The wonderful world of eggs :)

Well to make my revision more interesting I'm going to write features for each of the powerpoints in public health :). At least then it'll feel like less of a chore and hopefully I can make it more entertaining :D Lecture notes all taken from University of Liverpool Powerpoints, and I owe much of the content and some of the pictures to them :). 

Eggs

Firstly we will look at the reproductive tract of the hen. The picture below shows the typical laying hen reproductive tract. The egg typically takes 24-48 hours to develop. 

• Yolk with attached ovum develops in the ovary. The colour of the yolk comes from additives to the food of the chicken. The yolk is surrounded by the vitelline membrane. This is known as the zona pellucida in mammals. The fresher the egg, the stronger this membrane will be and it holds the yolk in place. The colour of the yolk can range from deep yellow to orange. It is a major source of vitamins and minerals. 
• Vascularisation in the yolk allows the yolk to develop. This recedes when the yolk is ready to be released further down the reproductive tract. A remaining blood spot is one of the defects that can be spotted upon sometimes cracking the egg. 
• The infundibulum is where the spermatozoa would be stored in the hen, and this would be where the egg would be fertilised if the hen had been mated. 
• As the yolk is released and continues down the reproductive tract pictured above, albumin is added in the magnum. This provides support for the egg yolk, antimicrobial agents and can also result in the occasional meat spot which is a part of the reproductive tissue which remains attached to the yolk.
• The isthmus is part of the oviduct. The shell gland is where the egg shell is formed, and the egg remains here for the longest period of time. 

Egg Structure

• The fresher the egg, the smaller the egg space. This can be used in candling to determine how fresh the egg is.
• The shell membranes are the second line of defence against bacteria. 
• The fresher the egg, the more noticeable the chalazas. 
• The fresher the egg, the more firmly the albumen (egg white) stands up firmly around the yolk when the egg is broken. The white is not watery. 

Egg Antimicrobial Defences

• The cuticle (outer egg) is made of glycoprotein and covers the shells pores. 
• The shell is composed mainly of calcium carbonate.
• The inner and outer membranes are thin layers of protein fibres. 
• Albumen viscosity impedes microbe movement. It has a PH of 9.2. 
• In the egg albumen; avidin-biotin system, lysozyme has antibacterial properties about gram positive bacteria, ovotransferrin is antibacterial, and ovoinhibitor inhibits fungal proteases. 

Eggs- Control & Public Health

The most important bit...?

Things can go wrong which affect egg production:

• Infertility.
• Disease (Infectious Bronchitis, Salmonella, Collibacilosis).
• Nutritional deficiencies.
• Stress.

The problems with egg production are multifactorial. 

There are a number of health risks associated which eggs which include:

1. Salmonella Enteritidis in vertical transmission (parent to offspring).
2. Contamination from microorganisms.
3. Residues- pesticides, PCBs, anthelmintics, coccidiostats and antibiotics. 

With a number of problems associated with egg production, there is a need for control. 

Salmonella is controlled on a farm level. Other factors are checked before marketing- contamination, shell integrity and 'age' of the egg. 

Salmonella Control

Salmonella is generally controlled at farm level. Generally aimed at the S.Enteritidis and S. tymphimuriumserovars. The national plan covers flocks with more than 350 birds. Samples are taken at a day old, two weeks before point of lay or being moved to laying accomodation, a further sample between 22 to 26 weeks of age and then at 15 week intervals during the laying period (N.B. laying age of hen usually ends at 52 weeks or around that figure). This was implemented from 2009. The target was to reduce the prevalence of salmonella by 10% each year. Fresh eggs from positive flocks can no longer be sold direct as table eggs for human consumption unless they are heat treated. 

Risk factors on farms included infected breeding stock and vertical, horizontal and airborne transmission. Also feed, wildlife, bad biosecurity, the people, rodents and so on. The main risk factors found were lack of attention to biosecurity and rodent control. Therefore protective clothing, footbaths and rodent control was implemented alongside strict biosecurity protocols. 

Egg Contamination 

• Faecal/environmental.
• Vertical transmission, damaged egg shell and cuticle, storage conditions or time of storage. 

Control of egg contamination:

• Eggs from infected farms, damaged eggs, old eggs, and dirty eggs all go for further processing.
• Temperature storage is at 20 degrees. 
• Storage can be no longer than 28 days. 

Further Egg Controls

• Egg marketing inspectors.
• Different egg categories based on shell and cuticle cleanliness, air space height, germ cell development, egg yolk location and foreign bodies, cleaning, smell & refrigeration. 
• Compulsory egg stamping with an individual producer code and method of production (O- organic, 1- free range, 2-barn 3-cage). Stamping and labelling of third countries sold at retail. 

Double Yolker :)

Egg Quality Faults & Abnormalities

1. Double Yolked Eggs. 
2. Blood Spots- in or around the yolk. Caused by one of the tiny blood vessels in the ovary breaking at the time of release. Free range hens have some protection from this due to rooting. High disturbance increases incidence.
3. Meat Spots- brown and associated with the albumen. More likely with brown laying hens and are small pieces of tissue from the oviduct. 

See meat spots in the albumen and blood spot in yolk

4. Watery Whites- Older birds, old eggs or with viruses such as IB. 

5. Abnormal Yolk Colour- green yolk due to excess green herbage.

6. Mobile and Bubbly Airspace- shell structure fault/abnormality. Airspace in albumen caused by ruptured inner membrane.

7. Bacterial/Fungal Contamination- black, green or yellow rots. Pass through egg shell. 

8. Misshaped Eggs- viral diseases.

9. Coated Shells- additional calcium causing calcium splashing or a lilac colour if the egg remains in the shell gland in young flocks for an extended period. 

10. Rough Shelled- Two eggs in the shell gland at the same time.

11. Pale Shelled- Brown laying hens, older hens in free range flocks or certain diseases.

12. Cracked Shell- Inadequate shell/poor shell handling- a common reason for downgrading eggs.

13. Dirty Shell- contamination.

Egg Processing

• Pasteurisation- reduced initial bacterial load. 69 secs @ 100 degrees. 5 day shelf life. 21 days shelf life in clean packs. 
• Drying- reduces water activity.