Blue discolouration met with in the canned body meat of crab (Scylla serrata) was due to copper content exceeding 1.8 to 2.0 mg/100g on dry weight basis. Bleeding the cleaned carcasses of crab thoroughly in running water brought down the copper content below this level and blue discolouration prevented. Addition of copper ions to the thoroughly bled crab meat to raise the copper content above this level caused discolouration. The incidence of blue discolouration was independent of the freshness of the animals used. Citric acid in high concentration prevented blueing to some extent, but is not advisable as texture and flavour were adversely affected

Balachandran, K.K.

Vijayan, P.K.

Aquatic Commons

Fish. Technol. (1981) 18, 117-122 	 117 
Studies on Blue Discolouration i Canned Body 
Meat of Crab (Scylla serrata) 
P. K. VIJAYAN and K. K. BALACHAN RAN 
Central Institute of Fisheries Technology, Cochin-682 029 
Blue discolouration met with in the canned body meat of crab (Scylla serrata) was 
due to copper content exceeding 1.8 to 2.0 mg/100g on dry weight basis. leeding the 
cleaned carcasses of crab thoroughly in running water brought down the copper content 
below this level and blue discolouration prevented. Addition of copper ions to the 
thoroughly bled crab meat to raise the copper content above this level caused discolou-
ration. The incidence of blue discolouration was independent of the freshness of the 
animals used. Citric acid in high concentration prevented blueing to some extent, but 
is not advisable as texture and flavour were adversely affected. 
Blue discolouration in canned crab meat 
is a common phenomenon which has engaged 
the attention of several workers in identi-
fying the causative factors and its preven-
tion. Most of the literature on the subject 
suggest that copper ion is mainly respon-
sible for its occurrence. Goringer & Dassow 
(1964) observed that the blued product in 
King crab (Paralithodes camtschatica) had 
some properties similar to those of copper 
proteins or biuret complexes. Osaka (1958) 
succeeded in preventing the blue discolura-
tion in canned crab meat by a "low tempera-
ture and fractional heating" of the carcasses 
from which carapaces were removed. 
Blackwood et al. (1959) recommended a 
fractional cooking method as a means of 
preventing the blue discolouration in canned 
Queen crab (Chinoecetes opilio) meat. 
Inoue & Motihiro (1970, a, b, c, d, e; 1971, 
1971a) studied in detail the cause and 
mechanism of the blue discolouration in 
canned King crab meat and established a 
relationship between the copper content in 
the meat and incidence of the blue dis-
colouration. However, Waters (1971), based 
on his studies on canned crab (Callinectes 
sapidus) inferred that only ferrous and ferric 
ions produced blue discolouration typical 
of that met with in canned crab meat. In 
his studies neither cuprous nor ammonium 
ions was found to cause significant blueing. 
Crab meat forms a sizeable proportion 
of the canned marine products exported  
from India. Its share in the total exports 
of canned products is presented in Table 1. 
Table 1. 	 Export of canned marine products 
from India* 
Item 	 1979 	 1978 	 1977 
Shrimp Q 139 204 128 
V 64.28 91.49 52.21 
Crab meat Q 56 42 50 
V 29.35 19.41 31.44 
Tuna Q NEG 14 22 
V 0.15 2.20 3.49 
Sardine Q — NEG NEG 
V — NEG NEG 
Fish Q 2 1 — 
V 0.80 0.20 — 
Mussel Q 2 NEG — 
V 0.48 0.13 
Total Q 199 261 211 
V 95.06 113.43 88.55 
*Sources: "Indian Seafood Exports Scale New 
Heights, Rs. 262 Crores." Marine Pro- 
ducts Export Development Authority, 
Cochin 
Q: Qty. in tonnes; V: Value Rs. in lakhs 
Sporadic occurrence of blue discoloura-
tion has been met with in the Indian canned 
crab meat when the body meat is canned 
separately or body and claw meats are 
canned together, the degree of incidence 
being higher in the latter case. Practically 
Vol. 18, 1981 
118 	 P. K. VIJAYAN AND K. K. BALACHANDRAN 
no research work has been carried out in 
India on this problem. Attempts to analyse 
the cause and possible methods of preven-
tion of this phenomenon in canned body 
meat of Scylla serrata is discussed in this 
paper. 
Materh is 	 Met ods 
