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1 Original article is available at spinger website doi: 10.1007/s11947-013-1220-7. Please cite this article as Joshi, N. D., Mohapatra, D., Joshi, D.C. & Sutar, R.F. 2014. Puffing Characteristics of Parboiled Milled Rice in A Domestic Convective-Microwave Oven and Process Optimization. Food and Bioprocess Technology,7(6):1678-1688.
Puffing Characteristics of Parboiled Milled Rice in A Domestic Convective-Microwave
Oven and Process Optimization
Nirav D. Joshi1, 2
, Debabandya Mohapatra1,*
, Dinesh C. Joshi
1, R.F. Sutar
1
1 College of Food Processing Technology & Bio-energy,
Anand Agricultural University, Anand-388110, Gujarat, India,
2 Sri G.N. Patel Dairy Science and Food Technology College
Sardarkrushinagar Dantiwada Agricultural University,
Sardarkrushinagar-385506, Dantiwada, Gujarat
*Current address of Corresponding Author,
Dr. Debabandya Mohapatra,
Senior Scientist, Agricultural Produce Processing Division,
Central Institute of Agricultural Engineering, Bhopal, Madhya Pradesh,
E-mail: [email protected]
2
Introduction:
Puffed rice, a whole grain puffed product from pre-gelatinized milled rice is one of the
convenient rice based popular product consumed in the most rice growing countries. It is a major
source of carbohydrate (89%), to some extent protein (5-9%), and other beneficial nutrients i.e.
dietary fiber, vitamins, minerals and phytochemicals, which have been known to have health
benefits (Houston et al., 1970; FDA 2006; Maisont and Narkrugsa 2009). Rice puffing includes
parboiling, drying, milling and roasting processes. Traditionally paddy is parboiled or gelatinized
by hydrothermal treatment before undergoing puffing. The hydrothermal treatment may include
only hot water soaking or hot water soaking and steaming or sand roasting of moistened grains
(Hoke et al. 2007). During this hydrothermal process/ gelatinization, the internal cracks of the
grains are sealed; leaving no cracks and the rice kernel acquires hardness (Juliano, 1985). The
rice kernel with hardened outer layer then acts like a miniature pressure vessel for the moisture
trapped inside the grains on application of heat (Hoke et al., 2005). During parboiling process the
grains usually attained about 30% (wb) moisture content, which is reduced to about 9-10% (wb)
moisture content (wb) (Hoke et al. 2005) by drying. The dried grains are then dehusked to
remove the outer husk and at least 6% bran was removed, before subjected to puffing (Hoke et
al. 2005). The puffing volume of the rice depends on many factors such as variety, pretreatments,
moisture content, salt concentration, puffing methods etc. (Chinnaswamy and Bhattacharya
1983a,b; Murugsan and Bhattacharya 1986; Chandrasekhar and Chattopadhyay 1991; Hoke et
al. 2005, 2007; Joshi et al. 2013). Expansion of rice plays a major role in consumer acceptance of
the product. A higher amylose contents for longer varieties are responsible for producing higher
expansion ratio of rice (Chinnaswamy and Bhattacharya, 1983b). It was observed from a study
by Murugsan and Bhattacharya (1986) that pre-drying the paddy to 9% (wb) moisture content
3
before adjusting it to the puffing–optimum moisture content of 14% (wb), puffing expansion
increased by as much as 30%. Chinnaswamy and Bhattacharya (1983a) found that a minimum of
6% degree of milling of rice is necessary to promote optimal expansion, apparently due to the
removal of the barrier generated by the bran layer. Above this value, the continued milling had
no appreciable effect. The findings were supported by Chandrasekhar and Chattopadhyay (1991)
and similar results from several researchers have been summarized by Hoke et al. (2005).
