Hits and Misses in Coffee

When you buy hot coffee at a cafe or burger joint, there are days when you think, "This is a good one," and days when you think, "Ugh, this is a miss..."
It's not just about the degradation of flavor; there are even days when it has an unusually strong diuretic effect or causes stomach discomfort.
Even when you aren't feeling unwell, why do these "misses" happen?

To put it simply, it is because coffee chemically changes significantly over time.
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Keeping it warm causes "only the water to evaporate, concentrating the components"


Coffee left for a long time on a warming pot or hot plate undergoes:
• Water evaporation

• Concentration of components

• An increase in the ratio of bitterness, astringency, and caffeineIn other words, the intensity of the stimulation changes even if you drink the same amount.This is a major cause of:

"Days when the diuretic effect is too strong"
"Days when it upsets my stomach"
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Coffee oils oxidize, changing the flavor


Coffee contains oils (coffee oils).
As time passes:
• Oxidation → The acidity becomes sharp

• Oxidation products → Off-flavors increase

Oxidized oils, in particular, are highly irritating to the stomach and are likely to be the cause of "feeling sick after drinking it."—

Aroma components (volatile molecules) escape


There are hundreds of volatile components in coffee aroma, but they escape into the air as time passes.
When the aroma is lost:

• Bitterness and astringency become prominent
• The flavor tends to become harsh

In other words, the balance between aroma and flavor is broken, leading to a phenomenon where "the flavor feels strangely intense."

—The pH (acidity) changes


Coffee is naturally weakly acidic, but its acidity changes further when kept warm for a long time.

As a result:
• It reacts more easily with stomach acid, making it prone to causing heartburn
• The flavor balance is also disrupted

Scientifically speaking, boiled-down coffee becomes "a different drink" with a different pH.
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Conclusion: The cause of hits and misses is "freshness + warming time"In short:

• Freshly brewed → Components are stable and stimulation is mild
• Time passes → Components concentrate, oxidize, and stimulation increases

Due to this degradation, both the flavor and the effect on the body change significantly.
"It feels a bit harsh today..."
"I'm going to the bathroom strangely often after drinking this..."

Such "miss coffee" is highly likely to be the result of chemical changes rather than your physical condition.
Looking at it this way, it is clear that even a single cup of coffee is a surprisingly delicate drink.