Screening ovaries means singling out each individual egg, i.e. performing
the procedure which makes roes into raw material for caviar type products.
There are several screening methods. However, the one used in most places
is manual dry screening. This is particu-larly true for the valuable salmonid
roes and certainly, nobody would ever mechanize screening of the expensive
and fragile sturgeon roe. Manual screening ensures an indi-vidual approach
to each ovary and hence minimal technological losses. A skilled worker
can screen manually 100 to 200 kg roe hourly. The time spent to screen
eggs constitutes only a fraction of the total processing time. The need
to mechanize the screening proc-ess may occur when caviar is processed
on board or when the landings are huge. Mechanization of screening is
used widely for lumpfish, pollack, cod and partially for salmon roe.
3.1 SCREENING OPERATION
Essentially intact ovaries free of any debris and blood clots are screened
after rinsing and draining. Before screening starts there is a final opportunity
to downgrade ovaries, or separate ones which are not of the same grade
as the lot. E.g., it might be worthwhile to segregate ovaries damaged
by the knife during extraction. If these ovaries are handled separately
less broken egg membranes would be generated while screening. To elimi-nate
impurities such as broken egg membranes, residuals of connective tissue,
or lumps of eggs still connected to connective tissues after screening
is finished, is a very difficult task, especially in salmon caviar processing.
Broken salmon egg membranes interfere with the processing and a lot of
effort is needed to get rid of them. These impurities require time consuming
operations (frequent washing of screens, using mesh paddles during brine
salting, letting the debris float while rinsing and decanting debris [sturgeon
caviar], letting the skeins sink in 100% brine before eggs are scooped
off [salmon caviar] from the salting tank, using tweezers to pick up manually
broken egg membranes during packing) for their elimination.
Thus, it is obviously advantageous in the first place not to generate
these impurities and to wash all tools as often as practically possible.
To diminish the rupture of eggs during screening it is advisable to bring
down the ovary temperature, say to 3 to 5°C. This will strengthen
weakened eggs. Cooling is done by rinsing ovaries in ice cold water or
better, light brine, or keeping them in cold storage. Cooling does not
improve egg quality, it only increases the yield. Presalting weak ova-ries
for 2-3 minutes in 100% brine before screening will strengthen them. Presalting
makes it easier to single out eggs from the connective tissue. Finally,
before screening the ovaries should be torn apart into two or more large
pieces. The outer ovary elastic film which envelops the eggs should be
opened up in order to expose all the ovary fragments (lobes) to the screen
surface. Otherwise the outer smooth ovary film will slide over the screen
and protect the eggs from being 'rubbed off by the mesh.
If there are large numbers of impurities the salted salmon eggs could
be run through the screening device for a second time.
3.2 SCREENING TECHNIQUE
The operation of singling-out individual eggs is called screening. It
starts by gently pushing the eggs toward the meshed screen so that the
eggs, still attached to the connec-tive tissue, stick out below the screen,
(see Figure 3-1). A gentle rubbing or rotating movement, combined with
light pressing, tears apart the connective membranes and frees the eggs
which fall through the screen. The connective tissue entangles in the
screen mesh. Overly harsh handling of weak eggs (immature and fatty or
not fresh ova-ries) results in excessive breakage. In this case the eggs
release interior liquid and the broken egg, becomes a whitish empty egg
membrane, which spoils the product. It is usual to collect the falling
eggs into a tray or basket which drains off excessive liquid.
FIGURE 3-1: Screening
a - Ovaries placed film up, b - Eggs pushed down through the screen
c - Eggs singled out by rotating ovaries
Screening requires considerable skill and experience. While the screener's
hands per-form rotating movements in alternating directions, the fingers
hold the ovaries lightly and measured pressure is applied using the palm.
By this technique the eggs are singled out but not broken. Screening is
executed with bare hands or using knitted cotton gloves. Sturgeon ovaries
are screened with bare hands only. To keep hands dry, sanitary latex gloves
may be put under the cotton gloves. While rotating, the ovaries are moved
all over the screen using spots which are not yet soiled with detached
debris. If screen-ing is done over the same spot for a long time, debris
could be squeezed through to-gether with the eggs. As often as possible
the debris should be removed manually or with a vacuum nozzle. To do this,
one should quickly rotate the debris until it forms a lump followed by
tapping the screen and pulling the lump off. This does not clean the screens
of small fractions of the debris and broken egg membranes. For further
cleaning, screens are removed. A clean spare screen is put in place and
the soiled one is passed on for high pressure jet cleaning. To save eggs
which stick to equipment surfaces or screens, light brushes or feather
whisks are used. Although light tapping of the screens makes the attached
whole eggs fall down from the first two screens and roll over the slope
screen, such tapping may introduce excessive debris into the lot. A better
solution would be to recover eggs and lumps still attached to the screens
separately for bait pro-duction. The described mechanism of egg detachment
is the same when screening is performed by machines. Machine screening
always generates larger losses.
