimic that taste as close as possible. Use alternate sugar sources (fructose, various cane sugar types, syrups like algave, corn, even miniscule amounts of maple syrup) to try and achieve a similar taste experience to how the hot beverage is perceived by you.
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One Response to Thoughts and Theories on Iced Coffee

  1. Kris says:

    This is one of the more sensible and in dept articles I’ve read and does a great job of explaining on human perception, I would love to know what a proper iced espresso would consist of in shops, mind you do you pour espresso into a coffee cup, paper cup, and then put ice in, since plastic will react with hot espresso, do you ask for a cold porcelain cup, my issue is a short iced americano, the water is usually warm not cold, and unlike a porcelain cup that is cooler or even warm, you are adding and literally “diluting” the espresso, which may or may not be a bad thing, preference wise but its still there, by contrast when you pour with just a few ice cubes, the coffee will either melt quickly , slowing, or instantly cool depending on the amount of ice, and what about multiple shots?

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