Why does this coffee become sweeter when it cools down? How does temperature affect our coffee tasting?

Have you ever experienced how the flavor and aroma of a hot cup of coffee changes once it starts to cool? At this point, you may start to notice subtle flavors and aromas that were not detectable when the coffee was hot. You may find that the coffee has a more pronounced sweetness, fruitiness, or floral flavor after it cools down.

Temperature fluctuations trigger specific chemical reactions that cause some changes in the flavor of the coffee.

How do we perceive flavor and aroma?

Coffee is a very complex beverage with over 1,000 aroma compounds, which are what we smell and taste when brewing and drinking it. But among these 1,000 compounds, 40 compounds contribute significantly to the aromas we enjoy in coffee.

Most of these aroma substances are produced during the roasting process. As the temperature rises during the roasting process, it will cause a stronger reaction with the sugars, carbohydrates and nitrogen compounds in the green beans. Due to the heat of the roasting process, the sugars will undergo a caramelization reaction, which will bring about a caramelized flavor and turn into brown.

This process produces volatile compounds that evaporate into gases at room temperature and dissipate into the air. In this form, our ability to perceive these aromas is enhanced. Through our tongue and nose, we sense the flavors and aromas of these aromatic compounds associated with sweetness, from chocolate to fruity sweetness.

The cells on the surface of our tongue are taste receptors for acidity, bitterness, and sweetness. They respond to the presence of certain chemicals and then transmit this sensation to our brain for us to analyze flavors and aromas. Chemicals in volatile compounds travel from our mouth to our nasal cavity to stimulate the olfactory system. This process is also the key to our ability to distinguish flavors and aromas.

The perception of other flavors is brought to our taste buds by various organic acids, sugars, oils and caffeine. For example, chlorogenic acid in green beans can produce a very spicy taste, while quinic acid produces a bitter taste. If there is a sour aroma in our coffee, it can increase our perception of acidity. This is because we understand the basic flavors that those volatile compounds will bring, just like we are always exposed to the aroma of lemon and the taste of sourness. Once we feel these two flavors together, our perception of flavor will increase.

How does temperature affect coffee extraction?

We all know that extraction can have a huge impact on the flavor and aroma of coffee. The water temperature you use to extract your coffee can also have a significant effect on the extraction rate of your coffee. Each molecule has an optimal amount to extract, depending on the water temperature. The warmer the water, the more extractable these compounds are. As the temperature of the water increases, the water molecules begin to gain more energy from the heat, and they begin to move faster, increasing the interactions between the water molecules and the coffee molecules.

The more these two molecules interact, the more substances will be extracted. The compounds dissolved by water molecules from coffee molecules will affect the taste and aroma of our coffee. If we use cold water, it will be more difficult to extract the volatile compounds that make our coffee taste interesting. The lower the temperature, the longer the extraction time will be needed to get the taste of the coffee we want.

This long-term low-temperature extraction will completely extract the sugar in the coffee, and the coffee extracted with proper time control will have a distinct caramel flavor. Therefore, the bitterness and astringency we feel in cold brew coffee are also weaker.

Aroma and flavor in coffee at high temperatures

According to the NCA, the ideal serving temperature for coffee is between 82°C and 85°C . However, this can burn your tongue, as temperatures in this range may exceed your tolerance for thermal pain. The most recognized way to drink coffee at this temperature is by sipping or cupping. When you drink, you inhale a small amount of liquid and a puff of air, which helps to quickly cool the liquid.

At around 76°C, coffee begins to develop aroma and releases a lot of steam, which intensifies your sense of aroma while suppressing your taste. Volatile compounds are released faster here and evaporate faster.

At a temperature of around 70°C, people can feel a more obvious flavor and aroma of coffee, and this full aroma state can be maintained until 60.4°C.

The aroma we perceive at 60°C is mainly bitter-related, and after that we experience a significant increase in bitterness. The bitterness is strongest at about 56°C.

Aroma and flavor in cold coffee

At temperatures below 50°C, you'll notice a big change in the flavor and aroma of the coffee. Aromas start to become harder to capture, mainly due to the reduction in steam produced as the coffee cools. Moving down further, bitterness begins to taper off, revealing a more complex flavor. Between 31-50°C, the most flavors can be captured, related to acidity and sweetness.

The sweetness of coffee is most prominent at around 44°C, and the bitterness is weakest at this temperature. Between 31 and 37°C, this subtle change is accompanied by the most interesting things about coffee - volatile compounds associated with sweetness, fruity aromas, floral aromas, herbal aromas, and nutty aromas are more prominent in this temperature range. Here, we really experience the decisive characteristics of coffee.

At lower temperatures, such as 25°C, the best acidity of coffee can be felt . For example, drinking a cup of Kenyan coffee at this temperature will make it more lively and the acidity more obvious.

So the same coffee tastes different at different temperatures. At higher temperatures, the flavor of the same cup of coffee may be less distinct. Whether you like your coffee hot or iced, it's important to understand the effect that temperature has on your perception of coffee flavor.

Next time you drink coffee, take note of the differences in the flavor you experience at different temperature ranges. Try to find the "ideal range" of bitterness, acidity, and sweetness for different coffees.