NIR Method Developed for Testing Frying Oil Quality

Michael A. Greenwood

French fry lovers, rejoice! A quick, reliable and relatively easy method has been developed that uses spectroscopy to monitor the quality of the frying oil into which so much tasty modern food is immersed.

Existing methods of testing frying oil often rely on chemicals and, although reliable, are time-consuming and costly and create hazardous laboratory waste. Overused oil becomes degraded, which negatively affects taste and nutrition and even poses potential harm to the consumer’s health. Researchers at the University of Nebraska-Lincoln led by Randy L. Wehling, a professor in the department of food science and technology, have developed an NIR spectroscopy technique that measures the content of two key indicators of frying oil quality: free fatty acids and total polar materials.

Using samples of FryMax, a commercially available soy-based frying oil, the researchers created different levels of degradation by heating the oil to 190 °C for varying lengths of time and storing it. To lessen temperature effects on spectral response, readings from all samples were taken under the same conditions. Prior to testing, the samples were warmed to 60 °C in a water bath. Each sample was placed in a 2-mm quartz cuvette, and data were collected immediately by taking the average of 32 spectral scans.

The researchers used a Foss scanning spectrometer and NIRSystems software. Data were taken from each sample as direct transmission measurements over the 400- to 2500-nm range. Calibration models were developed using both forward stepwise multiple linear regression and partial least-squares techniques.

The investigators found that predictions using longer wavelengths (in the range of 1100 to 2500 nm) were better than those using a shorter wavelength region. They also discovered that predictions for total polar materials were better than those for free fatty acids.

According to the researchers, there is a need for a method that can easily measure the quality of frying oil, as some 500,000 restaurants and institutions in the US alone use deep-fat frying to prepare assorted foods, including potato chips, french fries, doughnuts and chicken. They claim that their method can measure the oil in one to two minutes and that it does not harm the sample. Administering the NIR test also requires little training or expertise.

Further research into the technique is under way to determine whether frying different types of food affects the accuracy of the results.

Journal of Agricultural and Food Chemistry, Feb. 7, 2007, pp. 593-597.