microstructure ofsuwari andkamaboko sardine surimi gels

Microstructure of suwari and kamaboko sardinesurimi gelsCristina Alvarez, Isabel Couso and Margarita Tejada*Instituto del Frı́o (CSIC), Ciudad Universitaria, 28040 Madrid, Spain

Abstract: In a previous work it was suggested that the texture of kamaboko (set and cooked) gels made

from sardine surimi under varying setting conditions was predetermined by the speci®c matrix

forming in each suwari (set) gel. This paper describes the microstructure of the networks formed in

suwari and kamaboko gels set at 25, 35 and 40°C for 30 or 60min as examined by scanning electron

microscopy (SEM). Cooking conditions for kamaboko gels were ®xed at 90°C for 30min; other

preparation conditions were invariable. At low magni®cation (��500) the gel matrixes were compact,

with practically no differences among lots. At higher magni®cation (�20000), the suwari gel matrixes

formed at low temperature consisted of globules. At higher temperatures the globules joined up to form

®brillar structures (®bres) and zones of disordered globule aggregation (coagula); at longer setting

times, lateral bonding of the ®bres became apparent. Kamaboko gels produced from unstructured

globular matrixes exhibited only a few ®brillar zones and large areas of coagula. Where there was

already an incipient ®brous formation, these developed into individual ®bres or bundles of ®bres that

correlated with the best texture characteristics. Suwari gels with extensive lateral bonded ®bres gave

rise to kamaboko gels with a highly compact appearance under SEM; this correlated with a decline in

texture values. These different structures suggest that the protein±protein bonds in the suwari

networks have different levels of stability to heat, and these levels determine whether or not the

proteins can subsequently be reorganised when the kamaboko gel forms.

# 1999 Society of Chemical Industry

Keywords: sardine; gels; surimi; suwari; kamaboko; microscopy

INTRODUCTIONThe structural characteristics of the matrix and the

type of intermolecular interactions occurring under

different processing conditions determine the func-

tional properties and texture of gels. In heat-induced

gels, the proteins must unfold and aggregate to form

the network.1 Gelation starts with a reversible dena-

turation process, which becomes irreversible once the

denatured protein aggregates with other protein

molecules.2 Network characteristics vary according

to the rate of denaturation and aggregation. Rapid

unfolding of proteins followed by slow aggregation

favours the formation of networks with a ®ne stranded

structure, whereas gel matrixes tend to be coarse when

aggregation is rapid.3

The myo®brillar proteins of ®sh species are highly

unstable to heat, and therefore the application of

different heating conditions in the preparation of

thermo-irreversible gels can have a considerable effect

on the characteristics of the resulting gel, even where

all other gel preparation conditions are constant. In

previous studies we have established the types of

bonding among the major proteins4 and the prepara-

tion and composition conditions5 required to produce

thermo-irreversible sardine surimi gels with given

textural characteristics. Work has also been done on

the effect which the initial network formed in setting

conditions (suwari gels) has on the texture of set and

cooked (kamaboko) sardine surimi gels,6 from which it

was concluded that there is a range of optimum setting

temperatures and times that ensures kamaboko gels

with good textural characteristics.

In surimi gels attempts have been made to relate the

microstructure of the network as observed under

microscopy to certain texture characteristics of the

gels.7±9 Most of the work done on the ultrastructure of

surimi gels using scanning electron microscopy (SEM)

concerns gels made with added ingredients, where the

observed texture differences are related to changes

occurring in the network when the ingredient is

added.10±15 Very little work, however, has been

published relating the ultrastructure of networks

produced under different heating conditions to ob-

served differences in texture.10,13,17

The aim of this paper was to study by scanning

electron microscopy (SEM) the microstructure of

Journal of the Science of Food and Agriculture J Sci Food Agric 79:839±844 (1999)

* Correspondence to: Margarita Tejada, Instituto del Frı́o (CSIC), Ciudad Universitaria, 28040 Madrid, SpainE-mail: [email protected]/grant sponsor: Spanish Interministerial Commission for Science and Technology (CICyT); contract/grant number: ALI94-0954-CO2-01-02(Received 23 June 1997; revised version 21 July 1998; accepted 19 October 1998)

# 1999 Society of Chemical Industry. J Sci Food Agric 0022±5142/99/$17.50 839

suwari and kamaboko gels made from sardine surimi,

where the only variables were the heat setting

conditions, which had shown differences in texture.6

The object was to ascertain the effect of various setting

time±temperature combinations on the resulting

suwari and kamaboko networks and relate them to

differences in texture.

