You can determine the antioxidant activity of Capsicum annuum fruits using methods like DPPH (2,2-diphenyl-1-picrylhydrazyl) assay, FRAP (Ferric Reducing Antioxidant Power) assay, or ORAC (Oxygen Radical Absorbance Capacity) assay. These tests measure the ability of antioxidants in the fruit to neutralize free radicals or reduce oxidized compounds.

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Several tests have been used to check and measure the antioxidant potential. With a search, you can get them from the articles and finally use them.

Of course, it should be noted that there may be differences in performing an antioxidant test, for example, DPPH, in several articles.

For example, in this case, pay attention to the following methods for testing DPPH:

The following methods for performing the DPPH test have been carried out from 2 different articles to check the antioxidant potential of plants and algae:

1: DPPH-Radical Scavenging Activity (DPPH-RSA) Assay

The DPPH (2,2-diphenyl-1-picrylhydrazyl)-RSA assay is based on the bleaching of this radical under the presence of antioxidant-containing extracts. Thus, the DPPH-RSA activity of extracts was measured spectrophotometrically at 517 nm, against the stable nitrogen radical DPPH. In this technique, 25 µL of extracts were mixed with 200 µL of fresh DPPH ethanolic solution (40 mg/L) and let stand for 30 min in the dark. Results were expressed as Trolox equivalent (TE) per g of the dry extract. Determinations were conducted in triplicate.

Reference:

Barroso, M.F.; Ramalhosa, M.J.; Alves, R.C.; Dias, A.; Soares, C.M.D.; Oliva-Teles, M.T.; Delerue-Matos, C. Total antioxidant capacity of plant infusions: Assessment using electrochemical DNA-based biosensor and spectrophotometric methods. Food Control 2016, 68, 153–161.

2: DPPH-Radical Scavenging Activity

At first, 1 ml of the extract was added to 1 ml of 0.16 mM free radical methanol solution of DPPH and was mixed well and kept at room temperature and in the dark for 30 minutes. Then, the absorbance of the samples was read in the Elisa Reader device at 517 nm. The extract's free radical scavenging activity (DPPH) was calculated using the following formula. The results were expressed as a percentage (%) of radical scavenging activity (RSA).

Radical scavenging activity %= [1-(A sample- A sample blank)/A control)]×100

A sample=Absorbance of the DPPH solution plus test sample, A control= Absorbance of the DPPH solution without sample, and A sample blank= absorbance of the sample without DPPH solution

Reference:

Brand-Williams, W, Cuvelier, ME & Berset, CL 1995, Use of a free radical method to evaluate antioxidant activity. LWT-Food science and Technology, 28: 25-30.