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肉类研究
MEAT RESEARCH
　2009.12
Received: 11 November 2009
About the author: QU Nan Ocean University of China  email:qunanbetty@sina
Research Progress on Gel Properties of Surimi
QU Nan
（Ocean University of China , Qingdao 266003, China ）
Abstract ：Surimi is an important element of the aquatic deep processing industry. The gel property of surimi is an key factor for evaluating the character of surimi products.How to improve the gel properties in surimi processing is a focus issue nowadays. This article summarized the formation,in fl uencing factors and ways of enhancing the gelation of surimi.Key words: surimi ；gel strength ；elasticity
0　Introduction
S
urimi is made from the fi sh meat through the process
of mining, rinsing, dehydration, add an appropriate amount of sugar, poly-phosphate and other additives to prevent freeze denaturation of the protein so that  it can be preserved a long time at lower temperature conditions [1]. Because its convenient process and it can take full and effective use of aquatic resources, with high protein, low fat, eat convenience, portability and other characteristics, it has become a kind of promising modern aquatic processing foods which consumer favorite. However, at present the majority of the raw materials of surimi-based products are most dependent on import, which led to high production cost. In recent years, Domestic and foreign research scholars generally a
greed that surimi and its products is an important direction of the development of China's freshwater fish processing.The freshwater fish processed into surimi (surimi), can not only solve the contradictions of the freshwater fi sh supply and the different marketing seasons, but also enhance its value, reduce the cost, lower the price of surimi.
The evaluation of the quality of surimi-based products primarily through its elasticity, water holding capacity, flavor, color, the most important is the flexibility. The elasticity of surimi-based products is the comprehensive expression of hardness, flexibility, and adhesive. Depends primarily on gelation of surimi protein. The materials of surimi must have a certain ability to gel.
This article make a brief review of the gel ability, impact factors and improvement measures of surimi.
1  Surimi gelation
Surimi gelation mainly refers to gelation of myosin and actin molecules form a loose network structure before 50℃, and then turn into gel from sol. After the gelation of protein,α-helix of myosin molecules loosen slowly. When the temperature reaches 50-60℃, protein gels turn to a fracture network through the hydrophobic interaction and disulfide bond interaction, leading to the surimi modori. If the temperature continue to increase, gel will become orderly and non-transparent-like, gel strength increa
sed significantly. The gel properties of protein is the key factor to determine the quality of fish products, Because it has a direct impact on organizational characteristics, water retention, caking property and product yield of fi sh
products [2]
.
Myo fi brillar protein is a protein group with important biological features which myosin and actin form the main component. It is the most important muscle protein, which is known as salt-soluble protein. Gel properties of the muscle salt-soluble protein play an important role in structure features, determining the output, texture, adhesion, fat content and water retention of the product. Ferry [3] pointed out that the mechanism of the protein gel is myofibrillar protein heated lead to the dissociation of non-covalent bond, conformational changes to expose reactive groups, especially the hydrophobic groups of myosin which conducive to the interaction between
肉类研究
proteins, so that the denaturation protein-based group polymerization to a larger molecules gels.
Muscle protein myosin is the most important factor in gel, can form a good gel alone. Other myofi brillar proteins such as actin, regulatory proteins, and cytoskeleton proteins do not form gels, but have a signifi cant impact on the viscoelastic properties of gels[4].Lin[5] assume that the most important factor impacting the fi sh gel properties is the content of sarcoplasmic protein. Sarcoplasmic proteins reduce the gel-forming ability of myofibrillar protein and properties (such as strength, water retention, etc.) by affecting the aggregation of myosin. So it is critical to rinse to remove as much as sarcoplasmic proteins to increase the ability of the gel of myofi brillar protein.
2  Surimi modori
Modori is a common phenomenon in the process of surimi-based products which will lead to decline of the quality of surimi-based products. Overseas studies have shown that the softening of surimi-based products due to the hydrolysis of myosin caused by endogenous heat-stable protease. These protease include the lysosomal cysteine protease cathepsin B, L, or L-like, H and other protease [6 ~ 8]. For a long time, rinsing process is used in production to remove the gel softening induced protease in sarcoplasmic proteins to enhance the gel strength, in order to enhance the quality of surimi products.. However, many studies have shown that rinsing process can not completely remove the cathepsin B, L, L-like and H, in particular, cathepsin L [9-12]. However, Li-hong [13] have shown that silver carp suri
mi have the residue activity of the protease B, L, H after rinsing, consistented with the results of the study. In addition, it is also found that the activity of cathepsin L the highest residual rate, this may be because the white silver carp enzyme L can not easily removed by rinsing for its higher affinity with the organization of myosin..This is similar with the nature of the Pacific Alaska pollack’s cathepsin L[13]. There are also studies have shown that cathepsin L has no obvious effect on fi sh such as pollock. According to the different species, myosin may also be bound with other types of self-hydrolysis protease, such as myofi brils combination of serine protease (MBSP).
