How Fish Bone Structure Affects Boneless Fillet Processing Technology

Different fish species have different fish bone structures, which directly affect the thawing, deboning, and filleting processes in boneless fillet processing. This article classifies various fish in actual production according to fish bone structure, and discusses the optimal deboning, filleting, and thawing processes for each type of fish, providing reference for actual production.

I. Classification of Fish Bone Structure

1. Vertebral spine type

(1) Fish with longitudinal fish bones connected to the spine: This type accounts for the majority, and currently common ones in aquatic product production and processing include redfish, flounder, red drum, tilapia, etc.

(2) Fish with obliquely lateral fish bones connected to the spine: Mainly including horse mackerel, silver seabream, etc.

(3) Fish with fine bones connected to the spine: Mainly including Spanish mackerel, mackerel, Pacific saury, etc.

(4) Fish with no bones connected to the spine: This type mainly includes eel, etc.

2. Ventral spine type

(1) Fish bones in the belly growing obliquely inward, such as redfish, flounder, etc.;

(2) Fish bones in the belly lying flat against the belly, such as horse mackerel, silver seabream, etc.

Generally, most fish with longitudinal fish bones connected to the spine have the ventral spine type of bones growing obliquely inward in the belly, while fish with obliquely lateral fish bones connected to the spine have the ventral spine type of bones lying flat against the belly.

II. Methods for Removing Fish Bones

Fish with longitudinal spine bones generally also have obliquely inward-growing ventral bones, such as redfish, flounder, etc. When filleting this type of fish, the longitudinal vertebral bones are parallel to the fillet and can be removed together with the spinal column. However, the ventral bones are thin and long. If a deboning tool is used to pull them out one by one in production, it is very time-consuming, unsuitable for large-scale production, and very easy to break or miss, leaving a large amount of residue in the product, which reduces product quality. A deboning tool can be used to score a line above and below the ventral bones, so that when deboning, the flesh and bones between the two score lines are removed together, greatly improving labor efficiency. Using this method, the time to remove bones from each redfish fillet is only 8 seconds for a skilled worker. When scoring, the flesh between the two score lines should be as thin as possible, with the fish bones in between, to avoid loss.

For fish with obliquely lateral vertebral bones, the bones will be cut and left behind during filleting, such as horse mackerel, silver seabream, etc. The vertebral bones of horse mackerel, silver seabream, etc. are short and thick, and easier to remove than the ventral bones of redfish and flounder. When deboning, the tool should have a certain contact area with the fish bones, and the bones should not be clamped and broken. Pull the bones along the direction of the fish bones, otherwise the bones are easily broken. Some fish, such as horse mackerel, tend to have flesh that falls apart when over-thawed, so it is not easy to pull bones when completely thawed. However, if the fish body is too hard, it is also not conducive to bone removal. The degree of thawing should be properly controlled. When deboning this type of fish, the fish should not be picked up to pull the bones; the fish should be placed on a flat container to prevent the flesh from falling apart. For small-sized fish with obliquely lateral vertebral bones, pulling bones out one by one is likewise unsuitable for large-scale production. Lateral filleting can be used during filleting, with the fillet knife going around the fish bones. This filleting method reduces the yield of fillets, but small-sized fish are often low-value fish, and large-scale production can be ensured. The belly bones that lie flat against the belly, such as the belly bones of horse mackerel and silver seabream, can be removed during trimming, and can be detected by bone inspection in subsequent processes.

For fish with fine bones connected to the spine, representative species include: Spanish mackerel, mackerel, Pacific saury. Their spine bones are relatively fine and are close to perpendicular to the fillet. Under semi-thawed conditions, use a blade to score 2 cuts along both sides of the spinal line to remove the flesh and bones between the two cuts. This deboning method has high efficiency and low flesh loss. Deboning of Spanish mackerel, mackerel, etc. should be carried out while the fish body is still relatively hard, and is not suitable after complete thawing, because after complete thawing the flesh easily falls apart, affecting product quality. If there are subsequent processes such as cutting into sections, complete thawing is not conducive to product shaping.

III. Filleting Technology

There are roughly 2 manual filleting methods: the two-knife method and the side-filleting method. The two-knife method first cuts the fish open from tail to head or from head to tail, then uses the same knife method to remove the backbone from the other half. The side-filleting method cuts from both sides of the fish to the backbone (spinal column) respectively, then removes the backbone. Considering a single fillet, the two-knife method is faster than the side-filleting method, but the fillet cut by the two-knife method contains fish fins, and a dedicated person must be arranged in a subsequent process for trimming; the two-knife filleting knife advances in a sawing motion, leaving jagged edges on the fillet, making the fillet unattractive; the two-knife filleting knife stays close to the spinal column, and because the spinal column has a certain protrusion, this filleting method results in a high flesh-to-bone ratio on the fish bones and a low product yield.

IV. Thawing Technology

Thawing in boneless fillet processing can be divided into 2 types: complete thawing and semi-thawing. However, in actual operation, the specific thawing should be combined with deboning requirements, flesh properties, fish size, or factory facilities. Redfish, flounder, cod, etc. can be completely thawed. This type of fish is convenient for filleting and deboning under complete thawing, and complete thawing has little effect on fish body quality.

Thawing methods include still water thawing, running water thawing, shower thawing, natural thawing, etc., and several methods can also be combined for thawing. The temperature of the thawing medium should not be too high, to avoid excessive thawing of the outside of the fish body, especially for large-sized fish, which easily causes the drawback of the outside being over-thawed while the inside is not yet thawed. During still water thawing, ice can first be added to the thawing water, and after the temperature of the thawing water drops, the fish to be thawed can be added. For running water thawing and shower thawing, the medium temperature is generally required to be below 15°C. Natural thawing should preferably be carried out in a low-temperature room.

Atlantic cod requires running water thawing; this type of fish has a large amount of mucus on the body surface and a fishy smell. If thawed in still water, the fishy smell will enter the flesh. Spanish mackerel, mackerel, etc. are semi-thawed, with the core temperature controlled at around -7°C, and filleting and deboning are carried out while the fish body is still frozen. Horse mackerel thawing should be controlled to a state where it is not completely thawed through. Over-thawed fish bodies tend to fall apart during processing, and fish that are not thawed through are not conducive to bone pulling; factories should reasonably control this in actual production.