Starch source, screw configuration and injection of

steam into the barrel affect the physical quality of

extruded fish feed

Mette S�rensen1,2,3, Giao Nguyen3,4,Trond Storebakken1,3 & Margareth �verland1,3

1Aquaculture Protein Centre (APC), CoE, —s, Norway2No¢ma Marin, —s, Norway3Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences, —s, Norway4Center for Fisheries Post-Harvest Technology, Research Institute forAquaculture, Ho Chi Minh City,Vietnam

Correspondence:M S�rensen, Aquaculture Protein Centre (APC), CoE, —s, Norway. E-mail: mette.sorensen@no¢ma.no

Abstract

This experiment investigated the physical qualities(‘Holmen’ durability; ‘tumbling box’ durability; andpellet strength, length and diameter) of extruded¢shmeal-based feeds produced using di¡erentstarch sources, screw con¢gurations, with or with-out steam injected in the barrel. The starch sourcestested were native or pre-gelatinized potato starch(PGS), wheat starch (WS), whole wheat (WW)or a combination of WS and WW. Screw con¢gura-tions were a polygon element, followed by twoleft-pitched segments, Polygon____2L; a polygon, fol-lowed by one left-pitched and one right-pitchedelement, Polygon____LR; and no polygon section, alter-nating left- and right-pitched elements, LRLR. Starchsource and screw con¢guration a¡ected the physicalquality themost. Pellets producedwith LRLR had thehighest ‘Holmen’ durability (76%), hardness (30N)and the least diameter (5.3mm). The lowest durabil-ity (37%) and hardness (22N) were obtained withPolygon____LR. Potato starch yielded higher expansionthan cereal starch. The highest ‘Holmen’ durabilitywas seen for PGS (79%), while WS yielded thelowest value (44%). Injection of steam reducedhardness and ‘Holmen’ durability, but provided aminor overall contribution to physical quality com-pared with the starch source and screw con¢gura-tion. ‘Holmen’ durability appeared to be suitable tounveil variation (37^79%) in physical quality, while‘Tumbling box’durability (98.4^98.8%) did not provee⁄cient.

Keywords: extrusion, extrusion screw con¢gura-tion, ¢sh-feed, physical quality, Holmen durability,Tumbling box, starch source, steam injection

Introduction

Fish feeds used in intensive aquaculture productionshould be resistant to mechanical stress duringtransport, handling and in pneumatic feeding de-vices (Aarseth 2004; Aarseth, Perez, B�e & Jeksrud2006), but should still have a texture and sizethat can facilitate high feed intake (Hardy 1989) aswell as e⁄cient digestion by the ¢sh (Lovell 1989;Baeverfjord, Refstie, Krogedal & —sg�rd 2006).Waterstability as well bulk density in order to control sink-ing velocity and buoyancy control are also importantpellet quality characteristics that vary according tothe cultured ¢sh species, and the salinity and tem-perature of the water. Extrusion processing of ¢shfeed has been shown to meet several of these criteria.During extrusion, a combination of high tempera-ture (120^130 1C), high pressure (20^30 bar) andshear forces is used to transform ingredients into adough before the pellets are shaped in a die. A highmoisture content (25^30%) in combination with ashort residence time in the extruder (0.5^2min) en-sures that the process is not detrimental to the nutri-tional value of the feed (S�rensen, Lj�kjel,Storebakken, Shearer & Skrede 2002; Lj�kjel, S�ren-sen, Storebakken & Skrede 2004; S�rensen, Store-bakken & Shearer 2005).

Aquaculture Research, 2010, 41, 419^432 doi:10.1111/j.1365-2109.2009.02346.x

r 2010 TheAuthorsJournal Compilationr 2010 Blackwell Publishing Ltd 419

Physical quality is a¡ected by several variables, in-cluding the diet formulation (Refstie, Glencross,Landsverk, S�rensen, Lilleeng, Hawkins & Krogdahl2006; �verland, Romarheim, Ahlstr�m, Storebakken& Skrede 2007), extruder con¢guration (S�rensen,Stjepanovic, Romarheim, Krekling & Storebakken2009) and processing parameters (Aarseth, S�rensen& Storebakken 2006). Energy input into the materialduring extrusion is directly associated with cookingof the material and transformation of ingredientsfrom particulate solids into a dough. The two mainsources of energy in an extrusion system are of me-chanical or thermal origin. Input of mechanical en-ergy, reported as speci¢c mechanical energy (SME),is generated by friction caused by the dough in theextruder barrel as it is moved forward by the rotatingscrew. Dissipation of energy from the electrical motoris turned into heat, resulting in increased tempera-ture during the process. For most expanded products,thermal energy is added as steam in the pre-condi-tioner, where the mash is normally heated to 80^90 1C. Thermal energy is thus contributing to ap-proximately 2/3 and mechanical energy 1/3 of theenergy needed to obtain an extrusion temperatureof approximately 130 1C. The physical quality of ex-truded pellets is a¡ected by the extruder temperature(Aarseth et al. 2006). Elevated temperature during ex-trusion reduces the viscosityand increases the steampressure of the melt, which in turn directly in£u-ences expansion of the product. No previous studieshave investigated the quality of feed pellets whensteam is added directly to the barrel late in the extru-sion process.Screw con¢guration a¡ects transformation of the

feed dough inside the extruder barrel (Barres,Vergnes,Tayeb & DellaValle 1990), due to changes inresidence time (Olkku, Antilla & Linko 1980), degreeof ¢lling, energy input to the material (Erdemir,Edwards & McCarthy 1992; Yam, Gogoi, Karwe &Wang 1994) and shear rate (Gogoi, Choudhury &Oswalt 1996). Screw con¢guration can thus be usedto manipulate expansion and bulk density towardstargeted values in the pellets (Gogoi et al.1996).Starch is added to feed for carnivorous ¢sh primar-

ily used as a digestible binder and to facilitate expan-sion of the feed. However, the restricted capacity ofsalmonides to digest and metabolize starch (Hemre,Mommsen &Krogdahl 2002) limits the practical levelof inclusion of starch in the ¢nal feed to approxi-mately10%. Hence, starch sources with good bindingand expansion properties are needed. The functionalproperties di¡er among starches due to variations in