Mechanical screening of salmon roe is rarely used. The prevailing method
for salmon roe screening remains manual, because of the relative fragility
of the eggs. Only small size and strong eggs of the less valued roe e.g.
lumpfish, pollack, herring, cod - are screened mechanically.
3.3 SCREENING DEVICES
Historically screens were made, and continue to be made, of wooden frames
and cotton braided twines. This does not comply with Canadian Regulations.
The components for screen frames and the parts of screening devices should
be fabricated from materials such as aluminum alloy, stainless steel and
suitable plastic materials, to be rust-free and durable against frequent
In our laboratory the screen is made of stainless steel mesh products,
woven from round wires or perforated sheet metal. It could also be made
from single nylon monofilament weave or multiple twines. The mesh materials
used for screens, both wires or twines, should not have knots or be welded
at the intersections to avoid damage to the eggs. Nevertheless, some processors
claim not to have excessive damage when using the least expensive non-woven
square mesh wire cloth, which is point welded at the wire inter-sections.
This mesh wire cloth is not convenient because the hands slip along the
upper wire layer, see Figure 3-2a. At all times the screen has to be stretched
perfectly flat. Many screen designs provide screen stretching means, which
take out the slack which may occur.
Typical salmon caviar screen material is shown in Figure 3-2. The square
mesh wire cloth is formed from precrimped wires. When screens are fabricated
from cotton braided twines or nylon monofilament weave they should have
means to stretch them. Stretch-ing is done by tightening bars to which
the mesh cloth is attached.
FIGURE 3-2; Mesh Wire Cloth Types
Sturgeon roe is screened only through one screen. Figure 3-3. Singled
out eggs run through a metal funnel or straight to a bowl where eggs are
later rinsed and, after drain-ing, passed on for salting in similar bowls.
Salmon roe is more susceptible to breakage, generates more debris and
the screening device consists principally of three screens, which require
fabrication of a special stand or device. There are many designs. Figure
3-4. All three screens are easily removable for washing. The main upper
screen is usually at least 1x1 meters to provide, space for two workers,
FIGURE 3-3: Sturgeon Caviar Screens
FIGURE 3-4: Salmon Caviar Screens
The role of the second screen, located 15-20 cm lower, is to catch debris
and unscreened egg lumps. It is conceivable to have a third flat screen
to catch further debris. These lower screens should also be kept clean
at all times, otherwise eggs may start to pile up. The falling eggs roll
over the third slope screen towards the collecting basket. While rolling
down, the eggs are freed from excessive interior liquid generated by broken
eggs. This yolky liquid is directed to the drain by a tray which keeps
the place clean. Between the third screen and the tray there is a 1-2
cm clearance. While rolling down the slope screen, the eggs get rid of
the remaining debris and broken skeins. The slope angle is adjustable
30 to 50° to regulate the egg movement. It is essential to keep this
screen clean and extend its length to a maximum - as the further the eggs
have to roll down, the cleaner and dryer they will be. For that reason
salmon screening devices are often raised so there is enough space to
have a long slope screen and to install an egg collecting basket. This
requires the workers to stand on benches or elevated floors.
Screening is physically a tiresome job to do for a full day. To make
it easier the height of the screens should be adjustable to the height
of the person working.
Ovaries are screened in batches of 10 to 30 pieces. A new batch is put
on after the upper screen is cleaned of debris. Depending on egg quality,
changing of all three screens may take place every 0.5 to 2 hours. 150
Ibs per hour per worker is considered average salmon caviar screening
productivity. At maturity, stage IV, only part of the eggs of each ovary
are separated easily, namely the ones located away from the spinal column
and in the anterior ovary end. The desire to recover all eggs results
in substantial break-age towards the end of the screening process soiling
the first easily detachable and clean eggs with broken membranes. Thus
the lot as a whole would be downgraded. Russian salmon caviar screeners
suggested a simple solution - to screen those less mature ova-ries in
two steps. After easily detachable eggs are screened off for Grade No.
