Journal of Food Engineering 62 (2004) 143–150

www.elsevier.com/locate/jfoodeng

Effect of extrusion cooking of soy–sweet potato mixtures on availablelysine content and browning index of extrudates

M.O. Iwe a,*, D.J. van Zuilichem b, W. Stolp b, P.O. Ngoddy c

a Department of Food Science and Technology, University of Agriculture, P.M.B. 2373, Makurdi, Nigeriab Food & Bioprocess Engineering, Department of Food Science, Agricultural University, The Netherlands

c Department of Food Science and Technology, University of Nigeria, Nsukka, Nigeria

Received 1 June 2000; received in revised form 12 September 2001; accepted 9 May 2003

Abstract

Effects of three processing variables: feed composition (% sweet potato), screw speed, and die diameter on available lysine and

browning index were investigated following extrusion cooking of mixtures of defatted soy flour and sweet potato flour. Response

surfaces for the parameters were generated using a second degree polynomial. Results show that increase in screw speed and a

reduction in die diameter enhanced lysine retention. Increase in feed composition, and screw speed increased browning index, but

decreases in die diameter and feed composition increased browning index. Optimum extrusion conditions resulting in maximum

available lysine and minimum browning index were estimated.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Extrusion; Soy–sweet potato; Lysine; Browning index

1. Introduction

Extrusion cooking, as a heat treatment affects andalters the nature of many food constituents, including

starches and proteins, by changing physical, chemical

and nutritional properties (Harper, 1979; Sgaramella &

Ames, 1993).

Several reports had attempted to relate Maillard re-

action and discoloration or browning to loss of lysine

(Asp & Bjorck, 1989; Bjorck & Asp, 1983; Cheftel, 1986;

Hurrel & Carpenter, 1977; Noguchi, Mooso, Aymard,Jeunink, & Cheftel, 1982; O�Brien & Morrissey, 1989).

Furthermore, lysine loss has been related to extrusion

process parameters such as raw material, feed moisture,

screw speed, extrusion temperature, die diameter, feed

rate, screw compression ratio, torque and pressure,

energy input and pH (Asp & Bjorck, 1989; Camire,

Camire, & Krumhar, 1990).

Collins and Walter (1985) reported that the majornutritional change due to heat processing in sweet po-

tato was the loss of lysine, probably via reaction with

reducing sugars. In addition, Kays (1985) and Wolfe

*Corresponding author. Address: Department of Food Science and

Technology, Micheal Okpara University of Agriculture, Umudike,

P.M.B. 7267, Umuahia, Abia State, Nigeria. Tel.: +234-533-205-2047.

0260-8774/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0260-8774(03)00212-7

(1992) reported that discoloration could be a serious

problem during processing of sweet potatoes and stor-

age of their products. Discoloration occurs as a result ofsurface darkening due to oxidation. Bouwkamp (1985)

reported that the darkening was due to polyphenol oxi-

dase enzyme substrate complex, which is concentrated

in the cambial areas of the root. It was further proposed

(Bouwkamp, 1985), that the darkening reaction is a two

step process, the first being the enzymatic oxidation of

phenolic compounds to quinones. The second is the

nonenzymatic polymerization of quinones to melanin-like compounds. The effect of processing leading to the

loss of lysine and development of darkening and for-

mation of melanoidins seem therefore to be relevant in

sweet potato processing.

Extrusion of a soy–sweet potato system might favor

Maillard reaction and of course lysine loss due to the

presence of both reducing sugars and the epsilon-amino

group of lysine.The FDNB procedure has been considered a stan-

dard reference method for monitoring reactive lysine

(Hurrel & Carpenter, 1974; Hurrell & Carpenter, 1981),

however, Vigo, Malec, Gomez, and Llosa (1992) and

Morales, Romero, and Jimenez-perez (1995) showed

that the method is fairly complicated, time consuming,

and requires special precautions. Carbohydrate-rich

144 M.O. Iwe et al. / Journal of Food Engineering 62 (2004) 143–150

samples need to be dialyzed (Tomarelli, Yuhas, Fisher,& Weaber, 1985). To eliminate these inherent limita-

tions, Goodnoo, Swaisgood, and Catignani (1981) de-

vised a fluorometric assay (with a lot of advantages),

using o-phthaldialdehyde (OPA) to estimate reactive

lysine in proteins and a number of workers (Morales

et al., 1995; Swaisgood & Ctagnani, 1985; Vigo et al.,

1992) have favorably adopted it. This method could be

considered advantageous in estimation of lysine in car-bohydrate-rich foods such as the sweet potato, and also

in industrial routine analysis which requires limited

time.

