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A. Fresh Shucked Oysters

1. Washing of harvested oysters

Oysters harvested and transported directly to the processing plants should be reasonably free of sediments and other foreign substances. They can be cleaned with the use of an octagonal-cylindrical continuous rotary washing machine. The washing container is made of a rotating perforated cylinder of heavy construction with both ends open. The cylinder generally measures 4.4 m in length and 1.5 m in diameter but the dimension differs from plant to plant. A perforated water pipe is built into the cylinder from which water jets outward. The position of the cylinder is fixed at an angle of 7 degrees slope to allow oysters to slide down. A 3 HP power motor is needed to rotate the cylinder.

Sea or fresh water can be used for washing the oysters, but they must be clean enough from the standpoint of hygiene. The oysters from the washer are automatically rinsed when passing the high pressure water spray through the conveyor. They are placed in clean plastic baskets for shucking.

2. Shucking

Oysters are shucked in a manner that does not get them contaminated. Stainless steel tables are used for shucking. The table tops are smooth, well drained and high enough above the floor to prevent product contaimination. Shucking knives, which have fairly long blades and pointed tips are used. Shucking containers are most often plasticware or stainless steel vessel with a capacity of about 3 liters.

The opening process begins by placing the oyster on the table or holding it with the left hand with the concave or left shell down and with the hinge pointed toward the side of the opener. With the oyster in this position the adductor muscle is located about two thirds of the distance from the hinge towards the right. The knife is inserted between the shells at this point with a slight twist of the knife to keep the handle of the blade elevated. After the knife point has entered, moving the knife to the right and to the left will cut the adductor muscle on the flat or right shell. The knife is then turned until the blade is vertical. A prying motion will break the hold of the hinge to separate the two shells. Next, the adductor muscle on the other shell is cut and the oyster meat is flipped into the shucking pail.

Careful attention must be paid when the knife is slipped into the shell so that the knife blade will be against one shell or the other, in order not to injure the oyster meat and ensure that the adductor muscle is cut right against the shell.

3. Washing of Shucked Oysters

The air blowing system is used in washing shellfish meats. Shucking and packing operations are carried out in separate rooms or in sufficiently separated areas so that no shucked product or packing room equipment is contaminated by splashes or other activities. The shucked oysters are conveyed from the shucking room to the packing room where the blower is located, through a delivery window or by other means of conveyance. In large scale plants, the oyster meats are collected into 18-liter capacity plastic containers. The meats flow with the running water to the blower through the stainless steel chute which runs from the shucking room to the blower tank in the packing room. For more effective washing, the chute can be modified as it is built in one or two trap sections on which a grid is placed. Through this grid, shell particles, sand and others drop into a collecting box.

The blower is about 300 cm long, 40 cm wide, and 30 cm tall. The bottom section of the portion 21 cm below the top rim of the frame is built in the shape of a trough with a discharge valve on it. There must be a passage way at one end of the brim of the tank to enable the oyster meat to flow with the water. Two lines of perforated air pipe are laid above the true bottom, and a perforated plate is placed above the air lines. This plate serves as a false bottom. This horizontal type of blower has the advantage of continuous operation. The entire machine is made of stainless steel and built for easy cleaning.

For the washing operation, the tank is filled with water, the oyster placed in it, and the air turned on. The air bubbles agitate the water and the oysters, providing a cleaning action without damaging the meats. Small scale operators, however, use a tub of continuously overflowing water. The oysters placed in a shallow basket are stirred gently by hand or with a paddle in the tub.

4. Packing

After washing, the oysters are poured into a perforated stainless steel table. Bits of shell and other substances and rejected oysters such as those which are badly cut or of poor condition are removed. Grading is also done on the table. After grading, oysters are rinsed by spraying or dipping, or by soaking them in chilled sea water for a while. They are then drained and packed.

In general, packages are of two types, small and wholesale. For the small sized packaging, 250 or 300 g of shucked meat with some amount of chilled sea water is put into a polyethylene tube and sealed. This practice is carried out at shucking plants located in the field. Chain food companies use a small flat styrofoam tray on which 350 g of shucked meats purchased from a wholesale market is placed and sealed with thin vinyl wrapper.

