1 Extraction of gelatin from ﬁsh skins by high pressure treatment

7/28/2019 1 Extraction of gelatin from fish skins by high pressure treatment

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Extraction of gelatin from fish skins by high pressure treatment

M.C. Gomez-Guillen*, B. Gimenez, P. Montero

Instituto del Fro (CSIC), Ciudad Universitaria, 28040 Madrid, Spain

Received 7 August 2004; revised 17 November 2004; accepted 7 December 2004

Abstract

High pressure, at 250 and 400 MPa, for 10 or 20 min, was applied at either of two stages: during pre-treatment in acid at 10 8C to facilitate

destabilisation of acid labile crosslinks, or during extraction in water at 45 8C to accelerate collagen hydrolysis. The resulting gelatins were

evaluated in terms of yield of extraction, molecular weight distribution by SDS-PAGE, and viscoelastic properties of gelatin newly dissolved

and after overnight cold maturation. Pressure level and time of treatment induced noticeable changes in molecular weight distribution and

consequently affected viscoelastic properties. The use of high pressure to extract gelatin from fish skins is a useful alternative to the

conventional procedure. Its utility lies basically in that the longest phase of the treatment can be drastically shortened, thus making it possible

to produce a gelatin of high gelling quality in only a few minutes.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Fish gelatin; Extraction; High pressure; Viscoelastic properties

1. Introduction

Collagen, from which gelatin is derived, is the majorstructural protein found in skin and bones of animals.

Collagen molecules, composed of three a-chains inter-

twined in the so-called collagen triple helix, adopt a 3D

structure which is an ideal geometry for interchain hydrogen

bonding (Johnston-Banks, 1990; Te Nijenhuis, 1997). In

fact, the hydroxyproline content, through its OH group, has

been reported to be largely responsible for the triple helix

stabilisation (Burjandze, 1979; Ledward, 1986). There are

also very short terminal regions, called telopeptides, which

do not form triple helical structure, in which intra- and inter-

molecular covalent crosslinks, mainly involving lysine and

hydroxylysine residues, are located.Because of the acid lability of cross-linking in fish skin

collagen, mild acid treatment is normally enough to produce

adequate swelling and cleavage of non-covalent intra- and

inter-molecular bonds (Montero, Borderas, Turnay, &

Lizarbe, 1990; Norland, 1990; Stainsby, 1987). Subsequent

thermal treatment above 40 8C cleaves hydrogen

and a number of covalent bonds; this destabilises the triple

helix by means of a helix-to-coil transition, leading to

conversion into soluble gelatin (Djabourov, Lechaire, &Gaill,1993).Highmolecular weightpolymers mayarise inthe

resulting gelatin through the possible persistence of cross-

links depending on the nature and degree of solubilisation.

High pressure above 150 MPa has been shown to induce

protein denaturation by disturbing the balance of the non-

covalent interactions that stabilise the native conformation

of many proteins (Masson, 1992). However, collagen has

been reported to be scarcely affected by high pressure, given

that hydrogen bonds are chiefly pressure-insensitive (Gekko

& Koga, 1983; Heremans, 1995). At relatively low pressure

they could appear to be even somewhat strengthened, due to

an associated small reduction in volume (Cheftel & Culioli,1997). No significant effects by high pressure treatment

around 300400 MPa were found on intramuscular collagen

solubility from beef (Suzuki, Watanabe, Ikeuchi, Saito, &

Takahasi, 1993) or pacific blue whiting (Fernandez-Martn,

Perez-Mateos, & Montero, 1998). However, changes in

bovine collagen solubility have been shown to vary

with applied pressure; 200 MPa increased solubility,

whereas 400 MPa reduced it (Kwiatkowska, Jankowska, &

Korzeniowski, 2001).

0268-005X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.foodhyd.2004.12.011

Food Hydrocolloids 19 (2005) 923928

www.elsevier.com/locate/foodhyd

* Corresponding author. Tel.:C34 31 5492300; fax: C34 91 5493627.

E-mail address: [email protected] (M.C. Gomez-Guillen).
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Regarding high pressure utilisation in gelatin, there are a

few works focused on aspects related to induced

gelation (Montero, Fernandez-Daz, & Gomez-Guillen,

2002; Walkenstrom & Hermansson, 1997). However, no

published articles are available in relation to gelatin

extraction assisted by high pressure.

