Frying Operation Research Articles

A Field Study on Volatile Organic Compounds Generated During Cooking in a Large Pot Using PTR ToF MS

Various volatile organic compounds (VOCs), including aldehydes, are present in fried food being cooked in high-temperature oil. In this study, real-time VOC concentration was measured in the upper part of a large pot in the cooking room using Proton Transfer Reaction Time-of-Flight Mass Spectrometry (PTR ToF MS) for 3 days (roasted on the first day, fried on the second day, and simmered on the third day). The average concentration of diacetyl was the highest on the first day of stir-frying and steaming. The highest concentrations of formaldehyde was on day 3 when pork was cooked in sugar and sauce. Formaldehyde, 1,3-butadiene, acrolein, diacetyl, and naphthalene were detected during the frying process on the second day, and were detected in descending order of boiling point. In addition, various VOCs such as methanol were detected. The maximum/minimum concentration ratio was the highest for acrolein (3,030), so it was confirmed that many aldehydes were generated during frying. Although there is a limit to direct comparison with Occupational Exposure Limit as a result of area sample by PTR ToF MS, the mean concentrations of formaldehyde and diacetyl during the frying operation for 15 minutes were 232 ppb and 16 ppb, respectively, which was 80% of the American Conference of Governmental Industrial Hygienists Threshold Limit Value-Short Term Exposure Limit. After the frying was over, the VOC concentration began to decrease, and it took more than 3 hours to lower the VOC concentration to the level before the oil was heated. As various harmful gaseous substances are generated when cooking deep-frying, improvement methods such as using oil with high boiling points and developing respiratory protection programs should be devised.

Heat and mass transfer modeling and quality changes during deep fat frying: A comprehensive review

AbstractThe deep fat frying of food involves simultaneous heat and mass transfer between the food product and surrounding oil and hence the frying operation is complex in nature. Heat is transferred from the frying medium to the product surface by convection and from the surface to the core of the food through conduction. The specific heat, thermal conductivity, thermal diffusivity, and convective heat transfer coefficient are the major factors that greatly affect the rate of heat transfer. The heat transfer study for deep fat frying of food products revealed that the Fourier number was in the range of 0.1–0.5. The fat uptake and moisture loss are the primary modes of mass transfer during this process. For deep fat frying, the mass transfer coefficient for different food materials was found to be in the range of 10−4–10−6 m/s. The moisture diffusivity for various food materials was measured in the range of 10−6–10−10 m2/s. This review presents the different models implemented to assess the heat and mass transfer parameters during deep‐fat frying of different food materials. The effects of frying process parameters on color, texture, and different physical properties of fried food were discussed. Overall, this review includes information related to the frying of different food products that will assist the food processing connoisseurs and industries in predicting and optimizing the frying process parameters to obtain the desired quality in the product and make the process energy efficient.Practical ApplicationsFrying involves the convective heat transfer from the frying medium to the surface of the food product and conduction heat transfer within the food. During frying, the moisture and oil transport are controlled by diffusion gradient and capillary flow. Most of the oil penetrates into the food during cooling through the channels and pores of the product. The physicochemical changes in the fried product are regulated by frying process factors such as frying oil temperature, frying time, and thermal characteristics of the food. The intense heat of frying imparts a rich aroma and texture to fried foods. Modeling and optimization of frying is very important in order to produce high‐quality fried food products. Heat transfer parameters such as thermal diffusivity, heat transfer coefficient, Fourier number, and Biot number are important in analyzing and improving the product's frying operation. The food technologists would be able to analyze the mechanism of oil absorption and produce low‐oil fried food products if they had a better understanding of heat and mass transfer.

Performance of Olive-Pomace Oils in Discontinuous and Continuous Frying. Comparative Behavior with Sunflower Oils and High-Oleic Sunflower Oils.