Response surface analysis (RSA) is a system for op-

timizing variables which differs from the usual one-

variable-at-a-time experimental procedure (Henika,

1982; Joglekar & May, 1987). RSA tests several vari-

ables at a time, uses special experimental designs to cutcosts, and measures several effects by objective tests. A

computer takes the experimental results and calculates

models, using Taylor second-order equations which

define relationships between variables and responses

(Dziezak, 1990; Henika, 1982). The relationships are

quantitative, cover the entire experimental range tested,

and include interactions if present. Thus the models can

then be used to calculate any and all combinations ofvariables and their effects within the test range.

The goal of this research was to investigate the effects

of extrusion process conditions on available lysine and

browning index of extruded soy–sweet potato mixtures.

2. Materials and methods

2.1. Materials

Chips of orange fleshed variety of sweet potato (Ipo-moea batatas), and defatted soy flour were obtained

from commercial suppliers in The Netherlands.

2.2. Raw materials and preparation

Sweet potato chips were cleaned and coarsely crushed

in a Condiux tooth mill (Condiux, West Germany) and

then milled to pass a 1 mm sieve in a Retsch type ZM1

mill (Retsch B.V., The Netherlands). The milled sampleswere later extruded.

2.3. Extrusion of samples

The Almex-Battenfeld single screw extruder of a

screw length to diameter ratio (L=D) of 16:1 and com-

pression ratio of 1:1.15 was used for the runs (Iwe &

Ngoddy, 1998; Iwe, Wolters, Gort, Stolp, & Van Zuili-chem, 1998).

After stabilization of the extruder, runs were carried

out on 4 kg samples moistened to 18% moisture in a

pilot mixer. Extrusion temperature was set at 34, 54,100, and 100± 5 �C along the feeding, compression,

metering, and die zones respectively.

Extrusion screw speed adopted were 80, 92, 110, 127,

140 rpm respectively. Die diameter adopted was 6, 7, 8,

9, and 10 mm respectively. Each of the dies had an L=Dratio of 2.

2.4. Drying of extrudates

Cylindrical rods obtained were dried in a cabinet

dryer at 40± 1 �C to obtain dry extrudates of 5–7%

moisture (Iwe, 1997).

2.5. Preparation of extrudates for analysis

Preparation of extrudates for analysis followed the

reports of Iwe (1997) and Iwe et al. (1998). The dry

extrudates were coarsely ground in the Condiux toothmill and milled to pass through a 1 mm sieve in a Retsch

type ZM1 mill (Retsch B.V., The Netherlands). Reco-

vered materials were used for analyses.

2.6. Proximate composition

Proximate composition, of selected raw samples, such

as moisture, crude protein, ash, ether extract, carbohy-

drate were obtained by official methods of analysis

(AOAC, 1984). Energy was calculated based on Atwater

factors (Osborne & Voogt, 1978).

2.7. Available lysine

Available lysine was determined according to Vigo

et al. (1992) and Morales et al. (1995), with some

modifications. Homogeneous solution was obtainedfrom samples intended for use in protein digestibility

index assay (American Oil Chemist� Society, 1979). A1.5 ml sample of the solution was diluted with 5 ml of

distilled water, and 0.5 ml of the solution mixed with 1.5

ml of 16% sodium dodecyl sulphate (SDS) and stored

overnight at 4 �C in the refrigerator. Then 1.5 ml of

freshly prepared o-phthaldialdehyde (OPA) solution was

mixed with 50 ll of the sample solution and incubated at25 �C for 2 min, and the intensity read in a Perkin Elmer

Luminescence LS 50B Spectrometer (Perkin Elmer Ltd.,

UK). Relative fluorescence (RF) was read at 340 nm

(extinction) and 455 nm (emission) with error value of

2.5%.

A calibration curve was made using soy protein iso-

late (Purina Protein Europe) which contained 6.3 g/100

g of lysine. Percentage available/unavailable lysine wascalculated based on Eq. (1), taking into account that the

RF value of the unheated sample means 100% available

lysine (Morales et al., 1995):

Table 1

Experimental design for the response surface analysis (upper level: 10

maximum number of experiment: 23)

Run Cause variables

1 2 3

1 ) ) )2 + ) )3 ) + )4 + + )5 ) ) +

6 + ) +

7 ) + +

8 + + +

9 )a 0 0

10 +a 0 0

11 0 )a 0

12 0 +a 0

13 0 0 )a14 0 0 +a

15 0 0 0

Runs 1–8 are to be performed one time, runs 9–14 are to be performed

one time, run 15 is to be performed nine times.

