Effect of the thermoultrasonic treatment on the eggshell integrity and their impact on the microbial quality

Abstract

A study of the effect of thermoultrasonication on the ultrastructure and physical properties of the eggshell has been conducted as well as an evaluation of possible bacteria penetration including salmonellae, to the inside of egg. No effect of treatment was found in the physical properties of shell stored in different conditions nor was the structure of the eggshell affected, except for the cuticle. Most of the outer surface, including pores, of the thermoultrasonicated eggshell lacked the cuticle outer layer but cuticle residues remained within the mouth of the pore. This fact, probably led to maintaining the occlusion of the eggshell pores since both yolk and white remained sterile after three weeks of storage under several temperature and relative humidity conditions.

Industrial relevance

Contaminated eggshells are among the leading sources of human infection with Salmonella enterica, serovars Enteritidis and Typhimurium (S. Enteritidis and S. Typhimurium). We have successfully developed a system using heat and ultrasound, simultaneously applied (thermoultrasonication) to eliminate these organisms on the surface of intact eggs. Although, it would be necessary to develop equipment for larger scale application, this treatment provides many advantages from an industrial point of view because it is possible to pasteurize the surface of intact eggs by applying a thermoultrasonic treatment. This process yields a product that has a safe level of salmonellae with negligible damage to the thermolabile egg components.

Introduction

The avian eggshell is a biomineralised composite ceramic consisting of calcium carbonate embedded in an organic matrix. It is composed of several layers. The most extensive one is the hard shell proper, the palisade, which is organised upon the inside organic matrix with the next most external layer, named the cuticle, which has a variable thickness between 0.5 and 12.8 μm (Parsons, 1982), composed mainly by mucin (Moran and Hale, 1936, Cooke and Balch, 1970), i.e. a high molecular weight glycosylated protein associated with carbohydrates (Roussel et al., 1988). These differences in thickness are due to the presence of star-like grooves, which mark the beginning of the pores of the shell. The cuticle is easily removable by dipping the egg, e.g., in 5% EDTA for 90 min (Baker & Balch, 1962).

The main role of the cuticle is to contribute to the prevention of excessive water loss in the egg and to encourage the exchange of gases. There has been some indication that the removal of this cuticle could improve gas exchange if the humidity was adjusted appropriately in order to prevent excessive water loss (Peebles, Brake, & Gildersleeve, 1987). Furthermore, the cuticle could also operate as a barrier to protect the egg from bacterial invasion (Roussel et al., 1988).

The importance of the integrity of the cuticle is supported by the fact that the microbiological criteria in some countries, such as those of the European Union, do not recommend washing of the intact eggs to avoid the disruption of this structure. Yet, this could lead to more serious consequences than that of light surface dirt; it could facility the penetration of bacteria, pathogenic or not, through the pores.

We have previously reported (Cabeza et al., 2004, Cabeza, García, Hoz, et al., 2005) that the simultaneous application of heat and ultrasounds (thermoultrasonication) to intact eggs results in a reduction in the number of Salmonella enterica, serovars Enteritidis and Senftenberg present on the shell to a safe hygienic level in less severe conditions than those usually recommended for common pasteurization. Moreover, an equation has been found to integrate the most relevant external variables (temperature, time and ultrasonic energy) affecting ultrasound lethality (Cabeza et al., 2010), which allows to predict the effect of sonication under different process conditions and, therefore, is useful for the scale-up to the system. We have also reported (Cabeza, García, de la Hoz, Cambero, & Ordóñez, 2005) that this treatment causes negligible damage to the thermolabile egg components. However, we hypothetically supposed that the intact egg thermoultrasonication could cause a substantial disruption of the eggshell layers, consequently, the loss of its integrity, may encourage the passage of surviving bacteria to the egg inside. Therefore, this work was conducted with the aim of both to determine the extent of damage to the cuticle eggshell caused by the thermoultrasonication and to ascertain whether the bacteria located outside the thermoultrasonicated egg could pass through the pores during its commercial life.