Bagging coffee quickly after roasting limits its contact time with oxygen.
THE SCIENCE AND FLAVOUR OF COFFEE
04
HOW FLAVOUR WORKS
The subject of coffee flavour chemistry is broad enough to warrant a book many times the size of this one, never mind a section of these humble proportions. Over 300 flavour constituents have been identified in green coffee to date, rising to around 900 in the roasted coffee bean through the formation of new aromatic molecules. In isolation these compounds each possess unique smell and taste properties, but their interaction with one another may form countless new aroma sensations.
Traditionalists will argue that the success of a brewed cup of coffee should be measured in its enjoyment. Are the character traits of the coffee reflected in the cup? Are there defected or off-notes in the character of the coffee itself? Is the extraction balanced, not too bitter and not too bright? Is there sweetness in the cup and does the flavour feel ‘clean’? The human body has reached a state of rare perfection when it comes to analysing the stuff we put in our mouths. There is no instrument, manmade or otherwise, that is capable of achieving the multifunctional feats of the human oral and olfactory system. You don’t need expensive equipment or a chemistry degree to answer these questions; drinking coffee regularly can make anyone an acceptable judge of cup quality.
But (and it’s a big but) in cafés and at home, it’s nice, if not essential, to be able to repeat great results over and over again; the rituals that we develop to make our favourite brews (‘infuse for this long’, ‘dose that much’, ‘stir for 20 seconds with this spoon’) aim to do this. Some of us are happy with the results that we achieve as long as they remain consistent; the knowing why is secondary to the hassle-free enjoyment of a good cup. But for many of us, the pursuit of perfection demands a greater understanding of why different practices produce different results, because it’s the understanding why, that can unlock the secrets of how to make that great cup even better.
SWEETNESS
Sweetness is a confusing concept when it comes to coffee. On the one hand, it is estimated that over 50 per cent of coffee drinkers in the UK (and around 35 per cent in the US) add sugar to their coffee, but on the other hand we use words like ‘caramel’, ‘chocolate’, ‘nougat’ and ‘brûlée’ to describe an unsweetened cup.
A roasted coffee bean comprises approximately 0.2 per cent sugars, a relatively small amount by itself, and even smaller once brewed with water, meaning that a typical cup of French press coffee is only around 0.06 per cent sugars. I place the word sugars in italics, since we are not necessarily talking about the everyday white granules that come in a bag, but other longchain polysaccharides that exhibit some sugary characteristics, but with far less actual sweetness than the type of sugar we are familiar with.
A cup of coffee is not inherently sweet, but familiar ‘sweet’ aromatics created during roasting (see pages 54–57), coupled with trace amounts of complex sugars and caramels, give a perception of sweetness in the cup. A higher viscosity, i.e. thickness in the cup, likely amplifies this perception, too.
I personally see sweetness as an excellent objective for any kind of coffee brewing, with it seeming to peak at just the moment where all other contributing forces balance perfectly.
BITTERNESS
Bitterness is commonly used as a scapegoat for all manner of imperfections in a brew. For many of us, if a coffee tastes bad, it’s because it’s bitter. This is a little unfair, however, since the sensation which we describe as bitterness is often that of astringency or sourness, and in the case of espresso-based drinks the source of bitterness comes not from the coffee itself, but from overcooked (or ‘burnt’ milk).
Two compounds, trigonelline and quinic acid (the same stuff that provides tonic water with its bitterness), are thought to contribute much of the bitterness in coffee. Caffeine, though assumed to be flavourless, actually tastes bitter too (see page 66).
By itself, bitterness is an unpleasant sensation, but it can do a great job of focusing sweetness and taming acidity when balanced correctly. It’s the structure that bitterness offers that brings order to the other elements of a cup of coffee.
The presence of bitterness in the cup is often down to overextraction of the coffee. This means that a very slow espresso extraction, or a very long French press brew will produce a more bitter coffee. Likewise, a very quick extraction will exhibit low levels of bitterness. The grind size, brewing temperature and water (see pages 68–69) will also affect this, as will the darkness of the roast, since darker roasts have a higher solubility and in turn produce a more bitter cup. It seems plausible that bitter compounds are slower to extract than those of sweetness and acidity, but once they do come through, it is easy for them to dominate a cup.