the amylose^amylopectin ratio, size and shape of thegranule and other chemical components associatedwith the granule. A high amylose content increasesthe visco-elastic behaviour and £ow properties ofmelted starch (Chinnaswamy 1993; Kokini 1993).Wheat is extensively used by the feed industry dueto its functional and nutritional properties. Not onlythe composition of the starch, but also other consti-tuents, such as the gluten proteins, may contributeto this (Goesaert, Brijs, Veraverbeke, Courtin, Geb-ruers & Delcour 2005). To our knowledge, no studieshave been published investigating di¡erences be-tween pure wheat starch (WS) and wheat in ex-truded ¢sh feed. Potato starch (PS) may also have apotential in ¢sh feed, because it has a higher swellingpower and viscosity pro¢le thanWS (Gunaratne, Ra-naweera & Corke 2007). The gelatinization tempera-ture is similar for the two starches (Gunaratne et al.2007). The aim of this study was to investigate the ef-fect of various starch sources, screw con¢gurationsand in-barrel steam pressure on the physical qualityof extruded ¢sh feed.

Materials and methods

Production of extruded feeds

Five feeds based on ¢shmeal in combinationwith oneof the following starch sources: PS, pre-gelatinizedpotato starch (PGS),WS, wholewheat (WW) ora com-bination of wheat starch and whole wheat (WW_WS)were formulated (Table 1). All feeds were formulatedto be iso-nitrogeneous, iso-energetic and to have astarch content of 120 g kg�1 extruded feed. The ex-periment was designed according to a 3 � 5 � 2 fac-torial model, with three screw con¢gurations, ¢vecarbohydrate sources (PS, PGS,WS,WWorWW_WS)and either without or with steam injection into thebarrel. The three screw con¢gurations are shown inFig. 1. The screw consisted of ¢ve sections, and thethree di¡erent screws had the same con¢guration,except for the kneading area. One screw con¢gura-tion had a polygon element, followed by two left-pitched segments (Polygon____2L). The second screwcon¢guration had a polygon element, followed byone left-pitched and one right-pitched element (Poly-gon____LR). The third screw con¢guration had nopolygon section, but had alternating left- and right-pitched elements (LRLR). For the steam treatment,steam was injected into section four in order to ob-tain a steam pressure of1bar.When no steamwas in-jected, the steam pressure was 0.7 in section four.

E¡ect of starch source and processing conditions on ¢sh feed quality M S�rensen et al. Aquaculture Research, 2010, 41, 419^432

r 2010 TheAuthors420 Journal Compilationr 2010 Blackwell Publishing Ltd, Aquaculture Research, 41, 419^432

When steam was injected into the barrel, wateradded in the pre-conditioner was restricted in orderto obtain an acceptable structure of the pellets.The feeds were produced at the Centre for Feed

Technology at the Norwegian University of LifeSciences (UMB), —s, Norway. The ingredients weremilled using aMˇnch hammermill (HM 21.115,Wup-pertal, Germany) equipped with a 1mm screen,mixed in a Pegasus Menger (400 L, Sevenum, Hol-land) twin-shaft paddle mixer. After mixing, the feedmashwas reground in the hammer mill with a1mmscreen in order to ensure a more homogeneous parti-cle size distribution. The feed mash was conditionedfor 90 s in a Bˇhler double conditioner (BCTC10, Uz-wil, Switzerland), extruded in a ¢ve-section Bˇhlertwin-screw extruder (BCTG 62/20) with a 4mm die.The feeding rate and cutter speed were kept constantat 200 kg h�1and 800 rpm respectively. Other extru-der parameters are speci¢ed in Table 2. The pelletswere pre-dried in a Bˇhler £uidized bed dryer (OTW

50 05TSR2), before the ¢nal water content in the feedwas adjusted, in small dryers with a volume of 160 L(50 cm width � 80 cm length � 40 cm height), spe-cially manufactured for FoŒ rTek by the Department ofMathematical Science and Technology at UMB. Ap-proximately 25 kg feed was collected in each dryer,placed on a perforated tray located approximately10 cm from the bottom of the dryer. A10 kW heatingelement with a 400W fan (Viking 1862, Inelco AS,Fjerritslev, Denmark) was used to blow (air speed ap-proximately 1.5^2m s�1) hot air through the bed ofpellets (product temperature 50^60 1C) in order todry the feed to approximately 930 g kg�1DM. Duringadjustment of the drying time, dry matter waschecked using a Sartorius Moisture Analyzer (ModelMA50, Sartorius AG, Goettingen, Germany). The pel-lets were dried for approximately 30min to achievethe targeted dry matter content. After drying, thefeed was cooled to room temperature and packed inpaper bags. Representative samples for chemical andphysical analysis of the feed were taken from eachdiet and stored in sample bags of approximately

Table 1 Formulationand chemical composition of the feeds

Feed codeFeed 1 Feed 2 Feed 3 Feed 4 Feed 5WW PS GPS WS WW_WS

Formulation (g kg�1)

Fish meal� 749 854 854 846 796

Wheatw 242 0 0 0 125

Potato starchz 0 138 0 0 0

Gelatinized potato

starch‰

0 0 138 0 0

Wheat starchz 0 0 0 145 70

Vitaminsk 3.1 3.1 3.1 3.1 3.1

Mineralsk 4.5 4.5 4.5 4.5 4.5

Yttriumoxide�� 0.1 0.1 0.1 0.1 0.1

Carophyll pinkww 0.9 0.9 0.9 0.9 0.9

Composition

Dry matter (DM)

(g kg�1)

916.0 916.0 926.0 926.0 922.0

In DM (kg� 1)

Crude protein (g) 613.6 667.7 648.2 657.2 631.1

Crude fat (g) 76.4 87.3 76.7 76.7 79.2

Starch (g) 171.4 109.2 140.4 117.7 149.7

Ash (g) 126.6 146.3 144.7 146.9 136.7

�Norses

LT-94, low-temperature dried ¢sh meal, Norsildmel(Bergen, Norway).wDehulled, feed grade soft wheat, Felleskj�pet (Kambo, Norway).zFood grade, Lykeby Culinar AB (Fj�llkinge, Sweden).‰Lygel F60, Lykeby Culinar AB.zToro A/S (Bergen, Norway).kCommercial vitamins and minerals premix, Norsk Mineraln�r-ing AS (H�nefoss, Norway).��Sigma (St Louis, MO, USA).wwAstaxanthin preparation, DSM Nutritional Products (Kaiser-august, Switzerland).

