There are roast defects which we can all mostly agree on, and these should be avoided if desired. Here is the short list.
Baked: These are beans that have had too much time stuck at the same temperature, usually somewhere near the end of a roast. It’s a common issue as near the end of the roast we may try to extend development time by lowering the heat and if we lower too much we can stall. This basically removes the moisture from the beans without changing the color gradient much. This is said to be an invisible defect, only detectable in the flavor, which will be flat or possibly metallic, and lacking any sweetness. I find that its fairly obvious when this has happened as the moisture content is quite low and the beans will feel light and airy, and will not smell very nice once cooled.
Underdeveloped: This will have a vegetative flavor and aroma, sometimes grassy. Usually this will be combined with high levels of acid which is to some palates not very tasty. I do not enjoy underdeveloped coffee and will throw out beans with this issue. I find it to be a common issue with commercial fancy shops as they are trying to get the coveted floral or fruity flavors in the roast.
Overdeveloped: I find this to be very subjective and also somewhat misunderstood. Overdeveloping coffee in my opinion means that the development period has been too long, however often folks will say that very dark roasted coffee is overdeveloped. This is actually just dark roasted coffee. Overdeveloping a roast would be extending the overall development period, without baking and without adding darkness to the roast.
Scorched: Here is something we can all agree is not a good thing. When beans contact a very hot metal surface, it can instantly darken the contacted area. This can happen when roasters are preheated or charged to a high temperature and then beans are added and not mixed well enough. At a certain level of heat, no amount of mixing is going to prevent this. I find it adds a char flavor or smoke flavor and is visible early if it happens early by looking at beans which are just yellowing. If you see darkened spots on a yellow bean, it may be scorched. The other indicator is that when following a recipe or roast pattern, beans are darker than they should be, in other words, you dump your roast just off the crack and they are already quite dark when they should be a very light roast. It can certainly happen at any time during the roast but is most evident and also avoidable during the early stages as it is visible.
Tipping: This is when the ends of the beans are darker than the rest of the bean, and sometimes have a hole surrounded by a dark area. This happens from too much heat application with the possible addition of low airflow. This is an explosion of gas inside the bean which jets out superheated gas, blowing a hole in the bean and burning the surrounding area. This is something which should be avoided as it really changes the flavor of the beans. I find this happens if I am trying to “catchup” to a roast which is going to slowly for the desired profile. Slow and steady is the key to avoiding this.
Quakers: This is not really a defect from roasting but rather a material defect, however this has the ability to alter, often negatively the flavor of your roast. Really a quality control issue, these are beans that never ripened properly. They will often remain much lighter than the rest of the roast and will have a strong nutty taste and sometimes a powdery texture. It is a very strong flavor which can become pervasive in a brew, so after your roast is cooled, pick them out. I find that when getting my beans from a home bean supplier, I find very few of these, but when I get my green from a roaster, getting it from the sack (my favorite way to get green by the way) I may have more of these. It really all depends on your supply chain, but its so easy to solve, I don’t worry too much about it.
Uneven roast: This is a roast where there is a large color difference between beans in the same roast. This can happen from a lack of proper agitation, but also from a lack of balance between conductive and convection heat. Convection heat will typically produce a very even roast, whereas pure conductive heat, such as a frying pan, or a drum roaster will very low air flow may lead to an uneven roast. This may look like a mélange, but you will likely have the worst characteristics of each color, so really try to avoid it. A note on this is that natural process beans may have a more uneven appearance, but this is due to a natural variance in the product, so don’t assume your roaster is malfunctioning when roasting naturals.
So the takeaway from all this?
Apply steady even heat, agitate evenly and monitor your process all during the roast. Use a magnifying glass to view your results, and keep track of what you are doing the entire time so when you see a defect, or better yet, when you roast with no defect, you will know how to repeat it. Often times when I get a new bag of green I will pull samples every minute or two and save them in order, then go back and really examine them. If you do this, be aware that you are changing the volume in your roaster and it may want to race as you get near the end, so if you are doing a sample roast to determine timing, you may want to not pull samples out or your timing roast will be invalid.