Sourced from Sweet Maria’s Glossary
Crack: 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.
First Crack: 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 roasters. It has a sound more similar to the popping of popcorn, whereas the Second Crack that occurs around 440 to 450 Fahrenheit has a more shallow, rapid sound, like the snapping of Rice Krispies cereal in milk! First crack involves a rapid expansion of the coffee seed and marks the point where water and carbon dioxide fracture, leading to the liberation of moisture from the coffee in the form of steam. First crack opens the crease in the bean enough to release remaining silverskin, or chaff. First crack is a clue to the roaster-operator about the roast level, and it's termination generally marks the first stage (City Roast) where coffee is acceptably dark enough to enjoy.
Second Crack: Second Crack 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 patter, 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.
City Roast: City roast is what we define as the earliest palatable stage that the roast process can be stopped and result in good quality coffee. City roast occurs roughly between 415 and 425 degrees Fahrenheit in many coffee roasters with a responsive bean probe where First Crack starts in the 395 to 405-degree range. The benefits of City roasts are that the origin flavor of the coffee is not eclipsed by the development of strong roast flavors, but the risk is that sourness, astringency, or under-developed sweetness makes the cup unpleasant. City roast generally has a light brown color with strong surface texture, even dark creases in the bean surface, and only moderate expansion of the bean size. This varies greatly in different coffees though. As a very general rule, to reach City roast the coffee is removed from heat at the last detectable sound of First Crack, or very soon after, with no further development toward 2nd crack.
City+ Roast: City+ roast is an ideal roast level that occurs roughly between 425 and 435 degrees Fahrenheit in many coffee roasters with a responsive bean probe where First Crack starts in the 395 to 405-degree range. Also called a medium roast. This range of roast temperatures is after City roast (hence the + !) and indicates that the coffee has been allowed to develop further, anywhere from 10 seconds to 1 minute or more depending on roast method, after the last "pop" of First Crack was heard. These times and heat ranges vary greatly depending on the roast machine and green coffee. The benefits of City+ roasts is the balance between moderate roast flavor and the origin flavor of the green bean; astringent, sour or "baked" light roast flavors are reduced, yet the flavors specific to a particular coffee lot are still expressed in the cup flavor. City+ has a brown color and may not yet have the smooth surface that comes as further browning and bean expansion occur as the coffee approaches 2nd crack. This is a term Sweet Maria's basically invented (well, designating a +), and while used in the trade a bit, it has its context in our communications to home roasters more than anything.
Full City Roast: A coffee that has been roasted to the brink of second crack. The internal bean temperature that second crack normally occurs at is 446 degrees F. But in fact, second crack is a little less predictable than first crack, in my experience. Why? It could be explained as this: first crack is the physical expansion of the coffee seed as water and carbon dioxide split and CO-2 outgassing occurs. Second Crack is the physical fracturing of the cellular matrix of the coffee. This matrix is wood, also called cellulose, and consists of organized cellulose that reacts readily to heat, and not-so-organized cellulose that does not. Since every coffee is physically different in size and density due to the cultivar, origin, altitude, etc. it might make sense that the particular cell matrix is different too, and not as universally consistent in reactiveness as H-2O and CO-2.
Full City+ Roast: A roast slightly darker than Full City. At Full City+, the roast is terminated after the first few snaps of second crack. The main cue that distinguishes the difference between the Full City (or FC) and Full City + is audible, not visual. This is a term Sweet Maria's basically invented, and while used in the trade a bit, it has its context in our communications to home roasters more than anything.
Vienna Roast: Vienna roast occurs at the beginning of second crack. The Vienna stage (also called Continental) to Light French stage is where you begin to find origin character eclipsed by roast character. If you buy coffee for its distinct origin qualities, it makes sense that heavy roasting is at odds with revealing the full effect of the differences we can sense in coffee due to distinct origins. Nonetheless, some coffees are excellent at this stage. Vienna is a common roast level for espresso.