Frying performance of olive-pomace oils (OPOs) as compared to sunflower oils (SOs) and high-oleic sunflower oils (HOSOs) was studied in discontinuous frying (DF) and continuous frying (CF) for the first time. DF is used in household, restaurants and frying outlets, while CF is used in the food industry. Oil alteration during frying was determined by measurements of polar compounds (PC) and polymers. Fried potatoes were analyzed for oil absorption and alteration, color, and evaluated in an acceptability test. Results for DF showed that all SOs reached 25% PC at the 9th frying operation (FO), whereas HOSOs did between the 17–18th FO and variable results were found for OPOs since initial levels of diacylglycerols were different. Rates of formation of PC or polymers were the lowest for OPOs, thus showing the best performance in DF. Specifically for PC, relative rates of formation were 1.00–1.11, 2.46–2.71 and 1.37–1.41 for OPOs, SOs and HOSOs respectively. In CF, OPOs and HOSOs behaved similarly and better than SOs, although none reached 25% PC after 40 FO. The good performance of OPOs can be attributed to the high monounsaturated-to-polyunsaturated ratio, in common with HOSOs, and the additional positive effect of minor compounds, especially β-sitosterol and squalene.

Chemical Changes Associated with Repetitive Re-Use of Vegetable Oil during Deep Frying of Bean Cake, Plantain and Yam

Oil re-use is a common practice in households. Oxidative and hydrolytic reactions have been reported to cause significant changes in oil quality. This study was carried out to determine the chemical changes specific to beans cake (Vigna unguiculata), Plantain (Musa paradisiaca) and yellow yam (Discorea cayenensis) frying with repetitive re-use of oil. Refined vegetable and palm oil were used for five consecutive frying operations for bean cake, plantain and yam. Free fatty acids, iodine value, peroxide value and saponification value of the first, third and fifth frying operations were determined using standard methods. For refined vegetable oil, the FFA range for repeated frying operations obtained for bean cake, plantain and yam were 0.31-0.53, 0.37-0.52 and 0.40 to 0.53 % oleic respectively. While palm oil re-use had a range of 11.87 to 14.97 in the value of their FFA. Peroxide value (3.24- 4.92 meq/kg), Iodine value (91.09-107.00 mg/100g) and Saponification value (125.00-152.66 mgKOH/g) were obtained for re-use refined vegetable oil while Peroxide value (27.95-77.02 meq/kg), Iodine value (131.68-210.89 g/100g) and Saponification value (179.45-197.50 mgKOH/g) were obtained for re-use palm oil. Generally, frying operation led to increase in the FFA of refined vegetable oil. Frying operation for peroxide value significantly varied for all frying operation for plantain. Significant difference existed in iodine value at p<0.05 among frying operations for bean cake and yam. Changes in chemical properties of oil were affected by frying duration and food material fried.

A Review on Frying: Procedure, Fat, Deterioration Progress and Health Hazards

AbstractFor decades, frying has been a popular technique for the preparation of foods, both on domestic and industrial scales. The effects of edible oil type and frying operation conditions on the shelf‐life of fried products, the deterioration progress and the health hazards of fried foods have been studied by various researchers. To achieve a clear conception of the present situation, a collection of the results obtained from previous studies about product deterioration and safety issues due to the frying process, along with related tables, are of prime importance. According to the studies, it can be concluded that a vacuum, oil replenishment, frying temperature, and time of process are the most important parameters affecting the quality of oils and fried products; hence, by controlling them, it is possible to prevent the formation of toxic compounds and loss of nutritional components to a considerable extent during frying operation. Furthermore, according to recent studies indicating significant formation of toxic compounds before the chemical indices of the oil reach the disposal point, more studies are needed to determine whether they are formed at levels that compromise safety and affect consumers’ sensory perception before the established discard points.

Improvement in thermal stability of soybean oil by blending with camellia oil during deep fat frying

The degradation occurring in soybean oil, camellia oil, and binary blends at five ratios during deep frying of french fries at 180°C for five consecutive days was monitored. The decrease in polyunsaturated and increase in saturated fatty acids during the frying operation was more prominent in soybean oil as compared to the camellia oil and blended oils. Among the frying periods, camellia oil, blend (60:40) and blend (50:50) after frying did not change significantly in the overall fatty acid composition. In addition, results of physicochemical properties of the frying media, namely the free fatty acid, peroxide value, p‐anisidine value, total polar components, and color, indicate that camellia oil has the least degradation, while soybean oil has the highest. What is more, these results reveal that increase in oxidative stability is correlated with the increase in camellia oil levels, due to the high content of phenolic compounds and monounsaturated fatty acid. Therefore, the present study clearly indicated that partial replacement of soybean oil with camellia oil can significantly improve frying stability for deep frying of french fries.Practical applications: The results are useful for home cooking, restaurants, or fast foods sellers providing them with newly developed frying stability oils, camellia oil, and camellia/soybean blended oils for deep frying of french fries. Furthermore, discarding time was provided for soybean oil, camellia oil, and their blends when used as media for french fries frying, this suggestion allows preparing safe fried foods for consumers.Total polar compounds of oils during 5 days of usage for deep frying of french fries. Soybean oil (SO); camellia oil (CO); blended oils (soybean oil:camellia oil) SCO1‐90:10, SCO2‐80:20, SCO3‐70:30, SCO4‐60:40, SCO5‐50:50.

