Isabel Medina

Solid-phase microextraction method for the determination of volatile compounds associated to oxidation of fish muscle

A procedure for the determination of volatile compounds derived from lipid oxidation of fish muscle samples is presented. Analytes are concentrated on a solid-phase microextraction fiber employed in the headspace mode (HS-SPME), and selectively determined using gas chromatography in combination with mass spectrometry (GC-MS). The influence of several parameters on the efficiency of microextraction such as type of fiber, volume of sample, time, temperature, salting-out effect and stirring was systematically investigated. A saline extraction of fish muscle followed by incubation on a Carboxen-polydimethylsiloxane fiber during 30min at 60 degrees C gave the most effective and accurate extraction of the analytes. Quantification of them was performed by MS in the selected ion monitoring mode and by the internal standard method. Satisfactory linearity, repeatability and quantification limits were achieved under these conditions. The method was applied to the determination of the volatile compounds associated to oxidation of Atlantic horse mackerel (Trachurus trachurus) minced muscle and excellent correlations were obtained with chemical indexes for monitoring lipid oxidation as peroxide value and thiobarbituric acid reactive substances. This combined technique is fast, simple, sensitive, inexpensive and useful to monitor target compounds associated to fish rancidity as 1-penten-3-ol, 2,3-pentanedione or 1-octen-3-ol.

Effect of Lipophilization of Hydroxytyrosol on Its Antioxidant Activity in Fish Oils and Fish Oil-in-Water Emulsions

The effect of lipophilization of the antioxidant efficiency of hydroxytyrosol on fish oil enriched systems was studied. Hydroxytyrosol fatty acid esters with increasing size of the alkyl chain and different lipophilicity were tested in bulk fish oils and fish oil-in-water emulsions. Results showed a significant antioxidant activity of hydroxytyrosol esters in both systems especially in emulsions. The introduction of a lipophilic chain decreased the antioxidant effectiveness of hydroxytyrosol in homogeneous systems as fish oils. In emulsion systems, the presence of a short-medium lipophilic chain (acetate, butyrate or octanoate) improved the antioxidant efficiency of hydroxytyrosol favoring the physical location of the antioxidant in the interface, but longer alkyl chain (laurate) maintained or even decreased their antioxidant activity. A maximum of antioxidant efficiency seems to appear when the chain length of the hydroxytyrosol derivative is that of eight carbons which is probably associated with a preferential location of the diorthophenolic moiety in the right geometry. These results are of high importance for the optimum design of effective antioxidants for omega 3 enriched foods, which are very susceptible to suffer oxidation and, then, rancidity.

Influence of storage time and temperature on lipid deterioration during cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) frozen storage

Lean fish deterioration during frozen storage (−30 and −10 °C) for up to 1 year was studied by the assessment of lipid changes. Comparison between a formaldehyde (FA)-forming species (cod) and a non-FA-forming one (haddock) was carried out. Lipid damages were measured on the basis of free fatty acids (FFA), peroxide value (PV), thiobarbituric acid index (TBA-i) and fluorescent compounds. In both species at −30 °C, most lipid damage indices showed significant correlations with the storage time. However, at −10 °C, only the FFA and fluorescence detections provided significant correlations with the storage time. Comparison between the fish species showed higher lipid oxidation (PV and TBA-i) and hydrolysis (FFA content) in haddock than in cod at −10 °C; however, a higher fluorescence development was observed in cod at the same temperature. At −30 °C, little differences in lipid damage indices were detected between the two species. © 1999 Society of Chemical Industry

Fish species identification in canned tuna by PCR-SSCP: validation by a collaborative study and investigation of intra-species variability of the DNA-patterns

Analysis of single strand conformation polymorphism (SSCP) of an amplicon (123 bp) obtained by polymerase chain reaction (PCR) of the mitochondrial cytochrome b gene was used to identify the fish species in canned tuna. Single-stranded DNA (ssDNA) was separated by polyacrylamide gel electrophoresis, and visualised by silver staining. The reliability of the method was tested by a collaborative study in which eight European laboratories participated. Seven unknown samples (five from individual species and two mixtures of two tuna species) of canned tuna had to be identified by comparison with reference material. From a total of 72 cases, 65 (90.3%) were assigned correctly. Intra-species variability of SSCIP patterns was found in the case of Katsuwonus pelamis and Sarda sarda. As specimens from various fishing grounds gave two or three different patterns of ssDNA, the possibility of some variability of the DNA patterns has to be considered in SSCP analysis of these species.

Proton nuclear magnetic resonance rapid and structure-specific determination ofω-3 polyunsaturated fatty acids in fish lipids

Based on proton nuclear magnetic resonance (1H-NMR) spectroscopy, a rapid and structure-specific method for the determination of ω-3 polyunsaturated fatty acids (PUFAs) in fish lipids is presented. The different chemical shift observed for the methyl resonance of ω-3 PUFAs (δ=0.95 ppm) with respect to the methyl resonance of all other fatty acids (δ=0.86 ppm) has provided the possibility of proposing a new and rapid method for the determination of ω-3 PUFA content. Twenty-four fish lipid samples (raw, cooked and canned albacore tuna) produced results that showed good agreement between1H-NMR analysis and gas chromatographic determination. Raw and cooked samples showed significantly higher levels of ω-3 PUFA than canned tuna.

