There is so much that can go wrong with coffee.
Since roasting my own beans, I've realized that there are a lot of variables you need to consider when figuring out why your cup of coffee tastes poor.
The reasons could be the brewing technique, roaster error, or even faulty equipment.
There are many ways a roast can go wrong as far as roasting. Some common ones are when the beans get scorched or contain quakers. These defects are physically distinct and are noticeable when placed against the rest of the batch.
Scorching:
Scorching is when you burn the beans. When scorching happens, the beans impart a smoky, burnt flavor. One can scorch beans by either applying too much heat upfront or too much heat for too long. When a bean is scorched, the outer parts of the bean exhibit dark patches or dark discoloring. Scorching becomes hard to detect early in the roast as any burned dark patches later blend in color.
Roasters should be consistently checking the tryer to ensure no scorching is occurring.
Quakers:
Quakers are easy to pick out. Quakers are unripened beans grown in poor soil conditions and receive little sugar and starch development.
Quakers will be lighter in color than the other beans and affect the astringency and sweetness of the end coffee.
Then there are the less noticeable defects that can come from a roast.
Underdevelopment:
The coffee beans are constantly going through chemical changes during a roast. These changes correlate to the heat applied to the beans. If not enough heat is used, then specific reactions never happen as there's not enough energy to trigger required chemical reactions. A Mill City Roasters video mentioned they knew of a test where a roast was held at drying phase for an hour, and what resulted was no difference in end flavor. None of the flavor-developing chemical reactions were executed as there wasn't enough heat transfer to activate the chemical reactions. Albeit I'm a bit skeptical about the end test result, as if you go full out on drying, you'll dry out all the moisture, reducing conductive heat transfer due to lack of water, which I think has to have some effect on the end flavor.
What comes from an underdeveloped roast are grassy, green, vegetal flavors. It may seem strange to some to consider coffee tasting as so, but remember that pre-roasted coffee is green and is a seed.
Underdevelopment can be hard to pick out. Coffee beans have depth, meaning that they are dense and have mass inside their outer shell. This means that the inner parts of the bean will lag in development compared to the exterior. What could happen is you have a roast that outputs beans with a developed surface but an underdeveloped interior.
Below is a profile that resulted in an underdeveloped roast.
You can see that the bean temperature (violet line) never really crossed 200C, signaling that not enough heat was applied to this roast. If we were to determine the roast level from this article, it’d be between “drying” and “cinnamon", where cinnamon is the first point when coffee is drinkable.
At the time, I didn’t know the nuances between roasting a washed bean versus a natural (also known as dry-processed) bean.
The strategy I took was I thought I could use the same heat settings for a natural bean with a washed bean. Per the graph, we can see that’s not the case. Washed beans tend to have more moisture, so they require more heat.
Then in the cupping session, the natural bean (Cup 4, Ethiopia Natural) came out fine while Cup 2 (Ethiopia Washed) came out underdeveloped.
You can see in my blind cupping notes that I was able to pinpoint the underdevelopment through the grassy, sour, and hay-like flavors exuded from Cup 2.
Baked Coffee:
Baked coffee is just as bad as underdeveloped coffee. As far as I know, you can't tell a coffee is baked by looking at it; you have to cup it to determine if it's baked. Baked coffee tastes like bread and hay.
Baked coffee can happen when the roast is roasted in an uncontrolled way.
Below is a profile that resulted in bake flavors.
This was my very first roast on the Bullet. As you can see, I kind of went crazy with the power controls, resulting in a Rate of Rise (RoR) that went up/down.
Then when I cupped it, I got notes of baked flavors.
My results fall in line with what Rao points out in his article on baked coffee:
the main cause of baked coffee is a pronounced ROR crash, or more precisely, a drastic change in the slope of the ROR.
Another way to mistakenly bake coffee is if not enough heat was applied to get the roast to first crack. I, unfortunately, don't know more details behind this to elaborate on this point.
After roasting the beans, defects can still come out during the brew. You could over-extract the coffee, under extract the coffee, brew with stale beans, or make any other brewing mistake.
The number of points of failure makes you think about what needs to go right to get a good cup of coffee.
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