## 2 Materials and Methods
### Methods
#### 2.2.2 Extract Preparation, Antioxidant Capacity and Total Polyphenol Content  
About 500 mg of lyophilized grounded samples of each currant fruits were used for the preparation of acidified methanolic extract (70% methanol acidified with 0.1% formic acid \(v/v\), POCh, Katowice, Polska). All samples were extracted by shaking in a laboratory shaker (Elpan, type 357 Lubawa, Poland) for two hours, without light. After 2 h of extraction, samples were centrifuged (centrifuge type MPW-340, Warsaw, Poland). Then obtained samples were kept at \(-22^{\circ}\)C for further analyses, as was previously reported [18].  
The total polyphenol content in acidified methanolic extract of currant fruits was measured spectrophotrically using Folin-Ciocalteau reagent, at a wavelength of 760 nm using a RayLeigh UV-1800 spectrophotometer (UV-1800 spectrophotometer, Beijing Beifen-Ruili Analytical Instrument Co., Ltd., Beijing, China) [20]. The results are expressed as the chlorogenic acid equivalent (CGA) in mg per 100 g of dry matter (DM).  
The antioxidant activity of methanolic extracts of fruits of black, red and white currant fruits was measured by the following methods: with ABTS** radical (2,2'-azino-bis-(3-ethylbenzothazoline-6-sulfonic acid); (Sigma Aldrich, Saint Louis, MO, USA) [21]; the FRAP method (ferric-reducing antioxidant power) [22]; and the DPPH** radical method (2,2-diphenyl-1-picryl-hydrazyl-hydrate); (Sigma Aldrich, Saint Louis, MO, USA) [23] as previously reported [24; 25].  
The working solution for ABTS** analysis was prepared by diluting the ABTS** stock solution with 70% methanol (POCh, Katowice, Poland) to achieve an absorbance between 0.740 and 0.750 at 734 nm. A volume of 10-100 uL of the black, red and white currant fruit extract was transferred into a test tube and diluted to 1 mL with 70% methanol (POCh, Katowice, Poland). After mixing the diluted extract with 2 mL of the ABTS** working solution, the mixture was kept in the dark at 30 \({}^{\circ}\)C for 6 min. The absorbance of the sample was then measured at 734 nm.  
For the DPPH** analysis, the working solution was prepared by dissolving 6 mg of DPPH** in 100 mL of methanol (POCh, Katowice, Poland). The stock solution was then diluted with methanol to achieve an absorbance of 0.900-1.000 at 515 nm. A volume of 10-200 uL of the extract was placed in a test tube and diluted to 1.5 mL with methanol. After mixing the diluted extract with 3 mL of the DPPH** solution, the mixture was kept in the dark at room temperature for 10 min. The absorbance of the sample was measured at 515 nm.  
The total reducing capability, using the FRAP method, was determined as reported by [PERSON] and [PERSON] [22]. The extracts (10-200 uL) were placed in a test tube and diluted to 1 mL with 70% methanol (POCh, Katowice, Poland). After combining the diluted extract with 3 mL of the FRAP reagent working solution, the mixture was kept in the dark at room temperature for 10 min. The absorbance of the samples was then measured at 593 nm.  
The results obtained for ABTS**, FRAP and DPPH** methods were compared to the concentration-response curve of the standard Trolox dilution, and the results obtained are expressed in umol Trolox/g DM.