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(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.

The problem with this approach is that most of the chemicals in green coffee are designed to taste really awful to the critters that want to eat it. Those nice sugars and fruit acids are there, but so are the chlorogenic acids that get roasted out during the early stages of the 1st crack. These taste like the white part on an orange peel, dipped in bleach, and covered with sawdust. There's easier ways to train a palate.

After the halfway point of the first crack, the cellulose cell walls of the beans enter a "glass phase," in other words, they become a gooey liquid that expands as the remaining gasses inside the cells expand. If your roaster cannot rech that temerpature, then you end up with very hard, small beans.

Bean expansion happens when the cellulose is above the temperature where it goes soft, roughly 410F, and below the temperature where it starts breaking down, roughly 430F. This is independent of baking the beans at lower temperatures, or scorching and tipping them with higher environmental temperatures.

If you start timing the roast at the point the beans go from green or pale tan to a bright yellow, and check the time when the first crack starts, and you've run more than about 6 minutes, the chances are you've baked the roast. 3 to 5 minutes in this phase is about right. If the roaster is entirely unventilated, and the drum revolves very slowly, you can go longer and still get good roasts. But the entire process becomes a lot more hit and miss, since this is a roast using high drum temperatures and slow heat transfer.