c.~ PROJECT REPORT NO.8

Canada [Dept. of] Fisheries. Industrial Development Service PROJECT REPOR·

THE MECHANICAL DRYING OF LIGHT SALTED CODFISH

by

G.R. Peters Memorial University of Newfoundland, St. John's

and

W.D. McDougall Industrial Development Service,

Department of Fisheries of Canada, Ottawa

for

Industrial Development Seryice Department of Fisheries of Cangda, Ottawa

May 1967

THE MECHANICAL DRYING OF LIGHT SALTED CODFISH

Opinions expressed and conclusions reached by the author of this report are not necessarily

endorsed by the sponsors of this project

1

TH~ l~u.:.;GHAi~Ic}~L .tJltYiNG uF 1ICTHT -.->AV.cl~D CU]j1i'l-.->H

Il~Tl-(O J)UCT ruN

:Memorial Uni versi ty of Newfoundland and the Industrial

Development Service of the Department of Fisheries of Canada have

recently been en~aged in further studies of the mechanical drying

of salted codfish.

This report deals with the mechani.cal drying of liR'ht salted

codfish. A more detailed, technical report covering the mechanical

drying of sal ted codfish in general t will be published at a later date.

The solutions which will be advanced are not entirely

new in concept, but the problems have been studied in considerable

detail and the practical engineering aspects of the application

have been kept in mind.

Jill::) UL'1':-:>

It is essential to remove water rapidly from wet light

salted fish during the first few hours of mechanical drying if

sliming, and consequent downgrading of quality, are to be prevented.

During the summer, the outdoor air conditions may be such as to

impose serious limitations on the drying pot~ntial of the air and

''''hen IIbadtt dryin.?; conditions are experienced, sliming is very

likely to occur.

It is possible to determine ",hether any day is suitable for

drying wet li~ht salted fish by means of a simple instrument called

a "psychrometerTl. It consists of two thermometers mounted on a

frame so that ~ood air circulation is provided over the bulbs,

one of vlhich (the ·wet bulb) is kc;.:,-t wet by a vIick soaked \,lith

v.rater, (this thermometer 1di11 -'~ i ve a lovler reading than the dry

bulb unless the air is saturated); vlhi18 the other (the dry bulb)

2

is an ordinary thermometer. The dew point temperature and the

relative humidity can then be readily obtained from the dry bulb

and wet bulb readings by means of a suitable chart, tables or

slide rule.

The dew point is the point at which any reduction in

temperature 'til/ould cause condensation of some of the water vapor.

The relative humidity (expressed in percent) is the amount

of water vapor present in the air relative to the amount present

when fully saturated at the same temperature.

\'Then outside air is heated, the relative humidity is

decreased but the de,,', point temperature is unchan~. l'he

relative humidity of the air after it has been heated depends only

upon the dew point temperature and the temperature to which the

air is heated. The lower the dew point temperature, the lower is

the relative humidity which can be obtained. (See flg. 1). Also,

as indicated above, for any dew point temperature, the relative

humidity is decreased if the temperature of the air is increased.

For example, if the outside de"" point temperature is 55°F., the

relative humidity can be reduced to approximately 43 percent by

heating the air to 80°F. If the outside dew point is 50°F. the

relative humidity can be reduced to approximately 36 percent by o

heating to $0 F. (dee fig. 1). Note that it does not matter what

the relative humidity is outside.

It is essential that the drying rate be reasonably high

(to be discussed in detail later) during ~he early stages of

drying wet light salted fish in order to prevent slimine.

3

Good drying conditions are obtained when the dew point

temperature is low. In a dryer 'I1Tit~l an air velocity of about

300 feet per minute and an air temperature of about BOoF it is

unwise to attempt to dry wet light salted fish if the dew point

temperature is ov-er 55°F. At a temperature of BOOF, and a dew

point of 550F the relative humidity of the air before it reaches

the fish is 43 percent, assuming there is no recirculation, and

may easily be 55 percent or much higher at the exhaust end of

t.he dryer. The relative humidity increases and the dry bulb

temperature drops as the air moves over the fish due to the pick­

up of moisture. Experiments have shown that if the relative

humidity is in excess of 50 percent, and the air velocity is

about 300 feet per minute or lovver, the drying rate is severely

reduced and slimj.ng might occur.