For wholesale use, 8 kg of meat with some sea water contained in sealed polyethylene bag is placed in a polyethylene sack which serves as the inner pack. This is then packed into a styrofoam box with a capacity of 10 kg.

B. Frozen Oyster

The operations involved in processing frozen oysters are the same as those in processing fresh shucked oysters except for the steps after freezing. Processing frozen oysters for export is limited to the certified plants which must use oyster produced from approved grwing areas. This is in accordance with the “Regulations Governing Sanitary Control of Shellfish, Their Growing Areas, Harvesting and Processing of Shellfish Products for Export (MAFF Ordinance No. 699).”

Washed and graded oyster meat are individually placed on plastic plates designed for freezing oyster and frozen at -35 to -45 degrees Celsius. Freezing time varies with the type of freezing system: 50 to 60 minutes for air-blast freezer, 30 to 40 minutes for spiral freezer, and 15 to 20 minutes for belt freezer. After freezing is completed, the oysters are removed from the freezing plate by twisting the plate and immersing it in water at temperature of 0 to 5 degrees Celsius for glazing. The oyster are packed in polyethylene bags sized as a unit package (see Table 8-1). These unit bags are then packaged in cartons and stored in the cold storage room at -25 degrees Celsius.

Table 8-1. Standards of size for individually frozen oysters
Grade of sizeNo. of individual oyster per poundWeight of individual oyster
Extra large (XL)Under 25Over 18
Large (L)25 – 35Over 13
Medium (M)36 – 4510 – 13
Small (S)46 – 558 – 9
Tiny (T)56 – 756 – 7

Figure 8-1

Figure 8-1. Flowsheet showing the various stages of processing frozen oysters.

C. Canned Smoked Oysters in Oil

1. Preparation of raw materials

Most processing plants have been using the steam opened oysters for canning but fresh shucked oysters can be used after cooking at 95 degrees Celsius for 10 minutes. Oysters washed by the rotary washer slide from the washer into an empty steaming car. The loaded oysters are steamed into a horizontal or vertical retort at 105 degrees Celsius (at 5 lb steam pressure) for about 18 minutes. The steaming time varies, depending on size and condition of the oysters. The oysters are shucked using the knife. Shucked oysters are conveyed from shucking room to the blowing washer directly or through a chute in which water is running. Shell, sand and other debris are washed away from the oysters without damaging the meat. The washed meats are drained and graded.

2. Smoking

Graded oyster meats are placed in galvanized or stainless steel wire mesh bottom trays. Only one layer of oysters should be placed in a tray, spread so that the smoke can penetrate uniformly. The filled trays are brought into the smokehouse and smoked at 100 to 120 degrees Celsius for 10 to 20 minutes. Most processing plants have been recently employing the conveyor system continuous automatic smoking equipment which is 12.5 m long, 2 m high and 1 m wide and equipped with a five-step stainless steel wire mesh conveyor. The conveyor is designed in such a way that each succeeding step is longer than the previous one. In the mechanical smoking unit, oysters are smoked at 130– 150 degrees Celsius from 20 to 30 minutes depending on oyster size. Oak tree wood and oak tree sawdust are used mostly for smoking. Proper smoking produces oyster tinged with a light chocolate color.

3. Canning

After smoking and cooling, the meats are filled into No. 3-B Square Cans classified by count size into large, medium, small and tiny. Drained weight means the solid weight contained in the can after cutting. The fill-in weight, therefore, must be decided taking into account moisture content of the oysters after smoking and the changes that occur during processing. For these reasons, the smaller oysters have less shrinkage so that the amount of oil agglutinated with oysters after processing is more in smaller oysters than in larger ones. The standards of net weight, drained weight and count size for canned smoked oysters in oil are shown in Table 8-2. The amount of oysters to be placed in each No. 3-B Square Can is about 88 g for the count size of large, medium and small, and 86 g for the tiny size. After filling, about 20 g of edible vegetable oil (cotton seed oil, soybean oil or olive oil) heated at about 100 degrees Celsius is added into each container. If necessary, 1 to 2 g of table salt is added.