Gomez-Guillen and Montero (2001) reported a pro-cedure for extracting gelatin with high gelling capacity from

fish skins; the procedure was essentially based on a mild

acid pre-treatment for collagen swelling followed by

extraction in water at moderate temperature (45 8C). The

entire process takes around 24 h. The point of applying high

pressure in the fish gelatin extraction process is mainly to

cut down the treatment times and/or to improve the quality

of the gelatin produced. In conventional gelatin manufac-

turing procedure, high pressure can be applied at two stages:

either during pre-treatment in acid at 10 8C to facilitate

destabilisation of acid labile crosslinks, or during extraction

in water at 45 8C to accelerate collagen hydrolysis.

The aim of this work was to analyse the yield, molecular

weight distribution and viscoelastic properties of gelatin

extracted from fish skins using high pressure (250 or

400 MPa) either during mild acid pre-treatment at 10 8C or

during extraction in water at 45 8C.

2. Materials and methods

2.1. Gelatin extraction

Skins from Dover sole (Solea vulgaris) were collected

and stored frozen at K20 8C until use. Cleaning of thawed

fish skins and gelatin extraction procedure was carried out

as previously described (Gomez-Guillen & Montero, 2001).

Briefly, the method consist of a mild acid (50 mM acetic

acid) swelling step for 3 h and subsequent overnight

(1618 h) gelatin extraction in distilled water at moderate

temperature (45 8C). Extracted gelatin was air-dried until

moisture was less than 15%. This procedure was considered

as conventional treatment and the resulting gelatin was

called control gelatin, in order to compare with the

gelatins obtained by means of high pressure.

For the pressure-assisted extractions, fish skins were

frozen, thawed and cleaned as mentioned above. Pre-

treatment in 50 mM acetic acid at 10 8C was performed

during 10 min at 250 MPa, followed by extraction in water

at 45 8C during 1618 8C at atmospheric pressure (S250).

Samples extracted in water at 45 8C by high pressure were

all preceded by a pre-treatment (at atmospheric pressure) in

50 mM acetic acid at 10 8C during 3 h. In substitution of a

conventional overnight extraction during 1618 h, high

pressure was applied at 250 MPa for 10 min (E250-100) or

20 min (E250-200), and at 400 MPa for 10 min (E400-10

0) or

by two pulses of 5 min each (E400-2!50).

2.2. Electrophoretic analysis (SDS-PAGE)

Dry gelatin was dissolved in distilled water at 60 8C and

then 3-fold-concentrated loading buffer containing b-

mercaptoethanol was added until a final concentration of

2 mg/ml of gelatine is reached, as described previously by

Gomez-Guillen et al. (2002). Protein samples were heat-denatured 5 min at 90 8C and analysed by SDS-PAGE

according to Laemmli (1970) using 3% stacking gels and

5% resolving gels in a Mini Protean II unit (Bio-Rad

Laboratories, Hercules, CA) at 25 mA/gel. The loading

volume was 15 ml in all lines. Protein bands were stained

with Coomassie brilliant Blue R250. Type I and type III

collagens from fetal calf skin (Sigma Aldrich) were used as

markers ofa-chain, b- and g-component mobilities.

2.3. Viscoelastic properties

Dynamic studies were performed on a Bohlin CRS-10controlled stress rheometer rotary viscometer (Bohlin

Instruments Ltd, Gloucestershire, UK) using a cone-plate

geometry (cone angle 48, gapZ0.15 mm), according to

Gomez-Guillen et al. (2002). Solutions were prepared by

mechanical stirring for 1520 min of pre-weighed (6.67%

w/v) dry gelatin in water at 40 8C. Also the samples were

cooled in a refrigerator at 7 8C (maturation temperature) for

1618 h overnight to analyse the effect of maturation time

on gelatin.

Temperature ramps were performed at a scan rate of

1 8C/min, frequency 1 Hz, and oscillating applied stress of

3.0 Pa. The phase angle (8) was represented as a functionof temperature. Values of elastic modulus (G0) and viscosity

modulus (G00) were taken at 6 8C.

The ramps were implemented from 40 to 6 8C and back

to 40 8C for newly dissolved gelatin, to study gelatin

gelation and subsequent melting. Gelatin matured overnight

(1618 h) at 67 8C was also subjected to temperature

ramps from 6 to 40 8C and then from 40 to 7 8C, to evaluate

the effects of maturation time on the gelatin gel.

