A simple method for eliminating urea almost completely from elasmobranch muscle consists in desalting the initially salted muscle in 5% brine. The initial salting removes about 58% of the urea, which occurs to the extent of 5.8 to 7.5% D.W.B. in the fresh muscle, and the desalting further reduces the urea content to negligible quantities (0.2%)

Govindan, T.K.

Kandoran, M.K.

Suryanarayana Rao, S.V.

Aquatic Commons

SOME ASPECTS OF CURING OF 
SHARKS AND RAYS 
M. K. K.A.NDORAN, T. K. GoviND.A.N 
& 
S. V. SuRY.A.NAR.A.Y.A.N.A. R.A.o * 
Central Institute of Fisheries Technology Unit, Calicut-5 
[ A simple method for eliminating urea almost completely 
from elasmobranch muscle consists in desalting the initially 
salted muscle in 5% brine. The initial salting removeR about 
58% of the urea, which occurs to the extent of 5.8 to 7.5% D.W.B. 
in the fresh muscle, and the desalting further reduces the urea 
content to negligible quantities (0.2%) ] 
INTRODUCTION 
Catches of elasmobranch fish viz. 
sharks, rays, skates etc. amounted to 
40,761 tonnes accounting for 6.3% of the 
total marine fish landings in India for H362. 
Even though a part of this catch was 
consumed fresh, a good portion was salt 
cured and partly consumed in internal 
markets and partly exported to neigh-
bouring countries. 
The flesh of the elasmobran0h fish 
in general -posses3 a peculiar odour and 
flavour which are not relished by ma,ny fish 
consumers. The odour and flavour of 
fish are largely att.ributed to the various 
nitrogenous constituents which contribute 
to the non-protein nitrogen fraction of 
the fiFh muscle. In the case of clasmo-
branch fish, the non-protein nitrogen con-
tributes 30% of the total nitrogen of the 
muscle (Velankar and Govindan, 1958). 
Urea, trimethyalamine oxide and free cc-
amino acids are the major components 
constituting 40%, 17.5% and 5% respectively 
of the total non · protein nitrogen, the rest 
being accounted for by other minor consi-
tituents. What exactly contributes to the 
peculiar odour and flavour of elasmobranch 
fish is not known at present. However, 
high content of urea present in the flesh 
decomposes easily imparting a strong 
ammoniacal odour to tha flesh. Even 
though proper salting removes more than 
half of the total quantity of urea orginally 
present in the flesh, the remaining urea in 
the muscle is more than sufficient to pro-
duce fairly objectionable levels of am-
moniacal odour in the cured product 
by decomposition even in the first fort. 
night of storage. 
Even though some attempts have been 
made previously to remove urea from the 
elasmobranch fish by incorporating chemi-
cals like sodium bicar ban ate and sodium 
dihydrogen -phosphate in the curing salt, 
not much success has been attained. In 
the present work, a simple technique of 
desaltation of the originaHy salted flesh 
bas been found to give promising results 
as far as the removal of urea is concerned. 
* Present address: Central Food Technological Research Institute, Mysore, 
193 
MATERIALS A.ND METHODS 
Fresh sharks and rays from the land-
ing places at Calicut were used.. Moisture 
and salt were determined by the method 
described by Suryanarayana Rao et. al 
(1958) and total volatile nitrogen and urea 
nitrogen in the aqueous extract were de-
termined by Conway's Microdiffusion 
Method. The fish after gutting and 
washing were cut into slices about two 
inches thick and salted in the ratio 1:3 
(salt to fish). 
RESULTS A.ND DISCUSSION 
The results are given in Tables I 
11, HI, IV, V and VI 
