External Quality of Eggs

EXTERNAL QUALITY OF EGGS

  • A normal chicken egg is ovate in shape.
  • Wide variation in shape may lead to breakages during handling and transport. For easy transportation and merchandising of egg, the egg should possess normal external characteristic. If shape and size are abnormal, the eggs are liable to break.
  • Similarly, thin shell and weak shell egg or shell less eggs are liable to break and are unfit for transportation.
  • Moreover, eggs with thin shells loose their quality quickly.
  • Shell colour and other external characteristics will determine the market acceptability. Clean eggs will be sold at premium price, than dirty eggs.
  • The external characteristics of the egg will be assessed by the following methods:
    1. Egg size/ weight
    2. Shape
    3. Shell colour & Texture
    4. Cleanliness
    5. Volume
    6. Specific gravity
    7. Surface area

Egg size or weight

  • Each species of bird has its own standard egg weight.
  • Similarly the egg weight vary between breed and age of the bird.
  • Heavier birds produce heavier egg.
  • A normal chicken egg weighs 55-60gm depending upon the breed and age.
  • The chicken egg will be about 1/30th of the hen's body weight.
  • In case of ducks, the egg weight ranges from 65-70gm, depending upon the breed.
  • When compared to chicken, ducks lay heavier eggs which will weigh 1/25th of its body weight.
  • Goose lay eggs weighing 130-200gm, depending upon the breed.
  • Japanese quail eggs will weigh around 10gm, which will be about 1/15th of its adult body weight.
  • When compared to body size, Japanese quail lay heavier egg than other species.
  • Turkey egg will weigh 65-70g, ad it is only about 1/60th of its body weight.
  • In all species of birds, older birds lay heavier eggs than younger birds.
  • Record the weight of egg provided to 0.1gm accuracy using triple beam balance.
  • Some times the egg size will be extremely small or large as in the case of yolk less/ double yolked eggs which are difficult to transport because they will break during transit.
  • They are sold in the farm itself.

Shape

  • The usual egg shape is "ovate".
  • The shape of the egg plays a major role in packing and transport.
  • The normal shape of an egg can be marred due to diseases like Ranikhet and Infectious Bronchitis.
  • Too small or too large eggs are discarded in the farm itself. Egg shape is expressed as "Shape index'.
  • Here the process is to measure the maximum length of the egg using a vernier caliper and also the average width of the egg measured in two places.
  • It is measured to an accuracy of 1mm and the shape index is arrived at by using the formula

Shape index = ( (Average width / Average length) x 100

  • A normal egg will have a shape index of 72 (Range 70-74).
  • Egg which is spherical in shape will have a shape index of 75 and above; such of those eggs which are elongated/ elliptical will have lesser shape index of 70.
  • These eggs which do not fall into the normal range of the shape index cannot be used for hatching.

Shell colour and texture

  • It indicates smoothness and roughness of shell surface and also indicates shell quality.
  • Shell colour is due to the presence of pigments. Ooporphyrin gives brownish colour to the egg shell, which is normally seen in eggs laid by the Asian, the English and the American Class of birds.
  • The pigments Oocyan causes other blue colour in eggs seen in eggs laid by the breed Aracauna.

Cleanliness

  • This is essential for consumer satisfaction and also to improve and maintain the keeping quality.
  • A dirty egg may harbour harmful microbes which will spoil the egg and render it unfit for consumption.
  • Eggs collected from deep litter will be more dirty than caged eggs, obviously due to dirty wet litter and delayed collection of eggs

Volume

  • Volume is also one of the indicators of egg size.
  • Egg volume is directly proportional to the egg size.
  • Since the specific gravity of egg is more than one, the volume of a fresh egg will always be less than the egg weight. However a small egg will have greater volume by weight ratio.
  • The egg volume of different species will be around 90-95% of their fresh egg weight.
  • To measure the volume of the egg, fill a measuring cylinder of 500 ml/ 1 litre capacity with a known quantity of water.
  • After noting the lower meniscus of the water, gently slant the measuring cylinder and slide the egg carefully into the measuring cylinder and note the final reading of the water, the difference in value will give the volume of egg in cm3.

Specific gravity

  • This gives an indication of the egg shell quality, as well as its freshness.
  • Fresh eggs will have higher specific gravity than old and long stored eggs, because there will be a loss of moisture in the old eggs which inturn replaced by air.
  • So the air cell will become bigger as the egg is stored for a longer time.
  • Similarly, eggs having stronger shell will have higher specific gravity than thin shelled eggs.
  • The measure of specific gravity can be made by several methods.
    • By measuring the egg weight and then weighing the egg in water, to find the weight loss in water.
    • By dipping the eggs in a salt solution having several concentration of salt dissolved in it; having a specific gravity ranging from 1.0-1.1 with an interval of 0.02. A normal egg will have a specific gravity of 1.06. Any value less than this may indicate that the egg is old or the eggs are thin shelled; irrespective of other criteria, smaller eggs will have high specific gravity due to more uniform shell.
    • By knowing the egg weight and volume.

Specific gravity = Weight of the egg in gram / Volume in cc

Surface area

  • It is calculated by using the formula

Surface area = 12.6 x ( (length + width) / 4 ) 2

  • Where, 12.6 is a constant.
  • Surface area of an egg is directly proportional to egg size. The surface area will be more for elongated eggs than for spherical eggs.
Last modified: Thursday, 12 April 2012, 7:37 AM