Determination of fat content in fish feed by supercritical fluid extraction and subsequent lipid classification of extract by thin layer chromatography-flame ionization detection

Abstract

A supercritical fluid extraction (SFE) method employing carbon dioxide with an alcohol modifier was developed to determine fat content in fish feed. Fat content was determined for 14 dry extruded salmon or trout grower feeds with advertised fat contents ranging from 14% to 33% (w/w). Results with the new method were significantly higher than those obtained from Soxhlet extractions (AOCS Method Ba 3–38) with a difference in mean fat content of 1.9% ( P<0.001) and were similar but significantly higher than total fat determinations by acid hydrolysis followed by organic extraction (AOAC Method 922.06) with a difference in mean fat content of 0.3% ( P<0.001). Lipid extracts from the Soxhlet and the SFE methods were classified by thin layer chromatography-flame ionization detection into five lipid classes. Extracts from the SFE method contained a significantly greater amount of phospholipids (15–85%) than extracts from the Soxhlet method ( P<0.001). Advantages of the SFE method over conventional fat determination methods include an increased yield of lipid, shorter analysis times and the absence of hazardous chemicals. The SFE method has the additional advantage over acid hydrolysis methods of estimating total fat content with minimal chemical damage to the lipid extract.