Vegetable re-esterified oils in diets for rainbow trout: Effects on fatty acid digestibility

The present study aimed at determining the effect of re-esterified oils with different monoacylglycerol (MAG) content, produced from two different vegetable sources with different degrees of saturation, palm and rapeseed, on fatty acid digestibility in rainbow trout. Re-esterified oils were obtained from a chemical esterification process using acid oils (free fatty acids (FFA)-rich by-products from the refining of vegetable oils) and glycerol (by-product of biodiesel production). This process, which produces the formation of triacylglycerols (TAG), reduces the content of FFA present in the acid oil and generates a redistribution of the fatty acids in the glycerol molecule. This redistribution could increase the amount of saturated fatty acids (SFA) located at the sn-2 position of acylglycerols. Moreover, it allows obtaining fats with a certain proportion of MAG, known for being good emulsifiers. Therefore, a higher nutritive value of re-esterified oils than that of acid oils might be expected. A 21-day feeding trial where triplicate groups of rainbow trout were fed nine experimental diets formulated to contain a 21% of different experimental oils was carried out. For each source, four different types of oil were used: native, re-esterified low in MAG, re-esterified high in MAG and acid. A commercial fish oil was used for the control diet. Although re-esterified oils had better apparent digestibility coefficients (ADC) of SFA than their corresponding acid oil diets, no improvement in SFA digestibility was observed in rainbow trout fed re-esterified oils diets compared to those fed native oil diets, not even when a high content of MAG was present. Although this improvement did not occur, both palm and rapeseed re-esterified oils could be incorporated as a fat source in diets for rainbow trout without negatively affecting fatty acid digestibility values. The study concluded that fatty acid digestibility in the experimental oils was more affected by their degree of saturation than by their positional distribution and lipid class composition of the oils.