Vegetable re-esterified oils in diets for rainbow trout: Effects on fatty acid digestibility
Introduction
Nowadays, aqua feeds account for about 75% of the global consumption of fish oil (FO). Its production depends on the availability of wild fisheries, which has decreased since the mid-1990s. In spite of the progressive drop-off in the use of FO by aquaculture, its global demand and price have been increasing due to both the rapid expansion of aquaculture sector and its growing use by the nutraceutical industry. Thus, the price of FO is expected to rise 70% from 2010 to 2030 (FAO, 2014). As a consequence, the use of vegetable oils (VO) as an alternative source of energy to replace FO in commercial fish feeds has increased. VO are renewable sources produced in large volumes that have a lower price than FO, being palm, soybean, rapeseed and sunflower the most produced (Gunstone, 2011). They are mainly used for food and feed, although its use as feedstock for energy production has increased steadily (Behr and Gomes, 2010, Jayasinghe and Hawboldt, 2012).
Refining is an industrial procedure necessary to render VO to an edible form that has bland flavour and odour, clear appearance, light colour, stability to oxidation and suitability for frying (Brooks et al., 2013, FAO, 1994). It removes compounds other than triacylglycerols (TAG) in order to obtain a TAG-rich oil (by 99%). Some of these other compounds are valuable and can be recovered for subsequent use (Nuchi et al., 2009). They can be used for the production of special feed “technical” lipids such as calcium soaps, hydrogenated lipids, re-esterified, and mono- and diglyceride oils which can satisfy specific nutritional requirements (Parini and Cantini, 2008). Acid oils are free fatty acids (FFA)-rich by-products generated from the refining process (Nuchi et al., 2009). They can be considered a cheaper alternative to the use of vegetable native oils. However, studies performed in broiler chickens (Blanch, 1996, Wiseman and Salvador, 1991) reported that acid oils have a lower energy value than that of native oils, which have been related to a lower digestibility due to their high FFA content. The nutritive value of these oils might be improved by means of a chemical esterification process, which generates the formation of TAG and thus reduces the content of FFA. These TAG are formed after the reaction of FFA from vegetable acid oils with glycerol (Parini and Cantini, 2009), the latter being a by-product of biodiesel production. As this TAG synthesis is not selective, it can result in a different fatty acid positional distribution within the TAG molecule than their corresponding native oils. In native VO, SFA (palmitic and stearic) are mainly located in the external positions of the TAG molecules (sn-1 and sn-3) while the sn-2 position contains a high proportion of unsaturated fatty acids (oleic, linoleic and linolenic) (Hunter, 2001, Karupaiah and Sundram, 2007). During lipid digestion in mammals, pancreatic lipase hydrolyses the external positions of the TAG, being 2-monoacylglycerols (MAG) and FFA as the main products of the lipid digestion process. FFA that are mono- (MUFA) or polyunsaturated (PUFA) fatty acids will be mainly incorporated into micelles and absorbed. However, impaired digestibility is found for free long chain SFA due to its hydrophobicity, high melting point and the possibility to form insoluble soaps in the gut and thus be lost in faeces (Berry, 2009, Hunter, 2001, Small, 1991). The fatty acid located in sn-2 remains bound to the glycerol molecule as 2-MAG, which is directly absorbed (Schulthess et al., 1994). In fish, the predominant type and specificity of pancreatic lipase is still quite controversial and seems to vary greatly among species. Even so, a bile salt-dependent pancreatic lipase with sn-1,3-specific hydrolytic activity has been pointed as the main lipolytic enzyme in different species (Bogevik et al., 2007, Gjellesvik et al., 1992, Tocher, 2003). Thus, when a SFA is esterified in the sn-2 position, it may have a superior absorption, as it has been described in rats (Renaud et al., 1995), piglets (Innis et al., 1995), broiler chickens (Smink et al., 2008) and human infants (Kennedy et al., 1999). It could then be expected that the distribution of FA in chemically re-esterified oils resulted in a higher content of SFA in sn-2 position in the TAG and this could result in a higher digestibility of these oils compared to their native counterparts.
Another important difference between re-esterified and native oils could be the proportion of the different lipid classes – TAG, diacylglycerols (DAG) and MAG – that are present in the new re-esterified oil (Parini and Cantini, 2009). Chemical esterification process allows obtaining fats with the same fatty acid profile, but with different contents of TAG, DAG and MAG according to the process conditions (i.e. proportions of FFA and glycerol). As it has been pointed out, lipid digestion aims to reduce large lipid molecules (TAG and DAG) to smaller ones (MAG and FFA) for their absorption. Of these lipid classes, MAGs have been long known as good emulsifiers due to their amphiphilic nature and surface-active properties (Cruz-Hernandez et al., 2012, Hess et al., 1995, Martin et al., 2014), so digestibility values might improve when a major MAG content is present in the dietary fat.
To the best knowledge of the authors and to-date, there are no studies in the literature reporting the use of randomly re-esterified VO in fish diets. A higher nutritive value of re-esterified oils than of acid oils might be expected as a result of the reduction of the amount of FFA that takes place during esterification. Similarly, changes in their physicochemical properties compared to their corresponding native oils could be obtained due to both the higher amount of SFA in sn-2 and their higher proportion of MAG. Thus, the present study aims at determining the effect of re-esterified oils with different MAG contents, produced from palm and rapeseed acid oils, on fatty acid digestibility in rainbow trout (Oncorhynchus mykiss) as a first step to determine if they can be suitable fat sources for fish diets.
Section snippets
Experimental diets
Nine experimental diets were formulated to contain 48% protein and 21% lipid using the same ingredient composition except for the added lipid source. Oils used for the experimental diets came from two different vegetable sources with different degrees of saturation, palm (P) and rapeseed (R). For each source, four different types of oil were used: native oil (N), re-esterified oil low in MAG (EL), re-esterified oil high in MAG (EH) and acid oil (A), all resulting in eight experimental diets (
Characterization of experimental oils and diets
Because scarce information about re-esterified oils is available, a previous characterization of the experimental oils was necessary.
Characterization of experimental oils and diets
Results from the present study indicated that 1(3)-MAG was the main MAG isomer present in the oils, and especially in re-esterified oils, while 2-MAG only represented 7–12.5% of the total MAG in the oils. The predominance of 1(3)-MAG over 2-MAG could be related to the acyl migration of FA in sn-2 to sn-1 or sn-3 positions (Destaillats et al., 2010, Martin et al., 2014). As described by Cruz Hernandez et al. (2012), primary esters (sn-1(3)) are more stable than secondary esters (sn-2). In fact,
Acknowledgements
Authors would like to thank SILO S.p.a. (Firenze, Italy) for providing the experimental fats and Anna Simonetti from Skretting ARC Mozzecane (Italy) for the technical assistance. This study was supported by a FPI predoctoral research grant from Ministerio de Ciencia e Innovación del Gobierno de España (BES-2011-046806), a post-doctoral research grant from the Generalitat de Catalunya and the EU through the Beatriu de Pinós Post-doctoral Program (2011BP_B 00113) and by the financial contribution
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