Crabs of average length 15 cm caught from 
Cochin backwaters were processed after 
about 3 h. The carapaces and claws were 
removed and the resultant carcasses were 
halved and kept in running water for vary-
ing periods whenever required. The car-
casses were cooked in 2% boiling brine until 
the meat became firm enough to facilitate 
easy removal by scissors, mixed thoroughly 
and packed in lacquered cans (301 x 109) 
with parchment lining on all sides, filled 
with hot brine (2 % brine containing 0.1 % 
citric acid) and heat processed at 110° C 
for 60 min. 
Moisture was estimated by the method 
of A.O.A.C. (1975). The crab meat after 
drying was ashed, and the ash dissolved in a 
small volume of hydrochloric acid to give a 
0.1 N solution on dilution with distilled 
water. The analysis for copper was done 
directly using a Varian Techtrom atomic 
absorption spectrophotometer Model 1100. 
( ryan, G.W.). Visual observations were 
made on the colour of the canned meat and 
gradings given according to the intensity of 
colour. 
esuits a d I seussion 
Freshly caught crab was divided into 
three lots. line processed immediately 
and the second and third were processed 
after exposing to the atmosphere for 4 and 
8 h respectively. The first group of crabs 
processed were activity live, the second 
sluggish and the third near dead when pro-
cessed. The canned meat from these three 
lots were examined for any incidence of 
blue discolouration. The results are pre-
sented in Table 2. 
It was observed in several cases that meat 
picked from fresh crabs had the colour 
superior to those from sluggish or dead 
crabs. It is evident from the results that 
the blue colour is independent of the fre-
shness of the crab used for processing. 
Waters (1971) has reported that dead or 
decayed crabs do not cause any blue dis-
colouration in canned meat. It has been 
observed in the present study that blueing 
whenever occurred, was irrespective of the 
freshness of the meat. 
In another set of experiments, meat picked 
from fresh boiled crabs was packed in 2 % 
brine containing varying concentrations of 
citric acid to study the effect, if any, of citric 
acid on prevention of blueing. The results 
are summarised in Table 3. 
In one set of experiments the colour of 
samples turned blue when citric acid in brine 
was 0.3 % or less and at 0.4 % this phenome-
non was apparently controlled. 
Varga et al. (1969) carried out studies on 
the use of citric acid in preventing blueing 
in Queen crab (Chinoecetes opilio) meat. 
They observed that 0.1 % citric acid consi-
derably prevented the colour change and 
completely prevented the phenomena at 
higher concentration of the acid. Dipping 
the meat in acetic or citric acid solutions has 
been recommended for preventing blueing 
in European edible crab (Cancer pagurus) 
(Edwards & Early). i owever, this does 
not appear to be a recommendable process 
as the flavour and texture of the resultant 
product was very much adversely affected. 
Copper has been reported to be a major 
factor responsible for the blue discoloura-
tion in canned crab meat. The copper 
content in a number of samples of raw 
crab meat was estimated and reported in 
Table 4, which shows that the copper con-
tent in crab meat varied between 0.09 
and 5.696 mg/100g. In order to discern the 
relationship between the copper content 
in the meat and its influence on the blue colo-
uration of canned meat, meat showing blue 
discolouration in cans was separated out 
and the copper content estimated and com-
pared with the copper content in normal 
meat. Table 5 shows a definite relationship 
between copper content and the occurrence 
of blue discolouration. Canned meat 
with very light blueing had a copper content 
of 1.81mg/100g and the intensity of the colour 
increased with increase in copper. 
Inoue & Motihiro (1970) have established 
a relationship between the copper content 
FISHERY TECHNOLOGY 
BLUE DISCOLOURATION IN CANNED CRAB BODY MEAT 
	 119 
Table 2. Effect of time lapse between catching and processing on the incidence of blue discolour-
ation in canned body meat 
Experiment 	 State of 	 Colour of 	 Visual rating 
No. 	 crab used 	 meat 	 of blueing 
1 	 L 	 White 	 0 
	
