Pigott 1982 Aquacultural-Engineering 1

7/28/2019 Pigott 1982 Aquacultural-Engineering 1

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Aquacultural Engineering 1 (1982) 71-79

T H E R O L E O F A F O O D E N G I N E E R I N F I S H D I E T R E S E A R C HA N D F O R M U L A D E V E L O P M E N T

GEOR GE M. PIGOTTInstitute for Food Science and Technology, University of Washington, College of Fisheries,Seattle, Washington 98195, USA

ABSTRACTThis paper expands upon the role that food engineers play in the multi-discipline areao f aquaculture engineering. The author explains wh y it is necessary to apply basicengineering principles to those areas which have had only a minor emphasis in pastaquaculture research programs.

INT RODUCTIONF o r s o m e y e a r s w e h a v e b e e n h e a ri n g gl o w in g a c c o u n t s o f t h e t r e m e n d o u s f i s h f a r m i n gand a quacu l t u r e i ndus t ry t ha t w i ll be a m a jo r sou rce o f ' an im a l ' p ro t e in fo r the wor ld .Ove r t he pa s t 10 yea r s t he r e have been many s tud i e s by such g roups a s t he Techn i ca lA d v i s o r y C o m m i t t e e f o r F is h e ri e s R e s e a rc h (T A C , 1 9 7 4 ), t h e I n t e r n a t i o n a l D e v e l op -m e n t R e s e a r c h C e n t e r ( I D R C , 1 97 8) a n d t h e N a t i o n a l A c a d e m y o f S c ie n c es ( N A S ,1977) . I n gene ra l , s t ud i e s f o r t h i s pa s t 10 yea r pe r i od have d r awn the s ame conc lu -s ions : nam e ly , t ha t we need m ore knowled ge in r ep rod uc t i ve phys io logy , nu t r i t i ona lr equ i r em en t s , d is ea se and pa r a s i t e con t ro l , and r e l a t i onsh ip w i th env i ron m en ta lf ac to r s . The r e spon se t o t he se m an y s tud i e s w i th t he i r un ive r sa l conc lus ions ha s beento e s t ab l i sh r e sea rch cen t e r s an d r e sea r ch p rog ram s and t o ma ke m ore s tud i e s . Ye t ,r e la t iv e l y l it tl e p r o g re s s h a s b e e n m a d e t o w a r d c o m m e r c i a l m a s s p r o d u c t i o n o f f is h b ya q u a c u l t u re . W h y ?

Over t he pa s t two o r t h r ee decades , t he fo od p roces s ing i ndus t r i e s have unde rg onea m a j o r t r a n s i ti o n f r o m ' M a a n d P a ' o p e r a t i o n s t o l a rg e c o m p a n i e s p r o c es s in g m a s squan t i t i e s o f f o ods fo r an eve r - inc r ea s ing pop u l a t i on . Th i s re l a t i ve ly r ap id change int h e i n d u s tr i al c o m p l e x h as p r e s e n t e d m a n y p r o d u c t i o n p r o b l e m s , m o s t o f w h i c hinvo lve t he app l i ca t i on o f eng inee r ing p r i nc ip l e s i n t he f i na l p r ac t i c a l so lu t i ons . Wi th -

71Aquacultural Engineering 0144-8609/82/0001-0071/$02.75 O A pplied Science Publishers Ltd,England, 1982Printed in Grea t Britain


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72 G.M. PIGOTTou t co n t i nu in g t he se d r a s t i c t e chno log i ca l changes in ou r f oo d indus t ry , t he per cap itas u p p l ie s o f th e r ig h t k i n d s o f f o o d s f o r a n i m a l a n d h u m a n c o n s u m p t i o n w i ll c o n t i n u eto d im in i sh a s t he po pu l a t i on i nc r ea se s a t a g r ea t e r r a t e t han t he p ro duc t i on o f f ood .Th e m anu fac tu r e o f f i sh feed fo r aquacu l t u r e f a ll s i n t he same ca t ego ry as t he gene ra lf o o d i n d u s t r y . S a n i t a r y s t a n d a r d s a n d q u a l i t y c o n t r o l p r o c e d u r e s m u s t b e r i g o r o u s i no rde r t o i n su re hea l t hy , who le som e f i sh a s f oo d fo r hum ans . I n add i t i on f ish m us t bep rod uce d a t a p r ic e t h a t w i ll be mu tua l l y bene f i c i a l t o t he f i sh f a rm er and t he consu m er .

