The changes in AA, TAA and DHAA concentrations in different opened fruit juices stored for 24 and 48 h under refrigeration conditions are presented in Table 4. DHAA was not observed in any fresh juice (the results obtained for samples prepared with and without DTT were not statistically different). The decrease in AA of 0–2.9% and 4.1–6.4% in comparison to fresh juice was observed for juices stored for 24 and 48 h, respectively. The concentration of DHAA was from 0% to 5.4% depending on storage time and a kind of juice. It was reported that phenolic compounds protect AA from degradation but this protection depends on the type of compound (Miller and Rice-Evans, 1997 and Özkan et al., 2004). Moreover, it was found that apple juice had the lowest protective effect on AA in fortified fruit juices comparing to orange and blackcurrant juices. Apple juice phenolic acids were recognised as less efficient protectors of AA than flavonoids of orange and blackcurrant juices (Miller & Rice-Evans, 1997). These results may explain higher losses of AA in apple juices than in multifruit juices analysed in the present study. It was observed that oxidation process was only partly responsible for the loss of AA because the concentration of DHAA did not compensated the decrease of AA in fresh juice. This was especially apparent for multifruit juices, in which probably not only oxidation but also degradation of AA occurred. The changes in the concentration of AA in apple juices were mainly due to the oxidation thus the decrease in TAA during storage was not observed. Determination of vitamin C in fruit juices stored for at least 24 h under refrigeration may require determination of TAA because oxidation of AA may occur during storage.
The changes in AA, TAA and DHAA concentrations in different opened fruit juices stored for 24 and 48 h under refrigeration conditions are presented in Table 4. DHAA was not observed in any fresh juice (the results obtained for samples prepared with and without DTT were not statistically different). The decrease in AA of 0–2.9% and 4.1–6.4% in comparison to fresh juice was observed for juices stored for 24 and 48 h, respectively. The concentration of DHAA was from 0% to 5.4% depending on storage time and a kind of juice. It was reported that phenolic compounds protect AA from degradation but this protection depends on the type of compound (Miller and Rice-Evans, 1997 and Özkan et al., 2004). Moreover, it was found that apple juice had the lowest protective effect on AA in fortified fruit juices comparing to orange and blackcurrant juices. Apple juice phenolic acids were recognised as less efficient protectors of AA than flavonoids of orange and blackcurrant juices (Miller & Rice-Evans, 1997). These results may explain higher losses of AA in apple juices than in multifruit juices analysed in the present study. It was observed that oxidation process was only partly responsible for the loss of AA because the concentration of DHAA did not compensated the decrease of AA in fresh juice. This was especially apparent for multifruit juices, in which probably not only oxidation but also degradation of AA occurred. The changes in the concentration of AA in apple juices were mainly due to the oxidation thus the decrease in TAA during storage was not observed. Determination of vitamin C in fruit juices stored for at least 24 h under refrigeration may require determination of TAA because oxidation of AA may occur during storage.
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