Article pubs.acs.org/JAFC
Generation of Desired Aroma-Active as Well as Undesired Toxicologically Relevant Compounds during Deep-Frying of Potatoes with Different Edible Vegetable Fats and Oils Alice Thürer† and Michael Granvogl*,‡ †
Deutsche Forschungsanstalt für Lebensmittelchemie, Lise-Meitner-Straße 34, D-85354 Freising, Germany Lehrstuhl für Lebensmittelchemie, Technische Universität München, Lise-Meitner-Straße 34, D-85354 Freising, Germany
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ABSTRACT: Deep-frying leads to the generation of various degradation products providing desired properties, like aroma, taste, or color, but some can have adverse effects on human health. The study investigated the influence of frying oils differing in their fatty acid compositions on the generation of desirable and undesirable compounds during deep-frying of potato chips. Selected key odorants and toxicologically relevant compounds (acrolein, acrylamide, furan, and glycidamide) were quantitated by stable isotope dilution assays. Significantly higher concentrations of (E,E)-2,4-decadienal and (E,Z)-2,4-decadienal were found in chips fried with oils rich in linoleic acid, the precursor of the 2,4-decadienals. In contrast, the amounts of Strecker aldehydes and pyrazines were similar. Oils rich in linolenic acid revealed the highest amounts of the toxicologically relevant (E)-2-alkenal acrolein, whereas oils mainly consisting of monounsaturated or saturated fatty acids led to a clearly lower amount. Acrylamide and glycidamide concentrations in chips also showed a clear dependence on the used frying medium, in contrast to furan, whose amount was more or less similar in all chips. KEYWORDS: acrolein, acrylamide, deep-frying, fat and oil, food-borne toxicant, furan, glycidamide, key aroma compounds, potato chips
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INTRODUCTION During the frying process, a complex series of physical and chemical changes leads to the formation of various degradation products of the frying medium, which can have desired but also undesired attributes. On the one hand, desired aspects appreciated by consumers are aroma, taste, color, and often a crispy texture, which make them very popular.1 Besides the type of food, the choice of the frying medium has a substantial impact on the final aroma properties of the fried food. For example, 2,4-decadienal, an autoxidation product of linoleic acid, generating a typical, deep-fried aroma, was assumed as an important odorant in several sensory evaluations of French fries or potato chips. Thus, various research studies indicated that a certain amount of linoleic acid is essential for a desirable aroma development of fried potato products.2−4 Further studies showed higher concentrations of (E,E)-2,4-decadienal and (E,Z)-2,4-decadienal as well as of several pyrazines (e.g., methylpyrazine and 2,5(6)-dimethylpyrazine) in chips fried in palmolein compared to silicone oil, whereas the amounts of Strecker aldehydes remained more or less constant.5 On the basis of sensory experiments as well as calculated odor activity values (OAVs), studies of Wagner and Grosch6 clearly indicated that both 2,4-decadienal isomers caused the deepfried odor impression in the aroma profile of French fries. In contrast, although a high OAV was also calculated for the cooked potato-like smelling Strecker aldehyde 3-(methylthio)propanal (methional), the authors concluded on the basis of omission experiments that methional did not contribute to the aroma of French fries. On the other hand, it has become evident that some of the degradation products can have adverse effects on human health, © 2016 American Chemical Society
e.g., reducing the nutrient value of the food or favoring the generation of undesirable toxicologically relevant compounds (“food-borne toxicants”). Probably the best known compound is acrylamide, listed in group 2A (“probably carcinogenic to humans”) by the International Agency for Research on Cancer (IARC).7 Acrylamide can be formed during the Maillard reaction between naturally occurring ingredients, its precursor free asparagine and a wide range of carbohydrates or degradation products thereof.8−11 In 2008, Granvogl et al.12 proved a possible conversion of acrylamide into the genotoxic metabolite glycidamide via epoxidation of the double bond by fatty acid hydroperoxides generated during heat-processing of unsaturated fatty acids. Other compounds, also considered as toxicologically relevant, are acrolein (IARC, group 3: “not classifiable as to its carcinogenicity to humans”) and furan (group 2B: “possibly carcinogenic to humans”).13 The toxicity of the aldehyde is based on its ability to interfere with cell metabolisms and to modify proteins and nucleic acids by introducing intermolecular and intramolecular cross-links.14−16 Acrolein is formed during the combustion of fossil fuels and was identified in tobacco smoke, kitchen exhaust, and heated fats and oils. In vegetable oils, former studies showed a huge influence of the fatty acid compositions of the heated oils on the amounts of acrolein and, in addition, crotonaldehyde.17,18 Researchers at the US Food and Drug Administration (FDA) have identified furan in a Received: Revised: Accepted: Published: 9107