A s f o o d e n g in e e rs a n d t e c h n o l o g is t s w e m u s t c o n t i n u a l l y e n la r ge o u r k n o w l e d g e o ft he mu l t i - d i s c ip l i ne a r ea s neces sa ry fo r f il li ng the r equ i r em en t s f o r fi sh f eed manufac -t u r e . T h i s i nc ludes p r oc u re m en t o f r aw ma te r i a l s , hand l i ng , p roces s ing , packag ing ,t r ans po r t a t i on and m arke t i n g o f f i n i shed f eeds . The ab i l i t y t o app ly ba s ic eng inee r ingp r inc ip l e s i n t he se a r ea s i s mos t impor t an t t o t he hea l t hy g rowth o f aquacu l t u r e , bu tu n f o r t u n a t e l y h a s b e e n g iv e n m i n o r e m p h a s i s in p a s t a q u a c u l tu r e p r o g r a m s .I t is m y f i rm be l i e f t ha t p a r t o f the p ro b l em wi ll be re so lved by t h i s new jou rna ldea l ing w i th t he t e chno log i ca l a spec t s o f aquacu l t u r e and emph as i z ing t he ro le o feng inee r s, t e chno log i s t s and o the r s s choo l ed i n the app l i ed phys i ca l r a t he r t han t hebio logica l sc iences . This wi l l c rea te in te res t in the technology tha t wi l l be a majorcon t r i bu t i o n t o ha s t en ing t he p r ac t i c a l r e a l i t y o f com me rc i a l l y r ear i ng ed ib l e f ish andshe ll fi sh . E i t he r d i r ec t l y o r i nd i r ec t l y a ll o f t il e f a c to r s r e co m m end ed by t he p r ev ious lym e n t i o n e d s u r v e y s f o r c o n c e n t r a t e d e m p h a s i s in f u t u r e r e se a rc h a n d d e v e l o p m e n t a reheav i l y dependen t on f ac i l i t i e s de s ign , mechan i ca l ope ra t i ons , env i ronmen ta l con t ro l ,p r o c e s s i n g a n d h a n d l i n g o f f i s h f o r m a r k e t , a n d f e e d d e v e l o p m e n t a n d m a n u f a c t u r e . Ishou ld l i ke t o conc en t r a t e on t he l a t t e r i tem a s an examp le o f wh y Fo od Eng inee r s ar eso imp or t a n t t o t he r is e o r dem i se o f aquac u l t u r e , a l t hough a s t r ong case can be bu i l tf o r t e chno logy i n each a r ea .

The t yp i ca l co nce p t o f the aquacu l t u r i s t t ow ard f eed i s t ha t ing red i en t s a r e m ixed ,t hen hea t ed , d r i ed o r i n some manne r p roces sed p r i o r t o pe l l e t i ng and packag ing(Fig . 1) . In rea l i ty , the growth and hea l th of the f i sh as wel l as the economics of thep r o c e ss a r e d i r e c t ly d e p e n d e n t o n t h e p r o p e r t i e s o f t h e c o m p o n e n t i n g r e d ie n t s t h a t goin to t he f eed and t he subse quen t de t a i l ed p roces s ing te chn iques . Fo r exam ple , a sk ar e sea r che r do ing f eed s t ud ie s on d i e t s u t il iz i ng f ish me a l abou t t he sou rce o f t he mea l .The u sua l answ er i s t ha t ' i t is com m erc i a l f i sh me a l ' . The sou rce and p roces s ing ( e spec i-a l ly d ry ing ) t e chn iques a r e u sua l l y no t kno wn , n o r even ava i l ab l e t o t he pu rchase r .Ho wev e r , t he r e su l ts o f a t e s t i nvo lving g row th f ro m such a d i e t a r e im m ed ia t e lysuspe c t i f ex tens iv e chem ica l and b io logica l t es t ing is no t car r ied ou t s ince the ava i lab leo r d i g e st ib l e p r o t e i n a n d v a l u es o f o t h e r n u t r i e n t c o m p o n e n t s v a r y t r e m e n d o u s l yd e p e n d i n g o n t h e p r o c e ss i n g t im e a n d t e m p e r a t u r e a n d t h e t y p e o f d ry i n g t e ch n i q u eto wh ich t he d ry ing f i sh was sub j ec t ed .The proces s eng ineer i n aquacu l tu re