Generally, rice grains are puffed with (i) hot sand roasting (ii) frying in hot oil, (iii) hot
air, and (iv) gun puffing methods (Chandrasekhar and Chattopadhyay 1989; Hoke et al. 2005;
Solís-Morales et al. 2009). In the roasting process, grains are fed to the rotating drum containing
heated sand or about 250C (Chinnaswamy and Bhattacharya 1983a). Due to sudden thermal
gradient, the moisture in the rice grains evaporates and escapes out, causing starch to expand and
the rice to puff. In oil puffing method, grains are deep fried in hot oil (200-220C). In hot air
method, grains are subjected to hot air (250-300C) for the sudden expansion of moistened starch
(Hoke et al. 2005). In gun puffing method the rice samples were fed to a rotating drum after the
hydrothermal treatment and the drum outlet is closed and then it is heated. Due to heating super
heated steam is generated inside the grains, which can go up to 115C and the pressure inside the
drum may reach about 8 bar. On reaching the pressure of about 13 bar, lid of the cylinder is
opened through hydraulic press and the pressure is released causing puffing of grains (Hoke et al.
2005). Roasting has the risk of burning and producing defects, while the oil from frying can be
absorbed by the puffed rice and easily turn rancid on exposure to oxygen after a while during
storage. In the Indian sub-continent, the puffed rice industries mostly use sand roasting method,
which is based on conduction heating. Heating of the sand is achieved by the husk or wood chips
fired furnace that poses serious environmental hazard along with a risk of adherence of silica
4
with the puffed product (Hoke et al. 2005). In comparison, high temperature short time (HTST)
fluidized bed air puffing has better puffing efficiency as the product is uniformly exposed to
heating medium (Chandrasekhar and Chattopadhyay 1989; Brito-De La Fuente and Tovar 1995);
the fuel efficiency however, is limited. Therefore, there is a need of using clean and convenient
energy for processing that addresses the environmental and health issues as well as convenient
cooking. In this light, electromagnetic radiation provides better energy efficiency for thermal
processing. In fact, there is an increasing trend to use microwave (MW) energy for food
processing due to the fact that microwave heating is more efficient than the traditional heating
process with benefits that include: quicker start-up time, faster heating, energy efficiency, space
saving, selective heating and final products with improved nutritive quality (Sumnu 2001;
Maisont and Narkrugsa 2009). These factors make the microwave heating an ideal option for
puffing or popping of grains. Considering the above facts, a study was undertaken to develop a
method for producing puffed rice from parboiled and milled whole rice domestically and to
optimize the process variables.
Material and Methods:
Pre-Treatments
Gurjari variety of paddy samples as selected from previous study (Joshi et al. 2013), sourced
from Rice Research Centre, Nawagam, Anand Agricultural University, India, were cleaned and
graded. Parboiling was performed as a pre-treatment for production of puffed rice, for which
paddy samples were soaked in waterbath (80°C) for 45 min followed by steaming at 1
atmosphere for 10 min in autoclave as a method suggested by Kaddus Miah et al. (2002). The
paddy samples ( 31.5%, wb) then were shade dried in the laboratory (255°C, 65-75% Relative
5
Humidity) till they reach a moisture content of 10% (wb), as suggested from previous research
works (Hoke et al., 2005). Dehusking of the paddy was carried out in a lab scale rubber roll de-
husker. Polishing of rice to a degree of 6 %, was carried out with lab scale abrasion polisher (TM
05, Satake, Japan). Degree of polish denotes the extent of bran removal from the brown rice and
is usually determined by weighing the brown rice and milled rice mass. It is expressed as: (1 -
[weight of milled rice/weight of brown rice]) 100) (Mohapatra and Bal, 2007). The
hydrothermal treatment ensured fully gelatinization of the rice grains which was determined by
alkali spreading test by dipping five parboiled and milled rice grains in 1% KOH solution for
overnight in a petridish; the spread of rice starch indicative of degree of starch gelatinization
(Juliano 1985), was recorded.
The experimentation was divided into two parts; in the first part the puffing characteristics of
parboiled and milled rice at constant moisture content for different preheating temperatures,
microwave power level and residence time, was studied. From this study, the range of residence
time for the optimization study was selected. In the second part, optimization of the process
parameters were carried out, which are delineated in different sub-sections.