ACIDITY
Acidity might sound scary, but it is a very important element in a good cup of coffee, providing fruitiness, juiciness, roundness and one of my favourite features: refreshment! Those at the early stages of coffee discovery often find it quite surprising how clean, grippy acidity can set apart a cup of well-brewed coffee from what they might be used to.
High-acidity coffees tend to be grown at higher altitude and tend to have undergone the fully washed processing method (see pages 30–31). Some coffee producers, from countries like Kenya and Colombia, have built a reputation for producing bright, acidic coffees.
There are many different acids in roasted coffee, but the most abundant are citric (also found in citrus fruits), malic (also found in apples), lactic (found in dairy products) and acetic (vinegar). However, the concentration of acids within a cup of coffee is not an accurate gauge of acidity, since the buffering effect of salt, as well as the presence of bitterness and sweetness can all skew our perception of acidity.
A study by the Technical Unit of the International Coffee Organization in 1991 showed that grind size, brew time and brew temperature all have an effect on extraction of the 30 or so acids in roasted coffee. As you would expect, a finer grind produces a slightly more acidic coffee, but with increasingly higher temperatures and longer brew times, the concentration of acids peaks before 100°C/212°F and 14 minutes, respectively. What this means is that with very long brews and very high-temperature brews, some acids are denatured or destroyed altogether, reducing their overall concentration.
The acid concentration, or pH, of roasted coffee changes through the course of the roast. Green coffee generally has a pH of around 5.8, which drops to approximately 4.8 on or around the time of first crack.
AROMA
The concept of aroma is not limited to the simple act of smelling coffee as it wafts over towards us. Retronasal smell, or the smell that we experience as we breathe out through our noses when holding a mouthful of coffee is also a hugely important factor. Once combined with the sensory input from the tongue concerning taste, texture and temperature (as well as other stimuli), the brain is able to devise an accurate representation of the coffee, these are slight chemical changes in our grey matter, but a powerful emotive experience to our consciousness.
If you’ve ever held a drink in your mouth while holding a thumb and finger over your nose, you’ll have found that flavour is pretty boring when the nose isn’t involved. It is the hundreds of volatile aromatics in coffee interacting with the olfactory epithelium (one of the specialized types of tissue) in our noses that gives us the larger chunk of a coffee’s flavour profile.
The breadth and complexity of such things is certainly beyond my own understanding, and superfluous to the scope of this book. But in summary, many of the aromatic qualities that we associate with a cup of coffee come from the furan family (aromas including toffee and bran), pyrazine family (aromas such as earthiness, vegetal, walnut), and thiazole family (aromas like toast, nuts and brown meat).
THE SIGNIFICANCE OF SCALES
I think it goes without saying that taking time to accurately measure and record your adventures in coffee will make for tastier brews and improved co
nsistency in the future. Working with digital scales instead of measuring jugs/pitchers or spoons helps this process as it’s faster, more precise, less messy and requires no additional equipment. Scales are actually a more appropriate way of measuring things for all areas of culinary arts, but in the case of coffee they are notably of great use when directly comparing the mass of solids and liquids for brew ratio and extraction yield calculations (see pages 69–71). And indeed, a good set of scales has never been more imperative than in the minefield of espresso coffee, which presents challenges when attempting to do things without scales and by volume alone, since the dissolved gases that are part and parcel of the nature of the brewing method can give the illusion of a larger volume of liquid than is actually the case.
A good set of scales (or even three) will quickly become your best friend on the journey to tasty coffee.
With that in mind, if you take a flick through the later pages of this book, you’ll see that almost every brew method and recipe calls for ingredients to be weighed rather than measured. It might feel awkward at first, but I promise that once you get used to it, you will never look back – and that goes for cooking, too.