Table 8-2 Inspection standards for canned smoked oysters in oil
Can nameNet WeightDrained WeightNo. of individual oyster per can by size
No. 3-B Square
Over 105Over 90Under 1314–2021–30Over 31
No. 5-C Square
Over 70Over 55Under 89–1516–25Over 26

Note : Inner side of the can must be coated

4. Sealing and Processing

The filled containers are sealed by a vacuum sealing machine, maintaining a vacuum of 30 cm Hg in the vacuum chamber. The sealed cans are stacked in iron trays of heavy construction and taken to the retort for processing. Oysters are packed in No. 3-B Square Cans and if the initial temperature of can contents is higher than 60 degrees Celsius, sterilization takes 70 minutes at 115 degrees Celsius (at 10 lb steam pressure).

The pack is water-cooled rapidly in the retort to about 35 degrees Celsius. When cutting the cans after processing, the vacuum in the containers should not be less than 7 cm Hg.

D. Canned Boiled Oyster

Steam opened oyster meats arely used in the processing of canned smoked oysters, but fresh shucked oysters can also be used after cooking. The washed and graded oyster meats are placed in the cans (required drained weights are shown in Table 8-3). Taking into consideration shrinkage during processing (a 10 to 20 percent loss, for example), the amount of oysters to be placed in a No. 7 fruit can (8 oz.) in commercial practice is about 185 g for large, 180 g for medium and small, and 178 g for tiny size. Passed under a perforated pipe, the filled cans are added with 2.5 to 3 percent salt brine. The filled containers are sealed by a vacuum sealing machine. The conditions of retorting and cooling are the same as in processing smoked oysters. The vacuum in the cans, when they are cut, should not be less than 20 cm Hg.

Table 8-3. Inspection standards for canned boiled oysters
Can nameNet
No. of individual oyster per can by size
No. 7 Fruit (8 oz Tall)
Appollation: 211-4
65.4 dia.x81.3 mmH
Vol.: 249.3 ml
Over 225Over 145Under 1516 – 2021 – 3031 – 55
Appellation: 211-5
65.4 dia.x101.1 mmH
Vol.: 318.1 ml
Over 290Over 187Under 1920 –2627 – 4041 – 60
Appellation: 301-7
74.1 dia.x113.0 mmH
Vol.: 454.4 ml
Over 440Over 273Under 3031 –4041 – 5556 – 85

Note: Inner side of the can must be coated

Figure 8-2

Figure 8-2. Flowsheet showing the various stages of processing canned smoked oysters in oil.

E. Dried Oyster

1. Preparation of raw material

Cooked freshly shucked oysters can be used for the product, but most of the factories use steam-opened meats. They use the same washer described in the processing of fresh shucked oyster for washing the oysters. The washed oysters are loaded in steaming cars, and cars are pushed along the track to a horizontal retort of heavy construction with doors at each end. The oysters are steamed at 110 5 degrees Celsius for 7 to 10 minutes, and then shucked. During shucking, careful attention is taken to prevent the breakaway of the adductor muscles. The shucked meats are placed onto shallow mesh plastic trays and washed by shaking the tray in a water tank in which water is continuously overflowed. Clean seawater or 2–3 percent salt brine is used for washing the oyster meats.

2. Drying

The washed oyster meats are spread on the bottom of bamboo trays 80 cm by 100 cm in size. The trays are stacked on the cars for steaming. The loaded oysters are steamed in the retort for 5 minutes at 80 degrees Celsius to increase its firmness and reduce drying time. After steaming, the oysters in the trays are dried in the sun. At sundown the drying trays are stacked up to a convenient height and covered by tent or canvas if necessary. During the spring season the drying process requires four to seven days depending on the size of the oysters, for example four to five days for small and six to seven days for medium to bigger sizes.

If the weather is unfavourable, the oysters are dried in a hot-air dryer. The drying tray on which oysters are spread are stacked on the cars designed for holding the trays and taken into the drying chamber. At the beginning of the operation, temperature of the chamber is maintained at 27–38 degrees Celsius for one to two hours and then increased gradually to 60 degrees Celsius. For hot-air drying, the drying process is repeated based on the assumption that an hour's drying at 60 degrees Celsius is equivalent to one day of sun-drying. Because hot-air drying results in product dicoloration, it is recommended that drying be completed under the sun.

