Application of FT‐NIR spectroscopy in assessment of used frying fats and oils*

Abstract

NIR spectroscopy (NIRS) was used to develop a fast and cost‐effective method to assess the quality of deep‐frying oil. A large number of fresh, used and discarded oils, used in fast food units, restaurants, fisheries, bakeries, and other frying industries, were analyzed for relevant parameters of thermo‐oxidative fat degradation such as total polar compounds (TPC), di‐ and polymerized tri‐acylglycerols (DPTG), anisidine value (AnV) and acid value (AV) using the standard DGF methods. Additionally, the fatty acid composition, including trans fatty acids, iodine value (IV) and the contents of monomeric oxidized tri‐acylglycerols (MonoxTG) were determined to describe the nutritional value of fried food. MonoxTG is a mixture of oxidized triglycerides of different molecular weight forming two groups separated in GPC. It is suggested to call them total oxidized products (TOP) as both groups of monomeric oxidized TAGs characterize, objectively and reliably, the whole spectrum of oxidized products in a used frying oil in contrast to AnV. Typical concentrations of this group of oxidized monomeric tri‐acylglycerols (TOP) in frying oils are 3 up to 15%. High calibration accuracy was obtained for all NIRS determinations. All methods were validated using test set validations. Partial least squares regression (PLS) was used to correlate spectral data with TPC, TOP, DPTG, AV, and AnV. High calibration accuracy was obtained for the NIR determination of all parameters analyzing used frying oils. For the TPC, the coefficient of determination R2 was 97.6 with an RMSEP of 1.56% over a range from 2.0 to 50.3%. Also DPTG was predicted successfully with R2 = 99.4 and RMSEP = 0.47% (0–31.3%). The AnV was measured over a broad range of 0.1 to 173 with R2 = 95.7 and RMSEP = 6.8. For acidity expressed as AV covering a wide range from 0 to 5.7 the R2 value was 97.4% with an RMSEP of 0.17%. A new parameter to monitor the degree of oxidation (TOP) has shown an R2 = 96.8 and RMSEP = 0.57%Practical applications: NIRS is a useful tool for monitoring the thermo‐oxidative changes in used frying fats and oils during the frying process. This helps to optimize the frying process and to improve the quality of the food to be fried. From the results obtained in this study, it can be deduced that the determination of the TPC is not sufficient. The determination of all oxidized monomeric triglycerides TOP provide better information of those compounds which are discussed to have a detrimental effect to human health.