Microwave Aided Puffing of Parboiled Samples
Microwave aided puffing experiments were performed with use of convective-MW oven
(Samsung, CF1111TL, Korea). The oven has two modes of heating, one is convective and
another is microwave; so that one can preheat the oven to certain temperature (up to 250 C) and
then can put the product in the oven and subject it to either convective or microwave mode of
heating. The capacity of the oven was 19 litre and diameter of the glass disk was 0.29 m. Puffing
6
kinetics of Gurjari variety of rice was determined by performing preliminary trials to select the
sample size (data not shown), which was based on the critical difference between the different
samples with respect to specific energy consumption. Only head rice was used for puffing. The
dried polished samples having 10% moisture content was sprayed with pre-calculated amount of
2% (NaCl) salt water evenly and conditioned for 24 h with stirring in every 1 h for the first 4 h,
so that the grains at the bottom of the container will not absorb more water. The 2% salt solution
was added to the rice as suggested from the compiled works, reported by Hoke et al (2005).
Since the salt solution was sprayed over the grains in thin layers uniformly, it is assumed that all
the grains attained uniform moisture content. The moisture content and sample size of rice were
kept constant in all experiments at 14 (% wb) and 50 g, respectively, while preheating
temperature of the oven glass disk varied from 180, 200, 220°C, MW power levels were 300,
600, 900 W, and residence time varied in the range of 10-100 s, at 10 s interval. Preheating of the
oven disk was done by operating the oven in convective mode for stipulated time to reach the
desired temperature, which was measured by a infrared thermometer. The samples were then
placed on the heated glass disk and the rest of heating was carried out in microwave mode for the
puffing.
Volume Expansion Ratio:
The volume expansion was meassured by sand replacement method (Chinnaswamy and
Bhattacharya 1983a, Joshi et al., 2013). Initial volume of 50g unpuffed parboiled milled rice was
taken in a 500 ml cylinder and filled it with fine sand and the volume was noted. The same
sample was then puffed in a microwave oven with the process described in the earlier section.
Volume expansion ratio was calculated using following expression:
7
Volume expansion = 𝑉 𝑓𝑖𝑛𝑎𝑙
𝑉 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 (1)
Vfinal = Final volume of puffed rice
Vinitial = Initial volume of unpuffed rice
Puffing Yield
Puffing yield is of importance as it would indicate how many grains were fully puffed during the
microwave heating, as consumer would expect all the grains to puff to the maximum volume.
After puffing the unpuffed grains were handpicked and the weight of total puffed grains were
recorded. The grains were considered fully puffed when they did not have any unpuffed part and
the volume expansion ratio was 4 or more. Grains were considered semi-puffed when some part
of the kernel was puffed and some part still remained unpuffed and the volume expansion ratio
varied between 2 and less than 4. Puffing yield was determined considering fully puffed and
semi puffed grains:
𝑃𝑢𝑓𝑓𝑖𝑛𝑔 𝑦𝑖𝑒𝑙𝑑 = 𝑊𝑓𝑝𝑔 +𝑊𝑠𝑝𝑔
𝑊 × 100 (2)
Wfpg= weight of fully puffed grains
Wspg= weight of semi-puffed grains
Wupg= weight of un-puffed grains
W= total weight of grains after puffing= Wfpg +Wspg+ Wupg
Sensory Evaluation
The sensory evaluation of puffed rice for optimization study was conducted by a panel of 11
members comprising of staff and post graduate students. The panelists were regular consumer of
8
puffed rice, which was available in the market and hence could judge the product prepared in the
domestic-convection-microwave oven. The 46 experimental samples as obtained from the design
of experiments were coded using random three digit numbers and served to the panel members.
They were given written instructions and asked to evaluate the puffed rice for appearance, color,
taste and overall acceptability based on 9 – point hedonic scale, where score 1 represented for
unacceptable quality and score 9 represented excellent quality.