Figure 8-3

Figure 8-3. Flowsheet showing the various stages of processing dried oysters.

3. Coating

Coating means spraying the oysters with oyster extract during drying to improve brightness and prevent the product from hardening. The drained liquid obtained from the process of steaming the oyster meats before drying is used. Generally, spraying is done once or twice a day after one or two days of sun-drying. A total of four sprayings is generally made. In hot-air drying, spraying is done once after every drying operation.

4. Grading and Packing

After drying, the oysters are graded by count size (Table 8-4). The graded oysters in 3-kg units are put in a polyethylene bag as a unit pack and sealed. Ten bags of the unit pack are packaged in a carton box. The unit of weight for export trade is expressed in terms of pickle (60 kg). Two carton boxes containing 20 unit packs each weigh one pickle.

Table 8-4. Standards of count size for dried oysters
Weight of individual oyster by size (g)
Extra large
Over 5.54.5 – 5.43.5 – 4.42.5 – 3.4Under 2.5

5. Storage

The packaged products are normally stored in cold storage rooms at -20 degrees Celsius until shipping out.

F. Salt-Fermented Oyster

Fresh shucked oysters are washed thoroughly to remove bits of shell, sand and other foreign substances, and placed on baskets for draining. For low-salt fermentation, 10% table salt is added to the oysters and mixed evenly with or without spices (minced garlic, shredded green onion, red pepper powder, roasted sesame seeds and chopped ginger are commonly used as spices). The mixture is packed in a jar and aged for 3 to 10 days, depending on temperature. Those fermented without spices are seasoned by adding some spices before serving.

For high-salt fermentation, about 20% table salt is added to the oysters. After it is properly fermented, the liquid of hydrolysis that accumulates on the upper layer of the jar is collected and filtered through cloth. The filtrate is heated to interrupt the fermentation process, and then cooled. The heat-treated filtrate is put back into the jar and mixed. The jar is sealed and kept in a cool place.

G. Oyster Juice

Oyster juice is a concentrated oyster extract utilizing either the drained shell fluid, a by-product from steamed shellstock oysters, or the extract that comes from cooking fresh shucked oysters. Oyster juice is used as the raw material of oyster sauce, a popular seasoning.

1. Extraction

Extract from steamed oysters. This extract can be obtained as a by-product when the oysters are steamed for processing into canned and dried products. The fluid that drains out of the oysters while the retort temperature is coming up to about 110 degrees Celsius is discarded because the liquid drained at this temperature zone is mostly sea water in the shells. The liquid drained at or above 110 degrees Celsius is collected for the extract. About 3 percent of diatom earth is added to the extract and the mixture stirred. The mixture is either filtered or settled in a vertical tank to remove foreign substances.

Extract from shucked oysters. The boiled water used in cooking fresh shucked oysters processing is used as the extract. The extract is filtered and concentrated. Broken or rejected oyster meats may also be used as raw material for the extract. In this case, the oyster meat with about three times as much water is cooked slowly until the temperature reaches 80 degrees Celsius. Thereafter, the cooking is carried out vigorously for 30 to 40 minutes, and the extract is filtered and concentrated.

2. Concentration

There are two methods of concentrating the extract. The more common one is evaporation using a simple stainless steel tank equipped with steam pipes or steam jacket kettle. The other method uses a vacuum concentrator to reduce pressure concentration. The concentration is carried out until the product comes to 37% in brix and 22–24 degrees in Be (baume).

The product quality standards for oyster juice, as prescribed by the Fishery Products Inspection regulations, are as follows: salt content should be less than 15%, and extractives and total nitrogen should be more than 21 and 1% respectively.

3. Packing

After completing the concentration, the warm extract is poured into 18-liter square-shaped tin cans, with a small opening on top for pouring the liquid in. After the opening is capped and seamed, the packs are stored at either room temperature or in a cold storage room.

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