Deep-fat frying produces desirable or undesirable flavor compounds and changes the flavor stability and quality of the oil by hydrolysis, oxidation, and polymerization. Tocopherols, essential amino acids, and fatty acids in foods are degraded during deep-fat frying. The reactions in deep-fat frying depend on factors such as replenishment of fresh oil, frying conditions, original quality of frying oil, food materials, type of fryer, antioxidants, and oxygen concentration. High frying temperature, the number of fryings, the contents of free fatty acids, polyvalent metals, and unsaturated fatty acids of oil decrease the oxidative stability and flavor quality of oil. Antioxidant decreases the frying oil oxidation, but the effectiveness of antioxidant decreases with high frying temperature. Lignan compounds in sesame oil are effective antioxidants in deep-fat frying.

https://onlinelibrary.wiley.com:443/doi/pdf/10.1111/j.1750-3841.2007.00352.x

Chemistry of deep-fat frying oils

During frying, the degradation of oil produces harmful compounds. Improper monitoring of oil-discard times in restaurants either risks the public health or causes financial losses to industries. Measuring the oil quality is a complex problem and an online sensor is needed. The process of frying reviewed includes moisture, heat and fat/oil transfer, crust formation and various structural, textural and chemical changes in the product, and degradation of frying medium. Some of the European nations and the U.S. have specific regulations against the use of deteriorated frying oils. Due to the absence of a suitable online frying oil quality sensor for restaurant situations, it is difficult to implement any regulation against the use of deteriorated frying oil. Based on various regulations, a model regulation to increase the safety and quality of fried foods is discussed. Background and requirements for developing an online sensor to measure frying oil quality are discussed. Other related areas reviewed in this article are factors affecting oil penetration and absorption by the food, surfactant theory of frying, analytical indices, quick tests and acceptability of frying oil.

Regulating the use of degraded oil/fat in deep-fat/oil food frying

Early studies used extremely overheated fats which were toxic if fed to animals. Later fats and oils were heated in equipments for deep-fat frying under the conditions of good commercial practice. Such heated fats showed no detrimental effects even if fed in long-term feeding trials. The results are reported and critically evaluated. However, certain fractions of the heated fats, e.g. the so called Total Polar Materials caused clearly noticeable effects in the feeding studies: growth retardation, increased liver and kidney weights, and disorders of the enzyme system but only if fed in high doses. In the last years several research groups specialized in the use of cell cultures and enzymo-logical methods and gained insight into resorption and metabolism. Model compounds or compound mixtures have been synthesized. Their structure and/or composition deviated more or less from the genuine compounds or fractions of the heated fats and the results of these studies must not be generalized. New analytical procedures allow to isolate well-defined fractions, and quantitative data about their contents in heated fats are obtained: (a) harmless mono- and diglycerides and free fatty acids, (b) dimeric and polymeric triglycerides and dimeric fatty acids, harmless as well because of very low resorption rates and (c) oxidized triglyceride monomers. Their saponification and degradation products are oxidized monomeric and dimeric acids, oxidized cyclic fatty acids, and other polar compounds. These low molecular oxidized compounds are nutritionally suspect. Since 60 years and with considerable effort more or less systematic investigations have been performed in many countries to answer the question, if heated fats are detrimental to health.

Health aspects of thermoxidized oils and fats.

Seven commonly used frying oils and fats (beef tallow, canola oil, partially hydrogenated canola oil, corn oil, cottonseed oil, soybean oil, and partially hydrogenated soybean oil) were heated at t...

Effect of Heating on the Characteristics and Chemical Composition of Selected Frying Oils and Fats

Heat treatment of rapeseed (primor) and soybean oils resulted in the geometrical isomerization of linolenic acid. The geometrical isomers were isolated from a rapeseed oil heated at 240 C for 10 hr by a combination of thin layer chromatography (TLC) of the methoxy bromomercuric adducts of the total methyl esters and AgNO3-TLC. Three major isomers were identified after hydrazine reduction followed by ozonolysis in BF3-MeOH as 18:3Δ9c, 12c, 15t, 18:3Δ9t, 12c, 15c and 18:3Δ9t, 12c, 15t. These were accompanied by minor amounts of 18:3Δ9c, 12t, 15c, 18:3Δ9c, 12t, 15t and 18:3Δ9t, 12t, 15c. The 18:3 isomers were detected in both soybean and rapeseed oil heated at 240 C for 10 hr. At this temperature, the increase in time of the heat treatment from 10 to 40 hr resulted in a relative increase of the di-trans and in a decrease of the mono-trans isomers. Only minor quantities of these isomers were detected in the oils heated at 200 C for 10 hr. At this temperature, the increase in time of the heat treatment resulted in an increase of both the mono- and di-trans isomers.

Geometrical isomerization of linolenic acid during heat treatment of vegetable oils

Cyclic monomers derived from the intramolecular condensation of the C18 polyunsaturated fatty acids have been reported to elicit toxic responses when fed to laboratory animals at low dietary levels. This study was undertaken to quantitate the cyclic monomers formed by thermal oxidation induced during deep fat frying to assess the potential toxicity of commonly used vegetable oils. Two separate experiments were designed to study the effects of unsaturation and both intermittent and continuous heating on cyclic monomer formation. Both lightly hydrogenated soybean oil (iodine value [IV]=107) and refined, bleached and deodorized soybean oil were studied. The heated oil sustained substantial chemical and physical alterations, as indicated by IV decreases from 10–15 units, increases in free fatty acids of 5–10-fold and in noneluted material of 18–21%. Selected samples were completely hydrogenated and analyzed for cyclic monomers by gas chromatography. Under chromatographic conditions sufficiently sensitive to detect a cyclic monomer standard at less than 0.5% by weight, no cyclic monomers were detected in any of the heated oil samples. However, after concentration by low temperature crystallization of the hydrogenated samples to remove a major portion of the saturated components interfering in cyclic monomer resolution, about 0.3–0.6% cyclic acids, as well as 0.4–0.9% polar materials, were detected in the heated soybean oils. Components appearing in the gas chromatogram with the same retention time as those in a cyclic monomer standard were further identified by gas chromatography-mass spectrometry as disubstituted cyclic C-18 acids.

J. B. Meltzer

Papers

Analysis of thermally abused soybean oils for cyclic monomers