3 Affecting factors of surimi gel strength and
the strengthening omeasures
The gel strength of surimi and its products can be measured by fracture strength, degree of depression and the water holding capacity. Gelatin is the product of the breaking strength and the degree of depression, can also be measured by gel-based index and gel degradation index. The influencing factors of gel strength of surimi include species of fi sh freshness, rinsing process, heating methods, gels enhancer additions, etc. In addition, the high-pressure treatment can also enhance the gel strength.
3.1  The species of fish
Basic ingredients are also different between different types of fi sh, the diffi culty and facility of gel may be different. For example, gel-forming ability of marine fi sh is stronger than freshwater fish, white meat fish stronger than red meat fi sh, bony fi sh is stronger than cartilaginous fi sh.
3.2  The freshness of fish
Many scholars have already made a lot of research on protein denaturation of fish stored at low temperatures. These studies show that fi sh proteins will denature under storage at low temperature, mainly refl ected in the changes in the spatial structure[15-16]and solubility of myofibrillar protein[17], ATPase activity of actomyosin[16-20], the content of mercapto group, hydrophobicity of myofi brillar protein, and hydrolyze of myofibrillar protein under the effect of exogenous enzyme and endogenous enzyme, ect[22-23].
A large number of studies have shown that there is a certain correlation between the changes in the physical and chemical indicators of surimi protein which is frozen storage. Long time storage will lead to degeneration of myosin, hydrophobic area exposed hydrophobicity increased, the spatial structure of the head of myosin changes, leading to the decrease of ATPase activity, sulfhydryl group oxidation,
disulfi de bonds increased. Pan Jin-feng [24] found that there is some correlation between the decline in different physical and chemical properties of silver carp surimi protein. Silver carp surimi proteins form the disulfi de bonds during frozen storage, which led to a decline in salt-soluble and enzyme activity, and eventually lead to a decline in its water retention. Zhou Yan [25] found that the denaturation temperature of salt-soluble protein of surimi reduced as the increase of the amount of protein loss. Wang Yu [26] study on the optimum conditions of cross-linking reaction and gel-based of silver carp surimi. The result is that there exists an obvious positive correlation between cross-linking and gel strength of myosin.
In short, surimi gel reduced as the fall of the freshness, so fresh ingredients is necessary to get high-quality surimi.
3.3  Rinsing process
Gel breaking strength, sag and gel strength of the surimi which is unrinsed are of the lowest degree, with the highest rate of separated water. The breaking strength, sag and gel strength of surimi gel increase and rate of separated water decline after rinsed with different concentrations of CaCl
2
solution. This shows that due to the presence of a large number of water-soluble proteins in unrinsed surimi gel, salt-soluble myosin can not form a dense gel network. After rinsing, the soluble sarcoplasmic
protein, degradation factors of fi sh gel such as pigment are removad, furthur  more, the presence of Ca2 + activate the endogenous. The enzyme can catalyze the cross-linking of myosin heavy chain. In addition, Ca2 + also has the ability of bridging, reinforcing the protein network. Therefore, after rinsing, the surimi gel quality can be significantly increased. Baxter [27] studied the cleansing’s influence on the crabs gel, water-retaining property and strength of the crabs gel increased if cleaned no more than 3 times.
3.4  Heating conditions method
Heating conditions and manner have a signifi cant infl uence on the surimi gel properties. Due to various surimi has a strong gel-forming ability at 40℃ or so, therefore, fi rst to make surimi gel at low-temperature, then quickly heated through the gel deterioration temperature can signifi cantly increase its gel strength. There are many studies on the gelling properties of eight kinds of West Africa frozen surimi with different heating temperature, heating time and heating method. The results shows that the
surimi heated at 40℃ basically has a strong gel strength, 60～70℃ gel degradation. Heating 20min at 40℃and then heated at 90℃ for 40min, surimi-based products can have a high hardness, elasticity and gel strength.
3.5  Additives
some additives are often added in the surimi processing to enhance the gel performance. In the freshwater fi sh surimi processing, starch, dietary fiber, protease preparations, Ca2+-containing substances and reducing substances are commonly used. They have a important influence on the freshwater fish surimi gel properties and quality. In addition to consider the impact of the single factors on the freshwater fi sh surimi, their synergistic effect should also be taken into account.
3.5.1  Starch and dietary fiber
Starch is a commonly used thickener and excipients in meat products. It can affect the quality of carp surimi gel. Studies have shown that potato starch is better than corn starch and potato starch on increase the quality of carp surimi gel. Egg white protein is significantly better than the peanut protein with the same amount on the role of carp surimi gel. Carrageenan can improve the quality of carp surimi gel. the quality of the carp surimi is the best when the addition of Potato starch, egg white protei
n and carrageenan in proportion to 10:1:1 [28]. Dietary fi ber have water-retention capacity, can signifi cantly improve surimi gel strength and effectively delay the deterioration of surimi gel.3.5.3  Reduction
Adding a number of reduction at the begining of surimi gelation can inhibit thiol oxidized to disulfide bonds, restore the activity of frozen denatured protein. For example, add 0.08%～0.10% reduction of mercaptoethanol and sodium and so on at the begining of gelation can recovery the activity of oxidized sulfhydryl group in muscles of freshwater fi sh surimi after frozen storage.  the level is up to 80%.
