J. R. Daniel Kirk:
However, coffee bean weight is determined, to no little extent, by the water naturally present in the bean. When you roast a coffee bean, one effect of the roasting process is that the bean dries out.
The longer you roast the bean, the drier–and therefore lighter!–the bean becomes. …
So here’s the question: should we, in fact, measure coffee by volume rather than weight in order to produce more consistent coffee? Or, alternatively, should we vary the weight of coffee such that fewer grams are in play for darker roasts and more for lighter roasts?
Water doesn’t account for much of an unroasted bean’s mass. According to strangers on the internet:
Water accounts for between 8 to 14 percent of the weight of unroasted coffee. … The total loss weight loss for an average roast is approximately 16 percent.
I like to use about 12g of beans in my AeroPress. That would be about 14g of unroasted beans, and honestly, I don’t think I would notice the difference between 12g and 14g most of the time. And that’s the weight difference for the entire roasting process.
But the difference between light and dark roasts is very small relative to the entire roasting process. Most good roasts are likely to fall in the narrow band between 430–460°F.
All of the water, being heated far past its boiling point into steam, probably escapes long before the lightest usable roasting temperature is reached, leaving no meaningful water-weight difference between light and dark roasts.
But let’s assume that there’s a tiny difference. For a 12g serving, a 1% weight difference is about 1 coffee bean. Your kitchen scale probably isn’t precise enough to notice.
So there might be a water-weight difference between lighter and darker roasts, but it’s probably too small to matter when you brew.