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Publications (5)14.31 Total impact

  • Altunkaya, Arzu
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    ABSTRACT: The in vitro kinetics of lettuce PPO with respect to dissolved oxygen using catechin, chlorogenic acid, caffeic acid and gallic acid has been examined. In-vitro lettuce polyphenol oxidase (PPO) activity was determined by measuring the consumption of oxygen during the oxidation reaction. The effect of whey protein concentrate (WPC) was tested on the inhibition of lettuce PPO comparing with ascorbic acid (AA) and cysteine. A competitive model that considered inhibitors was the most appropriate model to explain reaction kinetics. Browning of lettuce was also monitored during storage for 24 h. Addition of WPC prevented loss of lightness in lettuce. Loss of identified phenolic compounds in lettuce was measured during the enzymatic browning process by high-performance liquid chromatography. Degradation of identified phenolic compounds followed first order kinetics during storage. Combination of WPC with cysteine was proposed for the protection of phenolics compounds against PPO-catalysed oxidation.
    Food Chemistry 10/2011; · 3.33 Impact Factor
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    ABSTRACT: Enzymatic reactions taking place in minimally processed vegetables are considered as a major problem, because they adversely affect sensorial and nutritional quality. Polyphenol oxidase (FPO), peroxidase (POD) and lipoxygenase (LOX) from lettuce were purified on a column packed with positively charged diethylaminoethyl (DEAE) cellulose by applying pH gradient elution from pH=4.0 to 9.0. The main purified fractions (PPO1 and PPO4, POD1 and POD2, LOX1 and LOX2) were characterized for enzyme concentration-reaction rate relationship, thermal stability, pH activity and kinetic parameters. Kinetic properties of each isoform were considerably different. Cysteine was found as the most effective inhibitor of both fractions of FPO. Kinetic parameters of lettuce POD were presented using guaiacol at various H(2)O(2) concentrations. beta-carotene directly influences lettuce LOX in the reaction medium available for the catalytic conversion of linoleic acid into hydroperoxides. Ascorbic and oxalic acids appear as effective PPO inhibitors, protecting phenolic compounds against oxidation in lettuce. Understanding the characteristics of deteriorative enzymes becomes important to maintain suitable conditions for fresh-like quality of lettuce. The results can be useful to keep the nutritional quality of minimally processed lettuce during shelf-life.
    Food Technology and Biotechnology 06/2011; 49(2-49):249-256. · 0.98 Impact Factor
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    ABSTRACT: High performance liquid chromatography (HPLC) was used to characterise phenolic profile and to select the most effective anti-browning compound(s) on fresh lettuce. Four anti-browning agents, ascorbic acid, cysteine, citric acid and oxalic acid were tested for their effectiveness on preventing loss of phenolic compounds in lettuce during processing and storage. Aliquots of the reaction mixture were withdrawn at different times varying from 0 to 24 h, and directly analysed by HPLC. Protocatechuic acid, chlorogenic acid, ferulic acid, caffeic acid, p-coumaric acid and phloridzin were identified in fresh lettuce. Degradation of phenolic compounds followed a first-order kinetic pattern. The effect of anti-browning agents on first-order degradation rates of phenolic compounds was determined. Lettuce treated with oxalic acid and ascorbic acid maintained a higher level of phenolic compounds than citric acid and cysteine. Interestingly, cysteine had no positive effect for the prevention of oxidation of phenolic Compounds even though it prevented browning in lettuce.
    Food Chemistry 11/2009; 9(1-117):122-126. · 3.33 Impact Factor
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    ABSTRACT: Combined antioxidative effects of lettuce extract and alpha-tocopherol, quercetin or ascorbic acid (AA) were investigated for peroxidation of L-alpha-phosphatidylcholine liposomes with oxidation initiated by lipophilic or by hydrophilic azo-initiators. Lettuce extract had a clear antioxidative effect as evidenced by a lag phase for formation of conjugated dienes, and alpha-tocopherol and especially quercetin acted synergistically in prolongation of the lag phase both following initiation in the lipid phase and in the aqueous phase. Combination of AA with lettuce extract showed in contrast a lag phase that was similar to that observed for AA alone. Storage of lettuce extract for 24 hat refrigerator or room temperatures resulted in a Decreasing lag phase with increasing storage time for both storage temperatures, an effect not counteracted by addition of quercetin or alpha-tocopherol. Heating of lettuce extract for 10 min at 80 degrees C did not affect the lag phase and heating of lettuce extract resulted in an increasing synergism for added quercetin and alpha-tocopherol indicating that thermal inhibition of polyphenoloxidase (PPO) increases antioxidant potential and interaction.
    Food Chemistry 06/2009; 115(1-115):163-168. · 3.33 Impact Factor
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    ABSTRACT: In this study, polyphenol oxidase (PPO) was isolated from fresh lettuce. Its optimum temperature and pH were found to be 40 degrees C and 7.0, respectively. Lettuce PPO was shown to use catechin, catechol, chlorogenic acid, caffeic acid and gallic acid as substrates. Among the substrates used, the greatest substrate specificity was observed with chlorogenic acid. Lettuce PPO was sensitive to some inhibitors. Ascorbic acid, cysteine, oxalic acid and citric acid were tested as potential inhibitors of lettuce PPO. Cysteine was the most effective inhibitor. Total phenol and total antioxidant activity contents were also determined in the presence of these inhibitors at room and refrigerator temperatures. Ascorbic acid and cysteine increased the total antioxidant activity of lettuce while citric and oxalic acids had no effect on the total antioxidant activity. Lettuce phenolics were protected from oxidation by ascorbic acid and cysteine.
    Food Chemistry 04/2008; 107(3-107):1173-179. · 3.33 Impact Factor