Rice Science, 2014, 21(4): 187−200 Copyright © 2014, China National Rice Research Institute Published by Elsevier BV. All rights reserved DOI: 10.1016/S1672-6308(13)60191-2

Traditional Parboiled Rice-Based Products Revisited: Current

Status and Future Research Challenges

Himjyoti DUTTA, Charu Lata MAHANTA (Department of Food Engineering and Technology, School of Engineering, Tezpur University, Napaam, Sonitpur, Assam 784 028,

India)

Abstract: Parboiling is an age old technique carried out to improve rice quality. Different grain

parboiling techniques have been traditionally followed and scientifically developed for preparation and

industrialization of rice. The state of Assam, India produces a large number of rice varieties, some of

which are traditionally processed into peculiar parboiled rice products like Hurum, Komal chaul, Bhoja

chaul and Sandahguri, which are of both ethnic and possible commercial importances. In spite of

extensive research carried out on parboiled rice, these products and their special parboiling techniques

have not been sufficiently explored. The status of research on parboiled rice as a whole with special

attention to these lesser known speciality products of Assam is extensively reviewed. Future scope of

research on these products is also identified.

Key words: rice; parboiling; rice product; technique

Rice (Oryza sativa L.) is staple food for more than

half of the world’s population. The International Rice

Genebank of the International Rice Research Institute

(IRRI) has a collection of more than 117 000 types of

rice, comprising of modern and traditional varieties,

also including wild relatives of the plant. The north-

eastern states of India are very rich in biological as

well as ethnic diversity. Among the north-eastern

states of India, Assam is unique in terms of a mixed

culture as well as both hilly and low land vegetation.

Located between 24º to 28º4′ North latitude and 89º4′

to 96º East longitude, the state shares international

borders with Bangladesh and Bhutan. More than 15

distinctly different tribes dwell in the land and each

has its own food and social habits. Anthropological

study by Bordoloi et al (1987) elaborated that many of

these tribes migrate from Tibet, China and Burma.

Hence, the agro-climatic location and continuous

migration of the different Mongolian races from

South-East Asia as well as Aryans from the Indian

subcontinent and abroad have brought the unique

racial intermixing and cultural exchange visible in

Assam. This also causes extensive transfer of crop

germplasm, resulting in a rich agricultural biodiversity.

Assam produces numerous varieties of rice from time

unknown. Ahmed et al (2011) have elaborated the

varieties and recent growing patterns of rice in Assam.

Rice grain is principally composed of starch granules

with very small size (about 2–5 µm). Starch is the

natural source of energy in plants and is primarily

composed of two molecular fractions: amylose and

amylopectin. On the basis of the apparent amylose

content, Juliano (1979) classified rice into high

amylose (> 25%), intermediate amylose (20%–25%),

low amylose (7%–20%) and waxy or glutinous (1%–

2%) groups. The cooking properties of rice grains

vary considerably from one group to another. Another

classification method was proposed by Bhattcharya

et al (1980), they classified hundreds Indian rice

varieties collected from different parts of the country

into eight groups. Group I cooks flaky and hard, while

group VIII cooks extremely sticky and soft. Group IV

consists of the aromatic rice varieties. It was observed

that only Assam produced varieties from all the eight

groups. Glutinous rice varieties of Assam and its

adjoining states are unique in India.

Parboiling is an age-old technique used for post-

harvest processing of rice grains. About 60% of the

total production of paddy rice grains is parboiled in

India. The country has more than 5 000 large scale

mills with parboiling facilities. The parboiling technique

is known to originate in India in the fourth or fifth

century AD (Bhattacharya, 1985). The basic and Received: 2 September 2013; Accepted: 5 March 2014

Corresponding author: Charu Lata MAHANTA (charu@tezu.ernet.in)

188 Rice Science, Vol. 21, No. 4, 2014

conventional parboiling process involves soaking

rough rice for 2–4 d or longer in water at ambient

temperature to attain saturation, followed by boiling

the steeped rice till the grain expands and the hull’s

lemma and palea start to separate. The grains are then

air- or sun-dried to a desirable moisture level and

milled. Advancement to the conventional parboiling

method is pressure parboiling, involving application

of high pressure and temperature to partially hydrate

rice, which reduces the processing time and increases

plant turnover. Another form of traditional parboiling

method called dry-heat parboiling involves brief

conduction heating of the soaked rice grains with or

without very hot sand (Bhattacharya and Ali, 1985). Fig.

