Oxygen content and oxidation in frying oil

Underutilised palm stearin as hard stock for deep-frying medium and its performance for oil uptake in instant noodles

BACKGROUND

As a by-product of the palm oil industry, palm stearin is often overlooked despite having several beneficial properties, such as excellent stability, which is critically essential to meet the demand of the global food trend in producing safer processed food. Specifically, deep frying of food is often associated with the production of toxic compounds that could potentially migrate into the food system when oils are degraded under continuous heating. The incorporation of palm stearin is regarded as a cost-effective and efficient method to modify the fatty acid composition of oils, enhance the frying qualities and lower the degradation rate.

RESULTS

This study blended 5% and 10% palm stearin into palm oil to investigate the deep-frying performance and impact on food quality. Increasing the palm stearin content improved the frying oil's oxidative and hydrolytic stability, evidenced by reduction of total polar material, free fatty acid and total oxidation value. Addition of palm stearin increased the slip melting point which improved the oil's oxidative stability but no significant increase in oil content of instant noodles was observed. Scanning electron microscopy and fluorescence microscopy showed the formation of larger pores in the noodle structure that facilitated oil retention.

CONCLUSION

Blending palm stearin into frying oil enhanced the frying stability and minimally affected the oil uptake in instant noodles. This article presents the viability of blending palm stearin into frying oils to develop longer-lasting frying oils. © 2024 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Elevated temperature extraction of β-carotene from freeze-dried carrot powder into sunflower oil: Extraction kinetics and thermal stability

β-Carotene, a precursor of vitamin A, can alleviate the deficiency of this vitamin prevalent worldwide. Earlier research studies have addressed the extraction of β-carotene at relatively low temperatures (up to 70°C) due to its perceived instability at higher temperatures, as a result of which extraction rates recorded are relatively low. This study models the net rate of β-carotene extraction by considering both extraction and degradation kinetics. The model developed, which accounts for degradation occurring in solid and extract phases, has been experimentally validated for the extraction of β-carotene from freeze-dried carrot powder into sunflower oil over a range of temperatures 90-150°C. This study also gives insights into the application of sunflower oil as a carrier for β-carotene during cooking and food processing, by monitoring and modeling the thermal degradation and isomerization of β-carotene at temperatures up to 220°C. The modeling of extraction kinetics shows that it is possible to achieve viable extraction rates by employing temperatures in the range (90-150°C) for relatively short times (<5 min). The degradation kinetics shows that almost 75% of the β-carotene can survive heating at 180°C for 10 min-indicating the possibility of using β-carotene enriched edible oils for frying. This study also reports on the formation of three isomers of β-carotene identified using HPLC: trans-, 9-cis, and 13-cis. The reaction network model developed in this study was able to account for the transient variation of the concentration of all three isomers. PRACTICAL APPLICATION: β-Carotene is a precursor of vitamin A and its consumption can potentially alleviate the deficiency of this vitamin prevalent worldwide. This study validates a model for the extraction of β-carotene in sunflower oil, which takes into account extraction as well as degradation occurring during extraction, so that a rational method is available for the design of efficient extractors for this purpose. This paper also establishes the thermal stability of β-carotene under frying conditions by quantifying its thermal degradation as well as isomerization.

Ethyl cellulose particles loaded with α-tocopherol for inhibiting thermal oxidation of soybean oil

Most endogenous antioxidants degrade and lose efficiency during frying. The study aimed to inhibit thermal oxidation of soybean oil by fabricating α-tocopherol loaded particles with ethyl cellulose (EC) of different viscosity grades (M9, M70 and M200) via anti-solvent method. As the viscosity of ethyl cellulose increased, particle size decreased from micrometer to nanometer. Confocal laser scanning microscope confirmed successful encapsulation and uniform distribution of α-tocopherol in the loaded particles. Differential scanning calorimetry and thermogravimetric analysis demonstrated that loaded particles protected α-tocopherol from oxidation and degradation. Meanwhile, Fourier transformed infrared demonstrated that α-tocopherol interacted with EC through hydrogen bond and hydrophobic effects. With excellent dispersibility in soybean oil, loaded particles effectively inhibited thermal oxidation of soybean oil and loaded M200 nanoparticles was the most effective, which performed far better than tert-butylhydroquinone (TBHQ). Therefore, the nanoparticles offered a promising way to enhance oxidative stability of oils during thermal processing.

