Why Your Coffee Tastes Bitter or Sour

Same bag of coffee, but one morning the cup is harsh and bitter, and the next it's sharp and sour. Is it the roast, the recipe, or both? We follow the water into the grounds to separate four things that are easy to mix up: what the roast already put in the beans; how much of the coffee dissolves (extraction); how evenly the water reaches the grounds; how concentrated the drink ends up (strength). CHAPTERS 0:00 Same coffee, two different cups 0:28 Extraction vs strength 1:15 What the roast already changed 1:52 Why grind changes the cup 2:26 The shortcut through the bed 3:05 Brewing isn't three neat steps 3:42 Temperature is one variable 4:14 How to troubleshoot, one change at a time 5:01 The answer 5:18 Next: inside the roaster QUICK TAKEAWAYS • Extraction (how much of the dry coffee dissolved) and strength (how concentrated the drink is) are different measurements. Adding water lowers strength without extracting anything more. • Roasting builds part of the flavor, including bitterness, before you brew. A recipe can't turn beans into a different roast. • Finer grounds usually slow the flow in a filter brewer. Very fine grounds can make extraction less even, not simply higher. • Early and late parts of a brew overlap in taste. Perceived sweetness isn't a sugar meter. • In one drip-coffee study, brews at 87, 90 and 93 °C, matched for strength and extraction, tasted little different. • To troubleshoot filter coffee, keep everything consistent and change one thing at a time. A sour or bitter cup is a clue, not a diagnosis. SOURCES 1. Frost S.C., Ristenpart W.D., Guinard J.X. (2020). Effects of brew strength, brew yield, and roast on the sensory quality of drip brewed coffee. J Food Sci 85(8):2530–2543. doi:10.1111/1750-3841.15326 2. Batali M.E. et al. (2021). Titratable Acidity, Perceived Sourness, and Liking of Acidity in Drip Brewed Coffee. ACS Food Sci Technol 1(4):559–569. doi:10.1021/acsfoodscitech.0c00078 3. Frank O. et al. (2007). Structure Determination and Sensory Analysis of Bitter-Tasting 4-Vinylcatechol Oligomers and Their Identification in Roasted Coffee. J Agric Food Chem 55(5):1945–1954. doi:10.1021/jf0632280 4. Cameron M.I. et al. (2020). Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment. Matter 2(3):631–648. doi:10.1016/j.matt.2019.12.019 5. Batali M.E. et al. (2020). Sensory and monosaccharide analysis of drip brew coffee fractions versus brewing time. J Sci Food Agric 100(7):2953–2962. doi:10.1002/jsfa.10323 6. Batali M.E. et al. (2020). Brew temperature, at fixed brew strength and extraction, has little impact on the sensory profile of drip brew coffee. Sci Rep 10:16450. doi:10.1038/s41598-020-73341-4 7. Guinard J.X. et al. (2023). A new Coffee Brewing Control Chart relating sensory properties and consumer liking to brew strength, extraction yield, and brew ratio. J Food Sci 88(5):2168–2177. doi:10.1111/1750-3841.16531