Statistical Design and Optimization
Analysis of variance (ANOVA) was carried out using Microsoft excel. Response surface
methodology was employed to optimize the process parameters. To optimize process parameters
for MW puffing of rice, Box-Behnken design for four parameters and 3 levels was used and the
optimization of process parameter was done using Design Expert 8.0.4.1 (Trial version
StatEase). Four independent variables (Xi), viz., moisture content (m), preheating temperature
(Tph), MW power (W), and residence time (t) were considered as important factors impacting the
quality of end product. Samples were conditioned to requisite moisture content by adding salt
solution, the preheated glass plate temperature was measured by an infrared thermometer
(Raytek, India). Microwave power and residence times were set by the convective-microwave
oven panel. The responses were puffing efficiency (Y1), volume expansion ratio (Y2) and sensory
score (Y3). Four mathematical functions ƒk (1, 2, 3, 4) were assumed to exist for Yk as shown in
eq 3.
Yk =ƒk (m, Tph, W, t) (3)
9
Where, m is the moisture content (%wb), Tph is the preheating temperature (°C), W is the MW
power (Watt), t is the residence time (s). A total of 46 treatments were performed for the
experimental study. Second-order polynomial (SOP) equation was used to approximate the
function ƒk using response surface methodology (RSM) (Myers and Montgomery, 1995).
3 3 32
k k0 ki i kii i kij i j
i 1 i 1 i j 1
Y b b X b X b X X
(4)
(k=0, 1, 2, 3….)
Where, bko, bki, bkii, bkij are constants coefficients and the Xi’s are the coded independent
variables.
All the independent variables were coded by giving numbers from -1 to +1. The objective of
coding the parameters was to use their weighted value, so that the analysis would not be affected
by their individual values. The initial moisture content of the parboiled milled rice samples were
varied between 12-16% (wb), preheating temperature of the glass base plate of the oven varied
between 180 to 220C, microwave power levels were varied between 300 W to 900W and
residence time was varied from 50 to 70 seconds as decided from the puffing characteristics
study; as it was observed that most of the grains puffed in that range. The common formula for
coding the variable is as follows (Mohapatra and Bal 2010):
𝑋 =𝑥− 𝑋𝑚𝑎𝑥 +𝑋𝑚𝑖𝑛 /2
𝑋𝑚𝑎𝑥 −𝑋𝑚𝑖𝑛 /2 (5)
Verification of the Model
The models for which lack of fit was non-significant (p>0.05), were considered for optimization.
Three representative trials were conducted at the optimized conditions. The sensory
10
characteristics, volume expansion ratio, puffing yield were calculated and residuals were
computed to test the goodness of fit of the developed model.
Results and Discussions
Puffing Characteristics of Parboiled Milled Rice
Puffing characteristics of pre-gelatinized (parboiled) Gurjari rice milled to 6% degree of polish,
for different preheating temperatures of the glass disk (180, 200, 220 C), microwave power
level (300, 600 and 900W) and residence time varying from 10-100 seconds for a sample size of
50 g parboiled milled rice, pre-conditioned to 14% (wb) moisture level with 2% salt solution was
studied.
Effect of processing parameters on puffing yield
The effect of preheating temperature, power level and residence time on the puffing yield is
depicted in Fig. 1 (a, b, c). No puffing was observed within 10s of MW exposure, whereas 70%
of the grains were found to be puffed by 30 s exposure at 900W for preheating temperature of
200 and 220°C, however it was more than 40 s for preheating temperature of 180°C. It is evident
from the graphs that with the increase in preheating temperature, puffing yield increased for
same level of residence time for a constant power level. When conditioned grains are subjected
to high thermal gradient, evaporation of moisture takes place instantaneously. The grain acts like
a pressure vessel and the steam generated due to thermal or pressure gradient built enough
pressure to blow up the starch before escaping out (Moraru and Kokini 2003). The puffing
11
process can be explained vis-a-vis corn popping in microwave environment. During MW
exposure, heat is generated mostly due to molecular friction of polar molecules like water and
some amount of ionic conduction (Oliveira and Franca 2002). The volumetric heat generated
inside the grain kernel produces superheated steam, which tries to escape out through diffusion.