MATERIALS AND METHODSMaterial usedFrozen sardine (Sardina pilchardus) surimi (SA grade),

prepared in one batch for this study by SCOMA

(Lorient, France), was air-freighted with solid CO2 to

the laboratory, cut into blocks, vaccum-packed in

Cryovac BB-1 bags (10.66kPa pressure) and stored at

ÿ20°C (�1) for up to one month. Cryoprotectants

added to the surimi were 40gkgÿ1 sucrose, 40gkgÿ1

sorbitol and 3gkgÿ1 Na tripolyphosphate. Crude

protein was measured by the Kjeldahl method,18

crude fat by the Bligh and Dyer method19 as modi®ed

by Knudsen et al20 and moisture and ash by AOAC.18

The proximate composition of the surimi was: crude

protein 131.3gkgÿ1; crude fat 30.5gkgÿ1; moisture

758.2gkgÿ1; ash 6.3gkgÿ1. The pH was 6.6.

Gel preparationSardine surimi was tempered for about 2h at 21°C(�2) until it reached-5°C (�1), then ground for 1min

(Stephan UM12 refrigerated vacuum cutter mixing

machine, Stephanu SoÈhne GmbH & Co, Hameln,

Germany, 10kPa, coolant temperature ÿ2°C). The

surimi was chopped for 1min at high speed (setting 1).

Salt was added (NaCl, 30gkgÿ1 surimi, equivalent to

24.4kgÿ1 in the ®nal gel) and ice ¯akes (as necessary to

adjust moisture content to 780gkgÿ1) were added and

the mixture was beaten slowly (setting 2) for 5min.

Temperature of the surimi sol was kept below 10°C at

all times. The sols obtained were heated in stainless

steel cylinders (30mm inner diameter�30mm height)

with screw-on tops and bottoms with te¯on rings to

seal, taking special care to ensure that sol was well

packed and free of air bubbles. Suwari and kamabokogels were obtained heat-setting for 30min (at 25, 35

and 40°C) and 60min (35 and 40°C) in a water bath

(Julabo F10, Labortechnik GmbH, Seelbach, Ger-

many). Kamaboko gels were obtained cooking the

heat-set gels at 90°C for 30min using a saturated

steam oven (Rational Combi-Master CM6, Grob-

kuÈchentechnik GmbH, Landsberg Lech, Germany).

After treatment, the gels were cooled under running

tap water and stored at 4°C (�1°C) for 24h before

analysis.

Scanning electron microscopy (SEM)Small gel blocks (2mm sided cubes) were cut from the

centre of the cylindrical gel specimen, ®xed with 2%

glutaraldehyde in a phosphate buffer (pH 7.2) then

dehydrated in solutions of acetone of increasing

strength, and critical-point dried (Balzer mod

CPD030) using CO2 as transition ¯uid. The prepared

samples were mounted on copper specimen holders,

sputter-coated with gold (Balzer mod SCD 004) and

examined using a JEOL JSM 6400 scanning electron

microscope operated at up to 15kV. Four replications

at 4 different magni®cations (�35, �500, �6000 and

�20000) were performed.

RESULTS AND DISCUSSIONAt low magni®cation (��500) there was very little

difference between the gels; the microstructure of all

lots exhibited a compact, continuous matrix (results

not shown) similar to those found in other sardine

surimi gels.14 At higher magni®cation suwari gels made

at 25°C for 30min exhibited a structure composed of

globular elements (mean diameter 0.22mm�0.06)

joined together without any de®nite orientation or

interglobular cross-connections (Fig 1). Heating at

35°C for 30min (Fig 2) initially produced ®brillar

structures (®bres) (width 0.57mm�0.25) with a

globular appearance on the surface. Where setting

was prolonged to 60min (Fig 3) the ®bres became

more clearly de®ned, with two coexisting size ranges:

thin ®bres (width 0.11mm�0.03) and ®bre bundles

(�1.00mm). These ®bres were longer than the ®bres

that formed in 30min, with amorphous end contours

forming compact zones. At 40°C and 30min setting,

the ®bres were composed of globular aggregates in

orderly lengthwise formation (width 0.40mm�0.09)

Figure 1. Ultrastructure of suwari gels set at 25°C for 30min. Bar=1mm.


840 J Sci Food Agric 79:839±844 (1999)

C Alvarez, I Couso, M Tejada

(Fig 4), whereas those heated for longer (60min) at the

same temperature exhibited lateral bonding of the

®bres, producing a very compact structure (Fig 5).