October 24, 2016 November 2, 2016 November 3, 2016 November 3, 2016 DOI: 10.1021/acs.jafc.6b04749 J. Agric. Food Chem. 2016, 64, 9107−9115
Article
Journal of Agricultural and Food Chemistry
zine, 2-ethyl-3,5-dimethylpyrazine, 2-ethyl-3,6-dimethylpyrazine, 2methylbutanal, 3-methylbutanal, and 3-(methylthio)propanal (Aldrich; Sigma-Aldrich Chemie); 2-mercaptobenzoic acid, silicone oil 47 V 20 (Rhodorsil) (VWR International, Darmstadt, Germany); and liquid nitrogen (Linde, Munich, Germany). All other reagents were of analytical grade. Stable Isotopically Labeled Internal Standards. The following standards were commercially obtained: [13C3]-acrolein (99%) (Isotec; Sigma-Aldrich Chemie); [13C3]-acrylamide (Aldrich), [2H4]-furan (Acros Organics; Fisher Scientific, Nidderau, Germany); and [13C3]glycidamide from Toronto Research Chemicals (North York, ON, Canada). The following compounds were prepared as previously described: [2H2−4]-(E,E)-2,4-decadienal and [2H2−4]-(E,Z)-2,4-decadienal,23 [2H3]-2,3-diethyl-5-methylpyrazine and [2H3]-2-ethyl-3,5(6)-dimethylpyrazine,24 [2H2]-3-methylbutanal,25 and [2H3]-3-(methylthio)propanal.26 High-Resolution Gas Chromatography−Olfactometry (HRGC-O). To identify the key odorants of potato chips, HRGC-O was performed as recently described.27 Quantitation of Aroma-Active Compounds by Stable Isotope Dilution Analysis (SIDA). Potato chips were frozen with liquid nitrogen and ground with a commercial blender. Dichloromethane (2 × 150 mL) and the stable isotopically labeled internal standards (dissolved in diethyl ether or dichloromethane; amounts depending on the concentration of the respective odorant determined in a preliminary experiment) were added to the potato chips (10 g for 1, 2, and 8 and 50 g for 3−5, respectively) (Table 2), and the mixture
number of thermally treated foods, especially in canned and jarred foods.19 Furan can be formed by multiple pathways including the breakdown of carbohydrates, breakdown of amino acids in the presence or absence of reducing sugars, oxidation of polyunsaturated fatty acids, or breakdown of ascorbic acid and related compounds.20−22 Although there have been many studies dealing with the formation of either aroma-active or toxicologically relevant compounds, up to now, a holistic approach in the analysis of fried food quality including studies on the influence of the frying oil on both aspects is still missing. For this purpose, six different fats and oils varying in their fatty acid compositions were used in the present study for deep-frying of thin potato slices. After the frying process, first, several key aroma compounds of the chips were quantitated by means of stable isotope dilution assays. Second, selected toxicologically relevant compounds were quantitated in the fried food (acrolein, acrylamide, furan, and glycidamide) as well as in the frying medium (acrolein and furan). In additional studies, the effect of the frying temperature on formed acrylamide amounts and key aroma compounds of potato chips was investigated in parallel.
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MATERIALS AND METHODS
Safety. CAUTION: Acrolein, acrylamide, furan, and glycidamide as well as [13C3]-acrolein, [13C3]-acrylamide, [2H4]-furan, and [13C3]glycidamide are hazardous and must be handled carefully. Food Samples. The vegetable fat and various oils differing in their fatty acid compositions were purchased at a local supermarket: coconut oil, extra virgin olive oil, rapeseed oil, safflower oil, linseed oil, and a hardened fat, particularly recommended for roasting, baking, and frying. Potatoes, chips, and precooked French fries were also from local supermarkets. Deep-Fried Potatoes. Thin slices of potatoes (peeled, 1.5 mm of cross section) were deep-fried for 2.5 min at 180 °C in a D320 electrical deep-fryer (Kenwood, Heusenstamm, Germany) containing 2.3 L of oil, which was preheated for 10 min prior to frying. The potato-to-oil mass ratio was about 1:7. To guarantee comparable conditions, always the sixth batch of fried food was used for analysis. Chips were frozen with liquid nitrogen and ground with a commercial blender. The fatty acid compositions of the oils used for frying are summarized in Table 1.
Table 2. Selected Ions (m/z) of Analytes and Stable Isotopically Labeled Standards as Well as Response Factors (Rf) used in Stable Isotope Dilution Assays iona (m/z) no. 1 2 3 4 5 6 7 8
Table 1. Fatty Acid Compositions of the Unheated Fat and Oil Samples concna (g/100 g of fat) sample coconut oil olive oil rapeseed oil safflower oil linseed oil frying fat
C12:0 50