1 gives the schematic diagram of the three types of

parboiling processes. Modern parboiling technologies

and equipments have been developed by research

institutes like Indian Institute of Technology,

Kharagpur and Central Food Technological Research

Institute, Mysore, etc (Kachru, 2010). Many variations

in processing conditions and techniques have also

developed and been studied. Introduction of modern

dryers, microwaves, fluidized hot beds etc. have made

the parboiling process more efficient in recent times

(Cheenkachorn, 2007; Sangdao and Krairiksh, 2008;

Fofana et al, 2011).

This article reviews the rice parboiling techniques,

their effects on the properties of the grain and

utilization of parboiled rice products supported by

research findings. The research possibilities on Assamese

traditional rice products which have unexplored

commercial and scientific significance are specially

mentioned.

Parboiled rice

Research on parboiled rice dates back to the early

1960s (Mecham et al, 1961; Bhattacharya and Subba

Rao, 1966; Bhattacharya, 1969). Although various

aspects of parboiled rice have been studied by numerous

researchers, most of the works revolve around the

physicochemical attributes of the products. Parboiling

results in easier processing (by hand), improves nutritional

profile, and changes texture. However, mechanical

processing becomes more difficult (Gariboldi, 1984).

Suitably parboiled rice should always give 100%

intact grains upon milling. Kaddus Miah et al (2002)

observed a large reduction in fissured grains on hot

soaking. Their study suggested that due to parboiling,

the void spaces in the endosperm are filled and cracks

are cemented, making the grain harder. Hence, it

requires greater force and time for milling and

polishing and therefore the nutritive epidermal layers

of the rice grains are partially retained even after

milling. Insect infestation is also reduced due to the

hardness. Ibukun (2008), however, reported that

prolonged steaming leads to the loss of nutrients in

parboiled milled rice. The changes that occur in the

rice grains on parboiling are broad in its morphological

(grain surface and internal biological arrangements),

structural (crystallinity change), physical (grain dimension,

density, translucence, colour, hardness, porosity, flow

and packing properties, milling quality, etc) and most

importantly, the physicochemical and nutritional

aspects (paste viscosity, texture, polymerization and

polymorphism, thermal properties, proximate composition

is widely used in the world. However, soaking of

paddy rice grains for days in ground water or tap

water has major drawbacks, making them unsuitable

for consumption. Long soaking periods result in

potent growth of microorganisms arising from the

water source or soil attached with the paddy husk

(Karunaratne, 2010). As rice seeds are hydrated and

the activities of hydrolytic enzymes like amylase,

protease, phosphatase and β-glucosidase increase,

simpler sugars and proteins are formed and released

into the soaking unit (Xavier and Anthoni Raj, 1995).

This makes the system an ideal medium for yeasts and

bacteria to grow. These microorganisms further form

certain flavour and odour compounds that are

considered unacceptable by many consumers. In

addition, certain parboiled products like Ofada of

Nigeria are relished due to the organoleptic flavour

and nutritional enhancement on account of the microbial

Soak in water at room temperature for 4–5 d

Fig. 1. Three basic parboiling processes.

Paddy rice grains

Himjyoti DUTTA, et al. Rice Parboiling: A Review 189

activity during soaking (Ebuehi and Oyewole, 2007).

