I have seen that in most of the research article, antioxidant activity of fresh fruits and vegetables is expressed as fresh weight basis. Why this is so?
Why not? It is easy and drying might influence antioxidant activity.
When we dry a plant, we can reduce the water content to a variety of percentages. This is not standardised. Therefore, expression of the fresh weight provides a more standardised benchmark.
Biochemical/nutrient content should be analysed at stage of marketable maturity. Since most of the fruits/vegetables are used as fresh, antioxidant activity should be quantified at marketable maturity and expressed as fresh weight basis.
The vegetables and fruits are mostly consumed as fresh, hence, antioxidant activity at fresh weight basis is opt.
To better standardize what base you are using - so that others are able to reproduce/validate/compare results, it would be good when expressing on a fresh weight basis, also express moisture content (and the methodology used to calculate it) so that people could change from fresh to dry basis. Again, it would be more for being able to reproduce/validate/compare results.
As mentioned above, F&V will typically be consumed fresh... but they might have different maturity levels based on location, etc. - thus, reporting on a fresh and dry basis helps to characterize your material. Also, as mentioned, depending on the drying methodology you might have slight changes in content... so in theory, the 'gold standard for drying' would be freeze-drying.
Though estimation based on fresh weight is recommended, it faces some practical problems. When you deal with large number of samples like 200 to 300 genotypes with their replications, practically it is not possible to estimate all these samples in a single day. So, here drying helps you. Once the sample is dried, it can be kept in dessicators for enough time. As Dr. Reyes says we can convert the AO activity from dry weight to fresh weight.
I also agree with the changes in the activity with drying. Nowadays several modern drying technologies like vacuum drying or freeze drying are available which can prevent the losses due to drying to a greater extent.
When you go for freeze drying, freeze the samples in liquid nitrogen and store them in -80 degree centigrade refrigerators.
The vegetables and fruits are mostly consumed as fresh, hence, antioxidant activity at fresh weight.
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