S 	 White/ 	 0 
dull white 
D Dull white 	 0 
2 	 L 	 White 	 0 
	
S 	 White 	 0 
D Dull white 	 0 
3 	 L 	 White 	 0 
	
S 	 White/ 	 0 
dull white 
D Dull white 	 0 
4 	 L 	 White with slight 
blueing in parts 
	
S 	 White with slight 
blueing in parts 
D Dull white with 
slight blueing 
in parts 
5 	 L 	 White with moderate 
blueing in parts 
	
S 	 White with moderate 
blueing in parts 
D Dull white with 
moderate blueing 	 + 
in parts 
L = actively live, processed without delay 	 0= no blueing 
	
S = sluggish, processed after 4 h 	 + = slight blueing 
D = dead/near dead, processed after 8 h 	 + + = moderate blueing 
Table 3. Effect of citric, acid in the prevention of blue discolouration 
Experiment 	 Concentration of 
	 Visual rating 
	
No. 	 citric acid in brine 	 of blueing 
0 – Control 	 0 
0.1 
	 0 
0.2 
	 0 
	
1 	 0.3 	 0 
0.4 
	 0 
0.5 	 0 
0 – Control 
0.1 
0.2 
	
2 	 0.3 
0.4 
	 0 
0.5 	 0 
0 = normal; — no blueing; += slight blueing; + + = moderate blueing 
Vol. 18, 1981 
120 	 P. K. VIJAYAN AN t K. K. BALACHANDRAN 
Table 4. Copper content in raw crab body 
meat 
Sample 
1 
Copper content (DWB) 
mg/100g 
2.190 
2 2.890 
3 5.696 
4 5.589 
5 5.023 
6 3.724 
7 3.524 
8 5.213 
9 0.090 
10 0.160 
Table 5. Correlation between copper con-
tent and blue colour in canned 
crab meat 
Sample 	 Copper 	 Visual rating 
content (DWB) 	 of blueing 
mg/ 100g 
1 0.06 0 
2 0.095 0 
3 1.62 0 
4 1.82 —+ 
5 2.043 
6 2.89 + + 
7 3.34 ++ + 
8 4.33 + 
0 = no blueing; — + = very slight 
blueing; + = slight blueing; + + = 
moderate blueing; + + + = heavy blueing 
and the incidence of blue discolouration in 
the meat of King crab. They found 0.49mg/ 
100g copper in normal King crab meal and 
2.8mg/100g in the blue crab meat and stated 
that the blue colour appears when copper 
exceeds 22mg/100g. Their studies on the 
cause and mechanism of blue discolouration 
in canned crab (Inoue & Motihiro, 1970a, 
1971, 1971a) showed 1970b, that it is due to 
the haemocyanin contained in crab haemo-
lymph, which can react with hydrogen 
sulphide to produce blue colour. They 
observed that the reflectance spectra of 
haemocyanin-sulphide complex closely rese-
mbled that of the blue meat and con  
eluded that the causative factor for blue 
discolouration in canned crab is a haemo-
cyani n -sulphide complex. 
Since the blood haemocyanin of crab is 
the source of copper in the meat, different 
methods were tried to bleed the animals to 
get rid of copper. In order to increase the 
exposed area to facilitate bleeding the 
cleaned carcasses were halved and then 
subjected to treatments as detailed in Table 6. 
The data indicates that it is advantageous 
to bleed the carcasses in running water for 
30 min. To find out the optimum time 
required for bleeding and to bring down 
the copper content to a satisfactory level, 
the cleaned carcasses were bled in running 
water for varying periods and the copper 
contents estimated after bleeding and the 
meat canned and observed for the incidence 
of blueing. The results are presented in 
Table 7. 
When the initial copper content was low, 
bleeding for 15 min in running water was 
sufficient to bring down the copper content 
to a satisfactory level when no blue colour 
developed in the canned meat. When the 
initial copper content was very high, bleed-
ing upto 45 min did not yield satisfactory 
results. Longer bleeding in such cases, 
not only failed to prevent the appearance of 
blue colour but also affected the texture 
and flavour of the meat. However, cases 
wherein such abnormally high copper con-
tent is met with are very rare and for pra-
ctical purposes bleeding for 30 min in 
