What do these large bubbles in my French press indicate? I've seen these bubbles when pressing coffee that I've home roasted in two different presses with two different water sources. I've seen it with coffee from multiple roasts now. The beans are the same in all cases (Rwandan single origin). I am grinding with a burr grinder. Do these large bubbles in the bloom indicate anything about the bean or the roast?
The bubbles are the result of "degassing" — carbon dioxide escaping from the coffee. Degassing will be especially dramatic in the first 24 hours after roasting, during which 40% of the CO2 leaves the bean. See this blog post: Understanding Degassing.