Temperature is one of the most widely discussed topics in coffee, particularly when we talk about green bean storage, roast profile, and brewing. As a general rule, there are a few set temperature ranges for each area which help us to preserve quality and improve consistency.
Once brewed, coffee naturally cools down over time. This process leads to irreversible changes in flavour. For example, a bright and acidic coffee will generally become sweeter and less aromatic.
Although it’s not possible to regain these original sensory characteristics once coffee cools, some people do choose to reheat it. And while it remains a contentious topic, microwaving coffee is often the easiest and most efficient way to do so.
Common experiences when microwaving coffee include a more pronounced and unpleasant bitterness, so is reheating it worth the risk? Are there better alternatives, or should we not be reheating coffee at all?
To learn more, I spoke with Dr Samo Smrke and Verônica Belchior, two industry professionals, both with backgrounds in coffee chemistry. Read on for their insights.
You may also like our article on how temperature can impact your experience of coffee.
How do microwaves work?
Microwave ovens are common kitchen appliances in many households across the world. First invented in the early 1970s, they provide an efficient means of cooking or reheating food and beverages.
Verônica Belchior is a co-founder of the coffee education project The Coffee Sensorium. She tells me how microwaves work.
“The microwave oven works by [rotating] the water molecules in foods and beverages,” she says. “This creates heat that is able to cook food.”
This transfer of energy to the water molecules comes from electromagnetic radiation. The radiation waves are reflected off the metal walls of the microwave, so they can then be absorbed by the water molecules in food and beverages.
Water has a high dielectric constant as it has strong hydrogen and oxygen bonds. Essentially, this means it readily absorbs microwave radiation. Foods with higher water levels, such as vegetables, are better suited to being heated in microwaves.
Samo Smrke is the Head of Coffee Transformation at the Zurich University of Applied Sciences’ Coffee Excellence Centre. He explains how coffee can be reheated in the microwave.
“Coffee is mainly water, which absorbs the microwaves. On a molecular level, the radiation causes these water molecules to rotate and spin faster.
“This gives them more energy, which means that the temperature of the liquid increases.”
Effects on flavour and aroma
In order to understand how microwaving coffee affects its aromas and flavours, we must first explore the chemistry behind volatile compounds in coffee.
Volatile molecules contribute to the aromas and flavours we experience when drinking coffee. These compounds have lower boiling points than non-volatile compounds, meaning they evaporate more quickly. Some volatile molecules immediately evaporate once coffee is ground or comes into contact with water.
Once coffee loses these volatile compounds, its chemical composition changes. This continues to happen as brewed coffee cools.
“Microwaving coffee accelerates the chemical changes in several molecules,” Verônica says. “It also speeds up the loss of other volatile molecules.
“It’s impossible to avoid the loss of these volatile compounds because the heat created from the [rotation] of water molecules influences the behaviour of those compounds.”
Samo explains that microwave radiation has no direct effect on coffee aromas and flavours. This is because volatile compounds have lower dielectric constants than water, so they are less susceptible to absorbing microwave radiation. The energy is instead passed from water molecules to the volatile compounds. He describes the process as being similar to conductive heat transfer.
“There have been many studies researching how microwaving affects molecules. Most found that there is only a heating effect, so microwaves do not have any significant specific effect on molecules,” he says.
However, it should be noted that after coffee has cooled for a considerable amount of time, more unpleasant flavours will begin to form.
Verônica explains: “Over time, coffee kept in thermos bottles or other servers can lead to bacterial growth, and oxidative notes will appear as well.”
She adds that oxidation causes coffee to taste stale and creates off-flavours.
Some solubles, like carbohydrates, are more easily dissolved in hot water when coffee is brewed, but yield prominent flavours as the liquid cools. The higher the number of total dissolved solids in the extraction, the more noticeable the reemergence of these solubles will be.
While some of these compounds dissolve again once the coffee is reheated, some will still remain insoluble. This can cause increases in astringency, as well as a lighter or thinner mouthfeel and body.
Why does bitterness increase?
It’s commonly known that reheating coffee, whether in the microwave or by another method, makes coffee taste more bitter.
This is due to the presence of chlorogenic acids. Coffee beans contain large concentrations of these acids, which belong to the aromatic phenolic acid group. These contribute heavily to coffee flavour and aroma.