Effect of Storage Time on the Physicochemical Properties of Waste Fish Oils and Used Cooking Vegetable Oils

Waste fish oils (FOs) and used cooking vegetable oils (UCOs) are increasingly becoming alternative renewable fuels. However, different physicochemical aspects of these renewable fuels, including the effect of storage, are not well-known. In this work, the effect of the storage period on physicochemical properties of selected samples of FOs and UCOs was investigated. The bio-oils were stored at 4 °C for up to five years before each experimentation. The chemical properties were characterized using capillary gas chromatography with flame ionization detection (GC-FID) and high-performance size exclusion chromatography including an evaporative light scattering detector (HPSEC-ELSD). Water contents and acid numbers of the bio-oils were determined using the Karl Fischer (KF) titration and the ASTM D 664 methods. Furthermore, the average heating values and surface tension of the bio-oils were determined. According to the results obtained, for all bio-oil types, the concentrations of polymerized triglycerides, diglycerides, and fatty acids and monoglycerides had increased during the storage periods. The physical properties of the bio-oils also showed a small variation as a function of the storage period. The overall results observed indicate that the deterioration of the physicochemical properties of bio-oils can be controlled through storage in dark, dry, and cold conditions.

Potential Adverse Public Health Effects Afforded by the Ingestion of Dietary Lipid Oxidation Product Toxins: Significance of Fried Food Sources

Exposure of polyunsaturated fatty acid (PUFA)-rich culinary oils (COs) to high temperature frying practices generates high concentrations of cytotoxic and genotoxic lipid oxidation products (LOPs) via oxygen-fueled, recycling peroxidative bursts. These toxins, including aldehydes and epoxy-fatty acids, readily penetrate into fried foods and hence are available for human consumption; therefore, they may pose substantial health hazards. Although previous reports have claimed health benefits offered by the use of PUFA-laden COs for frying purposes, these may be erroneous in view of their failure to consider the negating adverse public health threats presented by food-transferable LOPs therein. When absorbed from the gastrointestinal (GI) system into the systemic circulation, such LOPs may significantly contribute to enhanced risks of chronic non-communicable diseases (NCDs), e.g. cancer, along with cardiovascular and neurological diseases. Herein, we provide a comprehensive rationale relating to the public health threats posed by the dietary ingestion of LOPs in fried foods. We begin with an introduction to sequential lipid peroxidation processes, describing the noxious effects of LOP toxins generated therefrom. We continue to discuss GI system interactions, the metabolism and biotransformation of primary lipid hydroperoxide LOPs and their secondary products, and the toxicological properties of these agents, prior to providing a narrative on chemically-reactive, secondary aldehydic LOPs available for human ingestion. In view of a range of previous studies focused on their deleterious health effects in animal and cellular model systems, some emphasis is placed on the physiological fate of the more prevalent and toxic α,β-unsaturated aldehydes. We conclude with a description of targeted nutritional and interventional strategies, whilst highlighting the urgent and unmet clinical need for nutritional and epidemiological trials probing relationships between the incidence of NCDs, and the frequency and estimated quantities of dietary LOP intake.

Polydimethylsiloxane Shows Strong Protective Effects in Continuous Deep-Frying Operations

Polydimethylsiloxane (PDMS) was previously reported to show no protective effect in continuous deep-frying. In this study, we used canola oil with/without added PDMS to deep-fry shredded potato at 180°C either continuously or with 10-, 20-, or 30-min intervals between frying sessions for 6 h. In continuous deep-frying in canola oil not containing PDMS, far more oil vapor was generated from the oil and the water in the potato compared to frying with 20- and 30-min intervals between sessions and the oil in the fryer accordingly had a lower polar compound content (PC). The longer the oil was used to deep-fry potato, the more steam was generated from potato. Thus, polar compounds evaporated into the air in the steam, resulting in a low PC value of oil in the fryer. In contrast, both thermal deterioration and oil vaporization were remarkably inhibited in canola oil containing PDMS regardless of the frying pattern, and the PC value of the oil in the fryer increased in proportion to the amount of potato deep-fried. Canola oil with/without added PDMS was heated at 180°C for 6 h to confirm the effect of water released from potato on the oxidation of oil. A large increase in PC was observed in canola oil not containing PDMS when heated without water but this increase was inhibited to some extent when water was supplied continuously. On the other hand, the PC of canola oil containing PDMS was far lower than that of oil not containing PDMS, but the addition of water promoted an increase in PC. In conclusion, we observed superior protective effects of PDMS regardless of the deep-frying pattern employed, but the PC value nonetheless increased as the amount of food deep-fried increased. In addition, we confirmed that water in potato strongly correlates to PC increase of oil in the fryer.