Use of microbial starter cultures for quality enhancement

of such products has hence been practiced (Adeniran

et al, 2012). The increased phenol content in the rice

grains during soaking may lead to minor enzymatic

colour development (Ramalingam and Raj, 1996). The

modern process, therefore involves soaking of rice

grains at elevated temperatures (around 70 ºC) for a

few hours that saturates the grains fully or partially,

followed by open or pressure steaming for a brief

period (Tirawanichakul et al, 2012). Igathinathane et al

(2005) developed a combination of soaking procedures

which resulted in 67% reduction in soaking time as

compared with single stage soaking at 70 ºC. They used

initial soaking at 80 ºC for 45 min followed by 70 ºC

for 3.25 h. The combination is dependent on the

gelatinization temperature of the starch of any specific

rice variety. The traditional method of parboiling also

involves prolonged boiling under atmospheric pressure.

Work of Himmelsbach et al (2008) dealt with changes

in starch properties of rice with variable soaking and

steaming temperatures. They reported that soaking at

90 ºC followed by steaming for 12 min results in

enough gelatinization to prevent grain damage during

milling. The characteristic amber colour of parboiled

rice is mainly due to Maillard type browning reaction

between reducing sugars and amino acids that leads to

formation of Amadori coloured compounds (Messia et al,

2012). Colour development has been suggested as an

important parameter for determining steaming severity

by Lamberts et al (2006). The husk pigments also play

an important role in determining the colour intensity

as they migrate inwards into the endosperm during

steaming (Lamberts and Delcour, 2008). A non-

uniform steaming leads to a non-uniform product

colour. Other than that, parboiling severity alters the

gelatinization temperature and reduces the gelatinization

enthalpy of starch due to developed amorphousness

(Wirakartakus and Lund, 1978). A laboratory-scale

parboiling technique developed in our laboratory

involving special treatments resulted in lowered

cooking time of low amylose rice (Dutta and Mahanta,

2014). However, it is against the general findings due

to the developed grain integrity and hardness, and the

cooking time is increased after parboiling (Soponronnarit

et al, 2008; Tirawanichakul et al, 2012). Ong and

Blanshard (1995b) did not find any relations between

gelatinization temperature and texture of cooked rice.

Their extensive study (Ong and Blanshard, 1995a, b)

reported that the texture of cooked rice has strong

correlations with the apparent amylose content and

fine structures of amylopectin. The proportion of the

longest (DP = 92–98) and shortest (DP < 25) amylopectin

chains primarily determine the cooked rice texture.

Intermediate (DP = 43–68) chains do not have any

effects on texture. Cooked grains with harder texture are

high in amylose and with longer chains of amylopectin

that bind with other non-starch components. The

changing status of viscosity of rice paste on cooking

in excess of water and subsequent cooling as

measured with a rapid viscosity analyser (RVA) gives

the idea of the occurrence of starch gelatinization and

retrogradation. The ratio of amylose and amylopectin

of the rice flour plays the principal role in determining

different viscosity parameters (Reddy et al, 1994). Our

previous work on four rice varieties indicated that a

high amylose variety forms a harder gel than glutinous

ones (Dutta and Mahanta, 2012). In addition, amylopectin

chain length, moisture content, protein content, lipid

content and mineral content play significant roles in

the pasting properties of rice flour and starch (Suzuki

et al, 2006; Dautant et al, 2007; Ascheri et al, 2012).

