*2.9. Cell Viability*

Before the cellular antioxidant and anti-inflammatory experiments, it was necessary to find out if the plum powders were cytotoxic for both cell lines (HT-29 and RAW 264.7). With this purpose, cell viability was determined by MTT reduction assay as previously was described by Silvan et al. [23]. Confluent stock cultures (~90%) were trypsinized (Trypsin/EDTA), and cells were seeded in 96-well plates (~5 <sup>×</sup> <sup>10</sup><sup>4</sup> cells per well) and incubated in culture medium at 37 ◦C under 5% CO2 in a humidifier incubator for 24 h. Briefly, cell medium was replaced with serum-free medium containing PEP (1 mg/mL final concentrations), and the cells were incubated at 37 ◦C for 24 h under 5% CO2. Control cells (non-treated) were incubated in serum-free medium without plum powders. Thereafter, cells were washed with PBS, the medium was replaced by 200 μL of serum-free medium, and 20 μL of MTT solution in PBS (5 mg/mL) was added to each well for the quantification of the living metabolically active cells after 1-h incubation. MTT is reduced to purple formazan in the mitochondria of living cells. Formazan crystals in the wells were solubilized in 200 μL DMSO. Finally, the absorbance was measured at 570 nm wavelength by employing a microplate reader Synergy HT (BioTek Instruments Inc., Winooski, VT, USA). The viability was calculated considering controls containing serum-free medium as 100% viable. Data represent the mean and standard deviation of three independent experiments (*n* = 3). All experiments were carried out between passage 10 to passage 30 to ensure cell uniformity and reproducibility.
