GC–MS Profiling of 2,4-DNPH Derivatized Carbonyl Compounds in Fresh and Reused Frying Oils

Abstract

Repeated use of vegetable oil for frying has been reported as a potential contributor to many health hazards affecting human populations, particularly in developing countries where regulatory oversight may be weak or limited. Thermal oxidation of vegetable oils during repeated deep-frying promotes the formation of reactive carbonyl compounds, including aldehydes and ketones, which are associated with oxidative stress and related health risks. This study screened for short-chain aldehydes and other carbonyl compounds in derivatized re-used deep-fried culinary oil used by fast-food vendors, oil extracted from akara (bean cake) and fresh oil as a reference using GC-MS profiling. The three oil samples, namely fresh oil, oil extracted from akara and re-used deep-fried oil (100 mL each), were collected from roadside fast-food vendors in Enugu State, Nigeria. The samples were derivatized with 2,4-dinitrophenylhydrazine to form hydrazone derivatives, thereby improving volatility and detectability during GC-MS profiling. GC-MS analysis revealed greater abundance and diversity of aldehydes in re-used deep-fried oil than in fresh oil, including trans-2-decenal (0.10% area), trans-2-tridecenal (0.05% area), 2,4-decadienal [tt-DDE] (0.08%), cis-9-hexadecenal (0.07%), cis-1-chloro-9-octadecene (0.11%), hexanal and nonanal (0.74%). Akara oil showed pronounced alteration, characterised by 2-propenoic acid ester (31.47%), dodecanoic acid (28.68%), bis(2-ethylhexyl) phthalate (14.38%) and myristic anhydride (6.79%). Fresh oil showed minimal carbonyl presence. The observed increase in viscosity, foaming, altered taste, odours and darkening in colour are physicochemical changes associated with these culinary practices and indicate alteration of the fatty acid composition of the oil through hydrolysis, oxidation, polymerisation and tautomerisation. The chromatographic profiles indicate the presence of established toxic compounds and some novel compounds whose toxicities remain unknown. These findings indicate that repeated deep-frying markedly increases the formation of toxic carbonyl compounds, highlighting potential public health concerns associated with consumption of snacks prepared with re-used deep-fried oil.