The Textural Properties of Surimi

A predominant ingredient in the Orient is fish - used both fresh and comminuted as surimi fish mince.

Made for centuries by the Japanese and thought to date as far back as 1100AD, surimi is now spreading from Japanese to Western processed foods and is used to form extruded, shaped or cooked simulated shell-fish meat products such as crab, lobster, scallop or shrimp.

To gain consumer acceptance of imitation shell-fish, the texture, flavour and appearance of fresh shell-fish must be matched as closely as possible. 

This objective has successfully been achieved by Japanese processors, who produce surimi as an economic alternative to fresh fillets and imitation shell-fish which are barely distinguishable from the real thing. However, in the emulation of this success and creation of products with an authentic shell-fish texture, especially using surimi from non-Japanese species of fish, European food manufacturers are challenged. 

Europeans have not had the history to develop their skills in discrimination sufficiently to equal the quality. Instead, they are using analysis techniques at R&D, production and quality control stages. At each stage in surimi product development, production and quality control, food manufacturers can quantify textural parameters using the 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.

Gel Strength or “Stiffness” using Penetration Tests
The importance of the gel forming ability in surimi is vital, especially in the Japanese food Kamaboko. Strong gels can hold in water and maintain succulence and juiciness of the end-product. Loss of product quality, in the form of decreased gel-forming and water-holding capacity of surimi is attributable to two factors: temperature of heating and the addition of the incorrect level of cryoprotectants (added for the prevention of protein denaturisation during freezing).

1: Although a 5mm ball probe is
typically used for surimi testing,
a range of cylinder and ball
probe diameters are
available for use on
the texture analyser.

The “punch” or penetration test, although considered an empirical test, is the single most popular gel measurement technique used in the surimi industry for evaluating “gel strength” or stiffness. The test imitates the large deformations to failure involved in mastication. Many studies have been reported that correlate puncture or penetration methods 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.

By using a 5mm ball probe, the gel forming ability of surimi can be analysed by the penetration method. Surimi "Gel Strength" is traditionally calculated by Peak force (g) * Distance at the peak (cm) (as shown in Figure 2).

This Gel Strength does not provide significant meaning to the rheological properties of gels. However, it has been arbitrarily used in the surimi industry as a symbol of surimi quality.      

For instance, when this method is used on a range of different surimi sample (as shown in Figure 3) the results show significant differences in gel strength. A high gel strength value for example 1019gcm for ‘A’ grade surimi indicates a much higher degree of firmness compared to its ‘A’ grade counterparts. 

2: Typical curve obtained from a penetration test
to assess gel strength

This highlights the effect of adding ingredients to the product. The very high gel strength of suwari and the very low gel strength of modori when compared to the control surimi highlight the effect of processing time and temperature on the gel consistency of surimi.

However, it is possible that five different gels could have the same gel strength but the protein quality of the gels is significantly different.  

3: The above is the most traditional way to measure the
texture of surimi. However, there are other testing solutions
for the assessment of additional surimi properties.

When used as a single measurement, deformation indicates the quality of surimi proteins – the greater the deformation the higher is the quality. Force values depend on the quantity of proteins – a higher force indicating the least value surimi. Therefore, it is often wise to express force and deformation values individually to indicate the gel functionality of surimi and not solely in the form of gel strength.

The penetration method evalutes a small area, making it useful for showing differences due to location within a sample. However, because a ball probe is symmetrical about its central axis, it is not suitable for determining anisotropy which is better determined by a cutting/shearing test.
 
Surimi Industry Research Findings – don’t take our word for it...

An excerpt taken from ‘Surimi and Surimi Seafood’ (Ed. J. W. Park), 2nd edition:
“Park and colleagues, along with the U.S. surimi industry leaders, evaluated 12 samples of surimi gels with a wide range of quality using five units of conventional rheometer or rheotex and two units of TA.XTplus Texture Analyser. Soft gel could not be measured using any of the five conventional units, while two units of TA.XTplus were able to detect the gel values.  Indeed, the accuracy of the measurement, based on the standard deviation, was much higher with the TA.XTplus instrument. This accuracy might have been due to the ability to calibrate the instrument as needed.”