1 lot, the ovaries are passed onto another screen where the screening
is finished and a Grade No. 2 lot is formed which may be substantially
soiled with debris. The first lot may consti-tute 60-70% of total yield.
These lots are salted separately.
Screened eggs obtained from fresh, elastic, shiny ovaries look almost
perfectly round, transparent and have a shiny surface. When put on a sloped
surface they roll down. Screened eggs from weak, flabby and slimy ovaries,
taken from fish which have under-gone the first stages of autolysis, look
dull, are collapsed, and their surface is sticky and slimy. When put on
a sloping surface they do not roll but slide down. One should clean the
debris off the slope screen and adjust the slope to ensure the eggs are
rolling down. The only time the debris has a chance to become detached
and stay on the screen is when the eggs are rolling but, not slipping,
down. Some processors claim they have had good performance with slope
screens fabricated from oblong mesh wire cloth (Figure 3.2d).
The screening devices may have many deflecting surfaces, so eggs do not
bump around. All elements of screening devices should be easily washable
and accessible to water jets and brushes. The yolky, egg interior liquid,
if not washed off, becomes a hard, glue-like spot.
An important issue is to choose the proper mesh and wire size. With wire
cloth, mesh is sometimes specified with a number which refers to the number
of openings per linear cloth unit, e.g. per foot. In choosing screen sizes
for caviar our main concern is the size of the opening i.e., the actual
clearance between wires, which depends on wire diameter. For smaller eggs
smaller wire diameters are chosen. Wire or twine diameter ranges 0.6 to
0.9 mm. Practical examples for mesh sizes and screen designs are given
in Figure 3-2, and Table 3-1.
MESH WIRE CLOTH AND NETTING OPENINGS, in mm
CHUM CHINOOK ATLANTIC
COHO PINK TROUT
Netting, Metal sheet only for mechanized screening
Netting or metal sheet
Netting or metal sheet
Sheet material screens are usually used in screening machines. For
eggs of 2 mm diame-ter and lower sheet material screens are the only ones
suitable for both manual and ma-chine screening. The perforations are
round and mainly metals sheets are used. Suitable plastic sheet screens
could be employed. Perforations are usually located in a chess or-der.
The hole diameter is more than twice the size of egg diameter, e.g. for
herring cav-iar the hole diameter is 2.5 - 3 mm. Slope screens, used for
draining, could have elongated perforations, when made from sheet material
since it provides for better draining. Small square openings for both
wire cloth and stainless steel sheet screens tend to clog very rapidly
and drainage stops.
Holes of stainless steel sheet screens are fabricated by punching. This
results in a sharp edge of the hole on one side of the sheet. This sharpness
may rupture egg membranes and may also injure screener's hands. It is
advisable to polish off these sharp edges. Wire mesh doth frame edges
should be protected, e.g. with smooth strips, to avoid screener's hand
3.4 MECHANIZED SCREENING
There are several concepts and machines to mechanize salmon and all
kinds of small size fish egg screening. They are not used for sturgeon
roe. The Soviet Union developed two mechanical screening machines, which
are used both on and off shore. Figure 3-5 explains semi manual screening
principles of a machine for screening salmon ovaries.
FIGURE 3-5: Salmon Caviar Screening Machine
1 - Suspension spring, 2 - Motor/ 3 - Rotating screen, 4 - Ovaries,
5 - First screen, 6 - Second shaking screen, 7 - Third screen, 8 - Slope
adjustable screen, 9 - Tray, 10 - Egg collecting basket, 11 - Adjustable
The screening experiment is shown in Figure 3-6.
FIGURE 3-6: Screening Experiment
The screener uses the rotating mesh disc (3) to press ovaries which
are put onto the up-per screen (5) over a set of three screens attached
to a frame. The second screen per-forms back and forth movements (motor
driven) with a 90 mm stroke 70 times per min-ute. Simultaneously the screener
is moving the disc around the upper screen as needed. The eggs are singled
out in the same manner as manual screening. Debris remains on all three
screens and the slope screen (8), and eggs accumulate in basket (10).
Debris from the upper screen is collected using the disc. Every hour the
machine is washed. The ma-chine is operational in several processing plants
in Kamchatka, Russia. It relieves the screener from a stressful manual
job, however the yield is 10-15% less than for manual screening.
