Effect of oil replenishment during deep-fat frying of frozen foods in sunflower oil and high-oleic acid sunflower oil

Frying stability of sunflower oil (SO) with 23% oleic acid and 61% linoleic acid, and of high-oleic acid sunflower oil (HOSO) with 74% oleic acid and 13% linoleic acid was studied during 20 discontinuous deep-fat fryings of various frozen foods, with or without frequent replenishment of the used oil with fresh oil. Alterations of both oils were measured by column, gas-liquid and high-performance size-exclusion chromatography. Total polar content and compounds, related to thermoxidative changes, and diacylglycerides, related to hydrolytic changes, increased in all oils during frying but reached higher levels in SO than in HOSO. Nevertheless, the increased levels of diacylglycerides observed may result from the frozen potatoes prefried in palm oil. Oleic acid in HOSO and linoleic acid in SO significantly decreased, but the fatty acid modifications that occurred during the repeated fryings were not only related to thermoxidative alteration but also to interactions between the bath oil and the fat in the fried products. Data from this study also indicated that HOSO performed more satisfactorily than SO in repeated fryings of frozen foods. Moreover, frequent addition of fresh oil throughout the deep-frying process minimized thermoxidative and hydrolytic changes in the frying oils and extended the frying life of the oils.     

Digestibility and absorption coefficients of palm olein - Relationships with thermal oxidation induced by potato frying

Sets of 500 g of potatoes were discontinuously fried in 3 1 of palm olein using a zero turnover of fresh fat until a polar content of 25% was reached. A total of 90 fryings were required. Total alteration significantly increased (p?0.001) with the number of fryings from 9.27 mg/100 mg oil to 14.81 and 26.36 mg/100 mg oil after 40 and 90 fryings, respectively. Triglyceride polymers, triglyceride dimers and oxidized triglycerides significantly increased (p?0.001) from 0.12, 1.02 and 1.12 mg/100 mg oil in the fresh oil to 3.67,7.92 and 8.32 mg/ 100 mg oil after 90 fryings, respectively. When these palm oleins were administered to rats at the level of 1 g/100 g body weight, a significant decrease was noted after 4 h experiment in the true digestibility and true absorption coefficients (p<0.05). However, the content of intracellular gastrointestinal fat did not change. The changes in digestibility and absorption were negative and highly correlated with total alteration and with thermoxidative alteration compounds (p<0.01). Results suggest that palm olein with a high level of thermoxidative products was digested less efficiently than the fresh one but did not affect the clearance of intracellular gastrointestinal fat.     

A Comparative Study of Physicochemical and Flavor Characteristics of Chicken Nuggets during Air Frying and Deep Frying

Chicken nuggets were air-fried and deep-fried for various times at the same temperature. The moisture content, oil content, color, texture, volatile compounds, and lipids degradation of the chicken nuggets were investigated. The evolution rate of the moisture content, oil content, color, and texture were much higher during deep frying than during air frying. Air frying required more time to produce similar quality attributes as those of deep-fried chicken nuggets. On average, the oil content of the air-fried chicken nuggets was 25% lower than that of the deep-fried chicken nuggets. Of the 74 volatile compounds identified in the fried chicken nuggets, aldehydes, hydrocarbons, and heterocycles were the main flavor contributors. The total amount of volatile compounds and aldehydes detected in the air-fried chicken nuggets was much lower than that in the deep-fried samples, but the number of heterocycles produced by the Maillard reaction was higher during air frying. Sensory evaluation revealed that the air-fried chicken nuggets had lower oiliness and crispness, and 9 min was the optimal frying time to obtain air-fried chicken nuggets with similar sensory attributes (color, odor, juiciness, and overall acceptance) as those of the best deep-fried chicken nuggets. Air frying was a healthier frying method for the production of fried chicken nuggets, which could reduce oxidative degradation of lipids in chicken nuggets.     

