THE RELATION OF THE CANDLING APPEARANCE OF EGGS TO
THEIR THICK WHITE PERCENTAGE
By T. S. KRISHNAN, B.A., M.Sc., A.I.C., Poultry Research Section, Imperial Veterinary
Research Institute, Izatnagar
(Received for publication on 27 July 1942)
(With Plate XI and one text figure).
IN the modern marketing of eggs, grading is of
primary importance. The purpose of grading is
to classify eggs according to their size, external
appearance and interior quality. Effective grad-
ing enables the trade 'to quote prices based on
comparative values and offers an incentive to the
producer to give the best possible care to the eggs.
The consumer's reaction to the dependability of
quality is likely to outweigh his hesitancy about
buying the higher priced grades. Grading also
enables persons in the trade to sort the eggs in
accordance with the purpose for which they are
best suited. Sorting according to size and appear-
ance is comparatively easy but to classify eggs
according to their interior quality is not so simple.
Several methods for judging the quality of broken
out eggs are available but there are not so many
for grading eggs-in-shell according to their interior
quality. Those suggested by Wilcke [1936] and
Carr [1939] have not found any wide acceptance,
while the ' floatation test ' is somewhat crude.
The only method that is in general use is ' candl-
ing '. This makes possible rapid grading for trade
purposes.
In this method, the egg is held with the end
containing the air cell (usually the broad one)
uppermost, in an inclined position, in front of the
opening in the candling device (Plate XI, Fig. 1)
through which a beam of strong light issues. The
egg is then given a rapid twist to set the contents
rotating. This enables the observer to see all the
egg contents silhouted against the light. The
operation is best carried out in a darkened room
as the light then shows up the moving egg contents
more clearly. The size and condition of the air
cell, the shadow, size, outline, position and mobility
of the yolk, as well as the nature of the white can
be easily discerned. Cracks and checks are noted.
Meat spots, blood spots, developed embryos,
moulds, rots and other egg faults are also observed.
In a good fresh egg, the air cell should be small,
firm and round ; the yolk should be centrally
placed and should cast a pale shadow with a very
indistinct outline ; the white should be firm and
clear. The movement of the yolk should be free
but slow. No evidence of embryo development,
foreign bodies or other egg faults should be present.
As the interior quality of the egg deteriorates with
age or other cause, the air cell generally increases
in size, the yolk enlarges and the thick white
liquefies. Consequently the yolk approaches near-
er to the shell and casts a darker shadow, the out-
line of which becomes progressively more distinct
with loss of quality. Its movement, too, becomes
at first very rapid and later on sluggish. If the
egg is fertile and has been exposed for some time
to high temperatures, the embryo begins to develop
and shows either a developed embryo spot, a
blood ring, a dead germ or blood veins. This
method of judging the interior quality of eggs is
not by any means perfect but in experienced
hands 'candling' generally proves sufficiently
satisfactory for commercial purposes. Very good
and very poor eggs are separated with comparative
ease but intermediate grades are usually more
difficult to classify.
Several factors influence the candling appearance
of eggs. One of the most important is the effi-
ciency of the candling lamp. The design of the
candling lamp determines the efficiency of the
illumination of the egg contents by influencing
the amount, intensity, direction and colour of the
light used. As Graham [1934] has put it, 'To
solve the problem of candling, the help of the
physicist in devising a suitable lamp is indispen-
sible '. Doan [1934 1/2/3] has done a considerable
amount of work on candling equipment and has
devised a lamp [1936] which is said to increase
internal egg visibility and also relieve the strain
on the eyes of the operator. It does not, however,
allow the high speed grading usually attained with
the standard commercial types. Wilhelm and
Payne [1934] have also tested the efficiency of
different types of candling lamps fitted with clear,
frosted or red bulbs. The two latter were found
to be slightly more efficient than the first. Benja-
min and Pierce [1937] also refer to a candler
for accurate research work, developed by Shackel-
ton and Powell, in which a transparent colour scale
is used. This is said to enable the operator to judge
the internal quality more accurately. Almquist
[1933] worked out in detail the relationship of the
candling appearance of eggs to their actual internal
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