(Article from CoffeeScience.org)
If you are a coffee person (most of us are, anyway), your day probably won’t start before you brew that one tasty cup of sweet-smelling morning coffee, or grab one on your way out at your favourite coffee shop. The taste of your cup of coffee is influenced by many things, top among them the mineral makeup of the water used, the quality of coffee beans and the process the cup is made.
According to researchers at the University of Bath, water composition has a dramatic effect to the ultimate taste of coffee, with low levels of bicarbonate and high levels of magnesium being the key components in brewing a perfect cup of coffee. The study revealed that the mineral composition of water had the biggest impact in the extraction of six different chemicals. Water rich in magnesium improves the extraction of coffee bean flavours while sodium and bicarbonates, often found in bottled water, ruins the taste.
If you are wondering what makes the coffee you love so much a perfect drink to start your day, I am going to walk you through a journey of discovery that will completely change your understanding of coffee, how you make your coffee and the science behind the coffee.
As a coffee lover, you probably know that coffee beans have hundreds of different chemicals in them. The precise composition, however, depends on the type of bean, where it is grown, and how it is roasted. In the study done by researchers at the University of Bath, we learn that water composition is important to ensuring the right proportions of starches, bases, sugars and acids extracted from the coffee bean roasts. The flavour of a cup of coffee is determined by the level to which the water extracts the chemicals from the ground beans, a process that is largely influenced by the grinding process, temperatures, roast profile, brew time, and pressure.
Therefore, to make a perfect cup of coffee, you must ensure that you use the right quality of water. Hard water, or water with salt elements, is generally bad for coffee, but it is the type of hardness that matters. It is worthwhile noting that there is no one type of water that will consistently produce flavoursome extractions all roasted coffee; this explains why the flavour of your cup of coffee may vary widely even when brewed under almost identical conditions.
The best coffee brewing results can be attained by using different filtration techniques and manipulating reasonably hard tap water. Whether you are a diehard coffee drinker whose days cannot go well unless they start with a cup of coffee or just need to master the art of making the perfect cup, you must master the balance of the important variables that can make or break a perfect cup of frappé, long black, or latte.
Most of what we taste in coffee is actually smell. The only sensations that the mouth picks up are sour, bitter, salty, and umami. If you did not smell, the coffee, a cup would only taste sour or bitter due to the organic acids. If you doubt it, try holding your nose the next time you take your first sip.
The rich and satisfying taste of coffee comes almost entirely from the volatile compounds that are produced when the coffee beans are roasted. These compounds produced during the roasting process are a lot like other types of compounds formed when cooking food. For instance, the smell of bread is actually the smell of compounds that are made when a type of protein reacts with sugar in a process called Maillard reaction during baking.
There are about 800 different compounds produced during the thermal degradation reaction when coffee beans are roasted. Most of these compounds decompose proteins and sugar then react to form the volatile compounds that give a cup of coffee its unique smell. Most of the reactions take place inside the thick walls of the coffee bean cells, which, in their form, act as small pressure chambers. Each of the 800 compounds reacting to give coffee its aroma cause different responses to the olfactory membrane of the nose.
When you use unroasted (green) coffee beans to make your coffee, it will have a grassy taste. While the same organic compounds in roasted beans are present, and the coffee has about the same amount of caffeine, it lacks the desired sensation because there are fewer volatile compounds that are only released during roasting.
Roasted coffee has only about 20 major compounds, but it is the influence of the many minor compounds that give it it’s overall taste that we get to love so much. When the smells of these volatile compounds are analysed, they actually have a wide range of different odour qualities—compounds containing nitrogen e.g. pyridine have foul smells but others have a fruity smell. Others compounds have descriptors such as rancid and putrid. It is justifiable to conclude that the smell that you positively identify as “coffee” is a combination of hundreds of different smells given off by volatile compounds.
Now that we know what gives coffee its unique taste and smell, what are the variables that determine the results? With so many baristas with fine-tuned expertise of making espressos, it is hard to get a bad cup of coffee today. However, different people have different preferences as far as the taste of coffee goes. There are five important variables that you can control to brew the perfect cup of coffee:
Because the caffeine in water is highly soluble in water, most of it is extracted early. The volatile compounds and oils, which are responsible for the coffee cup’s complex aroma and flavour, are extracted more slowly. The organic acids that give coffee a bitter taste take the most time to extract. In this section, we will go through each of the four variables in turn to discover just how the perfect cup of morning coffee can be arrived at.