An eng inee r who i s go ing t o be ac t i ve i n t he de s ign , cons t ruc t i on and ope ra t i on o ff eed manufac tu r i ng f ac i l i t i e s shou ld be educa t ed i n t he food f i e l d a f t e r he ha s ga ined a


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FOOD ENGINEERS IN AQUACULTURAL ENGINEERING 73

143 Ib Whole F ish(Hake)D E B O N E R - + S K I N , B O N E S70% Reco very (43 Ib)

I100 Ib

H205 0 I b --~ H O M O G E N I Z E RI

150 Ib

HCI (1 L.) E N Z Y M E 4 hPepsin N.F. -)" R E A C T O R 4 8 C(0 .15 Ib , 1 :10000)

NaOH [ j -~ SLU DG E (141b ,( 10 g a l . 1 N ) C E N T R I F U G E L I P I D ( 16 I b)I

120 Ib

[ A N I O N 1 5 g a ' - + I A N ' O N E X C H A N G E w e tedC O N V E R S I O N re s in + - R E A C T O RI

NaC I, H20 118 Ib$

CO NC EN TR AT OR --> H2Ot (84 I b)I34 Ib

S P R A Y D R I E R j - + H ~ O f ( 23 1 b)

F I N A L P R O D U C T(11 tb)F~ . ] . ] ) [ lo t p lan t m a te r ia l balance and f low cha r t (bas is : ] 00 ] b deboned f i sh ( ] ] b = 0 .454 kg ) ) .

(Pigott and Bucove, 1978.)


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74 G.M. PIGOTTgood bas i c backg round in eng inee r ing . Th i s no rma l ly r equ i r e s s t udy beyond a fou ryea r deg ree p rog ram . On the o the r hand , t he f ishe r ie s b io log i s t and t he aquacu l tu r ema nage r shou ld have an exposu re t o t he app l i ca t i ons o f eng inee r ing p rinc ip l e s neces-sa ry t o manufac tu r e h igh qua l i t y f eed and t he subsequen t hand l ing , s t o r age and u sec o n d i t i o n s r e q u i re d t o m a i n t a i n t h a t q u a li ty . I n s u m m a r y , t h e t r e m e n d o u s m a r c hf o r w a r d t h a t m u s t b e m a d e i n a q u a c u lt u r e f e e d t e c h n o l o g y an d m a n u f a c t u r e o v e r t henex t decade mus t be made by b io log i s t s , aquacu l tu r i s t s and t e chno log i s t s work ingh a n d - i n - h a n d a n d n o t i n c o m p e t i t i o n . W e s h o u l d n o t a t t e m p t t o m a k e a n e n g i n e e rou t o f a b io log i s t no r a b io log i s t ou t o f an eng inee r , bu t we shou ld g ive each ana p p r e c i a t io n o f t h e o t h e r ' s f ie ld s o t h a t c o m m u n i c a t i o n b e t w e e n t h e t w o a r e a s is m o r ee f f ec t i ve .

Wherea s no rma l eng inee r ing app l i ca t i ons dea l w i th chemica l and phys i ca l a spec t s ,t he de s ign , cons t ruc t i on an d ope ra t i on o f com m erc i a l f e ed m anu fac tu r in g f ac il it ie smus t cons ide r t he b io log i ca l f a c to r s . F l avo r s , odo r s , co lo r , t ex tu r e , body , nu t r i t i ona lcomponen t s , e t c . a r e no t on ly r e l a t ed t o t he phys i ca l and chemica l changes a f f ec t i ngf ish feed ing bu t t o t he i n f l uence o f mic ro o rgan i sm s and the i r me tab o l i c p rodu c t s .

