Protein oxidation and protein-lipid interactions in different food models in the presence of berry phenolics

Antioxidant activity of different berry phenolics, including anthocyanins, ellagitannins, and proanthocyanins, toward both protein and lipid oxidation in different food model systems (liposomes and emulsions) was studied in this thesis. In addition, the partition of berry anthocyanins into different phases in the oil-in-water emulsion, and the effect of different milk proteins (lactalbumin, bovine serum albumin (BSA), and casein) on liposome oxidation were investigated. The location of tryptophan residues, the loss of anthocyanin colour, and the effect of berries on emulsion turbidity (stability) were measured during oxidation of the oil-in-water emulsions. All tested proteins acted as antioxidants toward liposome (lipid) oxidation. The inhibition was more pronounced with casein than with BSA and lactalbumin. In addition, casein itself was the most stable protein in the liposome model as determined by measuring the loss of tryptophan fluorescence or the formation of protein-carbonyl compounds. However, lactalbumin was chosen for further investigations on protein-lipid interactions with added phenolic compounds because its sensitivity toward oxidation and better applicability for the overall purpose of developing dairy products with berry phenolics. Different berry phenolic from bilberries (Vaccinium myrtillus), blackberries (Rubus laciniatus), blackcurrants (Ribes nigrum), lingonberries (Vaccinium vitis-idaea), and raspberries (Rubus idaeus) as well as different standard compounds inhibited protein and lipid oxidation in different food model systems (lactalbumin-liposomes and different oil-inwater emulsions). The nature of the phenolic compounds in berries had an impact on their antioxidant activity. However, the antioxidant activity of berries depends not only on the phenolic composition but also on the oxidative environment of choice as well as the oxidation products monitored. In general, the antioxidant effect toward lipid oxidation was more pronounced than the effect on protein oxidation in all food models investigated. In blackcurrants, blackberries and bilberries anthocyanins were responsible for the antioxidant activity. On the contrary, in raspberries ellagitannins and in lingonberries proanthocyanidins together with anthocyanins led to inhibition of protein and lipid oxidation. Partitioning of berry anthocyanins into the different phase of the oil-in-water emulsions affected their antioxidant activity. Most of the anthocyanins isolated from raspberries, lingonberries and blackcurrants were partitioned into the aqueous phase, and around 20% of the anthocyanins were incorporated with proteins at the oil droplets interface. The decrease of anthocyanin colour intensity, the increase of emulsion turbidity, and the change of tryptophan residues location in the oil-in-water emulsions during oxidation indicated that phenolic compounds were interacting with berry phenolics. Both the loss of anthocyanin colour and modification of tryptophan residues in aqueous environment may be due to covalent interaction between tryptophan residues and oxidized phenolic compounds (quinones). In hydrophobic environment non-covalent interactions are more favourable, and they may provide better protection toward oxidation. The total effect of berry phenolics on the oxidative stability of food products should be considered because at some concentrations phenolic compounds may not act beneficial.