Omega-3 Fatty Add Content of Intact Muscle of Farmed Atlantic Salmon (Salmo salar) Examined by 1H MAS NMR Spectroscopy

Abstract

The nutritional benefits of fish and fish oils have resulted in an increasing interest in seafoods and derived products generally focused on the level of omega-3 (n-3) fatty acids (FAs). In particular 20:5 n-3 (EPA) and 22:6 n-3 (DHA) are believed to play a natural, preventive role in cardiovascular diseases, and alleviation of other health problems [1,2]. Aquaculture opens up interesting possibilities for exerting a control over factors affecting the nutritional and sensory attributes of fish as food such as the quantitative and qualitative content of fat in the edible tissues. About 20% of the muscle lipids of farmed Atlantic salmon are n-3 FAs, with some variation due to the FA composition of the fish feed. The content of EPA and DHA in muscle of farmed Atlantic salmon has been found to be approximately 0.6 and 0.8 g/100 g of fillet, respectively [3]. Traditionally, gas chromatography (GC) has been used to obtain the FAs profile of lipids. This technique requires that the sample is pretreated, extraction and methylation of the lipids have to be included as part of the analysis [3]. Recently it has been demonstrated that high-resolution nuclear magnetic resonance (NMR) spectroscopy can be used to provide insight into the nature of lipid mixtures and offers the opportunity to study hetereogeneous lipid mixtures, oils and fat deposits without being destructive [4–7]. NMR measurements can be performed on intact muscle and allows the identification and quantification of muscle metabolites [4,7,8] addition to lipid fluidity studies in fish muscle stored at low temperatures [9]. In preliminary studies 13C NMR has been used to obtain the n-3 FA content of intact fish muscle [7,10]. The 13C NMR experiment on intact muscle is very time consuming and it would be an advantage to detect on 1H instead of 13C due to the facts that 1H NMR has the highest NMR sensitivity of any stable nucleus, and it has nearly 100% natural abundance. In our preliminary research it has been shown that high-resolution 1H NMR is a unique and rapid technique to quantify the total n-3 acid content of the lipid extracted from muscle of Atlantic salmon [5,7]. A traditional 1H spectrum of intact muscle will only result in broad signals containing fat/water proton resonances and individual FAs or groups of lipids are not observed. However, recent research indicates that magic angle spinning (MAS) NMR spectroscopy offers the opportunity to study intact tissue non-destructively to quantify components of the tissues. Ni and Eads [11,12] have studied fruit tissue and they have shown that low-speed MAS simultaneously relieves susceptibility broadening, improves resolution, produces accurate chemical shifts, and increases signal-to-noise ratio. In the present study, we have obtained 1H MAS NMR spectra of intact salmon muscle and quantified the total n-3 acid content from the spectra. The data from the MAS NMR analyses were compared to those obtained by estimating the fat content in extracts from equivalent muscle samples by 1H NMR and by GC. The results from MAS NMR and from 1H NMR were usually in good agreement, while the content of n-3 estimated by GC was in general higher than if estimated by the NMR techniques.