Wha t are engineer ing pr inc ip les?In genera l , engineer ing i s the appl ied aspec t of bas ic sc ience . Appl ied research i s a

con t inu a l e f fo r t o f t r y ing un t i l a s c i en t if i c a ll y sound and dup l i cab l e so lu t i on is f oun dfo r a p r ac t i c a l p rob l e m. The ba s ic l aws o f phys i c s , chem is t ry and b io logy as exp re s sedi n t h e l a n g ua g e o f m a t h e m a t i c s s h o w u s t he r e la t io n s b e t w e e n m a n y o f t h e e l e m e n t sc o m p o s i n g a ' s y s t e m ' w h e t h e r i t b e a s m a ll p i ec e o f m a c h i n e r y o r t h e p la n e t o n w h i c hwe live. H owe ve r , due t o m an y sh o r t com ings o f m an ' s know ledge o f t he se r e l a ti on -sh ip s , p r ec is e r e l a t ionsh ips a re no t de t e rm inab l e w i thou t t he u se o f expe r im en ta l l ydes igned cons t an t s and s econda ry r e l a t i onsh ips t ha t ba l ance t he equa t i ons and makethem use fu l f o r app l i ca t i on t o p r ac t i c a l s i t ua t i ons . Hence , when we r e f e r t o t he app l i -ca t i on o f ba si c eng inee r ing p rinc ip l e s, we mea n t he u se o f expe r ime n ta l l y de t e rm inedin fo rma t ion combined wi th t he ba s i c s c i en t i f i c l aws i n such a manne r t ha t we candes ign a succes s fu l p l an t , de s ign and bu i ld m ach ine ry fo r e c ono mica l l y hand l ing ,t r anspo r t i ng , p roces s ing and packag ing f eed and f eed i ng red ien t s . F ina l l y and pe rha psm o s t i m p o r t a n t o f a ll, w e m u s t c o m p i l e re c o r d s a n d i n f o r m a t i o n o n p r o d u c t i o npe r fo rmance so t ha t p roces s ing f ac i l i t i e s and t e chn iques can be improved o r newlyd e v e l o p e d .

The p ro b l em s o f u t il i zi ng t he eng inee r ' s t oo l s i n t he f eed i ndu s t ry a r e a ll t oo t r ad i -t i ona l. W e need an d mu s t r e ly upo n bas ic eng inee r ing da t a fo r t he s t udy o f p roces s ingt echn iques and t he com m erc i a l i z a t i on o f t he se t e chn iques . Th i s i nc ludes :

1 . The changes i n we igh t and compos i t i on t ha t occu r i n f eeds and componen t sdu r ing p roces s ing .

2 . Th e pack dens i t ie s and spec i fi c g r av it ie s and com pres s ib il i ti e s tha t a r e b rou gh tabou t i n l oad ing b in s , st o r age t anks and va t s .


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F O O D E N G I N E E R S I N A Q U A C U L T U R A L E N G I N E E R I N G 7 5

3. The safe loading depths for fresh or raw products during transportation andstorage.

4. The flow properties for various feeds and components in different forms andprocess states.

5. The properties of viscous materials such as purees, pulps, slurries, etc.6. The specific heat, heat capacity and heat conductivity of feed in all states of

preservation. This is particularly important where simultaneous mass and heattransfer occur.

7. The effects of particle size on various processes. This has a bearing on many pro-cesses, such as spray drying, grinding, filtering slurries and miscellanea anddisintegration operations.

These often unknown items are related to many important parameters in a feed suchas solubility, bindability, nutrient stability, and tendency to float or sink.

In the past, processing and handling machinery and equipment have all too oftenbeen based on history and tradition rather than efficient functioning, easy maintenanceand sanitary requirements. Sadly enough, major technical developments in the industry,as with many other industries, are often guided or determined by political rather thanby sound scientific and economic factors.

BASIC PROCESSINGTECHNIQUESThe quality of all components in fish feed must be known in order to formulate a dietmeeting certain specifications and then the effect of the final feed mill process must beconsidered. Processing involves a wide variety of operations. However, the steps inpreparing a feed can be separated into two categories. These are:

1. Preservation to extend shelf life and insure safety of feed by reducing, inactivatingor eliminating spoilage and disease organisms.

2. Mechanical handling, including formulation, mixing, particle size control, andpackaging or holding in containers to protect feed from damage and spoilageduring shipping and storage.

Prese rva t ion me thodsThere are four basic methods being used to preserve feeds. Most processes involve a

combination of two or more of the techniques which involve addition of chemicals,adding or removing heat, and removing water.

