Evaluation of food-grade vegetable oils using ultrasonic velocity measurement and fatty acid composition

Extra virgin olive oil (EVOO) is a high-value food commodity and is often a target for food fraud, in which the EVOO is adulterated with lower grade oils such as refined olive oil (ROO), pomace olive oil (POO) and other vegetable oils of nut or seed origin such as rapeseed or canola oil (RSO), peanut oil (PNO) and sunflower oil (SFO). The objective of this study is to investigate ultrasonic techniques to distinguish between different food-grade oils based on their fatty acid (FA) composition. An ultrasonic pulse-echo system was used to measure the propagation delay and hence the velocity of ultrasonic waves at 5 MHz in three different types of olive oil (EVOO, POO and ROO) and three other vegetable oils of nut or seed origin (PNO, RSO and SFO). The ultrasonic system was temperature controlled in a heated water bath at 23.5°C±0.05°C. The ultrasonic velocity was determined using the differential propagation delay from four 2.00 mm increments in the propagation path, determined using a micrometer to ±0.005 mm to eliminate any uncertainty in the initial propagation path. The FA content of each oil was determined using an ISO 12966-2 (2017) automatic BF3 transmethylation procedure followed by gas chromatography according to ISO 12966-4 (2015) using an Agilent HP7890A Gas Chromatograph. 80 different samples were tested, using extra virgin olive oil (n=30), refined olive oil (n=15), pomace olive oil (n=15), rapeseed/canola oil (n=10), sunflower oil (n=5), and peanut oil (n=5). The FA composition and ultrasonic velocity of each sample were measured. A statistically significant correlation between polyunsaturated fatty acid (PUFA) content and ultrasonic velocity, and a statistically significant negative correlation between monounsaturated and saturated fatty acid (MUFA and SFA) content and ultrasonic velocity, were noted. The ultrasonic velocity may thus be used to help distinguish between different food-grade vegetable oils that have a high PUFA content, such as sunflower oil and rapeseed/canola oil, and those with a high MUFA content such as olive oil and peanut oil. The FA composition appears to influence the density and compressibility of the oil, which determine the ultrasound velocity.