Protecting oil during frying: A comparative study

The effect of carbon dioxide blanketing (CDB) and vacuum frying (VF) on the frying performance of regular canola oil was evaluated. For 7 h daily and for 7 days French fries were fried in regular canola oil at 185 ± 5 °C without and with CDB and in a vacuum fryer. The extent of changes in the oil was assessed by analysis of total polar compounds (TPC), anisidine value (AV), color component formation and changes in composition of fatty acids and tocopherols. Frying under CDB reduced the amount of TPC by 54%, while 76% reduction was observed during VF compared to standard frying conditions (SFC). Similarly, lower oxidative degradation was observed when measured by AV. At the end of the frying period, the reduction in unsaturated fatty acid content was 3.8, 1.9 and 12.7% when frying under CDB, vacuum and SFC, respectively. The rate of tocopherol degradation was three and twelve times slower in VF when compared to CDB and SFC, respectively.     

Frying Quality Characteristics of French Fries Prepared in Refined Olive Oil and Palm Olein

The objective of this study was to compare two oils with different polyunsaturated/saturated (P/S) fatty acid ratios, refined olive oil (P/S 0.75) and palm olein (P/S 0.25), in frying French fries. The chemical qualities of the oil residues extracted from the French fries were assayed for five consecutive batches fried at 1-h intervals. The levels of total polar compounds, free fatty acids, p-anisidine value and phytosterol oxidation products (POPs) were elevated in French fries fried in both oils. The level of total polar compounds increased from 4.6 in fresh refined olive oil to 7.3% in final batches of French fries. The corresponding figures for palm olein were 9.8–13.8%. The level of free fatty acid in fresh refined olive oil increased from 0.06 to 0.11% in final products. These figures for palm olein were 0.04–0.13%. The p-anisidine value increased from 3.7 to 32.8 and 2.5 to 53.4 in fresh oils and in final batches of French fries in refined olive oil and palm olein, respectively. The total amount of POPs in fresh refined olive oil increased from 5.1 to 9.6 μg/g oil in final products. These figures were 1.9 to 5.3 μg/g oil for palm olein.     

Accumulation of 5-Hydroxymethylfurfural in Oil During Frying of Model Dough

5-Hydroxymethylfurfural (HMF), a thermal process contaminant, forms in food during frying as a result of the Maillard reaction and caramelization. Owing to its chemical properties, HMF formed in foods during frying partially transfers into frying oil. This study aimed to investigate the accumulation of HMF in oil during repetitive frying operations. A model dough composed of 25 % of glucose was fried at 160, 170, 180 °C for 50 frying cycles. Apart from total polar compounds (TPC), accumulation of HMF was determined in oils during repetitive frying operations. Increasing frying temperature also increased the amount of HMF formed in dough, and those transferred to oil. Prolonging frying cycles to 150, increasing amount of dough being fried to 100 g and frying time to 10 min caused the TPC content to reach 25 % at the 130th frying cycle at 180 °C. Under the same frying conditions, the concentration of HMF showed a rapid increase at the first 10th frying cycle. Its increase was at a slower rate until the 50th frying cycle reaching a plateau level exceeding 5.0 mg/L. The results revealed that HMF transferred and accumulated in the frying oil during repetitive frying. The HMF concentration exceeding 5.0 mg/L in oil may be considered as an indicator for heavily used frying oil. Correlation between TPC and HMF contents of frying oil showed no linear correlation.     

Anti-Rancidity Effects of Sesame and Rice Bran Oils on Canola Oil During Deep Frying

In this study, the effect of sesame oil (SEO) and rice bran oil (RBO) on the rancidity of canola oil (CAO) during the process of frying potato pieces at 180 °C was investigated. The SEO and RBO were added to the CAO at levels of 3 and 6%. Frying stability of the oil samples during the frying process was measured on the basis of total polar compounds (TPC) content, conjugated diene value (CDV), acid value (AV), and carbonyl value (CV). In general, frying stability of the CAO significantly (P < 0.05) improved in the presence of the SEO and RBO. The positive effect of the SEO on the stability of the CAO was more than that of the RBO. Increasing the amounts of SEO and RBO from 3 to 6% led to decreases in the TPC and AV, and increases in the CDV and CV of the CAO during the frying process. The best frying performance for the CAO was obtained by use of 3% of both SEO and RBO together (CAO/SEO/RBO, 94:3:3 w/w/w).     

