Food Safety Focus (29th Issue, December 2008) – Incident in Focus

Bacteria on Eggs – Should Eggs be Washed?

Reported by Dr. Ken CHONG, Scientific Officer
Risk Assessment Section,
Centre for Food Safety

On 18 November 2008, the media reported that a study conducted by a local university found high bacterial counts on egg shells as well as in liquid egg samples imported from various countries. The report raised some concerns over food safety of eggs.

llustration: Egg

Contamination of Eggs

Eggs are not laid in a sterile environment. In fact, eggs may be contaminated via two different routes: vertical transmission through the ovary or transovarian or horizontal transmission through the shell or trans-shell. Through vertical transmission, bacteria are introduced from infected reproductive tissues to eggs prior to shell formation. This form of transmission is mostly associated with pathogenic bacteria, namely Salmonella. Horizontal transmission usually occurs from faecal contamination on the egg shell as the eggs are released via the cloaca, where the excretion of faeces also takes place. It also includes contamination through environmental vectors, such as farmers, pets and rodents.

Bacteria may enter through pores or cracks on shell of eggs. Although there are numerous pores (ranging from 6000 – 10000) on the egg shell, there is an outer layer of protection called cuticle that can help to retard the penetration of bacteria on egg shells. As eggs with cracks on the shell allow the entry of bacteria into the egg content, cracked eggs should be removed from sale. In addition, bacteria may contaminate egg contents at breaking.

Bacterial Count and Food Safety

In the study concerned, the numbers of bacteria on the shells and inside the eggs were count. Aerobic colony count is a count of viable bacteria, which includes those that occur naturally in most foods and those present through contamination, based on counting of colonies grown on nutrient agar plate. The bacterial count is employed to indicate the sanitary quality but not safety of foods, because it does not indicate the presence of pathogen in the tested food samples. In addition, it is not surprising to find bacteria on the surface of the shell eggs since eggs can be tainted with bacteria during release and following exposure to the environment. The total numbers of bacteria found on the shells in the study were up to about 1.5 million colony-forming unit (cfu). However, bacteria normally exist in the environment. For instance, different areas of human body may have bacterial counts varying from ten thousand to one million cfu per cm².

Should Eggs be Washed ?

Since there may be plenty of bacteria on egg surface, should eggs be washed? Shell eggs need not be washed as any process that wets the shell may facilitate the entry of microorganisms, presumably due to water entry through the pores together with microorganisms by capillary action. Water left on shell surface may also enhance the survival of microorganisms on egg shells. In addition, improper washing may damage the cuticle.

Many eggs sold on the market have actually been cleaned during commercial egg processing. The cleansing work needs to be conducted in a controlled manner in order to minimise the entry of microorganisms. The eggs are washed as soon as possible after they are laid and eggs with cracks are removed. The surface of the eggs is sanitised with special detergents that do not damage the cuticle. Water used for washing should be warmer than the temperature of eggs, but not too hot that will damage the cuticle. Cooler water will facilitate the entry of bacteria into the eggs. After washing, the eggs are dried immediately. In some cases, the eggs may be sprayed with a layer of mineral oil to replace the original cuticle that may be lost during washing.

Advice to Consumers

• Purchase eggs from reliable and reputable suppliers. Observe the expiry date and storage temperature on the package/label of eggs.
• As a general rule, shell eggs need not be washed. However, if eggs are soiled with faecal matter, they can be washed (with household detergent if required). Washed eggs should be used immediately.
• Avoid eating raw or inadequately cooked eggs and egg products, particularly so for the elderly, infants and pregnant women. Eggs should be thoroughly cooked until the yolk and white are firm.

Advice to Trade

• Store shell eggs under refrigeration.
• Avoid cross-contamination between raw eggs and other food. The food contacting surface and utensils used for preparation of raw eggs should be cleaned thoroughly.
• Choose pasteurised eggs products or dried egg powder to prepare dishes not requiring heat treatment, in particular ready-to-eat desserts.
• Avoid using cracked eggs as they are more likely to be contaminated and thus present a higher health risk.