Investigation on the volatile oil of Makó onions

The authors have developed a method for determining the volatile oil content of onions. Using this method they found in the Hungarian Makó onions 0.019—0.031 % volatile oil, depending on storage. The composition of onion oil was studied by GC; GC-MS and TLC methods. GC-MS revealed 20 volatile sulphur compounds identified as 10 trisulphides, 6 disulphides, 3 tetrasulphides, and 1 thiophene derivative as well as one unsaturated aldehyde. Three of these: ethyl cis-1-propenyl trisulphide, ethyl trans-1-propenyl trisulphide, and butyl methyl trisulphide were described for the first time in onions. Carbonyl compounds were studied in the form of derivatives obtained with 2,4-dinitrophenyl hydrazine. On the basis of Rf-values and by addition, propanal, butanal, pentanal and hexanal were identified in the aldehyde fraction, while in the dicarbonyl fraction — beside an unknown compound — glyoxal and methyl glyoxal were identified. The presence of these two compounds in onion oil has been reported for the first time.