How to Stabilize Surimi with Cryoprotectants: Methods and Additives

Surimi is only as good as its frozen shelf life. This concentrated fish protein paste – the backbone of products like imitation crab, fish cakes, and kamaboko – is highly vulnerable to protein damage during freezing. Without adequate protection, the myofibrillar proteins that give surimi its characteristic elasticity and gel-forming strength begin to denature the moment temperatures drop. Cryoprotectants are the solution: a group of food additives mixed directly into surimi before freezing to shield its proteins and preserve functional quality throughout storage.

Why surimi proteins need protection during freezing 🔗

Surimi is a wet concentrate of myofibrillar proteins extracted from fish muscle through repeated washing, refining, and dewatering. These proteins – primarily myosin – are responsible for surimi’s ability to form strong, elastic gels when heated. But they are also structurally sensitive. During freezing, water within the surimi matrix reorganizes into ice crystals. As those crystals grow and expand, they exert mechanical pressure on the protein network, distorting and disrupting it. At the same time, the concentration of dissolved salts and other solutes rises in the remaining unfrozen water, creating chemical conditions that accelerate protein denaturation.

Long-term frozen storage of fish products causes denaturation of myofibrillar proteins, resulting in a loss of their functional properties. In practical terms, this means surimi that was once smooth, springy, and capable of forming a firm gel becomes tough, rubbery, or crumbly. The loss of salt-extractable protein (SEP) – a key indicator of myofibrillar protein integrity – tracks directly with this deterioration. Multiple freeze-thaw cycles further compound the damage, promoting protein and lipid oxidation while reducing Ca-ATPase activity, a marker of myosin functionality.

This is where cryoprotectants step in – not just to slow the damage, but to actively preserve the proteins’ structure and functionality.

The primary cryoprotectants: sucrose and sorbitol 🔗

The most widely used cryoprotectants in commercial surimi production are the sugars sucrose and sorbitol, typically applied in combination. Surimi freezing is done commercially using 4% sucrose, 4% sorbitol, and polyphosphates at around 0.2%, which protect fish myofibrillar proteins during extended frozen storage. Both compounds work by interfering with ice crystal formation and by stabilizing the water layer surrounding protein molecules, reducing the amount of free water available to freeze and concentrate damaging solutes.

Sucrose 🔗

Sucrose is a disaccharide with a strong affinity for water molecules. During freezing, it competes for the water that would otherwise form large, damaging ice crystals, effectively reducing ice crystal size and the mechanical stress they create. It also interacts directly with protein surfaces, helping maintain their native conformation. Cryoprotectants are food additives that prevent protein denaturation in surimi during frozen storage, with muscle proteins – mainly myosin – being particularly vulnerable to degradation. Sucrose is highly soluble and distributes evenly through the surimi matrix, making it practical to use at scale.

Sorbitol 🔗

Sorbitol is a sugar alcohol that provides cryoprotective effects similar to sucrose but with notably lower sweetness. This is an important practical advantage: elasticity and sweetness increased – and overall preference decreased – when the sugar mixture concentration exceeded 4%, which is why precise dosing matters in surimi formulation. Sorbitol molecules form hydrogen bonds with both water and protein molecules, creating a stabilizing network that helps proteins retain their native structure even at freezing temperatures.

The two are typically blended in a 1:1 ratio by weight. A concentration of 2 to 4% sucrose-sorbitol mixture is well-accepted by consumers in surimi sausage, patty, and cake, and at this range, surimi can be well-preserved at −20°C for at least 5 months. Going higher than 4% total sugar concentration tends to introduce excessive sweetness and can alter the neutral flavor profile that makes surimi versatile as a food ingredient.

The role of polyphosphates in cryoprotection 🔗

Sugars alone do not provide complete protection. Polyphosphates – specifically sodium tripolyphosphate (STPP) and tetrasodium pyrophosphate (TSPP) – are added to surimi alongside sucrose and sorbitol to enhance the overall cryoprotective effect.

Sodium tripolyphosphate (STPP) 🔗

STPP is one of the most functionally important additives in surimi processing. Phosphates increase moisture retention and the ability of proteins to reabsorb liquid when surimi is thawed or tempered. They also increase pH slightly, leading to improved gel-forming ability, gel strength, and cohesiveness due to enhanced water-holding capacity. By keeping water molecules associated with the protein structure rather than allowing them to migrate and refreeze, STPP reduces drip loss on thawing and helps maintain protein functionality throughout the cold chain.

Tetrasodium pyrophosphate (TSPP) 🔗

TSPP works synergistically with STPP. Together, the two phosphates prevent unwanted protein-protein interactions – specifically the aggregation of myosin chains during freezing, which is one of the main causes of textural degradation. Adding 0.2-0.3% polyphosphate in the form of STPP or a 1:1 mixture of STPP and TSPP works synergistically with carbohydrate cryoprotectants, and gel strength increases further when a phosphate mixture completely replaces STPP alone. This combination approach is now standard in commercial surimi operations.

