Atmospheric cold plasma inactivation of Escherichia coli and Listeria monocytogenes in tender coconut water: Inoculation and accelerated shelf-life studies

Abstract

In the current work we evaluated the effect of atmospheric cold plasma (ACP) generated in air and M65 (65% O2, 30% CO2, 5%N2) as working gases for inactivation of Escherichia coli and Listeria monocyotogenes in tender coconut water. A 5 log10 reduction in the population of both microbes was achieved in tender coconut water with addition of 400 ppm citric acid and plasma treatment with M65 gas for 120 s at 90 kV, followed by a 24 h post-treatment storage under refrigerated conditions. Optical emission spectroscopy was utilized in identifying reactive gas species of nitrogen and oxygen which were accounted for cellular leakage and morphological changes in microbes on plasma treatment. The plasma treatments on tender coconut water caused a lowering of pH and small changes in Hunter color parameters (a* and b*), while total soluble solids and total titratable acidity did not change significantly. Accelerated shelf-life studies (ASLS) carried out at 10 °C, 20 °C and 30 °C compared three batches: Batch 1(tender coconut water), Batch 2 (Tender coconut water + citric acid + ascorbic acid) and Batch 3 (Tender coconut water + citric acid + Plasma treatment with M65 + ascorbic acid). ASLS revealed that the rate constants for parameters namely percent transmission, ascorbic acid content, total titratable acidity and total color change decreased in the order Batch 1 > Batch 2 > Batch 3. Estimation of rate constants following Arrhenius model for Batch 3 at 5 °C was comparable to experimental results. A shelf-life of 48 days was predicted for Batch 3 at 5 °C considering 75% ascorbic acid content degradation. Thus, ACP was concluded to be a novel technology for tender coconut water processing.