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Journal of Food Engineering 80 (2007) 1282–1292
Optimization of oven toasting for improving crispness and other
quality attributes of ready to eat potato-soy snack using
response surface methodology
A. Nath *, P.K. Chattopadhyay
Department of Agricultural and Food Engineering, Post Harvest Technology Centre, Indian Institute of Technology, Kharagpur-721 302, India
Received 28 July 2006; received in revised form 21 September 2006; accepted 27 September 2006
Available online 14 November 2006
Abstract
Ready-to-eat potato-soy snacks were developed with high temperature short time air puffing process followed by oven toasting for
increasing crispness. Oven toasting experiments were conducted with varying temperature (85.86–114.14 �C) and time (12.69–
35.31 min) based on central composite rotatable design. The final product was evaluated in terms of quality attributes such as crispness,
moisture content, ascorbic acid loss, colour (L and DE) values and overall acceptability. The optimum product qualities in terms of crisp-
ness (38.7), moisture content (3.35%, db), ascorbic acid loss (20.87%, db), L value (52.03), DE (8.60) and overall acceptability (7.8) were
obtained at temperature of 104.4 �C and time of 27.9 min.
� 2006 Elsevier Ltd. All rights reserved.
Keywords: Optimization; Oven toasting; Crispness; Snack; Quality
1. Introduction
The simplest potato snack products are made from
doughs of dried potato derivatives and water. These
doughs are mixed to a thick consistency formed by extru-
sion, or sheeting, and cut into small pieces such as tubes
or discs. The moist dough pieces are passed into a frying
bath to be puffed and dehydrated to form crispy products.
Air puffing ideally creates an aerated, porous, snack-like
texture with the added benefits of dehydration. Blending
the puffed products with different flavours and marketing
them in moisture impermeable plastic film pouches pro-
vides enormous opportunities for increasing acceptance
and usage of puffed products (Arya, 1992).
Addesso et al. (1995) investigated production of chip-
like starch based snacks. The moisture content of the
dough sheets is reduced by heating in air, preferably in a
gas-fired oven, to obtain chip-like snacks, such as potato
0260-8774/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2006.09.023
* Corresponding author. Tel.: +91 9732652832.
E-mail address: amitsilchar2004@yahoo.com (A. Nath).
chips and corn chips, having low oil content, a blistered
appearance, and a crisp texture.
Soybean, being a rich source of protein and fat, seems to
be the right substitute for solving the problem of protein-
energy malnutrition. Soybean has been used as a food for
a long time, but only to the 20th century, has it been sub-
jected to a variety of processing technologies. It is a fairly
new crop for Indian consumers and few resources have
been directed toward enhancing utilization of soybean in
the daily diets of people in the country. Blending of potato
flour with soy flour improves the nutritional qualities of the
product.
Saimanohar et al. (2005) prepared a high protein nutri-
tional baked snack food comprised of vegetable sources
as wheat flour, roasted peanut paste, sesame seed, soy-
bean flour and well balanced mixture of vitamins, miner-
als and others. Ingredients dissolved in formula water
after powdering, dehulling as required etc. are mixed to
get homogeneous dough. Dough is sheeted and cut using
circular die of 3.0–4.0 mm dia. It is baked at 180–220 �C
for 4–6 min.
mailto:amitsilchar2004@yahoo.com
A. Nath, P.K. Chattopadhyay / Journal of Food Engineering 80 (2007) 1282–1292 1283
Oven toasting is commonly used in snack food for
increasing crispness of the product. In this process product
loses moisture content and thereby shelf life of the final
product is increased. Oven toasting is generally done for
specific period and with particular temperature with the
help of different types of oven depends upon product char-
acteristics and quantity. Commonly electric oven is used in
small-scale production while light diesel oil (LDO) oper-
ated oven with digital temperature indicator and timer is
used for large scale crispy snack production.
Mukherjee (1997) reported that oven toasting increased
crispness of the dehydrated puffed potato cubes and the
optimum levels of treatment parameters (temperature,
121.21 �C and time 16.55 min) for maximum crispness
(42.12 N/mm) as obtained by applying response surface
methodology.
Various researchers have examined the mechanical
property of crispness in snacks. Bourne, Moyer, and Hand
(1966) studied crispness of potato chips at different mois-
ture content by using the punch test, and observed a
decreasing initial slope from the force–deformation curve
as the water content increased. Bruns and Bourne (1975)
have used instruments to examine crispness and reported
that the initial slope of the force–deformation curve is a
good indicator of crispness. But the mechanical analysis
of potato chips did not produce any useful quantative
information, which might be used as an indicator of crisp-
ness intensity due to its irregular size, shape and curvature.
Anon. (1998) conducted a study on comparison of tex-
tural qualities of crisp samples by bulk compression using
an Ottawa cell. It is reported that, as compression pro-
ceeded, fractures were observed as a series of peaks. The
area under the curve was considered as an indication of
Crispness.
Prince, Chattopadhyay, and Mukherjee (1994) mea-
sured the hardness and crispness of rice-soya crackers using
Instron universal testing machine. It was observed that the
hardness (highest peak of force–deformation curve) and
crispness (steepness of force–deformation curve) of rice-
soya crackers decreased as percentage of soya in the mix
increased. These trends remained sharp up to 30% soya
in the mix, and then slowed down.
To impart crispness to the ready-to-eat (RTE) potato-
soy snack, oven toasting is a necessary processing step.
Although oven toasting is a common processing steps for
various snack foods, no published data are available for
the same. The present study was undertaken to optimize
the oven toasting process in terms of temperature and
retention time for improving the quality parameters of
RTE potato-soy snack namely, crispness, colour, overall
acceptability etc.