Hence, varietal and cultivar differences can be

considered as the major factor behind differences in

cooking behaviour of rice (Ashogbon and Akintayo,

2012). Prominent changes in the starch molecular

arrangement as well as migration and restructuring of

other components are suggested by the RVA patterns

(Himmelsbach et al, 2008). Lowering of starch

swelling at the heating phase indicated by lowered

peak viscosity and other parameters was evident,

which confirmed the findings of Bhattacharya and Ali

(1985) that part of the gelatinised starch had

retrograded during slow drying of the rice grains. The

molecular re-association during retrogradation explains

the phenomenon of longer cooking time of parboiled

rice in boiling water. In addition, a major part of the

starch is gelatinised in the parboiled rice grains, such

rice grains absorb water faster at temperatures below

the gelatinisation temperature of the rice. In a related

study, Lai et al (2001) proposed a relationship

between pasting properties and starch rigidity upon

hydrothermal treatment. Parboiling also affects the

level of disulphide bonds in proteins (Derycke et al,

2005). The protein network formed after parboiling

restricts the swelling capacity of parboiled rice flour

when heated, thereby also contributing to the lower

peak viscosity values. The viscosity studies by different

authors also suggested changes in gelatinization onset

temperature after the hydrothermal treatment (Shih et al,

190 Rice Science, Vol. 21, No. 4, 2014

2007; Himmelsbach et al, 2008). Thermal behaviour of

rice flour can be better understood by the differential

scanning calorimetery (DSC) of rice slurry. Heating of

starch slurry in the DSC melts any forms of crystallite

present. Along with the reduction in total crystallinity

as measured by X-ray diffractometry, newer crystallites

are also formed during retrogradation. The native A-

type X-ray diffraction spectra of raw rice starch

changes partially to B-type due to differences in water

content and newer arrangement after recrystallization

(Mahanta et al, 1989). Amylose-lipid complexes give

V-type X-ray diffraction pattern and additional

endothermic peaks in the DSC thermograms. The

formation of these new crystalline polymorphs depends

largely on the rice variety and the parboiling conditions

applied (Manful et al, 2008). Lamberts et al (2009)

have reported the formation of amylose crystallites in

parboiled rice.

Popular parboiled rice products

Asia as a whole is the major producer and consumer

of steam parboiled rice. Parboiled rice is industrially

produced, consumed and exported from almost all

Asian countries. African countries like Nigeria, Ghana,

Egypt, Niger and Benin are also significant producers

of rice, and parboiling is a very common technique

practiced in African households that plays an important

role in fighting undernourishment and food loss (Tomlins

et al, 2005). More recently, extensive research on

African parboiled rice has been carried out (Demont et al,

2012). Amongst the dry-heat parboiled products, the

most popular are the puffed rice and flaked rice (Fig.

2). The processing methods for both these products

have been upgraded (Bhattacharya and Ali, 1985).

Puffed rice

Paddy rice grains are soaked in warm water overnight

and the soaked grains are then roasted in sand on a

Bhatti in small batches to produce dry-heat parboiled

paddy rice grains. The paddy rice grains are allowed

to dry in mild sun or in air after spreading out. The

dried paddy grains are then milled in a huller. The

milled parboiled grains are gently heated on the Bhatti

without sand to reduce the moisture content to the

appropriate level, then taken out and mixed with a

proper amount of salt solution. After holding for some

times, the parboiled rice grains are again roasted on

the Bhatti in small batches with sand on a strong fire

for a few seconds to produce the puffed rice. The

improved process has been developed by the Central

Food Technological Research Institute, Mysore, India

(Chinnaswamy and Bhattacharya, 1983). It is found

that dry-heat parboiling gives better puffing expansion

of rice than steam parboiling, and pressure parboiling

gives better puffing expansion than dry-heat parboiling.

Paddy rice grains are first washed with water,

followed by pressure parboiling at 2.0‒2.5 kg/cm2 for

Puffed rice Flaked rice Hurum Komal chaul

Bhoja chaul Korai Sandahguri

Fig. 2. Traditional Asamese parboiled rice products.

Himjyoti DUTTA, et al. Rice Parboiling: A Review 191

15 min. The parboiled grains are then dried either in

sun or mechanical drier and milled. Before puffing,

the parboiled grains are mixed with saturated salt

solution (3‒4 mL of saturated salt solution per 100 g

of rice grains), and thereafter the moisture content of

parboiled grains is brought down to 10.5%‒11.0% and

puffed in 10 times of its weight with sand at 250 °C

for 10‒11 s. Pressure parboiled rice gives slightly

coloured puffed rice. To reduce the colour, the paddy

grains are washed with 2% sodium bisulfite solution.

