To some extent the quality of a given lot of coffee can be determined by just looking at it… are the beans close in size, or is there a wide range of sizes in the bag? Sometimes the single variety is screened so the beans are all within a certain range of sizes, other times they are not. This alone is not a reliable determinant of flavour. I’ve had some unscreened coffees that roasted up very evenly and had amazing flavour. Other times, not so much.
As to colour… after roasting… one of two things has happened: either, as the other Nico explained, less than careful picking includes too many unripes. This IS a quality determinant, as the hard quakers taste sour or otherwise “off”. Lower quality coffee can often include many unripes. The other possible explaination for colour variation is improper modulation of heat application during the roast. Gentler heat allos the beans to draw heat from the roaster more evenly, too rapid heat application can scorch some and leave others not heated enough throughout the roast. Learning to modulate heat application to keep Rate of Rise under control during the various parts of the roast is key to a good roast.
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