3.5.4  hydrocolloids
Hydrocolloids affected the gel properties of horse mackerel surimi. The addition of the available glue, tamarind gum, konjac gum, carrageenan, agar and CMC can improve the gel strength of horse mackerel surimi. Adding guar gum and sodium alginate on the horse mackerel surimi leads to slightly affected of its gel properties, but not signifi cant. Xanthan gum and pectin can lower the gel properties of mackerel surimi[30]. Konjac gum has little effect on the intensity of the myosin heavy chain band, but can make surimi to form dense, uniform gel network. At the same time, increasing the gel strength and water holding capacity, reducing the whiteness of surimi gel [31].
3.5.5  protein additive
Protein additives can make some improvement on the gel strength of surimi especially freshwater fish surimi. such as soybean trypsin inhibitor (STI) can inhibit the degradation of myofibrillar protein caused by myofibril-
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3.5.2  Cryoprotectants
Cryoprotectants are mainly sucrose, sorbitol, compound phosphate and other oligosaccharides. The -OH group of sucrose, sorbitol can form a stable structure with the water molecules around the myofi brillar protein of surimi. Compound phosphate can adjust the pH value of surimi, prevent the surimi protein denaturation; chelate metal ions in surimi, increase ionic strength; allows actomyosin decomposed into actin and myosin protein, it can also dissolved salt-soluble proteins.The flavor of surimi products will be affected after adding sugar and some other sweet substances as Cryoprotectants. so the non-sweet taste of Cryoprotectantsresearch is getting more and more attention.reactive materials studies
Rehalose is non-reducing disaccharide, with the ability of protect biological dehydration, inhibit the decrease of salt-soluble myofibrillar proteins, Ca2+-ATPase activity and sulfhydryl content and the incr
ease of the surface hydrophobicity during frozen, delaying the freezing-thawing of bighead carp myofi bril protern. More effective than sucrose, sorbitol and their mixtures[29]. There are also studies have shown that polyphenols can be added in the frozen surimi to signifi cantly lower the acid value, peroxide value, TVBN value. It also can inhibit the reduction of water content in frozen surimi and slow down the decrease of frozen surimi gel strength significantly, while the impact on the gel strength frozen surimi was not signifi cant.
肉类研究
bound serine protease (MBSP) in the muscle of silver carp. It is found that caspase can inhibit the modori of the marine fi sh, but the inhibition of modori of freshwater fi sh is less. Soy protein isolate (SPI) can enhance the intensity of MHC [31]. Studies have shown that tyrosinase has an impact on thermal stability and gel formation of myofibrillar protein of breast [32]. SDS-PAGE gel electrophoresis showed that myosin, troponin are of the most sensitive to tyrosinase while the actomyosin not changed signifi cantly. Globulin play a decisive role in gelation. Thus the addition of tyrosinase to breast can increase the gel strength, hardness, moisture retention. Egg albumin and bovine plasma protein could slow down the softening of surimi gel due to its inhibition on the cathepsins of fi sh.
The addition of TGase can catalyzes the formation of cross-linked bond between MHC of Eel surimi, thereby enhancing its gel properties. The gel strength of add 5% TGase Eel surimi is three times higher[33]. At the same time, additives and seasoning additions can affect or limit the production of fishy smell substances. Because they have the activity to inhibit microbial producing fi shiness substances, they can neutralize, transform, adsorption, embed or mask fi shiness substances to achieve the purpose of the reduction or removal of fi shiness smell.3.5.6 Pressure
Studies have shown that the appropriate pressure is conducive to protein gel. Because it lead to changes in the covalent structure of protein, which is conductive to to the interaction of protein. As a result, the elasticity of surimi gel is improved. But the excessive pressure may result in the denaturation of action, which is not conducive to protein gelation. Iwasaki et al [34] have reported a similar conclusion is that it is conducive to protein gel under the pressure of 200MPa, the pressure of more than 200MPa would inhibit the fl exibility of rubber.
4  Prospects and outlook
Improve gel strength become the focus of the study as the demands for surimi-based products increasing. Muscle salt-soluble protein is the decisive factor of surimi gel. The formation of myofi brillar
protein gel which is induced by heat is a complex dynamic process. There are a number of factors will affect the gel formation in varying degrees. Therefore, the myofibrillar protein gel formation and its infl uencing factors also need further research.
（Translated from Meat Research 2009.10，pp80-83）

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