Cyclic fatty acids in sunflower oils during frying of frozen foods with oil replenishment

Frying of frozen foods has become popular because it considerably reduces cooking time. Polymers and cyclic fatty acid monomers (CFAM) formed during frying are potentially toxic and therefore their production should be minimized. Twenty discontinuous fryings of different frozen foods were carried out over ten consecutive days, in sunflower oil (SO) and in high‐oleic acid sunflower oil (HOSO), by adding fresh oil after each frying to bring the volume of the fryer oil back to 3 L. CFAM methyl ester derivates were hydrogenated, isolated, concentrated and quantified by HPLC using a reverse‐phase column, followed by gas chromatography. After 20 fryings, significantly higher contents of polar material, polymers and CFAM (all p <0.001) were found in SO than in HOSO. Bicyclic compound formation was four times higher in SO (p <0.001). The fat from the fried potatoes presented a polymer content very similar to that of their corresponding oils. The 100‐g rations of the SO‐fried potatoes from the 20^th frying supply 49 or 15%, respectively, more polymers and CFAM and 1 mg more bicyclic fatty acids than the 100‐g rations of HOSO‐fried potatoes. Because digestion and absorption of polar material, polymers and CFAM occur, the data clearly show the advantageousness and advisability of frying with HOSO rather than SO.     

Quantitation and Distribution of Polar Compounds in an Extra Virgin Olive Oil Used in Fryings with Turnover of Fresh Oil

The alteration of an extra virgin olive oil used in 75 repeated and intermittent deep-fat fryings of potatoes (with a frequent turnover of fresh oil) was measured by column and high-performance size-exclusion chromatography (HPSEC). Total polar content increased slightly but continuously in the fryer's oil during the first 30 fryings from 2.76 ± 0.01 mg/100 mg oil (mean ± SD) to 6.60 ± 0.00 mg/100 mg used oil, followed by a tendency to reach a near steady state after a large number of fryings. Compounds related to thermoxidative alteration also increased significantly in the oil used 75 times to fry potatoes. Triglyceride dimers increased from 0.03 ± 0.01 mg/100 mg unused oil to 1.65 ± 0.05 mg/100 mg oil at the thirtieth fryings while triglyceride polymers and oxidized triglycerides did so during the first 60 fryings (0.07 ± 0.01 mg/100 mg oil and 0.56 ± 0.03 mg/100 mg oil in the unused oil respectively to 0.38 ± 0.01 mg/100 mg oil or 2.95 ± 0.15 mg/100 mg oil in the used oil, respectively). Both of these changes were followed by a tendency of these polar compounds to reach a near steady state in later successive fryings. Nevertheless, diglycerides and free fatty acids related to hydrolytic alteration remained quite stable throughout the frying process. Data from this study indicate that repeated fryings of potatoes in extra virgin olive oil with a frequent turnover of fresh oil throughout the frying slightly increased the level of polar material in the fryer oil during the first fryings followed by minor changes and by a tendency to reach a near steady state in successive fryings.     

High-performance size-exclusion chromatographic studies on polar components formed in sunflower oil used for frying

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.     

Performance of Regular and Modified Canola and Soybean Oils in Rotational Frying

Canola and soybean oils both regular and with modified fatty acid compositions by genetic modifications and hydrogenation were compared for frying performance. The frying was conducted at 185 ± 5 °C for up to 12 days where French fries, battered chicken and fish sticks were fried in succession. Modified canola oils, with reduced levels of linolenic acid, accumulated significantly lower amounts of polar components compared to the other tested oils. Canola oils generally displayed lower amounts of oligomers in their polar fraction. Higher rates of free fatty acids formation were observed for the hydrogenated oils compared to the other oils, with canola frying shortening showing the highest amount at the end of the frying period. The half-life of tocopherols for both regular and modified soybean oils was 1–2 days compared to 6 days observed for high-oleic low-linolenic canola oil. The highest anisidine values were observed for soybean oil with the maximum reached on the 10th day of frying. Canola and soybean frying shortenings exhibited a faster rate of color formation at any of the frying times. The high-oleic low-linolenic canola oil exhibited the greatest frying stability as assessed by polar components, oligomers and non-volatile carbonyl components formation. Moreover, food fried in the high-oleic low-linolenic canola oil obtained the best scores in the sensory acceptance assessment.     

