Vyncke, W.

Open Marine Archive

11539 
Reprinted from Laboratory Practice, Vol, 17, No 7, July 7968 
LABP 17-77 APPARATUS AND DEVICES 
813 
New Laboratory Method for Expressing Fish-Eluid 
by W. Vyncke , Fisheries Research Station {/Vlinistry of Agriculture), Ostend, Belgium 
Introduction 
Fish consists almost exclusively of protein and water; 
fatty species also contain variable quantities of lipids. As 
most biochemical processes take place in the liquid phase 
or have an immediate influence upon its composition 
(e.g. by absorption of degradation products), the study 
of expressible fluid is most important. In the laboratory 
it can be used to carry out numerous analyses, e.g. 
determination of total volatile bases (TVB), trimethyla-
mine (TMA), and volatile reducing substances (VRS) by 
microdiff'usion, bacteriological tests, assessment of 
mineral content, etc. Recently it was also utilized for the 
differentiation of fish species by electrophoresis (Lane et 
a/. 1966). It is further of interest for the study of the 
quality of frozen fish. 
A distinction must be made between free drip, which 
exudes in a natural way from the fish, and expressible 
fluid obtained when an external force is exerted. The latter 
procedure is generally applied as it gives considerably 
more yield. The fluid may be extracted by means of a 
hydraulic press, handpresses (household types) or 
electrical centrifuges (fruit juice centrifuges). The 
hydraulic press is generally used. A quantity of fish is 
wrapped in non-absorbent cloth, placed into the cylinder 
and covered by a tight-fitting piston. Holes are pierced 
into the bottom of the cylinder so that the fluid may flow 
out freely. 
TORSION METER 
f^tg 
^ 
% 
c 
V . 
1 
1 • 
r 
;>-xj_ 
' SPRING 
^ 
^ 
to 
120 
REMOVABLE CYLINDER 
DISCS 
S ) 
' 
COLLECTING PAN 
• ^ 
^^^fflSJL'^*'»»? '.^•"3 
Figure 1. Press for the extraction of expressible fluid. 
Banks (1955) designed a special apparatus, consisting 
of a perforated metal cylinder against which the fish 
sample is pressed by a rubber bag inflated to the desired 
pressure. The rubber bag allows a precise adjustment of 
this pressure. 
Preliminary experiments showed that these methods 
are rather tedious. Moreover for fatty fish (e.g. herring) 
and semi-fatty fish (e.g. redfish) an insufficient quantity 
of expressible fluid is obtained. The use of an electrical 
centrifuge, as proposed by Wittfogel and Gebhardt 
(1957), gave good results with lean fish (e.g. cod) but was 
impracticable with fatty and semi-fatty fish. 
For this reason an attempt was made to develop a 
simple technique suitable for all fish. 
After numerous tentative experiments a small hand-
press with discs seemed to fulfil the requirements excel-
lently. In order to determine the possible influence of 
varying pressure, pressing time, etc., an exhaustive series 
of tests was carried out to study the fish fluid expressed 
with this apparatus. Different quantitative aspects, viz. 
amount of expressible fluid, refractive index (RI), TVB, 
TMA, VRS and total solids, were examined. 
Experimental 
Apparatus and procedure 
The press used (Figure 1) is a modified small commercial 
handpress (Presse médicale AS, Paris, France) made of 
galvanized iron with slightly conical discs of 8.3 cm 
diameter. Pressure is applied by tightening a spring by 
means of a wing nut. In order to adjust the desired 
pressure, the press was fitted with a torque meter 
(Torquometer type TQC-l-FU, Snap-on Tools Corp., 
Kenosha, USA) connected via a pipe-spanner to the 
wing nut; on the latter a nut of 17 mm was welded. 
For each determination 100 g of fish cut into pieces of 
5 to 10 g was used. 
Species 
Cod {Gadus morfiua L.), redfish (Sebastes marinus L.) 
and herring (Clupea /larengus L.) were used. When 
necessary fish of varying degree of freshness were taken. 
Metiwds 
The following analyses were carried out on the expressible 
fluid: 
Total solids: 5 g was mixed with sand and dried at 
105° C until constant weight. 
Rl: determination on two drops of fluid at 20° C, in 
an Abbé refractometer. 
TVB and TMA: according to the diffusion method of 
Conway (1962). 
VRS: according to the method of Farber and Ferro 
(1956). The expressible fluid was neither filtered nor 
centrifuged before analysis. 
