Effect of storage on Texture: Seafood

Fish

The main structural similarities between fish and meat are in the form of the muscle fibres. The major structures of the two are quite different, however. Unlike meat, fish flesh is composed of muscle sheets running head to tail on each side of body, divided into segments by perpendicular sheets of connective tissue. 

Cooked fish has a tender, delicate texture for two reasons. Firstly, fish flesh contains much less connective tissue than is found in warm-blooded animals, and secondly, contains more thermally unstable collagen, which liquefies on heating. This causes the structure to fall apart when force is applied, as the bonds caused by connective tissue are no longer present. Connective tissue properties influence the texture of raw fish, whereas cooked fish is influenced by muscle fibre properties.

Fresh Fish

Once fish has been caught, it is placed on ice until purchased to minimise spoilage. Changes occur during this cold storage time, affecting fish texture. These changes are made up of three stages: pre-rigor, rigor and post-rigor. The texture of the fish at each stage is heavily influenced by the characteristics and treatment of the fish before and after death, including size, activity and post-mortem handling.

Freshly killed fish have soft and extensible muscles, but fish cooked straight after being caught is tough due to heat acceleration of rigor. Changes to texture begin to occur early in the post-mortem period, particularly in the physicochemical state of myofibrillar proteins.

Rigor mortis brings greater changes to texture, peaking at one or two days after catch. Chemical changes prevent muscle filaments from sliding past each other. Muscles contract, which leads to toughness, hardness and lack of extensibility. However, the elevated temperature of cooking brings about a partial resolution of rigor and a decrease in toughness. In contrast to a meat like beef, fish muscles do not appear to exhibit cold shortening.

Biochemical changes in the post-rigor period lead to softening and, occasionally, mushiness and tissue disintegration.

Frozen Fish

Fresh fish is highly subject to microbial spoilage and has a very limited storage life even when kept on ice. Consequently, other storage methods must be used in most cases. Air drying is an effective preservation method but causes irreversible changes to the texture of fish. It is not easily rehydrated, causing a tough, dry product when cooked. Freeze-drying gives a better product but still does not hold water well and has a poor texture. Smoking and canning produce acceptable products, but these have limited uses compared to fresh fillet.

Freezing is the most important commercial process for preserving filleted fish form. For retail trade, the fillets are wrapped or packaged individually. For wholesale and further processing, fish is frozen in blocks. The shelf life of frozen fish may be as long as two years.

The texture of frozen fish is affected by its condition upon freezing. It can be improved using a range of methods, such as vacuum packaging or storing in brine. If dehydration is not prevented, dryness can be observed in frozen fish. However, the textural change most commonly seen is a rubbery mouthfeel due to toughening. Textural deterioration can be caused by protein denaturation. Additionally, structural damage is caused by the formation of ice crystals; a faster freezing rate is less damaging because it results in finer ice crystal size and less disruption to the structure. Ice crystals grow further during storage. Temperature fluctuations are very detrimental. Additionally, the formation of ice removes water from the protein network, which weakens the system of hydrogen bonds, causing the protein structure to break down.

Enzyme activity contributes significantly to textural deterioration of fish, depending on the breed. One example is the catalysis of formaldehyde formation. The presence of even a low concentration of formaldehyde leads to texture degradation, causing toughening, leading to a rubbery texture.

There are many Texture Analysis test methods available for assessing fish in any state. The main sample group that will be tested is either fresh or defrosted fillet. Care must always be taken to test the sample in the same region of the fish as texture is known to vary greatly from one length of the fish to the other.

Finger Test

Performing a test on fish using a ball probe

A commonly-used industry quality test for fillet firmness is a finger press test on the side-line at the dorsal fin, applying 1kg for two seconds (600g for tuna). The fillet is characterised as elastic (the surface springs back to shape instantly), or as increasingly plastic (an undesirable characteristic) if the surface only springs back partially, or the finger goes right through the fillet. This test can easily be mimicked using a Texture Analyser and a hemispherical or spherical probe. Advised location of testing on a fillet using these probes is shown.

Deboning Test

Tensile test on a Texture Analyser

The majority of fish products sold on the market in the UK are prepared for direct cooking, which means that they are processed “pin-bone free”. Deboning is, therefore, an important step within the manufacturing process of fish. A Texture Analyser can be used to measure the pulling force and break point of bone samples, either by pulling bones from the fillet with a tensile grip, or by performing tensile tests on the bones themselves.

Testing fish using a Warner Bratzler blade

Cutting Test

The Warner-Bratzler shear blade can be used to assess the firmness and texture in salmon fillets. The blade is used to assess the cutting force, or ‘bite’ of fresh fish. This method is sensitive and yields accurate and repeatable data.

