Think of wine. The majority of wine is produced by the same grape: vitis vinifera. Yet, this same humble grape grown in Long Island brings a markedly different character from the same grape grown in Chardonnay. Much like the grape is the seed of vitis vinifera (there *are* other varieties), chocolate is exclusively the seed of the Theobroma cacao tree. Factors such as soil acidity, sunlight, temperature, rainfall, elevation and wind all affect the flavor of the cacao bean. Unlike the grape, however, chocolate is made from the seed itself, not the white flesh of the cacao fruit. There is less room for variation, and due to major subsequent processing, source is a little less important than other steps. Nonetheless, if you want to explore the importance of source, find a single chocolate maker (whom presumably processes all chocolates identically) and get a selection of samples from different sources. Sao Tome in Africa might be a little more fruity, South American chocolate is acidic, and chocolate from Malaysia might have a hammy or bacon-like note (these are not rules, just generalizations).
Like coffee, chocolate must be fermented. After harvesting the pods (pictures of chocolate pods!) , they are split and put on the ground, covered with banana leaves (or put in a box), and allowed to ferment for about 5 days. During fermentation much of the color and flavor of chocolate develops. What is going on? The temperature rises. The initial critters come from the environment (the machetes, the hands of the workers, the banana leaves, etc.). Yeasts, bacteria and fungi come in successive waves, each feasting and excreting bits and pieces of the pulp and creating a micro environment suitable for the next wave of little critters (e.g. suitable pH and temperature). Simple sugars and compounds are turned into much more complex flavors. The pectin/glue that holds the pulp together is digested, possibly assisting in the leeching of bitter alkoloids from the seeds themselves (unfermented cocoa is inedible). During this aerobic fermentation, tannins and polyphenols form brown pigments. Other pigments are degraded, such as purple anthocyanidin pigments. The cotyledon in the seed itself is bleached. Enzymes such as endoproteases, aminopeptidases, carboxypeptidases, polyphenol oxidase and glycosidases form flavor compounds and precursors to flavor compounds. At the end of 5 days, up to 1,000,000,000 microbes per gram of pulp is present, and the pulp is then uncovered and dried in the sun. But the fun doesn't stop there. Bacillus bacteria hang around, and molds come in and assist in the development of further flavor. Most enzymes are inactivated, but the endoproteases and glycosidases are still present, thus altering the character of the beans after they are separated from the pulp, dried, and stored for further processing.
Roasting is probably one of the greatest contributors to flavor. Temperatures range from 115 to 140C and roasted flavors are created through "Maillard" pathways. Mailliard browning is a type of non-enzymatic browning reaction that occurs in food (and people as we age!). The reaction is between (reducing) sugars, proteins (amines) and heat. The delicious flavors of chocolate, coffee, browned steak and roasted chicken all come from the Maillard reaction. About 3,500 volatile compounds are formed by this reaction and many have low sensory thresholds (i.e. can be tasted in small quantities). In chocolate specifically, a class of compounds known as pyrazines is formed which become the dominant identifier for "chocolate aroma."
4. Final Processing
Roasted beans are milled to separate the shell from the kernel, which produces cocoa nibs. The nibs are then ground (sometimes for days in artisan operations) to produce finer and finer texture. The cocoa fat melts and this combinations of finely ground cocoa and fat is known as chocolate liquor. Particle size is very important to the quality of the finished product. Smaller particles create a smoother chocolate with a more intense flavor (due to the surface area of the cocoa). Conching, or aerating the chocolate creates the final step in flavor, viscosity and color development by elimination of some volatile acids, breaking up crystals and removing moisture.
Tempering, the bane of home chocolatiers, is critical for texture of the final product. Through measured heating and cooling of the chocolate, crystals are formed which make the difference between chocolate with "snap" (think really good dark chocolate) that is shiny, and chocolate that is dull and has a much softer (more undesirable) texture. Milk and sugar are added to create differing levels of sweetness and additional flavor.
Finally, practice is the best way to do this. Using this tasting technique, get some high and low quality chocolates. Look at the ingredient labels and look for the most pure you can find. Side by side, there is a BIG difference between the good and bad. Think about all that goes into making that chocolate bar also!
Two videos to finish us up!
Case, Christine. The Microbiology of Chocolate. http://www.smccd.edu/accounts/case/chocolate.html
Reineccius, Gary. (2006). Flavor Chemistry & Technology, 2nd edition. Boca Raton, FL: CRC Press.