Effect of the Presence and Absence of Potatoes under Repeated Frying Conditions on the Composition of Palm Oil

The effect of repeated deep frying of potatoes versus repeated heating/quenching on the chemical profile of palm oil was investigated. The novelty of the work is that the frying and heating/quenching experiments were conducted under similar time-temperature profiles. The effects of the frying load (potato-to-oil ratio: 1/7 and 1/35 kg_potatoes/l_oil) and of the time-temperature profile were examined. Whole palm oil and its polar fraction were analyzed using high pressure size exclusion chromatography. Both repeated frying and repeated heating/quenching generated polar and polymerization products in palm oil. Interestingly, no hydrolysis or other decomposition products were generated under any of the examined conditions. The presence of potatoes during frying in palm oil increased the concentration of polymerization products and polar compounds compared to oils without potatoes significantly. The effects of frying load on oil quality depended on frying time. No significant effect of frying load was observed up to frying times of 13 h (or 10 frying batches). However, frying oil quality was affected by frying load once frying times exceeded 24 h (or 20 batches).     

Rapid Determination of Degradation in Frying Oils with Near-Infrared Spectroscopy

Measures of free fatty acids (FFA), total polar materials (TPM), and conjugated dienoic acids (CDA), typical indices of oil degradation, were analyzed in daily oil aliquots taken from soybean oils with different linolenic acid concentrations used to fry French fries. The oils also were scanned with a reflectance near-infrared spectrometer using a wavelength range of 350–2,500 nm. By using partial least squares and one-out cross validation, calibrations were developed to quantitatively determine FFA, TPM, and CDA by near-infrared spectroscopy (NIRS). The coefficients of determination (R ²) when compared to the standard methods were 0.973 for FFA, 0.984 for TPM, and 0.902 for CDA. NIRS was an accurate and fast method to determine FFA, TPM, and CDA in oxidized oils. The ability to obtain different parameters simultaneously makes NIRS a potentially valuable tool for food quality assurance.     

Impact of Canolol‐Enriched Extract from Heat‐Treated Canola Meal to Enhance Oil Quality Parameters in Deep‐Frying: a Comparison with Rosemary Extract and TBHQ‐Fortified Oil Systems

Canolol‐enriched extracts obtained from the extraction of fluidized bed treated canola meal with supercritical carbon dioxide were added to high‐oleic canola oil in different concentrations (200, 500 and 750 mg/kg). After 30 h of deep‐fat frying, oils fortified with canolol‐enriched extracts showed a two to three times better frying performance in comparison to the commonly used antioxidants (TBHQ, 200 mg/kg; rosemary extract, 40 and 200 mg/kg) and a control without antioxidants with regards to the formation of di‐ and polymer triacylglycerols, total polar compounds, secondary degradation products (anisidine value) and the iodine value. The canolol‐enriched extracts were also able to slow down the degradation of α‐ and γ‐tocopherol during frying resulting in significant amounts of tocopherols after 30 h of frying in comparison to the other oils. The influence of the canolol‐enriched extracts indicated strongly concentration‐dependent performance. With increasing concentration of the extract, the thermal stability of the fortified oil was improved. The only disadvantage of the addition of the extracts was an increase in the initial acid value, but within the frying time, only oil fortified with 750 mg canolol‐enriched extract/kg reached the limit given in different countries.     

Monitoring of Changes in Composition of Soybean Oil During Deep-Fat Frying with Different Food Types

Changes in the composition of soybean oil during deep‐fat frying with wheat dough (WD) and chicken breast meat (CBM) were comparatively investigated using gas chromatography–mass spectrometry and Fourier transform infrared spectroscopy (FTIR). The amounts of saturated fatty acids (FAs) and short‐chain FAs were increased. The amount of unsaturated FAs was decreased as the processing time increased. An increase in the amount of tetradecanoic acid and 9‐cis‐hexadecanoic acid was observed during the CBM frying only. The FTIR spectrum of frying oil was analyzed by extracting the entire information as the area ratios based on vibration absorptions of the specific functional groups. Changes in content of functional groups, namely cis C=C, trans C=C, C=O, C–O, O–H, and C–H, were studied by the FTIR‐based method. Based on the changes in the content of FAs and functional groups, soybean oil fried with CBM degraded more quickly than that fried with WD. Moreover, good linear correlations between the change in contents of functional groups and the mass percentages of FAs were also observed. The FTIR‐based method could be used in real time to monitor the quality of frying oil during the deep‐fat frying.     