Measurements were done at least in triplicate and results

were expressed as mean values and standard deviation.

3. Results and discussion

The utility of the high pressure treatment was tested at

two stages: during acid swelling to facilitate the conversion

of the insoluble collagen into a form suitable for subsequent

extraction; and during the actual gelatin extraction step in

water at 45 8C. As a control for comparison of the gelatins

from different extraction procedures, gelatin was extracted

conventionally following the procedure described by

Montero and Gomez-Guillen (2000), with a mild acid pre-

treatment followed by overnight extraction at 45 8C.

M.C. Gomez-Guillen et al. / Food Hydrocolloids 19 (2005) 923928924


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3.1. Yield of extraction

Pressure-assisted acid swelling was performed at

250 MPa/10 min at 10 8C (S250), followed by an overnight

extraction in water. These conditions were assayed with

50 mM acetic acid as in the control treatment, and also with

10 mM acetic acid. Preliminary results showed that no

significant amount of gelatin could be extracted at the low

acetic acid concentration, even with 20 min pressurising. All

the preliminary collagen swelling (pre-treatment) assays inthe extracting step were therefore performed with 50 mM

acetic acid.

As Table 1 shows, the yield (expressed as grams of dry

gelatin per 100 g of clean skin) was slightly higher when

pressurisation was used for collagen pre-solubilisation

(S250), than in conventional gelatin extraction (control).

On the contrary, the yield was considerably lower in all

cases when pressurisation was applied in the extraction step.

The general impression was that increasing the pressure or

prolonging pressurisation tended to reduce the yield. As

regards the pressure level, more gelatin was extracted with

250 MPa than with 400 MPa. Prolonging of treatment with250 MPa from 10 to 20 min also caused a reduction in the

extraction yield. In the case of pressurising at 400 MPa,

when samples were pressurised in two consecutive 5-min

cycles, the amount of gelatin extracted was only 2% greater

than when samples were pressurised continuously for

10 min in the same conditions.

3.2. Molecular weight distribution

The electrophoretic (SDS-PAGE) profiles of the various

gelatin preparations are shown in Fig. 1. Samples extracted

with high pressure at 400 MPa (E400-100 and E400-2!5

0)

were characterised by a typical gelatin/collagen type I

profile, with a notable amount of a-chains (a1/a2 ratio

around 2), a considerable presence of b-components (a-

chain dimers), and particularly of higher molecular weight

polymers including g-components (a-chain trimers)

(Gomez-Guillen et al., 2002). There were noticeably more

of these high molecular weight peptides than with the

conventional extraction procedure; the reason may be either

that high pressure favours more polymer extraction, or else

that there is some aggregation during hydrolysis. Both in the

swelling step (S250) and in the water extraction step (E250-

10 0), pressurisation at 250 MPa for 10 min led to gelatin

preparations in which peptides of MW%100 kDa predomi-nated and dimers and trimers were practically absent. The

electrophoretic profiles and the higher yield of these

pressure-assisted gelatin preparations indicate a higher

degree of collagen solubilisation or hydrolysis. When

pressurisation at 250 MPa was prolonged to 20 min (E250-

20 0) during extraction in water, the profile contained

appreciably more high molecular weight polymers, which

suggests a degree of pressure-induced aggregation.

3.3. Viscoelastic properties of newly dissolved gelatins

Dry gelatins were dissolved at 40 8C. Fig. 2 shows

changes in the phase angle (d) of the newly dissolved gelatin

preparations (concentration 6.67% w/v) upon cooling to

6 8C and subsequent heating. As the cooling ramp shows,

the transition curve from solution to gel was rapid and

similar in all samples, the only variation being in the gelling

temperatures. All samples presented low phase angles at

6 8C (d!58), indicating good gelling gelatins. The same was

true during subsequent heating; the studied gelatins differed

in melting temperature but all presented a sharp melting

transition. The onset of gelling and melting occurred at

slightly higher temperatures than in the control in gelatins

extracted under 400 MPa pressure, but at considerably

Table 1

Yield of extraction (expressed as g dry gelatin per 100 g of cleaned skins)

Samples Yield of extraction (%)

Control 21.3

S250 22.8

E250-100 10.2

E250-20

0 4.7

E400-100 3.4

E400-2!50 5.7

Fig. 1. Electrophoretic analysis of gelatin preparations (SDS-PAGE) in the

presence of 2-b-mercaptoethanol: (a) control, (b) S250, (c) E250-100, (d)

E400-100, (e) E400-2!5

0 and (f) E250-200.