The diffusion process however is restricted due to some physical barriers like horney endosperm
in case of corn (Pordesimo et al. 1991; Singh and Singh 1999) and husks in case of
paddy(Maisont & Narkrugsa 2010). As a result enough steam pressure is generated inside the
grain kernel that corresponds to a temperature of 177 °C which is equivalent to 135 psi pressure.
This condition propels the free and bound water to evaporate, resulting in expanded starch
(Hoseney et al. 1983). Though MW energy generates volumetric heat in the grain, it is essential
to reach a minimum temperature (180°C) for the puffing of grains to start. Therefore,
preheating the convective-MW oven before subjecting the parboiled milled rice grains to MW
heating would necessitate the requisite thermal gradient aided by MW energy. Thus, puffing of
parboiled and milled rice would be possible even without physical barrier like thick cuticle or
husk using MW energy aided with convection and conduction heat. It was observed from the
graphs that as the residence time increased for various power levels and preheating temperatures,
the puffing yield increased. Most of the samples achieved at least 80% of efficiency within 60 s,
at higher power level (900 and 600W), for low power level at 300W, however residence time
was much higher to achieve same yield level. Since at high power levels (600-900W) the effect
was non-significant on puffing yield, the residence time was chosen for the optimization process,
considering the two power levels, where minimum 80% puffing was achieved. The reason for
that may be shrinkage of kernel at higher residence time, for lower power level.
12
Pair comparison t-test was carried out to check the significance between the different
power levels and preheating temperatures. It was observed that preheating temperatures had
non-significant effect on the yield for the range tested (180-220C) for t- two tail and t-one tail
test (data not shown). This result implied that by choosing any of the preheating temperatures
considered in the study, there would not be much difference in the puffing yield of rice obtained
in a convective-microwave oven. This phenomenon can be explained as following; since the pre
heating temperature range was chosen considering previous findings (Moraru and Kokini 2003),
where a minimum temperature of 177°C was essential for achieving puffing. In this investigation
significant effect of pre-heating temperature on puffing yield and volume expansion was noticed.
This corroborates the earlier reports of using a temperature higher than 170°C for better puffing
(Chinnaswamy and Bhattacharya 1983a).
Power level was found to have significant (p<0.05) effect on the puffing yield for all
preheating temperatures, when tested between 900 and 300W and 600 and 300W, which implied
that while choosing the operating parameters, power level should be between 600 and 900W.
At lower power level (300W), the rice samples did not get continuous supply of energy as a
requirement for puffing. Similar results were reported by Maisont and Narkrugsa (2010) who had
observed better puffing yield for paddy at higher power level (800W).
One factor ANOVA test was carried out to check the effect of residence time on puffing
yield (Table 1), for different power levels and preheating temperatures. The results implied that
residence time did not have significant effect on puffing yield at higher preheating temperature
(200-220°C) and power levels (600-900W); however, the effect was significant (p<0.05) at
lower preheating temperature and power level. It was found that at a preheating temperature of
180°C and power level of 300W, the effect of residence time was significant (p<0.05). Thus, it
13
can be concluded that puffing is mainly governed by the power level and preheating temperature
rather than the residence time for a conditioned grain, provided the preheating temperature is
near about the puffing/roasting temperature of cereals grains, which is above 170°C
(Chinnaswamy and Bhattacharya 1983a).
Effect of processing parameters on volume expansion
The effect of preheating temperature, MW power level and residence time on the volume
expansion ratio (VER) is delineated in figure 2 (a,b,c). Pair comparison t-test showed that VER
was not affected by both preheating temperature and power level. It may be explained by the fact
that at certain moisture level, the expansion of starch is basically governed by the available
thermal energy. Since required thermal energy for expansion of starch was provided through
preheating and during MW heating, the VER did not show any significant deviation from its
behavior.
