updateMicrobialeggs &

egg products

10 International Food Hygiene — Volume 25 Number 4

Eggs are produced by most higheranimals as a part of their reproductivecycle. In terms of human food, it is

eggs from a variety of birds that we considerto be a good food source. The apparentsimplicity of the egg, a hard shellsurrounding a liquid centre, hides itscomplex nature which makes it a versatilestable food in its own right, or foodingredient used widely within the industry.

Egg structure

The egg shell is constructed of forms ofcalcium carbonate (approximately 95%)together with a stabilising protein matrix.The protein helps in maintaining thestructure of the shell and in reducing theeffects of physical damage. The shell is keyto maintaining the contents of the egg ingood condition. It prevents entry of micro-organisms into the albumen and yolk, and itfacilitates both gas and moisture transfer tothe egg contents, keeping the egg ‘fresh’ fora considerable period. Comparing eggs withother raw animal derived foods, there arefew others that remain fresh at ambienttemperatures over considerable periods oftime, without the help of considerableamounts of artificial preservatives. The egg isquite an amazing structure.The inner liquid parts of the egg are made

up of the ‘white’ (albumen) and the yolk.The white is made up of approximately 90%water into which is dissolved a variety ofproteins (approximately 10% of the white).The white contains virtually no fat and onlyabout 1% carbohydrate. After the shell, thewhite is the next level that the egg hasagainst microbiological attack. It contains arange of potent antimicrobial agentsincluding transferrins (these bind ironmolecules that are a key requirement formicrobial growth) and the enzymelysozyme.Lysozyme is a key antimicrobial enzyme

that has evolved to break apart the hardouter wall of bacterial cells (made up of thepolymer peptidoglycan). Lysozyme does notsimply prevent microbial growth, it killsbacteria by breaking their outer defensivewall.Egg yolk is composed of approximately 4%

Dr Roy Betts, Head of Microbiology,Campden BRI, Chipping Campden,Gloucestershire, GL55 6LD, UK.Produced as a service to the food industry by Thermo Fisher Scientific.

carbohydrate, 25% fat, and 10-15% protein.In a fertilised egg it would make up the mainnutritional component for the developingembryo. Its distinctive colour comes from anumber of carotenoid pigments that itcontains. The yolk can contain largenumbers of antibodies directed againstbacteria and this forms part of theprotection that eggs have against bacterialattack.

Egg microbiology

It has been stated (Board 2000) that eggscan become internally contaminated bymicro-organisms in three ways. l Trans ovarian infection – resulting fromthe yolk within an ovary follicle beinginfected, usually with organisms from theblood of the bird.l Oviductal infection – where the egg whiteis infected during its movement along theoviduct. Organisms can be introduced intothe oviduct via either the bird’s vascularsystem, or by upward migration from thebird’s cloaca, or via practices involved inartificial insemination.l Trans-shell infection. The egg shell will becontaminated immediately on laying bycontact with dust, faecal material and otherenvironmental agents. It is known that thelonger the egg is in direct contact with suchcontaminated material, the greater thechance of internal contamination occurring. This is due to the large number of pores

within the shell, which under the rightconditions can transfer organisms across theshell into the egg interior. This transfer canbe accelerated if eggs are placed in water. Warm eggs placed in cold water result in

an internal vacuum which can draw thewater (and any contaminants on the outside

of the shell) inside the egg, likewise anypressure changes can result in transfer intothe egg.

Egg spoilage

The main cause of egg spoilage arises fromthe growth of Gram negative bacteria withinthe egg contents. There have been manystudies on the types and causes of egg ‘rots’and Board (2000) gives excellentdescriptions of the various forms of eggspoilage.If an egg becomes internally contaminated,

there will be a lag before grosscontamination of the egg contents becomesapparent. In eggs contaminated by the trans-shell route, this lag period can be betweenseven and greater than 20 days in length. There are numerous reasons that have

been suggested for this lag time, but it islikely to be due to changes in the eggstructure during storage, making conditionsmore favourable to bacteria growth,perhaps together with some biochemicalchanges within the bacteria themselveshelping overcome the egg’s defensivemechanisms.

Pathogens and eggs

The predominant human pathogen that isassociated with eggs is salmonella. Thispathogen can be associated with eggs in twoways:l It can be found on the egg shell, whichcan become contaminated from the bird’sfaecal material.l It can be found within the egg contents,which become contaminated when the eggis being formed within the bird. This route is

known as trans-ovarian infection. Shellcontamination can be controlled to someextent by ensuring that laying areas/conditions are hygienic, and by carefullycontrolled washing of eggs (although greatcare has to be taken to avoid externalcontamination being drawn into the eggduring the washing process). Trans ovarianinfection can only be controlled by ensuringthat birds are free from salmonella.During the late 1980s and early 1990s, the

annual incidence of reported humansalmonellosis in the UK was at a record highof nearly 40,000 cases.A large majority of these were due to

