Why Does Coffee Become Sour When It Cools Down? What Determines the Flavor Changes in Pour-Over Coffee? What to Do When Coffee Is Too Sour?

Why Does Coffee Become More Acidic as It Cools?

Have you ever experienced this like FrontStreet Coffee has: when coffee is first brewed, it tastes perfectly balanced between sweet and sour, but as it sits, it becomes increasingly acidic. Eventually, the acidity becomes so sharp that one sip will make you feel like you've stepped into a famous painting!

Many friends might wonder: why does coffee become more acidic as it cools? Actually, not all coffee behaves this way! These are just isolated cases. Most coffees, when cooled, simply lose some aromatic compounds without significant changes in taste; some even transform from highly acidic to highly sweet as they cool! Therefore, we can conclude that becoming more acidic when cooled is just a special case for certain coffees. So, why do some coffees become more acidic as they cool?

Why Some Coffees Become More Acidic When Cooled

The fact that some coffees become more acidic when cooled is caused by two main factors working together! The first is the well-known effect of our sensory perception, and the second is the coffee's own characteristics.

FrontStreet Coffee has mentioned many times that our sensory perception of taste changes with temperature. Sour, sweet, bitter, salty, and umami are the five tastes we can perceive, among which sour, sweet, bitter, and salty are the four tastes we can find in coffee. The temperature at which these four tastes reach their peak perception are completely different. For example: sweetness is most perceived at temperatures close to body temperature; bitterness is most perceived at near room temperature; while sourness is special because its perception doesn't change significantly with temperature.

Reading this, some friends must wonder: if the perception of sourness doesn't change, why does cooled coffee become more acidic? Because the perception of sweetness decreases! Sour, sweet, and bitter are three mutually balancing tastes (saltiness accounts for a very small proportion in coffee, so it can be ignored). They suppress or become "stepping stones" for other tastes depending on their proportion in the coffee. In a cup of light roast coffee, bitterness is minimal due to insufficient roasting, so the taste is mainly dominated by sourness and sweetness. Initially, the coffee might have lemon, green tea, floral, and honey notes. But when the coffee temperature drops outside the peak range for sweetness perception, the coffee becomes more acidic due to reduced sweetness perception, leaving only lemon acidity and green tea aftertaste.

As FrontStreet Coffee mentioned earlier, not all coffee is like this! That's right, this isn't just a perception issue—it's also about the coffee itself. When a coffee has insufficient sweet compounds, it becomes sharply acidic when cooled. Initially, the coffee doesn't taste as acidic because the temperature is still within the high perception range for these sweets. As the temperature drops, sweetness perception weakens, and the可怜的少量 sweetness becomes nearly imperceptible, essentially non-existent. So sourness dominates and becomes "aggressive." However, if the sweetness itself is abundant, then even after the coffee cools, the overall taste won't be easily altered.

There are two reasons for this situation: one is that the beans themselves contain more acidic compounds, and the other is uneven extraction, where sweetness wasn't fully extracted. Both can lead to sharp acidity in coffee after cooling, mainly the latter. Cases caused by extraction issues are extremely common, so FrontStreet Coffee suggests that when we encounter this situation, we should prioritize adjusting extraction parameters. For details, you can refer to FrontStreet Coffee's previous articles, so I won't elaborate further here!

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