Μελέτη της επίδρασης ενδογενών και εξωγενών παραμέτρων στην οξείδωση λιπαρών σε γαλακτώματα
Authors: Δημάκου Χαρίκλεια 
Published: January 2014

The current PhD thesis concern parameters that affect the oxidative stability of oil-in-water emulsions. Lipid oxidation determined by measuring primary (conjugated dienes-CD and lipid hydroperoxides) and secondary oxidation products (thiobarbituric acid reactive substances and volatile products). The parameters studied were type (sunflower-, cottonseed-, corn- and olive kernel-oil) and concentration (10-60%) of lipid phase, type (Tween 20, Na-CAS, WPI) and concentration (0.5-3%) of emulsifier and pH (1.7-7.0). The sunflower oil-in-water emulsions oxidized faster due to their content of polyunsaturated fatty acids (60-63%), followed by emulsions with cottonseed- (57.7-58.0%), corn- (55-57%) and olive kernel-oil (12.1%). Increase of concentration in sunflower oil-based emulsions till 30% resulted to the reduction of oxidative rate, while emulsions with oil content 30, 40 and 60% oxidized at the same extent. Milk proteins, Na-CAS and WPC, as well as mixtures of emulsifiers, Na-CAS and Tween 20 at different ratio led to the formation of oxidatively stable emulsions in comparison to Tween-based emulsions. An increase in protein concentration (0.5,1,2%) or in protein proportion in the emulsifier mixture (25,50,75,100%) was found to inhibit proportionally the oxidative instability of the emulsions. Emulsions stabilized with preheated revealed a decrease of emulsion oxidative sensitivity. An increase of pH (3, 5.5,7) in Tween-stabilized emulsions led to higher rates of oxidative deterioration, in comparison to protein-stabilized emulsions at pH (1.7,5.7,7), where opposite results observed. Oxidation rate of emulsions stabilized with mixture of Na-CAS-Tween 20 (50:50) was unaffected by the pH (3.5, 6.5) change. The emulsions homogenized with ultrasonic agitation and had oil droplet size (2.30±0.15 μm), or under varying pressures (30,100,300,600,900 bars) had droplet size (from 3.23±0.25 μm to 0.67±0.25 μm). Droplet size had no effect on the oxidative deterioration of the emulsions, irrespectively of the type of emulsifier used. An increase of storage temperature (5,15,25,40,60 °C) led to higher oxidation rates. The rate constant of CD increase was temperature dependent according to Arrhenius equation with an activation energy equal to 37.5kJ/mol. Finally, natural compounds, known for their antioxidant capacities, like carotenoids and flavonoids were studied. B-carotene, lycopene and lutein, paprika, bixin-rich and norbixin-rich annatto extracts from the group of carotenoids (1g/L oil) and (+)-catechin, fisetin and quercetin from the group of flavonoids, were studied (1mmol/kg oil). B-carotene and lycopene presented low antioxidant activity in emulsions, while polar carotenoids presented good antioxidant activity in the order of lutein, paprika and annatto extracts. The effect of concentration of paprika and bixin extracts (0.25-1.5g/L oil) was studied. Antioxidant activity increased by increasing concentration up to 1g/L, while higher concentration did not offer any additional protection. Among flavonoids, quercetin presented the best antioxidant activity followed by fisetin and (+)-catechin. Paprika, annatto and quercetin were microencapsulated in Na-CAS, but microencapsulation did not ameliorate their antioxidant activity in oil-in-water emulsions.