The science behind… coffee!

Brewing a great cup of coffee depends on chemistry and physics

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What can you do to ensure a more perfect brew?
Chris Hendon, CC BY-ND

Christopher H. Hendon, University of Oregon

Coffee is unique among artisanal beverages in that the brewer plays a significant role in its quality at the point of consumption. In contrast, drinkers buy draft beer and wine as finished products; their only consumer-controlled variable is the temperature at which you drink them.

Why is it that coffee produced by a barista at a cafe always tastes different than the same beans brewed at home?

It may be down to their years of training, but more likely it’s their ability to harness the principles of chemistry and physics. I am a materials chemist by day, and many of the physical considerations I apply to other solids apply here. The variables of temperature, water chemistry, particle size distribution, ratio of water to coffee, time and, perhaps most importantly, the quality of the green coffee all play crucial roles in producing a tasty cup. It’s how we control these variables that allows for that cup to be reproducible.

How strong a cup of joe?

Besides the psychological and environmental contributions to why a barista-prepared cup of coffee tastes so good in the cafe, we need to consider the brew method itself.

Science helps optimize the coffee.
Chris Hendon, CC BY-ND

We humans seem to like drinks that contain coffee constituents (organic acids, Maillard products, esters and heterocycles, to name a few) at 1.2 to 1.5 percent by mass (as in filter coffee), and also favor drinks containing 8 to 10 percent by mass (as in espresso). Concentrations outside of these ranges are challenging to execute. There are a limited number of technologies that achieve 8 to 10 percent concentrations, the espresso machine being the most familiar.

There are many ways, though, to achieve a drink containing 1.2 to 1.5 percent coffee. A pour-over, Turkish, Arabic, Aeropress, French press, siphon or batch brew (that is, regular drip) apparatus – each produces coffee that tastes good around these concentrations. These brew methods also boast an advantage over their espresso counterpart: They are cheap. An espresso machine can produce a beverage of this concentration: the Americano, which is just an espresso shot diluted with water to the concentration of filter coffee.

All of these methods result in roughly the same amount of coffee in the cup. So why can they taste so different?

When coffee meets water

There are two families of brewing device within the low-concentration methods – those that fully immerse the coffee in the brew water and those that flow the water through the coffee bed.

From a physical perspective, the major difference is that the temperature of the coffee particulates is higher in the full immersion system. The slowest part of coffee extraction is not the rate at which compounds dissolve from the particulate surface. Rather, it’s the speed at which coffee flavor moves through the solid particle to the water-coffee interface, and this speed is increased with temperature.

The Coffee Taster’s Flavor Wheel provides a way to name various tastes within the beverage.
Specialty Coffee Association of America, CC BY-NC-ND

A higher particulate temperature means that more of the tasty compounds trapped within the coffee particulates will be extracted. But higher temperature also lets more of the unwanted compounds dissolve in the water, too. The Specialty Coffee Association presents a flavor wheel to help us talk about these flavors – from green/vegetative or papery/musty through to brown sugar or dried fruit.

Pour-overs and other flow-through systems are more complex. Unlike full immersion methods where time is controlled, flow-through brew times depend on the grind size since the grounds control the flow rate.

The water-to-coffee ratio matters, too, in the brew time. Simply grinding more fine to increase extraction invariably changes the brew time, as the water seeps more slowly through finer grounds. One can increase the water-to-coffee ratio by using less coffee, but as the mass of coffee is reduced, the brew time also decreases. Optimization of filter coffee brewing is hence multidimensional and more tricky than full immersion methods.

What do they know that we don’t?
Redd Angelo on Unsplash, CC BY

Other variables to try to control

Even if you can optimize your brew method and apparatus to precisely mimic your favorite barista, there is still a near-certain chance that your home brew will taste different from the cafe’s. There are three subtleties that have tremendous impact on the coffee quality: water chemistry, particle size distribution produced by the grinder and coffee freshness.

First, water chemistry: Given coffee is an acidic beverage, the acidity of your brew water can have a big effect. Brew water containing low levels of both calcium ions and bicarbonate (HCO₃⁻) – that is, soft water – will result in a highly acidic cup, sometimes described as sour. Brew water containing high levels of HCO₃⁻ – typically, hard water – will produce a chalky cup, as the bicarbonate has neutralized most of the flavorsome acids in the coffee.

Ideally we want to brew coffee with water containing chemistry somewhere in the middle. But there’s a good chance you don’t know the bicarbonate concentration in your own tap water, and a small change makes a big difference. To taste the impact, try brewing coffee with Evian – one of the highest bicarbonate concentration bottled waters, at 360 mg/L.

The particle size distribution your grinder produces is critical, too.

Every coffee enthusiast will rightly tell you that blade grinders are disfavored because they produce a seemingly random particle size distribution; there can be both powder and essentially whole coffee beans coexisting. The alternative, a burr grinder, features two pieces of metal with teeth that cut the coffee into progressively smaller pieces. They allow ground particulates through an aperture only once they are small enough.

Looking for a more even grind.
Aaron Itzerott on Unsplash, CC BY

There is contention over how to optimize grind settings when using a burr grinder, though. One school of thought supports grinding the coffee as fine as possible to maximize the surface area, which lets you extract the most delicious flavors in higher concentrations. The rival school advocates grinding as coarse as possible to minimize the production of fine particles that impart negative flavors. Perhaps the most useful advice here is to determine what you like best based on your taste preference.