The effect of residence time on volume expansion was investigated by a single factor
ANOVA analysis (Table 2). It was observed that residence time had significant effect on the
VER for all levels of preheating temperature and power levels considered. At lower residence
time (10 s) no significant change in VER was noticed. In fact, none of the grains puffed at all,
within 10 s of residence time for all preheating temperature and MW power levels. As the
residence time progressed the grains started puffing, which started only after 10 s of residence
time and by the end of 60 s most of the grains were found to be puffed. This could be due to the
fact that as the residence time progressed the starchy grain exposed to higher microwave energy,
which transformed into thermal energy in the rice kernel, for its expansion. At lower preheating
14
temperature and power level, even though at higher residence time, VER was less. The probable
reason could be the absence of enough thermal gradient and requisite moisture content in the
grains, which was necessary for the starch to expand. At lower preheating temperature (180C)
and power level (300W), parboiled and milled grains experience lesser thermal gradient. Since at
lower thermal gradient super heated steam pressure build up took some time; this provide
opportunity for the moisture to escape out in absence of any barrier. Since microwave heating is
dependent on the mostly on the water content of the product, in the absence of requisite moisture
content in the grains starch expansion was incomplete, which resulted grains with less VER.
Puffing yield and VER are the two major criteria for deciding the effectiveness of puffing in
microwave; this study provided some information of puffing characteristics of parboiled milled
rice in a domestic convective-microwave oven. The range of residence time for better expansion
from this finding was then carried forward for the optimization study.
Optimization of Process Variables for Producing Puffed Rice in a Convective-Microwave Oven
The experiments for puffing of rice were conducted under different combination of process
variables. Variation of responses (puffing yield, VER and sensory score) of puffed rice with
independent variables or process variables are moisture content, pre-heating temperature, power
level and residence time. A complete SOP model was tested for its adequacy to decide the
variation of response with independent variables. To aid visualization of variation in responses
with independent variables, series of three dimensional response surfaces were drawn using
Design Expert Software version 8.0.5.1.
15
It can be observed that puffing yield, expansion ratio of rice and sensory evaluation
values ranged from 25.78 to 100 (%), 2.23 to 5.00, and 2 to 5, respectively, for preheating
temperatures of (180-220°C), microwave power levels of (300-900W) and residence time
varying from 10-100 s, for a moisture level of 14% (wb). ANOVA was conducted on the
experimental data and the significance of moisture content, pre-heating temperature, power level
and residence times as well as their interactions on responses were calculated. The linear,
quadratic and interaction models were fitted to the experimental data and statistical analysis were
calculated for each response and were given in Tables 3. Three dimensional surface graphs were
generated for each of the fitted model as a function of two variables, while keeping third variable
at the central value.
Effect of process variables on puffing yield of rice:
Puffing yield data were collected to obtain efficient combination of process variables. The
puffing yield of Gurjari was 48.98 to 100 for the process variables considered in this
investigation (Fig 3 a). The results showed that, for the selected range of variables, power level
(W) and had highly significant effect at 1% loss of safety (LOS), residence time (t) pre-heating
temperature (Tph) had significant effect at 10% LOS whereas, moisture content (m) had no
significant effect on expansion ratio of rice as shown in Tables 3. Non-significant effect was
observed for all the interactions. The higher terms of moisture content was found to have
negative impact on the puffing yield (Table 3). Similar trend was noticed in case of preheating
temperature and power level. The negative impact of higher terms of moisture content on the
predicted model indicated that at higher mc (16%) there will be depreciation in the puffing yield.
16
The result affirms the earlier reports of Hoke et al. (2005), who have reported the optimum
moisture level for puffing of rice to be 10.5 – 14% (wb). This result also supports the hypothesis
that at higher moisture level, starch structure collapses producing lesser expansion due to
nucleation during corn popping process (Moraru and Kokini 2003).