Frying of Potatoes: Physical,Chemical, and Microstructural Changes

Frying is one of the oldest unit operations and is used not only in industry but also at home. The most commonly fried vegetable is potato, for important commercial products such as potato chips, par-fried potatoes, and french fries. Quality parameters of interest for fried potatoes include physical and chemical properties such as color, mechanical properties (e.g., crispness, hardness, etc.), structural properties (e.g., porosity and roughness), oil content, and water content, among others. Some chemical contaminants such as acrylamide and furan are heat-generated during the frying of potato slices or strips, leading to final fried pieces with considerable amounts of these contaminants. The controllable variables in industrial frying processes are generally potato variety, oil type, frying time, and frying temperature. Therefore, the study of the quality changes during frying is critical because knowledge regarding kinetics parameters will enable prediction of the final quality in fried potatoes and improvements in the final product value by selecting properly the processing conditions. Finally, modern techniques such as computer vision provide valuable tools to quantify and predict physical and chemical properties of potato pieces during frying in a fast and noninvasive way. In addition, computer vision can allow us to classify fried potatoes in different quality classes previously determined by sensorial panels.     

Identification of Ingredients for Stabilizing Frying Oil Through a New Screening Method

Frying oils are prone to quality deterioration; because of the high surface area, lipid molecules can interact with air, moisture, and oxidative reagents that promote various reactions. However, the search for effective ingredients for stabilizing frying oil has been slow, given the complexity of the frying processes and lack of effective tools to accurately predict the performance of emerging ingredients. On the other hand, demand for label‐friendly food ingredients has increased. This study aimed to develop a rapid method mimicking real frying conditions to allow the screening of a large number of treatments with improved throughput, while evaluating their effectiveness in stabilizing frying oils. The design utilizes multi‐well heating blocks of conventional OSI. Repeated frying was conducted using miniature baskets, and validations were conducted to examine the accuracy of the relative efficacy of individual treatments. Utilizing this method as a tool, natural plant extracts, standard antioxidants, and their combinations were evaluated and compared in frying palm oil, which demonstrated that the combination of tocopherols, rosemary extract, and oil‐soluble green tea extract was effective for the stabilization of frying oil.     

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.     

Frying Stability of Canola Oil Blended with Palm Olein, Olive, and Corn Oils

The fatty acid composition, peroxide value (PV), acid value (AV), iodine value (IV), total tocopherols (TT) content, and total phenolics (TP) content of canola oil (CAO), palm olein oil (POO), olive oil (OLO), corn oil (COO), and the binary and ternary blends of the CAO with the POO, OLO, and COO were determined. The blends were prepared in the volume ratios of 75:25 (CAO/POO, CAO/OLO, CAO/COO) and 75:15:10 (CAO/POO/OLO, CAO/POO/COO). The CAO and its blends were used to fry potato pieces (7.0 × 0.5 × 0.3 cm) at 180 °C. During the frying process, the total polar compounds (TPC) content, AV, oil/oxidative stability index (OSI), and color index (CI) of the CAO/blends were measured. In general, frying stability of the CAO was significantly (P < 0.05) improved by the blending, and the frying performance of the ternary blends was found to be better than that of the binary blends.     