2. Materials and methods
Dough with about 37% (w.b.) moisture content was
puffed from a blend of potato flour and 10% soy flour
added in total weight of potato flour with 2% NaCl. Dolly
pasta machine (LaMonferrina Make) was used to prepare a
rectangular shape (15–20 mm length, 10 mm width and
1.5 mm thickness) dimension chips from the dough. These
chips were air puffed in a high temperature short time
(HTST) fluidized bed air puffer specially designed and fab-
ricated for the purpose. The HTST air puffing system con-
sisted of four sections, viz., air supply unit, heating unit,
plenum chamber and whirling bed column (puffing cham-
ber). The chips were air puffed in hot air at 231 �C for
25 s to produce optimally puffed snacks with 3.7-fold
expansion. These snacks were subjected to oven toasting
for different temperature and time for optimization of the
oven toasting process.
Oven toasting experiments were conducted in household
electrical oven (Bajaj India, OTG-2900T, 230 V AC,
2000 W, 50 Hz) with inside chamber (400 · 350 · 300 mm)
having temperature range of 50–300 �C. Ambient tempera-
ture and relative humidity of 30 ± 2 �C and 65 ± 2%, respec-
tively, were recorded during experimentation. The air
temperature inside the oven was measured by 26 gauge
iron-constantan thermocouples connected to a six channel
digital temperature indicator having a range of 0–600 �C
with a least count of 1 �C. Potato-soy puffed sample of
100 g was selected for each oven toasting treatment. The
puffed material just after air puffing was placed over a perfo-
rated tray and placed in single layer inside the oven atdesired
temperature. The time was noted by a stopwatch (least count
0.1 s). As soon as the retention time reached the predeter-
mined level, the materials were taken out from the oven
and kept at room temperature for cooling before being
packed in air tight containers for further analyses. Changes
in crispness, moisture content, ascorbic acid content, colour
(L and DE) values and overall acceptability were measured.
2.1. Moisture content
The moisture content of the samples at every stage of the
process was determined by hot air oven method as
described by Ranganna (1995).
2.2. Crispness measurement
The texture characteristics of puffed RTE potato-soy
snacks in terms of crispness was measured using a Stable
Micro System TA-XT2 texture analyzer (Texture Technol-
ogies Corp., UK) fitted with a 25 mm cylindrical probe.
The studies were conducted at a pre-test speed of
1.0 mm/s, test speed of 0.5 mm/s, distance of 30% strain,
and load cell of 5.0 kg. Crispness was measured in terms
of major positive peaks (Anon., 1998; Cruzycelis, Rooney,
& McDonough, 1996). For measurement of crispness a
macro was developed which counts number of major peaks
obtained in the product during compression. Average val-
ues of five replication were reported. Sensory evaluation
was also carried out only after oven toasting by a panel
of 10 judges for measuring crispness in terms of texture
using standard method (BIS, 1971).
1284 A. Nath, P.K. Chattopadhyay / Journal of Food Engineering 80 (2007) 1282–1292
2.3. Ascorbic acid (AA) loss
The ascorbic acid content of the samples was determined
by visual titration method using 2,6-dichlorophenol-indo-
phenol (Ranganna, 1995). The reagents used were 3%
meta-phosphoric acid (HPO3) and the dye solution con-
taining 50 mg 2,6-dichlorophenol-indophenol in 200 ml
water. First the dye was standardized, by titrating a mix-
ture of HPO3 and ascorbic acid standard (5 ml each) with
the dye solution and an aliquot of 5 ml of the extract was
titrated with dye solution to a pink end-point. The ascorbic
acid content of the sample was calculated by the formula
Ascorbic acid content ðmg=100gÞ
¼ Titre�Dye factor� Volume made up� 100
Aliquot of extract� Volume of sample ð1Þ
2.4. Colour (L value and DE) measurement
The colour (L-value) and colour difference (DE) was
measured using a simple digital imaging method (Yam &
Papadakis, 2004). A high-resolution digital camera (Sony
Cybershot DSC-P40, 4.1 MegaPixel, 2· digital zoom)
was used to measure color by capturing the color image
of the sample under two 40 W florescent light. Once the
color images of the samples were captured, the color was
analyzed quantitatively using Photoshop (Adobe Systems,
2002). Photoshop can display L, a, and b in the Info Palette
and Histogram Window.
The Lightness (L), a, and b in the Histogram Window
are not standard colour values. However, they can be con-
verted to L*, a* and b* values using the formulas
L� ¼ ðLightness=255Þ � 100 ð2Þ
a� ¼ ð240a=255Þ � 120 ð3Þ
b� ¼ ð240b=255Þ � 120 ð4Þ
L, a, and b colour scale was selected for all measurements.
All measurements were replicated thrice and the mean
readings were taken. L-value and colour difference (DE)
parameter as described by Eq. (5) were used to describe
the colour of RTE potato-soy snack:
DE ¼ ½ðL� L�Þ2 þ ða� a�Þ2 þ ðb� b�Þ2�0:5 ð5Þ
DE indicates the degree of overall colour change of a sample
in comparison to colour values of an ideal sample having
colour values of L*, a* and b*. Fresh samples before puffing
treatments were taken as ideal sample in this case having L,
a, and b values of 49.46, 3.72 and 29.36, respectively.
2.5. Evaluation of overall acceptability (OAA)
The overall acceptability of the product was carried out
by the standard method (BIS, 1971). The product was eval-
uated by a panel of 10 judges. The effects of different pro-
cess parameters on overall acceptability score of oven
toasted product were evaluated. The process variables con-
sidered were: temperature (85–115 �C) and time (12–
36 min). A nine point hedonic scale was used in which
the sample scoring 1 was rated as disliked extremely, while
those scoring 9 as liked extremely.
