Pungency and volatile compounds in some Spanish onion cultivars

Onion (Allium cepa L.) is the most representative species which forms part of the Alliaceae family. Onion is one of the most important vegetal crops throughout the world. The presence of sulfur compounds in the alliums greatly influences its aromatic characteristics and provides the particular smell and spicy flavour known as pungency. The sulfur volatiles compounds are formed by enzymatic reaction, the hydrolysis by allinase of S-alk(en)yl-L-cysteine sulfoxides. This work characterizes the volatile compounds and pyruvic acid content of four cultivars of autochthonous onions from a vegetable germplasm bank in Zaragoza (Spain). Onions were grown in a greenhouse under the same conditions of time, temperature, climate, soil, and nutrient supply and samples extracted after peeling the onions. The content in acid pyruvic presented values between 4,96 and 13,8 μmol g-1 fw of acid pyruvic. The aroma volatiles of the cultivars were analysed by gas chromatography coupled with mass spectrometry and the aroma profiles were similar with a total of 106 volatile compounds identified in the four cultivars. Half of the volatiles were sulfur derived compounds. The higher pungency and pyruvic acid concentration was found in the cultivar ‘Betanzos’, which also contained the largest number of volatile organic compounds and 26 exclusive volatiles when compared with the other cultivars.