An egg’s shell has a natural protective coating (cuticle) that resists the entrance of bacteria and retains moisture inside.
Washing eggs with water removes this protection, and thus washed eggs should be eaten as soon as possible.
Whether eggs are wet- or dry- cleaned, they should be sold separately from naturally clean eggs, as their storage life is shorter.
The cuticle from the shell is a protein-fat substance, and the lack of a cuticle can therefore be detected with a simple ultraviolet (UV) lamp.
Washed eggs (without a cuticle) are red in colour under UV-light, while a blue colour indicates that the cuticle is still present.
Even with good flock management, some eggs will get dirty. I have already listed the risks of allowing water to touch the shell have already been mentioned.
Dry cleaning systems are preferred. Rubbing lightly with fine sandpaper or a rough cloth is better than wet cleaning. Cloth-backed sandpaper or emery paper can be wrapped around a block of foam rubber for dry cleaning by hand.
Steel wool and nylon dishwashing or bathroom scrubbing aids are also quite suitable. Care should be taken not to remove too much of the protective cuticle layer which covers the shell.
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Only the dirty patches should be cleaned. There are also motor-driven dry-cleaners commercially available. The simplest model consists of a spinning wheel of foam rubber.
A mixture of glue and sand is applied periodically to the foam wheel. The operator holds the egg against the spinning foam wheel to clean it.
Washing of eggs is only suggested under very well-controlled conditions.
The concern is to ensure that the washing water temperature (38 to 43 o C) is never below that of the egg. This avoids the wash water being sucked into the egg through the shell pores by the action of the egg contents shrinking (as happens if the egg is in contact with cooler water).
In addition, the washing machine must be able to monitor the detergent/sanitizer/disinfectant/antiseptic levels in the water to ensure that they are optimal.
Only special types of non-tainting chemicals can be used. The water itself must be changed frequently.
After washing, the shell should be pasteurized by dipping the eggs in water at 82 o C for a few seconds, then dried quickly with warm air before packing.
The eggs must also be clearly labelled as “washed”. Washing done in this way is complex and expensive, and is therefore only justified in large operations, although even then it involves risks.
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