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The quality of fish feed and the effectiveness of fish feed production process are important issues for both fish feed suppliers and fish farmers. A correct combination of protein, carbohydrate, fat, and moisture contents in fish feed is crucial for achieving a desirable growth rate and other key characteristics of farmed fish.

Water is the most abundant chemical substance in musclebased food such as fish fillet, and the interactions between water and macromolecules are influential to various food quality attributes like texture and juiciness. Information on water binding and distribution within the muscle is therefore important while studying raw material properties and quality changes during processing and storage. Low-field NMR transverse spin relaxation of the water-hydrogen nucleus has been succesfully employed in food research to provide information such as the relaxation time, which is dependent on the mobility of the water molecule and thereby of its binding to or entrapment by structural elements of the cells.

In the last 10 years, high-resolution carbon-13 and proton-NMR spectroscopy have been successfully applied to the analysis of fish oils and lipids [1-10]. The purpose of these analyses was focused not only on the rapid determination of lipid classes and fatty acid composition but also on the assessment of positional distribution of fatty acids along the glycerol moiety, fish quality, lipid oxidation, and lipolysis. ^13C and ^1H-NMR are complementary techniques in order to define lipid composition and chemical alterations. ^13C-NMR offers the advantage of higher resolution but lack in sensitivity and requires longer time of analysis to obtain quantitative spectra. Proton NMR, on the other hand, is very rapid and quantitative, but proton signals of lipid molecules are present in a narrow spectral range thus reducing resolution. Therefore, different applications have been carried out taking into account advantages and disadvantages of two techniques.

Fatty acids are key components of fish lipids, and the analysis of them is usually carried out for the characterization of fish oils. Fish oils contain a wide variety of fatty acids (Table 1). Thus, Ackman^[1] has reported as many as 50 or 60 components, though only about 14 of these are of importance in terms of weight percent of the total. They are low in saturated fatty acids (mainly myristic and palmitic acids) and high in unsaturated fatty acids, especially those unsaturated acids with long chains containing 20 or 22 carbons or more, and up to six double bonds.

Marine lipids are highly susceptible to lipid oxidation, and analytical methods describing the early stages of lipid oxidation are needed. Free radicals, which are major contributors in the initiation and propagation stages of lipid oxidation, might be detected by different electron spin resonance (ESR) spectroscopy techniques. High resolution (HR) nuclear magnetic resonance (NMR) spectroscopy is less sensitive compared to ESR, but can be applied to identify the different reaction products formed during lipid oxidation. Additionally, HR-NMR can provide detailed information about the chemical composition, which is valuable for predicting the susceptibility to lipid oxidation. This chapter will shed light on some of these techniques applied on marine material.

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].

Marine microalgae, or phytoplankton, have been termed the “grass of the sea” since they are the primary producers that form the basis of marine food chains. They are found naturally throughout all aquatic environments, and can be cultured for feed production or for experimental purposes. Some microalgae are toxic, such as those involved in shellfish poisoning. Others have long spines on the cell that can cause mechanical damage to fish gills, and some species form gelatinous aggregations which can hinder gas exchange in fish gills [1]. The nutritional value of microalgae depends primarily on the amount and composition of amino acids and fatty acids, and diatoms contain many of the desired compounds [2]. Since microalgae are so important both in terms of beneficial primary production but also as the cause of fish or human death by poisoning, we have developed several technological solutions to survey the distribution and amounts of microalgae in the oceans. On a larger scale, satellite measurements of chlorophyll a (Chi a ) and buoys equipped with spectroradiometers that measure light extinction are used to estimate the standing stocks of microalgae in the oceans and coastal areas. For more detailed information, laboratory studies often involve species identification and cell counts with light microscope, spectrophotometric measurements of Chi a and in vivo light absorption, or pigment chromatography for identification and biomass estimation based on group specific pigment composition [1,3]. There are, however, some drawbacks to these well-established methods, both related to the time spent on the analysis run and the use of organic solvents. Each method aims at specific cellular structures or metabolites and the different methods are therefore often combined to increase the information output. This means that even more time is spent on the analytical work, while with the HR MAS NMR spectroscopic methods it is possible to analyze complex samples such as tissue and cells in relatively short time, and the information output from each analysis includes data on many different metabolites.

The use of HR MAS NMR presents a unique tool for studying biological solid and semisolid samples [1,2]. The method gives information on different classes of compounds from the samples in question that would have taken multiple analyses to obtain otherwise [3]. Due to the lower sensitivity of ^13C NMR in comparison to ^1H NMR, the latter would be the method of choice for studying small metabolites and osmolytes, and biological processes involving these. Reported applications of ^1H HR MAS NMR to study microalgae have been discussed by Chauton and in HR MAS NMR Spectroscopy, Part 1, found in this book on pages XXX-XXX.

Although being less sensitive than many other analytical methods, magnetic resonance imaging (MRI) does have significant advantages such as being non-invasive and non-destructive. Furthermore, no sample preparation is needed prior to analysis and the method is not hampered by limitations regarding signal penetration depth. However, due to the high investment costs, the sheer size of the instrument and the necessary related infrastructure, MRI cannot presently be considered as a standard analytical tool in aquaculture or fish processing industry. As a research tool, however, taking advantage of the unique features of the method, we can obtain basic insight into a number of issues related to anatomical studies, composition and structure of tissues, distribution maps of fat, water, and salt as well as temperature profiles (mapping). Moreover, theoretical transport models can in turn be used to interpret the images. For the aquaculture industry, MRI studies may for instance be helpful to study the effect of feed composition (fat) and different feeding regimes on fat contents and distribution in tissues. In fish processing, MRI can be used as a tool for the optimization of various unit operations such as salting, freezing, and thawing. For a more detailed treatment of various MRI applications, see review by Hills [1].

Because food is so important for survival, food preservation is one of the oldest technologies used by human beings. The fish preservation techniques mostly used today are salting and freezing. Salting is an ancient preservation technique. Salting keeps on being used, but it is often replaced by freezing. Smoking merely adds flavour and colour and removes some water. Smoked fish are almost as perishable as fresh fish.