Decontamination effect of hot-air drying against bacterial pathogen and surrogate strains on basil leaves, from laboratory to pilot scale settings

Highlights

• Hot-air drying at > 60 °C reduced Salmonella Senftenberg by 4-log on basil leaves.

• Product moisture content influenced the inactivation effect of drying substantially.

• E. coli P1 was the most suitable surrogate to validate hot-air drying processes.

• Decontamination effect of drying was validated from lab to pilot scale settings.

Abstract

Drying technologies are often evaluated on their efficiency to remove moisture and impact on sensory quality of products. The decontamination effect against foodborne pathogens, however, is rarely investigated. This study investigated the inactivation of foodborne pathogens and surrogates on basil leaves during drying, from characterizing thermal resistance of strains in a_w-adjusted basil to collecting decontamination data under various drying processes. Salmonella Senftenberg was the most resistant pathogenic strain tested during heating on basil leaves. A treatment of 20 min at 60 °C reduced S. Senftenberg by 4.7 and 3.7-log on basil leaves at water activity of 0.99 and 0.95, respectively. The pronounced increase in strains’ thermal resistance at reduced water activity was also observed during dynamic hot-air drying. Overall, more than 4-log reduction of S. Senftenberg was achieved after 90 min, 40 min and 20 min drying in a laboratory oven at 60 °C, 80 °C and 100 °C, respectively. Upscaling experiments in a pilot dryer using surrogate strains provided additional confirmation on the decontamination efficiency and effect of product moisture content during drying. Results from surrogate strains suggested that Escherichia coli P1 showed to be a more suitable surrogate for hot-air drying processes of basil leaves than Enterococcus faecium.

Introduction

Dried herbs are frequently used as ingredients and added into minimally processed or ready-to-eat foods to enhance their sensorial and healthy attributes (Székács et al., 2018). As with many other agricultural products, herbs are exposed to microbial contamination during pre- and post-harvest (Gurtler, Doyle, & Kornacki, 2014). Prevalence results showed that around 1.5% (2/132) and 1.1% (31/2833) of dried herbs tested positive for Salmonella in production and retail samples, respectively (Sagoo et al., 2009). Between 1973 and 2012, 28 outbreaks implicating herbs and spices have been documented, which caused a total of 2228 reported cases, 134 hospitalization and 2 deaths (FAO, 2014). There have been increasing concerns regarding the microbial safety of dried herbs in recent years (Székács et al., 2018; Szűcs, Szabó, Lakner, & Székács, 2018). Processing measures that reduce the contamination level on dried herbs could provide added value for food safety, especially for products that undergo no additional heating steps (Little, Omotoye, & Mitchell, 2003).

Drying processes are frequently used in the production of herbs (Lewicki, 2006). It is assumed that hot-air drying with its thermal effect could reduce the presence of foodborne pathogens. However, the processing conditions are often designed out of technological nature to achieve desired product quality and accomplish long-term ambient stability. The decontamination effect against foodborne pathogens is rarely investigated (Bourdoux, Li, Rajkovic, Devlieghere, & Uyttendaele, 2016). Moreover, heat resistance of foodborne pathogens, such as Salmonella, is known to increase at reduced water activity (a_w) (Podolak, Lucore, & Harris, 2017). Considering that the moisture content of food products decreases with the progress of drying, it is important to identify the critical inactivation parameters in order to understand the fate of pathogens on herbs during dynamic drying. Besides, the process lethality of any given technology needs to be validated at an industrial level where the introduction of pathogenic strains is not allowed due to biological hazard concerns. Hence, the use of surrogate strains, which utilizes non-pathogenic proxies that response to a treatment in an equivalent or more resistant manner than the pathogen of concern, is of increasing interests (Busta et al., 2003; Hu & Gurtler, 2017; NACMCF, 2010). However, the proper selection of surrogate strains depends highly on the type of process, product and targeted pathogens. Evaluation of surrogate strains and paired comparison between surrogate and pathogenic strains on herbs during hot-air drying need to be established.

Using basil leaves (Ocimum basilicum) as a case study, the present study firstly compares the thermal resistance of various bacterial strains on fresh and semi-dried herbs. Inactivation kinetics of selected strains on basil leaves during hot-air drying were further evaluated in a laboratory oven. Finally, the process lethality of hot-air drying against bacterial surrogates was evaluated in a pilot scale dryer to validate the microbial reductions simulating an industrial drying process. These experiments fill in the missing information of the decontamination efficiency of hot-air drying process, providing insights on critical processing parameters and conditions needed to reach sufficient microbial reductions on dried herbs.