Abstract
Fish is an important dietary component and is a major source of protein and requires appropriate post-harvest handling, processing and storage. In this study, oven and freeze dried Atlantic salmon both untreated and treated with garlic, cinnamon and BHT as well as mixture (garlic + cinnamon + vitamin C) was stored for 24 weeks at 22 oC or 30 days at 40 oC. The chemical composition (moisture, protein, total lipids and ash) of the Atlantic salmon was also investigated. The mechanism of antioxidant activity was extensively studied through DPPH, H2O2, hydroxyl radical scavenging and metal chelating activity assays. It was demonstrated that garlic and cinnamon contain large amounts of polyphenols and flavonoids that reflect directly on the assays mentioned above. The PV value, protein, TBARS and carbonyl contents of the antioxidant-treated and untreated Atlantic salmon were studied. Atlantic salmon (control) without antioxidants deteriorated quickly resulting in high PV value; BHT was the most effective antioxidant, followed by the combination of garlic, cinnamon and vitamin C; garlic and cinnamon in oven- and freeze-dried salmon. However, cinnamon prevented TBARS formation more than garlic after 30 days storage (p<0.05). Garlic, cinnamon, the antioxidant mixture and BHT all reduced protein denaturation as shown by a lower carbonyl content and loss of solubility. In general, freeze drying was more effective than oven drying in maintaining the nutritional value of the stored Atlantic salmon. Structural, thermodynamic and texture changes that occurred during fish storage were assessed by FT-Raman spectroscopy, differential scanning calorimetry (DSC) and small deformation rheology respectively. FT-Raman spectroscopy showed that proteins were less denatured in the presence of different antioxidants; the tyrosine doublet ratio decreased and β-sheet structure (990 and 1239 cm-1) increased, suggesting the involvement of the hydroxyl groups in hydrogen bonding. Raman spectroscopy also confirmed that the structure and composition of salmon flesh lipids were directly affected by storage time and protected by the addition of antioxidants. However, DSC analysis showed that with the exception of garlic and BHT (Peak1 myosin), antioxidant-treated salmon fillets were significantly more denatured or cross-linked, with low ΔH values, compared to untreated control salmon fillets. Small deformation rheology showed an increase in the elastic modulus G’ over the viscous modulus G” indicating an initial gel nature of the dried fillet paste. An increase in G’ confirmed protein denaturation and muscle toughness during storage; the values were significantly higher than those observed in samples treated with different antioxidants. Finally, BSA was subjected to the previously discussed techniques and used as a model protein to show that protein-polyphenol (500 ppm cinnamon and cinnamaldehyde) interactions occurred mainly via aliphatic, aromatic and tyrosine amino acids. Protein denaturation was evidenced by higher G’ and ΔH values of protein-polyphenol mixtures compared to the control BSA.