LRLRPOLYGON_2L POLYGON_LR

60L20

60L20

80R80

60R20

60R20

60L20

80R80

100R100

60L20

80R80

60R20

100R100

60R60

60R60

60R60

60R60

80R80

80R80

60R60

60R20

P45-4/R120 P45-4/R120

60L20

80R80

80R80

Kneading

Figure 1 Screw con¢gurations Polygon____2L, Polygon____LRand LRLR, from inlet (top) to outlet. The outlined area re-presents section 4, where the con¢guration di¡ers amongthe three screws.

Aquaculture Research, 2010, 41, 419^432 E¡ect of starch source and processing conditions on ¢sh feed quality M S�rensen et al.

r 2010 TheAuthorsJournal Compilationr 2010 Blackwell Publishing Ltd, Aquaculture Research, 41, 419^432 421

Table

2Ex

trusion

parametersuseddu

ring

theprod

uctionof

theexperimen

talfeeds

SCF

Starch

source

Steam

Water

Con

.(kgh�1 )

Steam

Con

.(kgh�1 )

Diepres

s(bar)

SME

(kWht�

1 )Temp.co

nd.

(1C)

Temp.B2

(1C)

Temp.B3

(1C)

Temp.B4

(1C)

Temp.B5

(1C)

DieTemp.

(1C)

Revolution

screws

(rpm

)Torque

(%)

Poly

gon_2L

WW

S20

15

32.6

49.0

89.0

88.2

134.0

120.6

126.1

125.9

274.0

86.3

Poly

gon_2L

NS

26

15

33.5

47.3

85.8

88.1

132.1

118.1

126.6

128.9

273.0

85.9

Poly

gon_2L

PS

S20

15

30.0

45.4

89.9

89.5

131.7

120.9

126.4

122.0

273.9

80.0

Poly

gon_2L

NS

26

15

29.2

42.8

86.8

88.5

128.3

117.5

124.9

123.0

273.5

77.6

Poly

gon_2L

GP

SS

20

15

35.4

48.9

88.5

89.6

133.9

121.0

130.4

128.4

274.0

86.1

Poly

gon_2L

NS

26

15

34.5

45.8

86.9

89.8

131.5

120.2

130.3

128.0

273.2

83.3

Poly

gon_2L

WS

S20

15

32.0

45.5

90.6

91.6

135.1

122.3

129.1

131.0

273.0

80.5

Poly

gon_2L

NS

26

15

28.1

41.0

89.1

91.6

132.3

120.8

126.7

128.5

273.0

74.4

Poly

gon_2L

WW

_W

SS

20

15

31.1

45.4

91.0

92.1

135.8

122.1

127.9

130.3

273.4

80.2

Poly

gon_2L

NS

26

15

29.8

43.3

88.9

90.9

133.0

120.3

126.9

128.9

273.5

78.6

Poly

gon_LR

WW

S16

15

30.7

33.7

90.0

91.4

123.0

124.2

135.3

127.4

203.0

78.7

Poly

gon_LR

NS

26

15

35.7

34.9

87.2

90.1

119.6

111.6

130.3

121.3

203.0

85.2

Poly

gon_LR

PS

S16

15

32.6

34.1

88.2

90.4

121.0

123.2

135.6

125.9

203.0

79.7

Poly

gon_LR

NS

26

15

34.9

31.6

87.8

89.4

116.4

110.4

126.2

118.4

203.0

77.1

Poly

gon_LR

GP

SS

16

15

34.7

35.2

92.0

91.6

122.0

124.1

135.6

123.0

203.0

82.1

Poly

gon_LR

NS

26

15

39.3

34.3

89.0

91.0

117.6

111.4

128.6

119.7

203.0

83.8

Poly

gon_LR

WS

S16

15

30.8

30.1

94.0

92.9

120.9

124.8

133.3

123.2

203.0

70.4

Poly

gon_LR

NS

26

15

34.0

29.4

90.2

92.1

116.1

111.0

124.5

116.9

203.0

71.8

Poly

gon_LR

WW

_W

SS

16

15

31.5

32.7

91.0

91.9

122.0

124.6

133.9

124.9

203.0

76.5

Poly

gon_LR

NS

26

15

34.4

32.4

90.2

91.5

119.0

114.4

129.1

120.3

203.0

79.2

LR

LR

WW

S16

16

30.2

36.1

93.2

91.9

130.6

123.0

129.3

121.9

203.0

84.3

LR

LR

NS

16

16

31.7

37.3

93.1

93.0

132.2

123.0

131.7

124.7

203.0

87.1

LR

LR

PS

S16

16

30.8

35.7

92.9

91.2

129.8

122.9

129.9

120.3

203.0

83.5

LR

LR

NS

26

16

31.0

33.3

89.0

90.4

122.9

110.4

123.7

118.0

203.0

81.4

LR

LR

GP

SS

16

16

36.4

39.7

91.6

91.8

131.3

121.0

132.6

124.2

203.0

92.7

LR

LR

NS

26

16

36.4

36.7

89.4

91.7

126.6

114.0

129.1

121.8

203.0

89.7

LR

LR

WS

S16

16

32.0

36.4

92.7

93.1

132.1

122.2

132.0

125.4

203.0

85.0

LR

LR

NS

26

16

29.6

31.9

91.0

92.7

125.5

112.1

125.8

121.0

203.0

78.0

LR

LR

WW

_W

SS

16

16

33.2

37.2

92.7

93.3

132.9

122.8

133.4

128.3

203.0

86.9

LR

LR

NS

26

16

32.5

34.8

90.0

92.6

126.8

114.1

129.9

122.6

203.0

85.1

� Water

Con

.(kg

h�1 ),

water

addition

incondition

er;S

team

Con

.(kg

h�1 ),

steam

addition

incondition

er;D

iepress(bar),diepressure;S

ME(kWht�

1 ),speci¢cmechan

ical

energy

;Tem

p.cond.

outlet

(1C),

temperature

attheou

tlet

ofcondition

er;Tem

p.B2,

temperature

insection2of

theextruderba

rrel;Tem

p.B3,

temperature

insection3of

theextruderba

rrel;Tem

p.B4,

temperature

insection4of

the

extruderba

rrel;Tem

p.B5,

temperature

insection5of

theextruderba

rrel;Die

temp.