The experimental design is exact rotatable and nearly orthogonal and

requires 23 experiments of which one is to be performed at each cube

point, one at each axial point and nine at each center point. The above

is the essential part of the design. Each row states the adjustment levels

of the factors at one run.

M.O. Iwe et al. / Journal of Food Engineering 62 (2004) 143–150 145

%Unavailablelysine ¼ 100� FRs

FRb

� �� 100 ð1Þ

where FRs is the sample fluorescence, and FRb is un-

processed sample fluorescence.

2.8. Browning index

Browning index (BI), was determined according to

Palombo, Gertler, and Saguy (1984) on 1 g sample. The

optical density of centrifuged, clear filtrate was read on aCecil (CE 2020, Cecil Instruments, Cambridge England)

spectrophotometer at 420 and 550 nm. Water was used

as blank and browning index, was calculated as:

BI ¼ A420 nm � A550 nm ð2Þ

For practical purposes BI was expressed as OD/g dry

solids (Palombo et al., 1984).

Table 2

Proximate composition of raw samples of soy and sweet potato flours

Feed comp (% swt pot) Protein (%) Fat (%)

100 4.55 0.5

75 16.57 0.5

50 28.83 0.5

25 40.12 0.5

0 50.92 0.5

Feed comp¼ feed composition; swt pot¼ sweet potato.

Carb.¼ carbohydrate.

Carbohydrate calculated by difference, and Energy by Atwater factor.

2.9. Experimental design and statistical analysis

A second-order central composite exact rotatable and

nearly orthogonal design was developed on a CA-

DEMO package (Rasch, Nurnberg, & Williams, 1993),

as shown in Table 1. Effects of the process variables

(feed composition (fc), screw speed (ss) and die diameter

(dd)), on the response variables: available lysine and BI

were investigated according to Myers (1976). TheRSREG procedure (SAS, 1990) was used in fitting the

model (Iwe et al., 1998).

The process was optimized for maximum values of

available lysine and minimum browning.

Plots of the fitted response surfaces were made as

contained in an earlier report (Iwe et al., 1998).

3. Results and discussion

3.1. Proximate composition of raw samples

The proximate composition (dry weight basis) of the

raw soy and sweet potato samples used in this research isshown in Table 2. The protein content of the soy flour

was about 11 times more than the sweet potato flour,

hence mixing soy flour with sweet potato flour contri-

buted about 72% protein in the raw mixture. As ex-

pected, the addition of soy flour raised the nutritional

status of sweet potato, as for other low protein foods

(Asp & Bjorck, 1989).

3.2. Available lysine

The percentage values of available lysine in the pro-

cessed samples ranged from 68% to 100%. It was ob-

served that there is a limit within which soy addition

affected lysine availability. The estimated regression

coefficients and ANOVA of the response function,

available lysine, in terms of the studied variables are

shown in Table 3.

Results indicated that both linear and cross producteffects of screw speed and feed composition were sig-

nificant (p6 0:10). A re-computation of the responses

showed that the cross product of screw speed and die

Ash (%) Carb. (%) Energy (kJ)

2.3 80.12 1441.36

3.0 72.15 1509.4

4.0 60.20 1514.6

5.0 48.17 1502.17

7.0 34.85 1497.36

Fig. 1. Effect of screw speed and die diameter on available lysine

in extrudate.

Table 3

Estimated regression coefficients and ANOVA for available lysine (AL), using the variables: feed composition (% sweet potato) (fc), screw speed (ss)

and die diameter (dd)

Source Coefficient Std error df P value

Regression coefficients:

Regression on constant 133.6265 117.3687

fc 0.120110 1.310474 1 0.2771

ss )0.080358 0.872322 1 0.3751

dd )14.3197 13.89824 1 0.3232

fc*fc )0.00629 0.006034 1 0.3171

ss*fc 0.012070 0.005153 1 0.0372

ss*ss )0.00393 0.002682 1 0.1682

dd*fc 0.044328 0.091730 1 0.6376

dd*ss 0.124134 0.061325 1 0.0658

dd*dd )0.12009 0.603399 1 0.8456

R2 0.9390

ANOVA and canonical value (CV)

Factor df Sum of squares F value CV

fc 4 1285.6902 0.0000 98.49

ss 4 475.9105 0.0003 84.55

dd 4 56.4005 0.2691 2.26

146 M.O. Iwe et al. / Journal of Food Engineering 62 (2004) 143–150

diameter was also significant (Eq. (3)). The resulting

polynomial, after removal of the non-significant terms

and re-computation, becomes:

AL ¼ 198:58� 0:53fc� 1:67ss� 12:69dd

þ 0:01fcssþ 0:12ssdd ð3Þ

Feed composition showed a high significant effect, fol-lowed by screw speed. The cross product effect of screw

speed and die diameter was however higher than that of

feed composition and die diameter (Eq. (3)).