From this analysis it can be concluded that choosing any of the higher level of preheating
temperature, power level and residence time will not cause any major deviation in the puffing
yield value (48.98 to 100%) regard to this study. Preheating temperature (Tph) above 200°C,
power level above 600W and residence time at least 60 would yield better puffing yield,
considering the range of parameter of this investigation. This confirms the earlier findings
discussed in previous sections. The lack of fit was found to be non-significant which means the
model is adequately fit. Model F-value 31.53 of the value can be explained by this variation. The
regression equations describing the effects of process variables on puffing yield of puffed rice in
terms of coded values of variables are given as:
Puffing yield= 76.56 + 4.94 m + 10.65 Tph + 25.57 P + 9.75 t (6)
Effect of process variables on VER:
The expansion ratio of Gurjari was found to be in the range of 3 to 5, for the experimental range
(Fig 3b). As reported from the previous study (Joshi et al 2013) coarse variety having
intermediate amylose content (20-25%) gives better volume expansion as compared to the other
varieties. The results showed that, for above selected range of variables, moisture content (m)
had highly significant effect at 1% LOS, residence time (t) had significant effect at 10% LOS,
17
whereas pre-heating temperature (Tph) and power level (W) had no significant effect on
expansion ratio of rice as shown in Tables 3. The findings of this study is in agreement with the
study Arimi et al (2008), where they had mentioned that the expansion of imitation cheese in
MW is not always dependent on the starch matrix, but moisture could be the critical factor for
the expansion. Similar studies by Arimi et al (2010) had shown the importance of moisture
content on expansion of starch-protein containing imitation cheese. They had found that the
expansion of imitation cheese in MW was optimum at 14% (wb). The results from this
experimentation also are in agreement reports by various researches on puffing of rice as
compiled by Hoke at al. (2005). The quadratic terms m2 had highly significant (p < 0.01) effect at
1% LOS while Tph,2, P
2 and t
2 had no significant effect on expansion ratio of rice. Non-
significant effect was observed for all the interactions.
According to ANOVA Table 3 the coefficient of linear terms of moisture content (m) and
residence time (t) are negative therefore increase in moisture content and residence time may
reduce the expansion ratio of rice. Since co-efficient code variable of P2
is positive, a maximum
expansion will occur at the higher power level selected in this study. Since higher terms of
moisture content have negative correlation with VER, puffing of rice should be carried out at the
lower level i.e. 12-14% moisture levels.
This shows that a minimum level of moisture in the grain is essential for puffing, in this
case 12% (wb). The present study supports hypothesis that at higher moisture level collapse of
starch structure, limits the starch expansion (Moraruand Kokini 2003). It may be inferred that,
moistening the grains to 14% (wb), puffing at 220°C pre-heating temperature and 900W power
level would yield fully puffed rice. This implied at higher level of moisture (more than 14% wb),
the products VER will be negatively impacted due to collapse of starch matrix.
18
Preheating temperature, power level and residence time did not show any significant
effect on the expansion, considering the fact that the ranges were chosen to ensure proper
expansion of rice starch. Lack of fit was found to be non-significant for the SOP model to predict
VER. Model F-value 35.65 indicates that the model is significant.
The regression equations describing the effects of process variables on puffing yield of puffed
rice in terms of actual levels of variables are given as:
VER= 5 - 0.79 Tph – 0.075 Tph - 0.10P – 0.20t + 0.000mTph + 0.00mP + 0.29mt –
0.17TphP – 0.057Tpht - 0.13Pt - 0.83 m2 – 0.094 Tph
2 + 0.000333P
2 – 0.22t
2 (7)
Effect of process variables on sensory score of rice:
The results showed that sensory score of Gurjari increased from 5 to 8 in the given process
parameter range (moisture content 12-14% wb, preheating temperature 180-220°C , power level
300-900W and residence time 50-70s) (Fig 3c). The results concluded that, for the selected range
of variables, power level (P) had significant effect at 10% LOS, while, moisture content (m), pre-
heating temperature (Tph) and residence time (t) had no significant affected on sensory score of
rice as shown in Table 3 The quadratic terms of m2 had significant effect at 5% LOS while Tph
2,
P2 and t
2 had no significant effect expansion ratio of rice. Non-significant effect was observed
for all the interactions. The higher terms of variety and moisture content have the impact on
sensory score. Moreover, with increase in moisture content above 14% the product was rejected
by the sensory panel (sensory score less than 5). The reason for this could be the VER, which
was negatively affected by the higher moisture content, which resulted in collapsing of starch
matrix, thus not expanding fully, which was discussed in the previous section. Consumer prefer
19
puffed rice with good volume expansion and without any browning, whereas at higher moisture
content the volume expansion was not maximum and there was also browning due to over
exposure to higher power level. As a whole the sensory score of the puffed rice was largely
controlled by its appearance and taste, and the indicator was volume expansion ratio and colour
of the product.