(1C),temperature

infron

tof

thedie;

rpm,revolution

sof

screw

perminute;Torquerealtiv(%

),relative

energy

con-

sumption;

WW,w

holewheat;PS

,potatostarch;G

PS,

gelatinized

potato

starch

;WS,

wheatstarch;W

W_W

S,wholewheatan

dwheatstarch.

E¡ect of starch source and processing conditions on ¢sh feed quality M S�rensen et al. Aquaculture Research, 2010, 41, 419^432

r 2010 TheAuthors422 Journal Compilationr 2010 Blackwell Publishing Ltd, Aquaculture Research, 41, 419^432

10 kg. These samples were again mixed thoroughlybefore samples were divided into appropriate samplesfor each of the analyses carried out.The feed sampleswere stored at 4 1C until they were analysed. Thediets used for analysis of physical quality were notcoated with fat in order to avoid fat leakage whenanalysing the durability of the feed.

Chemical and physical analysis of the feed

Chemical analysis of the feed

Chemical analyses were carried out at Analycen,Moss, Norway. Five feeds were analysed for dry mat-ter (EU DIR 71/393), nitrogen (Kjeldahl; 93/28/EEC),crude fat (EU DIR 98/64), starch (AOAC 996.11) andash (EU DIR 71/250). All the feeds were analysed fordry matter (to constant weight at105 1C).

Physical quality of the feed

Strength at rupture (hardness) was measured by dia-metral compression using a Texture-Analyser (TA-XT2

s

, Model 1000 R; SMS Stable Micro Systems,Blackdown Rural Industries, Surrey, UK), ¢tted witha 25 kg load cell and a principal component (PC)-op-erated remote control. Analyses using a cylinder(d550mm) were performed by pressing the cylinderonto the pellet at a constant speed of 2mm s�1 toachieve 60% compression. The strength applied onthe pellet was progressively increased. The strength^time graphs were recorded by a computer and ana-lysed using the TEXTURE EXPERT for WINDOWS (version1.15, Stable Micro Systems), and strength at rupturewas recorded on 30 pellets, and was reported as theaverage of 3 � 10 pellets.Length and diameter were measured on the same

pellets that were used for the hardness analysis.Length was manually measured using an electronicdigital calliper. The diameter of the pellets was auto-matically measured using the texture analyser.‘Holmen’ durability (HDI) was measured in a Hol-

men pellet tester (Borregaard Lignotech, Hull, UK).Sifted (5mmmesh size) samples of100 g were placedin the Holmen pellet tester at a 60-s testing time. Thesample was removed, and the empty Holmen testerwas run for another 30 s in order to collect dust es-caping the collector in the ¢rst round. The samplewas sieved using Retsch sieving equipment (RetschGmbH Haan, Germany) using three sieves with meshsizes of 5mm (¢rst sieve), 0.5mm (second sieve) and0mm (collector), for 30 s with a 1.5mm amplitude.

The dust in the collector, the fraction of broken pel-lets in the second sieve and the unbroken pellets inthe ¢rst sieve were weighed on an electronic scale(SJ-H/SJ-HS, A&D, Japan).‘Holmen’durability was cal-culated as the percentage of pellets remaining on the5.0mm screen.The experiment was conducted in tri-plicate for each of the feeds.‘Tumbling’durability (TBDI) was measured using a

Tumbling BoxTester (Seedburo USA) and proceduresfollowing ASAE Standard S269.4 (ASAE 1997). Sifted(5mm) samples of 500 g of pellets were placed ineach of four boxes. Five hexagonal iron nuts (eachwith a diameter of 8mm and a weight of 5 g) wereadded in each box to enhance the pellet attrition dur-ing tumbling. The feed samples were tumbled for10min at a speed of 50 rpm. After testing, the feedsamples were sieved to remove dust following thestandard sieve sizes, amplitude and running time ofthe sieving tests.‘Tumbling’durability was calculatedas the weight percentage of pellets remaining on a5mm screen. Three replicate measurements weremade per feed.

Data analysis

Analysis of variance (ANOVA) was performed by usingthe GLM procedure of SAS computer software (SAS1990). The experiment was arranged in a 3 � 5 � 2factorial design with three screw con¢gurations(Polygon____2L, Polygon____LR and LRLR), ¢ve carbohy-drate sources (WW, PS, PGS, WS and WW_WS) andeither with steam injection into the barrel or withoutsteam injection into the barrel. Signi¢cant (P � 0.05)di¡erences among means were ranked by Duncan’smultiple range test and are indicated by di¡erentsuperscript letters in the tables.Type I sumof squareswas used to calculate the proportionof total variation(corrected total SS) that was explained by the maine¡ects.Partial least squares regression (PLS 2) was per-

formed in the UNSCRAMBLER statistical package(UNSCRAMBLER, WINDOWS v 9.7, 2007, Camo AS, Oslo,Norway). Partial least squares regression is a bi-linearregression-modelling tool in which systematic co-variations from two data tables are decomposed intolatent variables called PCs. A large number of origi-nal variables are reduced to a fewer number of un-ob-servable variables (PCs) that are linear combinationsof the original ones. The ¢rst few PCs provide infor-mation about the most important interactions andphenomena in the data set. In the PLS 2 procedure,

Aquaculture Research, 2010, 41, 419^432 E¡ect of starch source and processing conditions on ¢sh feed quality M S�rensen et al.

r 2010 TheAuthorsJournal Compilationr 2010 Blackwell Publishing Ltd, Aquaculture Research, 41, 419^432 423

the information from the latent variables is used tostudy the relation between two data tables X and Y(Martens, H�y,Westad, Folkenberg & Martens 2001).The latent variables decomposed from the X data (de-sign variables) table were in turn used for modellingtheY-variables (physical quality of feed).The extrudervariables were excluded from X data table, becausethe explained Y-variance was reduced when the ex-truder parameters were used in the model. Moreover,interactions between the design variables as well asthe squared design variables were also excluded fromthe model since they did not add more information tothe explainedY-variance. All variables were centredand scaled to unit variance. The calibration modelwas validated by full cross validation (Martens &N�s 1989). Signi¢cant variables in the PLS predic-tions were identi¢ed using the modi¢ed Jack-knifeUncertainty test (Martens & Martens 2000).

