As i have not much knowledge about the biological activity of any food material, but as  chemistry  point of view, I can say that at lower pH phenolic compounds will have less susceptibility towards oxidation because hydroxyl groups will be shielded by protonation. So pH will definitely affect the antioxidant activity of any sample containing phenolic groups.

It depends how complicated is your sample and what antioxidants are present there? Is this a plant material? Are the major compounds polyphenols or vitamins (C, E, carotenoids)?

wojciech Koch

major compounds are polyphenols (phenolic compounds and flavioniods). the sample is honey

If the pH of a honey is natural it will not influence the analysis or You would like to change the pH according to some analysis protocol?

the ph of honey ranges from 3-4. now i want to change the ph from 2-5 to check the effect on antioxidant property

I understand that You would like to check the influence of pH on antioxidant properties of a honey? I don't think that such a change will influence significantly on antiradical scavenging activity but please try and then You will see

i think more the acidic, significant will be the antioxidant

some studies indicate that acid pH are more stable polyphenols, I believe that reducing the pH 3 to 2.5 shall not affect the antioxidant activity of your sample

As i have not much knowledge about the biological activity of any food material, but as  chemistry  point of view, I can say that at lower pH phenolic compounds will have less susceptibility towards oxidation because hydroxyl groups will be shielded by protonation. So pH will definitely affect the antioxidant activity of any sample containing phenolic groups.

brother...effect of pH on Polyphenols does vary with type of poly-phenol. The effect is either way depending on type. Rifat Akbar is rt in explanation but it also vary a little bit in complex systems. actualy the fate of activity depends on resonance stabilization of phenoxide ions which is intermediate product in acidic conditions.  As u might be aware about the mild acidic nature of Polyphenols, which plays imp role in complex systems, they behave differently under different media conditions. Can u let me know which polyphenols are predominant in your product. And one more interesting thing about Polyphenols, if we dont go for extreme reaction conditions, polyphenols resist in change because of RESONANCE, and i think it is natures gift that these compounds act as AO in almost every condition (Except in extreme cases like very low pH of high pH, high temp, ect)

brother...effect of pH on Polyphenols does vary with type of poly-phenol. The effect is either way depending on type. Rifat Akbar is rt in explanation but it also vary a little bit in complex systems. actualy the fate of activity depends on resonance stabilization of phenoxide ions which is intermediate product in acidic conditions.  As u might be aware about the mild acidic nature of Polyphenols, which plays imp role in complex systems, they behave differently under different media conditions. Can u let me know which polyphenols are predominant in your product. And one more interesting thing about Polyphenols, if we dont go for extreme reaction conditions, polyphenols resist in change because of RESONANCE, and i think it is natures gift that these compounds act as AO in almost every condition (Except in extreme cases like very low pH of high pH, high temp, ect)

Infact, in acidic pH, the antioxidant activity of polyphenols will be more significant and will decrease monotonously with increase in pH, due to deprotonation of hydroxyl groups. And yes resonance effect have direct control on it but depends on which other electronegative group is attached to these compounds.

Thanx Rifat Akbar for your valuable answers

In fruit and vegetable samples, a part of the antioxidant activity is due to the presence of vitamin C (ascorbic acid). This vitamin is not stable at alkaline pH. Hence, raising the pH above 7 will - in addition to the effect described by Rifat - further decrease the antioxidative cpacity of food samples.

 Dear Gulzar

Sir, Sorry to come late to the discussion.  The effect of pH on antioxidant capacity is interesting. Your question elicited a wide range of important answers. Be encouraged on your research. Obviously, there is a need for more research in this area.

We recently made a small contribution on the effect of pH on antioxidant capacity. For tea infusions increasing pH increased antioxidant capacity . Other investigators also  reported similar phenomenon for, pure antioxidant reference compounds and whole foodstuffs, e.g. wine. You will find a summary this work in the paper "A FRAP assay at pH 7..etc" available from Research Gate. Best W and we look forward to reading your research./ R.

What do you know about pH in cranberry commercial juices??