2.6. Experimental design and optimization
The independent variables considered for this investiga-
tion were: temperature (85–115 �C) and time (12–36 min).
The experimental design was applied after selection of the
ranges. Thirteen experiments were performed according
to a second order central composite rotatable design
(CCRD) with two variables and five levels of each variable.
Experiments were randomized in order to minimize the
effects of unexplained variability in the observed responses
due to extraneous factors. The center point in the design
was repeated five times to calculate the repeatability of
the method (Montgomery, 2001). Experiments were con-
ducted according to the CCRD design and response sur-
face methodology (RSM) was applied to the experimental
data using a commercial statistical package, Design Expert
– version 7.0 (Statease Inc., MI, USA).
The following second order polynomial response surface
model (Eq. 6) was fitted to each of the response variable
(Yk) with the independent variables (X)
Y k ¼ bk0 þ
X2
i¼1
bkiX i þ
X2
i¼1
bkiiX 2i þ
X2
i6¼j¼1
bkijX iX j ð6Þ
where bk0, bki, bkii, and bkij are the constant, linear, qua-
dratic and cross-product regression coefficients, respec-
tively and Xi’s are the coded independent variables of X1
and X2.
Regression analysis and analysis of variance (ANOVA)
were conducted for fitting the models represented by Eq.
(6) and to examine the statistical significance of the model
terms. The adequacy of the models were determined using
model analysis, lack-of fit test and R2 (coefficient of deter-
mination) analysis as outline by Lee, Ye, Landen, and
Eitenmiller (2000) and Weng, Liu, and Lin (2001). The lack
of fit is a measure of the failure of a model to represent data
in the experimental domain at which points were not
included in the regression or variations in the models can-
not be accounted for by random error (Montgomery,
2001). If there is a significant lack of fit, as indicated by a
low probability value, the response predictor is discarded.
The R2 is defined as the ratio of the explained variation
to the total variation and is a measure of the degree of fit
(Haber & Runyon, 1977). Coefficient of variation (CV)
indicates the relative dispersion of the experimental points
from the prediction of the model. Response surfaces and
contour plots were generated with the help of commercial
statistical package, Design Expert – version 6.0.4 (Design
Expert, 2002). The numerical and graphical optimizations
were also performed by the same software.
Numerical optimization technique of the Design-Expert
software was used for simultaneous optimization of the
A. Nath, P.K. Chattopadhyay / Journal of Food Engineering 80 (2007) 1282–1292 1285
multiple responses. The desired goals for each variables
and response were chosen. All the independents variables
were kept within range while the responses were either
maximized or minimized. In order to search a solution
maximizing multiple responses, the goals are combined
into an overall composite function, D(x), called the desir-
ability function (Myers & Montgomery, 2002), which is
defined as:
DðxÞ ¼ ðd1 � d2 � . . . :� dnÞ
1
n ð7Þ
where d1, d2. . .dn are responses and n is the total number of
responses in the measure.
The numerical optimization finds a point that maximizes
the desirability function. The characteristics of a goal may
be altered by adjusting the weight or importance (Design
Expert, 2002).
3. Results and discussion
Response surface analysis was applied to the experimen-
tal data (Table 1) and the second order polynomial
response surface model (Eq. 6) was fitted to each of the
Table 1
Treatment combinations for oven toasting with 2 variable 2nd order RSM de
Experiment Number Process variables Response
Temperature (�C) Time (min) Cr peaks
1 110.00(1) 32.00(1) 45
2 110.00(1) 16.00(�1)33
3 90.00(�1) 32.00(1) 39
4 90.00(�1) 16.00(�1) 29
5 114.14(1.414) 24.00(0) 41
6 85.86(�1.414) 24.00(0) 32
7 100.00(0) 35.31(1.414) 43
8 100.00(0) 12.69(�1.414) 30
9 100.00(0) 24.00(0) 35
10 100.00(0) 24.00(0) 34
11 100.00(0) 24.00(0) 35
12 100.00(0) 24.00(0) 34
13 100.00(0) 24.00(0) 34
Figure in the parentheses denote coded level of variables: Cr, crispness; MC,
acceptability.
Table 2
ANOVA and regression coefficients of the second-order polynomial model fo
Variables DF Estimated coefficients
Cr MC AA L DE OAA
Model 5 34.40 4.09 12.40 55.94 5.45 7.0
X1 1 2.84 �1.05 8.61 �3.51 2.37 0.2
X2 1 5.05 �1.12 6.77 �3.06 1.92 0.4
X1X2 1 0.50 0.26 �0.42 �1.96 2.40 �0.1
X 21 1 1.05 0.49 5.39 �0.74 1.29 �0.4
X 22 1 1.05 0.52 1.52 �1.21 1.60 �0.2
Lack of fit 3
R2 0.99 0.94 0.95 0.96 0.93 0.9
Adj R2 0.98 0.90 0.92 0.94 0.88 0.8
CV% 2.05 9.62 3.830 1.92 15.87 3.3
* Significant at p < 0.05.
** Significant at p < 0.01.
*** Significant at p < 0.001.
response variable (Yk). Regression analysis and ANOVA
were conducted for fitting the model and to examine the
statistical significance of the model terms. The estimated
regression coefficients of the quadratic polynomial models
for the response variables, along with the corresponding
R2 and coefficient of variation (CV) values are given in
Table 2. Analysis of variance showed that the models were
highly significant (p 6 0.001) for all the responses (Table 2)
except for overall acceptability which was significant
(p 6 0.01).