Hoke et al (2005) reviewed the optimum conditions

set for different types of rice puffing (dry-heat,

microwave and gun-puffing) from different works.

Effects of amylose and protein content are also

evident in the quality of puffed rice, suggesting

varietal differences to be considered for puffing rice

(Chinnaswamy and Bhattacharya, 1983; Villareal and

Juliano, 1987). A more recent and sophisticated

analysis on the same variety by Mahanta and

Bhattacharya (2010) clarified the earlier doubts on the

variable product qualities with processing conditions.

The study indicated that the puffing is promoted by

gelatinization and amylose-lipid complex formation,

and retarded by amylose retrogradation and probable

starch breakdown due to high temperature conduction

heating. Addition of different salt solution increases the

puffing ability of rice (Chinnaswamy and Bhattacharya,

1983; Murugesan and Bhattacharya, 1986). Presently,

use of microwave for puffing has also gained much

popularity. Maisont and Narkugsa (2010) studied the

effects of salt, moisture content and microwave power

on puffing qualities of a variety of waxy rice. The

authors tried various combinations of the three parameters

and found that the most suitable combination of 2%

salt solution, 13% moisture content and microwave

power of 700 or 800 W gives the optimum product

with high puffed yield and expansion volume with

moderate hardness.

Flaked rice

For flaked rice, soaked rice grains are precooked by

roasting followed by flaking in an edge runner. The

process also removes the brittle outer husk. One major

drawback of passing through the edge runner is the

low yield (63%‒65%). In the edge runner, the roasted

grains have to pass between the roller and pan edge

repeatedly round and round for a minute. So, even the

dehusked and flattened grains pass through the edge

runner repeatedly. As a result, there is severe abrasion

at the edges of the grain along with grain breakage.

An improvement of this process is the installation of

another idle roll in the edge runner which increases the

yield to 66%‒67%. Roasting of the soaked paddy

grains in this method is done in a continuous roaster.

The roasted paddy grains are successively dehusked in

a centrifugal dehusker and polished in a polisher. An

aspirator removes all the husk and bran particles. The

modified dehusker gives 100% dehusked roasted rice.

The polished rice is then roller flaked. This process

gives undamaged flaked rice. Ali and Bhattacharya

(1976a, b) reported certain additional changes

occurring in flaked rice which were not perfectly

described then. Mujoo and Ali (1998) suggested that

rice roasting resulting in gelatinization, causes

exposure of starch component to enzymatic digestion.

However, the additional step of roller flaking for rice

flake preparation causes the formation of certain

starch-protein complexes with molecular weights

greater than 4 × 107 kDa. These complexes decrease

the susceptibility of the product to enzymatic hydrolysis

(Mujoo et al, 1998).

The Central Food Technological Research Institute

at Mysore, India has conducted extensive studies on

dry-heat parboiled rice. An early work by Ali and

Bhattacharya (1976a) suggested that roasted rice gives

higher degree of gelatinization than conventional

parboiled samples as indicated by alkali degradation

test. It also shows higher values of saturated equilibrium

moisture content. The impact of retrogradation is

minimised as the water required for retrogradation is

insufficient due to rapid dehydration during roasting.

