Marijan Šeruga

Ultrasound extracted flavonoids from four varieties of Portuguese red grape skins determined by reverse-phase high-performance liquid chromatography with electrochemical detection

Several flavonoids present in red grape skins from four varieties of Portuguese grapes were determined by reverse-phase high-performance liquid chromatography (RP-HPLC) with electrochemical detection (ECD). Extraction of flavonoids from red grape skins was performed by ultrasonication, and hydrochloric acid in methanol was used as extraction solvent. The developed RP-HPLC method used combined isocratic and gradient elution with amperometric detection with a glassy carbon-working electrode. Good peak resolution was obtained following direct injection of a sample of red grape extract in a pH 2.20 mobile phase. Eleven different flavonoids: cyanidin-3-O-glucoside (kuromanin), delphinidin-3-O-glucoside (myrtillin), petunidin-3-O-glucoside, peonidin-3-O-glucoside, malvidin-3-O-glucoside (oenin), (+)-catechin, rutin, fisetin, myricetin, morin and quercetin, can be separated in a single run by direct injection of sample solution. The limit of detection obtained for these compounds by ECD was 20-90 pg/L, 1000 times lower when compared with photodiode array (PDA) limit of detection of 12-55 ng/L. RP-HPLC-ECD was characterized by an excellent sensitivity and selectivity, and appropriate for the simultaneous determination of these electroactive phenolic compounds present in red grape skins.

The influence of interactions among phenolic compounds on the antiradical activity of chokeberries (Aronia melanocarpa)

In the present work, interactions between phenolic compounds from chokeberries and their influence on the antiradical activity was studied. Three fractions were isolated from chokeberries containing different classes of phenolic compounds. The first fraction contained a major part of phenolic acids and flavonols, the second anthocyanins, and the third insoluble phenols and proanthocyanidins. The phenolic compound content was determined using high-performance liquid chromatography, and the antiradical activity using the DPPH test. In order to evaluate the effects of interactions between phenolic compounds on the antiradical activity, the antiradical activity of individual phenolic fractions was compared with that obtained by mixing phenolic fractions. Phenolic mixtures showed the decrease in the antiradical activity in comparison with the individual phenolic fractions. These results suggest the existence of complex interactions among phenolic compounds that caused the decrease of the antiradical activity. Interactions among chokeberry phenols promoted a negative synergism.

Influence of Anthocyanins, Flavonols and Phenolic Acids on the Antiradical Activity of Berries and Small Fruits

One fraction enriched with flavonols and phenolic acids; and the fraction enriched with anthocyanins, were isolated from red raspberry, blackberry, sour cherry, strawberry, chokeberry, elderberry, and blueberry to study their antiradical activity using the free radical, 2,2-diphenyl-1-picrylhydrazyl, test. The phenol content was determined using high-performance liquid chromatography. The antiradical activity of anthocyanins was 7 to 25 times higher than the antiradical activity of flavonols and phenolic acids. High correlations were found between the content of anthocyanins, flavonols, phenolic acids, quercetin derivatives, caffeic acid derivatives, and the antiradical activity (r = 0.99, 0.93, 0.83, 0.93, 0.94, respectively). Among the phenols studied in this work, anthocyanins showed the highest influence on the antiradical activity in all fruits. Other phenolics with relatively strong influence on the antiradical activity of fruits were quercetin derivatives and caffeic acid derivatives (chlorogenic acid) in chokeberries and blueberries, quercetin derivatives in blackberries, quercetin derivatives and caffeic acid derivatives (chlorogenic acid) in elderberries, and hydroxycinnamic acids in sour cherries.