The machine is also used to screen pollack and cod roe. In this case
the screens are re-placed with netting of a smaller mesh size.
A compact fully automated screening machine, employed on the Soviet processing
boats, mainly to process pollack caviar is shown in Figure 3-7. Ovaries
from a bin are delivered onto a horizontal round stainless steel perforated
sheet-table. A vertical shaft is rotating at 40-50 rpm. A rotating ring
holds six adjustable rubber blades. Eggs fall down over a second, third
and slope screen. The output is 900 kg/hour for capelin and 350 kg/hour
for salmon. The average yield is 88 and 75%, respectively.
FIGURE 3-7: Vertical Shaft Machine
Lumpfish ovaries are screened in Iceland, Canada and other countries
using horizontal shaft drum type screeners, see Figure 3-8. The horizontal
shaft is equipped with blades or blades and brushes positioned at an angle
which rotate inside a drum made of perfo-rated stainless steel sheet material.
The blades have means to regulate the clearance to-wards the drum surface
so as to apply the required pressure to rub through the eggs.
FIGURE 3-8: Horizontal Shaft Machine
The debris remains inside the drum and is pushed towards the outlet located
at the lower part of the drum end. By regulating blade angle towards the
drum axis one can achieve the best yield and regulate the intensity of
debris removal. The eggs fall into a funnel and further into a collecting
basket. The drum is inclosed in a protective cylinder. The screening machine
output depends on its size. Details on lumpfish screening machine performance
are given in Section 8-2. None of the described methods of mechanized
screening used water or brine while screening. 'Dry screening must be
lowering the yield, especially in the case of weak eggs. That is why several
attempts were made to introduce 'wet' screening technologies which substitute
gentle agitation of ovaries in so-lutions for the harsh mechanical screening.
Simultaneous salting (brining) of the eggs takes place and, in fact these
methods claim to avoid any handling procedures leading to the claim 'ovaries
in - caviar out'.
Figure 3-9 shows schematically a salmon roe processing system, which
could utilize both pure brine solutions or brine solutions containing
enzymes. In fact, these technolo-gies do not employ the screening procedure
and should be rather defined as fish egg separating systems.
FIGURE 3-9: Fish Egg Separating System
1 - Ovaries, 2 - Brine-enzyme separation tank/ 3 - Debris, 4
- Floatation-rinsing tank, 5 - Air supply, 6 - Draining, 7 - Brine
salting, 8 - Caviar
The use of enzymes to decompose connective tissues of salmon and trout
ovaries has been known since 1960. The technology aims to utilize less
mature ovaries providing yields higher than when manual screening is used.
It also claims lower screening and egg handling labour costs. Although
it is claimed that the enzyme solution only decom-poses the connective
tissues and does not affect the eggs itself, very thorough washing of
screened eggs exposed to the enzyme action is required.
Proteolytic enzymes such as papain and bromelin could be used for this
process. In principal proteases of animal origin are used. Enzyme activity
depends largely on con-centration and temperature. Sometimes adjustment
of solution pH is necessary.
The temperatures used should not exceed 50°C so that egg interiors
won't coagulate. At the same time enzyme activity usually falls drastically
at low temperatures. It is reported that enzymes produced from cold-water
fish viscera and psychrophilic microorganisms are active at relatively
low temperatures in comparison with enzymes produced from warm-blooded
animals or plants. Treatment time will depend on enzyme concentration
and effectiveness of the stirring vessel and may range from several minutes
to half an hour. The enzyme concentration in solution ranges from 0.02
The flotation principle in these systems is widely used because eggs
and debris (tissue, broken egg shells etc.) have different floating properties.
The described process is known to be used for processing caviar from farmed
trout, fatty chinook salmon and generally fresh but weak eggs.
There is no information on the shelflife of caviars processed using enzyme
separating systems vs caviars processed by means of conventional methods.
The following consid-erations require one to treat the effectiveness of
these methods with caution and may explain why these methods are not used
- The conventional screening methods for mature eggs provide high recovery
at low labour and negligible investment costs.
- Immature and weak eggs may undergo partial decomposition of the egg
membrane, which will overexpose them to brine. This may result in egg
yolk leaching and an undesirable watery taste.
- The process involves lengthy exposure of eggs in brine, which makes
it extremely difficult to achieve the desired salinity and viscosity.
The use of hot enzyme solutions might be efficient in preparation of
salmon eggs for bait.