The lack of fit (Table 2), which measures the fitness of
the model, did not result in a significant F-value in case
of crispness, moisture content, ascorbic acid loss, colour
(L and DE) values and overall acceptability, indicating that
these models are sufficiently accurate for predicting those
responses. The coefficient of determination (R2) values of
all responses are quite high (>0.91) indicating a high pro-
portion of variability was explained by the data and the
RSM models were adequate (Table 2). As a general rule,
the coefficient of variation (CV) should not be greater than
10%. In this study, the coefficients of variation were less
than 10% for all the responses except degree of brownness
sign
s
MC (%, db) AA loss (%, db) OAA L value DE
3.39 32.26 7.5 46.39 14.19
4.47 18.22 7.0 55.08 6.83
5.06 20.32 7.4 57.46 4.04
7.18 4.59 6.4 58.31 6.30
3.70 38.95 7.3 49.28 11.48
6.56 8.35 6.2 59.00 5.61
3.17 24.53 7.7 47.93 12.79
7.24 7.27 6.3 58.47 5.54
4.16 13.51 8.0 55.70 5.78
4.37 13.78 7.6 55.18 6.32
3.43 12.35 7.6 56.46 4.61
4.19 9.00 7.6 57.25 4.40
4.28 13.37 7.5 55.13 6.13
moisture content; AA, ascorbic acid; DE, colour difference; OAA, overall
r the response variables (in coded units)
F-values
Cr MC AA L DE OAA
6 105.26*** 21.75*** 27.19*** 36.42*** 18.82*** 14.78**
8 120.08*** 43.24*** 69.01*** 89.08*** 34.22*** 12.64**
3 379.14*** 48.87*** 42.77*** 67.64*** 22.38** 30.08***
3 1.86 1.32 0.08 13.92** 17.56** 1.24
1 14.26** 8.02* 23.56** 3.46 8.85* 22.67**
8 14.26** 9.31* 1.87 9.26* 13.60** 10.84*
2.85 2.06 3.78 1.81 2.60 1.76
1
5
8
1286 A. Nath, P.K. Chattopadhyay / Journal of Food Engineering 80 (2007) 1282–1292
where CV was recorded to be 15.87 (Table 2), a relatively
lower value of the coefficient of variation indicates better
precision and reliability of the experiments carried out.
3.1. Crispness
Maximum crispness of 14 (no. of +ve peaks) was
recorded for optimized air puffed product before oven
toasting. The observations for crispness with different com-
binations of temperature and time during oven toasting are
presented in Table 1. It varied from 29 to 45 within the
combination of variables studied. The regression equation
describing the effect of the process variables on crispness
of RTE potato-soy snack in terms of actual level of the
variables are given as:
Cr ¼ 120:3� 1:97T � 0:78t þ 6:3E � 03Tt þ 0:01T 2 þ 0:02t2
ð8Þ
The non-linear regression equation developed for Cr (Eq.
8) was solved for predicting process variables for obtaining
the maximum Cr, using Microsoft Excel (Solver). The pro-
cess variables determined by regression analysis was found
to be at a temperature of 114.14 �C and retention time of
35.31 min and the predicted maximum Cr of 50.75 was
obtained from Eq. 8. It can be observed from ANOVA
(Table 2) that temperature and time is most significant
parameter affecting the crispness (p 6 0.001) at linear level,
while, quadratic terms of temperature and time (p 6 0.01)
however, there was no significant contribution at interac-
tion term. Regression model explained 99% of the total
variability (p 6 0.001) in crispness of RTE potato-
soy snack (Table 2). Fig. 1 shows crispness of RTE
potato-soy snack as a function of temperature and time.
Maximum (46.61) and minimum (28.76) values of crispness
were observed at temperature = 107–115 �C/time = 32–36
min and temperature = 85–97 �C/time = 12–14 min (Fig. 1a
and b) respectively. Objective method showed that oven
toasting increased crispness of air puffed potato-soy snack
by 167.69 percent. Sensory evaluation of the oven toasted po-
85.00 92.50 100.00 107.50 115.00
12.00
18.00
24.00
30.00
36.00
Crispness
Temperature (˚C)
 T
im
e 
(m
in
.)
30.6
33.7
37.9
42.7
46.6
28.8
4
5
 C
ris
pn
es
s 
 
Fig. 1. Contour plots (a) and response surface (b) for the effect of
tato-soy snack had crispness score of 7.4 in nine point hedo-
nic scale, showing it to be highly acceptable crisp snack.
Mukherjee (1997) obtained maximum crispness of
45.12 N/mm (measured with Instron Universal testing
machine) while optimizing RTE dehydrated puffed potato
cubes made from whole potato by oven toasting method
at temperature of 125.21 �C and time of 16.55 min,
Khodke (2002) recorded maximum crispness of 37 during
the production of optimized RTE dehydrated potato
cubes. These findings were in accordance with the present
study. The present findings revealed that both temperature
and time were equally important parameters responsible
for crispness of potato-soy snack. Increase in temperature
and time caused loss of moisture from the product impart-
ing toughness to improve crispness.
3.2. Moisture content
Moisture content of the product before oven toasting
was 11.53% db. Values of moisture content (%, db) during
oven toasting at different combinations of temperature and
time are presented in Table 1. Minimum moisture content
(3.17%, db) was found to be at a temperature of 100 �C and
time duration of 35.31 min while maximum moisture con-
tent (7.24%, db) was recorded at a temperature of 100 �C
and time duration of 12.69 min. The regression equation
describing the effect of the process variables on moisture
content of RTE potato-soy snack in terms of actual level
of the variables are given as:
MC ¼ 79:13� 1:16T � 0:86t þ 3:3E � 03Tt þ 4:9E
� 03T 2 þ 8:2E � 03t2 ð9Þ
The process variables determined by regression analysis for
minimum MC was found to be at a temperature of
108.54 �C and retention time of 30.85 min and the pre-
dicted minimum MC (3.16% db) was calculated from Eq.