Results

The feeds were formulated to contain approximately12% of starch. The chemical analyses showed thatthe starch content varied slightly among the feeds(Table 1). The lowest starch content was observed forthe feeds with PS and the highest content was ob-served for the feed withWW. During feed production,adjustable extruder parameters such as feeding rate,extruder revolutions of screw per minute (rpm) andwater addition to the conditioner were kept constant.Changing screw con¢guration a¡ected the genera-

tion of SME (Table 2). The highest SME was obtainedwith screw con¢guration Polygon____2L (45.4 kWh t�1),followed by screw con¢guration LRLR (35.9 kWh t�1)and screw con¢guration Polygon____LR (32.8 kWh t�1).Revolutions of screw per minute was increased whenPolygon____2Lwas used in order to obtain anacceptablequality of the feed (Table 2). The higher rpm thus con-tributed to an increased SME when screw con¢gura-tion Polygon____2L was used.When steam was injectedinto the barrel, water addition to the conditioner wasreduced from 24 to 20 kg h�1for screw con¢gurationPolygon____2L and 16 kg h�1 when screw con¢gura-tions Polygon____LR and LRLRwere used (Table 2). Themain e¡ects of starch source, screwcon¢gurationandsteam injection are presented inTable 3.The HDI showed considerable variation among the

feeds (Table 3). ‘Holmen’ durability di¡ered signi¢-cantly among all ¢ve feeds with di¡erent starchsources. In general, the feeds made from the PSs hadhigher HDI compared with the feeds withWSs. The

highest PDI was shown for PGS, followed by PS,WWand WW_WS. The lowest HDI was shown for WS.Starch source explained 33.2% of the variation inthe HDI (Table 3). Screw con¢guration explained thegreatest proportion (43.4%) of variation in the HDI(Table 3). Screw con¢guration LRLR resulted in thehighest HDI, whereas Polygon____LR yielded the lowestHDI. Steam injection into the barrel explained a min-or (4.8%), but signi¢cant proportion of the variationin the HDI (Table 3). ‘Holmen’ durability was signi¢-cantly reduced by steam addition.The TBDI was in general high (98.4^98.8%), and

showed a low variation among the feeds (Table 3).Starch source provided a considerable contributionto the overall variance (31.1%). The ranking amongthe feeds was as follows: WW5gelatinizedPS4WW_WS4WS4PS. Screw con¢guration alsosigni¢cantly a¡ectedTBDI (Table 3). In numerical va-lues, the di¡erences among the screw con¢gurationswere low, but screw con¢guration LRLR resulted inthe signi¢cantly highest PDI and Polygon____2L thelowest. Injection of steam into the barrel provided aminor contribution to the overall variance (0.5%) inthemodel (Table 3); hence, the in£uence of heat treat-ment only slightly a¡ected theTBDI.Starch source was the most important factor ex-

plaining hardness of the feed and contributed to37% of the overall variance (Table 3). The signi¢-cantly highest strength of rupturewas obtained withPGS. No signi¢cant di¡erences were observed amongthe feeds with WWand PS, but these two carbohy-drate sources resulted in signi¢cantly higher hard-ness compared with the feeds with WS or WW_WS.No signi¢cant di¡erence was observed between thetwo feeds formulatedwithWS orWW_WS. Screwcon-¢guration provided an almost equal contribution tothe overall variance in hardness as the starch source.Hardness was signi¢cantly a¡ected by screw con¢g-uration (Table 3). The highest hardness was shownfor screw con¢guration LRLR, followed by screw con-¢guration Polygon____2L and screw con¢guration Poly-gon____LR. Steam injection resulted in a signi¢cantreduction in the hardness.Starch source signi¢cantly a¡ected the expansion

of the pellets, resulting in di¡erent pellet diameters,and contributed to 30% of the total variance. Exceptfor WW and WW_WS, there were signi¢cant di¡er-ences among the starch sources. The highest dia-meter was shown for PGS, followed by PS,WS,WWand WW_WS. Screw con¢guration explained thehighest proportion of variance (42.5%) of the dia-meter (Table 3), and was signi¢cantly di¡erent

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among the three screw con¢gurations. The least ex-panded pellets were obtained with screw con¢gura-tion LRLR and the highest was screw con¢gurationPolygon____2L. The steam injection into the barrel pro-vided a minor contribution to the overall variance(1.6%), but had a signi¢cant e¡ect on the diameter ofthe pellets (Table 3). No steam injection resulted in alower diameter compared with steam added to thebarrel.The length of the pellets wasmainlya¡ected by the

carbohydrate source (Table 3). The longest pelletswere obtained with WW and WW_WS. The shortestpellet was obtained with WS, which tended to beshorter than PGS. No di¡erences in length were ob-served between the PS and the PGS. Length was sig-ni¢cantly a¡ected by screw con¢guration, withscrew con¢guration Polygon____2L resulting in signi¢-cantly longer pellets than screw con¢gurations Poly-gon____LRand LRLR.The length of the pellets producedwith the two latter con¢gurations did not signi¢-cantly di¡er from each other. Steam injection into

the barrel resulted in signi¢cantly shorter pelletscompared withwhen no steamwas injected.The water content varied among starch sources,

screw con¢gurations and steam injection to the ex-truder barrel despite the attempt to standardize thedrymatter content of the feeds.The lowest water con-tent was found for WW_WS (5.7%), followed by WS(5.9%) andWW (6.1%). Feed with PS had the highestwater content (8.3%), while feed with PGS had a low-er water content than the PS but higher than thefeeds withwheat (7.8%). Pellets produced with screwcon¢guration LRLR had the highest water content(7.5%), followed by screw con¢guration Polygon____2L(7.0%).The lowest water content was found for pelletsproduced with screw con¢guration Polygon____2L(5.6%). When steam was injected into the barrel,water addition in the pre-conditioner was restricted.This also resulted in a lower water content in the ¢n-ished pellets (5.9%) compared with when no steamwas added (7.5%). Variations in feed water contentwere expressed by PC4 (data not shown) in themulti-