The problem then in drying wet light salted fish is to

prevent sliming on days when the dew point is high. A record

of temperatures taken during the sununer months at the Department

of Fisheries Experimental Plant at Valleyfield, Newfoundland,

indicates that dew point temperatures are frequently in excess

of 55OF. as shown in Table 1.

rfABLE 1

NUlvIHER OF DAYS VmE Nil/SAX IJ.\'iUIvI D1!:V[ POINT TEl'TPERATUfill

\/AS 220F • OR HIGHt:R

Year 1959 1960 1961 1962 1963 1964 1965 1966 Month

,-' ~ June 0 3 4 0 1 1 ,-"",

4

Year 1959 1960 1961 1962 1963 1964 1965 Month

July 19 16 15 2 10 10 10

:l August 11 15 9 9 11 6 September 4 4 7 3 7 3 !F

To'rAL 34 3$ 35 14 29 20 ...-) fL; ,Y'

IvIOI::i'l'Ulill LOS':; GURVES

The most useful way to present drying data is by a graph

of moisture loss with drying time which will be discussed in

more detail in the following section. (See !i'ig. 3).

Note that the "moisture loss is presented on a "dry basislt.

1966

13 -, -lJ

:1

Although the "wet basislt is used in the trade for moisture content,

it is impossible to draw a moisture loss curve on a "wet basislt

except for individual cases. This is because the total weight,

which is the basis for "wet basis lt is changing as drying proceeds.

The Itwet basis" refers to the percent of moisture by weight of

the wet material. The "dry basis" refers to the percent of

moisture by weight of the Itbone dry" material; thus, 60 percent

on the "wet basis" means 60 pounds of moisture "in 100 pounds

of the wet material, while 60 percent on the "dry basis!t means

60 pounds of moisture for every 100 pounds of the !tbone dry"

material or in 160 pounds of the wet materia.l.

To convert from one basis to the other the following

formulas may be applied:

(1) % moisture ("wet basis") ::; ~~~~~~~~~~~~~~~

( 2 ) % mo i st ure (" dry bas is 11) = ~~:..::.=~:::...;:~~~~b~ar..;:s;;.;:i~s:;..II-:t):......;:;x~l:.;:0rT0 wet basis

5

Th{s relationship is shovm in a graph in Fig. 2 which

can be used to find both moisture content and moisture loss

on one basis if the value on the other basis is known. For

example a moisture loss of 80 percent (udry basisTf) reduces

the moisture content from 70 percent (ttwet basis") to

approximately 60 percent ("wet basistf) as illustrated by the

dashed lines in Fig. 2.

EFFECT OF RELATIVE HUMIDITY ON DRY1NG

In a salt fish dryer, without dehumidification equipment,

relative humidity can only be kept down to a level permitted by

the outside dew point. Thus, while the relative humidity may be

increased by recirculating air, it cannot be lO\'1ered once the

limit imposed by the dew point is reached.

The effect of relative humidity on the moisture loss

curve is shown in Fig. 3. The curves were obtained by drying

light salted fish ( salt content approximately 25 percent

ttdry basis") of medium size under the conditions indicated.

The moisture loss curves for 35 percent and 50 percent are quite

close together, but the curve for 65 percent is very much lower.

VIet lie;ht salted fish would slime if drying were as slow as that

indicated by the 65 percent curve. To be safe, the 50 percent

curve in Fig. 3 will be taken as the "minimum moisture'loss curve".

In other words, wet light salted fish should lose about 80 percent

6

(ndry basisTr) in the first twelve hours of drying. For light

salted fish with an initial moisture content of 70 percent

(ftwet . basislf), this means that it should be dried to about

60 percent ("wet basis") in twelve hours.

As already indicated, the relative humidity of the air

increases as it passes over drying fish, so that it is the fish

in the region of maximum relative humidity (at the exhaust end

of a tunnel type dryer) which must dry at least as fast as that

indicated by the 50 percent curve of Fig. 3.

The rise in relative humidity depends mainly on the amount

of fish being dried. It therefore follows that when drying

conditions are poor, some improvement can be gained by reducing

the dryer load.

The problem of preventing spoilage new reduces to over

coming the effect of high relative humidity so that the moisture

loss curve can be kept above the minimum loss curve of Fig. 3

ONE ~OLUTION - HIGH~R AIR V~LOCITY

\Vhile any part of the fish surface is wet, the drying rate

is influenced by both relative hwnidity and the air velocity.

\Vhen the relative humidity in the dryer is low (that is on days

when the dew point is low) the air velocity is not very important,

since the drying rate will be sufficiently high to prevent sliming.

7

Whe'n the relative humidity in the dryer is high {that is

on days when the dew point is high} the drying rate can be

maintained at a reasonably high level by increasing the air

velocity. Figures 4A, 4B and 4C show the effect of air velocity

on the moisture loss curves at 65 percent, 50 percent and 35

percent relative humidities at air velocities of 300, 600, 900

and 1200 feet per minute. The dashed curve is the "minimum loss

curve" previously discussed.