High-performance size-exclusion chromatographic studies on a high-oleic acid sunflower oil during potato frying

The behavior of a high-oleic acid 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 content isolated by column chromatography. The total polar content increased in the oil from 3.6 ± 0.1 (mean ± SD) mg/100 mg unused oil to 7.6 ± 0.4 mg/100 mg oil after being used in 20 repeated fryings, followed by a tendency to reach a near-steady state throughout the successive fryings. Further, the polar fraction was examined by high-performance size-exclusion chromatography. Triacylglyceride dimers increased continuously from 0.18 ± 0.01 mg/100 mg unused oil to 2.42 ± 0.12 mg/100 mg oil at the 40th frying with no further significant changes. The amount of triacylglyceride polymers increased from 0.03 ± 0.00 mg/100 mg unused oil to 0.70 ± 0.01 mg/100 mg oil at the 60th frying, but did not increase further with continued frying. Oxidized triacylglycerides also significantly increased from 1.13 ± 0.06 mg/100 mg oil to 3.58 ± 0.09 mg/100 mg oil at the 50th frying to reach a near-steady state in successive fryings. Diacylglycerides and free fatty acids levels, related to hydrolytic alteration, did not change from the starting oil after continued fryings. Data from this study indicated that repeated fryings of potatoes in high-oleic sunflower oil with a frequent turnover of fresh oil throughout the frying slightly increased the level of polar material in the fryer oil during the first fryings, followed by minor changes and a tendency to reach a near-steady state in successive fryings.     

Chemical reactions involved in the deep fat frying of foods. I. A laboratory apparatus for frying under simulated restaurant conditions

A laboratory apparatus has been designed which can be used to quantitatively collect the volatile decomposition products produced during deep fat frying under simulated restaurant conditions. In order to study the chemical reactions of frying fat without any inter-reaction with the food fried, moist cotton balls were fried in corn oil. The oil used for frying was shown to differ considerably from oil which was continuously heated. The latter had a darker color and higher viscosity. It foamed significantly while the oil used for frying did not. Furthermore, the continuously heated oil had a much lower free fatty acid content than did the oil used for frying under simulated restaurant conditions. The volatile decomposition products collected during frying of cotton balls in corn oil were separated into acidic and nonacidic compounds. Each group exhibited a definite gas chromatographic pattern after only a short period of frying. Part of the degradation products, particularly those of higher boiling points, were found to remain in the frying oil. Paper of the Journal Series, New Jersey Agricultural Experiment Station, Rutgers. The State University of New Jersey, Department of Food Science, New Brunswick.     

Sunflower oil used for frying: Combination of column,gas and high-performance size-exclusion chromatography for its evaluation

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 mg/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 mg/100 mg oil) and oxidized triglycerides (3.4 ± 0.2 to 7.6 ± 0.3 mg/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 mg/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.     

Influence of used frying oil quality and natural tocopherol content on oxidative stability of fried potatoes

Sunflower oil (SO) and high-oleic sunflower oil (HOSO) were used to prepare fried potatoes by either discontinuous or continuous laboratory frying. Fried potatoes that had been fried in oils of differing quality were stored at 60°C for up to 30 d and evaluated for polar compounds, polymers, peroxide value, oil stability index, and α-tocopherol content. Results obtained through the various methods applied were consistent and indicated that the length of the induction period could not be explained only on the basis of the degree of unsaturation or polar compound levels in fried potatoes before storage. α-Tocopherol content also had a significant influence as potatoes fried in HOSO, with 16% polar compounds and only 10 mg/kg α-tocopherol at the starting point of storage, were oxidized more rapidly than potatoes fried in SO with a comparatively higher degradation level, 19% polar compounds, and 100 mg/kg α-tocopherol.     

Polar content vs. TAG oligomer content in the frying-life assessment of monounsaturated and polyunsaturated oils used in deep-frying

The frying-lives of olive oil (OO), sunflower oil (SO), and a blend of these oils (BO) were assessed and compared by measuring the polar content (PC) and the TAG oligomer content (TOC) in the oils. Oil was replenished with fresh oil every 10 uses in all three oils. Changes in the PC and TOC in relation to the number of frying uses were fitted to different curvilinear models. The power model yields the highest R ² value in the three oils. The 25% PC was surpassed after 32.2 fryings [95% confidence interval (95% Cl) 29.1–36.8] in the OO, 22.5 (95% Cl, 21.0–24.6) fryings in the SO, and 27.5 (95% Cl, 25.5–30.1) fryings for BO. However, according to the 10% TOC cutoff point, OO should be discarded after 25.0 (95% Cl, 22.8–28.7) fryings, SO after 15.0 (95% Cl, 14.8–15.4) fryings, and BO after 17.7 (95% Cl, 17.0–19.1) fryings. Nevertheless, changes in PC and TOC were different only between OO and SO (P<0.05 and P<0.02, respectively), indicating that OO performs better than SO, and that BO can be used as an alternative when both frying-life and price are under consideration. Present findings suggest the need to unify criteria in different countries for oil disposal because the 25% PC corresponds to a TOC higher than 10%.     