The response surface for these values is shown in Fig

1. It was practically observed that the increase in the

level of sweet potato increased lysine retention. Higher

screw speed also enhanced lysine retention, especially at

increasing levels of sweet potato in the mixtures (figure,not shown).

High retention of lysine at increasing sweet potato

levels could be attributed to the lower levels of lysine in

the sweet potato raw material, since the losses were more

pronounced at increasing levels of soy addition (Table

4), which apparently has higher lysine content (Iwe, van

Zuilichem, Ngoddy, & Lammers, 2001).Increase in screw speed increased lysine retention,

owing possibly to reduced residence time of the feed

mixture in the extruder, since operating temperature was

more or less kept constant. Olkku, Antila, Heikkinen,

and Linko (1980) and Bartels, Janssen, and van Zuili-

chem (1982) had reported that an increase in screw

speed could either reduce residence time or have no ef-

fect, and hence influence lysine retention. However, Aspand Bjorck (1981) found a correlation between lysine

loss and screw speed, owing possibly to indirect effect of

starch hydrolysis at higher shear. Even though increased

shear leads to more severe conditions, the corresponding

reduction in residence time, as a result of increase in

screw speed, limits the duration of heat treatment and

resulting in high lysine retention (Table 4). The apparent

positive effect of increasing screw speed due to reducedresidence time are available in several reports (Asp &

Bjorck, 1989; Bounie & Cheftel, 1986; Noguchi et al.,

1982; Pham & Del Rosario, 1984b). The effect of screw

speed in the present research could therefore be ex-

plained in line with already published reports.

Canonical analysis of the response surface showed

that the critical values of the independent variables were

estimated at 98.49% sweet potato level, 84.55 rpm and2.26 mm die diameter.

It is observed in Fig. 1 that there was a slight increase

in lysine retention with smaller die diameter and lower

screw speed. This effect could not be explained as Tsao,

Frey, and Harper (1978) had reported that increase in

die diameter increased lysine retention in single screw

Table 5

Estimated regression coefficients and ANOVA for browning index (BI), using the variables: feed composition (% sweet potato) (fc), screw speed (ss)

and die diameter (dd)

Source Coefficient Std error df P value

Regression coefficients:

Regression on constant 2.206583 0.781832

fc )0.00377 0.008929 1 0.6799

ss )0.01858 0.005811 1 0.0077

dd )0.19889 0.092581 1 0.0528

fc*fc )0.00002 0.000040 1 0.7130

ss*fc )0.00006 0.000034 1 0.0962

ss*ss 0.000099 0.000018 1 0.0001

dd*fc 0.001565 0.000611 1 0.0249

dd*ss 0.000281 0.000409 1 0.3266

dd*dd 0.004111 0.004019 1 0.2288

R2 0.8711

ANOVA and canonical value (CV)

Factor df Sum of squares F value CV

fc 4 0.0047 0.0769 67.37

ss 4 0.0157 0.0012 103.73

dd 4 0.0119 0.0037 7.82

Table 4

A description of the experimental model based on selected process variables and their response (independent) variables

fc swt pot (%) ss RPM dd (mm) BI (OD/g) s (s) AL (%) DG (%) PT (�C)

60 110 8 0.33 30.4 64.4 49.0 115.0

100 110 8 0.31 45.3 95.3 81.6 115.3

80 80 8 0.39 62.5 72.9 81.4 108.7

80 140 8 0.44 43.9 88.7 74.9 117.6

80 110 6 0.29 47.2 83.3 92.9 110.8

80 110 10 0.39 42.7 84.4 97.3 122.0

80 110 8 0.33 48.7 82.0 77.0 113.3

fc¼ feed composition; swt pot¼ sweet potato; ss¼ screw speed; dd¼ die diameter; BI¼ browning index; s¼mean residence time; AL¼ available

lysine; DG¼degree of gelatinization; PT¼ product temperature.

M.O. Iwe et al. / Journal of Food Engineering 62 (2004) 143–150 147

extrusion of lysine-fortified rice. However elimination

of the die effect from the estimated coefficients did not

affect the shape and magnitude of the plot, hence the

resulting polynomials after re-computation becomes:

AL ¼ 97:0253� 0:5329fc� 0:6758ssþ 0:0121fcss ð4Þ

Following the re-computation, lysine retention was

shown to be highly influenced by feed composition and

screw speed. It is also evident that available lysine relatesto the browning of extruded sample, owing to Maillard

reaction. This relationship could be inferred from the

fact that it was those extrusion variables which affected

browning index that affected available lysine also,

however at differing magnitude (Eqs. (3) and (4)).

