Short answer Reheating coffee is never ideal, as it breaks down the chemical makeup of the liquid, turning a rich cup of joe into something bitter and carboard-like. In particular, the reintroduction of heat speeds up the degradation of the chlorogenic acids into quinic acid and caffeic acid, which have a sour, “rubber burnt” taste. And reheating cooks off the balance of VOCs (volatile organic compounds) that give aroma, leaving heavy bitter notes behind. Although not poisonous, reheated coffee is a different chemical composition from fresh and much more bitter, which upsets the stomach.
For the Frugal Home Brewer: Economics of Oxidation
The Main Problem:
You make a big pot because it saves time and money, but what do you do with the extras without feeling wasteful? Either you’re wondering if the ”cost” of reheating is a loss of flavor, or if you’ve really ruined the coffee.
The Hidden Mechanism:
Many think coffee is “bad” only when it has mold. But as soon as it is ground, coffee starts to “rust,” or oxidize.
Oxidation: Coffee, like a sliced apple turning brown, reacts with oxygen. It can be even quicker if coffee sits on a hot plate (such as with a typical drip machine).
The 30-Minute Rule: Coffee is in its “Goldilocks zone” where the flavor and temperature are just right for around 30 minutes after brewing. Beyond that, the hydrogen ions in the water react further with the coffee oils to become evermore acidic.
Why Reheating Fails:
If you reheat such an oxidized liquid on the stovetop or in a microwave, it gives energy required to accelerate these oxidative reactions 200-300%. You’re not only making it hot; you are rapid-aging the coffee.
The Protocol: The “Coffee Ice Cube” Approach
Instead of re-warming, CONFUSE HER METABOLISM by freezing prior to the point of maximal oxidation.
- Time it: After 45 minutes of brewing, pour any coffee that remains into a liquid measuring cup.
- Cool it: Let cool to room temp (do not put hot coffee directly in the freezer as that will lower the temp and affect other food).
- Freeze it: Transfer to an ice-cube tray.
How to use it: The next time you make hot coffee, drop one of these bad boys in your cup to cool it down to drinking temp sans dilution. Or for iced coffee, take them for a spin instead. This “captures” the chemical profile, instead of destroying it through heat.
For the Distracted Office Worker: The Microwave Physics
The Core Issue:
You are busy. Your coffee gets cold. The microwave is the weapon of choice. You need a recipe that renders the coffee not unpalatable, even if it is far from perfect.
The Critical Analysis:
Your microwave does not heat liquids evenly. They heat by dielectric heating — spinning water molecules.
The Hot Spot Problem: Microwaves cause localized regions of extreme heat (superheating) while the rest of the liquid is merely hot.
Oil Breakdown: Coffee has fragile oils (lipids). In a superheated pocket in a microwave, however, these oils can scorch faster than heat spreads to the rest of whatever you’re microwaving, leaving behind a telltale “ashy” taste.
The “Low-Power Pulse” Technique
If you have The other option is to microwave the fish, but prep it properly to ensure there are no #put the “hot spot” method.
- Power Down: Never use the “High” (100%) setting as a default. Adjust your microwave to 50 percent power.
- The 15-Second Rule: Warm it up in 15-second increments.
- Agitation: Vigorously stir your coffee between each 15-second interval. This spreads the heat and avoids burning of the oils in spots.
- The End Point: Stop when the coffee is between lukewarm and warm, not hot. Getting them above a certain temperature (around 175°F / 80°C) ensures that all the chemicals in the stone are completely broken down.
Better Solution:
Get a mug warmer (a conduction coaster). Unlike a microwave, this uses conduction to keep it at temperature and never cools down enough to need putting in the microwave.
For those Who Enjoy a Good Taste: The Chemistry of Bitterness
The Core Issue:
You realize that reheated coffee is sour, astringent and flat. You want to know what made you catch in your molecular throat, so that you can stay ahead of it.
The Chemical Breakdown:
The deterioration of taste is influenced by the change of CGAs.
Raw State: In raw coffee, CGAs contribute to a nice, mild acidity and some fruitiness. They are antioxidants.
Heat Impact – When coffee is held hot, or reheated CGA’s break down (hydrolyze) further into Caffeic Acid and Quinic Acid.
The Taste Result: Quinic acid is most to blame for the bitterness and sourness felt in the pit of your stomach, and the astringency on your tongue.
The Aroma Vacuum:
Flavor is approximately 80% smell. It’s the smell of coffee and they are Volatile Organic Compounds (VOCs).
Volatility: These are very volatile substances, and they evaporate at a high rate of speed even at room temperature, violently when heated.
The Second Boil: By the second boil, time to reheat gets you past any of the pleasant VOCs (those fruity, floral, nutty notes) that have not already evaporated. Reheating allows the heavier, less pleasant sulfurous compounds to escape from what was left. You’re essentially packing in all the bad flavors.