A WORD ON IMPERIAL AND METRIC
I have become increasingly disenchanted with the imperial system over the past years for a number of reasons. Firstly, pints, cups, pounds and ounces fall very short of the mark when it comes to precision measurement, with some areas of coffee requiring measurements in 1/100 of an ounce. Secondly, the metric system is also vastly superior in its ability to scale measurements by only changing the prefix – 140 g is equal to 0.14 kg, for example – and calculating brew ratios.
I recommend acquiring two sets of digital scales (although it is possible to get by with only one): one for measuring small quantities of coffee grounds (this scale should have increments of 0.1 g) and one for measuring larger quantities, such as water in a pour-over brew (this scale will get by with 1 g increments but should be capable of measuring up to 5 kg). Scales needn’t be expensive – both sets should cost you no more than £30/$47, but do read the reviews. Also note that scales need to be serviced from time to time to keep them on the money, so get a set of calibration weights for this.
CAFFEINE
The term ‘caffeine’ was originally coined by the German chemist F. F. Runge in the mid-19th century. The word itself is conjunction of the German kaffee (coffee) and the chemical suffix -ine. The French translated it to caféine and, in turn, the English caffeine. An alkaloid (a naturally occurring chemical compound, many of which are stimulants), caffeine, is designed to be a natural deterrent against insects, but I suppose the added bonus is that larger animals (who readily digest coffee beans and prepare them for germination) seem to love the stuff. Almost all plants containing mind-altering alkaloids are grown in the tropics, where the competition for survival is so fierce that the plants have developed increasingly elaborate ways to defend themselves. Caffeine can be found in over 60 other plants besides coffee, including tea, kola nuts, yerba maté (a leaf similar to tea, popular in South America), and guarana berries, to name a few – but given the word’s etymological origins, it’s is fair to say that the very definition of caffeine is ‘coffee chemical’.
And that ‘coffee chemical’ is now the most widely consumed drug in the world, surpassing that of nicotine and even alcohol. New Scientist magazine state that around 90 per cent of North Americans consume caffeine on a daily basis (approximately 75 per cent of which is coffee). It’s mankind’s obsession with caffeine’s stimulating effect that has led to an increasing number of consumer products containing a synthesized form of the drug, from the obvious sodas and soft drinks through to the new wave of energy drinks, as well as ice cream, gum and even shaving gel.
A 200-ml/7-fl. oz. cup of filter coffee can vary wildly in terms of caffeine content, but may typically contain around 120 mg (0.12 g) of caffeine, or 600 mg/litre. However, it is estimated that 120,000 tons of caffeine are consumed globally every year - equivalent to up to 800 billion cups of coffee. Caffeine typically accounts for between 1.2–2.5 per cent of the dry matter of green coffee and around 0.7 per cent of the dry coffee cherry. The robusta species has, in the past, been celebrated for its overinflated caffeine content, and this is indeed true, but evidence suggests that other growing factors, variety and processing methods also have a part to play. Recent studies seem to agree that darker roasts do cause some sublimation of caffeine molecules, which have a boiling point of 178°C/352°F, but this is offset by an overall reduction in bean weight and density, so the effect is somewhat negated. Besides, anyone selecting coffee based on caffeine content and its correlation to roast profile would be somewhat missing the point.
Since caffeine is highly soluble, the brewing technique, within reason, makes little or no difference to the caffeination of the final beverage. Of course, the brew ratio (ratio of coffee to water) will affect the mg/litre of caffeine in the final drink, and for this reason it’s better to relate caffeine content to the grams of coffee that you are consuming, rather than the volume of the drink. The common belief is that caffeine is flavourless, but this is in fact incorrect. Pure caffeine has a very strong bitter flavour and even its modest presence in coffee is thought to provide around 10 per cent of coffee’s bitterness – certainly enough to subtly differentiate the beverage from those absent of caffeine.
