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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

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 washing.

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, simultaneously.

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.

TABLE 3-1

MESH WIRE CLOTH AND NETTING OPENINGS, in mm

	STURGEON	SALMONIDS*		WHITEFISH	LUMPFISH	HERRING
		CHUM CHINOOK ATLANTIC	COHO PINK TROUT
Type of Screen	Netting	Netting, Metal sheet only for mechanized screening asdf		Netting or metal sheet	Netting or metal sheet	Metal Sheet
Opening mm	6-8	10-12	7-9	6-7	5	2.5-3

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 injuries.

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 3-6: 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

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 to 0.1%.

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 widely:

The use of hot enzyme solutions might be efficient in preparation of salmon eggs for bait.

3.0 SCREENING