Why Does My Coffee Taste Bitter

We received an email from a customer that said the coffee he ordered was the best coffee he had ever had in his whole life and could we send him two more pounds. He made our day!
The next day we got an email from another customer who said our coffee was bitter and he wanted his money back. Both orders were from exactly the same roast batch. They were also from the roast we sent to Coffee Review and they rated it at 95. We didn’t argue. We gave him his money back, but wondered if he had considered other possibilities besides the beans.
Bitterness can be the result of poor picking, processing or roasting. Blaming the beans for bitter coffee is understandable but there are other reasons for bitter coffee. Some of the most common reasons that coffee tastes bitter are:
Medical Alert
There are a number of medical conditions, including Covid 19, where a symptom is a change in your perception of aroma and/or taste. If your coffee tastes bitter it may be an early alert to a medical problem. Sometimes bad coffee is bad coffee, but at times bitter coffee may be an alert to go talk to your doctor. More at Is Bitter Coffee a Medical Alert?
Mouth Chemistry
Coffee flavor can be affected by conditions in your mouth. Gum disease and dry mouth syndrome will create bitter tastes.
The problem may be even more mundane. Just brush your teeth, your coffee is going to taste bad. Ate scampi last night, your first sip of coffee is going to taste bad. Sometimes you have to give the coffee a chance.
Under Extraction
Not all of the flavors in coffee are good, some are bitter. There is a sweet spot between under extracted and over extracted coffee where the bitter flavors are muted. Getting to that sweet spot requires a combination of great coffee beans, the right grind, the right water, the right extraction time and clean equipment. If you don’t perfectly sync all of these factors you may get bitter coffee.
The taste characteristic of under extracted coffee is sour and weak. Under extraction can occur in a number of ways
  • Too little water
  • Too much ground coffee
  • Too coarse a grind
  • Water temperature too low
  • Steeping too short
Over Extraction
The taste characteristic of over extracted coffee is bitter and harsh. Over extraction can also occur in a number of ways.
  • Too much water
  • Too little coffee
  • Too fine a grind
  • Water temperature too high
  • Steeping too long
Oils, Mold and Fungus
My grandmother never washed her percolator because she believed that the patina built up by years of coffee brewing added to the flavor of the coffee. She was right. Her coffee was bitter and rancid.
Coffee oils build up on coffee makers. Rinsing in water doesn’t remove the oils. Coffee equipment needs to be washed with hot water and soap. Alternatively, you can use commercial coffee cleaners or white vinegar or citric acid.
Brewers with water reservoirs build up mold and fungus in the tank and in the pipes. Coffee brewed with moldy water tastes moldy. Running white vinegar though the brew cycle will clean out any mold or fungus buildup.
Storage
Coffee is like a sponge. It will pick up any flavors from nearby items. Store ground coffee in the refrigerator near aromatic items like onions, garlic, salami and etc it will pick up those flavors. Put coffee in the spice cabinet and it will pick up the spice flavors. Put it near oil or gasoline and it will pick up that taste.
So if your coffee tastes bitter, give the beans a break and consider whether the bitterness comes from something else.

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