Composition of phospholipids of white muscle of six tuna species

A comparative study of the phospholipids of white muscle of six of the comercially utilized tuna species, including quantitative analyses of phospholipid classes and studies of the acyl composition of the major components. Plasmalogen compounds also were identified and quantified. Choline and ethanolamine glycerophospholipids were the most abundant classes in all the samples, as well as the only molecules containing plasmalogens (16∶0, 18∶0, and 18∶1 alkenyl ether chains). The patterns of fatty acid distribution within each of the phospholipid classes showed general similarities in the species analyzed. However, ratios between certain saturated and polyunsaturated fatty acids in different phospholipid classes showed remarkable diffences. The high content of n−3 polyunsaturated fatty acids in the principal phospholipids, such as the plasmalogens, and taking into account the fatty acids possible importance in human nutrition, indicates that the white muscle of tuna species may be a potentially important dietary item.

Hydroxytyrosol prevents oxidative deterioration in foodstuffs rich in fish lipids.

Hydroxytyrosol, a natural phenolic compound obtained from olive oil byproduct, was characterized as an antioxidant in three different foodstuffs rich in fish lipids: (a) bulk cod liver oil (40% of omega-3 PUFAs), (b) cod liver oil-in-water emulsions (4% of omega-3 PUFAs), and (c) frozen minced horse mackerel ( Trachurus trachurus) muscle. Hydroxytyrosol was evaluated at different concentration levels (10, 50, and 100 ppm), and its antioxidant capacity was compared against that of a synthetic phenolic, propyl gallate. Results proved the efficiency of hydroxytyrosol to inhibit the formation of lipid oxidation products in all tested food systems, although two different optimal antioxidant concentrations were observed. In bulk oil and oil-in-water emulsions, a higher oxidative stability was achieved by increasing the concentration of hydroxytyrosol, whereas an intermediate concentration (50 ppm) showed more efficiency, delaying lipid oxidation in frozen minced fish muscle. The endogenous depletion of alpha-tocopherol and omega-3 polyunsaturated fatty acids (omega-3 PUFAs) was also inhibited by supplementing hydroxytyrosol in minced muscle; however, the consumption of the endogenous total glutathione was not efficiently reduced by either hydroxytyrosol or propyl gallate. A concentration of 50 ppm of hydroxytyrosol was best to maintain a longer initial level of alpha-tocopherol (approximately 300 microg/g of fat), whereas both 50 and 100 ppm of hydroxytyrosol were able to preserve completely omega-3 PUFAs. Hydroxytyrosol and propyl gallate showed comparable antioxidant activities in emulsions and frozen fish muscle, and propyl gallate exhibited better antioxidant efficiency in bulk fish oil.

Comparison of natural polyphenol antioxidants from extra virgin olive oil with synthetic antioxidants in tuna lipids during thermal oxidation

Polyphenols extracted from extra virgin olive oil (EVOO) were tested for their ability to inhibit lipid oxidation of canned tuna. Hydroperoxide formation during oxidation was monitored by measurement of peroxide value and decomposition of hydroperoxides by static headspace gas chromatographic analysis of volatiles. In tuna oxidized at 40 and 100 degrees C, 400 ppm of the EVOO polyphenols was an effective antioxidant as compared with 100 ppm of a 1:1 mixture of the synthetic antioxidants butylated hydroxytoluene and butylated hydroxyanisole. However, at concentrations <100 ppm, the EVOO phenolic compounds promoted hydroperoxide formation and decomposition. The EVOO polyphenols were effective antioxidants when added to heated tuna muscle in the presence of either brine or refined olive oil. The oxidation rate in tuna muscle packed in brine was higher than that of tuna packed in refined olive oil. The EVOO polyphenols had higher antioxidant activity in the brine samples than in the refined olive oil. The higher antioxidant activity of EVOO polyphenols in tuna packed in brine may be explained by their greater affinity toward the more polar interface between water and the fish oil system.

One-step conversion of fatty acids into their 2-alkenyl-4,4-dimethyloxazoline derivatives directly from total lipids

Abstract 2-Alkenyl-4,4-dimethyloxazoline derivatives of fatty acids were obtained by direct reaction of 2-amino-2-methylpropanol with oils or total lipid extracts. The yields were similar to those obtained by the formation of oxazoline derivatives from fatty acid methyl esters after transesterification of the starting lipids. The procedure is especially useful in the analysis by gas chromatography-mass spectrometry of samples rich in lipids containing α-unsaturated ethers (alkenylglycerols), as it avoids the formation of the corresponding dimethylacetals that can complicate the chromatograms and the mass spectra of the fatty acid oxazoline derivatives.

Caffeic Acid as Antioxidant in Fish Muscle: Mechanism of Synergism with Endogenous Ascorbic Acid and α-Tocopherol

In an emulsion of corn oil in water with the addition of caffeic acid (Caf-OH) and alpha-tocopherol (alpha-TOH), Caf-OH was found to be very active in delaying lipid oxidation without affecting significantly the kinetics for alpha-TOH degradation. In contrast, Caf-OH addition to fish muscle retarded both the degradation of endogenous alpha-TOH and the propagation of lipid oxidation, measured by peroxide value (PV) and thiobarbituric acid reactive substances (TBARS), with increasing effect with increasing Caf-OH addition (55.5-555.1 micromol/kg). Electron spin resonance (ESR) spectroscopy confirmed a higher capacity of Caf-OH to regenerate alpha-TOH via reduction of the alpha-tocopheroxyl radical compared to other cinnamic acid derivatives (o-coumaric, ferulic, and chlorogenic acids). Degradation of endogenous ascorbate (AscH(-)) was accelerated at higher concentration of Caf-OH in fish tissue, suggesting a role of AscH(-) in the regeneration of Caf-OH. These results indicate that the antioxidant mechanism of Caf-OH implies the protection of endogenous alpha-TOH localized in tissue membranes where lipid oxidation is initiated and, at the same time, Caf-OH regeneration by the endogenous AscH(-). These combined effects result in a stronger antioxidant protection against lipid oxidation by favoring, as a final point, the protection of alpha-TOH, which is suggested as the last defense of fish muscle against lipid oxidation.