High hydrostatic pressure and phage SALMONELEXTM as a combined system towards salmonella inactivation in egg white and liquid whole egg

Abstract

Salmonella spp. still represents a major public health concern as a causative agent of foodborne diarrheal illnesses. Salmonella Enteritidis is the most common serovar associated with non-typhoidal salmonellosis, through the ingestion of raw, undercooked eggs or egg-derived food products. The consumption pattern has changed worldwide, towards a growing demand for healthy and minimally processed foods. The association of high pressure processing and natural antimicrobial agents, namely bacteriophages, has been studied as suitable biocontrol, energy-sustainable approaches. The aim of the work presented in this thesis was to develop a novel non-thermal technology through the combination of high hydrostatic pressure (HHP) with SALMONELEXTM towards Salmonella inactivation in liquid whole egg and egg white.A preliminary set of experiments allowed to establish the most adequate parameters to be employed in the developed synergistic system. Concerning the HHP resistance pattern of thirteen food and clinical Salmonella strains belonging to serovars Enteritidis, Typhimurium and Senftenberg following exposure to pressure magnitudes in the range of 200 to 600 MPa, a prominent intrastrain heterogeneity was observed and as expected, higher pressures elicited a lower survivability. The potential to exploit SALMONELEXTM in combination with HHP for Salmonella elimination was evaluated through the assessment of the pressure impact on the bacteriophage viability, which pointed out its notable baroresistance up to 500 MPa. Concerning the structural integrity and the morphological features, transmission electron microscopy analysis, unveiled that processing at 500 and 600 MPa elicited a detrimental impact on the bacteriophage integrity. With respect to the pH (5-8) stability assays, it was noteworthy the barotolerance (300 MPa) of SALMONELEXTM previously exposed to different pH values, which proved not to undermine its infectivity. Regarding the influence of the principal egg components (albumin, lysozyme and alkaline pH) on the bacteriophage inactivation induced by HHP (300 MPa), a scarce effect on the bactericidal activity of SALMONELEXTM was observed. The promising results highlighted the notable potential of SALMONELEXTM to be employed in pair combination with HHP. The effect of pressure processing on the food matrices physical properties was also assessed and it was found that pressures above 400 MPa presented a detrimental impact in both liquid whole egg and egg white.VIThe second part of the present work sought to investigate the effect of the combined treatment - mild high hydrostatic pressure (300 MPa, 5 min, 10 ºC) and SALMONELEXTM – on the inactivation of a four-strain Salmonella cocktail (comprising the most baroresistant isolates herein identified) in egg white and liquid whole egg. In the challenge assays performed - egg white (104 colony forming units (CFU) g-1) and whole egg (104 and 105 CFU g-1) - the application of the high pressure processing proved to be notably effective towards the complete inactivation of the Salmonella cocktail and therefore it was not possible to observe the potential bactericidal effect of SALMONELEXTM. Concerning the same experiment performed in the egg white (107 CFU g-1), the high pressure treatment per se was not able to reduce the pathogen below the detection limit of the enumeration technique, and hence the bacteriophage effect could already be noticed. The observed effect of the combination of HHP and SALMONELEXTM was determined to be a synergism.To the best of our knowledge, no other study has been performed concerning the association of mild high hydrostatic pressure to a bacteriophage towards Salmonella spp. elimination in egg white or liquid whole egg. This is also the first study documenting the impact of high pressure treatment on the stability of SALMONELEXTM, along with its characterization. The combination of the two hurdles (HHP and SALMONELEXTM) resulted in a novel non-thermal technology as a suitable alternative to egg white pasteurization since the synergistic system accomplished the Salmonella 5-log reduction requirement of the FDA.

Date of Award	14 Jan 2021
Original language	English
Awarding Institution	Universidade Católica Portuguesa
Supervisor	Paula Teixeira (Supervisor)