The peroxidation of different polyunsaturated fatty acids (PUFA) after photoirradiation in aqueous solution was evaluated by measuring fatty acid loss and malonaldehyde production in medium. The oxidation rates of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), two highly unsaturated fatty acids of the n−3 series, were surprisingly lower (14 and 22%, respectively) than the oxidation rates of linoleic, α‐linolenic, γ‐linolenic, dihomo γ‐linolenic, and arachidonic acids (62–90%). The quantities of malonaldehyde (MA) produced were assayed simultaneously by gas chromatography (GC) and high performance liquid chromatography (HPLC). MA production was found to be related to both the degree of unsaturation and the metabolic series of the fatty acid. The maximum value was observed with arachidonic acid (MA production from 2 mM arachidonic acid in aqueous solution was estimated at 44.9±6.0 μM by GC and 46.8 ±4.0 μM by HPLC). Eicosapentaenoic acid and docosahexaenoic acid produced lower MA quantities compared to arachidonic acid (MA production from 2 mM EPA and 2 mM DHA was estimated at 17.9±1.5 μM and 37.9±0.7 μM, respectively, by GC, and 26.3±4.9 μM and 37.3±4.2 μM, respectively, by HPLC). The MA yield, defined as the amount of MA (nmols) produced per 100 nanomoles of oxidized fatty acid, was used to express the susceptibility of individual PUFA to peroxidation. The MA yield correlated well with the degree of unsaturation, but was independent of carbon chain length and metabolic series. The study suggests that adequate assessment of lipid peroxidation cannot be achieved by measuring MA formation alone, but it also requires knowledge of the fatty acid composition of the system studied.

Interesterification was previously found to impact stearic acid absorption in a randomized cross-over study, when human volunteers consumed a 70 : 30 wt% high-oleic sunflower and canola stearin blend (NIE) compared to the same blend which had undergone either chemical (CIE) or enzymatic (EIE) interesterification. In this research, in vitro lipid digestion, bioaccessibility, and changes in undigested lipid composition and melting behavior of these same test fats were investigated using the dynamic, multi-compartmental TIM-1 digestion model and compared with the previous human study. Overall, TIM-1 bioaccessibility was higher with interesterification (p < 0.05). Oleic acid bioaccessibility was higher than stearic acid bioaccessibility for NIE, and vice versa for the interesterified blends (p < 0.05). Stearic acid was more concentrated in the undigested triacylglycerols (TAG) from NIE, corresponding to a relatively higher melting temperature of the undigested lipids. The results confirm the impact of TAG composition, fatty acid position and/or physical properties on lipid digestion. TIM-1 bioaccessibility was linearly correlated (R(2) = 0.8640) with postprandial serum TAG concentration in the human study. Therefore, the in vitro digestion model offered predictive insights related to the impacts of lipid interesterificaton on absorption.

Questions remain as to the impact of lipid structure, including crystallinity, on digestibility and metabolic response. This study was undertaken to determine the impact of triacylglycerol crystallinity on digestibility using undercooled (liquid emulsion, LE) and crystalline (solid emulsion, SE) particles exposed to an in vitro model simulating upper gastrointestinal tract (GIT) digestive conditions. By hot microfluidization, 10 wt% tripalmitin oil-inwater emulsions (D 3,2~0 .115 nm) with 0.9 wt% sodium dodecyl sulfate (SDS) were prepared. SE demonstrated complex melting behavior, was predominantly in the beta polymorph, and consisted of a heterogeneous mixture of anisometric particles. In vitro duodenal lipolysis was more extensive for the spherically shaped LE droplets vs. SE (P < 0.05), despite the fact that exposure to simulated gastric conditions (at pH 2, but not at pH 7) induced partial crystallization. Therefore, lipid droplet physical state impacted and was impacted by exposure to gastrointestinal conditions, with differences observed in fatty acid digestive release and implications for lipid absorption. Keywords Structure-functional properties Á Food and feed science Á Nutrition and health Á Emulsions Á Colloids Á Fat crystallization Á Polymorphism Á Lipid chemistry Á Lipid analysis J Am Oil Chem Soc (2018) 95: 161-170.

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