S. enteritidis originating from poultry andeggs. At that time action was taken bypoultry and egg producers, a vaccinationprogramme against salmonella, for poultrybreeding flocks ( beginning in 1994) and egglayers (beginning in 1998), has resulted in adrastic reduction in contaminated poultryand eggs. The results of this have been a huge

success in reducing levels of contaminatedeggs. A comparison of surveys of Salmonellacontamination in raw shell eggs done for theUK Food Standards Agency in 1995 and2003, show a three fold reduction incontamination over that period. The resultwas a dramatic decrease in cases of humansalmonellosis from those seen in 1990(approximately 40,000 cases) to that seen in2012, of just over 8,000 (confirmed figuresfor 2013 are likely to be closer to 7,000cases). In the UK eggs produced from vaccinated

flocks will bear the ‘Lion Mark’, and therehas been concern, fuelled by a number ofsalmonella outbreaks, that eggs importedinto the UK may have higher rates ofcontamination.In 2006 a report on a UK FSA surveillance

study on eggs imported into the UK, clearlyindicated that imported eggs from somecountries had a higher incidence of

salmonella contamination than UK producedeggs.It is obviously the responsibility of egg

users to understand the risks involved inobtaining shell eggs from various sourcesand take appropriate management strategiesto minimise the risk to the end consumer.

Egg products

The term egg products covers a wide rangeof food types that would be impossible tocover in any detail. In the commercialmanufacture of any product containing egg,the main consideration will be what formthe ‘egg material’ is in when used. Fewindustrial producers would use shell eggswithin their products, most will user eitherpasteurised liquid egg or powdered egg. Pasteurisation of eggs should provide an

ideal way of extending shelf life byeliminating spoilage organisms and ensuringfreedom from salmonella. However, pasteurisation has to be very

carefully controlled as higher temperatureswill coagulate egg proteins and make anunusable product. Different countries willhave different guidelines or standards onrequirements for pasteurisation, and theseshould always be consulted whenconsidering use of pasteurisation for eggs. The required pasteurisation temperatures

for whole liquid egg will lie between 60°Cand 65°C for between 2.5 and 3.5 minutes.The higher temperatures will destroy theenzyme amylase, the absence of which canthen be used as a marker for correctpasteurisation. The addition of salt or sugar to the liquid

egg before pasteurisation can increase theheat resistance of micro-organisms, andcare must be taken to ensure that thepasteurisation process is sufficient for suchproducts. Egg white is more sensitive toheat treatment than other egg products and

generally is treated at lower temperatures.Its higher pH, however, means organismssuch as salmonella are more sensitive toheat and the lower temperatures can givean acceptable kill. The complexity of the pasteurisation issues

can be appreciated when it is understoodthat the pH of the white can vary with theage of the egg; faster shipment of eggs fromfarm to processor can result in lower pHand less antimicrobial effect at a giventemperature. Generally pasteurisationprocesses for liquid egg are designed to givean approximate 4-5 log reduction ofsalmonella. It is recommended that anyoneconsidering pasteurising egg or usingpasteurised egg, consults local national orinternational guidance on egg heattreatments and obtains an assurance thatany process used has been correctlyvalidated by a competent organisation thatunderstands egg processing requirements.

Conclusions

Eggs are an excellent food, nutritious, easyto use, and with a excellent natural shelf life.We know that they are prone to spoilage bya range of micro-organisms, but goodproduction, transportation and distributiontogether with chilled storage in the homecan minimise this issue. With respect to microbiological food

safety, the main issues with eggs have alwayscentred on salmonella. In the late 1980s theUK had a large and increasing problem withhuman salmonellosis which was largely dueto contamination of poultry and eggs. Intervention actions based on vaccination

of laying flocks against salmonella hasdrastically reduced this problem, makingBritish produced eggs far less likely tocontain the pathogen than they were twodecades ago. It is, of course, stillrecommended that eggs are properlycooked before consumption to lower therisk still further, but it is fair to say that eggsnow form a very safe and nutritious foodsource. A major concern has been that eggs

imported into the UK may not have beenproduced under the same stringentconditions as UK eggs and may have agreater chance of containing salmonella. A number of outbreaks of food poisoning

have been linked to imported eggs, andsurveillance would tend to back this up. It isup to users to ensure that they are aware ofthe risks from the shell eggs they use, andare taking appropriate actions to minimiserisk to consumers.Pasteurisation increases the shelf life of

liquid egg and provides a pathogen free rawmaterial for production of a range of eggproducts. Producers and users shouldalways ensure that the pasteurisationprocesses used comply with local guidanceor standards and gain expert help invalidation of such processes. nb food.testing.admin@thermofisher.com

International Food Hygiene — Volume 25 Number 4 11

Bacterium Type of egg spoilage

Pseudomonas fluorescens Pink rot

Ps putida Fluorescent green rot

Ps aeruginosa Fluorescent blue rot

Stenotrophomonas maltophilia Green rot

Aeromonas liquifaciens or Proteus vulgaris Black rot

Aerobacter spp. formally Cloaca spp. Custard rot