Surimi Processing Techniques for Dark Muscle Fish

Surimi, the washed and concentrated fish muscle protein used to make everything from fish balls to imitation crab sticks, has traditionally relied on lean, white-fleshed species like Alaska pollock. But with those stocks under increasing fishing pressure, the seafood industry has been pushing into new territory – specifically, small pelagic species like Indian oil sardine (Sardinella longiceps) and mackerel (Rastrelliger kanagurta). These dark muscle fish are abundant and affordable, especially in tropical coastal regions, but they bring a set of processing challenges that require tailored techniques to overcome. Understanding how surimi is made from these species – and why it demands a different approach – is essential for anyone working in fish processing technology.

What makes dark muscle fish different? 🔗

Dark muscle fish are so called because of the higher proportion of red or dark lateral muscle tissue in their bodies. This muscle is physiologically designed for sustained, active swimming – which is why pelagic species like sardines and mackerel are constantly on the move. The biochemical consequences of this, however, are significant for surimi processing.

These species accumulate two key problem components: high lipid (fat) content and high myoglobin content. Studies on the proximate composition of oil sardine and Indian mackerel have found that sardine flesh can carry a lipid content as high as 10.1%, while mackerel contains substantially more polyunsaturated fatty acids (PUFAs), which are highly susceptible to oxidation. These fats, if not removed during processing, interfere directly with surimi quality.

On top of this, the high dark muscle content means a higher concentration of heme pigments – primarily myoglobin and hemoglobin – that are significantly harder to remove than the iron pigments found in white-fleshed fish. These pigments cause the finished surimi to appear greyish or brown rather than the desirable white colour, and they actively accelerate lipid oxidation, contributing to rancidity and fishy off-flavours.

There is also a pH problem. After death, the muscle pH of sardine and mackerel drops rapidly to around 5.8, well below the 6-8 range needed for adequate gel formation. This is caused by high glycogen reserves in the dark muscle tissue, which produce lactic acid during post-mortem metabolism. The result is a weakened gel structure – a critical quality defect in surimi intended for downstream products like fish cakes or seafood analogs.

The surimi processing flow for dark muscle fish 🔗

Despite these challenges, the basic processing sequence for dark muscle fish follows the same broad framework as conventional surimi, with targeted modifications at key stages.

Fish selection and pre-processing 🔗

Freshness is non-negotiable. Post-rigor fish is always preferred for minced meat extraction, as pre-rigor or spoiled fish results in poor gel-forming ability. The fish are headed, gutted, and washed with chilled water. Since sardines and mackerel are small pelagic fish, the filleting and deboning step often uses mechanical separators with perforation diameters of 4-7 mm to separate the edible muscle from bones, fins, scales, and skin.

Mincing 🔗

The separated meat is minced to achieve a uniform particle size. This increases the surface area of the muscle, which is critical for efficient washing in the next stage. At this point the mince is grey-coloured, heavily loaded with lipids, pigments, sarcoplasmic proteins, and off-flavour compounds – none of which are desirable in the final surimi.

The critical role of washing in dark muscle fish surimi 🔗

Washing – also called leaching – is where dark muscle fish processing diverges most sharply from conventional surimi production. The washing stage removes blood, pigment, connective tissue, off-flavour compounds, and sarcoplasmic proteins, while simultaneously increasing the concentration of myofibrillar proteins, the proteins responsible for gel formation. For white-fleshed fish, simple cold water washing (at 5-10°C) repeated two to three times is usually sufficient. For dark muscle fish, this is not enough.

Why plain cold water washing falls short 🔗

Research on Indian mackerel surimi has shown that although increasing the number of cold water washing cycles does progressively reduce lipid and myoglobin content, plain water is only partially effective. Myoglobin is closely bound to myofibrillar proteins in dark muscle tissue, making it resistant to simple aqueous removal. This residual myoglobin not only darkens the surimi but continues to catalyse lipid oxidation in the finished product.

There is also a trade-off: too many washing cycles reduce gel quality because excessive washing also strips out some functional myofibrillar proteins along with the unwanted components. Getting the washing right is therefore a balancing act between purity and protein functionality.

Sodium bicarbonate washing for fat removal 🔗

One of the most widely recommended techniques for processing dark muscle fish like Indian oil sardine is washing with sodium bicarbonate solution. Sodium bicarbonate washing effectively removes most of the fat from the mince, while also raising the pH of the muscle tissue – helping to bring it back into the 6-8 range needed for gel formation. Studies on sardine surimi have confirmed that the addition of sodium bicarbonate significantly improves the whiteness of the product, reducing the dark coloration caused by residual pigments and oxidised myoglobin.

Specialised washing strategies 🔗

Beyond sodium bicarbonate, researchers have developed several advanced washing approaches to further improve dark muscle fish surimi quality.

Salt (NaCl) washing: Adding sodium chloride to the washing medium has been shown to substantially improve myoglobin and lipid removal. Research on tropical mackerel found that washing with cold carbonated water containing 0.6% NaCl in the first washing cycle, followed by two plain cold water cycles, achieved approximately 65% myoglobin reduction, 80% lipid reduction, and a 45% improvement in whiteness – significantly outperforming conventional water washing.

