The effects of temperature and relative humidity on the rate of drying of split open fish and salted fish in a tunnel dryer have been studied at a constant air velocity. By a judicious combination of these two, the rate of drying could be considerably accelerated, 10 to 12 hours only being required for drying to moisture levels below 30% in the case of mackerel, lactarius, otolithes and kilimeen (Nemipterus japonicus

Balachandran, K.K.

Aquatic Commons

AN ACCELERATED METHOD OF HOT 
AIR DRYING OF FISH 
K. K. BALACHANDRAN 
Central Institute of Fisheries Technology, Ernakulam, Cochin-11 
The effects of temperature and relative humidity on the rate of 
drying of split open fish and salted fish in a tunnel dryer have been 
studied at a constant air velocity. By a judicious combination of 
these two, the rate of drying could be considerably accelerated, 10 to 
12 hours only being required for drying to moisture levels below 30% 
in the case of mackerel, lactarius, otolithes and kilimeen (Nemipterus 
japonicus) 
INTRODUCTION 
Drying has, perhaps, been the oldest 
method of preservation of fish practised 
extensively in the fishing villages of India 
and elsewhere. Sun drying of fish is 
practised even todate in more or less the 
traditional way with little or no modifi-
cation to the process. Inspite of the rapid 
development of freezing and canning 
industries in our country in recent years, 
more than 40% of our fish landings is still 
preserved by either simple sun drying or 
salt-curing and drying. Being the cheapest 
means of preservation of surplus fish, 
drying is likely to stay, especially in a 
developing country like ours for quite a 
long time. Even though we are exporting 
considerable quantities of dried fish to the 
far eastern countries like Ceylon, Burma 
etc, very little of it is exported to the 
Western markets, because our products 
VoL VI No. 2 1969 
fail to conform to the quality specifications 
of those markets. This is mainly attribut-
able to the fact that our fish drying 
industry is still being carried on in the 
traditional way. Even within the country 
there is a large demand for good quality 
dried fish which cannot be produced by 
the methods now in use. 
Much of the earlier work done on 
mechanical dehydration of fish was con-
fined to cooked minced fish. Later some 
attempts were made to dry whole fish. 
Legendre (1955, 1961) studied the drying 
characteristics of light salted cod fish and 
Cambodian fish in an artificial dryer where 
the effects of temperature, relative humi-
dity, air velocity etc on the drying charact-
eristics of these fish have been discussed. 
Prabhu et a/ (1963) studied the drying 
characteristics of some impor!ant commerc-
124 
ial fishes of India in a laboratory tunnel 
dryer. In all these investigations the main 
difficulty encountered was the unduly long 
time required for drying, though in contrast 
to sun drying the time consumed was 
considerably less. To overcome this 
difficulty a method has been reported 
(Anon; 1963) where the fish was subjected 
to successive higher temperatures of drying 
during the falling rate period. This was 
achieved by passing the fish through 
different regions of progressively higher 
temperature over a conveyor belt in a 
tunnel of about 401 length. This method 
has since been applied in the case of fish 
of temperate and cold countries. The 
present communication deals with the 
application of the above principle to 
tropical fish. By a detailed study of 
different factors lih temperature and 
relative humidity and a combination of 
these, a method has been worked out for 
the accelerated dehydration of fish which 
is much faster than the methods reported 
earlier. 
MATERIALS AND METHODS 
The tunnel dryer reported by Prabhu 
et a! (Joe cit) was employed in these studies 
after effecting suitable modifications. 
Provisions were made for closer control of 
temperature and relative humidity where 
the wet bulb temperature was automa~ 
tically controlled. Uniform air velocity 
inside the tunnel was ensured by making 
