Investigations on protein-lipid interactions under oxidative conditions

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Title: Investigations on protein-lipid interactions under oxidative conditions
Author: Gürbüz, Göker
Contributor: University of Helsinki, Faculty of Agriculture and Forestry, Department of Food and Environmental Sciences
Thesis level: Doctoral dissertation (article-based)
Abstract: Oxidative reactions in food systems during processing and storage constitute a significant problem that determines the nutritional and sensory qualities of the food product as well as the textural and functional properties. Proteins and lipids, as essential components of foods, are highly prone to oxidative degradation that results in undesired modifications in food systems. Although lipid oxidation as a topic has been given a widespread attention, protein oxidation and its consequences in foods have been studied relatively recently. In particular, co-oxidation of food proteins and lipids, in terms of their interactions within the complex mechanism of oxidative reactions, has been gathering interest only lately. The behavior of proteins from various food sources and technological pre-treatments as well as the outcomes of this behavior under oxidative conditions in the presence of lipids is a much required subject on which to focus both by academia and industry. The purpose of this work was to elucidate the degradation of proteins and lipids within the oxidative system, and to characterize the consecutive modifications of oxidative interactions between food proteins and lipids in model investigations. In order to achieve this, experimental oxidation models were set up that investigate firstly the molecular level interactions between lipid oxidation product malondialdehyde (MDA) and whey protein, β-lactoglobulin (β-Lg) peptides. In this work, LC-MS/MS methods were employed to characterize and identify the interaction products. Later studies focused on the oxidative behavior of plant proteins such as quinoa, amaranth and faba beans, utilized in emulsification of rapeseed oil. Results from the analyses of primary and secondary volatile lipid oxidation products as well as tryptophan fluorescence of proteins were gathered to assess the oxidative process. The results of the first study showed that two distinct compounds were generated through Schiff-base adduct formation between MDA and β-Lg peptides, one of which had strong fluorescent properties. Emulsions studies demonstrated that amaranth proteins produced oxidatively and physically more stable emulsions compared to quinoa proteins. However, due to protein oxidation neither type of emulsions were as stable as the commercial emulsifier Tween®20. Emulsions with proteins of faba beans that had undergone pre-treatments were less oxidized compared to native samples which was due to hindering of the endogenous enzyme activity. Proteins located at the lipid droplet interface were degraded more than those in the water phase. This thesis demonstrates that interdependent relations between proteins and lipids such as adduct formation, free-radical transfer, and reactions between oxidized species have a significant effect on the overall course of oxidation of the food system which affect the final observed modifications. Therefore, customized solutions against oxidation should regard the intricate relations of protein-lipid co oxidation in a food system that contains proteins and lipids as major constituents.Ruuan valmistamisen ja säilytyksen aikana tapahtuu erilaisia kemiallisia reaktioita, joista hapettuminen on merkittävin ruuan ravitsemuksellisiin ja aistinvaraisiin ominaisuuksiin vaikuttava reaktioiden sarja. Hapettuminen heikentää myös ruuan rakenteellisia sekä toiminnallisia ominaisuuksia, kuten liukoisuutta tai kykyä muodostaa emulsioita. Ruuassa herkkiä hapettumiselle ovat sekä proteiinit että lipidit. Lipidien hapettumisesta on runsaasti tutkimustietoa. Sen sijaan proteiinien hapettumista ja proteiinien ja lipidien välisisiä vuorovaikutuksia on tutkittu verrattain vähän. Väitöskirjatutkimuksessa valmistettiin härkäpavusta, kvinoasta ja amarantista eristetyistä proteiineista ja rypsiöljystä emulsioita, joiden hapettumista seurattiin mittaamalla sekä proteiinien että lipidien hapettumistuotteita. Amarantin proteiinit tuottivat kestävämpiä emulsioita verrattuna kvinoan proteiineihin. Hapettumisreaktioita aikaansaavat myös kasvikunnan entsyymit kuten lipoksigenaasi. Tämän entsyymin tuhoaminen lämpökäsittelyllä tai mikroaalloilla edisti härkäpavun proteiineista valmistetun emulsion kestävyyttä. Tutkimuksessa selvitettiin myös molekyylien tasolla proteiinien ja lipidien välisiä hapettumisreaktioita. Maidon heraproteiinista, beta-laktoglobuliinista, eristetyn peptidin voitiin LC-MS/MS -menetelmän avulla osoittaa muodostavan reaktiotuotteita (addukteja) lipidien hapettumistuotteen, malondialdehydin (MDA), kanssa. Reaktiotuotteiden todettiin syntyneen nk. Schiffin emäsreaktion kautta. Tutkimustulosten mukaan ruuan lipidien ja proteiinien keskinäisten reaktioiden hallinta vaikuttaa merkittävästi elintarvikkeiden hapettumiseen ja siten ruuan laatuun.
URI: URN:ISBN:978-951-51-4121-7
Date: 2018-03-23
Rights: This publication is copyrighted. You may download, display and print it for Your own personal use. Commercial use is prohibited.

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