3.3. Browning index

The BI of the extrudates varied from 0.3050 to 0.4370

OD/g. Browning was observed to be higher at increasing

sweet potato content and screw speed; and decreasing

die diameter (10–16 mm) respectively.

The estimated regression coefficients of the indepen-

dent variables are shown in Table 5. Linear effects of

screw speed and die diameter were significant (p6 0:10).The quadratic effect of screw speed, and cross product

effect of die diameter and feed composition were sig-

nificant (p6 0:10).Resulting polynomial after removing insignificant

terms is:

BI ¼ 1:77� 0:01fc� 0:02ss� 0:10dd þ 0:00ss2

� 0:00fcssþ 0:00fcdd ð5Þ

Eq. (5) accounts for 85.13% of the total variation in

browning index. The response surface for the significant

variables are shown in Figs. 2 and 3.Changing screw speed affected browning index, al-

though higher level of browning was achieved at higher

screw speed and at increasing feed composition (Fig 3).

Fig. 2. Effect of feed composition (% sweet potato) and screw speed

on browning index (BI).

Fig. 3. Effect of feed composition (% sweet potato) and screw speed

on browning index (BI).

148 M.O. Iwe et al. / Journal of Food Engineering 62 (2004) 143–150

Higher screw speed translates into higher shear,

although residence time is reduced. The increase in

browning due to increase in screw speed could be ex-

plained according to Areas (1992) and Mitchell and

Areas (1992) who reported that only degraded poly-

saccharide can take part in the browning reaction. From

a previous report (Iwe et al., 1998), it was shown that

most processed samples were up to 70% gelatinized. Ourresult as shown in Table 4 confirms this. At a high screw

speed, and moderate feed composition and die diameter,

a BI value of 0.44 OD/g was achieved. This value cor-

responded to a mean residence time of 43.89 s, available

lysine of 88.67%, starch damage of 74.89% and product

temperature of 117.6 �C (Table 4).

The increase in browning index at lower levels of

soy addition might be due to the initial color of the rawsweet potato, as explained earlier. It should have been

expected that browning index should be higher athigher soy addition due to the presence of e-amino acids

in the soy. The present situation might have occurred as

a result of the oxidation of polyphenols in the sweet

potato raw material during preliminary processing

(Bouwkamp, 1985; Kays, 1985), which was carried into

the extruder.

In relation to feed composition, die diameter affected

browning (Fig. 3). Smaller die diameter and less sweetpotato significantly raised browning index. This effect

should be directly attributed to die diameter which also

affects residence time and therefore increased interaction

between reducing sugars and amino acids. Berset (1989)

reporting the work of Maga and Cohen (1978) stated

that die opening affected color development because a

large expansion or puffing tends to spread out dark

pigments.Canonical analysis of the response surfaces showed

that the critical values of the independent variables were

estimated at feed composition value of 67.36%, screw

speed of 103.72 rpm, and die diameter of 7.82 mm. The

optimum point was however a saddle point.

4. Conclusion

Available lysine retention during extrusion cooking

of soy–sweet potato mixtures was significantly influ-

enced by screw speed, feed composition and die diame-

ter respectively. Optimum available lysine was estimatedat a feed composition of 98.49%, screw speed of 118.98

rpm, and die diameter of 2.25 mm. Similarly, increase in

feed composition and screw speed and a decrease in die

diameter also increased browning index. Optimum val-

ues of the process variable for a minimum value of

browning index was estimated at feed composition,

screw speed and die diameter values of 67.36%, 103.73

rpm and 7.82 mm.

Acknowledgements

The authors are grateful to Drs. F.J. Morales andA. A. Metwalli, Dairy and Food Physics Department,

Wageningen Agricultural University, The Netherlands,

for their advice on lysine and color measurements; Dr.

G. Gort of the Sub-Department of Mathematics, Wa-

geningen Agricultural University, The Netherlands, for

the generation of the response surfaces, the Nigerian

National Universities Commission/World Bank and the

University of Agriculture Makurdi, Nigeria for pro-viding the sponsorship; and the Department of Food

Science Wageningen Agricultural University, The

Netherlands, for research space and hospitality.

M.O. Iwe et al. / Journal of Food Engineering 62 (2004) 143–150 149

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