When the maximum relative humidity in the dryer is as high

as 65 percent (Fig. 4A) the air velocity should be at least 900

feet per minute. When the maximum relative humidity is 50 percent

(Fig. 4B) an air velocity of 300 feet per minute is sufficient.

ALTERNATIVE SOLUTION - PROGliAIVj}JiliD AIR 'l'.I!;l~1P~HATURE

When a material is drying the cooling effect of the

evaporating water keeps the temperature of the material below

the air temperature. The temperature of wet light salted fish

during the first few hours of drying is slightly above the wet

bulb temperature, and gradually approaches the dry bulb temperature

as drying proceeds. 1 Legendre proposed a drying system in which the fish

temperature would be maintained at SOoF. by inserting a temperature

sensitive probe (thermocouple) into the fish flesh and using this

to air control temperature. Since the difference between the ,air

1. Journal of Fisheries Research Board o£ Canada, 15(4) Pg.543

and fish t~mperature gradually decreases, the air temperature will

be high at first and will gradually fall to near gOoF. There are,

however, some practical difficulties with the thermocouple system; f

mainly that of properly insertin~ the probe into the fish flesh,

and that of overcoming the decrease in sensitivity of the control

sJrstem due to the burying of the controll ing element in the flesh.

In an attempt to eliminate the above mentioned difficulties,

experiments were carried out to determine just how the air temp-

erature should be controlled to keep the fish temperature near,

but never above gOoF. The curves in Fig. 5 represent the pro­

grams which have been tested at Valleyfield.

The greatest advantage would be gained by having an

automatic control system vrhich would take the temperature through

the desired program; however, the ordinary thermostat type control

can be used to approximate the program. For example, a very simple

"programtt would be to set the dryer thermostat at 90oF. for the

first five hours, g5 0 F. for the next five hours, and 80°F for

the remainder of the drying period. Using this simple method,

there would be practically no danger of slimin3 of light salted

fish under the worst summer drying conditions, that is with dew

point temperatures up to 600 F. and even slightly higher.

Fig. 6 shows the moisture loss curves obtained for light

salted fish drying under the 77° and 800 programs shown'in

Fig. 5. It can be seen that the air velocity makes very little

difference, so that high air velocities a~e not required with this

method. The dashed curve is the minimum moisture loss curve

previously discussed.

..

9

COi'.'IPAH.ISON 01" THL 'l'i'lO SOLUTION::>

The air velocity in an existing dryer can be increased by

either increasing the size of the fan and motor or decreasing

the cross-sectional area of the dryer. The latter is not al\1<lYs

desirable or practical. If the fan capacity is increased, the

volume of air (cubic feet per minute) moving through the dryer

will be increased and , the heat demand will be increased in

proportion. Thus a new heater might be required. Increasing

fan capacity thus automatically increases electrical power arid

fuel consumption. Power and heat consumption can be kept to

a minimwn by either of the following arranBements.

(I) THE INJ'l'ALLATION OF A VJ..l·nABL~ SPbLD Dl{IV.c; It'vI\ TH~ FAN

A variable speed drive is an arrangement such as a variable

pitch pulley which permits variation of the fan speed and lower

cost operation when high air velocity is not required.

( 2 ) A TV.JO Dli yr:H S YS T~I\II

One of the dryers would be a small capacity, hi~h air

velocity type and the other dryer would be a much larger low air

velocity dryer. The small high velocity dryer could be used for

the complete drying of small lots of fish, in vvhich case the fan

would ooerate a high speed during the early sta'se and at low

speed for the final stage. When the plant was producing to

capacity, the small dryer would be operated entirely as a high

velocity dryer, with the fish being move~ to the large low air

10

velocity dryer after 8 to 12 hours, depending on the type of fish

and drying conditions.

The second solution of overcoming poor drying conditions,

-programmed air temperature- does not require any electrical energy

change since it is not necessary to use high air velocity. The

heater load, however, is proportional to both the amount of air

to be heated and the temperat.ure rise required. Heat consumption

will thus be increased due t.o the increased air telilperature. How­

ever, most existing dryers would have a heating system capable of

maintaining; a temperature of 90°F'. during the summer.

This programmed air temperature method can therefore be

applied to a dryer with relj.able temperature and humidity controls

and good air circulation without any new equipment, if the operator

is willing to reset temperature controllers as drying proceeds.

The danger or disadvanta[e of the above method lies in the

risk of overheating and resulting "fish burn". It must be borne

in mind that the use of hi8h temperatures depends upon the cooling

effect of evaporatinE water.