French Roast: Sugars are heavily caramelized (read as burned) and are degraded; the woody bean structure is carbonizing, the seed continues to expand and loose mass, the body of the resulting cup will be thinner/lighter as the aromatic compounds, oils, and soluble solids are being burned out of the coffee and rising up to fill your house with smoke. 474 is well beyond any roast I do on the Probat. Second crack is well finished. I will go as high as 465 on a couple blends, and that's my limit.
Quakers: A quaker is an industry term to describe under-ripe, undeveloped coffee seeds that fail to roast properly. These are most often the result of unripe, green coffee cherry making it into the final product. Normally, these are skimmed off as floaters (in the wet-process) or visually removed in the dry-process method. They are removed on the density table (Oliver table) as well. They occur much more often in dry-process coffees due to the lack of water flotation of the fruit, and the difficult task of removing them visually. Even the best coffees might have occasional quakers, and they can be removed post-roast when they are easy to see. Under-developed coffees do not have the compounds to have a proper browning reaction in the roaster (Maillard Reaction, caramelization), so they remain pale in color.
Roast Profile: Roast Profile refers to the relationship between time and temperature in coffee roasting, with the endpoint being the "degree of roast". Roast profiling is the active manipulation of the "roast curve" or graphed plot of bean temperature during the roast, to optimize the results in terms of flavor. Two batches might be roasted to the exact same degree of roast, temperature endpoint or time, and have very different cup results due to different roast profiles. It's not just important how dark a coffee is roasted, it is equally important how it got there, and that is expressed in the roast profile.
Caramelization: Caramelization is slower than Maillard reactions and requires higher temperatures. These reactions involve only sugars. They really begin up around 150C to 180C, with water being lost from the sugar molecule beginning the chain of events. In all cases the sugar is converted to a furfuryl. These are a type of furans that have a caramelly, slightly burnt and also slightly meaty notes. The same compound is produced via a different route in the Maillard reactions. However, it is with prolonged high temperature that many other types of aromas are generated. Caramelization is more predictable than Maillard reaction due to less variation in the starting compounds. Without the sulfur or nitrogen found in the amino acids caramelization is unable to produce flavors as meaty as Maillard reactions. It is interesting to note how the sugar solutions taste changes in caramelization. A sugar solution initially will be sweet with no aroma. Through caramelization it becomes both sour and a little bitter, as a rich aroma develops. Generally, the longer sugar is caramelized the less sweet it tastes, so the key is to balance the benefits of uncaramelized sugar sweetness while avoiding light roast astringency and sourness.
Maillard Reaction: The Maillard reaction is a chemical reaction between an amino acid and a reducing sugar, induced by heat in the coffee roasting process. It results in the browning color of coffee (from melanoidins, which are key to espresso crema too), as well as many volatile aromatics and flavors. It is not unique to coffee, and is at work in a variety of food conversion or cooking operations: toasted bread, malted barley, roasted or seared meat, dried or condensed milk.
Strecker Degradation: The Strecker Degradation is an interaction of amino acids (AKA proteins) with a carbonyl compound in an environment with water, resulting in the creation of CO2 and an Aldehyde or Ketone. The later two components are important for volatile aromatics and flavors, and the Strecker Degradation contributes to browning. It involves compounds formed in the Maillard reaction and is therefore necessarily linked to it in coffee roasting.
Pyrolysis: Pyrolysis is the chemical decomposition of a condensed substance by heating. It is a special case of thermolysis and is most commonly used for organic materials. At lighter levels, caramelization of sugars is an important result of the pyrolysis of coffee, the release of CO-2, and a host of other chemical and physical changes in the coffee. There are two stages of pyrolysis in coffee which we call "First Crack" and "Second Crack." Extreme pyrolysis, which leaves only carbon as the residue, is called carbonization and leads to charred flavors in very dark coffee roasts.
Pyrolysis often occurs spontaneously at high temperatures, for example in fires and when organic materials come into contact with lava in volcanic eruptions, and has been assumed to take place during catagenesis, the conversion of buried organic matter to fossil fuels. It is an important chemical process in several cooking procedures such as baking, frying, grilling, and caramelizing. It's important to note that chiefly involves the conversion of carbohydrates (including sugars, starch, and fiber) and proteins.