Abstract
ABSTRACT Essential oils from plants are an important alternative source of antimicrobial compounds against important pathogens for food safety. This study aimed to evaluate the in vitro antimicrobial activity of thyme essential oil (Thymus vulgaris) and to apply it in the sanitization of minimally processed arugula (Eruca sativa), aiming to increase the safety and shelf life of arugula. The in vitro antibacterial activity of the essential oil was evaluated by disc diffusion assays and determining the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against different foodborne pathogens. Thereafter, a “challenge test” was carried out to evaluate the antibacterial activity of the thyme essential oil (0.2%) on the sanitization of arugula inoculated with Escherichia coli (3 log CFU mL-1) compared to sodium hypochlorite (120 ppm of active chlorine). Afterwards, an experiment of applying the thyme essential oil (0.2%) as a sanitizing agent in minimally processed arugula was carried out to evaluate the vegetable shelf life compared to sodium hypochlorite (120 ppm of active chlorine). The arugula was stored at 8 °C for 7 days, and periodically, the number of aerobic mesophilic microorganisms, pH and titratable acidity were determined, and the color of the samples was evaluated. The thyme essential oil had strong antimicrobial activity in vitro against all species of bacteria tested. In the experiments in situ, the thyme essential oil (0.2%) and sodium hypochlorite (120 ppm of active chlorine) were not efficient in eliminating the inoculated E. coli from minimally processed arugula, and they did not result in an extension of the shelf life of the arugula in comparison to the control (p>0.05). However, the growth rate of the E. coli population over time on the arugula samples sanitized with the thyme essential oil (0.2%) was low, which indicates a positive effect of the essential oil, which may be optimized with future adjustments in the concentration and immersion time for the arugula sanitization.
Highlights
The constant search for healthy food without residual chemicals which can assure the nutritional and sensorial properties, has motivated the food industry to substitute bactericides and bacteriostatic chemicals for natural products (Tanja et al, 2019)
It is important to highlight that none of these studies have evaluated the effect of thyme essential oil on minimally processed arugula far; this study aimed to (a) evaluate the in vitro antimicrobial activity of thyme essential oil (Thymus vulgaris L.) against gram-positive and gram-negative bacteria; (b) determine the antimicrobial effect of thyme essential oil compared to chlorine in the survival of E. coli inoculated on minimally processed arugula; and (c) evaluate the shelf life of sanitized arugula with a concentration of thyme essential oil compared to the sanitization with sodium hypochlorite
The following important bacterial species in food were tested to evaluate the antimicrobial activity of thyme essential oil: (a) E. coli ATCC 25922; (b) Salmonella Typhimurium ATCC 14028; (b) S. aureus ATCC 25923; (c) B. cereus ATCC 11778; and (d) L. monocytogenes ATCC 19117, which were obtained from the microorganisms collection from the Laboratory of Food Microbiology of the IFC, Concórdia Campus
Summary
The constant search for healthy food without residual chemicals which can assure the nutritional and sensorial properties, has motivated the food industry to substitute bactericides and bacteriostatic chemicals for natural products (Tanja et al, 2019). Another concerning factor is the bacterial resistance to the majority of existing antimicrobials due to the low doses of antimicrobials in food hygiene stages in addition to the residue of these products being discharged into the environment, compromising public health (Hyun, 2015). Several plant extracts have already been tested in studies, leading to different results, and some essential oils have been used in food preservation (Khan et al, 2014; Cui et al, 2018; Hu et al 2019). Cui et al (2016) highlighted that the bactericidal effect of thyme essential oil is associated with changes that result in cellular decomposition, such as increasing cell membrane permeability, leading to intracellular leakage
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