Changes in Quality Characteristics of Different Deep Frying Fats During Frying and Regeneration Potentials of Different Adsorbents in Wasted Frying Oils

Regeneration potentials of synthetic magnesium silicate, acid activated bleaching earth and natural bleaching earth for the fried soybean oil after progressive laboratory scale frying. The wasted palm olein and sunflower oils which were obtained from two different restaurants were investigated. As indication for development of deterioration reactions in oil during frying and regeneration potentials of so-called adsorbents, free fatty acid content (FFA), total polar material (TPM), dielectric constant (DC) and color measurement values (L, a and b) were measured. According to the results, synthetic magnesium silicate (SMS), acid activated bleaching earth (ABE) and natural bleaching earth (NBE) have different adsorbent characteristics that were significantly affected by the origin, physical and chemical properties and production method of that adsorbent. Laboratory scale frying operations were significantly increased FFA, TPM and DC as a result of thermal oxidation reaction. The usage of SMS in the wasted soybean oil reduced FFA content, TPM and DC values while lighten the color. Nevertheless, the reductions of these values were much more evident for in wasted sunflower oil from Restaurant A (WA) and wasted palm olein from Restaurant B (WB) when compared to wasted oil obtained from laboratory scale frying operations. All values were reduced with the use of adsorbents while L values of WB is increased with adsorbent usage. Generally SMS showed a higher reducing potential on FFA content, TPM and DC values which are considered as deterioration indices of oil quality during frying operation.

Performance of palm olein and modified rapeseed, sunflower, and soybean oils in intermittent deep‐frying

The frying performance of rapeseed, soybean, and sunflower oils with modified fatty acid composition, and palm olein (PALMO) was compared during a rotational frying operation. The frying was conducted at 185 ± 5°C for 6 days where French fries, battered chicken, and fish sticks were fried in succession. At the end of the frying period, high‐oleic rapeseed and sunflower oils exhibited a significantly higher frying stability than PALMO and other modified oils, based on total polar components (TPC), polymers, and non‐volatile carbonyl compounds formation (anisidine value (AV)). The rate of TPC formation was 2.9, 2.9, 3.2, 3.2, and 3.4% per frying day for high‐oleic low‐linolenic rapeseed (HOLLRAP), high‐oleic sunflower (HOSUN), mid‐oleic sunflower (MOSUN), low‐linolenic soybean (LLSOY), and PALMO, respectively. Although the contents of free fatty acids (FFA) in the used oils were significantly below the regulatory discard level, in PALMO formation of these compounds was 1.7 times higher compared to the modified oils. Color component formation and tocopherol degradation were also observed to be the highest in palm olein. A 15‐member consumer panel awarded HOLLRAP and HOSUN the highest overall sensory acceptance scores, while for LLSOY and PALMO the lowest.Practical applications: Although several frying oils are available in today's market, only a few of them can deliver satisfactory performance during extended frying operation. Thus, the search for the ideal frying oils/fats is an ongoing task. The present study assessed frying performance in the quest for the appropriate frying oils/fats in order to deliver healthy fried products with optimized nutritional qualities.

Phosphatidylcholine and dihydrocaffeic acid amide mixture enhanced the thermo-oxidative stability of canola oil

Recently, we reported the synthesis of a series of dihydrocaffeic acid amides and evaluated their performance as antioxidants for frying applications using a model frying. In the present study, the possibility of a synergy between the amide, N-propyl-N-benzyl-3-(3,4 dihydroxyphenyl)propanamide (DCA) and phosphatidylcholine (PC) was explored in a 6-day actual frying operation. As measured by the amount of polar components (TPC), anisidine value (AnV), changes in fatty acid composition, residual tocopherol and hydroxynonenal (HNE), canola oil containing the formulated antioxidant was twice as stable compared to the regular unfortified oil. At the end of the frying period, the amount of HNE detected in regular canola oil and the fortified sample was at 5.7 and 2.5μg/g, respectively. Thus, the mixture containing phosphatidylcholine and dihydrocaffeic acid amide is a promising antioxidant for frying application.

