The modification of a sunflower oil used for 75 repeated deep‐fat fryings of potatoes, with a fast turnover of fresh oil during frying, was evaluated by measuring the total polar components isolated by column chromatography. The total polar components increased rapidly during the first 20 fryings from 5.09±0.21 (mean±SD) mg/100 mg unused oil to 15.99±0.40, followed by minor but also significant changes until the thirtieth frying (17.99±0.41 mg/100 mg oil). The level did not increase further with continued frying. Further, the polar fraction was examined by high‐performance size‐exclusion chromatography. Triglyceride polymers increased from 0.10±0.01 mg/100 mg unused oil to 1.65±0.13 and 3.44±0.17 mg/100 mg oil at the twentieth and seventy‐fifth fryings, respectively. Triglyceride dimers also increased significantly from 0.75±0.12 mg/100 mg unused oil to 6.25±0.28 (mg/100 mg oil) at the twentieth frying and to 7.09±0.31 mg/100 mg oil at the thirtieth frying, with no further significant changes. Oxidized triglycerides also significantly increased, but at the twentieth frying reached a near‐steady state of 6.26 mg/100 mg oil. Diglycerides and free fatty acid levels, related to hydrolytic alteration, did not increase with continued fryings. The results indicate that during deep‐fat frying of potatoes with fast turnover of fresh sunflower oil, more thermoxidative than hydrolytic processes take place. A dramatic leap of total polar content and a change of compounds related to thermoxidative alteration of the oil were found during the first twenty fryings, followed by minor changes and by a tendency to reach a near‐steady state throughout the successive fryings.

Thermoxidative and hydrolytic alterations of a sunflower oil used in sixty repeated and discontinuous deep-fat fryings of potatoes were evaluated by column and high-performance size-exclusion {HPSE) chromatography. Successive fryings of potatoes in sunflower oil, without turnover of fresh oil during the performance of fryings, increased the level of total polar components in the oil from 3.75% to 27.28% (w/w). Triglyceride polymers, triglyceride dimers, oxidized triglycerides and diglycerides increased after sixty fryings 89.8, 21.8, 4,9 and 1.7 times, respectively. These increases were well correlated with the number of fryings. However, there was not significant correlation between levels of free fatty acids and the number of fryings. Polar compounds were highly (r = 0.9691) and significantly (P<0.01) correlated with triglyceride polymers and also highly (r = 0.9969 and r = 0.9738) and significantly (P < 0.01) with triglyceride dimers and oxidized triglycerides, respectively. Nevertheless polar compounds were not significantly correlated with free fatty acids. Data suggest that an intensive thermoxidative rather than a hydrolytic process takes place in experimental deep-fat frying of potatoes.

Peroxidation of LDL and other lipoproteins is thought to play a central role in atherogenesis. Dietary thermally oxidised oils may increase atherogenic risk in consumers by increasing their oxidative status. The present paper compares the effects of two diets containing unused sunflower-seed oil (US) or sunflower-seed oil repeatedly used in frying (FS) (both 15 g/100 g diet) on weight gain, food efficiency ratio, serum lipid levels and lipoprotein composition, and the content of thiobarbituric acid-reactive substances (TBARS) in the liver, serum, and lipoproteins in growing Wistar rats. After sixty potato fryings the FS contained 27·7 g polar material/100 g oil and 16·6 g oligomers/100 g oil. The FS-fed rats had a significantly lower weight gain and food efficiency ratio. Liver-TBARS increased due to the consumption of the highly altered oil and showed a significant linear relationship (all r . 0·68; P,0·002) with the ingestion of thermally oxidised compounds. Serum-, VLDL-, LDL-and HDL-TBARS were significantly higher in the FS-fed rats (all P,0·001). Concentrations of serum total and non-esterified cholesterol and phospholipids were significantly higher in the FS-fed rats (P, 0·05, P, 0·05, and P,0·001, respectively). Serum triacylglycerol concentrations did not vary between the two dietary groups. Total and esterified cholesterol and phospholipid levels increased significantly in the HDL fraction (P, 0·05, P,0·05, and P, 0·001, respectively) of the FS-fed rats. HDL-cholesterol and HDL-phospholipids were significantly correlated with liver-TBARS (r . 0·747; P, 0·0001), VLDL-TBARS (r . 0·642; P, 0·003), LDL-TBARS (r . 0·475; P,0·04), and HDL-TBARS (r . 0·787; P, 0·0001). The data suggest that the rat increases HDL as a protecting mechanism against the peroxidative stress induced by the consumption of a diet containing the thermally oxidised oil.Deep-fat frying: Serum and lipoprotein peroxidation: Serum and lipoprotein lipids: Liver peroxidation

The alterations of a sunflower oil were evaluated by column, gas and high-performance size-exclusion chromatography after being used for deep-fat frying fifteen repeated and discontinuous times. Polar compounds increased significantly (6.2 _+ 0.3% to 18.7 --_. 0.8% in oil). Linoleic acid decreased (53.8 + 0.2 to 48.1 ___ 0.8 rag/100 mg oil) while oleic acid remained unaltered after 15 fryings. Saturated fatty acids such as palmitic and stearic, also remained unaltered. Triglyceride polymers (0.1 + 0.0 to 2.4 +_ 0.2 mg/100 mg oil), triglyceride dimers (1.0 --+ 0.2 to 6.7 +--0.3 rag/100 mg oil) and oxidized triglycerides (3.4 +_ 0.2 to 7.6 +_ 0.3 rag/100 mg oil) increased significantly in the oil used 15 times to fry potatoes. These thermoxidative compounds correlated well with the number of fryings (r = 0.9864, r = 0.9535 and r = 0.9758, respectively). Diglyceride compounds remained unaltered, while free fatty acids increased from 0.4 __ 0.0 to 0.6 --+ 0.0 rag/100 mg oil. Both of these, which are characteristic of hydrolytic alteration, did not correlate significantly (r = 0.5985 and r = 0.4261, respectively) with the number of fryings. These data suggest that a thermoxidative process, rather than a hydrolytic one, took place in this study.KEY WORDS: Column chromatography, deep-fat frying, gas chromatography, HPSEC, polar compounds, sunflower oil, thermoxidation. polar fraction was evaluated by the column chromatography method of Waltking and Wessels (8), with a modified proportion of petroleum ether/dietyl ether used to fill the column and to elute the nonpolar fraction.

Standard chemical and physical indexes, such as color index, acid value, and K270 were compared with the polar content and the specific thermoxidative and hydrolytic compounds originated during 75 potato fryings with sunflower oil. The color index, acid value, K270 and total polar content showed a significant increase (p < 0.05) after the first 30 fryings, followed by a tendency of the last three indexes to reach a near-steady state. Triglyceride polymers and triglyceride dimers showed a significant increase (p < 0.05) after 30 fryings, while the oxidized triglycerides were not increased significantly after 30 fryings. The amount of diglycerides and free fatty acids related to hydrolytic alteration did not change significantly throughout the frying operations. The significant correlations between acid value and total polar content, triglyceride dimers, triglycerides polymers and oxidized triglycerides, or between color index and polar content and triglyceride polymers or between K270 and triglyceride polymers indicate that these simple and standard methods can be applied and they are as useful as more specific methods of evaluation in the monitoring of frying operations, provided that initial values of all of these indexes are available.