Impact of antioxidants on oxidative stability of structured lipids

Abstract

There is a growing interest among lipid technologists, lipid chemists, and nutritionistsconcerning structured lipids (SL) due to the physiological benefits these lipids offer.Compared to a mixture of medium- and long-chain TAG, SL provide improved absorptionof fatty acids and an increased delivery of total fat and essential fatty acids to the peripheraltissues [1], enhanced absorption of lipid-soluble vitamins and lipophilic antioxidants [2], andthey help to reduce muscle catabolism and improve nitrogen utilization during metabolicstress [3].Several authors have reported that SL are characterized by a lower oxidative stabilitycomparedtotheinitialoil/fatsources[4].SuchfindingshavebeenobservedforSLsynthesizedfromoliveoilandn-3PUFAorcaprylicacid,ricebranandcaprylicacid,saffloweroil,coconutoil, soybean oil, and fish oil [5–7]. According to Martin et al. [4], themethod employed in theproduction of SL is the main determinant of lipid oxidation. Yet, the results in the literatureare inconsistent. Sometimes the SL product from random chemical synthesis generates fasterandmoreextensivelipidoxidationthanlipase-mediatedsynthesisoftheSL.Atothertimestheoxidativestability ofSLrecoveredfromlipase preparationsisworsecomparedtotherandom-ized method [8].During the processing of SL, natural antioxidants present in the starting oils/fats aredegraded. The most serious concern is the loss oftocopherols and tocotrienols. This has beenobservedduringtheproductionofSLfromamixtureoflowerucicacidrapeseedoilandtallow[9], olive oil and caprylic acid [10], soybean and caprylic acid [11], as well as borage andevening primrose oils containing n-3 fatty acids [12]. According to Hamam and Shahidi [13],during the production of SL tocopherols react with carboxylic acids, giving rise to tocopherolesters. These reaction products can be removed from the reaction mixture during short-timedistillation.Byung and Akoh [14] reported that the oxidative stability of purified SL prepared fromroasted sesame oil and caprylic acid was lower than that of sesame oil itself. There were nodifferences in the contents of unsaponifiables, including tocopherols and phytosterols.However, total sesame lignans content was decreased in the SL due to the loss of sesamolwhen compared to sesame oil. A dramatic decrease of phenolic compounds in SL preparedfrom extra virgin olive oil and CLA or caprylic acid was described by Lee et al. [10].Among the food-grade antioxidants available to protect SL against oxidation,a-tocopherol is employed most often; it has been added at a level between 50 and1000 mg/kg to substrates for SL synthesis such as fish oil and caprylic acid, canola oil andcaprylicacid,soybeanoilandcaprylicacid,sunfloweroilandcaprylicacid,milkfatandlinseedoil, as well as olive oil and n-3 PUFA [11, 15, 16]. In the study of Turan et al. [11] on theoxidative stability of SL synthesized by a specific sn-1,3-lipase catalyzed interesterificationof soybean oil with caprylic acid, no significant difference was observed between theinduction time of SL and SL plus different a-tocopherol concentrations (ranging from50 to 1000 mg/kg). Antioxidant mixtures of a-tocopherol (50 mg/kg) and d-tocopherol(500 mg/kg) with ascorbyl palmitate and lecithin have been reported to effectively decreasethe peroxide value, conjugated diene content, and p-anisidine value during oxidation of SLat 60