61014182226Temperature (C)

Phase

angle

()

0

10

20

30

40

50

60

70

30

40

50

60

6 10 14 18 22 26

Temperature (C)

Control

S250

E250-10

E250-20

E400-10

E400-2*5

Fig. 2. Changes in phase angle d, monitored upon cooling from 40 to 6 8C

and subsequent heating from 6 to 40 8C, of newly dissolved gelatins at

6.67% w/v.

M.C. Gomez-Guillen et al. / Food Hydrocolloids 19 (2005) 923928 925


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lower temperatures than the control in gelatins extracted

under 250 MPa pressure for 10 min (S250 and E250).

However, transition temperatures were higher when the

pressurisation time was prolonged (E250-20). As reported by

Johnston-Banks (1990), the increase in the gelling/melting

temperatures is directly related to the increase in the average

molecular weight of the studied gelatins.

Table 2 shows gelling and melting temperatures and

elastic modulus (G0) and viscous modulus (G 00) values at 6

and 40 8C. The samples pressurised at 250 MPa for 10 min,

both during swelling (S250) and during extraction in water

(E250-100), presented significantly lower G 0 and G00 values as

measured at 6 8C than the other gelatins. In samples

pressurised at 400 MPa in two 5-min cycles, viscoelastic

properties were again lower than with continuous pressur-

isation for 10 min. On the other hand, values of G 0 in 40 8C

samples were low upon subsequent heating, indicating that

the gels were all perfectly thermo-reversible and melted

completely.

3.4. Viscoelastic properties of overnight cold matured gels

The transition curves of the matured gelatin gels upon

melting and subsequent cold renaturation (Fig. 3) again

revealed differences among the samples, predominantly in

the thermal transition temperatures rather than in the slope.

Both melting and gelling temperatures of S250 and E250-100

gels were again notably lower than in the others. Also, phase

angle values at 6 8C were higher in these samples, indicating

poorer overall gel forming capacity as a result of cold

maturation.

In sample pressurised at 400 MPa for 10 min (E400-100),

the cooling phase angle transition indicated the onset of

collagen renaturation at around 20 8C, which was consider-

ably higher than with the conventional treatment.

As shown in Table 3, values of G 0 measured after

overnight maturation at refrigeration temperature (around

6 8C) in E250-200 and E400-10

0 gelatin gels were significantly

higher than in the control gelatin. Again, all gels were

completely melted at 40 8C. Upon subsequent cold rena-

turation, the E400-100 sample attained the highest values of

G 0 and re-gelling temperature, which was in fact higher

than in the corresponding newly dissolved gelatin. There is

clearly a relationship between the effect of pressure and the

molecular weight of gelatin fractions. According to Stainsby

(1987), the greater presence ofa-chain dimers and trimers

in these gelatins should make them more susceptible to

renaturation to the full native collagen form. In addition, the

presence of a significant amount of higher molecular weight

components acts also in favour of their slightly higher

melting/gelling temperatures (Johnston-Banks, 1990). On

the other hand, Sims, Bailey, and Field (1997) indicated that

the generation of short triple helices by many interactions of

free chains, rather than rapid completion of a longer triple

helix, will form a more stable gel. In this sense, the way of

triple helix formation upon cooling in E400-100 and E400-

2!50 samples having similar molecular weight profile, may

explain the difference in their renaturation ability.

High pressure-assisted extraction at 400 MPa for 10 min

produced considerably less amount of gelatin than conven-

tional extraction over 1618 h. However, the quality of the

gelatin extracted in that short time was high, comparable to

that of conventionally extracted gelatin as reported byMontero and Gomez-Guillen (2000). The molecular weight

distribution of the gelatin extracted by high pressure was

characterised by a high presence ofa-chain, together with

b- and g-components (dimers and trimers of a-chain,

respectively), as well as a certain amount of greater

polymers. All of them are key factors for high viscoelastic

properties, gel strength and melting/gelling temperatures.