Progressive retrogradation of starch in the roasted rice

grains occurs only when stored at room temperature at

above 18% moisture content (Ali and Bhattacharya,

1976b). They proposed that the high rate of moisture

absorption by dry-heat parboiled rice is due to

gelatinization effects, and the properties of conventional

parboiled rice are due to retrogradation. The authors

also developed a sedimentation test for pre-gelatinized

rice products (Bhattacharya and Ali, 1976). Roasted

rice gives high sedimentation volume in a dilute acidic

solution. Fine and coarse brown rice grains were

roasted by Guha and Ali (1998) where clear

differences were observed between the two. Bran

layers might have behaved as partial insulators during

the process. The products exhibit lesser cooking time

than steam parboiled rice due to high water absorption

characteristics developed. Chitra et al (2010) worked

on in vitro starch digestibility of popped rice,

expanded rice and flaked rice prepared by sand

192 Rice Science, Vol. 21, No. 4, 2014

roasting. Lower starch content in the products than the

raw rice is attributed to the adhering bran layers and

formation of resistant starch in them. The products

were hence suggested to be used in preparation of

diabetic foods. Domestic pan roasting and using

microwave were found to significantly reduce the

rancidity development in rice bran (Ahmed et al,

2007). These hence have scope of application for rice

products as development of rancidity in parboiled rice

during storage is considered as a serious problem.

Parboiled rice products of Assam

In Assam, the high and intermediate amylose rice

varieties are consumed as staple foods, and the low

amylose and waxy varieties are processed to make

speciality products. Steam parboiled rice is consumed

by a significant mass of the Assamese population in

their staple diet. A recent study was carried out on the

effect of open and pressure parboiling on four rice

varieties that are native to Assam and belong to high

amylose, low amylose and waxy categories (Dutta and

Mahanta, 2012). Differences in parboiling severity

result in distinctly different product characteristics.

Pressure parboiled glutinous samples show simultaneous

loss in crystallinity and increased hydration at low

temperatures. Unique taste and texture are attained

when common dry-heat parboiled rice products like

moori (puffed rice) and chira (flaked/beaten rice) are

prepared using the glutinous varieties which are

commonly eaten in Assam whereas moori and chira

made from non-glutinous rice are popular throughout

the Indian subcontinent. Further, there are certain

speciality rice products, namely Hurum, Komal chaul,

Bhoja chaul, Korai and Sandahguri (Fig. 2), which

despite of having possible ethno-economic importance

have not been extensively researched. Special

parboiling treatments develop peculiar hygroscopic

and sensory characteristics in these products. A few

such products and their present status in the scientific

scenario are detailed below.

Hurum

Hurum is an expanded rice product made from waxy

Bora rice of Assam (Fig. 2). It is distinctly different

from the more popular moori both in the processing

method and the product quality. The basic traditional

parboiling method (Fig. 3-A) comprises the following

steps: full soaking of grains, parboiling, dehusking at

high moisture, immediate flaking, rubbing fat to the

flaked rice and expansion in sand. This product can

therefore be called as ‘expanded flaked’ rice. A

previous study identified the essential steps of Hurum

making in the laboratory using paddy grains processed

by normal, dry-heat and pressure parboiling methods

(Mishra et al, 2000). Pressure parboiling was found to

give comparatively higher expansion to the product.

However, varietal differences were observed. Based on

the findings, a process was further optimized for

cottage scale production of Hurum (Mahanta and

Goswami, 2002). The traditional practice of dry-heat

parboiling is chosen over pressure parboiling as the

later is not feasible for cottage scale processing in

rural Assam of that time. The optimized process is

given in Fig. 3-B. Instead of a longer soaking period

and intermediate simmering in water, paddy grains are

soaked overnight in freshly boiled water and allowed

for self-cooling to room temperature to allow optimum

moisture absorption. It is directly followed by

vigorous roasting in sand on the Bhatti to bring the

moisture content down to 21%‒23%. The paddy

grains are then simultaneously flaked and dehusked in

an edge runner in a flaked rice mill. The flaked and

dehusked grains are rubbed with an optimum amount

of hydrogenated oil. The oiled and flaked grains are

then roasted on the Bhatti to expand for a short time to

obtain Hurum. The product is traditionally relished

with milk and sugar or jaggery. The extensive

expansion and translucency acquired during processing

are the distinctive characteristics of Hurum. The

translucency is due to the negligible amylose content

in waxy rice. No other scientific literature is available

on this product till date.