the ends of the dryer elliptical by 
providing suitable baffles. The air velocity 
was maintained constant at 130 linear mf 
min. Trays made of galvanized iron mesh 
were used to spread the fish while drying 
in the tunnel. 
Marine fish caught by the trawlers 
operated by the Institute off Cochin as 
well as those landed near about Cochin 
by country fishing boats were used in 
these studies. Fish were kept well iced till 
used for experiment. The common com-
125 
mercia! varieties like pallikora (Otolithes 
argenteus), Kilimeen (Nemipterus Japo-
nicus), mackerel ( Rastrel!iger kanagurta), 
and lactarius(Lactarius lactarius) were used. 
PREPARATION OF FISH FOR DRYING. 
The fish was cut open from the dorsal 
side, intestines and gills removed and 
washed clean of blood and slime. The 
split open fish was dried as such and when 
salted fish was employed for drying, salting 
was done in the ratio 1:5 (salt : fish) for 
18 hours since maximum water loss for 
split fish occurred during this period (Sen 
et at, 1960). After salting; the fish was 
lightly rinsed with tap water to remove 
aoy adhering salt. In both cases (salted 
and unsalted) the fish were spread on the 
trays, flesh side upwards and loaded in the 
dryer whieh was operated at the desired 
conditions. 
The loss in weight due to the removal 
ofwater was determined at regular inter-
vals and the moisture loss curves obtained 
by plotting the loss in kg per 100 kg of 
dry matter against time in hours. 
RESULTS AND DISCUSSION 
Several experiments were carried out 
at temperatures ranging from 40°C - 60°C 
keeping both R. H. and air velocity const~ 
ant at 50% and 130 linear m/min respecti-
vely, the latter two being selected at 
random. Fig. 1 shows the effect of temp-
erature on the drying rate of split open 
pallikora (Otolithes argenteus) (av. weight: 
105 g), It can be seen from the figure that 
the moisture loss increased with increases 
in temperature. However, temperatures 
above 50°C resulted in cooking of the 
muscle resulting in brittleness of the end 
product. 
Unsalted fish on drying exhibited two 
distinct phases of drying, viz., the constant 
rate period, during which the loss of 
moisture per unit weight of dry matter per 
FISHERY . TECHNOLOGY 
200 
\60 
100 
90 I 40 ••' 
II ~0 ,,• · 
~~--~--~~~~~ 
1~ I() 20 24 
TtmG In,..,~,_,, 
unit time remained constant and the falling 
rate period, during which th~ loss gradu-
ally decreased (Prabhu eta/, /oc cit.) Drying 
rate of fish at the constant rate period 
is equal to that from a saturated surface of 
the same shape (Jason, 1958). Tt is the 
falling rate period that determines the 
Pxtent or duration of drying. Legendre 
(loc cit) has observed that relative humidity 
(RH) influences drying during the early 
periods of drying and has recommended 
a higher RH for the later stages of 
drying. It is needless to say that any such 
increase in RH will automatically result in 
the lengthening of the drying period since, 
evidently at higher RH the moisture uo 
take by the circulating air will be lessened. 
In order to make up for the slow moisture 
loss during this period, the temoerature 
can be slowly stepped up to accelerate th~ 
diffusion of moisture from the interior to 
the surface from where it can be evaporated 
at the same rate. 
Drying characteristics of Otolithes 
(1 05g av. wt., split open unsalted) as affected 
by RH is represented in Fig. 2. the temp-
erature being maintained at 50°C and the 
air velocity at 130 linear mimin. The 
curves indicate that the duration of drying 
is hi~hly influenced by RH. The higher 
the RH, the more the time required for 
drying. During a period of 20 hours the 
VoL VI No. 2 1969 
--,---~---.-----r-=--~F-~ 
4 e 12 16 .::c• 24 
'rtm&mhovn 
FtQ t' [F"FECT OF n H ON DRYING RAT£: 
amount of water lost per 100 kg of dry 
matter was 150 and 202 kg respectively at 
60% and 50% RH, but at 40% RH 200 kg 
of water are lost even in 16 hours, How-
ever, the phenomenon of case hardening 
was conspicuous at this RH. Even some 