Table I gives the maximum pene-
tration of salt and the removal of urea. 
Table II gives reductbn of urea on 
pirkling in s11turated brine after initial 
·salting (1:3) and light washing. 
Table III gives the reduction in 
urea on deslating of the originally salted 
shark slices and on resalting of the de-
salted samples in the ratio 1:7. 
Table IV gives the relation between 
the size of the slices and reduction in 
urea. In this case, the shark flesh was 
cut into longitudinal fillets as is done 
commercially and then cured. 
Table V gives the behaviour of ray 
fish on salting, deslating and resalting. 
Table VI gives distribution of urea 
in flesh and brine at different stages of 
salting, desalting and resalting of shark 
flesh. 
Results obtained in prPliminary ex-
periments showed 1 hat incorporation of 
chemicals like sodium bicarbonate and 
sodium dibydrogen phosphate with curing 
salt at 5% level does not have any bene-
ficial effect as far as removal of urea is 
concerned. The penetration of salt is also 
not affected to any extent by the pre-
194 
sence of these chemicals. However, taste 
panel studies showed a better consumer 
appeal for the bicarbonate trea.ted sample. 
From table I it was observed that 
the maximum penetration of salt and re-
moval of urea takes place wi~hin the first 
24 hours of salting after which the changes 
are rather negligible. Hence prolonging 
the time of salting beyond 24 hrs. did 
not have any appreciable effect on the 
removal of urea. 
Figures in table II show that there 
is only a slight reduction in the nrea 
content even a.fter pickling for 15 days. 
This may be because the brine being sa-
turated, there is no considerable exch'mge 
of matter between the bdne and thq fle'lh. 
It may also be noted that spoilage takeB 
place in the pickle as shown by the in-
crease in T.V. N. values. 
Deslating the originally salted shark 
slicPs is very effective in removing urea 
retained in the muscle after salting as 
shown by figures in Table III. During 
desalting, the e:1;:cess salt in flesh comes 
out so as to establish equilibrium with 
the brine and in doing so, the residu<1l 
urea also comes out. 5% brine is the op-
timum strength that can be used for de-
salting, became use of 10% brine does not 
bring down the urea level as low as 5% 
brine does; and moreover, from the point 
of .view of economy also a weaker brine is 
preferable. But weaker brine than 5% 
brings about some spoilage in the flesh as 
shown by the T. V. N. value. The desalted 
sample can very well be used for making 
edible fish flour or for immediate con-
sumption as it is practically free of urea. 
If it has to be dried and preserved, it 
must be resalted; as otherwise, the salt 
content will be too low to give any 
preservative action. Moreover, it bas been 
observed that the desalted sample take 
up an undesirable dark colour and a 
TABLE- I EFFECT OF SALTING TJME ON MOISTURE, SODIUM CHLORIDE, 
T.V. N. AND UREA IN SHARK FLESH 
Moisture NaCI% T.V. N. mgN/ Urea% 
·samples description % (D. W. B.) 100 g. (D. W. B.) 
muscle 
Fresh 72.13 0.3 15.0 5.80 
After salting for 24 hours 57.13 15.59 Nil 2.45 
.After salting for 48 hours 54.37 17.14 Nil 2.40 
After salting for 72 hours 55.60 16.90 Nil 2.36 
'TABLE-II EFFECT OF PICKLING SALTED SHARK FLESH ON ITS MOISTURE, 
SODIUM CHLORIDE, T.V. N. AND UREA CONTETNS 
Moisture NaCI% T.V. N. mgN/ Urea % 
·sam pie description % (D. W. B.) 100 g (D. W. B.) 
muscle 
·Fresh 73.00 0.20 10.0 5.90 
After salting for 24 hours 58.42 17.24 Nil 2.50 
After pickling for 7 days 62.50 15.40 15.0 2.35 