9. The ANOVA for moisture content were obtained and
is presented in (Table 2). It can be observed from ANOVA
that temperature and time is most significant parameter
 85.00
 92.50
 100.00
 107.50
 115.00
12.00 
18.00 
24.00 
30.00 
36.00 
28.0 
34.3 
40.5 
6.8 
3.0 
 Temperature (˚C) 
 Time (min.) 
temperature and time on crispness of RTE potato-soy snack.
A. Nath, P.K. Chattopadhyay / Journal of Food Engineering 80 (2007) 1282–1292 1287
affecting the moisture content (p 6 0.001) at linear level,
while, quadratic terms of temperature and time (p 6
0.05) however, there was no significant contribution at
interaction term. Temperature and time duration were
the main variables affecting the moisture content as re-
vealed by the respective regression coefficient and F-value.
The negative coefficients of the first order terms of temper-
ature and time (Table 2) indicated that moisture content
decreases with increase of these variables. Regression mod-
el explained94% of the total variability (p 6 0.001) in
moisture content of RTE potato-soy snack (Table 2).
Fig. 2 shows moisture content of RTE potato-soy snack
as a function of temperature and time. Maximum
(9.03%, db) and minimum (3.28%, db) values of moisture
content were observed at temperature = 85–89 �C/time =
12–15 min and temperature = 103–113.5 �C/time = 27–34
min (Fig. 2a and b) respectively.
Optimum moisture content of 0.0456 kg/kg (d.b.) was
obtained for RTE dehydrated puffed potato cubes at the
optimized temperature and time combination by oven
toasting method (Mukherjee, 1997). This finding was in
accordance with the present study. Both the temperature
and time for oven toasting were responsible for bring down
moisture content of RTE potato-soy snack to the desired
level, which is but obvious.
3.3. Ascorbic acid loss
Loss of ascorbic acid (%, db) values during oven toast-
ing at different combinations of temperature and time are
presented in Table 1. Before oven toasting, the RTE
potato-soy snack contains ascorbic acid content of
29.48 mg/100 g, d.b. During oven toasting, highest loss
was observed to be 38.95%, d.b. at a temperature of
114.14 �C and time duration of 24.0 min while minimum
loss was 4.59%, d.b. at a temperature of 90 �C and time
duration of 16.0 min, respectively.
The regression equation describing the effect of the pro-
cess variables on ascorbic acid loss of RTE potato-soy
snack in terms of actual level of the variables are given as:
85.00 92.50 100.00 107.50 115.00
12.00
18.00
24.00
30.00
36.00
Mc (%)
 Temperature (˚C)
T
im
e 
(m
in
.)
4.33
5.37
6.54
7.61
9.03
3.28
3.61
Fig. 2. Contour plots (a) and response surface (b) for the effect of tempera
AA¼ 446:23�9:98T þ0:24t�5:3E�03Ttþ0:05T 2þ0:02t2
ð10Þ
Eq. 10 was solved for predicting process variables for
obtaining the minimum AA loss during oven toasting of
potato-soy snack. The minimum (1.93% db) AA loss was
estimated at the process condition of temperature
(91.47 �C) and retention time (12.69 min). The ANOVA
for ascorbic acid loss were obtained and is presented in
(Table 2). It can be observed from ANOVA that tempera-
ture and time is most significant parameter affecting the
ascorbic acid loss (p 6 0.001) at linear level, while, qua-
dratic terms of temperature (p 6 0.01) however, there was
no significant contribution at interaction term. Tempera-
ture and time duration were the main variables affecting
the ascorbic acid loss as revealed by the respective regres-
sion coefficient and F-value. The positive coefficients of
the first order terms of temperature and time (Table 2) indi-
cated that ascorbic acid loss increases with increase of these
variables. Regression model explained 95% of the total var-
iability (p 6 0.001) in ascorbic acid loss of RTE potato-soy
snack (Table 2). Fig. 3 shows ascorbic acid loss of RTE po-
tato-soy snack as a function of temperature and time. Max-
imum (42.00%, db) and minimum (3.29%, db) values of
ascorbic acid loss were observed at temperature = 111–
115 �C/time = 29-36 min and temperature = 86–98 �C/
time = 12–16 min (Fig. 3a and b) respectively.
Mukherjee (1997) reported maximum ascorbic acid loss
of 24.8 (%,d.b.) for RTE dehydrated puffed potato cubes at
the optimized temperature and time combination by oven
toasting method. Haase and Weber (2003) and Laing
et al. (1978) also observed degradation of ascorbic acid
during processing of French fries and potato chips. During
processing total losses of AA were about 52% for French
fries and about 26% for potato chips. These findings were
in accordance with the present study. Loss of vitamin C
during processing depends on the degree of heating, leach-
ing into the cooking medium, surface area exposed to oxy-
gen and any other factors that facilitate oxidation
(Eitenmiller & Laden, 1999).
 85.00
 92.50
 100.00
 107.50
 115.00
12.00 
18.00 
24.00 
30.00 
36.00 
3.10 
4.90 
6.70 
8.50 
10.30 
 M
c 
(%
) 
 
 Temperature (˚C) Time (min.) 
ture and time on moisture content (%, db) of RTE potato-soy snack.
85.00 92.50 100.00 107.50 115.00
12.00
18.00
24.00
30.00
36.00
Ascorbic acid loss (%,db)
 Temperature (˚C)
T
im
e 
(m
in
.)