Plasma Modified Membrane for Daily Recovery of Oil from Repeated Frying Operation with Frequent Oil Replenishment

AbstractSunflower oil was used for deep frying of potatoes at 170 ± 5 °C and for 8 h per day for 5 days in a fryer with an automatic oil filtration system. Three different frying operations were performed: operation (OP)‐1, OP‐2 and OP‐3; that correspond to the oil unfiltered at the end of each frying day, the oil filtered through the fryer's own filter (passive filtration) and the oil firstly subjected to passive filtration and then filtered through a polyethersulfone membrane modified with hexamethyldisiloxane via radio frequency plasma (75 W‐5 min, discharge power–time), respectively. The performance of each operation was investigated in terms of free fatty acids (FFA), conjugated dienoic acids (CD), TOTOX value, total polar content (TPC), Hunter color, viscosity, fatty acid composition, and tocopherol content. The results showed that OP‐3 could decrease FFA, CD, TOTOX, TPC, L*a*b* value, viscosity and linoleic acid (18:2)/palmitic acid (16:0) ratio in 29.6, 11.7, 25, 30.8, 6.1*11.3*20.8*, 7.8, 12.2 %, respectively, compared to the unfiltered oil (OP‐1). Regenerated oil from OP‐3 had a frying life approximately 17 h more than oils from both OP‐1 and OP‐2.

Intensification of pistachio by deep frying

Hot air roasting is the classical way for processing pistachio. In this study, the classical operation was compared with a frying operation that was assumed to be as efficient as classical roasting and permit flavour intensification. The final moisture content at 180 °C was 5.6% at 5 min for fried samples and at 160 °C was 6% at 11 min for roasted samples. The breaking force of pistachio kernels was found to decrease from 2,008 N for raw pistachio to 780, 860, 975 and 1,030 N and then increased from there to 930, 1,460, 1,930 and 2,130 N at 140, 160, 170 and 180 °C, respectively, during frying. The results are similar for roasting. Oil content values ranged between 47 and 48% for the raw pistachio kernels and 53, 54, 55 and 56% after 5 min of frying at 140, 160, 170 and 180 °C, respectively. The results suggest that frying is an efficient intensive and innovative processing technique.

Effect of Flow Characteristics on Ultrafine Particle Emissions from Range Hoods

In order to understand the physical mechanisms of the production of nanometer-sized particulate generated from cooking oils, the ventilation of kitchen hoods was studied by determining the particle concentration, particle size distribution, particle dimensions, and hood's flow characteristics under several cooking scenarios. This research varied the temperature of the frying operation on one cooking operation, with three kinds of commercial cooking oils including soybean oil, olive oil, and sunflower oil. The variations of particle concentration and size distributions with the elevated cooking oil temperatures were presented. The particle concentration increases as a function of temperature. For oil temperatures ranging between 180°C and 210°C, a 5°C increase in temperature increased the number concentration of ultrafine particles by 20-50%. The maximum concentration of ultrafine particles was found to be approximately 6 × 10(6) particles per cm(3) at 260°C. Flow visualization techniques and particle distribution measurement were performed for two types of hood designs, a wall-mounted range hood and an island hood, at a suction flow rate of 15 m(3) min(-1). The flow visualization results showed that different configurations of kitchen hoods induce different aerodynamic characteristics. By comparing the results of flow visualizations and nanoparticle measurements, it was found that the areas with large-scale turbulent vortices are more prone to dispersion of ultrafine particle leakage because of the complex interaction between the shear layers and the suction movement that results from turbulent dispersion. We conclude that the evolution of ultrafine particle concentration fluctuations is strongly affected by the location of the hood, which can alter the aerodynamic features. We suggest that there is a correlation between flow characteristics and amount of contaminant leakage. This provides a comprehensive strategy to evaluate the effectiveness of kitchen hoods in capturing cooking oil fumes, which is based on an assessment of the entire hood face exposure instead of on breathing-zone sampling alone.

