Fish Bones as a Mineral Source
Commercial fish filleting from large as well as small fish result in huge amount of fish bones which is generally discarded as a waste. Fish frame account for approximately 10–15 % of total fish biomass. The bones are mainly composed of calcium phosphate and collagen protein with some special carbohydrates and lipids. Thus, the waste could be used as mineral source for food and biomedical industries while giving an added value to fish processing by-products. Fish bone consists of 60–70 % of inorganic substances, mainly calcium phosphate and hydroxyapatite (Toppe et al. 2007). Hydroxyapatite (HA) and calcium phosphate-related ceramic material have earned much attention in various biomedical applications due to their close similarity to composition of natural bones. In particular, the composition [Ca10 (PO4)6 (OH)2] and Ca/P molar ratio of HA are more similar to inorganic part of bone and teeth, and hence, this biological HA could be used as an implant material for orthopaedic and dental applications. Even though much effort has been paid to obtain synthetic hydroxyapatite, fish bone provides a cheap source for extraction of biological HA with preserved chemical characteristics and many advantages (Boutinguiza et al. 2012). Pallela et al. (2011) have developed polymer-assisted thermal calcination method to isolate microand nanostructured HA form tune (Thunnus obesus) bones. Further, findings proved that HA isolated with polymer-assisted method shows less toxicity and high biocompatibility. In addition, high level of calcium in fish bone indicates that it would be useful as a potential source to obtain calcium for dietary supplements. As most of the regular diets are calcium deficient, several calcium supplementations have been commercialized. Nevertheless, bioavailability of calcium of these products is not clearly studied. It is well known fact that small fish is a good source of balanced calcium, and fish-derived calcium is readily absorbed into human body. Thus, bones from fish processing waste can be used to produce fortified products with high biological value, and several convenient methods have been developed to soften the fish bone to convert it into an edible form.
Omega-3 Fatty Acids
Cold water oily fish, such as salmon, herring, mackerel, anchovies, and sardines, and fish oil derived from these fish have been recognized as well-balanced sources of omega-3 fatty acids, especially docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Due to enhanced health benefits of omega-3 fatty acids, world fish consumption has reached to a level which threatened the marine fish sources, and thus, fishing for oil extraction is not encouraged. Fish processing by-products have been identified as an ideal candidate for extraction of fish oil rich in omega-3 fatty acids while giving a positive insight into sustainable marine fisheries (Immanuel et al. 2009). The processing leftovers, such as head, skin and internal organs, are rich sources of omega-3 fatty acids, and method and conditions for the extraction are determined base of the nature of fish source. Presently, several methods, such as high-speed centrifugation, Soxhlet extraction, low-temperature solvent and supercritical fluid extraction, are employed to extract fish oil. Among them, wet reduction followed by pressing and centrifugation is the most common method used to produce fish oil from waste materials (Chantachum et al. 2000). Purification of omega-3 fatty acids from extracted fish oil is a challenge due to the presence of complex mixture of triacylglycerols and vulnerability of free fatty acids EPA and DHA to oxidize into hydroperoxides. A recent study shows that enzymatic deacidification of high-acid crude fish oils is an effective approach to extract high amount of n-3 fatty acids (Wang et al. 2012).
The inverse relationship between high level of omega-3 fatty acids present in bold and chronic disease has been reported in several studies. Findings suggest high levels of EPA and DHA in blood which in turn reduced rate of coronary heart diseases have an association with inhibition of lipid-rich atherosclerotic plaques growth, reduction in formation of thrombus, improving vascular endothelial function and lowering blood pressure (Lavie et al. 2009). Moreover, there is evidence for therapeutic value of omega-3 fatty acids. For instance, beneficial effects against diabetes mellitus, anti-inflammatory action and thereby protection against autoimmune diseases and potent activity against human carcinomas including prostate, lung, colon and breast have been reported. Basically, documented health benefits of fish oil are a result of high content of EPA and DHA, and fish processing by-productderived oil fall under the agreement of expected fatty acid composition for biomedical treatments (Byun et al. 2008; Wu and Bechtel 2008).