“Reheating coffee will break down chlorogenic acids, which releases caffeic and quinic acids,” Verônica explains.
However, Samo points out that the discernible increase in bitterness also relates to the human perception of flavour. As coffee cools, acidity and sweetness become much more pronounced, so we then naturally pick up on the heightened bitterness after reheating the coffee.
Samo also adds that when brewed coffee is stored in closed containers for longer periods of time, other flavour changes may occur when the coffee is reheated.
“The aromas are not able to evaporate because the container is closed, so flavours might change due to chemical reactions occurring in the coffee.”
Should you reheat high-scoring coffees?
While reheating lower-quality coffees may result in fewer noticeable changes in flavour and aroma, should we be more concerned about microwaving higher-scoring coffees?
Samo believes that it depends on the coffee. “The outcome will be dependent on the coffee,” he says. “If a particular coffee scores highly, but has a delicate aroma, this is caused by smaller quantities of these volatile compounds. Reheating the coffee will destroy these volatile compounds, ultimately reducing the aroma.
“On the other hand, a coffee with a cup score of 86 points or above may have a rich, complex, and stable aroma, which becomes dull once cooled,” he says. “This is because fewer aromatic compounds are released into our olfactory systems when drinking a cold beverage, compared to a warm one. For such a coffee, reheating could be beneficial for heightening aroma.”
Verônica explains that there are chemical differences between high and low-scoring coffees, so reheating them has different effects.
“For coffees scoring over 86 points, the difference between freshly brewed and reheated flavours might be even more noticeable because the bitterness has been intensified,” she says.
“It should be noted that coffees scoring above 86 points tend to have fewer chlorogenic acids than coffees scoring between 80 and 83 points. The 80 to 83 coffees might also become more bitter once reheated because the sweet and fruity notes were less prominent.”
But ultimately, Verônica emphasises that increasing bitterness is inevitable when you reheat coffee, no matter how you do it or which coffee you use.
“There is no way to avoid the bitter perception by reheating coffee, even with good coffee.”
Are there better ways to reheat coffee?
Although reheated coffee will never taste as vibrant or aromatic as freshly brewed coffee, is there a method which can better mitigate the more unpleasant flavours?
Samo believes that microwaving is the best way to reheat coffee. “Microwaving applies heat more homogeneously throughout the liquid, especially when you compare it to reheating coffee on a stove.”
He also points out that the length of time you reheat coffee for is important.
“The best method for microwaving coffee would be to heat it fast, but to not let the temperature get too high,” he says. “Heating quickly is better than heating slowly, so that we lose as little aroma as possible.”
If coffee becomes too hot during reheating – or is brewed using water which is too hot – the loss of volatile compounds will increase.
This is because higher temperature water has more kinetic energy, which transfers to the volatile molecules. With more kinetic energy, these molecules move faster – meaning they are more easily “lost”.
“In addition, it’s important to only reheat coffee to drinking temperature,” Samo explains. He says that in order to experience the full range of coffee flavour and aroma, coffee should ideally be consumed at around 60°C (140°F).
“If you reheat coffee to a higher temperature than this, you lose more aroma through evaporation. Since coffee already loses a significant amount of its aroma in the initial cooling phase, overheating it will lose even more.”
Verônica agrees that using a microwave is the most effective way to reheat coffee.
“Different microwave ovens work in the same way. Although hot coffee is meant to be freshly consumed, other alternatives for reheating coffee will alter its chemical compounds,” she says.
It is possible for microwave radiation to only heat certain areas in the liquid more than others, and potentially miss some areas altogether.
Using a rotating plate when microwaving coffee will help to ensure the liquid will reheat evenly. In addition, stirring your coffee once reheated will allow for more even heat distribution and reduce the risk of burns or scalds.
People will always reheat coffee, but ultimately, Verônica advises against doing it altogether.
“For coffee professionals and people who just love coffee, the only way to consume coffee is fresh, as this maintains the initial flavours.”
Samo, meanwhile, says that doing it is a matter of personal preference. “There is no health-related reason why we should or shouldn’t reheat coffee, unless the coffee is considerably old.”
Ultimately, the decision is yours. Whether you reheat your coffee or not, however, just remember: it doesn’t matter how you do it – it will always be more bitter.
Enjoyed this? Then read our article on how to limit water temperature variation for better coffee.
Perfect Daily Grind
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