Alkaline and ozone washing: For fatty species like sardine and horse mackerel, alkaline salt washing and ozone washing have been shown to produce higher-quality surimi than standard water washing. These methods are particularly effective at removing heme proteins that are otherwise tightly bound to muscle tissue.

Antioxidant-infused washing solutions: A more recent approach involves combining carbonated water and NaCl with antioxidants such as EDTA, sodium erythorbate, sodium tripolyphosphate, or gallic acid. These compounds act as iron chelators, delaying hemoglobin-mediated lipid oxidation and reducing rancidity in the finished surimi gel. Treatments using such antioxidant-infused soda-saline solutions have produced mackerel surimi with textural characteristics comparable to white-fleshed fish surimi.

Ultrasonic-assisted washing: Single-cycle washing assisted by ultrasonication, combined with carbonated water, salt, and antioxidants, has been shown to produce mackerel surimi quality comparable to three-cycle conventional washing, while using less water and producing less processing waste – a significant advantage for sustainable production.

Mechanical methods for dark muscle removal 🔗

In addition to chemical washing, mechanical approaches such as the rotary freeze method, pressure washing, and density gradient separation are used to physically remove the dark muscle tissue from the mince prior to or alongside the washing step. These methods can reduce the initial load of pigment and lipid entering the washing stage, making subsequent chemical washing more effective.

Dewatering, refining, and cryoprotectant addition 🔗

After washing is complete, the mince is dewatered – typically by centrifugation or screw pressing – to bring moisture down to an acceptable level, usually around 80%. The dewatered mince is then refined through a strainer or refiner to remove any remaining connective tissue, bone fragments, or skin particles.

The most critical post-washing step is the addition of cryoprotectants. Cryoprotectants are food additives that prevent the denaturation of myofibrillar proteins during frozen storage by bonding with protein molecules via surface functional groups, increasing hydration and reducing protein aggregation. The standard commercial cryoprotectant blend is a mixture of sucrose and sorbitol (typically 4% each by weight), often combined with sodium tripolyphosphate (STPP) at around 0.2-0.25%.

This is particularly important for dark muscle fish surimi, as the myofibrillar proteins of pelagic species already have relatively poor stability during frozen storage compared to white-fleshed fish. Research on tropical fish surimi has confirmed that a sucrose-sorbitol mixture at 2-4% is sufficient to preserve protein functionality at −20°C for up to five months, making reduced-sugar formulations feasible without compromising storage stability.

Quality characteristics and grading of dark muscle fish surimi 🔗

Even with optimal processing, surimi from dark muscle fish like Indian oil sardine and mackerel is generally of inferior quality compared to surimi from white-fleshed species, primarily because of the high fat content and residual myoglobin that are difficult to completely eliminate. The finished surimi typically shows lower whiteness, weaker gel strength, and a slightly more pronounced fishy odour than pollock or threadfin bream surimi.

The key quality parameters assessed in surimi grading are gel strength, whiteness, moisture content, and lipid content. For Indian mackerel, research has confirmed that two washing cycles can achieve equivalent gel strength and whiteness to three cycles while retaining a higher protein yield – suggesting that a two-cycle optimised wash protocol offers the best balance of quality and resource efficiency.

Despite the lower quality grade, dark muscle fish surimi holds genuine commercial value. Pelagic species including mackerel, sardine, and scad are increasingly used as raw materials for surimi production in major processing countries like Vietnam and China, where their low cost and local abundance make them economically attractive. In these markets, lower-grade dark fish surimi is commonly blended with higher-quality white fish surimi to produce affordable products like fish balls – a combination that has driven the growth of mass-market surimi-based foods.

Storage conditions for finished dark muscle fish surimi 🔗

After cryoprotectant blending, the surimi is shaped into blocks, quick-frozen at −35°C, and then stored at −20°C. Cryoprotectants such as sucrose and sorbitol allow frozen surimi to be stored for up to one year under these conditions. For dark muscle fish surimi specifically, the residual lipids remaining after washing make oxidative rancidity the main storage risk. This is why antioxidant inclusion during washing – or as a separate addition before freezing – is increasingly common in commercial dark fish surimi production.

The surimi block is the intermediate product. It is later thawed and processed into final products – fish cakes, kamaboko, fish sausages, imitation seafood – by downstream manufacturers, who may blend dark fish surimi with white fish surimi or add starches, seasonings, and texturisers to achieve the desired product characteristics.

What do you think? Given that dark muscle fish like sardines and mackerel are far more abundant and affordable than traditional white-fleshed surimi species, do you think wider adoption of optimised washing techniques could make dark fish surimi economically competitive with pollock-based surimi in global markets? And with ultrasonic and antioxidant-assisted washing already showing strong results in research settings, what might be the main barriers to scaling these methods up in a typical tropical fish processing facility?

References

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