9.76
17.82
25.88
33.94
42.00
3.29
6.09
 85.00
 92.50
 100.00
 107.50
 115.00
12.00 
18.00 
24.00 
30.00 
36.00 
1.00 
13.50 
26.00 
38.50 
51.00 
 A
sc
or
bi
c 
ac
id
 lo
ss
 (
%
,d
b)
 
 Temperature (˚C) Time (min.) 
Fig. 3. Contour plots (a) and response surface (b) for the effect of temperature and time on ascorbic acid loss (%, db) of RTE potato-soy snack.
1288 A. Nath, P.K. Chattopadhyay / Journal of Food Engineering 80 (2007) 1282–1292
3.4. Colour (L and DE) values
Variation of colour (L and DE) values during oven
toasting at different combinations of temperature and time
are presented in Table 1. Before oven toasting, the opti-
mum L and DE values for RTE potato-soy snack were
recorded as 60.43 and 12.84, respectively. After oven toast-
ing, the maximum values of 59.0 and 14.19 at temperature
of 85.9 �C and 110.0 �C and time duration of 24.0 min and
32.0 min, respectively was recorded for L and DE values
while, the minimum values of 46.39 and 4.04 at tempera-
ture of 110 �C and 90.0 �C and time duration of 32.0 min
and 32.0 min, respectively.
The regression equation (Eqs. 11 and 12) describing the
effect of the process variables on L and DE values of RTE
potato-soy snack in terms of actual level of the variables
are given as:
L¼1:72T þ2:98t�0:02Tt�7:4E�03T 2�0:02t2�43:64
ð11Þ
DE¼191:92�3:07T �3:97tþ0:03Ttþ0:01T 2þ0:03t2 ð12Þ
The maximum L value (59.44) was recorded at temperature
of 85.86 �C and retention time of 23.06 min while, mini-
mum colour difference (4.32) at 88.11 �C and 26.34 min ob-
tained by solving the regression Eqs. 11 and 12 during oven
toasting of potato-soy snack. The ANOVA for L and DE
values were obtained and is presented in (Table 2). It can
be observed from ANOVA that temperature is the most
significant parameter affecting the L and DE values
(p 6 0.001) and time (p 6 0.001 and p 6 0.01, respectively,
for L and DE values) at linear level, while, interaction term
[(temperature)(time), p 6 0.01] and quadratic terms of time
(p 6 0.05) for L values and temperature (p 6 0.05) and time
(p 6 0.01) for DE values. Temperature was the main factor
affecting the L and DE values as revealed by the respective
regression coefficients and F-values. The negative and posi-
tive coefficients of the first order terms of temperature and
time (Table 2) indicated that L values decreases with in-
crease and DE values increases with increase of these vari-
ables, respectively. However, negative and positive
coefficients of their quadratic terms (temperature and time)
and interaction term suggested that increase and decrease
of these variables resulted in decrease and increase of L
and DE values, respectively. Regression model explained
96% and 93% of the total variability (p 6 0.001) in L and
DE values, respectively, for RTE potato-soy snack (Table
2). Figs. 4 and 5 showed L and DE values of RTE pota-
to-soy snack as a function of temperature and time. Max-
imum (58.8 and 17.32) and minimum (47.9 and 8.36) L and
DE values, respectively, were observed at tempera-
ture = 85–91 �C and 109–115 �C/time = 16–30 min and
32–36 min and temperature = 103–115 �C and 85–91 �C/
time = 25–36 min and 12–15 min (Figs. 4a and b and 5a
and b) respectively.
Decrease in L value indicating darker product up to a
certain limit could enhance its quality attributes as reflected
by increased overall acceptability score. The decrease in L
value and increase in DE value was due to non-enzymatic
browning reaction that took place during oven toasting.
These observations are consistent with previous studies
(Chandrasekhar & Chattopadhyay, 1990; Khodke, 2002;
Mukherjee, 1997). Effect of time on degree of brownness
during dehydration of potato was reported by Mishkin,
Saguy, and Karel (1983) who observed that browning
occurredonly after a certain time exposure which was more
than 40 min with the drying air temperature of 80 �C.
These observations supported the present findings regard-
ing the effects of process variables (temperature and time)
on the colour of RTE potato-soy snack.
3.5. Overall acceptability
Overall acceptability score values during oven toasting
at different combinations of temperature and time are pre-
sented in Table 1. During oven toasting, maximum overall
acceptability was observed to be 8.0 at a temperature of
100.0 �C and time duration of 24.0 min while minimum
was 6.2 at a temperature of 85.86 �C and time duration
of 24 min, respectively.
85.00 92.50 100.00 107.50 115.00
12.00
18.00
24.00
30.00
36.00
Color difference
Temperature (˚C)
 T
im
e 
(m
in
.)
8.36
8.36
10.82
14.07
17.32
4.53
5.30
6.32
6.32
 85.00
 92.50
 100.00
 107.50
 115.00
12.00 
18.00 
24.00 
30.00 
36.00 
4.00 
9.00 
14.00 
19.00 
24.00 
 C
ol
or
 d
iff
er
en
ce
 
 Temperature (˚C) Time (min.) 
Fig. 5. Contour plots (a) and response surface (b) for the effect of temperature and time on colour difference (DE) of RTE potato-soy snack.
85.00 92.50 100.00 107.50 115.00
12.00
18.00
24.00
30.00
36.00
Color (L-value) 
Temperature (˚C)
 T
im
e 
(m
in
.)
51.1
54.0
56.3
57.9
57.9
58.5
58.8
47.9
 85.00
 92.50
 100.00
 107.50
 115.00
12.00 
18.00 
24.00 
30.00 
36.00 
37.0 
42.8 
48.5 
54.3 
60.0 
 C
ol
or
 (
L-
va
lu
e)
 
 Temperature (˚C) Time (min.) 