The coarseness of the coffee beans grind, and the duration of extraction are inextricably linked. The finer the coffee grinds, the greater the surface area in contact with water. Contrariwise, the larger the grind, the lesser the surface area.
This essentially means that if you grind coffee beans into fine powder, you maximise the surface area that is available for extraction, leading to a speedy extraction of the target compounds. This can be a good thing or a bad thing depending on how long the extraction time is. Turkish coffee is often fine-ground and boiled. This is what makes it very strong and bitter. The fineness also gives it the muddiness (suspended solids) that pass through coffee filters and even block filters, resulting in long extraction durations because water is not allowed to pass through freely enough.
Coarsely ground beans on the other end of the spectrum have a very small surface area in contact with water. Given enough time, it is possible to extract unground coffee, but it would be a wasteful enterprise because water will not penetrate the interior of the bean. In such a case, most of the elements will still be inside the bean when it is thrown out as waste material.
The optimum grind or the coarseness of coffee beans lies somewhere between the finely ground powder and the very coarse beans. The other element, the time that the hot water passes through the ground coffee, determines the strength of the cup of coffee. The more finely ground the beans, the more the caffeine, volatile oils, and organic compounds will be present in the cup of coffee.
A cup of coffee produced from good quality beans but is too insipid or weak means that the beans are too coarsely ground and/or there was a high flow rate. If the cup of coffee is extremely bitter, it contains too high levels of organic acids because the beans were too finely ground.
Supposing we keep all the variables except temperature constant, what would happen what would happen when we brew coffee at different temperatures?
Temperature greatly affects the solubility and rates of compound extraction. If you use ice water, you can still make a cup of coffee, but it would not be as rich as a cup made from boiling water and it would take ages just to make a decent cup. Cold brewed coffee is made by placing ground beans in the cold water then allowing it to brew inside the refrigerator for hours—even days because the extraction rates are very low.
Temperature moderately affects the solubility of caffeine and strongly influences the solubility of organic acids in water. You would, therefore, expect a cup of coffee brewed overnight in a fridge to have low caffeine levels and much lower bitterness compared to coffee brewed with boiling water. When you use boiling water, everything—caffeine and organic acids—are extracted much faster, resulting in high levels of caffeine and bitter organic acids.
The temperature variable has a complicating factor that must be taken into account when setting the temperature to brew coffee—the volatility of oils. When coffee boils, the aroma and flavour compounds evaporate away in the steam, resulting in coffee with a weak taste yet rich in organic acids and caffeine. This means that the perfect cup of morning coffee should not be brewed in extreme temperatures.
The ideal cup of coffee is one that has the maximum amounts of volatile oils and caffeine but limited amounts of bitter organic acids. Therefore, keeping the coarseness of the ground coffee beans and brewing temperature constant, you can influence the amounts of these elements by altering the duration of the brewing time. This is the length of time that the water is in contact with the ground beans to allow the oils, caffeine, and acids to dissolve.
If the extraction time is too short, you will get a cup of coffee that is rich in caffeine but has a weak flavour, bitterness, and aroma because the elements that influence the taste did not extract properly. If you were to extract coffee for too long, your cup would have high levels of organic acids, which makes it too bitter.
The right time a perfect cup of coffee should take to brew is determined through trial and error. Other variables on this list such as the temperature and coarseness of coffee beans, of course, play a part in determining the duration of extraction. To consistently make a perfect cup of coffee, you would have to keep all the other variables constant while experimenting with the duration of brewing for the same quality of coffee beans.
Considered the most important subject in making the perfect cup of coffee, the coffee-to-water ratio is essentially that—how much coffee for a unit of water. When there is too little amounts of coffee, even when all the other variables are optimised, your cup of coffee will taste weak. When there is too much coffee and little water, the brew will be overpoweringly strong. The ideal ratio of water to coffee depends on your choice of extraction method and the rest of the variables on this list.
If you are using a drip filter to extract coffee, you need a lower coffee to water ratio because the temperature of water is higher and the rate of extraction high.
If using a plunger or a French press, you need to have more coffee for a unit of water because the temperature of water drops fast.
If using a modern espresso machine, you can adjust the volume of water to meet your taste. The water temperature is often maintained at around 97 C.
The bottom line is: to make the perfect cup of morning coffee, grind the beans to the right coarseness, match the water temperature and the duration of extraction, and adjust the coffee to water ratio depending on the extraction method. It’s easy!