water was quite sufficient to prevent the 
incidence of blue colour in the product. 
The "low temperature and fractional 
heating" method described by Osakabe 
employs the principle of bleeding of the 
crab carcasses to get rid of the copper 
making use of the difference in the coagula-
ting temperatures of blood and muscle pro-
teins. Similar method has been recommen-
ded by Blackwood et al. (1969) for preven-
tion of blueing. However bleeding the 
cleaned carcasses of crab in water was found 
to be simple and sufficient from this study. 
Tables 5 and 7 indicate that the blue dis-
colouration becomes apparent when the 
copper in the meat is around 2 mg/100g 
Known amounts of copper in the form of. 
FISHERY TECHNOLOGY 
BLUE DISCOLOURATION IN CANNED CRAB ID ODY MEAT 	 121 
Table 6. Copper content in the body meat of raw crab after various treatments 
Copper content mg/100g 
(DWB) 
Treatment 	 Experiment 	 Experiment 
1 	 2 
Raw body meat (control) 	 2.89 	 3.524 
Bled in running water for 30 min 	 1.80 	 1.910 
Dipped in 2 % brine for 30 min 	 2.31 	 2.620 
Dipped in 2 % brine containing 
0.1 % citric acid for 30 min 	 2.29 	 2.710 
Table 7. Effect of duration of bleeding on the copper content and the incidence of blueing 
Copper content mg/100g (DWB) and visual rating of blueing 
I 
Time of 	 Copper 	 Visual 	 Copper 	 Visual 	 Copper 	 Visual 
bleeding 	 rating 	 rating 	 rating 
min 
10 2.19 0 2.89 + 5.199 + + + 
15 1.829 0 2.17 0 3.720 + + 
30 1.62 0 1.81 0 2.920 -+- 
45 1.50 0 1.72 0 2.160 0 
0 = no blueing; 	 slight blueing; + + = moderate blueing; -i- + += heavy blueing 
Table 8. Effect of added copper ions on 
blueing 
Copper 	 Copper in 	 Visual 	 rating 
(as CuCl 2) 	 canned meat 	 of blueing 
added 	 mg/100g (DWB) 
mg 
Control 	 1.628 	 0 
0.5 	 1.916 
	 —+ 
1 	 2.54 	 + 
1.5 
	 4.36 	 +++  
0 = no blueing; — + = very slight 
blueing; + --- moderate blueing; + + + = 
heavy blueing 
cupric chloride solution were added to the 
meat picked from thoroughly bled and 
cooked crab carcasses to find out the effect 
of added copper ions in the occurrence of 
blue discolouration and also to determine 
the critical level of copper needed for the 
appearance of blue colour. It is observed 
from Table 8 that by introducing copper ions 
blue colour occurred in the sample with a 
copper content of 1.916mg/100g.. This is 
almost in agreement with the observations 
recorded in Tables 5 and 8. 
From the experiments and observations 
made above it can be reasonably concluded 
that the blue discoloration in canned crabs 
is due to copper content around 2 mg/100g 
and by bringing down the copper content 
below this level by bleeding the meat i 
water, the phenomenon of blue discoloura-
tion can be prevented in canned crab meat. 
References 
A.O.A.C. (1975) Official Method of Ana-
lysis. Association of 0 cial Ana-
lytical Chemists, 12th edn., Washington 
Blackwood, C. M., Varga, S. & ewar, 
A. B. (1969) Meeting on Atlantic 
Crab Fishery Development, Frederi-
cton-N. B. March 4-5 
Vol. 18, 1981 
122 	 P. K. VIJAYAN AND K. K. BALACHANDRAN 
ryan, G. W. (1973) J. mar. biol. Ass. 
U. K. 53, 145 
Edwards, E. & Early, J. C. Torry Advisory 
Note No. 26; Torry Research Station, 
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Goringer, H. S. & Dassow, J. A. (1964) 
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FISHERY TECHNOLOGY 


Studies on blue discolouration in canned body meat of crab (Scylla serrata)

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Studies on blue discolouration in canned body meat of crab (Scylla serrata)

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