Quantitative Fourier Transform Infrared Analysis for Anisidine Value and Aldehydes in Thermally Stressed Oils I. Dubois, F.R. van de Voort*, I. Sedman, A.A. Ismail, and H.R. Ramaswamy McGill IR Group, Department of Food Science and Agricultural Chemistry, Macdonald Campus of McGill University, Ste. Anne de Bellevue, Quebec H9X 3V9, Canada ABSTRACT: A Fourier transform infrared (FTIR) transmission- based spectroscopic method was investigated for the simultane- ous monitoring of aldehyde formation and the determination of anisidine value (AV) in thermally stressed oils. Synthetic cali- bration standards were prepared by adding known amounts of hexanal, t-2-hexenal and t,t-2,4-decadienal to canola oil (these compounds considered representative of aldehydic compounds formed during oxidation) plus random amounts of other com- pounds representative of oxidation by-products. The standards were analyzed for their chemical AV. With the partial least squares (PLS) technique, an FTIR spectrometer was calibrated to predict both the concentrations of individual aldehyde types and AV, with the individual aldehyde contributions being re- lated to the chemical AV by multiple linear regression to derive "apparent" AV values. The predictive capability of the PLS cali- brations was assessed by analyzing canola oils that were ther- mally stressed at 120, 155, and 200~ The apparent AV, pre- dicted for these samples, matched the chemical AV values within +I .65 AV units. A PLS calibration also was derived by using thermally stressed samples as calibration standards. This approach provided similar predictive accuracy as the use of synthetic calibration standards. As such, quantitative determi- nation of AV by FTIR spectroscopy was shown to be feasible, and the synthetic calibration approach provided additional in- formation on the aldehyde types present in a sample and al- lowed the use of a simple gravimetric approach for calibrating an FTIR spectrometer. This study provides the basis for the de- velopment of a rapid, automated FTIR method for the direct analysis for AV of thermally stressed fats and oils in their neat form without the use of chemical reagents. The implementation of such a method as a quality control tool would eliminate the use and disposal of hazardous solvents and reagents, required by the conventional chemical method, and drastically reduce analysis time (-2 min/sample). Possible applications include monitoring of the oxidative state of frying oils or evaluation of oxidative stability of biodegradable lubricants. JAOCS 73, 787-794 (I 996). KEY WORDS: Aldehydes, anisidine value, fats and oils, FTIR spectroscopy. *To whomcorrespondenceshould be addressedat McGill IR Group,Depart- ment of Food Science and Agricultural Chemistry,Macdonald Campus of McGill University, Box 187, Ste. Anne de Bellevue, Quebec H9X 3V9, Canada. Lipid oxidation has important economic consequences for the edible fats and oils industry because the end result is rancid- ity, manifesting itself by a cumulative sensory defect that is characterized by unpleasant and unacceptable off-flavors and odors. Thermal stress speeds up oxidative reactions, and, thus, lipid oxidation is of major concern in frying operations and in industrial applications, such as biodegradable lubri- cants or hydraulic fluids. The primary products of autoxida- tion, hydroperoxides, are subsequently a source of short-chain aldehydes, ketones, fatty acids, alcohols, and hydrocarbons, which contribute to the rancid off-flavors in edible oils (1). Of these compounds, aldehydes are largely considered re- sponsible for the off-flavors in fats and oils due to their low sensory threshold values (2). As autoxidation is a complex re- action that can take many routes, depending on the lipid in question and the conditions under which the lipid is undergo- ing oxidation, it is difficult to unambiguously assess the ox- idative state of fats and oils. In general, two measures are made, one related to the initial or primary oxidative products, i.e., hydroperoxides, plus a second measure of accumulated secondary breakdown products, usually carbonyl-type com- pounds (3). A variety of chemical methods are available that attempt to monitor secondary oxidation products, including the thio- barbituric acid test, the Kreis Test, and various other methods that attempt to address both total and volatile carbonyl com- pounds, many of which have been reviewed by Gray (4). Some of the methods developed for carbonyl compounds have been shown to be quite sensitive, quantitative, and well- correlated to compounds associated with the development of rancidity, particularly the determination of anisidine value (AV). Based on a method originally developed by Holm et al. (5), and later modified to use p-anisidine instead of carcino- genic benzidine acetate as the reactive reagent, the AV test is a widely accepted AOCS method (6). Although the method is relatively simple, the procedure requires substantial precision and analytical time and uses relatively noxious reagents. AV is a combined measure of mostly 2-alkenals and 2,4-dienals and, to a more limited degree, saturated aldehydes, because ultraviolet (UV) absorption of the p-anisidine/aldehyde reac- tion products varies with the aldehyde type. A double bond in the carbon chain conjugated with the carbonyl double bond Copyright 9 1996 by AOCS Press 787 JAOCS, Vol. 73, no. 6 (1996)