814 W. Vyncke LABORATORY PRACTICE 
Statistical tests 
All experiments were repeated 20 to 30 times and the 
results were tested statistically by the method of the 
paired comparisons. The significance of the observed 
differences was each time examined by means of the t-
test according to: 
d. 
t = ; , with n - 1 degrees of freedom 
Sa / V n 
da = mean difference between paired experiments 
Sa = standard deviation of the diff^ erences 
n = number of pairs 
A significant difference was indicated in the tables by 
the conventional signs and (respectively 95 and 99 per 
cent probability). 
Results and discussion ' 
Factors proper to the method and to the raw material 
respectively were investigated. 
Factors proper to the method 
Influence of pressure 
The effect of two different pressures (0.35 and 0.50 
kg/cm2) was examined. These particular pressures were 
chosen as at ca. 0.35 kg/cm^ the first drops of expressible 
fluid were expressed, and at a pressure higher than 
0.50 kg/cm2 pieces of flesh were extruded. The two 
pressures were adjusted by means of the torque meter. 
The corresponding moments were 0.10 and 0.15 kgm 
(8 discs were used). Owing to the loss of fluid pressure 
decreased gradually and after 10 and 20 minutes respec-
tively the spring was tightened again. After 30 minutes 
pressing was interrupted as practically no more fluid was 
liberated. 
The results for cod are given in Table I and indicate 
that the amount of expressible fluid is markedly higher at 
a pressure of 0.50 kg/cm^. The average difference was 
24.4 per cent. 
Total solids, RI, TVB and TMA appeared not to be 
influenced, whereas for VRS a very significant difference 
of 3.2 (xeq or 20 per cent was observed. An increase in 
pressure results in relatively more volatile reducing sub-
stances being liberated in the expressible fluid. The 
reason for this was not investigated in the present work. 
Banks (1955b), Love (1955) and Connell (1957) indicated 
that free drip as well as press drip consist mainly of 
intracellular fluid. Connell reports a value of 90 per cent. 
However as pressure is increased more and more intra-
cellular fluid mixes with the expressible fluid owing to the 
rupture of cell walls or by ultrafiltration. The possibility 
exists that under the conditions described certain 
components of this intracellular fluid only affect VRS and 
not TVB nor TMA. 
The further values obtained are similar to those of 
Banks (1955) who also ascertained that for cod more 
expressible fluid was liberated as the pressure was 
increased, but that total solids remained nearly constant 
up to a pressure of ca. 0.50 kg/cm^. With higher pressures 
however more and more water was squeezed out. 
Herring and redfish were also subjected to the same 
experiments and gave analogous results. 
It may thus be concluded that pressure need be 
adjusted accurately only for the VRS determination; for 
the other determinations this seems to be of no conse-
quence. 
TABLE I 
Influence of two pressures (0.35 and 0.50 kg/cm^) on the expressible fluid of cod 
Determination 
Expressible fluid (ml) 
Total solids 
RI 
TVB (mg N per 100 ml) 
TMA 
VRS (i^eq per 5 ml) 
Difference between paired experiments 
Min. 
+ 2.0 
- 0.24 
- 0.0003 
- 1.4 
- 0.9 
0 
Max. 
+ 10.5 
+ 0.19 
+ 0.0006 
+ 4.4 
+ 2.1 
+ 7.5 
Average 
+ 5.3 
0 
-0.0001 
+0.5 
+0.3 
+ 3.2 
Average values 
0.35 
21.7 
7.86 
1.3474 
23.5 
6.2 
16.2 
0.50 
27.0 
7.86 
1.3473 
24.0 
6.5 
19.4 
TABLE n 
Influence of mincing the fish on the expressible fluid of cod 
Determination 
ml 
Total solids (%) 
RI 
TVB (mg N %) 
TMA (mg N %) 
VRS ([xeq/5 ml) 
Difference between paired experiments 
Min. 
0 
- 0 . 0 2 
-0.0002 
- 0 . 1 
- 0 . 1 
- 3 . 0 
Max. 
+4.0 
+0.97 
+0.0026 
+ 3.5 
+2.4 
+ 1.5 
Average 
+ 2.1 
+0.50 
+0.0015 
+0.4 
+0.2 
- 0 . 5 
Average values 
Control 
25.5 
7.53 
1.3468 
31.8 
6.1 
18.8 
Minced 
27.6 
8.03 
1.3483 
32.2 
6.3 
18.3 
VOL. 17 No. 7 Method for Expressing Fish Fluid 815 
As the higher pressure (0.50 kg/cm^) liberates more 
expressible fluid it was chosen for further experiments. 
Influence of the number of discs 
Preliminary experiments showed that it is more con-
venient to operate with from 5 to 8 discs. In order to 
determine if changing the number of discs (and thus the 
contact surface) influences the composition of the 
expressible fluid, a series of comparative tests with 5 and 8 
discs respectively was carried out on cod. 
