Quality of thyme (Thymus vulgaris L.) and black pepper (Piper nigrum L.) during storage as affected by the combination of gamma-irradiation and modified atmosphere packaging

Thyme and black pepper are among the common herbs and spices irradiated commercially for decontamination. Irradiation performed in the air atmosphere has adverse effects on some quality aspects, and this can be minimized if the treatment is delivered under an O₂-free atmosphere. The effects of packaging under a modified atmosphere (100% N₂, MAP) or air (AP) prior to gamma-irradiation (at 0, 6 or 14 kGy) on total phenolic content (TPC), 2,2-diphenyl-2-picrylhydrazyl (DPPH) scavenging activity, β-carotene bleaching test-based antioxidant activity (BCLA-AA), antimicrobial activity, microbial load, color, and sensory properties of thyme and black pepper were investigated during 6-month storage. Irradiation under the MAP increased essential oil yield and DPPH scavenging activity of the samples at the beginning of the storage. Moreover, the TPC of black pepper and antimicrobial activity of thyme were increased by 14 kGy irradiation in both packages at the beginning of the storage period. Total bacterial and yeast-mold counts were reduced to below 1 log cfu/g after 14 kGy irradiation from an initial contamination level of 7.7 log and 5.2 log, respectively. Reductions in the essential oil yield, DPPH scavenging activity, and antimicrobial activity of thyme during 6-month storage were noted. The BCLA-AA of thyme was increased during the storage period, whereas that of black pepper in AP was decreased. Overall, gamma-irradiation can be applied under modified atmosphere packaging (100% N₂) to maintain or improve some of the functional properties of thyme and black pepper without compromising sensory and microbial quality.