The kamaboko gels set at 25°C for 30min exhibited a

matrix with sparse ®brillar structure (width

0.16mm�0.03) with large cavities and large cauli-

¯ower-like areas of haphazard globule aggregation

(coagula) (Fig 6); this microstructure was very

different from the network in the corresponding suwarigel, which was composed largely of globular elements

without any de®nite ®brelike or coagula structures.

The kamaboko gels set at 35°C for 30min exhibited a

®brillar structure in which the ®bres were thinner

(width 0.25mm�0.05) but longer than in the suwarigels; these were composed of globular particles in

lengthwise formation, producing a structure of con-

tinuous ®bres with inter-®brillar connections and

lateral association of ®brillar zones (Fig 7). With

60min setting, the ®brillar structure of the kamabokonetwork was more compact than with shorter setting,

due mainly to greater lateral association of adjacent

®bres (Fig 8). This structure was incipient in suwarigels, but here the ®bres were less clearly de®ned and

the structure more compact. The kamaboko gels set at

40°C for 30min (Fig 9) exhibited a morphology in

which ®brous zones (width 0.15mm�0.04) coexisted

with coagula of disordered aggregates which were




Figure 6. Ultrastructure of kamaboko gels set at 25°C for 30min andcooked at 90°C for 30min. Bar=1mm.



J Sci Food Agric 79:839±844 (1999) 841

Microstructure of suwari and kamaboko sardine surimi gels

more clearly de®ned than in the corresponding suwarigel. With 60min setting the kamaboko gel exhibited

more lateral ®brillar association, lending the gel a more

compact appearance with less inter-structure cavities

(Fig 10).

Since the work of Ferry,3 it has been generally

accepted that formation of a heat-induced gel requires

protein denaturation, which is accompanied by a

conformational change and exposure of reacting

groups, followed by a second stage in which the

denatured proteins establish protein±protein inter-

actions; these lead to aggregation, which is the origin

of the network's three-dimensional structure and in

which there must be a balance of attracting and

repelling forces. The required thermal conditions must

exist for aggregation to occur in such a way that the

molecules are correctly oriented for the formation of

gel structures; if the time or temperature is insuf®cient,

the molecules do not successfully form an orderly

network (gel), and if the temperature is too high, the

proteins aggregate randomly to form disordered

coagula.

The process of making thermo-irreversible gels from

surimi normally consists of two steps. The ®rst is

setting of a salt±surimi sol (�40°C), a process in which

the proteins, denatured by the action of the salt and the

mixing, are oriented; this ensures a gradual sol±gel

transition so that an orderly initial protein mesh is

formed. The second step is high-temperature

(�80°C) cooking to form the ®nal kamaboko network.

It is generally accepted that the network formed at

setting temperatures is only loosely structured and that

subsequent heating to obtain kamaboko gels serves to

stabilise this network. Formation of the set network

involves secondary interactions, disulphide bridges

and non-disulphide covalent bonds in varying propor-

tions depending on the time-temperature conditions,

the species from which the surimi is obtained and the

quality of the surimi. At high temperatures, there is

believed to be large-scale formation of disulphide

bonds and hydrophobic interactions.4,6,21,22

Our results show that in gels made with sardine

surimi the structure of the suwari networks differed

according to the heating conditions. When the setting

temperature was low (25°C), the gel had a globular

structure, probably because although denatured myo-

®brillar proteins did establish links with adjacent

molecules, the setting time was too short to allow the

orientation necessary to form higher structures. At

higher temperature (�35°C), ®brillar structures ap-

peared either individually or in bundles formed by a

lengthwise aggregation of globules. This suggests that

there was some element responsible for the way that

the globules arranged themselves to form ®bres. When

setting at 40°C was prolonged, the network became

compacted through the formation of higher structures,

apparently formed by lateral association of structures

of ®brillar appearance

The effect of setting temperature was appreciable as

a different microstructure in kamaboko networks; at

low temperature (25°C), there were ®brous zones with

large areas of disordered aggregation (coagula),

whereas at high temperature (40°C) or at prolonged

setting time (60min), there was lateral compacting of

the previously-formed ®bres and larger areas of

disordered aggregation (coagula). These structures

are thought to be due to the bonding of adjacent

molecules in the set gels. The formation of ®bres in

kamaboko gels would be the manifestation of length-

wise orientation of the globules along an axis, while

areas of ®brillar compacting or coagula would indicate

larger numbers of connecting points between adjacent

molecules whether or not these are already in ®brous

formation.

