Abstract

The past few years have seen an increase in the search and validation of clean label antimicrobials for their application in ready-to-eat meat products. Essential oils (EOs) from plants have been found to be effective against foodborne pathogenic bacteria such as Salmonella and Listeria under laboratory conditions. In the present study, the effect of thyme EO, pH, and isolated soy protein (ISP), and their interactions on the inactivation of either a Listeria or Salmonella cocktail were studied through a Box–Behnken experimental design. It was observed that as the amount of ISP decreased and thyme EO concentration increased, the number of microorganisms declined; furthermore, at higher ISP concentrations, a higher concentration of thyme EO was needed. This suggests an interaction between ISP and thyme EO that reduces bacterial inactivation. The results obtained from this research are a first step to establish appropriate thyme essential oil concentrations for its application as an antimicrobial on ready-to-eat meat products depending on their pH and protein content.

Keywords: thyme essential oil, antimicrobials, model system

#### 1. Introduction

Ready-to-eat meat products such as cold cuts and deli style ham or sausages have been implicated in the risk of foodborne diseases caused by the ingestion of pathogenic bacteria such as Salmonella and Listeria [1], especially when sliced at retail points [2]. Antimicrobial compounds used as ingredients in the product formulation have helped diminish foodborne disease cases, but the consumers' demand for "clean label" products has led to the search and validation of alternative compounds such as plant essential oils [3] since they not only possess generally recognized as safe (GRAS) status and wide acceptance from consumers [4] but, as plant secondary metabolites, they also contain antimicrobial components, which are important for plant defense [5], and nowadays as potential antimicrobial ingredients for food application.

Oregano and thyme have been reported as among the most effective essential oils as antimicrobials, suggesting that low concentrations of them may be needed to achieve the desired effect. This, in turn, is an advantage since high concentrations of an EO can have a negative impact on the sensory acceptability of food products [4]. Although some studies have shown the efficacy of EOs as antimicrobials in meat products, others have reported very low antimicrobial activity and the need for high concentrations of the EO to accomplish the desired effect [3].

In this context, there is a need to evaluate EOs directly in food products or in model systems that simulate food composition [4]. The use of food model media before food application is important since conditions can be controlled in order to determine the interactions between EOs and food components that could influence their antimicrobial activity.

In this regard, the main objective of this work was to, by means of a Box-Behnken design (BBD) of response surface methodology, evaluate the effects of pH, protein, and thyme essential oil concentration on the log of colony-forming units (CFU)/mL obtained after either a Salmonella or Listeria cocktail was subjected to the selected factors in model culture media.

(Autoplate 4000, Spiral Biotech, Norwood, MA) for each concentration of thyme

NutriOmics group

Laboratory collection Salmonella spp. Benemérita Universidad Autónoma de Puebla (BUAP) Food

Looking for the Killer Combination: pH, Protein, and Thyme Essential Oil Interactions that…

Salmonella typhi (ATCC ® 19,430) BUAP Chemical Sciences Faculty Food Microbiology

by Dra. María Lorena Luna Guevara

Universidad de las Américas Puebla Food Microbiology

Microbiology Laboratory collection, isolated from tomatoes

Instituto Tecnológico de Estudios Superiores de Monterrey

MIC was considered as the lowest concentration of EO at which no bacterial growth was observed after 24 h of incubation at 35°C and MBC as the lowest concentration, which resulted in no visible growth on the plate and no growth on trypticase soy broth after streaking the plate surface and further incubated under

Plate preparation and inoculation: The media for plates were prepared by mixing isolated soy protein (ISP) adjusted to the desired concentration, trypticase soy broth as the nutrient provider, bacteriological agar as solidifying media, NaCl to adjust water activity to 0.98, and distilled water. The mixture was heated until boiling under continuous agitation and sterilized by autoclaving for 5 min at 17 lb./

. After the media cooled down to 45°C, pH was adjusted with citric acid (10%), and EO to the desired tested concentration was added during continuous agitation with a magnetic stirrer. The media was poured in Petri dishes and left to cool for at least 2 h prior to inoculation. Plates at room temperature were inoculated with 30 μL (in three drops of 10 μL) of the cocktail dilution which, by previous determination,

Experimental design: A Box-Behnken design (BBD) of response surface methodology was applied using Minitab 18.1. (Minitab, Inc., State College, PA) to study the effect of medium pH, isolated soy protein (ISP) concentration (% w/w), and EO concentration (% w/w), as well as their interactions on the growth of Salmonella or Listeria cocktails measured as fraction of surviving microorganisms when compared

Each factor was investigated at three different levels (1, 0, +1) shown in Table 2. The experimental design included 15 sets of test conditions, including 3

A Isolated soy protein (% weight/weight) 10.0 11.5 13.0 B pH 5.5 6.0 6.5 C Essential oil (% weight/weight) 0.19 0.22 0.25

1 0 +1

Factor Independent variable Coded level

replicated center points as randomized experiments to avoid bias.

Studied independent variables and their levels for tested box-Behnken design.

EO tested, 0.065–0.093% (w/w).

Salmonella Typhimurium (ATCC®

Studied bacterial strains and providers.

13,311)

Table 1.

Strain Provider

DOI: http://dx.doi.org/10.5772/intechopen.90099

Laboratory collection Listeria monocytogenes (CDBB-B-1426)

L. monocytogenes (Scott A) Salmonella typhimurium (ATCC ® 14028)

allowed the count of 20–70 CFU in each drop.

to CFU/mL found in each cocktail.

optimal conditions [10].

in2

Table 2.

95