If there is an area of fish ~lere water does not evaporate

from the surface, heat damae;e may result. For example, fish

should not be laid over the tray frame and care should be taken

to avoid "dead spoto;; or areas of little or no air circulB.tion

in the dryer, since heat dalila. f~e is likely to occur in these

regions. These restrictions \'Jill probably JClean a slt?:ht reduction

in the usual dryer load • .:lince the dryer . is beinz nnerated at hi [';h

tempel'o.ture, the control system must be reI i R.ble. In r eneral the

11

programmed, air temperature method would require more care and

supervision than the high air velocity method but it would

require less capital expenditure in modification to an existing

dryer if the existing controls were adeque,te.

The total time required to dry the product to the final

moisture content is not greatly reduced by either .nethod, compared

to conventional drying, although some small reduction is possible.

This is due to the fact that once the fish surface is dry, the

drying rate is controlled more by the properties of the fish itself

than by external conditions. The advantage of the methods described

above lie not in the reduction of total drying time, but rather

in the elimination of the sliming problem during the drying of

wet light salted fish.

Moisture loss curves "lere determined for velocities ranging

from 300 feet per minute up to 1200 feet per minute at a maximum

dry bulb temperature of gOoF. and for programmed air temperature

drying at air velocities up to 1200 feet per m~nute.

Results i'ndicated that either method can be used to dry

wet light salted codfish in mechanical dryers, without de­

humidification, under the worst sun~er drying conditions normally

encountered (up to 600 F. dew point temperature) without danger

of sliming and consequent quality deterioration. Normal air

velocities (approximately 300 feet per minute) are sufficient

with the prograrruned air temperature method.

12

Light salted fish should lose about 80 percent (lldry basis")

in the first tV/elve hours of drying. This is equivalent to about

a 10 percent ("wet basistl) loss for 1 ight sal ted fi sh \"li th an

initial moisture content of 70 percent ("wet basislf).

If the dew Doint temperature of the air is 55°1" or hit:her

the drying of wet li~ht salted fish in ordinary mechanical dryers

(80°}'. maximtIDl dry bulb temperature and 300 feet per minute air

velocity) is unwise.

1 )

90--~--~~~--~~----~-------------------------------

FIGURE I

RELATIVE HUMIDITY WITH TEMPERATURE FOR SEVERAL DEW POINT TEMPERATURES

80~---4--~---4~~----~------~------~------~--- - ---

70~----~+---+-~+---+-~------~--------+-------+-------

~60 ~----~--~~-;~~~~~----~-------r-------+­... -Q

:e ::> :E:

LtJ 50 > -~ ...I LtJ ~ 40 r-----~r-----~----~~-T--~~~F----+-------+-------

t­Z LtJ ' (.) ~ LtJ __ ______________________ _

~ 30r-----~~----~------~--~~~----~

20 r-----~-------1------~---~---+----~~~~--~--~~~

10 ~----~r_----~------_4--_4--_4------_+------_+------~

o 50 60 70 80 90 100 110 12C AIR TEMPERATURE - of

1)

90--~--~~~--~~----~------------------------------~

FIGURE I

RELATIVE HUMIDITY WITH TEMPERATURE FOR SEVERAL DEW POINT TEMPERATURES

80 ~---+~f---\----+___I--+------+---------r--------"---------.----- -

70~----~~--+-~+---+-~-------~--------+-------+------~

~60 ~----~~~~-4~~~~r-----~-------+-------+------~ l-e :e ::> :r: 1LI 50 > ~ ..J ------L&J ~ 40 ~----~---------I----~-1~--~~~~---+-------+------~

I­Z 1LI (.)

~ 1LI ________________________ _

~ 30 ~----~~-------I------~--~~~----~

20 ~----~~-------I------~--~--~----~~--~--~--~~-1

10 ~----~~----___I------~--~--~------_+------_+------_4

o 50 60 70 80 90 100 110 120 AIR TEMPERATURE - of

450

400

350

-en en ~ 300

>-a: Q -I-~ 250 I-'z o (.)

w g; 200 I-en -o ::!:

I-z 150 w u 0::: W Q.

100

50

14

FIGURE 2 " RELATIONSHIP BETWEEN

MOISTURE CONTENT ON WET AND DRY BASES

/ --(j)-04 ----------------------I en

0 -I -wen

i--- 0:: (j) :::><{ 1-0) en 0>-~o::

0 ~-0 0

--~-.- --~--- 1----- -------_.-I t I I . II

V I 1

/ I I

~ / + ~ I. ..