Electroanalytical Oxidation of p‐Coumaric Acid

Abstract The mechanism of the electrochemical oxidation of p‐coumaric acid on a glassy carbon electrode was investigated using cyclic, differential pulse, and square wave voltammetry at different pHs. The oxidation of p‐coumaric acid is irreversible over the whole pH range. After successive scans, the p‐coumaric acid oxidation product deposits on the electrode surface, forming a polymeric film that undergoes reversible oxidation at a lower potential than p‐coumaric acid. This polymeric film increases in thickness with the number of scans, covering the electrode surface, and impeding the diffusion of the p‐coumaric acid and its oxidation on the electrode. The oxidation of p‐coumaric acid is pH dependent up until values close to the pKa. For pHs higher than pKa, the p‐coumaric acid oxidation process is pH independent. An electroanalytical determination procedure of p‐coumaric in pH 8.7 0.2 M ammonium buffer was developed, and a detection limit, LOD=83 nM, and the limit of quantification, LOQ=250 nM, were obtained.

Aluminium content of soft drinks from aluminium cans

ZusammenfassungEs wurde der Aluminiumgehalt in Erfrischungsgetränken aus Aluminiumdosen im Laufe von zwölf Monaten Lagerzeit durch GFAAS gemessen. Der Aluminiumgehalt nimmt in allen Proben im Laufe der erwähnten Lagerungszeit zu als Folge des Lösens von der inneren Dosenseite her durch das VorhandenScin von aggressiven Bestandteilen in Erfrischungsgetränken, vor allem durch Säuren. Der Aluminiumgehalt steigt mit der Säurekonzentration, d. h. mit der Abnahme des pH-Wertes, in Getränken. Die Tagesaufnahme von Aluminium (0,8 mg) durch den Konsum dieser Getränke ist allerdings bedeutungslos im Vergleich mit der Gesamttagesaufnahme von Aluminium durch die Nahrung sowie im Vergleich mit der zugelassenen Tagesaufnahme. Aus diesem Grunde stellen Erfrischungsgetränke aus Aluminiumdosen keine beachtenswerte Quelle der Aluminiumaufnahme dar.AbstractThe aluminium (A1) content of soft drinks from A1 cans has been measured during 12 months of storage, by the graphite furnace atomic absorption spectrophotometry (GFAAS) method. The results show that the A1 content in all soft drinks increased during the whole storage time. This increase was a result of dissolution of A1 from the can wall due to the presence of aggressive ingredients in the soft drinks, mainly acids. The A1 content rose with increasing acid concentration and decreasing pH value of the soft drinks. The evaluated possible daily intake of A1 (0.8 mg) through consumption of these drinks was practically negligible in relation to total daily dietary Al intake, as well as to tolerable daily intake. Thus, soft drinks from A1 cans are an insignificant source of dietary A1 intake and it appears that the A1 intake from this source should not be a cause for concern in regard to A1 toxicity for the human body.

Aluminium levels in human milk

Aluminium levels in human milk in the winter period of 1992/93 ranged from 0.004 mg/l to 2.67 mg/l and the mean value was 0.38 mg/l. The nursing women were divided into several groups according to the results of a questionnaire, i.e. according to their social status (if they were refugees or not), number of deliveries, post partum day, the weight they put on during pregnancy, their age, and smoking habits. The mean levels of aluminium for each group were presented. The measurement of aluminium was done by graphite furnace atomic absorption spectrophotometry.

Electrochemical and surface properties of aluminium in citric acid solutions

The electrochemical and surface properties of aluminium in 0.05 M citric acid solutions of pH 2–8 were studied by means of open circuit potential (OCP), potentiodynamic polarization and potentiostatic current–time transient measurements. The OCP reached a steady-state value very slowly, probably due to the slow rate of detachment of surface complexes into the solution. Aluminium exhibits passive behaviour in citric acid solutions of pH 4–8. Tafel slopes bc were characteristic for hydrogen evolution on aluminium covered by an oxide film. The corrosion kinetic parameters Ecorr, icorr and ba suggest that surface processes are involved in the dissolution kinetics, especially in the pH range 3–6. Current–time transient measurements confirm that, in citric acid solutions of pH 3–6, the dissolution is controlled by surface processes, that is, by the rate of detachment of surface complexes, while in solutions of pH 2, 7 and 8 dissolution is under mass-transport control. The addition of fluoride ions to citric acid changes the controlling steps of the dissolution process. Citrate and fluoride ions compete for adsorption sites at the oxide surface, and adsorption of these ions is a competitive and reversible adsorption.