A d d i n g c h e m i c a l s . Chemical additives are used in feed primarily to stabilize thelipid portion of the diet. However, other effects of chemicals used in processing areoften more important than realized. For example, consider a product such as a hydro-lysate extracted at a low pH and then neutralized prior to drying (Fig. 1). The


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76 G.M. PIGOTT

TABLE 1Fish protein hydrolysate: am ino acid profile versus pHtreatment (Gabaudan et al., 1980)Am ino ac id Am ino ac id con ten t (g 100 g pr o te in ~j

p H 7 .0 p H 3 . 8Arg 6-0 5.7Iso 3-7 5.3Leu 6.1 7.2Lys 7.3 7-8M et 1.9 2.3Phe 2-8 3.5Th r 3.9 4.7Val 3.6 4.4

no rma l ly ava i l ab l e p roduc t , i n t h i s c a se F i sh P ro t e in Hydro lysa t e (FPH) , i s neu t r a l a ta pH o f 7 -0 whe rea s t he supe r io r amino ac id r e t en t i on (Tab l e 1 ) i s i n t he p roduc t a tt h e p r o c e ss i n g c o n d i t i o n o f p H 3 . 8 . W e h a v e d e m o n s t r a t e d t h a t t h e n o n - n e u t ra l i ze d ,a n d m u c h c h e a p e r , p r o d u c t i s e q u a l o r s u p e r i o r t o t h e s t a n d a r d m a t e r i a l w h e n u s e d a sa b inde r and p ro t e in sou rce i n a l a rva l f e ed be ing deve lo ped a t the U n ive r s i ty o fW a s h i n g t o n ( G a b a u d a n e t a l . , 1980) .

A d d i n g h e a t . P r o b a b l y t h e p r e s e r v a t io n p r o c e s s t h a t h a s t h e m a j o r e f f e c t o n t h enu t r i t i ona l va lue o f f e ed i ng red i en t s and f i na l p r epa red f eed i s t he add i t i on o f hea t .Hea t i ng is u sed t o p r e se rve foo d by (1 ) pa s t e u r i za t i on t o r educe o rgan i sms , t husex t en d ing she l f l if e and m in imiz ing pub l i c h ea l t h r isk s and (2 ) s t e r i l i z a ti on t o k i l l a llo rgan i sms , t hus a l l owing l ong - t e rm s to r age o f a p rope r ly packaged f eed . The cook inge f f e c t o f h e a t i n g c h a n g e s t h e t e x t u r e , t a s te a n d a c c e p t a b i l i ty o f s o m e p r o d u c t s . A l s o ,a s w i l l be d i s cussed l a t e r , mo s t co nven t i ona l d ry ing o pe ra t i on s i nvo lve hea t i ng t oinc r ea se t he r a t e o f d ry ing .

H e a t n o t o n l y a l te r s t h e n u t r i ti o n a l v a lu e o f c o m p o n e n t m a t e r i a l s b u t a l so c a u se sp h y s i c a l a n d c h e m i c a l c h a n g e s t h a t a f f e c t t h e e c o n o m i c s o f g ro w i n g f is h . T h e e x t e n to f p r o t e i n d e n a t u r a t i o n b y h e a t c h a n g es t h e w a t e r s o l u b i li t y a n d b i n d i n g c h a r ac t e ri s -t ic s , t hus a f f ec t i ng t he wa t e r so lub i l i ty and s t ab i l i ty o f t he f eed pa r t ic l e s . He a ti nc r ea se s the ox id a t i on r a t e f o r f a t s , de s t roys ma ny o f t he hea t l ab i le v i t amins andp r o m o t e s t h e M a il la r d b r o w n i n g r e a c t io n .

R e m o v i n g h e a t (r e fr i ge r a ti o n ). R e m o v i n g h e a t f r o m a p r o d u c t r e d u c e s t h e g r o w t ho f m i c r o o r g a n i s m s a n d t h e r e a c t i o n r a t e s o f e n z y m e a c t i o n . A f t e r s u ff i ci e n t h e a t isr e m o v e d t o f r e e z e a f e e d , t h e m i c r o o r g a n i s m s b e c o m e d o r m a n t b u t t h e e n z y m e scon t i nue t o be ac t i ve . Thu s , deg rada t i on o f nu t r i en t i ng red i en t s con t i nu es wh i l e a r awm ate r i a l o r a p r ep a red f eed i s be ing he ld i n the f ro zen s t a t e . I t is i n t e r e s t i ng t o n o t etha t t he on ly ma jo r nu t r i en t damage t o a f e ed du r ing f r eez ing i s a i r ox ida t i on i f t he


