Previous studies have demonstrated that presence of water on the shell surface can undermine the egg’s physical defences. Water contaminated with significant amounts of iron or organic matter enters the egg and the egg’s chemical defences can be compromised. The main line of egg defence is the presence of the cuticle layer that plugs pores and prevents bacterial penetration taking place. Washing eggs can erode the cuticle however the incidence of internal contamination may be reduced significantly provided certain basic rules are followed. In particular, eggs should not be washed in water which is cooler than the egg contents because this can draw wash water into the egg as the egg contents cool and contract. Also, if wash water is too hot, thermal cracking of shell can occur.
Studies were conducted using a Rotomaid 100 bucket style egg washer – this style of washer alls the egg to become completely submerged in water. Washing agent was egg wash powder. Cage and free range eggs were collected within 24 hours of lay, candled pre- and post-washing with cracked eggs excluded from analysis. Eggs grossly contaminated with organic material were not used as they are normally discarded. Experiments were conducted for bucket washing in commercial conditions. The experiments were designed to assess the risk of bacterial contamination of egg contents if bucket washers are operated under inappropriate washing conditions.
When cage eggs were washed under manufacturer recommended conditions, there was significant reduction in total bacteria load associated with shell surface. Although bucket style washing does not sterilise the shell surfaces, egg washing under ideal conditions offer benefits in terms of reducing shell surface contamination and possibly cross-contamination between eggs. Under conditions of low or no sanitiser, bacteria could transfer from the surfaces of dirty eggs to shells of clean eggs via wash water. All free range eggs tested had visible soiling, when these eggs were washed the results were less clear cut. When clean and dirty eggs were analysed together and compared, there was a general trend for lower coliforms but not significantly lower.
Eggs which were deliberately washed in 5 day old water, lacked active sanitiser with residual bacteria which did not grossly contaminate egg contents. The role of temperature in prevention of water and ingress into the contents was investigated by immersing eggs into dye solutions (Coomassie blue) at different temperatures. Dye solution that was 15 degrees Celcius lower than egg contents and immersion of 3 minutes was required for visible staining of egg contents (mainly in air sac end of egg). Maintenance of increased temperature difference for the wash water over the egg contents was probably a contibutory factor in protecting the egg contents from bacterial contamination.
One very surprising finding of these studies as that it was very difficult to contaminate the egg contents using a bucket washer.