Komal chaul

Quick cooking rice has come to international knowledge

in recent times. Komal chaul (soft rice) is a whole

grain, ready-to-eat product, which needs no cooking

and can be consumed after simply soaking in cold to

lukewarm water (Fig. 2). It is a special rice product of

Assam. Recently, the Central Rice Research Institute

of India reportedly developed Komal chaul varieties

by using panicle progeny methods on waxy rice

varieties like Aghuni Bora (Yadugiri, 2010). However,

in Assam, the name Komal chaul designates the parboiled

rice product and not a group of rice varieties. The

tradition of preparation and consumption of Komal

chaul in Assam is age-old. Low amylose rice varieties,

locally termed as Chokua rice varieties, are preferred

over the waxy Bora varieties to prepare this product.

Himjyoti DUTTA, et al. Rice Parboiling: A Review 193

The Chokua varieties result in a better, less-sticky

Komal chaul than Bora varieties. Komal chaul has

already been considered and detailed as a potential

geographical indicator (GI) for the region by

Samaddar and Samaddar (2010). The authors have

taken it up as a bio-cultural product which has yet to

attain its GI registration. The traditional parboiling

technique is given in Fig. 4-A. Traditionally, Chokua

rice grains are soaked in water at room temperature

for 3–4 d to attain an acceptable moisture level. The

excess water is drained, and the grains are put in fresh

water and cooked over wood fire till the husks start

splitting. The water is again drained and the grains

dried under the sun on the same day. Dried grains are

milled in a dheki, the traditional foot pounding

machine to get the Komal chaul product. Drying of the

boiled grains is done on the same day so that there is

negligible retrogradation and the milled product

attains soft texture on simple soaking in water at room

temperature. Das et al (2005) analyzed the nutrient

profiles of a few Assamese dishes. Komal chaul is

generally relished with curd and jaggery. The dish is

high in carbohydrate and minerals, especially iron. A

recent work from our laboratory was on developing a

laboratory scale method (Fig. 4-B) for processing of

the product using low amylose Kola Chokua rice

grains (Dutta and Mahanta, 2014). Briefly, the grains

are cooked in boiling water for 1–3 min, and the

boiling vessel is then covered with a gunny bag to

retain the temperature for a longer time. Optimum

moisture level (37%, wet basis) can be obtained after a

soaking period of 18 h. Excess water is drained, and

A B

Fig. 3. Traditional method (A) and developed laboratory-scale method (B) of Hurum making.

194 Rice Science, Vol. 21, No. 4, 2014

the grains are steamed in an autoclave using steam.

Both open (100 °C/0 psig) and pressure steaming

(121 °C/15 psig) for 10 to 20 min are employed

followed by shade drying for 48 h and subsequent

milling to obtain Komal chaul with acceptable quality.

When the product is soaked in water at 50 °C for 20

min, a texture similar to normal cooked rice is

obtained. The flour of the raw untreated rice is highly

resistant to α-amylolysis. However, steaming pressure,

whether open or under pressure, and the severity of

steaming have an effect on the extent of amylolysis.

While in open steamed Komal chaul, steaming

severity decreases the starch hydrolysis rate, indicating

the formation of enzyme-resistant fractions, and

pressure steamed Komal chaul shows higher digestibility

with treatment severity.

Works on process optimization for different instant

rice products have been carried out by various researchers

(Amornsin, 1994; Prasert and Suwannaporn, 2009),

wherein raw milled rice was processed to get instant

rice. None of the processes however fall under

‘parboiling techniques’ as used for Komal chaul. This

is because milled raw rice was used instead of brown

rice as a raw material and parboiling strictly involves

cooking the grain inside the husk followed by drying.

