(These 101s are a work in progress: please feel free to comment. Once they are done, they'll go into the how to section)
What actually happens in light and dark roasts? It's easier to find out for oneself (especially those just starting to home roast) by using a simple analogue for coffee roasting: caramelizing sugar and citric acid. Here's how:
What you need:
-- A small saucepan, three cups, a whisk, a table spoon, a stovetop.
-- 1 Heaping tablespoon of sugar
-- 1 pinch of citric acid and 1 tablespoon of water or 2 table spoons of lemon juice
Here's what to do
-- put the sugar, acid and water in the saucepan, whisk, and taste -- it should taste roughly like a pez
-- put it on the burner, full power.
-- whisk till it starts to boil, and keep whisking
Here's how the cooking caramel looks:
-- When you see it get slightly yellow (left side), take it off the heat, and put a 1/2 teaspoon into the first cup
-- Return it to the heat, when it gets tan/brown (right side), add a 1/2 teaspoon to the second cup
-- Caramelize it till it's middle to dark brown (bottom), and add a 1/2 teaspoon to the third cup.
-- Now add a teaspoon of water to each cup.
-- The light caramel will dissolve in about 20 minutes, the dark ones in 35 and 45 minutes.
-- Taste each one. Try to compare the sweet, sour, and acid tastes.
Here's how the caramel "grounds" and "brews" look
Sugar starts browning around 375F, and the citric acid starts cooking off at the same temperature. This is true of the acids and sugars in coffee, so this "coffee roasting analogue" will deliver flavor balances comparable to what you get in lighter and darker roasts. You will note that the lightest "roast," tastes very sweet and tart. In the middle roast, the sweetness is reduced, so is the tartness, and one starts getting bitter caramel flavors. In the darkest roast, the bitterness is more prominent than the sweetness, the tart flavor is nearly gone, and one gets odd or interesting burned flavors, equivalent to the dry distillates dark roasted coffee.
The flavors in this experiment are very simple, so it's easy to tell how they change. Coffee has a thousand or so flavor compounds, and the changes from light to dark roast can be much harder to track. So doing this tasting experiment can help by giving a simple "taste summary" of what you'll find when cupping coffee.