Changes in Fatty Acid, Tocopherol and Xanthophyll Contents During the Development of Tunisian-Grown Pecan Nuts

Among oil compounds, fatty acids, tocopherols and xanthophylls (lutein and zeaxanthin) are of special interest due to their nutritional properties. The identification and quantification of these compounds in pecan nuts (Carya illinoinensis) could therefore be very useful to produce functional foods rich in compounds of this type. This paper reports studies on their accumulation and the effect of ripening on the content of these high value-added compounds. The total lipid content increased during the ripening. Saturated and polyunsaturated fatty acids decreased significantly, whereas, monounsaturated fatty acids increased during the ripening of pecan nut fruit. Maximum levels of total tocopherol (279.53 mg/kg oil) and xanthophyll (6.18 mg/kg oil) were detected at 20th weeks after the flowering date. These amounts decreased gradually as ripening advances. The early stages of pecan ripening seem to have nutritional and pharmaceutical interests. These results may be useful for evaluating the pecan nut quality and determining the optimal period when the pecans accumulated the maximum of these nutritional and healthy compounds.     

Frying stability of canola oil in presence of pumpkin seed and olive oils

Frying performance of canola oil (CO) was investigated in the presence of 5, 10, and 15% levels of virgin olive oil (VOO) and pumpkin seed oil (PSO) during frying of potatoes at 180°C. Acid value, carbonyl value, total polar compounds content, and total tocopherols content of the oil samples were determined during the frying process. VOO and PSO addition improved the frying stability of the CO. Frying performance of the CO increased more in the presence of PSO than in the presence of the VOO. The PSO levels higher than 5% exerted pro‐oxidant effects, indicating the necessity of investigation at lower levels. The better antioxidative effect of PSO was attributed to its probably different phenolic composition.     

Quality of Rapeseed Oil Produced by Conditioning Seeds at Modest Temperatures

Conditioning rapeseed can significantly increase the amount of bioactive compounds in the crude oil, but if the conditioning temperatures are too high, they can cause unwanted side effects such as darker color and sensory defects. Modest conditioning temperatures may be more suitable, but little is known about the effects on the quality and bioactive composition of the resulting oil. Oil was recovered from five rapeseed cultivars by cold pressing (CP) or by pressing seeds conditioned at 80 °C for 30 min (HP). Conditioning rapeseed increased oil yield without changing fatty acid composition and increased the amount of total sterols by 16 %, total tocopherols by 20 %, and the levels of polyphenols. Levels of the polyphenol canolol were up to 55-fold higher in HP oil than in CP oil. These higher levels of bioactive compounds gave HP oil higher radical scavenging activity. Although HP oil also had higher free fatty acid contents, peroxide levels, and specific UV extinctions (K values). The quality parameters of HP and CP oils were within codex limits indicating high quality. Modest conditioning temperatures can be used to produce rapeseed oil with high quality and radical scavenging activity.     

Frying stability of soybean and canola oils with modified fatty acid compositions

Pilot plant-processed samples of soybean and canola (lowerucic acid rapeseed) oil with fatty acid compositions modified by mutation breeding and/or hydrogenation were evaluated for frying stability. Linolenic acid contents were 6.2% for standard soybean oil, 3.7% for low-linolenic soybean oil and 0.4% for the hydrogenated low-linolenic soybean oil. The linolenic acid contents were 10.1% for standard canola oil, 1.7% for canola modified by breeding and 0.8% and 0.6% for oils modified by breeding and hydrogenation. All modified oils had significantly (P<0.05) less room odor intensity after initial heating tests at 190°C than the standard oils, as judged by a sensory panel. Panelists also judged standard oils to have significantly higher intensities for fishy, burnt, rubbery, smoky and acrid odors than the modified oils. Free fatty acids, polar compounds and foam heights during frying were significantly (P<0.05) less in the low-linolenic soy and canola oils than the corresponding unmodified oils after 5 h of frying. The flavor quality of french-fried potatoes was significantly (P<0.05) better for potatoes fried in modified oils than those fried in standard oils. The potatoes fried in standard canola oil were described by the sensory panel as fishy.     