Finally, the freshness of the coffee itself is crucial. Roasted coffee contains a significant amount of CO₂ and other volatiles trapped within the solid coffee matrix: Over time these gaseous organic molecules will escape the bean. Fewer volatiles means a less flavorful cup of coffee. Most cafes will not serve coffee more than four weeks out from the roast date, emphasizing the importance of using freshly roasted beans.

One can mitigate the rate of staling by cooling the coffee (as described by the Arrhenius equation). While you shouldn’t chill your coffee in an open vessel (unless you want fish finger brews), storing coffee in an airtight container in the freezer will significantly prolong freshness.

The ConversationSo don’t feel bad that your carefully brewed cup of coffee at home never stacks up to what you buy at the café. There are a lot of variables – scientific and otherwise – that must be wrangled to produce a single superlative cup. Take comfort that most of these variables are not optimized by some mathematical algorithm, but rather by somebody’s tongue. What’s most important is that your coffee tastes good to you… brew after brew.

Christopher H. Hendon, Assistant Professor of Computational Materials and Chemistry, University of Oregon

This article was originally published on The Conversation. Read the original article.

Wasp research dilemmas

We faced abuse for asking people to kill wasps for science – here’s why it was worthwhile

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Adam Hart, University of Gloucestershire and Seirian Sumner, UCL

When we launched a citizen science project earlier this year, we didn’t expect to get in so much trouble.

We wanted to public to help us find out more about social wasps (the kind that bother us at picnics and BBQs) and so we launched the Big Wasp Survey. Social wasps are essential pest-controllers and pollinators, but some species are declining while others are expanding their populations and range. Without basic data on the abundance and distribution of these wasps, we can’t conserve (or control) them.

Yet we know relatively little about social wasps in Britain. So we asked the public to set out beer-filled traps for a short period of time when mostly old and soon-to-die worker wasps would be active. This approach would provide essential data that we need to manage social wasp populations. But beer traps kill wasps, and that seemed to upset a lot of people.

Asking the public to kill wasps in the name of science led to high–profile national media condemnation. But our negative experiences were relatively mild – some scientists studying invertebrates have been subjected to torrents of social media abuse for “killing in the name of science”.

It seems our study played into an old stereotype of an entomologist as a Victorian-style net-wielding naturalist, capturing and killing six-legged victims that are then pinned and banished to dusty drawers. More a lethal stamp-collector than a scientist.

Outdated stereotype.
Shutterstock

The reality is modern entomologists are involved in science that underpins pressing societal and environmental issues including medicine, genetics, ecology and climate change. Unfortunately, this research still relies on killing insects, a practice accepted as a necessary evil by scientists but easily criticised by others, as we found.

There are three main reasons why entomologists sometimes have to kill what they study. First, many insects can only be identified by microscopic examination, for example by the shape of their genitalia. A photograph simply isn’t enough for this. We need a dead specimen.

Second, we often need a lethal approach to catching insects, using techniques such as pan traps (open pans of water) or pitfall traps (sunken traps filled with fluid to kill and preserve insects that fall in). Otherwise it’s much too difficult to catch them.

It’s a trap!
Shutterstock

Finally, scientists have learnt a great deal about some important and fundamental aspects of biology and medicine by killing insects. Data on the effects of agriculture, habitat change, the effects of pollution, predator-prey dynamics, and the ecological effects of climate change come directly from studies that leave dead insects in their wake.

The field of genetics would also be nowhere without the fruit fly, which have died in their billions to provide DNA samples in our quest to unravel the fundamental mechanisms of life. Likewise, the American cockroach, the Indian cricket and the mosquito have all died to develop our understanding of nervous systems, ageing, development and disease.

In the case of the Big Wasp Survey, relying on untrained citizen scientists to observe wasps without killing them wasn’t an option. We needed a standard method that everyone could follow and it isn’t possible to reliably observe and count individuals without trapping them. Although there are only eight common species of social wasp in the UK, it’s surprisingly difficult to identify them from living specimens. Without proper wasp identification, our study would be scientifically obsolete.

If we can collect a colony’s worth of wasps we can generate fundamental science to help manage and conserve these important insects. But, again, this would be completely impossible without the actual (dead) specimens for us to accurately identify and use to find out which species are where. We also couldn’t develop any additional research, such as looking at how wasp colour varies in different places, which might reflect pollution levels.

Reduce, refine, replace

Biological research on vertebrate animals (such as fish, mammals and birds) is underpinned by the environmental principle of the Three Rs (reduce, refine, replace). Insect scientists also adopt this principle where they can.

For example, you can use statistical maths to work out the minimum number of individuals (or samples) required to test a particular theory. Improved photography can let us identify some insects such as butterflies without killing them. We can even now use non-lethal methods to take minute quantities of DNA from some insects, allowing us to identify them without killing them.

Every day, billions of insects die splattered on vehicles, poisoned by insecticides or casually swatted for no scientific benefit. In contrast, the tiny number killed by entomologists help us to understand, among many other things, genetics, disease and ecology. The Big Wasp Survey has already collected data from several thousand locations across the UK, engaged millions of people with the value of social wasps and sparked off a number of potential new scientific collaborations with ecologists across Europe.

The ConversationEntomologists have long been troubled by the need to kill insects, and are seeking ways to reduce, refine and replace fatal sampling and identification methods. In the meantime, and in the face of censure and condemnation from those that do not understand the science, entomologists will have to continue to kill insects to make meaningful scientific advances.

Adam Hart, Professor of Science Communication, University of Gloucestershire and Seirian Sumner, Reader in Behavioural Ecology, UCL

This article was originally published on The Conversation. Read the original article.