Effect of frying conditions on the physico-chemical properties of rice bran oil and its blended oil.

The changes occurring in rice bran oil and its blend with sunflower oil during repeated frying cycles of dried and moist potato chips were monitored. The parameters assessed were: Colour, Refractive Index, SpecificGravity, Oryzanol Value, Free fatty acid, Iodine Value, Peroxide value, anisidine value, Saponification Value, trans fats and fatty acid composition. No significant changes (p≤0.05) were observed in the refractive index and specific gravity of rice bran oil, sunflower oil and their model blend. The colour of blended oil was lesser than RBO and the intensity of color increased after each frying cycle during the deep fat frying of moistened and dried potato chips. The oryzanol content and iodine value decreased with the frying cycles. The decrease in oryzanol value during the frying operation was more prominent in rice bran oil as compared to the blended oils. The increase in p-anisidine value was more in rice bran oil as compared to blended oil. No significant changes (P<0.05) in the myristic, palmitic and stearic acid composition was observed during the repeated deep fat frying cycles in both the rice bran oil and blended oils samples. The amount of unsaturated fatty acid decreased gradually during repeated deep fat frying cycles in both the oils. The trans fat increased with repeated deep fat frying cycles in both the rice bran and blended oils, when used to fry moistened and dried potato chips. Both the oil samples showed greater formation of trans fatty acids when the moistened potato chips were used during frying.

Plasticized methylcellulose coating for reducing oil uptake in potato chips

As a result of consumers' health concerns and the trend towards healthier and low-fat food products, research has been undertaken to reduce the amount of fat absorbed in fried foods. This work focused on studying the efficacy of sorbitol and glycerol as plasticizers of methylcellulose coatings used to reduce oil uptake during the frying process of potato chips Changes in color, mechanical properties, water activity and lipid oxidation during storage were monitored. Also, an explanation regarding the different performances between both methylcellulose coatings with and without plasticizer was attained and techniques from the field of packaging films such as dynamic mechanical analyzer (DMA) and Fourier transform infrared spectroscopy were applied to analyze the behavior of coatings submitted to the frying operation. The application of a methylcellulose coating was an adequate choice to reduce oil absorption in fried potato chips. The most effective formulation was 10 g L(-1) methylcellulose with the addition of 7.5 g L(-1) sorbitol. With the incorporation of this formulation, oil absorption was reduced by 30%. Neither the sorbitol concentration nor the presence of the MC coating affected the puncture maximum force and color parameters L and a*. The results of the sensory analysis indicated that the panelists could not distinguish between the coated and uncoated potato chips. Methylcellulose-based coating plasticized with sorbitol could be an alternative for obtaining healthier potato chips.

Formation of trans fatty acids in edible oils during the frying and heating process

To assess an impact of heated edible oils on intake of trans fat, the formations of trans fatty acids (TFAs) in cooking conditions was estimated by a frying and heating model system. For the frying model, sliced raw potatoes (10% of the frying oil (w/w)) were fried in commercially available canola oil at 160, 180 and 200 °C, and the 10 frying cycles were performed. The TFAs contained both in fried potatoes and in frying oils were measured by gas chromatography (GC). Lipids content of raw potatoes was about 0.1% (w/w) and TFAs in the raw potatoes were negligible. On the other hand, fried potatoes contained lipids at the level of 8.8%–9.2% and their fatty acid composition was mostly in correspondence with that of the frying oil. The TFAs amount of potatoes fried by the tenth frying operation was at the level of 0.99–1.05 g/100 g lipids. When 100 g potatoes fried in this process were consumed, the TFAs intake was estimated at less than 0.1 g. After 10 frying operations, TFAs content, acid values and peroxide values of the frying oils were measured and compared with those of corresponding heated canola oils without food. The amounts of trans 18:1 FAs contained both in the frying oil and in heated oil were less than the quantitative limit (0.047 g/100 g oil). The increases of trans 18:2 FAs and trans 18:3 FAs of the used frying oil were 0.02 g/100 and 0.05 g/100 g, respectively, compared with those of the fresh oil. trans 18:2 FAs accumulation in the heated oil was slightly less than that in the frying oil. To elucidate TFAs accumulation in various edible oils during cooking, six kinds of commercially available edible vegetable oils were heated to 180 °C in glass test tubes. Small changes in TFAs amounts were observed after four hours heating. These results suggested that an ordinary frying process using unhydrogenated edible oils has little impact on TFAs intake from edible oils.