cracks appeared on the surface of the fish 
at the later sta[!e of cfrying at this RH 
resulting in poor appearance Of the 
of the material. Even at 50°C, higher RH 
of the order of 60% and above caused 
certain amount of cooking of the flesh in 
the case of certain fishes. 
R. H. influences the drying rate as 
long as the surface remains wet. Once 
the surface of the fish gets dried up, further 
dryin~ is largely governed by the mechani-
sm of diffusion of water from inside the 
muscle which can be accelerated by in-
creasing the temperature of the circulating 
air. But an unregulated increase in drying 
temperature and reduction in RH aggre-
vates the phenomenon of case hardening. 
Curve II in fig. 3 shows the effect of 
phased drying of Otolithes (105 g av. wt., 
split open, unsalted) by increasing the 
temperature slowly as drying progressed, 
the RH and air velocity remaining constant 
at 50% and 130 linear m/min. Curve I 
shows the course of drying of the same 
fish at constant temperature of 50°C, 
keeping other conditions unchanged. 
126 
I 
F 10 3 f rFEC T OF PHASED ORVING 
~,_.... _____________ __, 
By increasing the temperature of the air 
slowly after the constant rate period 
(4 hours as shown by the straight line 
portion of the curve) it was found possible 
to accelerate the moisture loss considerably 
thereby effecting substantial saving in the 
drying time. The phenomenon of cooking 
of muscle did not take place during this 
process. Since considerable amounts of 
moisture evaporate in the initial stages of 
drying and the latent heat required for this 
is taken from the circulating air, the temp-
erature of the muscle does not rise to any 
appreciable extent. At the later stages 
when much of the water has been removed 
from the fish, increase in temperature does 
not affect the muscle as at the earlier stages 
of drying, since the residual water content 
is not sufficient to cause cooking of the 
muscle. 
Since the above technique effects con-
siderable reduction in drying time, the 
method was extended to the dehydration 
of salted fish also. The results of some 
experiments carried out with different 
varieties of fish are represented in fig. 4. 
Salted fish did not exhibit any constant 
rate period. The initial high/ rate of drying 
suddenly fell and slow drying proceeded 
(Prabhu eta/Joe cit). From the figure it 
can be seen that this fall in rate can be 
compensated to a large extent by the in-
127 
100 
~ 100 
E 
'o 
0 
0 
2 
.. 
n 
0 
FiO ~ PHASED DRYING OF SALTED FISH 
crease in temperature as observed with 
unsalted fish. The time taken by different 
fishes to dry to moisture contents of 30% 
or below varied from 10-12 hours depend-
ing upon size. 
SUMMARY 
High temperature accelerated the dry-
ing of fish considerably, but temperatures 
above 50°C caused a certain amount of 
cooking of the muscle. The lower the 
RH, the higher the rate of moisture loss. 
But RH below a certain level resulted in 
case hardening. Unduly high RH values 
increased the duration of drying. By 
increasing the temperature of drying after 
the constant rate period, the drying time 
could be considerably reduced without 
sacrificing the product quality. Extension 
of this method of drying to salted fish 
worked well in substantially reducing the 
drying time. 
REFERENCES 
Anon 1963. World fishing, 12, (5), 37-46. 
Jason, A. C. 1963. Fundamental Aspects 
of the Dehydration of Food Stuffs, 
Society of Chemical Industry, London. 
FISHERY TECHNOLOGY 
Legendre, R. 1955 J. Fish Res. Bd. Canada, 
12, (I), 68-74. 
Legendre, R. 1961. J. Fish Res. Bd. Canada, 
18 (2), 147-62. 
Prabhu, P. V. Swaminathan, M. Venkatas 
raman, R. and Bose, A. N. 1963. 
VoL VI No. 2 1969 
Indo. Pac. Fish. Coun. Current Affairs 
Bulletin No. 37. 
Sen, D. P, Visweswariah, K. and Lahiry, 
N. L. 1960 Food Sci. (Mysore) 10· 
123-31. 
128 


An accelerated method of hot air drying of fish

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An accelerated method of hot air drying of fish

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