.After pickling for 15 days 64.45 13.04 32.2 2.20 
TABLE-III CHANGES IN MOISTURE, SODIUM CHLORIDE, T. V N. AND 
UREA IN SHARK FLESH ON SALTING, DESALTING AND RESALTING 
Moisture NaCI% T. V.N.mgN/ Urea. % 
.Sample description % (D.W.B.) 100 g (D.W.B.) 
muscle 
Fresh 75.0 0.14 Nil 6.20 
After salting 58.3 17.20 Nil 2.60 
After desalting in 10% brine 68.0 7.20 Nil 0.54 
After desalting in 5% brine 70.4 6.10 Nil 0.23 
After desalting ia 2% brine 71.5 4.50 35.0 0.19 
After rel"alting the desalted 
(5% brine) sample 59.2 16,50 Nil 0.12 
195 
TABLE- IV EFFECT OF THE SIZE OF FILLETS ON SALT PENETRATION 
AND REMOVAL OF UREA DURING SALTING, DESALTING AND 
RESALTlNG OF SHARK FLESH 
Moisture NaCI % T.V. N. mgN/ Urea% 
Sample description % (D.W.B.) 100 g (D. W. B.) 
muscle 
Fresh 73.5 0.4 20.0 7.50 
After salting 59.3 16.2 Nil 2.75 
After desalting 70.2 6.3 Nil 0.22 
After resalting 59.5 16.3 Nil 0.14 
TABLE- V BEHAVIOUR OF RAY FISH FLESH ON SALTING, DESALTING 
AND RESALTING 
Moisture NaCI% T.V.N.mgN/ Urea% 
Sample description 0/ ,o (D. W. B.) lOOg (D.W.B.) 
mmele 
Fresb 78.50 0.25 10.00 3 70 
After salting 59.89 15.70 Nil 2.10 
After desalting 76.25 4.90 Nil 0.19 
After resalting 58.50 17.20 Nil 0.10 
TABLE- VI DISTRIBUTION OF UREA IN THE FLESH AND BRINE AT 
DIFFERENT STAGES OF SALTING. DESALTING AND 
RESALTING OF SHARK FLESH 
Sample description 
Fresh 
On salting 
On desalting 
On resalting 
Urea % on Dry Weight Basis 
In Flesh In brine Total 
7.20 7.20 
2.70 4.30 7.00 
0.18 2.50 2.68 
0.12 0.05 0.17 
196 
Remarks 
Compare with 7.2. 
in fresh 
Compare with 2.7· 
before desalting. 
Compare with 0.18. 
before resalting. 
rather sticky nature if dried without re-
-salting. 
Table IV shows that the size of the 
-slices does not affect the course of removal 
of urea and table V shows th'1t ray fish 
also beh'tves in a similar manner as shark 
in these experiments. The figures in Table 
VI show that the removal of urea from 
-shark flesh is by leaching out of the 
urea into the brine surrounding the flesh. 
'SUMMARY 
The incorporation of chemicals like 
sodium bicarbonate and sodium dihydrogen 
phospate in the curing salt, prolonging 
the salting time, and pickling the salted 
flesh in saturated brine had no beneficial 
(lffect for removing urea from elasmobranch 
flesh. 
Desalting the heavily salted flesh in 
S% brine has been found to be very 
·effective in removing the residual urea 
from the flesh. The desalted sample has 
however to be resalted if it is to be dried 
197 
and preserved. The size of the slices does 
not affect .the course of removal of urea. 
Ray fish behaves exactly like shark in 
these experiments. The removal of urea is 
attained by leaching out into the brine. 
The technique of deslatation described in 
this paper is a simple and effeetive means 
to remove urea almost competely from 
the elasmobranch flesh, thus providing 
longer shelf life for the cured product. 
AcKNoWLEDGEMENTs 
The authors wish to acknowledge 
their indebtednesss to Dr. A. N. Bose, 
Director, Central Institute of Fisheries 
Technology, Ernakulam, for his keen 
interest in the problem; to Shri A. P. 
Valsan for helpful discussions and to Shri 
P. Sadanandan for technica.l assistance. 
REFERENCES 
Suryanarayana Rao et, a/ .. 1958 Ind. J. Fish 
5, 169. 
Velankar, N. K. and Govindan, T. K. 1958 
Proc. Ind. Aca. Sci.) XLVII (4) s· 


Some aspects of curing of sharks and rays

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Some aspects of curing of sharks and rays

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