Table 3 Physical quality of pellets produced with di¡erent screw con¢gurations, carbohydrate sources and thermal treat-ment presented as mean values and proportion (type I SS) of the total variation (corrected total SS) explained by main e¡ectsand interaction (%) on Holmen durability

HolmenDurability (%)

Tumbling boxdurability (%) Hardness (N) Length (mm) Diameter (mm)

Starch source (starch)

Whole wheat 49.5c 98.8a 26.7b 7.6a 5.3d

Potato starch 69.6b 98.4d 27.8b 7.2c 5.6b

Pre-gelatinized potato starch 79.1a 98.8a 32.9a 7.1cd 5.7a

Wheat starch 44.1e 98.5c 23.0c 7.0d 5.5c

Whole wheat1wheat starch 46.5d 98.7b 23.3c 7.4b 5.3d

Screw configuration (SCF)

Polygon_2L 60.0B 98.6B 28.0B 7.5A 5.7A

Polygon_LR 37.1C 98.5C 22.1C 7.1B 5.5B

LRLR 76.1A 98.7A 30.1A 7.1B 5.3C

Steam pressure (steam)

No steam 63.0X 98.6Y 28.2Y 7.4X 5.5

Steam 52.4Y 98.7X 25.3X 7.1Y 5.5

Two-way ANOVA

Root MSE 2.1 0.1 1.8 0.1 0.1

Proportion (type I SS) of variation (corrected total SS) explained (%)

Starch source 33.2��� 31.1��� 37.0��� 33.3��� 30.0���

SCF 43.4��� 13.9��� 32.6��� 23.7��� 42.5���

Steam 4.8��� 0.5� 6.1��� 16.9��� 1.6���

���P � 0.001; �P � 0.05.ffiffiffiffiffiffiffiffiffiffi

MSEp

is the square root of the mean square error.a,b,c,d,eDi¡erent superscripts within starch indicate signi¢cant di¡erences among treatment means (P � 0.05).x,yDi¡erent superscripts within steam/no steam indicate signi¢cant di¡erences among treatment means (P � 0.05).A,B,CDi¡erent superscripts within screw con¢gurations indicate signi¢cant di¡erences among treatment means (P � 0.05).ANOVA, analysis of variance.

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variate analysis. The main factors described in PC4were that steam injection into the extruded barrelyielded a lower water content in the feed after dryingand the wheat and PSs di¡ered in the ¢nal water con-tent.The physical quality of the feeds was related to the

design parameters by PLS 2 regression. The designvariables (carbohydrate source, screw con¢gurationand steam injection into the barrel) were used as X-variables. ‘Holmen’ durability, strength, diameter,length and dry matter content of the ¢nished feedswere used asY-variables (Table 3).The optimumnum-ber of PC was four. Approximately 39% of the varia-bility in the total Y-variables was explained by PC1,42% by PC2,64% by PC3 and 66% by PC4. Inclusionof more than four PCs did not improve the prediction.The score plots visualized the projection of the sam-ples, while the corresponding projections of the vari-ables were shown in the corresponding loading plot.The signi¢cant variables, identi¢ed by jack-knife/cross-validation, are marked with circles in the load-ing plots. A positive association between physicalquality and design parameters is obtained if both arelocated in the same area of the plot. A negative asso-ciation exists if the two are on opposite sides of theco-ordinate axis.‘Tumbling’durability was poorly de-scribed by the model and was therefore omitted fromtheY-matrix during the calculations.The ¢rst PC in the loading plot (Fig. 2a) mainly de-

scribed variation in the physical parameters of thepellets such as strength and HDI. These physicalquality parameters were strongly correlated to PC1,as shown by their relatively similar loading in PC1(Fig. 2a). ‘Holmen’ durability and strength were alsopositively related to the drymatter content of the feedalong PC1. The most important additional informa-tion the PLS 2 analysis provided in addition to theANOVA analysis was the huge impact of screw con¢g-uration to explain the variation in hardness and PDI(Fig.2a). High quality was associatedwith screwcon-¢guration LRLR and PGS, whereas screw con¢gura-tion Polygon____LR had the highest negative loading inPC1. A negative association also existed among thephysical quality parameters and the diets with starchfrom cereal (the mixture with WW_WS,WS and theWW) as well as steam. Bothwheat and steam signi¢-cantlya¡ected the qualityof the pellets, but had a lowin£uence on PC1and PC2 (Fig. 2a). Potato starchwaslocated on the positive side of PC1, but was not de-tected as being signi¢cant by the jack-kni¢ng proce-dure.The second PC can be described as a‘size factor’,describing variation in the diameter and length of the

pellet. The size factor was positively associated withscrew con¢guration Polygon____LR and negatively as-sociated with screw con¢guration LRLR. The corre-sponding score plot (Fig. 2b) showed that thesamples were clustered in three main clusters ac-cording to screw con¢guration. Screw con¢gurationLRLRwas located to the right in the loading plot andscrew con¢guration Polygon____LR to the left, whilescrew con¢guration Polygon____2L was mainly or-iented above the screw con¢gurations Polygon____LRand LRLR clusters.Within each cluster, the carbohy-drate source followed the same pattern with respectto pellet quality, in whichWS was oriented to the left(associated with low quality) and gelatinized PS tothe right (associated with high quality).Principal component 3 can be interpreted as the

‘shape factor’, describing variation in the diameterand length of the pellets. Diameter and length werelocated on the opposite side of each other in PC3(Fig.3a), and were mainly described by starch sourceand injection of steam into the barrel. Diameterwas positively associated with GPS, steam and WS,while length was positively associated withWWandthe WW_WS feed, but the latter with less in£uence.The corresponding score plot (Fig. 3b) showed thatsteam injection into the barrel in general increasedthe diameter, but the highest loading was observedfor the feeds with PGS, PS and WS, extrudedwith screw con¢guration Polygon____LR. Length waspositively associated with no steam addition to thebarrel and feeds withWWorWS showed the highestloading.

