Abstract
Fish oil contains the bioactive omega-3 (n-3) fatty acids that are essential for human normal growth and development. Awareness of the positive relationship between fish oil and human health has increased the consumption of fish oil and its associated forms. Food products fortified with omega-3 concentrates from fish oil have also been introduced and marketed as functional foods. However, these products may be susceptible to oxidation due to the high polyunsaturated fatty acids (PUFA) content in the fish oil. Hence, it is important to know the physicochemical character of fish oil before it can be incorporated into the food products.
Hoki is a commercial demersal fish species in New Zealand and hoki oil has been processed as an omega-3 concentrate through ethyl ester (EE) preparation. However, information on the physicochemical properties of hoki oil is quite scant. Tuna is a global commercial pelagic fish. Physicochemical characterisation of refined hoki oil (RHO) batch 1 (RHOB1) and batch 2 (RHOB2), unrefined hoki oil (UHO) and unrefined tuna oil (UTO) were analysed in this study.
Analysis of lipid classes by thin layer chromatography (TLC) showed that no phospholipids were found in the fish oils sampled. UTO contains a higher percentage of PUFA compared to hoki oils, which have higher percentages of monounsaturated fatty acids (MUFA). All oils showed a good ratio of n-3 to n-6 fatty acid. Cholesterol contents in UHO and UTO were higher than in the RHOB1. Processing reduced the amounts of cholesterol in hoki oil. UTO has a higher concentration of natural α-tocopherol but lower concentration of vitamin A than hoki oils. Higher percentages of unsaponifiable matter (USM) were found in RHOB1 and UHO compared to UTO. The hoki oils appear more yellow than UTO, which is darker by comparison. Moisture, p-anisidine value (p-AnV) and free fatty acids (FFA) contents in the hoki oils were lower than UTO. Other indicators of oxidative stability showed that the hoki oils were more stable compared to UTO despite the peroxide value (PV) being lower in UTO.
Positional distribution of fatty acids in RHO, UHO and UTO were conducted via pancreatic lipase treatment and 13C Nuclear Magnetic Resonance (NMR) spectroscopy. Results of DHA and EPA positional distribution in the hoki oils by pancreatic lipase treatment were consistent with NMR data, where DHA was preferentially located at sn-2 position and EPA was randomly distributed. For UTO, DHA and EPA were preferentially located at sn-2 position by NMR spectroscopy. However, both DHA and EPA in UTO were incompletely recovered after the pancreatic lipase treatment. This indicates the pancreatic lipase treatment is an unreliable method for positional distribution determination of tuna oil.
In the present study, enrichment of the n-3 fatty acid content of RHOB2 intact triglycerides (TG) or via FFA and EE preparations was carried out through dry fractionation (DF), low temperature solvent crystallisation (LTSC), and urea complexation (UC) methods. Results showed that n-3 fatty acids were enriched in liquid fractions of all methods except the DF, and the highest enrichment was obtained via UC method. The FFA form of RHOB2 produced a higher concentration of n-3 fatty acids via the LTSC method compared to the TG form.
Melting and crystallisation characteristics of RHOB2, its EE and component fractions, UHO and UTO were carried out using Differential Scanning Calorimetry (DSC) from -60° to 40°C under nitrogen atmosphere. The melting and crystallisation of UTO occurred over similar temperature ranges to hoki oils but display different profiles due to differences in fatty acid composition. For RHOB2, its EE and component fractions, the melting and crystallisation profiles of the liquid fractions were shifted to a lower temperature range than their parent oils and solid fractions.
Oxidative stability of RHOB2, its EE and component fractions, UHO and UTO were investigated using DSC and Thermogravimetric Analysis (TGA) at 80°C under air atmosphere. The onset time for oxidation (to) in hoki oils occurred earlier in the TGA than the DSC as the sample gained weight prior to thermal decomposition. Conversely, UTO was rapidly oxidised as it was thermally decomposed prior to weight gain. The to and times for maximum (tmax) thermal decomposition (DSC) or weight gain (TGA) of EE were earlier than RHOB2. Both liquid and solid fractions from RHOB2 and EE had earlier to and tmax than their parent oils. Prediction of shelf life can be calculated based on the to of isothermal heating using DSC and TGA at different isothermal settings. Measurement of the oxidative stability of fish oils by DSC and TGA follow the Q10 law on the relationship between temperature and rate of chemical reaction.
Thermal decomposition of RHOB2, its EE and component fractions, UHO and UTO were carried out using TGA. Results showed that thermal decomposition of the triglyceride (TG) oils occurred in three thermal stages, which may represent a progressive degradation of PUFA, MUFA and saturated fatty acid (SFA), followed by volatilisation of polymerisation and pyrolysis products. There is unlikely any antioxidants protection at the temperature where the significant thermal decomposition began. The profiles of EE oils occurred earlier than the TG oils and had only two stages possibly due to volatilisation as opposed to decomposition.
Correlation between unsaturated fatty acids, vitamins A and E recoveries in RHOB2 and its component fractions with antioxidant activity were carried out by 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid), known as ABTS. The findings showed that antioxidants activity of the liquid and solid fractions were lower than the parent oil. Despite its high PUFA, the liquid fraction had similar stability to the solid fraction due to higher vitamin A and tocopherol protection.