Variation in the Bioactive Compound Content at Three Ripening Stages of Strawberry Fruit

During the harvest season of two consecutive years, five strawberry cultivars (‘Arosa’, ‘Elsanta’, ‘Marmolada’, ‘Miss’ and ‘Raurica’), grown in the continental part of the Republic of Croatia, were examined. Strawberry fruits quality was evaluated by individual phenol compounds, individual anthocyanins and fruit color. Fruits were harvested in three different periods. Analyzed strawberry cultivars show very good average values of the studied phenolic acids and flavonoids with predominant caffeic acid and epicatechin content in all researched strawberry cultivars. Considering the content of individual anthocyanins, pelargonidin 3-glucoside is predominant in strawberry extract followed by cyanidin-3-glucoside and pelargonidin 3-rutinoside. The correlation between individual anthocyanin content and chromaticity parameters was detected in all strawberry cultivars, additionally correlation coefficients and statistical significance were much lower. The results show a positive association between cultivar and harvest time on strawberry pulp color, with each of the color variables, a, b, a/b ratio, C, L and h° values.

Corrosion of aluminium in soft drinks

The corrosion of aluminium (A1) in several brands of soft drinks (cola- and citrate-based drinks) has been studied, using an electrochemical method, namely potentiodynamic polarization. The results show that the corrosion of A1 in soft drinks is a very slow, time-dependent and complex process, strongly influenced by the passivation, complexation and adsorption processes. The corrosion of A1 in these drinks occurs principally due to the presence of acids: citric acid in citrate-based drinks and orthophosphoric acid in cola-based drinks. The corrosion rate of A1 rose with an increase in the acidity of soft drinks, i.e. with increase of the content of total acids. The corrosion rates are much higher in the cola-based drinks than those in citrate-based drinks, due to the facts that: (1) orthophosphoric acid is more corrosive to A1 than is citric acid, (2) a quite different passive oxide layer (with different properties) is formed on A1, depending on whether the drink is cola or citrate based. The method of potentiodynamic polarization was shown as being very suitable for the study of corrosion of A1 in soft drinks, especially if it is combined with some non-electrochemical method, e.g. graphite furnace atomic absorption spectrometry (GFAAS).

Quality of strawberries produced applying two different growing systems

The quality of strawberry fruits (cv. “Clery” and cv. “Asia”) was studied and compared under the conditions of two different growing systems (under a high plastic tunnel and in the open field). Several chemical parameters were determined in the harvested fruits: dry matter, soluble solids, total acidity, ratio between sugars and acids, vitamin C, total phenols, non-flavonoid phenolic compounds, total anthocyanins, reducing sugars, sucrose, colour of fruits, and HPLC analysis of flavonols, flavanols, and phenolic acids content. The fruits of cultivar “Clery” had more antioxidant compounds than the fruits of cultivar “Asia” when grown in the open field. The fruits of both cultivars grown under a tunnel generally had good properties; however, the total phenol content and the non-flavonoid fraction were somewhat lower. Among the phenolic compounds, (+)-catehin, (−)-epicatehin, ellagic, p-coumaric acid, quercetin, and kaempferol were identified and quantified using HPLC equipped with Photo Diode Array detection. The highest content of phenolic compounds was found in cv. “Clery” from the open field production. Analysis of antioxidant compounds has not shown statistically significant differences as a result of growing system or grown cultivar. Samples of strawberries grown under a high plastic tunnel, regardless of examined cultivar, had better basic chemical parameters. The mash colour also depends on cultivar and growing system.