1Department of Animal Product Quality and Transformation Technology,
CLO, Melle, Belgium
2Laboratory of Food Microbiology and Food Preservation,
Department of Food, Technology and Nutrition,
University of Ghent,
Ghent, Belgium
Washing eggs in sterile plastic bags with diluent is an efficient sample preparation method for the determination of the bacterial contamination on eggshells. The total count of aerobic bacteria and the total count of Gram-negative bacteria on the eggshell can be used to detect critical contamination points in the egg production chain. The number of eggs to be sampled in a point of the production chain was determined on a statistical basis and fixed on 40 for non-graded eggs and on 20 for graded eggs. In two production chains, one cage production and one organic production system, critical contamination points were identified. The influence of the housing system on the bacterial contamination of the eggshell at the stable was studied. A positive correlation was found between the initial bacterial eggshell contamination and the concentration of bacteria in the air of the poultry houses. With the exception of heavily soiled shells, like shells from ground eggs, there is a poor correlation between the level of bacterial contamination and the visual eggshell contamination.
Introduction
External eggshell contamination could be important for the shelf life and the food safety of consumption eggs and egg products. In literature few data are published about the bacterial contamination on the shell of consumption eggs. The shell can already be infected when passing through the vent, but many researchers suggest that the main contamination occurs within a short period after laying due to contact with dirty surfaces (Harry, 1963; Board et al., 1964; Quarles et al., 1970 and Gentry and Quarles, 1972). It is hypothesized that bacterial contamination of the internal egg content could be the result of the penetration of the shell by bacteria deposited on the surface of the egg after it has been laid (Haines, 1938; Harry, 1963; Schoeni, et al. 1995). Because of the introduction of different alternative housing systems for laying hens, the study of the bacterial contamination on the shell of consumption eggs is important.
The aim of our study was to develop the methodology to quantify the bacterial contamination on the eggshell and to detect critical points of contamination in the entire production chain; this means from the production, grading and transport up to the sales-outlets. Also the influence of the housing system on the bacterial contamination of the eggshell of consumption eggs was studied.
Material and Methods
Determination of bacterial eggshell contamination
Different methods for the recuperation of bacteria from the eggshell, different counting media and incubation temperature/time combinations were compared. Based on these results the most efficient and practical method was used for application in further experiments.
Number of samples and statistical analysis of data
To produce statistically reliable results, a minimum of eggs need to be sampled at a certain point in the production chain. The minimum number of samples is based on the number of samples from which the standard error starts converging to an asymptotic value (Grijspeerdt and Verstraete 1997). Standard differences were assessed using an analysis of variance (ANOVA).
Collection and transport of eggs
In the points of the production chain before packaging, the eggs were picked up with the fingertips and placed in new carton filler-flats. Between each sampling point the fingertips were disinfected. In the points after packaging closed cartons (first category eggs) or carton filler-flats (second category eggs) filled with eggs were sampled. The eggs were brought by car, in ambient conditions, to the laboratory were they were kept for maximum 56 hours in ambient conditions before analysing.
Influence of time, temperature and atmospheric humidity on the bacterial eggshell contamination
The influence of storage of the eggs till expire date at room temperature with an average atmospheric humidity of 50% and in a refrigerator at 5°C with an average atmospheric humidity of 85% was studied.
Sampling through the production chain
Sampling was performed in a caged layer house and an organic production unit, housing respectively the brown-shell breeds Warren Sex A Line and Bovans Goldline hens. At least 7 points were sampled in the entire production chains.
Influence of the housing system on the eggshell contamination
To study the influence of the housing system on the bacterial eggshell contamination, three different systems, - enriched cages, perchery and cage production -, arranged on the same farm were followed during several months.
Results and discussion
Determination of bacterial eggshell contamination
The most efficient and practical method for the recuperation of bacteria from the eggshell was based on a washing procedure of the intact egg in a plastic bag with 10 ml diluent. The diluent was plated by a spiral-plater on Nutrient Agar for the determination of the total count of aerobic bacteria and on Nutrient Agar with 0,0001% crystal violet for the total count of Gram-negative bacteria. Plates were incubated for 3 days at 30°C.
Number of samples
To produce statistically reliable results, minimum 40 eggs need to be sampled at a certain point in the production chain with non-graded eggs. The required number of samples for graded eggs was set at 20.
