Skip to 0 minutes and 6 secondsCoffee contains over 1000 aroma compounds, and a variety of antioxidants, the most abundant being chlorogenic acids - a family of structurally similar esters. We will see that chlorogenic acids are mainly responsible for coffee's bitter taste. When coffee beans are roasted, chlorogenic acids and the numerous other organic compounds undergo a whole host of chemical reactions. This includes Maillard browning reactions that produce a range of compounds, which affect the taste of coffee, such as brown-coloured and very bitter-tasting polymers called melanoidins. Interestingly, melanoidins are very poorly characterised due to their complexity, and their chemical structures remain largely unknown. The majority of the coffee aroma is found in the coffee oil, which makes up about 10% of the roasted beans.

Skip to 0 minutes and 59 secondsThe aroma of coffee is a result of volatile (low-boiling) compounds. Non-volatile (high-boiling) compounds in the coffee give the sour, bitter and astringent flavours. The final temperature of the roasting process correlates directly to the colour of the beans; at higher final temperatures, the beans have a darker colour. The roasting time can be anywhere between 90 seconds and 40 minutes. The length of time influences the reactions that occur in the beans; roasting the beans for longer periods result in bitter coffees, which lack a pleasing aroma, whereas a very short roasting period may not give enough time for all the reactions happening in the roasting process to be completed, which can result in underdeveloped coffee.

Skip to 1 minute and 43 secondsThe non-volatile compounds in coffee, that gives the sourness, bitterness and astringency, include caffeine which provides body, bitterness and strength to brewed coffee; carboxylic acids, such as ethanoic acid and maleic acid, which provide sourness; and nicotinic acid, which has a nutritional value (as it is a vitamin of the B complex). Alongside these, there are many other non-volatile compounds in coffee that contribute to the taste sensation and feel of the final cup. Interestingly, caffeine is able to bind the adenosine receptor in the body, as it has a similar chemical structure to adenosine. Adenosine suppresses physiological activity, resulting in drowsiness as is it slows down nerve cell activity.

Skip to 2 minutes and 27 secondsBy blocking the action of adenosine at its receptor, caffeine causes a stimulant effect and helps to prevent tiredness. Volatile compounds in coffee provide the aroma. The major mechanisms for the formation of coffee aroma include the Maillard reaction and the Strecker Degradation. There are other contributing mechanisms and interactions, which results in a very complex system for producing coffee aroma. Gas-chromatography-olfactometry techniques have been used to identify individual odourants in coffee. Scientists achieved this by sniffing the eluted compounds, which allowed 18 different aroma compounds to be identified. The mass spectrum for each compound was compared to authentic standards, along with their odour activities and retention indices.

Skip to 3 minutes and 16 secondsIn gas chromatography there is a mobile and a stationary phase, the retention indices of compounds are the times it takes for a compound to move through the column. Different compounds have different elution times because they will all interact differently with the mobile and stationary phases. Identifying these odorants showed that the aroma change between roasted and brewed coffee is due to a change in concentration of the odourants present. Using these 18 odourants, a synthetic mixture was produced that followed the same odour profile as coffee brews. Four out of the eighteen identified aroma compounds are shown here.

Skip to 3 minutes and 56 secondsThese are sotolon, which has the smell of toast, vanillin, which smells of vanilla, furaneol, which has a caramel smell and finally, abhexone, which smells of honey. Omission studies have been carried out on these synthetic coffee mixtures. An omission study is when a synthetic cup of these compounds is created but a specific compound is excluded. An expert in coffee aromas tastes the mixtures to determine if the flavours are significantly affected by the loss of each compound. Through these emission studies, it has been determined that the removal of certain compounds from the brew, including 2,3-butadione and vanillin did not result in a significant loss in coffee flavour.

Skip to 4 minutes and 43 secondsOverall, out of over 900 volatile components in coffee, amazingly, fewer than 20 have actually been determined to be relevant to the aroma of the brew.

The organic flavour compounds in coffee

Most of us have a favourite style of coffee and every cup will taste different because of the roast of the coffee bean, the grind, the amount of bean used, the water, the temperature and many other factors. At home, you may have tried to reproduce the coffee goodness from the coffee houses, but do we actually know the best method to use in order to extort the captivating flavours perfectly? Here are some pointers for how to make these excellent coffees.

Cappuccino: This drink consists of equal parts of steamed and frothed milk and shot of coffee. The milk is poured on top of the coffee shot and dusted with nutmeg, cinnamon or chocolate powder.

Mochas: The easiest way to do this is to make it with quality hot chocolate, add a shot of coffee with steamed milk poured in and top with whipped cream, then lightly dust with chocolate powder. It looks great in a clear glass-mug with a long stemmed spoon.

Lattes: Foam and steam milk to 75 °C. The ratio is worked out as 50% coffee and 50% milk. Slowly pour the milk down the side of the coffee cup or glass so it infuses with the coffee shot. The main difference between a latte and a cappuccino is a latte blends the milk and coffee together, whereas the cappuccino keeps the two apart.

Espressos: Using a finely ground good quality coffee bean and a high pressure espresso pot, gives a coffee shot with a golden-brown foam on top, to be served in an espresso cup to keep the coffee warm as it is being drunk.

Steaming milk involves introducing hot water vapour into cold milk until it reaches the ideal temperature for the drink. It is not as simple as it sounds as different milks (with different amounts of proteins, carbohydrates, and fats) require different amounts of steaming time. For example, fat molecules can stabilise the formation of foam by surrounding the air (injected during the steaming) and entrapping it in a bubble - milks with higher fat content lead to stable foams at temperatures below room temperature, while milks with lower fat contents (like skim milk) are better at stabilising foam at higher temperatures. The temperature is key as too high a temperature can scald the milk, which denatures the enzymes and causes curdling (as denatured milk proteins clump together).

At the end of the day, making coffee correctly is all up to you and the way your flavour buds take to the coffee taste, or indeed, if you like coffee in the first place! A survey of our second year York undergraduate chemists, showed that water was their most popular non-alcoholic drink, followed by tea then coffee. They preferred black tea to green or herbal teas, and almost all put milk in their tea in preference to adding lemon or drinking it black. By far the most popular coffee was latte followed by cappucino and only 5% of the students knew what bulletproof coffee was.

So, what is your favourite non-alcoholic drink, and why? Do you have a particular routine (such as a Turkish coffee approach) for making your favourite brew and/or do you use any unusual ingredients? For example, I wonder how many of you enjoyed an eggnog latte at Christmas?

For those of you with an interest in Latte Art why not post a picture of your creation, showing the design you have created by pouring steamed milk into a shot of espresso?

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This video is from the free online course:

Exploring Everyday Chemistry

University of York