The Strategy: Thermal Insulation
Do not, I repeat, do NOT own a coffee maker with a hot plate.
Switch Gear: Switch to a thermal carafe or a double-walled vacuum-insulated French Press.
The Physics: Because of the vacuum insulation, heat cannot be transferred by conduction or radiation. It will keep your coffee hot at about 180°F (82°C) for hours without adding any outside energy. In the absence of external energy CGAs decomposition to Quinic acid is greatly retarded.
For the Health-Obsessed Drinker: Acidity and Physiology
The Bottom Line:
So the issue is that you are worried reheating coffee makes it toxic for your body or aggravates a health condition like acid reflux or GERD (Gastroesophageal Reflux Disease).
The Physiological Reality:
Second-day coffee won’t give you cancer or lead to any long-term cellular damage. BUT, it’s a lot harder on your stomach and intestines.
pH Shift: Fresh black coffee has a pH range of 5.0 to 5.5 (mildly acidic), but reheating the drink may lower its pH value (increase acidity) as quinic acid continues to concentrate as stated previously.
Gastric Reaction: For those with sensitive stomachs, the higher levels of quinic and caffeic acids cause excess gastric acid to be generated. This causes heartburn, indigestion, and another type of “jittery” sensation that many attribute to intake too much caffeine.
The “Cold-Hot” Compromise
Never drink reheated coffee if you have a sensitive stomach. Instead, adapt elsewhere in your brewing, perhaps by making Cold Brew, even if you wish to enjoy it hot.
The Logic: A cold water extraction extracts about 65 percent less acid from the coffee than hot water because the acidic compounds dissolve less easily in cold water.
The Process: Make a cold brew concentrate (steep coarse grinds in cold water for 12 to 24 hours).
The Heating: When you have a hankering for hot coffee, dilute the cold concentrate with boiling water.
Why this works: You are warming the coffee for the first time. Since the acids were not mashed initially, heating it now is not going to form them spontaneously. What you receive is a hot glass of low acid coffee and gentle on your stomach.
Frequently Asked Questions
Why does coffee that has been reheated in a microwave taste bitter and burnt?
Heat and time both break down chlorogenic acids in coffee into quinic and caffeic acids, which contribute to sour/astringent flavors in the cup. The process also steams off the volatile organic compounds (VOC) that make it smell nice, leaving only heavy, sulphurous components behind.
Does re-heating coffee make it bad for your stomach?
Although not dangerous, re heating coffee is much more acidic than fresh because all the quinic acid has gone into solution. This sudden acidity can upset the stomach, prompt increased production of gastric acid, and result in heartburn or indigestion in sensitive persons.
How do you properly reheat cold coffee in the microwave?
Do a “Low-Power Pulse” to avoid scorching the delicate oils: run the microwave at only 50 percent power while cooking in short bursts of 15 seconds. Stir well between each burst of heat to spread the wattage and stop when the coffee is not piping hot, but rather lukewarm.
For how long does brewed coffee remain fresh before it becomes bad?
After coffee is made, there’s a “Goldilocks zone” that lasts for approximately 30 minutes. After this, oxidation rapidly increases and hydrogen ions interact with coffee oils to increase acidity: at 60 minutes the aging doubles by speed during storage, while in recirculating systems it is 200-300% faster.
What is the most effective way to store unused coffee without ruining it?
Do not reheat, instead use the “Coffee Ice Cube” trick. Pour leftover coffee into a liquid measuring cup, cool it to room temperature and freeze it in an ice cube tray to preserve the chemical profile for use in iced coffee or to chill hot one later.
References
- Title: Chlorogenic acids: separation and degradation in the production of instant coffee.
Source: Journal of Agricultural and Food Chemistry.
Conclusion: The thermal processing and high-temperature holding of coffee considerably degraded 5-caffeoylquinic acid (5-CQA) to quinic acid and other reaction products, which were directly proportional to the increase in bitterness and astringency.
Date: 2009 (Validating the classical principles of food chemistry). - Original Message—– From: Rossop, Nancy Sent: Monday, April 07, 2003 8:32 AM To: ‘Lol conjunction.org’ Subject: Loss of volatile compounds in coffee.
Source : Food Research International.
Conclusion: Studies of headspace analysis performed on coffee have shown that certain aroma compounds (aldehydes and ketones) considerably decrease immediately after the percolation process, and upon reheating these substances are not produced again, but contrary to this has been generating sulfur-like deterioration products. - Subject: Acidity and Gastrointestinal Irritation.
Body: American Chemical Society (ACS).
A finding: Presentations to ACS meetings have also noted that dark roast coffees and cold brew preparations contain unique chemical markers (such as N-methylpyridinium) that have their own separate influence on stomach acid secretion, pointing out again that the longer coffee is subjected to high heat, the more bitter and toxic it becomes for your digestion.