Discussion

Di¡erent methods were used in order to investigateattrition and degradation of pellets exposed to var-ious stresses (Table 3). The greatest variation in phy-sical quality was observed when pellet quality wasevaluated using HDI or strength (Table 3). The tum-bling box procedure resulted in a low variationamong the feeds although hexagonal nuts wereadded to increase the impact. The low variation indurability among the feeds resulted in a low correla-tion coe⁄cient betweenTBDI and HDI (r50.44) andbetween TBDI and strength (0.47), and may also ex-plain why TBDI was poorly described and was thusomitted from the PLS-2 model. The strength of feedmeasured with a compression test correlated wellwith the HDI (r50.86). These two methods thereforegave the best models to explain variation in physical

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quality (Fig.2a).The results suggest that for uncoatedfeed, HDI and strength provide good indications ofdi¡erences in the physical quality of the feed, andshould be preferred methods for the evaluation ofphysical quality for extruded feed.Analysis of data using PLS models enabled visuali-

zation of complex structure in the data and thereforeprovided more information than what is obtainedusing conventional regression analysis or ANOVA. Theloading plots showed how the design parameterswere associated with the physical quality of pellets,whereas the score plots presented the signi¢cant im-pact of screw con¢guration and starch source, andthat steam injection was of lesser importance. Thescore plots portrayed three clusters according to

screw con¢guration. However, within each maincluster, the samples were oriented according to theirassociationwith feed quality.The greatest HDI and strengthwas observed when

feed was produced with screw con¢guration LRLR(SME 35.9 kWh t�1), followed by screw con¢gura-tions Polygon____2L (SME 45.4 kWh t�1) and Poly-gon____LR (SME 32.8 kWh t�1). Screw con¢gurationa¡ects mixing, residence time and energy inputs tothe dough, and thus cooking of starch and proteins(Gogoi et al. 1996; Chao-Chi Chuang & Yeh 2004). Areduction in reverse screw elements with Poly-gon____LR resulted in a lower SME in line with Gogoiet al. (1996). Speci¢c mechanical energy is expressingmechanical energy input (the product of torque and

Figure 2 (a) Loading plot for the two principal components (PC)1and (PC)2 showing the relationships between designparameters and physical quality parameters diameter, length, hardness, ‘Holmen’durability and the dry matter content.For a description of the design parameters and physical quality, seeTables 2 and 3. (b) Distribution of the feed samples inthe score plot. Ingredient composition and processing of the diets are described inTables1and 2 respectively.

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rpm) divided bymass £ow rate (Mercier, Linko & Har-per 1989). The higher rpm used when feed was pro-duced with screw con¢guration Polygon____2Lconsequently increased SME (Table 2). Increasedscrew speed has resulted in increased expansion instarch-based products (Ainsworth, Ibanoglu, Plun-kett, Ibanoglu & Stojceska 2007). Hence, the greaterdiameter for feeds produced by screw con¢gurationPolygon____2L may be explained by the higher rpm.The SME input is considered to be an important para-meter in extrusion, because it is related to energy in-puts, followed by product transformation in theextruder barrel (Godavarti & Karwe1997). In the pre-sent experiment, the highest PDI and strength was

obtained with screw con¢guration LRLR, generatingan intermediate SME.The results suggest that SME isnot well correlated to the physical quality of the feedwhen di¡erent ingredients are used, because it de-pends both on the extrusion conditions and on thematerial structural characteristics such as chemicalcomposition and amylose:amylopectin ratio (Mat-they & Hanna1997).Although starch level was targeted at 120 g

kg�1DM chemical analysis revealed a variation be-tween 109 and 171g DM�1. The variation in starchcontent did not signi¢cantlya¡ect the overall results,because the main variation in physical qualitymainly was explained by screw con¢guration and

Figure 3 (a) Loading plot for the two principal components (PC)3 and (PC)1showing the relationships between the sizeof the pellet, diameter and length, and the design parameters. For a description of the design parameters and physicalquality, seeTables 2 and 3. (b) Distributionof the feed samples according to the shape of pellets in the score plot. Ingredientcomposition and processing of the diets are described inTable1and 2 respectively.

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starch source. The wheat and PSs were clearly sepa-rated along PC1, with the PSs positively andWSs ne-gatively associated with strength and durability. Forthe feed industry, it is important to use starch sourceswith desired functional properties at the lowest pos-sible inclusion level, due to the previously describedmetabolic limitations in the utilization of starch.The lower PDIand strength observed for feeds con-

tainingWSs compared with the PSs (Table 3; Fig. 2aand b) underlines that starch sources di¡er in func-tionality, which moreover a¡ects the physical qualityof the feed. The functionality of starch is a¡ected bythe structure and size of starch granules (Swinkels1985), amylose:amylopectin ratio (Visser, Suurs, Brui-nenberg, Bleeker & Jacobsen1997), starch molecularweight (Blennow, Bay-Smidt & Bauer 2001) and thedegree of amylopectin branching (Fredriksson, Sil-verio, Andersson, Eliasson & Aman 1998). The ratioof amylose:amylopectin content in potato and wheatwas reported to be approximately 20:80 and 25:75 re-spectively (Thomas & Atwell1999). Amylopectin mo-lecules in PS are slightly negatively charged due tophosphate monoester groups resulting in more elec-trostatic repulsion (Liu 2005) and weaker intermole-cular bonding between the amylopectin molecules.In the presence of moisture and heat, water mole-cules easily disrupt the weak bonds, and break upthe inter-chain bonds, resulting in rapid swelling ofPS granules to yield high-viscosity pastes (Bemiller& Whistler 1996). Wheat starch is more stable, be-cause it is dominated by type A starch with highercrystallinity compared with type B starch that isdominating the PS (Liu 2005). This may also explainthe higher swelling power of PS. Swinkels (1985) re-ported a swelling power of 21 for wheat and1153 forPS, suggesting that the tightly packed A type struc-ture of the amylopectin inWS limits the disruptionof the crystalline structure of starch granules whenthey are gelatinized. Liu (2005) classi¢edWS amongthemoderately swelling starches (type II group) char-acterized by a lower peak viscosity and less thinningduring cooking. Potato starchwas classi¢ed with thehigh swelling starches (type I group) characterizedby a high peak viscosity, followed by rapid thinningduring cooking (Liu 2005). The lower expansion ob-served for thewheat-based feeds may be explained bya lower viscosity. The lower viscosity of the wheatmay also explain the lower PDI and strength ob-served for the wheat-based feeds in the present ex-periment. Viscosity is positively associated withimproved binding between particles (Thomas & vander Poel,1996; Svihus, Uhlen &Harstad,2005). Signif-

