After First crack in one of two distinct heat-induced pyrolytic reactions in coffee. It is distinguished by a cracking or popping sound in the coffee, and occurs between 390 and 410 degrees Fahrenheit in most coffee More, a roast reaction around 440 to 450 degrees that is distinguished by a snapping sound. Second An audible popping sound heard during roasting. In coffee, one refers to "first crack" and "second crack," which come from two different classes of chemical reactions.: An audible popping sound heard during roasting. In coffee, More is the second audible clue the roaster-operator receives about the degree-of-roast, following First Crack. Whereas First Crack sounds a bit like popcorn popping, Second Crack has a faster, shallower pattern, much like Rice Krispies in milk, electrical sparking, a snapping sound. Second crack is a further stage of the pyrolytic conversion of compounds in coffee and occurs around 440 to 450 degrees Fahrenheit. The 2nd crack is a physical fracturing of the cellular matrix of the coffee, and results in an eventual migration of oils to the outside of the bean, as they are freed from their chambers within the coffee. When second crack is volatile, it can blow small discs off the coffee bean.