Reducing the oil content in coated and deep‐fried chicken using whey protein

Whey protein has been investigated for its oil barrier properties in fried foods. Whey protein has been implemented successfully non‐film forming applications to reduce the fat content of deep‐fried chicken. As a 10% postbreading dip, whey protein solutions significantly lowered oil absorption of boneless patties by 37% compared to undipped controls. In bone‐in chicken thighs, 10% whey protein postbreading dip caused a 15% reduction in oil absorption during frying which was doubled by 10% β‐lactoglobulin postbreading dip at pH 2. A 30% lipid reduction was observed in deep‐fried battered and floured chicken strips when the strips were dipped in a 10% solution of denatured whey protein prior to frying compared to control chicken strips. When added to the batter of deep‐fried chicken nuggets, a significant fat reduction was observed for nuggets to which 3% whey protein was added to the batter compared to the control. In chicken strips that were marinated in a 15% solution of whey protein, a 35% fat reduction was observed. In spite of these results, questions that still remain. These studies have been undertaken in fully cooked products, not in raw coated or par‐fried products. The impact of rethermalization method is not known. Only preliminary research on the impact of WPI on the frying oil with respect to the formation of polar materials has been undertaken. On the positive side, the processes for introducing whey protein as an oil inhibition agent utilize processing steps that are already very common, e.g. incorporation into the batter or a marinade. Whey protein has the potential to be employed as an oil inhibiting agent in fried foods especially as it is relatively inexpensive and easy to come by.     

Review of stability measurements for frying oils and fried food flavor

Measurements of degradation in frying oils based on oil physical properties and volatile and nonvolatile decomposition products were reviewed. Rapid methods by means of test kits were also considered. Factors that affect the analysis of total polar components (TPC) in frying oils were examined. Relationships between TPC, free fatty acid (FFA) content, Food Oil Sensor readings (FOS), color change (ΔE), oil fry life and fried-food flavor were evaluated. Flavor scores for codfish, fried in fresh and discarded commercial frying oil blends, were dependent upon individuals in the consumer panel (n=77). Part (n=29) of the panel preferred the flavor of fresh fat; others (n=24) didn't; the rest (n=24) had no preference. FFA, FOS and TPC were analyzed in two soybean oils and in palm olein during a four-day period in which french fries were fried. Flavor score and volatiles of potatoes fried on days 1 and 4 in each oil were also determined. TPC, FFA and FOS significantly increased (P<0.05) in all oils during the frying period. TPC and FFA were highest in the used palm olein, and flavor of potatoes fried in palm olein on day 1 was less desirable than those fried in the soybean oils. Potatoes fried in day-1 oils had significantly higher concentrations (P<0.10) of several pyrazines and aldehydes than those fried in day-4 oils. Presented at the 84th Annual Meeting of the American Oil Chemists' Society, Anaheim, California, April 25–29, 1993.     

Effect of ascorbyl palmitate on the quality of frying fats for deep frying operations

The addition of 0.02% ascorbyl palmitate (AP) reduced color development of frying fat (animal fat/vegetable oil [A-V] shortening) and vegetable oil (partially hydrogenated soybean [V-S] oil) in simulation studies. It also reduced peroxide values, development of conjugated diene hydroperoxides (CDHP) and their subsequent degradation to volatile compounds, such as decanal and 2,-4 decadienal, indicating that AP has the ability to inhibit thermal oxidation/degradation of frying fats and oils. A commercial french fry fat had lower CDHP values compared to A-V fat in simulated studies, and fried chicken oil had lower CDHP values than the V-S oil. Peanut oil had higher thermal stability than the other fats and oils.     

Thermoxidative and hydrolytic changes in sunflower oil used in fryings with a fast turnover of fresh oil

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.     