icant di¡erenceswere observed among the feedswithWW,WS or WW_WS (Table 3). Whole wheat as thestarch source in the feed resulted in higher PDI andhardness than the WS or WW_WS in the feed. How-ever, the slightly higher starch content in the feedcontainingWW may also contribute towards an ex-planation of the improved quality.Although important for expansion, starch source

and screw con¢guration a¡ected the diameter of thepellets more than steam injection. The highest in-crease in diametric expansion was observed for thefeeds with PS. Among the feeds with WS, the feedwithWWyielded the highest diameter and feed, withWS yielding the lowest expansion. These ¢ndings arein contradiction to the results presented by SinghandSmith (1997). They found higher expansion for WScompared withWW. The di¡erent results may be ex-plained by the use of di¡erent starch sources. Thefunctional properties of wheat are a¡ected by a vari-ety of parameters. Examples are chemical composi-tion (Magnus, Br�then, Sahlstr�m,Vogt & Faergestad2000),WS composition (Sahlstr˛m, B�vre & Br�then2003a, b) as well as the composition of wheat protein(Tronsmo, F�rgestad, Scho¢eld & Magnus 2003),which di¡ers not only among cultivars (Holtekj�len,Uhlen, Br�then, Sahlstr�m & Knutsen 2006) but alsodue to climatic conditions such as temperature (An-ker-Nilssen, F�rgestad, Sahlstr�m & Uhlen 2006).In contrast to previously reported studies, feed

with the highest diameter containing gelatinized PSalso had the highest PDI and hardness (Figs 2a, b and3a, b). Other studies have shown an inverse relation-ship between PDI and hardness on the one hand anddiameter on the other (Yoshitomi 2004; Aarseth2006; S�rensen et al.2009).The present results there-fore suggest that carbohydrate sources with highswelling power, such as PS, can be used for dual pur-poses, increased expansion and improved physicalquality of extruded feed. Pre-gelatinized potatostarch has other processing properties than nativestarch due to the loss of granular integrity. Hence,PGS can be redissolved in coldwater without cooking(Bemiller & Whistler 1996). The PGS most likely ab-sorbed more water and faster than the PS, yieldingviscosity and cohesive properties for an improvedbinding in extruded ¢sh feed.Expansion of the pellet as it leaves the die is caused

by evaporation of water. Small droplets of water aredispersed in the dough in the extruder, and as the pel-let leaves the die it goes from high pressure to atmo-spheric pressure. The reduction in pressure causeswater to evaporate, leaving small pores in the pellets

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that can be ¢lled with fat in high-energy diets.Wateris an important plasticizer, and is added to reduce theglass and melt transition temperatures of proteinsand starches during extrusion (Strahm1998). Despitea reduction in water addition to the pre-conditionerwhen steamwas injected into the barrel, a good qual-ity was obtained as the pellets left the extruder. Theslightly lower water addition in the pre-conditionerwas partly compensated by the steam injection intothe barrel. Hence, the slightly lower water content inthese feeds did not result in problems with £owthrough the extruder. Apparently, the higher steampressure in the dough improved the melt as the SMEwas reduced (Table 2). Although small numerical dif-ferences were observed between the steam and theno-steam treatments (only signi¢cantly di¡erentwith two decimals,Table 3), the diameter of the pelletwas clearly associated with injection of steam intothe barrel (Fig.3a). The diameter of the pellet was ne-gatively associated with the length as previously ex-perienced (S�rensen et al. 2009). Moreover, whensteamwas injected into the barrel, the ¢nal dry mat-ter contents of these diets were lower compared withwhen no steam was added. Hence, a shorter dryingtime was needed when less water was added to theprocess. In particular, diets made from cereal starchhad a higher dry matter content than diets from PSs,and dry matter was high in feeds produced withscrew con¢guration Polygon____LR.Water in the pelletsbinds particles together by a liquid bridge (Pietsch1983). Hence, overdrying reduces the adhesionstrength of the liquid bridge, resulting in lowerbreaking strength and durability. The results clearlydemonstrated that the drying time and the generaldrying conditions should be carefully adjusted ac-cording to the screw con¢gurationand feed composi-tion, and in particular the carbohydrate source.

Conclusion

Starch source and extruder screwcon¢gurationwerethe most important factors a¡ecting the physicalquality of salmonid ¢sh feed, measured as ‘Holmen’durability, hardness, diameter and length. The feedswere ranked into three main clusters according toscrew con¢guration, and within each cluster, carbo-hydrate source was arranged in the same pattern,with PS positively associated with strength and dur-ability. Injection of steam into the extruder barrelalso signi¢cantly a¡ected the physical quality, but toa lesser degree than starch source and screw con¢g-

uration. Tumbling box durability did not prove e⁄-cient to explain the variation in physical quality.

Acknowledgments

This work was ¢nancially supported by theAquacul-ture Protein Centre, CoE (Project No.14949 fromTheResearch Council of Norway). Ismet Nikqi and DejanMiladinovic at FoŒ rTek are highly acknowledged forskilful technical assistance during the production ofthe experimental diets.

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