Fig. 4. Contour plots (a) and response surface (b) for the effect of temperature and time on colour (L value) of RTE potato-soy snack.
A. Nath, P.K. Chattopadhyay / Journal of Food Engineering 80 (2007) 1282–1292 1289
The regression equation describing the effect of the pro-
cess variables on overall acceptability score of RTE potato-
soy snack in terms of actual level of the variables are given
as:
OAA ¼ 0:88T þ 0:42t � 1:6E � 03Tt � 4:1E � 03T 2
� 4:4E � 03t2 � 43:23 ð13Þ
The non-linear regression equation developed for OAA
(Eq. 13) was solved for predicting process variables for
obtaining the maximum OAA, using Microsoft Excel (Sol-
ver). The predicted maximum OAA score (7.85) was deter-
mined by regression analysis at temperature of 102.36 �C
and retention time of 29.79 min The ANOVA for overall
acceptability score were obtained and is presented in (Table
2). It can be observed from ANOVA that time is most sig-
nificant parameter affecting the overall acceptability score
(p 6 0.001) followed by temperature (p 6 0.01) at linear le-
vel, while, quadratic terms of temperature (p 6 0.01) how-
ever, there was no significant contribution at interaction
term. Time was the main variable affecting the overall
acceptability score as revealed by the respective regression
coefficient and F-value. The positive coefficients of the first
order terms of temperature and time (Table 2) indicated
that overall acceptability score increases with increase of
these variables while negative coefficients of their quadratic
term (temperature and time) suggested that excessive in-
crease of these variables resulted in decrease of overall
acceptability score. Regression model explained 91% of
the total variability (p 6 0.01) in overall acceptability score
of RTE potato-soy snack (Table 2). Fig. 6 shows overall
acceptability score of RTE potato-soy snack as a function
of temperature and time. Maximum (7.54) and minimum
(4.98) values of overall acceptability scores were observed
at temperature = 101–105 �C/time = 27–32 min and tem-
perature = 85–88 �C/time = 12–15 min (Fig. 6a and b),
respectively.
The above finding revealed that time was the major
parameter responsible for overall acceptability of potato-
soy snack while temperature had comparatively lesser effect
within the experimental ranges of the variables. Increase in
temperature and time up to certain limit caused sufficient
loss of moisture from the product to produce desired crisp-
ness. With low temperature but more time the product had
85.00 92.50 100.00 107.50 115.00
12.00
18.00
24.00
30.00
36.00
OAA score
Temperature (˚C)
 T
im
e 
(m
in
.)
4.98
5.49
5.97
6.57
7.06
7.54
7.42
 85.00
 92.50
 100.00
 107.50
 115.00
12.00 
18.00 
24.00 
30.00 
36.00 
4.40 
5.20 
6.00 
6.80 
7.60 
 O
A
A
 s
co
re
 
 Temperature (˚C) Time (min.) 
Fig. 6. Contour plots (a) and response surface (b) for the effect of temperature and time on overall acceptability score of RTE potato-soy snack.
1290 A. Nath, P.K. Chattopadhyay / Journal of Food Engineering 80 (2007) 1282–1292
lighter colour due to insufficient browning reaction while
higher temperature beyond a certain limit caused develop-
ment of dark brown colour, burn flavour and deformation
of the tissue as well as the cells along with other biochem-
ical changes.
3.6. Optimization
Numerical and graphical optimization was carried out
for the process parameters for oven toasting for obtaining
the best product. To perform this operation, Design-Expert
program (V 6.0.4) of the STAT-EASE software was uti-
lized (Design Expert, 2002) and was used for simultaneous
optimization of the multiple responses. The desired goals
for each variable and response were chosen and different
weights were assigned to each goal to adjust the shape of
its particular desirability function. Table 3 shows software
generated three optimum conditions of independent vari-
ables with the predicted values of responses. Solution
no.1, having the maximum desirability value was selected
Table 3
Solutions for optimum conditions
S. No Process variables Responses
Temperature (�C) Time (min) Cr Mc (%, d
1 104.44 27.91 38.70 3.35
2 102.72 29.14 37.21 3.53
3 105.36 26.54 39.29 3.29
Table 4
Comparison of experimental with predicted values
Response Predicted value Actual value ± SD Standa
Cr 38.70 35.0 ± 3.54 1.58
MC (%) db 3.35 3.54 ± 0.114 0.05
AA (%) loss 20.87 21.22 ± 0.581 0.26
L value 52.03 51.38 ± 0.661 0.29
DE 8.60 8.58 ± 0.259 0.12
OAA 7.82 7.6 ± 0.548 0.24
as the optimum conditions of oven toasting for developing
RTE potato-soy snack.
3.6.1. Verification of the model
Oven toasting experiments were conducted at the opti-
mum process condition and the quality attributes of the
resulting product were determined. The observed experi-
mental values (mean of 5 measurements) and values pre-
dicted by the equations of the model are presented in
Table 4. One sample T-test was conducted using the statis-
tical software ‘SPSS’ to compare the mean actual values of
the responses with the predicted value. The null hypothesis
was tested and there was no significant difference recorded
between the actual and the predicted values (test value). No
significant difference between the actual and predicted val-
ues were found except for moisture content in which case
there was a significant difference at p 6 0.05. Closeness
between the experimental and predicted values of the qual-
ity parameters indicated the suitability of the correspond-
ing models.
Desirability
b) AA (%) L value DE OAA
20.87 52.03 8.60 7.82 0.962
21.68 53.51 7.29 7.81 0.954
22.40 51.37 9.21 7.81 0.917
rd error % Variation Mean difference Sig. (2 tailed)
10.57 �3.7 0.079
5.37 0.19 0.020
1.65 0.35 0.249
1.27 �0.65 0.093
0.23 �0.02 0.871
2.89 �0.22 0.421
85.00 92.50 100.00 107.50 115.00
12.00
18.00
24.00
30.00
36.00
 Temperature (˚C)
T
im
e 
(m
in
.)
