Lipoxygenase activity in menhaden (Brevoortia tyrranus) and its contribution to oxidation of omega-3 polyunsaturated fatty acids in menhaden oil
Menhaden is the major source of fish oil in the United States. Due to a high amount of polyunsaturated fatty acids which are highly susceptible to autoxidation, menhaden oil deteriorates rapidly, leading to objectionable off-odors and off-flavors. The purpose of this study was to investigate if the enzyme lipoxygenase is present in menhaden gill tissue and if it is a contributing factor in menhaden oil oxidation.
Peroxide, TBA and anisidine values of undeodorized and deodorized menhaden oils exhibited two maxima during 20 weeks of storage at 30°C. Peroxide values of the undeodorized oil peaked at week 1 with 6.71 meq/kg and at week 12 with 21.50 meq/kg, while in the deodorized oil it peaked at week 8 (9.28 meq/kg) and week 20 (18.71 meq/kg). TBA maxima were observed at week 2 (1416 μMol/kg) and week 12 (4951 μMol/kg) and at week 8 (1397 μMol/kg) and week 20 (4284 μMo/kg) for undeodorized and deodorized menhaden oil respectively. Anisidine values showed maxima at the same weeks. These results indicate that lipid peroxidation of the deodorized oil lagged a few weeks behind the undeodorized oiL In this study, the conjugated diene and fluorescence analyses were found to be poor indicators for monitoring lipid oxidation in menhaden oil.
Enzyme assays indicated that lipoxygenase activity is present in menhaden gill tissue with maximum activity at pH 9-10, resembling that of soybean lipoxygenase-l. A sensory panel judged omega-3 fatty acid ester concentrates treated with the enzyme extract as having a significantly (p < 0.03) stronger smell than the control ester for the first four weeks of an eight week study. However, no significant difference was found between the TBA values of the esters.
Of the 60 volatile compounds identified by GC-MS in the undeodorized menhaden oil, 19 were aldehydes, 9 were alcohols and 8 were ketones. Volatiles that are potentially Ii poxygenase derived, namely 2-octenal, 1-octen-3-01, 2-nonenal, 2,6-nonadienal (E,Z), and 2,5-octadien-l-ol were among those identified in the undeodorized menhaden oil. The deooorized oil contained fewer total volatiles, and fewer aldehydes (6), ketones (1) and alcohols (8), but more long chain aliphatic compounds such as hydrocarbons, many of which were not possible to positively identify. No lipoxygenase derived volatiles were identified in the deooorized oil. Most of the volatiles in the omega-3 fatty acid ester concentrates were identified as esterified short chain fatty acids. No difference in the amount of total volatiles was found between four esters that were treated with and without the enzyme extract, a boiled enzyme extract and an enzyme extract that was inocculated with esculetin. However, in a repetition of just the control and the enzyme treated ester, a significantly (p < 0.02) higher amount of total volatiles was found in the enzyme treated ester, supporting the results of the sensory analysis. It was not possible to identify specific volatiles in the enzyme treated ester that were present in larger concentrations than in the other ester treatments. Volatiles identified in EPA and DHA ethyl esters were similar to those volatiles found in the undeodorized and deodorized menhaden oil as well as the omega-3 fatty acid ester concentrates, but no lipoxygenase derived volatiles were found.
While lipoxygenase activity was found in the gill extract of menhaden, and sensory analysis was able to distinguish between a control and an enzyme incubated oil, the enzymatic activity was low (apparent Km = 16.7 μMol) and volatile analysis of various oils did not support the hypothesis that lipoxygenase is a major contributor to menhaden oil oxidation. Future research should include isolation and purification of menhaden gill lipoxygenase and the study of model systems to develop a better understanding of the contribution of lipoxygenase activity to oxidation of menhaden oil.