Evaluation of a sunflower oil used for frying by different analytical indexes and column and gas chromatography.

The alteration of a sunflower oil used repeatedly and discontinuously for frying potatoes on 15 successive occasions was studied. For this purpose, standard analytical indexes, such as fatty acids, gas chromatography, refraction and color indexes, and acid value were compared with a chromatographic method that quantifies the polar compounds originated during fryings. Total polar content increased significantly (p < 0.05) from 6.2 +/- 0.3 mg/100 mg oil to 18.7 +/- 0.8 mg/100 mg oil in the last frying. Linoleic acid decreased significantly (p < 0.05) from 53.8 +/- 0.2 mg/100 mg oil to 48.1 +/- 0.8 mg/100 mg oil at the 15th frying, while oleic acid concentration remained unaltered throughout the frying operations. The color index, and acid value, showed a significant increase (p < 0.05) after 15 fryings. Color index, acid value, and total polar content highly and significantly correlated with the number of fryings (0.981 > r > 0.933; p < 0.01). Linoleic acid concentrations also significantly correlated (r = -0.692; p < 0.05) with the number of fryings performed. Acid value, color index, and linoleic acid concentration also showed high and significant correlation with the percentage of total polar component of the oil (r = 0.9272, r = 0.9065 and r = 0.764 respectively; all p < 0.01). These data suggest that standard methods such as acid value and color index can be applied and are as useful as silica-gel chromatographic evaluation in the monitoring of frying operations, provided that initial values of both the color index and acid value are available.