Accumulation of 2-tert-Butyl-1,4-Benzoquinone in Frying Oil and Fried Food during Repeated Deep Fat Frying Processes

2-tert-Butyl-1,4-benzoquinone (TBBQ), the main oxidation product of tert-butyl-hydroquinone (TBHQ) during frying, is cytotoxic and its residual levels in frying oils and foods are unknown. In this study, TBBQ residues have been evaluated during the preparation of french fries. Results showed that frying at 140 °C resulted in the highest TBBQ peak concentration (48.42 mg kg⁻¹) compared with frying at 190 or 170 °C. This unexpected finding can be attributed to more extensive hydrolytic reaction when frying at the lower temperature, generating more peroxyl radicals. TBBQ concentrations proved to be independent of the oil type among various unsaturated oils. However, higher TBBQ levels were observed in saturated palm oil and crude soybean oil than in unsaturated oil or refined oil. Continuous frying leads to the accumulation of a large amount of TBBQ in fried food. After frying 1–5 batches, TBBQ levels in both the frying oil and fries were above 10 mg kg⁻¹, exceeding its critical cytotoxic concentration (IC₅₀ value of 10.71 mg kg⁻¹ for RAW 246.7 cells in our previous study), warranting concern with respect to the safety of fried food. FTIR has been utilized as an effective tool for visually monitoring the degree of oxidation in the frying medium with respect to its hydrogen peroxide level, which contributes to the increased level of TBBQ derived from TBHQ therein.     

Improving the Frying Performance and Oxidative Stability of Refined Soybean Oil by Tocotrienol‐Rich Unsaponifiable Matters of Kolkhoung (Pistacia khinjuk) Hull Oil

Increasing consumer awareness for all natural products has quickly led to growing research on new resources of potent and profitable natural antioxidants. In this context, for the first time, the Kolkhoung hull oil (KHO) (Pistacia khinjuk)‐unsaponifiable matters (USM) (UHO) (100, 200, and 400 mg kg^?1) were incorporated into refined soybean oil (RSO) and the oxidative stability of prepared oils was measured during 32 hours of frying. Then, the obtained results (oxidative stability) were compared to the samples containing tert‐butyl hydroquinone (TBHQ) (100 mg kg^?1) as a common synthetic antioxidant. According to the results of oxidative stability assays of acid values, conjugated diene values and carbonyl values, and total polar compounds, the incorporation of UHO, particularly at a concentration of 200 mg kg^?1, was more efficient in improving the oxidative stability compared to TBHQ. The tocol content of KHO (2043.4 mg kg^?1) was higher than the reported amounts of other conventional edible oils. Furthermore, by incorporation of UHO into RSO, as compared with TBHQ, a better protection of naturally occurring antioxidants (tocopherols and sterols) was found after adding UHO to RSO. This fact was mainly attributed to the UHO's tocotrienol fraction. Hence, the USM of KHO can be used as a potent antioxidant to improve the oxidative stability of frying oils.     

Quality of Wood-Pressed Rapeseed Oil

This study is the first report on the quality indices of Chinese wood‐pressed rapeseed oil. Nineteen representative wood‐pressed rapeseed oil samples (representing 80% of the factories that produce this oil in China) were collected and investigated for their physicochemical properties (acid value, peroxide value, color value, and oxidative stability index), fatty acid composition, as well as contents of tocopherols, phytosterols, polycyclic aromatic hydrocarbons (PAH), aflatoxin B₁, arsenic, and lead. Significant differences were observed in the quality results of 19 wood‐pressed rapeseed oil samples. The acid value, peroxide value, and color values, as well as the oxidative stability index were found in the ranges of 1.10–3.04 mg KOH/g, 0.84–9.34 mequiv O₂/kg, 3.50–5.30 (Y = 35, R), and 4.43–10.34 h (120 °C), respectively. Sixteen of the analyzed oil samples had greater than 2% erucic acid. Tocopherols content ranged from 50.93 to 97.54 mg/100 g and phytosterols content ranged from 453.86 to 735.65 mg/100 g. Meanwhile, benzo[a]pyrene and PAH4 were 1.00–13.57 and 7.47–33.55 μg/kg, respectively. Fourteen samples exceeded the maximum allowable limit of benzo[a]pyrene and PAH4 (chrysene, benz[a]anthracene, benzo[b]fluroranthene, and benzo[a]pyrene) according to the European Union (EU) standards. However, the contents of aflatoxin B₁, arsenic, and lead of the tested oil samples were all in conformance with the EU, Codex Alimentarius, and China national standards. Results showed that more attention should be paid to the problem of high PAH4 contents in wood‐pressed rapeseed oil. The inspection and monitoring of raw materials and processing equipment, as well as the establishment of quality standards for the production of high‐quality wood‐pressed rapeseed oil are necessary.