Owing to the larger contact surface with eight discs 
the amount of expressible fluid was distinctly higher and 
differed significantly at the 95 per cent level. The average 
difference was 5.7 ml or 26.7 per cent. 
The other determinations were almost uninfluenced 
and no significant difference was recorded. This indicates 
that the composition of the expressible fluid is not 
modified by changing the number of discs. As eight discs 
give distinctly more expressible fluid, this number is to be 
preferred for practical reasons. Similar results were 
obtained for herring and redfish. 
Influence of pressing time 
Pressing times of 10 and 30 min. respectively were tested. 
Preliminary experiments had shown that an almost maxi-
mum quantity of expressible fluid was obtained in 30 min. 
Between 30 and 60 min. only an additional 2 or 3 ml were 
squeezed out. 
As expected, a larger quantity of fluid was obtained 
with cod after 30 min; on average this was 28.5 ml 
against 23.7 ml for 10 min.; i.e. 20.2 per cent more. Total 
TABLE m 
Reproducibility of the method 
Fish species 
Cod 
Redfish 
Herring 
Number 
of dupli-
cations 
150 
82 
105 
Average 
difference 
{ml) 
1.6 
1.7 
1.6 
Maximum 
difference 
(ml) 
4.5 
6.0 
4.5 
Standard 
deviation 
(ml) 
1.38 
1.51 
1.43 
solids, RI, TVB, TMA and VRS showed a slight decrease, 
which appeared not to be at all significant. For herring 
and redfish analogous results were obtained. The pressing 
time is of no importance and needs not to be strictly 
adhered to. As after 30 minutes the amount of expressible 
fluid is distinctly higher, it is preferable to take this time 
approximately into account. 
Influence of quantity offish 
Usually 100 g of fish was introduced into the press, and 
in order to check if this quantity had to be weighed 
accurately a series of tests with 100 and 110 g of fish was 
carried out. The amount of fluid from 110 g was higher 
for the three species; the difference however was only 
significant at the 95 per cent level for cod and averaged 
3.6 ml. This is certainly due to the fact that lean fish have 
a greater moisture content than redfish or herring and 
that their firmer consistency hberates the expressible fluid 
more easily. The other determinations showed nowhere 
any significant differences. 
The conclusion may thus be drawn that the quantity 
of fish need not be weighed accurately. This offers an 
important practical advantage and allows the use of the 
press aboard fishery research vessels, where weighing 
generally causes serious difficulties. 
Influence of mincing the fish 
Instead of pieces of fish for the extraction of expressible 
fluid, minced fish may also be used. Comparative experi-
ments were carried out with both techniques. An electrical 
meat grinder was used. Minced redfish and herring 
caused serious difficulties as, owing to the softer consis-
tency of these fish, a large quantity of fish tissue was 
extruded when pressure was applied. The amount of 
expressible fluid decreased considerably and had to be 
filtered but this method was suitable for cod. The results 
of the comparative tests are given in Table II. 
It was not only noticed that more expressible fluid was 
liberated (on the average 8.2 per cent), but also that the 
fluid contained more dissolved substances: total solids 
were on the average 7.1 per cent higher and RI increased 
by 0.0015 units. This is probably due to the presence of a 
higher percentage of intracellular fluid as mincing cer-
tainly damages more cells. It does not appear to affect 
TVB, TMA and VRS-determinations significantly. 
TABLE rv 
Influence of storage on the amount of expressible fluid and its total solids content 
Fish species 
Cod 
Redfish 
Herring 
Determination 
Expressible fluid (ml) 
Total solids (%) 
Total solids 
(g per 100 g fish) 
Expressible fluid (ml) 
Total solids (%) 
Total solids 
(g per 100 g fish) 
Expressible fluid ml) 
Total solids (%) 
Total solids 
(g per 100 g fish) 
Storage time (in days) 
0 
22.7 
8.60 
1.95 
21.2 
8.95 
1.90 
0 
15.7 
11.92 
1.87 
2 
26.5 
7.29 
1.93 
19.5 
8.94 
1.74 
2 
15.2 
10.74 
1.63 
7 
30.5 
6.40 
1.95 
19.2 
8.83 
1.70 
4 
14.5 
10.09 
1.46 
9 
31.4 
6.20 
1.94 
18.9 
8.75 
1.65 
6 
13.8 
9.40 
1.30 
816 W. Vyncke LABORATORY PRACTICE 
The use of minced fish has the advantage that a more 
homogeneous sample from a greater number of fish can 
be taken. An important disadvantage is that this method 
can only be applied to lean fish species. 