Polyphosphate added at 0.5% provides the greatest gel strength, but 0.3% is optimal for gel strength and flavor when sodium tripolyphosphate and trisodium pyrophosphate are used in combination. Exceeding these levels does not proportionally improve quality and may affect flavor.

Step-by-step: the cryoprotectant addition process 🔗

The stabilization of surimi with cryoprotectants follows a clear sequence that must be executed carefully to protect protein integrity from start to finish.

Step 1 – Dewatering the fish meat 🔗

Dewatering is a critical step to avoid excess moisture in surimi before cryoprotectants are added. The washed fish mince is pressed or centrifuged to reduce its moisture content, typically to around 75-80%. Removing excess water is essential: too much residual moisture dilutes the cryoprotectants, reducing their concentration and protective effectiveness. It also ensures that the additives are distributed evenly rather than being diluted unevenly across the batch.

Step 2 – Mixing in the cryoprotectants 🔗

Once the fish meat is properly dewatered, the cryoprotectant blend – sucrose, sorbitol, and the polyphosphate mixture – is added and mixed thoroughly. The sucrose-sorbitol mixture is blended at a 1:1 proportion by weight, while STPP is added at 0.25% and mixed in a silent cutter for 10 minutes. The mixing must be thorough enough to achieve even distribution, but not so aggressive that it damages the protein matrix. It is critical to keep the mixing temperature below 13°C throughout this stage, either by using refrigerated equipment or adding ice during mixing. Even a brief rise in temperature can trigger premature protein denaturation.

Step 3 – Block formation 🔗

After mixing, the stabilized surimi is packed into moulds and formed into standardized blocks – typically weighing around 10 kg – for consistent commercial handling. Cryoprotectants such as sucrose, sorbitol, and sodium tripolyphosphate are added followed by freezing as blocks or rectangular slabs. The block format ensures uniform freezing rates across the product and makes downstream handling, transport, and portioning more efficient.

Step 4 – Freezing 🔗

The formed blocks are then blast-frozen or contact-plate frozen as quickly as possible. Rapid freezing is critical because it minimizes the size of ice crystals that form, reducing mechanical damage to the protein network. Samples are typically blast-frozen at −35°C for two hours and then stored at −20°C. The expected shelf life of frozen surimi is about one year if stored at −25°C – a shelf life made possible by the combination of rapid freezing and effective cryoprotection.

How cryoprotectants maintain surimi’s functional properties 🔗

The ultimate goal of cryoprotection is not just to prevent visible damage, but to preserve the specific functional properties that give surimi its commercial value – particularly gel-forming ability, water-holding capacity, and protein solubility.

Cryoprotective additives reduce protein denaturation by preserving the extractability of salt-soluble proteins during both chilled and frozen storage. In a study comparing different cryoprotectant formulations over 4 months at −20°C, untreated surimi showed a sharp decline in salt-extractable protein from 69.8% down to 46.3%, whereas surimi treated with sucrose, sorbitol, and polyphosphate maintained significantly better protein integrity. Compared to unprotected samples, cryoprotectant-treated surimi showed substantially higher protein solubility, Ca²⁺-ATPase activity, and sulfhydryl content – all indicators that the protein structure has been preserved – while surface hydrophobicity remained lower, indicating less protein unfolding.

These measurable improvements translate directly into product quality: firmer gels, better texture in the final product, and reduced drip loss after thawing. The addition of sucrose-sorbitol cryoprotectants significantly inhibits carbonyl formation and lipid oxidation, and maintains Ca-ATPase activity – demonstrating effective protection against both protein structural damage and oxidative deterioration.

Emerging and alternative cryoprotectants 🔗

The conventional sucrose-sorbitol-polyphosphate system is effective but has limitations. The high sugar load increases caloric content and can add excessive sweetness when concentrations are not carefully controlled. Increasing concerns over sugar intake and the demand for clean-label products have driven the development of natural alternatives, including polysaccharides, antifreeze proteins, and protein hydrolysates, which effectively inhibit ice crystal formation while stabilizing protein structures.

Research into compounds such as carboxymethyl chitosan oligosaccharide, flaxseed protein hydrolysate, and mannan oligosaccharides has shown promising cryoprotective results comparable to – or in some cases exceeding – the conventional mixture. These alternatives are particularly attractive where flavor neutrality and reduced sugar content are priorities. However, cost, regulatory status, and scalability mean that sucrose, sorbitol, and polyphosphates remain the dominant system in commercial surimi manufacturing worldwide.

What do you think? Given the growing demand for low-sugar food products, how do you think the surimi industry should balance the proven effectiveness of sucrose-sorbitol blends against consumer pressure for cleaner, lower-calorie formulations? And if alternative cryoprotectants like protein hydrolysates prove equally effective at scale, what factors beyond performance should guide their adoption in commercial surimi processing?

References

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