Our bodies metabolize caffeine readily and the effects can be felt almost immediately, but it typically reaches peak performance after about 30 minutes; after three hours, the effects are half what they were at the point of peak performance. It can take over 12 hours for the drug to leave your system altogether, but the exact time varies according to lots of other factors: hydration, food, exercise, smoking, alcohol, race, age and even gender. In The World of Caffeine: The Science and Culture of the World’s Most Popular Drug, authors Bennett Alan Weinberg and Bonnie K. Bealer muse that a nonsmoking Japanese man drinking his coffee with an alcoholic beverage would likely feel caffeinated ‘about five times longer than an Englishwoman who smoked cigarettes but did not drink or use oral contraceptives.’
The psychological and physiological influence that caffeine has on the brain and sympathetic nervous system is of such an acute precision that it almost seems a shame not to regularly exercise it. Caffeine first disarms the natural mechanisms that cause us to feel drowsy, and then takes command, firing neurons that stimulate the pituitary gland, raising our heart rate and causing a sudden release of blood sugars and adrenaline.
It achieves this by being chemically similar to the molecule adenosine, which tricks neurons (nerve cells) in the brain. Adenosine appears in higher concentrations when we have expended lots of energy and clings to neurons like expanding foam, causing the slow-down of brain functions and the feeling of tiredness. Caffeine behaves like an evil-twin, however, fooling the neurons and latching on in place of adenosine. This anomaly is known as ‘competitive inhibition’ because the caffeine is inhibiting the effects of adenosine by competing with it for places on the neurons’ adenosine-receptors.
With the impostor in place, one might think that the neuron would behave normally, but the kicker is that caffeine has the opposite effect of adenosine, and actually provokes the neuron into firing faster than usual. The pituitary gland, a pea-sized gland at the base of the brain, becomes aware of this and stimulates the sympathetic nervous system into ‘fight or flight’ mode – an emergency protocol that dilates the pupils, increases heart rate, and releases sugar stored in the kidneys. It’s this two-pronged attack mechanism that makes caffeine such an effective stimulant.
Since as far back as the 1800s, the negative effects of caffeine in coffee have been cited by coffee’s opponents, such as Charley Post, the inventor of the caffeine-free ‘healthy’ coffee alternative known as Postum, also known as ‘America’s Favorite Coffee Substitute’. Like most drugs, caffeine use (and abuse) is open to potential side-effects, along with the danger of withdrawal symptoms. Those of us who have developed
a tolerance to caffeine have actually developed additional adenosine receptors – our bodies’ way of giving the drowsy molecule a better chance of slowing us down when we deserve a rest. No problem if you’ve plenty of coffee on tap, but it can spell bad news for caffeine junkies unable to get a fix, since the adenosine will have a greater number of receptors to bind to and the feeling of tiredness can be amplified – in other words, you feel withdrawal. Very heavy caffeine consumers may even experience mood swings when deprived of caffeine, since the drug is linked to the production of serotonin, which regulates such things as mood, appetite and sleep.
Clinical trials have failed to find lasting negative effects from overconsumption of caffeine, however, but a great deal of new research seems to indicate that caffeine might be a powerful positive force in the fight against various mental illnesses. It has for some years been suggested (and subsequently proven) that caffeine slows the memory decline in old age, but more recent studies have found that moderate caffeine intake in mice prevents brain-cell deposits of a protein that is a specific hallmark of Alzheimer’s. Trials also indicate that caffeine may be a powerful preventative and therapeutic drug in the fight against Parkinson’s disease.
WATER
Discussing the quality of water you use in coffee normally provokes a roll of the eyes from those who have yet to discover the monumental impact it really has. Water fulfils two roles in coffee brewing: first as an ingredient, where it represents at least 90 per cent of the cup of coffee, and second as a solvent, where it is used to extract soluble flavour from ground coffee.
The first of these roles can easily be managed by a simple taste test; if your water tastes bad before you brew with it, it will almost certainly be reflected in the resulting cup. Water used to make coffee should contain no chlorine taste, so I’d advise using bottled mineral water in your kettle instead of water straight from the tap/faucet, or filtering your water before using it. Either of these steps will help to improve the flavour of your coffee.
The Curious Barista's Guide to Coffee Page 9