Beyond controlling for freshness, grind, roast profile, time, and temperature, there's a well-known psychological aspect to perceiving bitter coffee: the color of the cup. Our perception of taste comes not merely from the chemical aspects of what we consume and how those bear on our body's senses of taste and smell, but from our own mental state.
Turns our that white cups provide greater contrast for the dark color of coffee, and thereby predispose us to perceive stronger, and often more bitter, tastes from the coffee we're drinking. By contrast, using clear glass to drink from predisposes us to imagine that we're drinking something lighter and sweeter than we'd otherwise detect. The research on the effects of color on coffee taste
is compelling, and I can personally vouch for the effects, not just in myself, but having observed hundreds of people at our tastings over the past few years.
As I have a tremendous sweet tooth, it should come as no surprise that my cup of choice is a clear, 5-ounce juice glass. No bitter mugs for me!