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 enzymological 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.

The level of petroleum ether insoluble oxidized fatty acids in used frying fats is one of the recommended criteria for assessing their quality in Germany; however, the method of determination is time consuming and inaccurate. Gel permeation chromatography (GPC) allows the determination of dimeric and oligomeric triglycerides in a heated fat irrespective of the presence of oxidized material. An indication of the total amount of polar and oxidized compounds can be obtained by liquid chromatography (LC) on a silica gel column in connection with a moving‐wire detector. A comparatively simple and quick method is the separation of polar and unpolar components in a used frying fat by means of column chromatography (CC) on silica gel. Over a large number of investigations, a good correlation was seen between the results obtained with GPC, LC, and CC and the amount of petroleum ether insoluble oxidized fatty acids isolated from used frying fats. Limits of the analytical data obtained by the new methods are proposed which indicate deterioration of used frying fats.

Die üblichen Raffinationsverfahren für pflanzliche Speisefette und ‐öle haben während der Entschleimung, der Entsäuerung und der Bleichung keinen unerwünschten Einfluß auf die Zusammensetzung des Raffinats. Während der Dämpfung hingegen werden in Abhängigkeit von Temperatur und Zeit geringe Mengen von dimeren Triglyceriden und trans‐Fettsäuren gebildet. Unter den in der Praxis üblichen Verfahrensbedingungen ist die Menge der gebildeten Nebenprodukte jedoch unbedeutend. Bei küchentechnischer Verarbeitung von Nahrungsfetten ist die Belastung je nach dem Garprozeß unterschiedlich. Während das Kochen und Backen keine, und das kurzzeitige Braten nur unwesentliche Qualitätseinbußen hervorruft, besteht beim Fritieren, besonders bei zu langer Verwendung des Fritierfetts, die Gefahr von starken Veränderungen. Über die dabei auftretenden chemischen Reaktionen gibt es zahlreiche Untersuchungen; im wesentlichen sind dies Isomerisierungen sowie Oxidations‐ und Polmerisationsreaktionen. Zur Beurteilung des Verdorbenheitsgrades haben die Deutsche Gesellschaft für Fettwissenschaft e.V. und der Arbeitskreis Lebensmittelchemischer Sachverständiger der Länder und des Bundesgesundheitsamtes Empfehlungen veröffentlicht, die neben einer sensorischen Beurteilung auf analytisch zu bestimmenden Kennzahlen beruhen. Seit 60 Jahren untersucht man mehr oder weniger systematisch, aber stets mit beträchtlichem experimentellen Aufwand, ob der Genuß erhitzter Fette die Gesundheit des Menschen gefährden kann. Dabei wurden praxisüblich erhitzte Fette oder daraus isolierte Fettfraktionen, meistens über mehrere Generationen, an Versuchstiere verfüttert. Die Ergebnisse der aussagekräftigsten Langzeitstudien werden kritisch ausgewertet.

Used frying oil can be separated by means of column chromatography on silica gel into an unpolar fraction, which contains predominantly the unaltered tri-glycerides, and a polar fraction consisting of all oxidation and decomposition products formed during the heating process. The size of the polar fraction indicates the degree of fat deterioration. A similar procedure was applied to obtain fractions from a heated oil to be used in long-term feeding experiments. Several tons of a sunflower oil which had been used in the industrial production offish fingers were separated into a polar fraction(l) and an unpolar fraction (2). The sunflower oil had not been overheated and was taken at the moment when the production would have been stopped, according to factory practice, and the oil discarded. Fractions 1 (group U) and fraction 2 (group P) as well as the original unheated sunflower oil (group F) and the heated sunflower oil (group H) were fed to rats over 18 months at a level of 20% in the diet. Fraction 1 caused a highly significant reduction in weight gain of the animals as compared with unheated sunflower oil but had only an insignificant detrimental effect upon the many biochemical, histological and clinical parameters. The order of the weight gain caused by the four samples was: U < H < P < F. The changes of other parameters as well as the implications of these long-term feeding studies will be discussed in detail.

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