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Concentration (in parts per trillion) at which humans can detect wine aroma compounds called pyrazines. Professor Marian W. Baldy, PhD compares this extraordinary capacity to “sniffing out a one-cent error in your $10 billion checking account.” Pyrazines (short for methoxypyrazine) are the compounds responsible for the green, herbal, savory aromas (like bell pepper) in many Bordeaux varieties such as cabernet sauvignon, merlot, and sauvignon blanc.