~ I MOISTURE 1 LOSS

II(wET BASIS)

20 30 40 50 60 70 80

PERCENT MOISTURE CONTENT (WET BASIS)

CJ)

CJ)

~ al

>-a: 0

t-Z lLJ (.)

a:: lLJ Q.

CJ) CJ)

0 -I

I.LJ 0:: ::> I-en 0 :lE

15

180 --------~----~~----~~------------------------·----~ FIGURE 3

EFFECT OF RELATIVE HUMIDITY AIR VELOCITY = 300 FPM AIR TEMPERATURE = 80° F

160~------~----~~----~~----~------~------~-------4

120

100

80

60

40

20~~--~~------~----~~----~~----~------~------~

o~ ______ ~ ______ ~ ______ ~ ______ ~ ____ ~~ ____ ~~ ____ ~ o 5 10 15 20 "25 30 35

DRYING TIME - HOURS

(/)

U)

16

180--------~----~~------------------------------------

FIGURE 4A

EFFECT 0 F AIR VELOCITY AT 65% RELATIVE HUMIDITY

160~----~~----~~------~------~------~------~----~

140~------r-------~------~------~-------~------~----~

~120~------~------~------~--~--~~~--~~------~----~

>-0:: o

f­ffiIOOr-------~--------~ u 0:: LIJ Q.

A...~ ..

~ U) 801------+--0 r-----bL--+---':-. ----1--. U)

o ~ ...J ~

IAJ 0:: :)

~ 60~------H-~~--r----~--~------~------~------+--------~ U) -o ~

40~--~7J~--~--~------+-------+-------+-------+-----~

- - - MINIMUM LOSS CURVE FOR LIGHT SALTED FISH

20~~--r-~------~------+-------+-------+-------+-----~

O~ ____ ~~ ______ ~ ______ ~ ______ .. ______ ~ ______ ~ ____ ~ o 5 10 15 20 25 30 35

DRYING TIME - HOURS

t

,

17

180~----~~----~------~----------------------------~ FIGURE 48

EFFECT OF AIR VELOCITY AT 50% RELATIVE HUMIDITY

160~----~~----~-------+-------~------~------~-----~

140~----~------~--------+--~~~--~~-r~----~----~

tn 120 -en « III

)0-n: o 100

~ z w <.> a: IJJ

80 a.. I

en en 0 ...J

LLJ 60 a: :::> 1-CJ)

0 ~

40

10 15

- - - MINIMUM LOSS CURVE FOR LIGHT SALTED FISH

I

20 25 30

DRYING TI ME - HOURS

180

160

140

CJ) -CJ)

<t 120 m

>-0: C

t­ZIOO L&J o 0: L&J a.. I

~ 80 o ...J

L&J 0::' ::> t- 60 CJ) -o ::E

40

20

o o

/ I

~I

18

FIGURE 4C

EFFECT OF AIR VELOCITY Al 35% RELATIVE HUMIDITY

/" ~

;;::: v/ V

~ // V// " d~

// ,<~. /

,/

I 00 /

G ~ / 0 / , iii

/ if / If 7 / ~/

/ vi I

/ / ---MINIMUM LOSS CURVE FOR ·

LIGHT SALTED FISH

5 10 15 25 30 35

DRYING TIME - HOURS

19 . 106

FIGURE 5

104 AIR TEMPERATURE PROGRAMS FOR SEVERAL FISH TEMPERATURES

102

• 100

-98

96

94

92

l£. o 90

L\. 88 :2: w .... 0: 86 <C

84

82

80

\ \\ \\ ,\ \ 1\ \ \\ 1\\ \\ ,\ ' ~

\ \\ \ '\ _. \ "\.

\\ ~\ ~ ~ - -

\ ~\ ~ .....

~ ~ TEMP. 80°F.

\ ~ ~ --r----" ~~5~-~r-.

r---. r--

78 ,

76

74

72 1--

70 o 2 4 6 8 10 12 14· 16 . 18 20 22 24 26

TIME - HOURS

180

160

140

0 . (J)

~120

>­a:: c

f­ZIOO I.rJ (.)

a:: I.rJ Q.

. I en 80 (J)

o ...J

I.rJ a:: ::> f- 60 (J)

o ~

40

20

o o

/ I'

/

: I FIGURE 6

MOISTURE LOSS CURVES FISH TEMPERATURE 80°F.

// ~

/ ./

'.

/ /' V/

,/ /

/ // /'

~ ~/ ~. ~~7 0.0.

~. e:>.S7 ~ : ~7

~ 00 ~~/ II P 9 ".)...

G Y 0. ~ N.

l} ~y'

~ ~ ,

~ ~'1 ~

~~

II

/ /

/

5 10 15 20 25 30 35

DRYING TIME - HOURS