The concentrations and pro‐oxidative effects of free fatty acids in commercial krill oil are not well defined. We now report that krill oil free fatty acids account for 2–13% of total lipids in commercial krill oil (n = 8) that these compounds are enriched in eicosapentaenoic acid (+7.1%) and docosahexaenoic acid (+6.3%) relative to whole oils; and that this composition make them highly pro‐oxidizing in marine triacylglycerol oils, but not in krill oil, which derives oxidative stability from both its phospholipids, and neutral lipids (the latter because of astaxanthin). Specific fatty acid esterification patterns showed that krill oil free fatty acids predominantly (88–93%) originated from phospholipids, mainly from the sn‐2 position, which was eight‐fold more hydrolyzed than the sn‐1 position. Lipolysis was not ongoing in stored oils. Adding small amounts of krill oil (1–5%) to marine triacylglycerol oils significantly increased their oxidative stability and also their resistance to free fatty acid‐mediated pro‐oxidative effects.

Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are commercially important omega-3 fatty acids found in fish oils. Here we demonstrate that a handheld Raman spectrometer can be used to quantitate these compounds in intact fish oil capsules, avoiding oxidizing risk. Partial least squares regression models were prepared by relating Raman spectral variance to EPA and DHA concentrations determined using gas chromatography–mass spectrometry (GC-MS) analysis of fatty acid methyl esters in 15 commercial samples containing 145–473 mg g−1 EPA and 101–260 mg·g−1 DHA. Handheld Fourier transform (FT)-Raman models had root mean square errors of cross-validation of 38 mg g−1, 24 mg g−1, and 32 mg·g−1 for EPA, DHA, and EPA+DHA, respectively. Models generated from a benchtop FT-Raman spectrometer had corresponding errors of 32 mg·g−1, 22 mg·g−1, and 26 mg·g−1. By comparison, average standard deviations from triplicate GC-MS analyses were 11 mg·g−1 for EPA and 9 mg·g−1 for DHA.

In the wild, larval and juvenile marine fish forage on a wide range of planktonic and zooplanktonic prey to gather the lipids required for growth. These include copepods, cladocera, barnacle nauplii, gastropod larvae and oceanic tunicates (Turner, 1984). In aquaculture, juvenile marine fish also require live feed, with rotifers (usually Brachionus plicatilis) commonly used for this purpose (Lubzens

The concentrations and pro-oxidative effects of free fatty acids in commercial krill oil are not well defined. We now report that krill oil free fatty acids account for 2–13% of total lipids in commercial krill oil (n = 8) that these compounds are enriched in eicosapentaenoic acid (+7.1%) and docosahexaenoic acid (+6.3%) relative to whole oils; and that this composition make them highly pro-oxidizing in marine triacylglycerol oils, but not in krill oil, which derives oxidative stability from both its phospholipids, and neutral lipids (the latter because of astaxanthin). Specific fatty acid esterification patterns showed that krill oil free fatty acids predominantly (88–93%) originated from phospholipids, mainly from the sn-2 position, which was eight-fold more hydrolyzed than the sn-1 position. Lipolysis was not ongoing in stored oils. Adding small amounts of krill oil (1–5%) to marine triacylglycerol oils significantly increased their oxidative stability and also their resistance to free fatty acid-mediated pro-oxidative effects.