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FOOD ENGINEERS IN AQUACULTURAL ENGIN EERING 77produc t i s exposed t o a i r . Howeve r , a cons ide r ab l e deg rada t i on t akes p l ace du r ingf rozen s t o r age w hen bo th chem ica l and phys i ca l deg rada t i on is c aused by t he chang ingre l a t i onsh ip be tw een f r ee wa t e r and enz ym es . The s t o r i ng o f f rozen f eeds is expens ive ,no t on ly i n t he cos t o f ma in t a in ing and ope ra t i ng f r eez ing and co ld s t o r age f ac i li t ie sbu t a l so i n t he l o s s o f nu t r i t i ona l va lue .

R e m o v i n g w a t e r ( d e h y d r a t i o n o r d r y in g ) . Dry ing i s a wo rd t ha t i s un ive r sa l ly unde r -s t ood fo r t he r emo va l o f wa t e r . I n i t s b roades t s ense , it r e f e r s t o t he r emova l o f al iqu id f rom a so li d by evap o ra t i on . T he r equ i r ed hea t o f evap o ra t i on is p rov ided t ot h e m a t e r ia l b e i n g d r ie d b y c o n d u c t i o n f r o m a s o li d s u rf a c e , b y c o n v e c t i o n f r o m ag as , ra d i a t io n f r o m a n o t h e r s u r fa c e , o r s o m e c o m b i n a t i o n o f th e s e tr a n s f e r m e c h a n i s m s .Feeds a r e d r i ed o r dehydra t ed fo r ( 1 ) p r e se rva t i on t o i nc r ea se s t o r age l i f e , ( 2 ) r educ -t i on o f t he cos t and t he p rob l e m s a s soc i a ted w i th packag ing , hand l i ng , s t o r i ng ands h i p m e n t a n d ( 3 ) p r e p a r a t i o n o f p r o d u c t s h a v i n g c e r ta i n d e s ir e d c h e m i c a l a n d p h y s i c a lp r o p e r t i e s i m p o r t a n t f o r m a x i m u m f e e d e f f i c ie n c y u n d e r c e r t a in e n v i r o n m e n t a lcond i t i ons o r f o r spec i f i c r equ i r emen t s o f t he spec i e s be ing f ed .

T w o f u n d a m e n t a l a n d s i m u l t a n e o u s p r o c e s s e s o c c u r d u r i n g d r y i n g : ( 1 ) h e a t i st r ans f e r r ed t o ev apo ra t e l i qu id and (2 ) m ass is tr ans f e r r ed a s a li qu id o r vapo r w i th inthe so l id and a s a vap o r f r om the su r f ace . The f a c to r s gove rn ing t he ra t e s o f t he sep r o c e ss e s t ak i n g p l a ce d e t e r m i n e t h e d r y i ng r a t e . T e m p e r a t u r e , h u m i d i t y a n d v e l o c i t yo f a i r, su rf ace a r ea , p r e s su re o r vacu um , and d i r ec t i on o f a ir mov em en t a r e t he ex t e rna lc o n d i t i o n s w h i c h d e t e r m i n e t h e r a t e o f e v a p o r a t i o n . D i f f u s i o n i n c o n t i n u o u s h o m o -geneou s so l id s , c ap i l la ry f l ow in g r anu l a r so l i ds , g r av i t y sh ri nkage and p r e s su re g r ad i en tde t e rmine t he i n t e rna l l i qu id f l ow .

The i n t e r r e l a t i onsh ips o f t he con t ro l l i ng f ac to r s in d ry ing o f a so li d p r ec lude on es m o o t h c o n t i n u o u s d r y i n g r a te . H o w e v e r , t h e re a re t w o d i s ti n c t ph a s e s o f d r y in g t h a thave a ma jo r bea r i ng on t he qu a l i t y o f t he f i nal p ro du c t and on the cos t e f f i c i ency o fd ry ing . A we t so l id lo se s m o i s tu r e f i rs t by evapo ra t i on f rom the s a tu r a t ed su r f ace .Dur ing t h i s pe r i od t he e vapo ra t i on p rocee ds i n a mann e r s imi la r t o tha t o f a l i qu idw h e r e b y t h e e v a p o r a t i o n i s c o n t r o ll e d b y t h e a m o u n t o f h e a t i n p u t . T h is , th e s o-ca l led co ns t an t r a t e pe r i od , i s e s sen t i a ll y i nde pend en t o f the na tu r e o f t he so li d and iscon t ro l l ed by t he r a t e o f hea t t r ans f e r t o t he evap o ra t i ng su rf ace .