The model F-value of 6.05 implies that model is significant. There is only a 0.01%
chance that a “model F-value” this large could occur due to noise. The regression equations
describing the effects of process variables on puffing yield of puffed rice in terms of actual levels
of variables are given as:
Sensory score = 5 - 0.50 m+ 0.25 Tph + 0.67P+ 0.083t – 0.25mTph – 0.50mP +
0.025mt – 0.025 Tph t – 0.25 P t - 0.29 m2 - 0.67 – 0.67
Tph2–
0.29 P2– 0.17 t
2 (8)
Optimization of process parameters based on analysis and statistical data
From the results obtained through various analytical data discussed above, a suitable
combination of moisture content, pre-heating temperature, power level, and residence time was
calculated, for the puffed rice product with optimum quality and optimum values for the
responses stated and discussed above. During the optimization process, the specific constrains
were applied on the variables.
20
The optimum solutions was moisture content 14% (w.b), pre-heating temperature 220 °C,
power level 900W, residence time 60 s , with puffing yield 93.83%, expansion ratio 4.64,
sensory score 7.60.
Verification of model analysis of optimized puffed rice product
The verification of the optimum solution was also done by conducting the experiment at the
optimized point for the variables considered and it had been observed that the experimental
values obtained were similar to predicted values which are presented in Table 4. Therefore, it can
be concluded that the developed models were good fit to predict the puffing characteristics of
rice puffed in a convective-MW oven. The models had r2 value more than 0.9 and lack fit terms
were non-significant, that the models are good fit to the experimental values and the verification
of the results also had residual values less than 5% , which is in the acceptable range (Mohapatra
and Bal 2010). The present study fulfills all the conditions for getting desirable results.
Photograph of the product from the optimized solutions and non-optimized products are
presented in fig 4a and 4b.
Conclusions
From the experimentation it can be concluded that starchy grains like rice needs sufficient
thermal gradient for expansion, which corroborated earlier reports. Though physical barriers like
horney endosperm in corn and husk in paddy helped to contain the pressure to build up before
expansion, however, grains like milled rice requires additional thermal energy for expansion of
starch. For all preheating temperature and power levels, no puffing was observed within 10s of
21
residence time, implying the initial thermal gradient provided in the form of conductive as well
as convective heat was not sufficient to evaporate the moisture and expand the product. When
this was aided by the MW energy; the puffing could be observed after 30s of operation where
70% of the grains were found to be puffed. Better puffing yield was achieved when the power
level was more than 600 W and preheating temperature more than 180°C for residence time of
50 to 70s. This again reaffirms the effect of thermal gradient in the expansion of starch. From the
response methodology solutions, the optimum conditions for the puffing of Gurjari variety of
rice in a convective microwave oven was at IMC level of 14%, oven preheating temperature of
220°C, microwave power level of 900W and residence time of 60 s. This study also corroborates
the importance of moisture content in the expansion of starchy material, since at higher moisture
level (16%), collapse of starch granules due to nucleation could have detrimental effect in
achieving maximum expansion. This resulted in a puffing yield of 93.83%, VER of 4.64 and
sensory core of 7.6. Oven preheating temperature above 180°C and MW power level between
600-900W for residence time of 60 s yields puffed rice with acceptable volume expansion and
sensory score.
Acknowledgement: The authors wish to express their sincere thanks to the Head, Rice Research
Centre, Nawagam, Anand Agricultural University for proving different varieties of paddy and
facility to pre process the grains at their laboratory.
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