Traditionally this roast color has been considered under-roasted. However, it has become vogue in certain circles and especially among artisan roasters who entered the craft during the last decade. The coffee taste at this level is overly bright (acidy) and out of balance, like an unripe pineapple.
Roasting is our way of ripening the coffee bean. At about 405°F starches within the bean convert to sugars. The process is called pyrolysis.
Ultra Light coffee is not brought to a temperature that is high enough to convert all of its starch compounds to sugar. Bringing beans to a finish below 405°F skews the balance because sugar development was incomplete. Result: the taste of candy corn with straw and wood notes.
Starch is a form of carbohydrate that is not sweet (unripe banana, potato). At high temperatures, the long chain molecules of starch break down into simple sugars. Coffee flavors are directly related to this process. Sugars, as the heat rises, caramelize (light and medium roasts), then carbonize (Vienna and French roasts), then burn at about 475° F producing first, flat black beans and then FIRE!
Ultra Light Roasts are unbalanced, with overly bright, acidy mouth-feel. Mostly so unbalanced they are impossible to enjoy at 100% of the brew.
Long chain molecules of starch = starch is a form of carbohydrate, known by the chemical formula C22H12O11 (C=carbon, H=hydrogen, O=oxygen).
Simple sugars = are a form of carbohydrate, known by the chemical formula C6H12O6 (C=carbon, H=hydrogen, O=oxygen).