Factors proper to the used fish 
Influence of species 
The higher the moisture content of the fish, the larger is 
the quantity of expressible fluid. For very fresh fish 
80 per cent of the observations (about 300 per species) 
varied between the following limits per 100 g: 
cod: 24-28 ml (average: 26.2 ml) 
redfish: 17-21 ml (average: 19.5 ml) 
herring: 12-15 ml (average: 13.8 ml) 
The great advantage of the new method is emphasized 
when comparing the values obtained with those from 
other techniques. Banks (1955) with his special equipment 
collects an average of 7.1 g expressible fluid per 100 g cod 
after 6 hours at a pressure of 0.5 kg/cm^. Connell (1957) 
on the other hand obtained about 10 g expressible fluid 
with a hydraulic press after 60 minutes at 1,600 kg/cm^. 
When pressing time is rather long the influence of 
temperature must be taken into account and precautions 
must be taken to prevent evaporation losses. This is 
superfluous for the method described as pressure is only 
applied during a short period (10 to 30 minutes), thus 
providing a supplementary advantage. 
The reproducibility of the method was also determined 
by means of duplicate analyses carried out during the 
diff"erent stages of experimentation. 
The standard deviation (Table III) was determined 
according to: 
d = difference between duplicate analyses 
n = number of pairs 
Application of the F-test showed the standard devia-
tions not to be significantly diflerent. The combined 
standard deviation was 1.42 ml. The reproducibility of 
the method may thus be considered very satisfactory. 
Influence of the degree of freshness 
A series of storage experiments in ice was carried out. 
For cod and redfish the experiment lasted nine days, the 
fish being approximately five days old at the start. Herring 
was approximately three days old and was kept for six 
days. Ten fish were used for each experiment. 
From Table IV it appears that the amount of expres-
sible fluid and total solids are affected by the degree of 
freshness of the fish. After nine days a considerable 
increase in the number of ml (38.3 per cent) was observed 
for cod, whereas total solids proportionally decreased 
indicating that more water was present in the expressible 
fluid. As it was observed that the total solids in 100 g of 
fish remained practically constant (1.93 to 1.95), it may 
be concluded that the increase in the amount of expres-
sible fluid is primarily due to the liberation of water. 
This is caused by the fact that the water-holding capacity 
of the proteins decreases as deterioration progresses. 
These data are fairly identical with those of Banks 
(1955b), who ascertained that after some 12 days the 
amounts of expressible fluid increased from 7.2 to 
10.2 g or 41.6 per cent. 
Redfish and herring however gave a different picture. 
Both the amount of expressible fluid and total solids 
decreased during storage. The number of ml dropped by 
10.8 per cent for redfish and by 12.1 per cent for herring; 
total solids also decreased. As the percentage of total 
solids fell, it may also be concluded that for both species 
the expressible fluid is more and more diluted by exuded 
water. This phenomenon is thus analogous to that in the 
lean species (cod). The decrease.in the number of ml can 
undoubtedly be attributed.to the fact that the expressible 
fluid is retained by physipal forces due to the softer and 
more gelatinous consistency of semi-fatty fish (redfish) 
and fatty fish (herring); this consistency becomes softer 
and softer as deterioration progresses. 
Other advantages of the method 
Besides the advantages mentioned above, several addi-
tional practical advantages should be emphasized. 
The press is very easy to operate, it can be dismounted 
and remounted quickly without difficulty. For the same 
reason it is very convenient to clean and easily sterilized. 
It is moreover very cheap thus allowing a great number 
of presses to be put into service simultaneously. 
Finally, it should be noticed that the press can be 
used successfully for other fishery products such as 
smoked fish, cooked shrimps, raw and cooked prawns, 
etc., and that larger models allowing bigger quantities of 
fish to be pressed, can also be used. 
REFERENCES 
Banks, A. (1955). J. Set. Food Agric, 6, 282. 
Banks, A. (1955b). y. 5c(. Fooi//l^nc., 6, 584. 
Connell, J. (1957). J. Sci. Food Agric, 8, 701. 
Conway, E. (1962). Microdiffusion Analysis and Volumetric Error, 
Crosby Lockwood, London. 
Farber, L. and Ferro, M. (1956). Food TechnoL, 10, 30."!. 
Lane, J. P., Hill, W. and Learson, R. (1966), Com. Fisher. Rev., 
28 (3), 10. 
Love, R. (1955). J. Sci. Food Agric, 6, 30. 
Wittfogel, H. and Gebhardt, R. (1957). Arch. Lehensmittelhyg., 8, 
241. 


New laboratory method for expressing fish fluid

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