Active Cooling of Oil after Deep-frying

Oil used for deep-frying is often left to stand after cooking. A major concern is oxidation during standing that might be avoidable, especially in the case of oil used repeatedly for commercial deep-frying as this involves large volumes that are difficult to cool in a conventional fryer. This paper describes a method to minimize oil oxidation. French fries were deep-fried and the oil temperature decreased in a manner typical for a commercial deep-fryer. The concentration of polar compounds generated from thermally oxidized oil remarkably increased at temperature higher than 100°C but little oxidation occurred below 60°C. Heating the oil showed that the peroxide and polar compound content did not increase when the oil was actively cooled using a running water-cooled Graham-type condenser system to cool the oil from 180°C to room temperature within 30 min. When French fries were fried and the oil was then immediately cooled using the condenser, the polar compound content during cooling did not increase. Our results demonstrate that active cooling of heated oil is simple and quite effective for inhibiting oxidation.

Polydimethylsiloxane Droplets Exhibit Extraordinarily High Antioxidative Effects in Deep-Frying

The addition of more than about 1 ppm polydimethylsiloxane (PDMS) into oil results in PDMS forming both a layer at the oil-air interface and droplets suspended in the oil. It is widely accepted that the extraordinarily strong and stable antioxidative effects of PDMS are due to the PDMS layer. However, the PDMS layer showed no antioxidative effects when canola oil did not contain droplets but rather was covered with a layer of PDMS, then subjected to heating under high agitation to mimic deep-frying. Furthermore, no antioxidative effect was exhibited by oil-soluble methylphenylsiloxane (PMPS) in canola oil or by PDMS in PDMS-soluble canola oil fatty acid ester during heating, suggesting that PDMS must be insoluble and droplets in oil in order for PDMS to exhibit an antioxidative effect during deep-frying. The zeta potential of PDMS droplets suspended in canola oil was very high and thus the negatively charged PDMS droplets should attract nearby low molecular weight compounds. It was suggested that this attraction disturbed the motion of oxygen molecules and prevented their attack against unsaturated fatty acid moiety. This would be the reason in the deep-frying why PDMS suppressed the oxidation reaction of oil. PDMS droplets also attracted volatile compounds (molecular weight below 125 Da) generated by heating canola oil. Thus, adding PDMS to oil after heating the oil resulted in the heated oil smelling less than heated oil without PDMS.

Inhibition of Frying Oil Oxidation by Carbon Dioxide Blanketing

The oxidation of oil starts, in general, from the penetration of atmospheric oxygen into oil. Inhibition of the vigorous oxidation of oil at deep-frying temperature under carbon dioxide flow, by disrupting the contact between oil and air, was first demonstrated using oil in a round bottom flask. Next, the minimum carbon dioxide flow rate necessary to blanket 4 L of frying oil in an electric fryer (surface area 690 cm(2)) installed with nonwoven fabric cover, was found to be 40 L/h. Then deep-frying of potato was done accordingly; immediately after deep-frying, an aluminum cover was placed on top of the nonwoven fabric cover to prevent the loss of carbon dioxide and the carbon dioxide flow was shut off. In conclusion, the oxidation of oil both at deep-frying temperature and during standing was remarkably inhibited by carbon dioxide blanketing at a practical flow rate and volume. Under the deep-frying conditions employed in this study, the increase in polar compound content was reduced to half of that of the control.

