Bacteriology of Washed and Unwashed Eggs

Abstract

The ability of the eggshell to resist penetration of Salmonella bacteria was studied in unwashed and in hand washed eggs, and in eggs, which had been subjected to industrial, large-scale machine washing under strictly controlled conditions. In one part of the investigation, concerning unwashed and machine washed eggs, a defined amount of a broth culture of Salmonella typhi murium was applied on the shell surface, whereafter the eggs were incubated at 4°C for eight weeks or at 30°C for 12 days. At the end of the storage period, the egg contents were examined for the presence of Salmonella. No Salmonella bacteria were detected in the 120 eggs investigated. In another part of the investigation, dealing with 200 unwashed, 200 hand washed and 200 machine washed eggs, a broth culture of Salmonella was applied on the shell surface of eggs, whose contents had been poured out through an opening. In every second egg of each kind, the shell membranes had been scraped off over an area, corresponding to the placement of the bacterial inoculum. The eggshells were filled with enrichment broth and the occurrence of Salmonella bacteria in this broth was examined after incubation. In the eggs with intact shell membranes, the frequencies of Salmonella positive enrichment broths were for unwashed eggs 0.12, for machine washed eggs 0.10, and for hand washed eggs 0.22. The corresponding frequencies in eggs with removed shell membranes were 0.27 (unwashed), 0.42 (machine washed), and 0.60 (hand washed). No statistical difference occurred between unwashed and machine washed eggs when the shell membranes were intact. When the shell membranes were removed this difference was statistically almost significant. The difference between machine washed and hand washed eggs with intact shell membranes was statistically almost significant, while the same difference was highly significant in eggs with removed shell membranes. The author assumes, from the results of the present and a preceding investigation, that the applied washing procedure in no way harms any essential storage properties of uncracked eggs. Considering the known occurrence of hand washing of eggs in Swedish farms — a treatment with documentedly unfavourable influence — it would be desirable if all eggs could be subjected to machine washing under strictly controlled conditions at the grading stations. Such an arrangement ought to result in an increase in the average bacteriologic-hygienic quality of eggs for market.