Cr: 37
Mc (%): 3.30
AA: 20.00
OAA: 7.80
L : 51.4769
Db: 8.5
Db: 8.5
X1 99.74
X2 32.63
85.00 92.50 100.00 107.50 115.00
12.00
18.00
24.00
30.00
36.00
Temperature (˚C)
T
im
e 
(m
in
.)
Cr: 37
Mc (%): 3.30
AA: 20.00
OAA: 7.80
L : 51.4769
Db: 8.5
Db: 8.5
X1 99.24
X2 31.97
85.00 92.50 100.00 107.50 115.00
12.00
18.00
24.00
30.00
36.00
 Temperature (˚C)
T
im
e 
(m
in
.)
Cr: 37
Mc (%): 3.30
AA: 20.00
OAA: 7.80
L : 51.4769
Db: 8.5
Db: 8.5
X1 105.12
X2 27.21
85.00 92.50 100.00 107.50 115.00
12.00
18.00
24.00
30.0036.00
Temperature (˚C)
T
im
e 
(m
in
.)
Cr: 37
Mc (%): 3.30
AA: 20.00
OAA: 7.80
L : 51.4769
Db: 8.5
Db: 8.5
X1 105.61
X2 27.92
Fig. 7. Regions of best combinations of oven toasting process variables for production of optimized RTE potato-soy snack.
A. Nath, P.K. Chattopadhyay / Journal of Food Engineering 80 (2007) 1282–1292 1291
3.6.2. Graphical optimization
Fig. 1a–6a presents the effect of temperature and time
on crispness, moisture content, ascorbic acid loss, colour
(L and DE) values and overall acceptability scores, respec-
tively. Superposition of these contour plots was carried
out to obtain Fig. 7, which was utilized to determine
the best combination of oven toasting process for devel-
oping RTE potato-soy snack. Variables obtained from
the superimposed contours are as follows: temperature =
99.74 �C/time = 32.56 min, temperature = 99.24 �C/time =
31.96 min, temperature = 105.61 �C/time = 27.88 min and
temperature = 105.12 �C/time = 27.23 min, respectively
(Fig. 7a–d). The optimum ranges obtained are temperature
Table 5
Quality changes during oven toasting
Response Before oven toasting ± SD After oven toasting ± SD
Cr 13.20 ± 0.422 35.0 ± 0.817
MC (%) db 11.53 ± 0.106 3.54 ± 0.052
AA (%) loss 29.66 ± 0.108 24.26 ± 0.165
L value 59.87 ± 0.368 51.38 ± 0.187
DE 12.52 ± 0.079 8.58 ± 0.103
OAA 7.2 ± 0.094 7.6 ± 0.133
SD (standard deviation).
(�C): 99.24–105.61 �C and time (min): 27.23–32.56 min.
The optimum combination of temperature and time of
oven toasting was derived averaging those values: temper-
ature = 102.43 �C and time = 29.91 min for maximum
crispness, minimum moisture content, minimum ascorbic
acid loss, maximum L values and maximum over all
acceptability.
3.7. Quality changes during oven toasting
Different quality changes of RTE potato-soy snack
occurred due to oven toasting (Table 5). Oven toasting
caused significant (p 6 0.01) changes in crispness, moisture
Standard error % Variation Mean difference Sig. (2 tailed)
0.249 167.69 21.8 0.000
0.035 �69.30 7.99 0.000
0.061 �18.21 5.40 0.000
0.090 �14.18 8.49 0.000
0.045 �31.15 3.94 0.000
0.058 5.56 0.40 0.000
1292 A. Nath, P.K. Chattopadhyay / Journal of Food Engineering 80 (2007) 1282–1292
content, ascorbic acid content, colour and overall accept-
ability of potato-soy snacks. Oven toasting increased crisp-
ness (167.69%) and overall acceptability (5.56%) of the
snack whereas, ascorbic acid content (18.21%), moisture
content (69.3%), L value (14.18%) and colour difference
(31.15%) decreased due to oven toasting.
4. Conclusions
Central composite rotatable design was found suitable
for the process optimization of oven toasting process vari-
ables viz. temperature and time for developing ready-to-eat
(RTE) potato-soy snack. The second order polynomial
models for crispness, moisture content, ascorbic acid loss,
colour (L and DE) values and overall acceptability
obtained using Design Expert – version 6.0.4 were found
to be statistically significant. The process conditions were
optimized by numerical and graphical optimization meth-
ods and the optimum product qualities in terms of crisp-
ness (38.7), moisture content (3.35%, db), ascorbic acid
loss (20.87%, db), L value (52.03), DE (8.60) and overall
acceptability (7.8) were obtained at temperature of
104.4 �C and time of 27.9 min. Oven toasting of potato-
soy snack at the optimum process condition could signifi-
cantly improve crispness, colour and overall acceptability
of the air puffed potato-soy snack with minimum ascorbic
acid loss.
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	Optimization of oven toasting for improving crispness and other quality attributes of ready to eat potato-soy snack using response surface methodology
	Introduction
	Materials and methods
	Moisture content
	Crispness measurement
	Ascorbic acid (AA) loss
	Colour (L value and Delta E) measurement
	Evaluation of overall acceptability (OAA)
	Experimental design and optimization
	Results and discussion
	Crispness
	Moisture content
	Ascorbic acid loss
	Colour (L and Delta E) values
	Overall acceptability
	Optimization
	Verification of the model
	Graphical optimizationQuality changes during oven toasting
	Conclusions
	References