The Antioxidation Mechanism of Polydimethylsiloxane in Oil

Strong and stable antioxidation effects of polydimethylsiloxane (PDMS) are widely accepted and utilized in commercial frying oil; however, the mechanism is not fully established. On the other hand, canola oil contains about 700 ppm (mg/kg-oil) of the natural antioxidant, tocopherol. Canola oil containing 0, 1 and 10 ppm added PDMS was heated at 180°C for 1 h under stirring, then left for 2-3 days at room temperature; this treatment was repeated 5 times. Compared to pure canola oil, PDMS-containing canola oil exhibited remarkably lower peroxide, p-anisidine and acid values, a lower decrease in tocopherol content but a higher oxygen content during the heating experiments, implicating low oxygen consumption for the oxidation. While PDMS has not been known to exhibit antioxidative effects at ambient temperatures, the present results show that PDMS prevents autoxidation as well as thermal oxidation. In addition, PDMS, not tocopherols, provided the major antioxidative effect during intermittent heating, and the decrease of tocopherols was significantly inhibited by PDMS. Phase contrast microscopy confirmed that PDMS contained in canola oil was suspended as particles. Also, the oxygen content in standing PDMS-containing canola oil decreased as the depth of oil increased, corresponding to the PDMS distribution, which also decreased as the depth of oil increased. Moreover, PDMS had a higher affinity for oxygen than canola oil in a mixture of canola oil/PDMS, 1:1 v/v. Thus, it is suggested that PDMS restricted the behavior of oxygen dissolved in canola oil by attracting oxygen in and around the PDMS particles, which is wholly different from the radical scavenging antioxidation of tocopherol.

Monitoring of quality and stability characteristics and fatty acid compositions of refined olive and seed oils during repeated pan- and deep-frying using GC, FT-NIRS, and chemometrics

Refined olive, corn, soybean, and sunflower oils were used as cooking oils for deep-frying at two different temperatures, 160 and 190 °C, and for pan-frying of potatoes at 180 °C for 10 successive sessions under the usual domestic practice. Several chemical parameters were assayed during frying operations to evaluate the status of the frying oils. Refined olive oil, as frying oil, was found to be more stable than the refined seed oils. In fact, this oil has proven the greatest resistance to oxidative deterioration, and its trans-fatty acid contents and percentages of total polar compounds were found to be lower at 160 °C during deep-frying. Finally, chemometric analysis has demonstrated that the lowest deterioration of the quality of all refined oils occurred in the refined olive oil during deep-frying at 160 °C and the highest deterioration occurred in the refined sunflower oil during pan-frying at 180 °C.

Study on the effect of polydimethylsiloxane from the viewpoint of oxygen content in oil

It has been reported that polydimethylsiloxane (PDMS) inhibits oxygen dissolution into oil by forming a monolayer on the surface of the oil, thereby reducing thermal oxidation. In the present study, the distribution of PDMS was determined by the inductively coupled plasma atomic emission spectroscopy in standing PDMS-containing canola oil. PDMS did not disperse in the oil uniformly, but there was a tendency that the PDMS concentration decreased as the depth of oil increased, and the concentration of the bottom part was the lowest. When canola oil was covered with PDMS by dropping it gently on the surface of the oil and kept at 60°C, the oxygen content and oxidation of the oil were lower than those of the control canola oil. PDMS-containing canola oil and canola oil were heated with stirring from room temperature to 180°C, and then allowed to stand while cooling. Oxygen contents of both oils increased up to 120°C then dropped abruptly. While cooling, oxygen contents sharply increased at 100°C and approached the saturation content, although the increase for PDMS-containing canola oil was a little slow. Likewise, the thermal treatment of PDMS-containing canola oil and canola oil at 180°C for 1 h under stirring was repeated 5 times with standing intervals for 2-3 days at room temperature. Oxidation of the former was less than that of the latter in spite of its high oxygen content. In conclusion, the oxygen content of oil with/without PDMS addition increased, but oxidation of PDMS-containing canola oil was inhibited both during heating and standing with intermittent heating. It was suggested that PDMS exerted its antioxidative effect regardless of whether it covered the oil or was dispersed in it.

Heated vegetable oils and cardiovascular disease risk factors

Cardiovascular disease (CVD) is one of the leading major causes of morbidity and mortality worldwide. It may result from the interactions between multiple genetic and environmental factors including sedentary lifestyle and dietary habits. The quality of dietary oils and fats has been widely recognised to be inextricably linked to the pathogenesis of CVD. Vegetable oil is one of the essential dietary components in daily food consumption. However, the benefits of vegetable oil can be deteriorated by repeated heating that leads to lipid oxidation. The practice of using repeatedly heated cooking oil is not uncommon as it will reduce the cost of food preparation. Thermal oxidation yields new functional groups which may be potentially hazardous to cardiovascular health. Prolonged consumption of the repeatedly heated oil has been shown to increase blood pressure and total cholesterol, cause vascular inflammation as well as vascular changes which predispose to atherosclerosis. The harmful effect of heated oils is attributed to products generated from lipid oxidation during heating process. In view of the potential hazard of oxidation products, therefore this review article will provide an insight and awareness to the general public on the consumption of repeatedly heated oils which is detrimental to health.