Probl Hig Epidemiol 2011, 92(4): 848-851pl

Effect of onion on nutritional value of rapeseed oil

Małgorzata Stec, Ewa Kurzeja, Piotr Druszkowski, Katarzyna Pawłowska-Góral

Katedra i Zakład Żywności i Żywienia, Wydział Farmaceutyczny z Oddziałem Medycyny Laboratoryjnej, Śląski Uniwersytet Medyczny w Katowicach

Summary
Introduction. Rapeseed oil contains large quantities of polyunsaturated fatty acids which influence its nutritional value but are perishable during storage and thermal processing. Common onion, widely used as a vegetable and spice, contains biologically active substances (including flavonoids and organic sulfur compounds), which may make it a natural antioxidant.
Aim. The influence of crushed fresh common onion on the nutritious value of rapeseed oil during storage in room temperature, in a well-lit or dark place, and thermal processing afterwards.
Material & methods. The tested materials were rapeseed oil and fresh common onion. During the tests, conducted on oil directly after the opening of bottles, after storage with fresh onion and then after heating it to the temperature of 178±2°C and processing in that temperature for 15 and 30 minutes, the following were measured: organoleptic properties, acid value (LK) - the indicator of triacyloglycerole hydrolysis as well as peroxide value (LN), anisidine value (AV) and malondialdehyde concentration (MDA) - the indicators of oil oxidation.
Results. Adding fresh onion to rapeseed oil gave it an intense smell and taste of onion, which became less pronounced with the time of its exposure to heat, until they disappeared. The presence of fresh onion in rapeseed oil decreased the intensity of its oxidation processes while the oil was stored and decreased generation of primary and secondary products of the fatty acid oxidation.
Conclusions. Adding fresh onion to rapeseed oil may be used to obtain "onion oil", which could be used with raw products, for example in salads. Despite the protective influence of onion, rapeseed oil should not be thermally processed due to the appearance of large amounts of the fatty acid oxidation products.

Key words:  common onion, rapeseed oil, lipid peroxidation