Direct observation of the surface structure of French fries by UV–VIS confocal laser scanning microscopy

Abstract Observations of raw, fried and air-dried potato parenchyma samples were made with a UV–VIS confocal laser scanning microscope in reflective and fluorescence modes. Distinction between oil and cells was made possible by the addition of a fluorescent probe in the frying oil and the use of the natural fluorescence of cells. Observations were recorded at different depths and were treated to make volumetric reconstructions at different product scales. The results showed that after frying, the cellular structures were well conserved in terms of shape and size. Oil was shown to be mainly located in the first cell layer occupying around 50% of its volume. Oil remained in the cut cells emptied during the washing or the frying operation or in the damaged cells partially filled with starch. Cell detachment, because of starch swelling and dehydration, seemed to be the preferential connection between cell layers. The positive effect of air-drying toward oil uptake reduction could be the restriction of these connections.

L’huile de colza : une huile multi-usages en devenir

Ageing tests have been carried out with refined rapeseed oil in different conditions (transparent and non transparent bottles, moderate and dim natural light exposure, room and refrigerator temperature). The results mainly based on sensory evaluation show that rapeseed oil keeps a rather good quality if kept in a non transparent bottle and best if stored in the refrigerator. Results of frying tests under household conditions show that chemical degradation of rapeseed oil is lower than for sunflower oil. But for both oils, the level of degradation compounds are below the 25% polar compounds limit, assuming that the consumer does respect the recommendations on the label i.e. temperature ≤ 180°C and not more than 10 fryings. Although the French regulation does not allow until now to use oils for frying purpose with more than 2 % of linolenic acid of their fatty acid composition, results demonstrate that rapeseed oil could be used worthwhile for frying. However on the basis of room odor tests, rapeseed oil might not be accepted by some consumers due to its fishy odor during frying operation. Therefore from a marketing point of view, it is advisable to limit the use of rapeseed oil to seasoning and cooking purposes.

New theoretical and practical aspects of the frying process

AbstractThe deep‐frying process, normally carried out at 140–200 °C, is a very complex system due to the combination of heat and mass transfer between food and frying medium. The system becomes more complicated as the frying operation proceeds, because the composition of the food being fried and the frying medium is changing continuously due to the progressive deterioration of the frying medium. Apart from a variety of chemical reactions occurring, several changes take place in the frying food, such as gelatinisation of starch, denaturation of protein, and decrease of moisture. These changes bring about swelling of the product, formation of a crusty layer, and the appearance of a golden colour, good texture and taste. The precise control of the fryer enables these physical and chemical changes in the frying of food to convert it into a desirable finished product. This article discusses various types of reactions occurring in the food frying operation, possible mechanisms, a new realistic method – OSET index for measuring heat stability of frying oils – and the protective behaviour of substances that enhance the frying stability of oils.

Deep frying: the role of water from food being fried and acrylamide formation

The formation of acrylamide during food frying is generally influenced by food type, thermal treatment and equipment. The acrylamide concentration is increased when frying oils containing a higher level of polar materials or silicone or larger amounts of diglycerides. This effect may be caused by moisture escaping from food that has an enhancing effect on the heat transfer. It was noticed that if the moisture in the frying operation was bound by special adsorbents, the acrylamide content could be reduced by more than 50%. The effects of several additives like citric acid on the formation of acrylamide during frying of chips were also investigated. The mechanism of acrylamide formation in fried foods is discussed to explain these findings.

Détection et dosage des résidus de pesticides dans les matières grasses

The formation of acrylamide during food frying is generally influenced by food type, thermal treatment and equipment. The acrylamide concentration is increased when frying oils containing a higher level of polar materials or silicone or larger amounts of diglycerides. This effect may be caused by moisture escaping from food that has an enhancing effect on the heat transfer. It was noticed that if the moisture in the frying operation was bound by special adsorbents, the acrylamide content could be reduced by more than 50%. The effects of several additives like citric acid on the formation of acrylamide during frying of chips were also investigated. The mechanism of acrylamide formation in fried foods is discussed to explain these findings.