After maturing overnight at 67 8C, the modulus of

elasticity (G 0) of the gel from pressure-processed gelatin

was greater than that of a conventional gelatin, and

0

10

20

30

40

50

60

70

80

6 10 14 18 22 26 30

Temperature (C)

Phase

angle

()

Control

S250

E250-10

E250-20

E400-10

E400-2*5

6101418222630

Temperature (C)

Fig. 3. Changes in phase angle d, monitored upon heating from 6 to 40 8C

and subsequent cooling from 40 to 6 8C, of gelatin gels (6.67% w/v, 168 h

maturation at 67 8C).

Table 2

Viscoelastic properties of newly dissolved gelatines

Samples Cooling ramp Heating ramp

G 06 8C (Pa) G00

6 8C (Pa) g.t. (8C) G0

40 8C (Pa) G00

40 8C (Pa) m.t. (Pa)

Control 2520G63 54.6G1.4 13 0.64G0.02 0.41G0.01 22

S250 868G22 35.6G0.9 9 0.67G0.02 0.31G0.01 18

E250-100

731G18 45.9G1.1 8 0.66G0.01 0.35G0.02 17E250-20

0 2720G68 69.4G1.7 12 0.35G0.01 0.42G0.03 21

E400-100 2700G70 63.6G1.6 13 0.54G0.03 0.42G0.02 23

E400-2!50 1630G41 35.7G0.9 13 0.47G0.02 0.45G0.02 23

g.t., gelation temperature; m.t., melting temperature.



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the melting temperature was slightly higher. This gelatin

also presented greater renaturation ability upon subsequent

cooling, but only with continuous extraction (E400-100);

when extraction was carried out at 400 MPa in two 5-min

cycles (E400-2!50) there was no improvement in renatura-

tion ability even although the molecular weight profile wassimilar to that of gelatin E400-10

0, from which we deduce

that it is not only the amount but also the configuration of

these fragments that matters.

Pressurising at 250 MPa for 20 min produced a gelatin

with less amount of dimers and trimers than with 400 MPa,

however, viscoelastic properties were similar, and the value

of G 0 after overnight maturation was even higher. With

pressurising at 250 MPa, time was a key factor in the quality

of the extracted gelatin: with 10 min pressurisation in these

conditions, the peptides extracted were largely of relatively

low molecular weight and there were insufficient a-chains

for proper annealing of collagen molecules upon cooling.Although the yield was higher, this gelatin exhibited

considerably poorer gel-forming ability than any of the

others, including the control.

In light of these results, we conclude that the use of high

pressure to extract gelatin from fish skins is a useful

alternative to the conventional procedure, essentially in that

it drastically reduces the duration of the longest part of the

process, so that a high-quality gelatin can be produced in a

matter of minutes. Successive extractions at atmospheric

pressure and increasing temperatures would improve the

yield from original raw material. Further studies would be

needed to optimise pressure/time/temperature settings so asto increase the yield in the first extraction and in successive

extractions from the same original material.

Acknowledgements

This research was financed by the Comunidad de Madrid

under project 07G/0052/00, and by the Spanish Comision

Interministerial de Ciencia y Tecnologa under project

AGL2000-1497.

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Table 3

Viscoelastic properties of overnight matured gelatin gels

Samples Heating ramp Cooling ramp

G06 8C (Pa) G00

6 8C (Pa) G0

40 8C (Pa) G00

40 8C (Pa) m.t. (8C) G0

6 8C (Pa) G0

6 8C (Pa) g.t. (Pa)

Control 5240G131 361G9.0 0.72G0.02 0.50G0.02 24 2870G72 61.6G1.5 14

S250 566G14 104G2.6 0.66G0.02 0.30G0.01 20 312G7.8 24.9G0.6 8

E250-100

882G22 169G4.2 0.52G0.01 0.33G0.01 20 602G15.5 45.6G1.1 9E250-20

0 8030G201 413G10.3 0.48G0.02 0.28G0.02 23 3460G86 94.3G2.3 13

E400-100 6160G154 318G7.9 0.53G0.03 0.67G0.03 24 4720G118 122.6G3.1 17

E400-2!50 5370G134 303G7.6 0.44G0.02 0.60G0.02 24 2680G67 54.9G1.4 15

g.t., gelation temperature; m.t., melting temperature.

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1 Extraction of gelatin from ﬁsh skins by high pressure treatment

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