1. Critical evaluation of thermal and non-thermal accelerated lipid oxidation in fish oil
Main goal
Today saturated fatty acids are increasingly replaced by unsaturated fatty acids, resulting in a higher susceptibility to lipid oxidation of the food product. This lipid oxidation leads to the formation of undesired flavors and aromas and eventually makes the product unsuitable for consumption. Since lipid oxidation takes up to several months, accelerated oxidation tests are required in order to determine the oxidative stability and the possible influence of additives in foods. Today most accelerated lipid oxidation tests are based on high temperatures (e.g. Rancimat approach). However, several studies indicate a poor correlation with the natural oxidation mechanism. Recent studies also show that antioxidant properties can be falsely interpreted when applying elevated temperatures. In this work, a thermal accelerated lipid oxidation test (Rancimat based) is compared to a new highly innovative technique, non-thermal plasma (NTP), which in recent studies proved to oxidize lipids in a faster manner at moderate conditions, hence possibly inducing changes closer to natural oxidation mechanisms.
Experimental work and results
Natural aging (reference)
Fish oil kept in the dark , 22°C, 11 weeks
1 batch enriched with a-Tocopherol (0.1%)
Formation of many volatiles due to lipid oxidation (aldehydes, ketones, alcohols, …)
Lipid Indicator Compounds were chosen ( 2-Butenal, E-2-pentenal, 1-octen-3-ol, E,E-2,4-heptadienal, E-2-octenal, 2-nonenal, 2-decenal, …)
Enriching the fish oil with a-Tocopherol (0.1%) resulted in an antioxidant effect (figure 1)
Rancimat based test
Fish oil heated up to 100 °C for 15 hours while air is bubbled through at 13.5 L/h
Formation of various volatiles due to lipid oxidation
Many VOCs formed are not found in the naturally aged fish oil
Formation rate of LICs is dependant of temperature
Stripping of the oil sample, leaving a matrix that can not be evaluated on its organoleptic properties anymore
All volatiles are captured in water
Addition of a-Tocopherol (0.1%) enhanced the oxidation process (prooxidant effect)
Ing. Jeroen Vandamme*
Dr. Ir. Jim Van Durme
Department of microbial and molecular systems
(M²S)
Research cluster food & biotechnology
Research group molecular odour chemistry
Figure 1: Concentration of 1-octen-3-ol in function of time during natural aging of fish oil
Figure 2 : Formation rate of E-2-pentenal in function of temperature during Rancimat based test in fish oil
Figure 3: Concentration of 1-octen-3-ol in function of time during Rancimat based test in fish oil
Innovative NTP approach
Fourth state of matter
Gaseous mixture of different reactive species
DBD (Dielectric Barrier Discharge), 50kHz/500W/15kVp
Temperatures between °C
Doping Argon plasma with oxygen (molecular oxygen) or water (OH radicals) to induce the lipid oxidation process
Fish oil samples (50 mL) were treated with the plasma jet for 60 minutes (one batch enriched with a-Tocopherol)
Formation of volatile organic compounds within minutes
Enriching the fish oil with a-Tocopherol (0.1%) resulted in an antioxidant effect, in accordance with the naturally aged oil
Stripping of the oil sample
Tungsten capillary
Ground elektrode
Quartz capillary
Plasma jet
Gebroeders De Smetstraat 1
B-9000 Ghent, Belgium
Afterglow
Figure 4: Plasma reactor setup
Nonanal A B
Figure 5: A: Chromatogram overlay of fresh fish oil ((black) and Ar/O2 plasma –treated fish oil for 60 minutes, no antioxidant (blue); B: Chromatogram overlay of fresh fish oil (black and Ar/O2 plasma treated fish oil for 60 minutes, addition of 0.1% a-Tocopherol (blue)
Figure 6: Concentration of nonanal in function of treatment time with Ar/O2 plasma in fish oil with out antioxidant
Conclusions
The natural aging test of fish oil indicates the complexity of the lipid oxidation ILLUSTRATED BY A WIDE RANGE OF NEWLY FORMED, SECONDARY OXIDATION VOLATILES. Measuring their concentration in function of time is a good indication for the progress of the oxidation mechanism. BASED ON NATURAL OXIDATION EXPERIMENTS, AN ANTIOXIDANT EFFECT OF a-Tocopherol ADDITION WAS OBSERVED. CONTRARY, a thermal accelerated lipid oxidation (Rancimat based approach), HAD A UNREALISTIC OUTCOME COMPARED with the natural aging process, due to different oxidation kinetics and variant antioxidant properties. The addition of a-Tocopherol even induced a prooxidant effect. Preliminar experiments with NTP indicate that this method is capable of oxidizing lipids in an accelerated and non-thermal way. However, THE IDENTIFIED oxidation products are not IDENTICAL AS the ones DETECTED AFTER the natural aging process. The addition of a-Tocopherol induced an antioxidant effect, in accordance with the natural aging test. Further research, including a reactor optimization and parameter study, will lead to further profound insights of the NTP-based accelerated lipid oxidation.
Aknowledgements
WE would like to aCknowledge promotor Prof. Luc De Cooman for his support during this project . Also, WE THANK Prof. Christophe Leys and Dr. Anton Nikiforov (research group of applied plasma physics, Ghent, Belgium) for their technical support with the plasma reactor.