The structures composed primarily of ®bres or ®bre

bundles with clearly-de®ned contours found in gels set

at 35°C for 60min or 40°C for 30min, correlated with

the highest gel strength (GS), yield strength (YS) and

yield deformation (YD) values in these suwari gels; the

globular structure (25°C) or the linking of ®bres to

form more compact structures (40°C for 60min)

correlated with lower GS, YS and YD measured in

these gels.6 Thus gels with different microstructures

can give rise to similar fracture properties, as observed

in globular protein gels.23

Heating of the suwari gels to form kamaboko gels

produced a lengthwise arrangement of globules and

additional aggregation over the network already



842 J Sci Food Agric 79:839±844 (1999)


formed. The result was that kamaboko gels showed a

more ®brous appearance, the ®bres being composed of

globules with more clearly de®ned contours that in the

suwari gels. This structural modi®cation of kamabokonetworks was appreciable in texture as increased GS,

due mainly to the fact that YS values were higher than

in the corresponding suwari gels.6 The kamaboko gels

with the best texture characteristics were those whose

matrix structure was ®brillar, texture becoming poorer

where there were large areas of coagulation or lateral

compacting of ®bres, in either of which cases the GS,

YD and YS values of the gels were similar.6 The lowest

response of the gels to compression occurred in gels

where there was more compacting of ®brillar struc-

tures. The textural weakening of the gels observed

when high-temperature setting was prolonged was

previously considered to re¯ect a destruction of the

network formed at shorter time.6 Microscopic analy-

sis, however, showed that it actually consisted in

compacting of the previously formed ®brillar struc-

tures

The results suggest that the ®nal structure of

kamaboko networks was the outcome of denatura-

tion-aggregation during setting of the surimi sol, and

that this determined the possibility of reorganisation of

the molecules to form the ®nal network. If denatura-

tion occurred during setting but the intermolecular

bonds were predominantly unstable to heat (predo-

minance of hydrogen bonds), the large-scale protein

denaturation occurring when the gel was heated to

produce the kamaboko gel would cause rapid aggrega-

tion of unoriented molecules, since at this stage there

was very little heat-stable bonding to secure the

structure. The result would be areas of coagula held

together by heat-stable bonds. If, due to high

temperature or prolonged setting, the molecules

aggregated by means of heat-stable bonds, in the later

cooking stage to form kamaboko gels, reorganisation

will be limited so that only connection to adjacent

®bres would be possible and the ®brillar structure

which formed during setting would be compacted.

CONCLUSIONSThe network structure of sardine kamaboko gels was

previously laid down during setting. The best texture

characteristics of kamaboko gels were related to a

®brillar microstructure with continuous, clearly con-

toured ®bres or ®bre bundles. In sardine surimi sols,

setting gave rise to different morphologies depending

on the heating conditions in which setting took place.

If setting was such that the globules formed practically

no higher structures (low temperature, short time), the

matrix of the subsequent kamaboko gel had few ®brillar

zones and large areas of coagula, in which case texture

was characterised by lower GS, YD, YS. If ®brillar

structures began to form during setting, these stabi-

lised in the kamaboko gel, producing a range of ®brillar

structures of varying thickness, and the texture

characteristics of the gels was optima. On the other

hand, when extensive compacting of ®bres during

setting (long time, high temperature) was produced,

the network structure of the kamaboko gels was also

characterised largely by areas of ®brillar compacting

and the gels texture was similar to those of gels with a

predominance of areas of coagula formed at low

temperature.

ACKNOWLEDGEMENTSThanks are due to Dr B Fernandez and Dr MT Solas

of the Departmento de BiologõÂa Celular, Facultad de

Ciencias BioloÂgicas, and Dr C Barba of the Centro de

MicroscopõÂa ElectroÂnica, both at Universidad Com-

plutense de Madrid, for their valuable help in the

preparation of the SEM samples and the under-

standing of the results. The Spanish Interministerial

Commission for Science and Technology (CICyT)

®nanced this research under project ALI94-0954-

CO2-01-02.

REFERENCES1 Liu MN, Foegeding EA, Wang SF, Smith DM and Davidian M,

Denaturation and aggregation of chicken myosin isoforms. J

Agric Food Chem 44:1435±1440 (1996).

2 Lepock JR, Ritchie KP, Kolios MC, Rodhl AM, Heinz KA and

Kruuv J, In¯uence of transition rates and scan rate on kinetic

simulations of differential scanning calorimetry pro®les of

reversible and irreversible protein denaturation. Biochemistry

31:12706±12712 (1992).

