A rapid screening technique for determining the lipid composition of soybean seeds

An efficient method to determine the fatty acid composition of a large number of small seed tissue samples is needed to facilitate the genetic manipulation of seed lipid composition in soybean and other oilseeds. A rapid procedure has been developed to simultaneously extract and transmethylate the neutral lipids from soybean cotyledons at 90°C in a mixture of hexane and 1% H₂SO₄ in methanol (pretreatment of cotyledon pieces with hexane prior to methanolysis was found to improve yields). Fatty acid methyl esters were analyzed by capillary or packed column gas chromatography. The method required less than 10 mg of cotyledon tissue which could be taken from opposite the embryo axis so the sampled seed remained viable. This direct transmethylation method was compared to Soxhlet extraction and subsequent acid methanolysis and to methods requiring separate steps for extraction [with hexane alone, hexane:isopropanol (3:2) or chloroform:methanol (2:1)], washing, solvent removal and methanolysis. Determinations of fatty acid content and composition were similar among the four methods. Approximately 30 times more samples could be analyzed (more than 2000 samples per month) using the direct transesterification method than were possible with the Soxhlet procedure. This direct transmethylation method is more rapid, requires less tissue, and provides results comparable to other inexpensive methods. It should be especially useful in breeding studies where large numbers of small samples are encountered.