A t a po in t du r ing t he d ry ing p roces s a c r i ti c a l mo i s tu r e con t en t is r e ached a t wh ichp o i n t d i f fu s i o n o f w a t e r t o t h e s u r fa c e is n o t r a p id e n o u g h t o m a i n t a i n t h e c o n s t a n tevap o ra t i on r a t e . A t t h is c r i t ic a l mo i s tu r e con t en t wh ich d i f f e r s f o r e ach ma te r i a l , t hed r y i n g is n o l o n g e r c o n s t a n t a n d t h e r a te o f d r y i n g b e c o m e s d e p e n d e n t o n t h e r a t e o fwa t e r d i f fu s ion t o t he su r f ace . Th i s mass t r ans f e r con t ro l l ed po r t i on o f t he d ry ingcyc l e i s c a l l ed t he f a l l i ng r a t e pe r i od . The e f f ec t s and changes o f d i f fu s ion , c ap i l l a ryand p r e s su re g r ad i en t s c aused by sh r inkage and t he na tu r e o f t he ba se s t a r t ing ma te r i a la r e mo s t im po r t an t du r ing t he f aUing - ra t e pe r i od . F ina l nu t r i t ive va lue o f a d r i ed f eedo r f e e d c o m p o n e n t ( i. e. f is h m e a l ) is q u i te d e p e n d e n t o n t h e k n o w l e d g e a b l e c o n t r o l o ft he se d ry ing va r i ab le s .


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78 G.M. PIGOTT

Mechanical handlingWh ereas t he p r e se rva t i on m e tho ds d i r ec t l y a f f ec t t he phys i ca l and chemica l p rope r -

t i e s a s we l l o f t he nu t r i t i ve va lue , t he mechan i ca l a spec t s o f hand l i ng p r imar i l y a f f ec tt he phys i ca l p rope r t i e s t ha t r e l a t e t o p r e sen t a t i on o f a f e ed t o t he f i sh . Mechan i ca lope ra t i ons i nc lude :

1 . Mix ing i ng red i en t s t o ensu re a un i fo rm d i e t.2 . Gr ind ing , homog en i z ing o r o the rwi se r educ ing pa r t i c l e s iz e.3 . Screening to se lec t des i red par t ic le s ize ranges .4 . Ex t rud ing , pe l l e t ing o r o the rwi se fo rm ing pe l l e t s o f a de s i red pa r ti c l e siz e and

dens i t y .5 . Con vey ing i ng red i en t s o r f i n i shed d i e t by mech an i ca l o r pne um a t i c me ans .6 . Weighing and packaging f in i shed products .

T h e t w o p r i m a r y p r o b l e m s in t h e se a n d o t h e r m e c h a n i c a l h a n d li n g a n d p r o ce s s in gt echn iques a re pa r t i c le a t t r i t i on an d hea t i ng due t o f r i c t ion . A t t r i t i on , o f cou r se ,r e su lt s i n pa r t ic l e s iz e changes t ha t r educe t he e f f i c i ency o f a d i e t f o rmu la t ed fo r aspec i f i c s i z e f i sh . Hea t i ng o f t he p roduc t s occu r s du r ing g r i nd ing and pe l l e t i ng ope ra -t i ons. The u l t ima t e adve r se e f f ec t o f ove rhea t i ng i s t o r educe t he d i e t e f f i c i encyth rough de s t ruc t i on o f c e r t a i n nu t r i t i ve va lue s . Good eng inee r ing de s ign and s e l ec t i ono f f ac i l i t i e s and ca r e fu l p l an t ope ra t i on can subs t an t i a l l y r educe p rob l ems a s soc i a t edwi th a l l phase s o f f e ed p roduc t i on .