Low oil content potato chips produced by infrared vacuum pre-drying and microwave-assisted vacuum frying

The effects of vacuum infrared radiation (VIR) pre-drying on the microwave-assisted vacuum frying (MVF) potato chips were investigated to study its possibility of decreasing the oil content of fried potato chips. The moisture evaporation, oil content, texture, color, surface temperature, shrinkage, and sensory analysis of fried products were evaluated. Results showed that the VIR pre-drying significantly reduced the oil content in MVF products, a decrease from 22.38 to 13.49?g oil/100?g dry solid. The application of VIR pre-drying accelerated the dehydration rate, and increased the mechanical breaking force measured with a texture analyzer with 20–30?min pre-drying. The VIR pre-drying resulted in an increase in the total color change and the shrinkage of MVF potato chips. The sensory analysis showed that the proper duration of VIR pre-drying would achieve a higher consumers’ acceptance. Comparing with the current industrial applications, vacuum frying, and atmospheric deep-fat frying, the combined VIR and MVF technology would be an alternative frying method for producing healthier fried products with less oil content and high quality.     

Iron accumulation in oil during the deep-fat frying of meat

Iron accumulation in oil is a potential problem when frying food containing substantial amounts of iron. Selected meat products (skinless chicken breast, beef liver, and lean beef) were ground and fried (ca. 2-cm spheres, ca. 10 g/sphere) in partially hydrogenated soybean oil (PHSBO). Samples (450 g) of ground meat were fried 3 times/h for 8 h/d for 3 d. Oil samples were collected for analysis for iron (every 8 h) and oil degradation (every 4 h) and replaced with fresh oil. The iron contents of oil samples after 3 d of frying were approximately 0.11, 0.48, and 4.01 mg of iron/kg of PHSBO for the oil used to fry chicken, beef, and liver, respectively. There was a notable darkening in color and an increased tendency to foam for the beef liver oil sample compared with the other samples. After frying, the acid values were 0.9, 1.1, and 1.4 for the oil samples for chicken, beef, and liver, respectively. After frying, the p-anisidine values were 11.5, 12.8, and 32.6 for the oil samples for chicken, beef, and liver, respectively; the food oil sensor values were 0.96, 0.96, and 0.83 for the oil samples for chicken, beef, and liver, respectively.     

The Effect of Type of Oil and Degree of Degradation on Glycidyl Esters Content During the Frying of French Fries

The aim of the study was to determine the effect of oil degradation on the content of glycidyl esters (GEs) in oils used for the frying of French fries. As frying media, refined oils such as rapeseed, palm, palm olein and blend were used. French fries were fried for 40 h in oils heated to 180 °C in 30‐min cycles. After every 8 h of frying, fresh oil and samples were analyzed for acid and anisidine values, color, refractive index, fatty acid composition, and content and composition of the polar fraction. GEs were determined by LC–MS. Hydrolysis and polymerization occurred most intensively in palm olein, while oxidation was reported for rapeseed oil. The degradation of oil caused increased changes in the RI of frying oils. Losses of mono‐ and polyunsaturated fatty acids were observed in all samples, with the largest share in blend. The highest content of GE found in fresh oil was in palm olein (25 mg kg^?1) and the lowest content of GE was found in rapeseed oil (0.8 mg kg^?1). The palm oil, palm olein and blend were dominated by GEs of palmitic and oleic acids, while rapeseed oil was dominated by GE of oleic acid. With increasing frying time, the content of GEs decreased with losses from 47 % in rapeseed oil to 78 % in palm oil after finishing frying.     

Comparison of the Frying Stability of Sunflower and Refined Olive Pomace Oils With/Without Adsorbent Treatment

The aim of this study was to compare the frying behaviors of refined olive pomace (ROP) oil to sunflower oil in a 5-day trial, and to evaluate the refresh capacity of a new adsorbent mixture composed of diatomaceous earth, zeolite and lime. Frying trials were performed as control and adsorbent-treated groups. Oil samples collected after each day of frying were analyzed for physical characteristics (viscosity, turbidity, refractive index, smoke point), color values (L, a* and b*), and chemical properties (free acidity, peroxide value, conjugated dienoic acids, total polar materials). The amount of oil absorbed by the fried dough was measured. In general, results indicated that ROP oil had higher thermal stability. There were no significant differences between the quality parameters of control and adsorbent treated samples for either oil sample. Effectiveness of the new adsorbent mixture on frying oil refreshing was limited only to free acidity, viscosity, and color. Total polar materials measured with probe sensor were determined to be as accurate as the official technique, and can be recommended for quick, easy and cheap analysis. Also, there were some significant correlations between the measured oil quality parameters.