Influence of thermal treatment on polyphenolic compounds and antioxidant properties of Quercus robur kernels

Abstract

Polyphenolic compounds and antioxidant properties of dried MeOH extracts of native (QN) and thermally treated (QT) kernels of English Oak, Quercus robur were investigated. The content of total and simple polyphenols and tannins was determined spectrophotometrically using FC-reagentr, while gallic acid content was estimatecl usipg HPLC2. After thermal treatment tannin content was decreased (from 20,4 to 19,70 ), with significant increase of simple polyphenols and gallic acid content (from 1,9 to 4,004, and from 0,5 to 5,lYo, respectively). Antioxidant activity was estimated using FRAP test (total antioxidant activity, TAA), scavenging of DPPH radical and TBA test (inhibition of lipid peroxidation, LP in liposomes)r. Sample-s QN and QT showed substantial antioxidant activify. TAA was 2.44 and. 3.76 pmol F.'*/mg, respectively. Comparing to quercetin and L-ascorbic acid these values were ca 2-3 times lower. Both samples concentration-dependently scavenged stable DPPH radical (IC5s:8.04 and 5.77 pglml), and inhibited LP (IC50:32.13 and I 6.39 1tg/m| respectively). Higher antioxidant activity of extract of thermally treated kernels (QT) could be explained as a consequence of thermal degradation of hydrolysable tannins and, therefore, increased gallic acid content.

Ispitivani su polifenolni sastojci i antioksidantna svojstva suvih MeOI.l ekstrakata nativnog (QN) i termidki tretiranog (QT) semena luZnjaka, euercus robur. SadrZaj ukupnih i netaninskih polifenola i tanina odredivan je spektiofotometrijski, kori5cenjem Folin-Ciocalteu reagensa', a sadraaj galne kiseline tecnonr hromatografijom (HPLC)2. Konstatovano je da nakon termicktg tretmana sadrZaj tanina opada (sa 20,4 na 19,7o ), a znalajno raste kolidina netaninskih polifenola 1sa f ,9 na 4,0oA) i galne kiseline (sa 0,5 na 5 ,loh). Za procenu antioksidante aktivnosti odredivani su: ukupna antioksidanta aktivnost, UAA (FRAP test), sposobnost neutralizacije DPPH radikala i inhibitorni efekat na lipidnu peroksidaciju (LP) u lipozomima (TBA tesr).r Uzorci QN i QT ispoljili su znadajnu antioksidantnu aktivnost, i to UAA je iznosila 2,44 i 3,76 pmol F.'*/rg, respektivno. Dobijene vrednosti aktivnosti su oko 2- 3 puta niZe u poredenju sa kvercetinom i vitaminom C. lJzorci eN i eT su koncentraciono zavisno neutralisali DPPH radikal (IC5s:8,04 i 5,77 pglml) i inhibirali LP (ICs0:32,13 i I 6,39 ltglml, respektivno). Veia antioksidantna aktivnost ekstrakta termidki tretiranog semena (eT), moLe se objasniti veiim sadržajem galne kiseline, koja se oslobada razgradnjom hidrolizujucih tanina tokom termidke obrade.