Collection and transport of eggs
Collecting the eggs by hand did not influence the results significantly. Also no significant difference in the bacterial flora was found on eggs examined immediately and after keeping for 56 hours in ambient conditions at the laboratory.
Influence of time, temperature and atmospheric humidity on the bacterial eggshell contamination
Contrary to the total count of aerobic bacteria, the Gram-negative bacteria on the eggshell decreased statistically significantly at room environment (from 4.04 to 3.23 log cfu/eggshell) but not at refrigeration environment (from 4.04 to 3.66 log cfu/eggshell). This was probably due to the lower humidity at room temperature.
Sampling through the production chain
Cage production:
Figure 1 shows an increase in total count of aerobic and Gram-negative bacteria, at the moment the eggs enter the grading, candling and packing area ('metal mat' in Figure 1). For both parameters the increase was statistically significant (ANOVA). This point in the production chain was indicated as a critical control point.
Organic production:
Compared to the caged layer house, the bacterial eggshell contamination through the organic chain showed less fluctuations. The initial contamination with aerobic bacteria (5.8 log cfu/eggshell) of the eggs on the covered conveyor of the nest boxes was 1 log higher compared to the eggs on the conveyor of the stable in the cage production. This raise of initial contamination makes the nest boxes in the organic housing system a critical point for bacterial eggshell contamination. The higher initial contamination was also reflected in the air where a much higher contamination (2.2×104 cfu/50l air) was measured compared to the caged stable (1.4×103 cfu/50l air). Despite the clear difference in critical control points for bacterial contamination, the total bacterial count on the eggshell for the 2 production systems was comparable at the end of the chain. With the exception of heavily soiled shells, like shells from ground eggs, a poor correlation was found between the level of bacterial contamination and the visual eggshell contamination.
Influence of the housing system on the eggshell contamination
During August 2001 till June 2002 the bacterial eggshell contamination, total count of aerobic bacteria and Gram-negative bacteria, was analyzed bimonthly for eggs from a cage production and enriched cages with nest boxes containing mats or grids, all productions arranged on the same farm with the same hen breed.
In a period of eight months no statistical differences were found between the three housing systems.
Only on the last sampling date the eggshell contamination at the cage production was significantly higher. More pronounced was the seasonal influence on the eggshell contamination with a statistically significant decrease in the winter period. A second study is running, including also a perchery system. The first results indicate a statistically higher contamination of the eggshells, with total aerobic flora, of the perchery system compared to the cage production and the enriched cages.
The first data are shown in Figure 2.
In all studied poultry houses a positive correlation was found between the initial bacterial eggshell contamination and the concentration of bacteria in the air of the poultry houses.
Acknowledgments
This abstract would not have been possible without the work of especially Ann Van de Walle and Wouter De Laender, but also Jurgen Baert, Willy Bracke, Daisy Alleman and Vera Van de Mergel are acknowledged.
References
Board, R.G., Ayres, J.C., Kraft, A.A. and Forsythe, R.H. (1964). The microbiological contamination of egg shells and egg packaging materials. Poultry Science, 43 , 584-594.
Gentry, R.F. and Quarles, C.L. (1972). The measurement of bacterial contamination on egg shells. Poultry Science, 51, 930-933.
Grijspeerdt, K. and Verstraete, W (1997). Image analysis to estimate the settleability and concentration of activated sludge. Water Research, 31, 1126-1134.
Harry, E.G. (1963). The relationship between egg spoilage and the environment of the egg when laid. British Poultry Science, 4, 91-100.
Haines, R. B. (1938). Observation on the bacterial flora of the hen's egg with a discription of new species of Proteus and Pseudomonas causing rot in eggs. Journal of Hygiene Cambridge, 38, 338-355.
Schoeni, J. L., Glass, K.A., McDermott, J.L. and Wong, A.C.L. (1995). Growth and penetration of Salmonella enteritidis, Salmonella heidelberg and Salmonella typhimurium in eggs. International Journal of Food Microbiology, 24, 385-396
Quarles, C.L., Gentry, R.F. and Bressler, G.O. (1970). Bacterial contamination in poultry houses and its relationship to egg hatchability. Poultry Science, 49, 60-66.
From Proceedings of the "XVI European Symposium on the Quality of Poultry Meat" and the "X European Symposium on the Quality of Eggs and Egg Products", Saint-Brieuc Ploufragan, France.