Tensile Gaping of Fillet

Pizza Tensile Rig

While the Warner-Bratzler shear blade is effective in measuring firmness and predicting post-smoking texture, a tensile test provides the highest levels of accuracy when predicting post-mortem gaping (the appearance of tears or slits within the fillet on handling). This test, which uses the TA.XTplus Texture Analyser along with the Pizza Tensile Rig to pull samples apart while measuring the force required to do so, is a valuable addition to the existing pool of instrumental methods for fish flesh quality assessment. A large amount of salmon is downgraded during secondary processing due to soft flesh or gaping. Soft flesh results in a poor quality product with an unpleasant, mushy mouthfeel, while gaping causes unsightly fillets with low customer acceptability. Fillets with significant gaping are also unable to withstand the demands of mechanical handling and industrial processing.

Shellfish

Shellfish is perishable by microbial spoilage to a greater extent than fish. It can be prevented by keeping the crustaceans alive up to the point of cooking or freezing. Raw shellfish becomes soft and mushy if refrigerated for a long time after harvest due to protein breakdown, but this time can be extended with a modified atmosphere.

Freezing is the most common method of preservation for shellfish. However, as with other animal muscle based products, freezing causes the formation of ice crystals, leading to structural damage and degradation of texture. As with fillet, slow freezing causes larger crystals, leading to tissue dehydration and shrinkage. This causes toughening.

Kramer Shear Cell testing prawns

Shearing Test

A cutting test using a multiple blade fixture such as the Kramer Shear Cell allows an averaging effect of anisotropic samples, or samples with non-uniform geometry when measuring the hardness or toughness of shellfish. Additionally, the test area is large such that the influence of local irregularities is reduced.

Shellfish Analogues

Surimi is a large sector of the modern fish market. This is the manufacture of shellfish analogues from fin fish, including crab legs, lobster tail, scallop and shrimp analogues. Surimi is produced by forming a fish protein solution and then forming a protein gel network. Frozen storage is detrimental to gel-forming ability, but surimi does have to be stored frozen. Cryoprotectant additives help to preserve this gel structure.

Storage conditions affect surimi quality. For example, gel hardness and cohesiveness are lower when formed from fish kept on ice for a long time, as protein breakdown and denaturation occur.

Surimi is mainly deteriorated by microbial spoilage. Packaging solutions are used to prevent this including controlled atmospheres and low temperature storage.

5mm Ball Probe for indentation testing

At each stage in surimi product development, production and quality control, food manufacturers can quantify textural parameters using a TA.XTplus Texture Analyser. This equipment can provide accurate data about, for instance, the effects of gel moisture content, salt concentration, cooking temperature and length of time heat is applied during processing which cuts out the guess-work. Texture analysis is applicable both near-line and in individual laboratory tests for surimi manufacture, measuring specific toughness, elasticity and gel strength of surimi-type products.

Surimi Penetration

The importance of the gel forming ability in surimi is vital. Penetration test results can be correlated with the sensory properties of surimi gels. This attribute of the test, coupled with its convenience, has made it popular for quality control within the surimi industry. Surimi gel strength can be measured by multiplying peak force and distance to peak from a 5mm ball probe indentation test. This indentation method evaluates a small area, making it useful for showing differences due to location within a sample. This measurement does not provide significant meaning to the rheological properties of gels. However, it is often used arbitrarily in the surimi industry as a symbol of surimi quality. 

Knife Blade 

Surimi Cutting

As a ball probe is symmetrical about its central axis, it is not suitable for determining anisotropy in surimi. This is better determined by a cutting test, which can be used to assess whether a product is too tough. Using a Knife Blade, this test simulates the action of cutting with a knife or initial bite when placed between the front teeth.

Surimi Elasticity

P50 Cylinder probe

Shellfish has a characteristically chewy texture and manufacturers need to imitate this to create a convincing analogue. In texture analysis terms, chewiness is measured by elasticity. High elasticity produces an item with a rubbery consistency whereas low elasticity creates an undesirably brittle product. In a compression test, the ‘hold distance until time’ setting will show the product’s relaxation properties. The percentage of force relaxation represents the index of elasticity, providing information important in the development of new surimi products. In these tests, samples are placed under a cylinder probe that fully covers their surface. This test examines the recovery of the surimi samples after compression for a certain length of time. It is also suitable to assess the effects of the compression of products during transport and handling and the ability of a product to retain or regain its shape.

By determining the cutting strength, elasticity and gel strength of surimi, manufacturers can make informed decisions on ingredients and processes used.

Texture Analysis is an advantageous step in the quality control process of any fish supplier, enabling quick and easy measurements of the most important quality parameters in any given product, and reducing the capacity for human error.

Reference: Food Storage Stability, Chapter 8 “Effect of storage on texture” – Alina S. Szczesniak

 

There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of solutions.

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For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

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