3 Ferry JD, Protein gels. Adv Prot Chem 3:1±78 (1948).

4 Careche M, Alvarez C and Tejada M, Suwari and kamaboko

sardine gels. Effect of heat treatment on solubility of networks.

J Agric Food Chem 43:1002±1010 (1995).

5 Alvarez C, Couso I and Tejada M, Sardine surimi gels as affected

by salt concentration, blending, heat treatment and moisture. J

Food Sci 60:622±626 (1995).

6 Alvarez C and Tejada M, In¯uence of texture of suwari gels on

kamaboko gels made from sardine (Sardina pilchardus) surimi. J

Sci Food Agric 75:472±480 (1997).

7 Okada M and Migita M, Photomicrographic examination of ®sh

meat jelly. Bull Jap Soc Sci Fish 22:265±268 (1956).

8 Suzuki T, Fish and Krill Protein: Processing Technology, Applied

Science Publishers Ltd, London, UK (1981).

9 Sato S and Tsuchiya T, Microstructure of surimi and surimi-

based prducts, in Surimi Technology, Ed by Lanier TC and Lee

CM, Marcel Dekker, New York, USA, pp 501±518 (1992).

10 Montejano JG, Hamann DD and Lanier TC, Thermally induced

gelation of selected comminuted muscle systems. Rheological

changes during processing. Final strengths and microstructure.

J Food Sci 49:1496±1505 (1984).

11 Verrez-Bagnis V, Bouchet B and Gallant DJ, Relationship

between the starch granule structure and the textural proper-

ties of heat-induced surimi gels. Food Struct 12:309±320

(1993).

12 Alvarez C, Couso I, Tejada M, Solas MT and Fernandez B

Action of starch and egg-white on the texture, water-holding

capacity and microstructure in surimi gels, in Quality Assurance

in the Fish Industry, Ed by Huss HH, Jacobsen M and Liston J,

Developments in Food Science 30, Elsevier Science Publish-

ers, Amsterdam, The Netherlands, pp 449±457 (1992).

13 Alvarez C, Couso I, Tejada M, Solas MT and Fernandez B

In¯uence of manufacturing process conditions on gels made

from sardine in 4th Symposium on Food Proteins. Structure

Functionality Relationship, Ed by Schwenke KD and Mother R,

Reinhardsbrunn, Germany (1992).

J Sci Food Agric 79:839±844 (1999) 843

Microstructure of suwari and kamaboko sardine surimi gels

14 Alvarez C, Couso Y, Solas MT and Tejada M, Waxy corn starch

affecting texture and ultrastructure of sardine surimi gels Z

Lebensm Unters Forsch A 204:121±128 (1997).

15 Lee CM, Wu MC and Okada M, Ingredient and formulation

technology for surimi-based products in Surimi Technology Ed

by Lanier TC and Lee CM, Marcel Dekker, New York, USA,

pp 273 (1992).

16 Couso I, Alvarez C, Solas MT, Barba C and Tejada M,

Morphology of starch in surimi gels. Z Lebensm Unters Forsch

A 206:38±43 (1998).

17 Tejada M, Alvarez C and Couso I, Solas MT, Behaviour of

sardine surimi during heating, International Conference

Upgrading and Utilization of Fishery Products (FAR), 12±14

May, Noordwijkerhout, The Netherlands (1992).

18 AOAC Of®cial Methods of Analysis of the Association of Of®cial

Analytical Chemists, 12th edn, AOAC International, Washing-

ton, DC, USA (1984).

19 Bligh EG and Dyer WJ, A rapid method of total lipid extraction

and puri®cation Can J Biochem Physiol 37:911±917 (1959).

20 Knudsen L, Reimers K, Berner L and Jensen NC, A modi®cation

of Bligh and Dyer's oil extraction method reduting chloroform

vapour outlet. Abstracts of papers. 15th Western European Fish

Technologists' Association Meeting, Hamburg, Germany

(1985).

21 Niwa E, Chemistry of surimi gelation, in Surimi Technology, Ed

by Lanier TC and Lee CM, Marcel Dekker, New York, USA,

pp 389±428 (1992).

22 Tejada M, Gelation of myo®brilar ®sh proteins. Rev Agroquim

Technol 34:257±273 (1994).

23 Foedgeding EA, Bowland EL and Hardin CC, Factors that

determine the fracture properties and microstructure of

globular protein gels. Food Hydrocoll 9:237±249 (1995).

844 J Sci Food Agric 79:839±844 (1999)


microstructure ofsuwari andkamaboko sardine surimi gels

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