Puffing Characteristics of Parboiled Milled Rice in a Domestic Convective–Microwave Oven and...

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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 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: debabandya@gmail.com

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

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

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

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

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:

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

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

Yk =ƒk (m, Tph, W, t) (3)

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

(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

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

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.

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

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

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.

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.

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,

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.

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

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

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

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

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.

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

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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