Financial Times

Good food, bad food debate
More facts about eggs
The little knowledge gained from working for about eight years as a microbiologist in two major integrated poultry operating companies in Sri Lanka has prompted me to write this short note on The Sunday Times FT article recently headlined “Eggs covered with poultry dung.”

Salmonella and eggs:
The egg of a hen is an excellent product that normally is well protected. Externally, a fresh egg has three structures each of which is effective to some degree in retarding entry of microorganisms.

The most nutritious and susceptible component of the egg to microbial spoilage is the yolk. Though the freshly laid eggs are generally sterile, within a short period of time after laying, numerous micro-organisms may be found on the outer side of the egg depending on the cleanliness of the pen and handling.

The entry of micro-organisms into the whole egg is favoured by high humidity, under which conditions, growth of the micro-organisms on the surface of eggs is favoured, followed by penetration through the shell and inner membrane.
More bacteria are found in the yolk than in the egg white because egg white has antibacterial constituents. Upon storage, the thick egg white loses water to the yolk, resulting in a thinning of yolk. Once the bacteria is in the yolk, they grow and produce by-products of protein and amino acid metabolism such as hydrogen sulfide and other foul-smelling compounds.

The effect of significant growth of bacteria in the yolk causes the yolk to become “runny” and discoloured. Depending on the type of bacteria growing in the yolk, spoilage could be called “green rot”, “pink rot”, “colourless rot”, “black rot” “red rot” and “custard rot”.

I used a simple technique; agar moulding technique, to determine the amount of shell penetration occurring in naturally contaminated floor egg. The technique involves the removal of the content of the egg through a small hole made on one end of the egg and filling it with a sterile bacteriological medium in which a colourless dye’ tetrazolium salt has been incorporated.

After closing the hole, the shells with its agar medium are incubated for 48 hours. The red spots on the inner membrane (after breaking the shell) shown in the photographs are caused by the microbial reduction of tetrazolium salt.
When a bacteria grows on a medium in the presence of this dye, the colour of that place where bacteria grow turns red. These eggs were purchased from shops in the Kiribathgoda area which included eggs from a super market as well. Though the eggs look apparently clean, bacteria had penetrated through the membrane. When the shell is highly contaminated with poultry dung, the number of penetrations could also be high.

Reports from various countries indicate that 2.6% - 7.0% of eggs are contaminated with Salmonella, particularly S.typhimurium. Duck eggs have been reported to have higher contamination rates reaching 20%.
The situation with S.enteritidis is different; the bacteria can invade the ovaries and oviducts of chicken with the result that the contents of the intact egg may be infected by transovarian infection.

A few years back, some blood serum samples taken from poultry breeding flocks (in Sri Lanka) showed positive results when tested for antibodies against S.enteritidis using ELISA technology (Enzyme Link Immunosorbant Assay). This means that our poultry operations are not S.enteritidis free. Most egg associated infections have involved either consumption of raw eggs or of products made with raw eggs such as ice cream and by consumption of lightly or “hard” cooked eggs.

Almost all Salmonella species are destroyed by normal cooking but when an egg is boiled, the yolk of egg is unlikely to reach a sufficiently high temperature to inactivate the bacteria until after about 7 minutes of cooking. This means that domestically cooked eggs with “runny” yolk may contain Salmonella bacteria.

All species and strains of Salmonella may be presumed to be pathogenioc for man and it remains the most important reported cause of food poisoning. And contamination through hen eggs is one of the major vehicles of infection. The primary habitat of Salmonella is the intestinal tract of animals such as birds and reptiles, farm animals and man.

Different species of Salmonella are responsible for diseases of man and animals. Salmonella pathogenicity to man affects in three ways;

* Salmonella species adapted and cause diseases only to man such as S.typhi and S.paratyphi which usually cause serious disease with septisemic typhoidic syndrome (enteric fever) – water borne and person to person transmission being more important.

* Salmonella species that are ubiquitous such as S.typhimurium affect both man and animals causing gastrointestinal infections of varying severity. These are involved in infantile and travellers diarrhoea.

* Salmonella species which are highly adapted to an animal host, such as S.gallinarum in poultry usually produce mild symptoms in man.

D.C. Hettiarachchi
Consumer from
Kiribathgoda

Back to Business