FUTURE APPLICATIONS

Aq uacu l t u r a l eng inee r ing ha s no t been p r ec i s e ly de f i ned and w il l p ro bab ly evo lve ove rm an y yea r s a s a comp os i t e o f m an y eng inee r ing and s c i en t i f ic d i s cip li ne s. I t i s dou b t fu lt ha t any one i nd iv idua l w i ll a ccum ula t e t he va s t back g roun d neces sa ry to hand l e a l leng inee r ing t a sks i nvo lved i n a comple t e aquacu l t u r e ope ra t i on . A pa r a l l e l example o fspec i al iz ed eng inee r ing a r ea i nvo lves the Agr i cu l t u r a l E ng inee r w ho i s p r im ar i l y i n t hemechan i ca l a r ea i nvo lv ing f a rm mach ine ry de s ign , deve lopmen t , and ope ra t i on . TheAquacu l t u r a l Eng inee r may we l l a l so be o r i en t ed t o mechan i ca l eng inee r ing whe rebyhe wou ld be ac t i ve i n f i sh hand l i ng , f e ed ing , ha rves t i ng and env i ronmen ta l con t ro lmachinery , equipment and fac i l i t i es . As in the case of agr icu l tura l engineers , the c iv i leng inee r ing a spec t s o f de sign and cons t ruc t i on and t he p roces s ing eng inee r ing a r eai n v ol v in g f is h p r o c e ss i n g a n d f e e d d e v e l o p m e n t a n d m a n u f a c t u r e w o u l d b e r e q u i r e d tocom ple t e t he eng inee r ing com pl im en t . How eve r , since a l l o f t he se a r ea s a r e r e l a t ed tothe a pp l i ca t i on o f eng inee r ing t o aquacu l t u r e , l i ke t he Agr i cu l t u r a l Eng inee r ing pub li -ca t i on , t he Aqu acu l t u r a l E ng inee r ing j ou rn a l shou ld p rov ide a sound ing boa rd fo r al leng inee r ing app l i ca t i ons t o t he f i e l d o f aquacu l t u r e . Hence t h i s new jou rna l i s aw e l c o m e a d d i t io n t o m a n y e n g in e e rs , l ik e m y s e l f , w h o h a v e h a d t o p u b l i sh s o m e o fthe i r eng inee r ing pape r s i n mo re ba s i cal l y s c ien t if i c j ou rna l s t ha t a r e no t r e a l ly o r i en t edtoward t he se app l i ed a r ea s .


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FOOD ENGINEERS IN AQUACULTURAL ENGINEERING 79The first million years in which the earth was inhabit ed by man has ended with a

popu lati on of over four billion people and a large majority of the prime agriculturalareas have been exploited by this population. However, this population figure willmost likely continue to increase to the extent that we could find ourselves facing amost bleak future indeed. Analysis of the shortages of food that will become disastrous,depletion of conve ntion al sources of power and raw materials, and an expanding popu-lation to feed, clothe a nd house, leads one to conclude that the future of mankin d onour earth is quite dep ende nt on the applied application of engineering principles forthe design, construction and operations related to both economics and the effect onthe e nvir onme nt. Aqua cultura l engineers can be most effective in helping us to realizethe tr eme ndou s poten tial of raising desperately needed high-quality protei n food onsuch a large scale as to have a major impact on the survival of man kin d.

REFERENCESGabaudan, J., Pigott, G. M. & Halver, J. E. (1980). The effect of processing on protein ingredientsfor larval diets. Proceedings o f W orm Maricul ture Soc ie ty Ann ua l Mee t ing , New Orleans, La.,USA, 5-9 March.IDRC (1978). Fi sh Farmi ng , ed. B. Stanley, W. H. Alsop & F. Brian Davy, International Develop-ment Research Center, Ottawa, Canada, IDRC-120E.NAS (1977). World food and nut rit ion study - the potential contributions of research, preparedby the Steering Committee on World Food and Nutrition of the Commission on InternationalRelations, National Reseaxch Council, National Academy of Sciences, Washington, DC.Pigott, G. M. & Bucove, G. (1978). Engineering a plant for enzymatic production of supplementalfish proteins. J. Fd. Proc. and Pres. , 2, 33-54.TAC (1974). Consultative Group on International Agricultural Research, Technical Advising Com-mittee, Aquaculture Research Programme: Report of the TAC Subcommittee on Aquacultm'e,D D D R / I A R / 7 4 / 2 5 Restricted.

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Pigott 1982 Aquacultural-Engineering 1