In package atmospheric cold plasma outperforms direct exposure for microbial decontamination of bread

Abstract:

Bread is a high water activity staple that is recontaminated by airborne bacterial and fungal propagules as soon as it leaves the oven, and its short mould free shelf life is conventionally managed with chemical preservatives that increasingly conflict with clean label expectations. This study evaluated a pins to parallel plate high voltage atmospheric cold plasma (CAP) reactor as a non thermal, residue free surface decontamination step for freshly baked bread. Bread cubes (5 × 5 × 5 cm) were exposed to an atmospheric glow discharge struck across a 40-45 mm inter electrode gap at 25 kV for 30, 45 and 60 s, either directly (T₁-T₃) or after sealing in the retail package (A₁-A₃), and compared against an untreated control. Moisture content, total plate count (TPC), yeast and mould count and 9 point hedonic sensory scores were determined. TPC fell from 1.7 × 10⁵ cfu/g in the control to 0.5 × 10⁵ cfu/g (0.53 log; −71%) after 60 s of direct exposure and to 0.3 × 10⁵ cfu/g (0.75 log; −82%) after 60 s of in package exposure. Yeast and mould became undetectable after 60 s of direct exposure but after only 45 s of in package exposure. Moisture content was consistently and highly significantly (P ≤ 0.01) higher in the in package samples (13.05-13.42%) than in directly exposed samples (12.05-12.13%) at every exposure time; within the in package series moisture declined significantly with treatment time (F = 13.392, P ≤ 0.01), whereas the directly exposed series showed no significant time effect (F = 1.311, NS). Sensory scores declined progressively with direct exposure beyond 30 s but were retained in the in package series, which peaked at 45 s. Confining the discharge within the package therefore delivered a greater microbial reduction at a lower moisture and sensory cost than open air exposure, and 45-60 s at 25 kV in package is identified as the most favourable operating window for pins to parallel plate CAP treatment of bread.