AROMA PROFILES AND PUNGENCY IN DIFFERENT “TROPEA” RED ONION (ALLIUM CEPA L.) ECOTYPES AND COMMON RED ONIONS.

Onion (Allium cepa L.), from Liliaceae family, are the second most important horticultural crop in the world, eaten for its unique aroma and taste (1). The effects of onions consumption in protection against cancer, coronary heart disease, diabetes as well as ageing, mainly attributed to organosulphur compounds, flavonoids, vitamins and some minerals (2). “Tropea Red Onion” is a sweet red onion characterized by a large red envelope and internal almost white tunics and excellent organoleptic characteristics, mainly cultivated, in three different ecotypes, in the tyrrhenian coastal areas of Calabria region (Southern Italy) For its tenderness, crispness and sweetness is considered one of pride italian horticultural products and then one of the most counterfeited. The aim of this study was to evaluate the flavour (aroma and pungency) differences among Tropea red onion ecotypes and between them and the common red onions grown under the same conditions. The onion’s pungent aroma is produced when onion cells are ruptured releasing the enzyme alliinase. Alliinase reacts with flavor precursors, S-alk(en)yl-cysteine sulfoxide, to produce many volatile S-compounds, along with pyruvic acid and ammonia (3-4). This enzyme cleaves the precursors to produce alkenyl sulphenic acids. These intermediate rearrange rapidly to form thermally unstable thiosulphinates (Ti)(3-4). In the onion, 1-propenylsulphenic acid rearranges to give mostly isomers of di(1-propenyl)Ti (3-4), zwiebelanes (ZW) and (Z,E) thiopropanal-S-oxide, the lachrymatory factor (LF). The LF is considered the main compound responsible for onion pungency, but a lot of sulfur volatile compounds contribute to pungency of the onion (3-4). To evaluate the different pungency degrees between Tropea onions ecotypes and common red onions, the pyruvic acid concentration in onion juice (µmol/ml) as an indicator of pungency (5), by HPLC, and the others products of the enzymatic reaction, by SPME/GC/MS and GC/MS, were estimated. The main differences in volatile fraction of three Tropea ecotypes consist in the amount of Thiopropanal-S-oxide (LF) and zwiebelane isomer (ZW). The organic extract analysis by GC/MS, showed the presence, in the common red onions, of two interesting compounds one of these identified as propenyl-propanethyosulfinate, both absent in all Tropea ecotypes. The results are in good agreement with pyruvic acid amounts that in all three Tropea ecotypes (2,26 – 2,57 µmol/ml) were lower than the common red onions (3,57 µmol/ml).