Rewthong et al (2011) used hot-air dryer and freeze

dryer to make quick-cooking rice, which makes the

processing quicker but costlier. Prapluettrakul et al

(2012) developed a process for preparing instant rice

for young children. The product is prepared by boiling

jasmine rice followed by freezing for 24 h at -20 ºC,

drying at 70 ºC, and allowing for rehydration by

boiling for 3 min. These conditions are most suitable

for development of desired texture. Sripinyowanich

and Noomhorm (2012) compared various modern

drying techniques and found that product quality of

unfrozen cooked rice that was subjected to vibro-

fluidized bed drying at 160 °C is superior to the other

techniques that involved freezing prior to drying.

Recently, Boluda-Aguilar et al (2013) developed a

similar product using jasmine rice wherein the rice

was cooked with microwave rather than simple

soaking in water as in the case of Komal chaul. A

positive attribute of these works over the developed

method for Komal chaul lies in the much shorter

soaking times employed.

Bhoja chaul

Bhoja chaul in simple terms is a dry-heat parboiled

rice product (Fig. 2). Unlike the common puffed rice,

A B

Fig. 4. Traditional method (A) and developed laboratory-scale method (B) of Komal chaul making.

Himjyoti DUTTA, et al. Rice Parboiling: A Review 195

Bhoja chaul grains do not undergo excessive structural

and morphological disorganization during the process.

The conventional technique for processing Bhoja

chaul is given in Fig. 5-A. Waxy rice grains are

soaked for 3–4 d at room temperature after the water

is drained out. The moistened grains are roasted in an

iron vessel over wood fire with constant stirring.

Roasting is stopped when the grains sufficiently dry

up. The roasted grains are spread over mud floor to

cool for about 30 min before milling in a dheki (a

hand and foot operated pounding and milling device)

to get Bhoja chaul. However, many variations can be

seen in the time and temperature of soaking and

roasting in different rural households. Soaking in hot

water overnight or boiling the grains till very fine split

in a few husks occur are also practiced by some. The

roasting temperature is controlled by the intensity of

the wood fire. The prepared Bhoja chaul is soaked in

water at room temperature prior to consumption to let

it hydrates sufficiently to an optimum level. The

excess water is squeezed out with hand during which

the sticky grains cling to one another forming an oval

and flat shaped lump. These lumps are eaten with milk,

cream, curd and jaggery. According to the rural

household processors, the desirable characteristics of

Bhoja chaul are the roasted aroma and colour, a sticky

and chewy texture and appearance of the rice grains

clinging together to form the lump.

In a laboratory scale process developed, a motor

operated, electrically heated, drum type roaster was

used for roasting. Out of many aged waxy grain

samples soaked at 98 °C overnight and roasted

between 145 °C–175 °C for 6–11 min, a few samples

gave high hydration capacity (35%–41%). However,

none had the desirable sensory characteristics. The

raw starch taste was indicative of insufficient

hydrothermal treatment. To increase the severity of

the process, paddy grains were added to boiling water,

cooked for 1 min and left overnight to hydrate, which

increased the moisture content of the grains upto 37%.

These grains were then processed into Bhoja chaul

after roasting at temperatures between 145 °C–165 °C

for 9–13 min. Based on the developed colour and

water uptake, a roasting temperature of 145 °C and

roasting time of 12 to 13 min at 39 r/min speed of

roaster was found to be appropriate. It brings down

the kernel moisture to 11%–12%, which is found to be

suitable for higher head rice yield and longer storage

of the product. It is also found that the colour of the

product intensifies with roasting time. The developed

A B C

Fig. 5. Traditional method (A), developed laboratory-scale method using aged waxy grains (B) and developed laboratory-scale method using

new waxy grains (C) of Bhoja chaul making.

196 Rice Science, Vol. 21, No. 4, 2014

method is given in Fig. 5-B.

Newly harvested waxy rice exhibits acceptable

texture after roasting at 145 °C for 10.5 min. This

necessitated trial for different combinations of roasting

time and temperature for new waxy rice. A roasting

temperature of 130 °C and processing times of 12.5,

13.5 and 14.5 min were hence tried. A higher roasting

time is required for a product with higher hydration

capacity, aroma and colour which is only possible

when the soaked grains have higher initial moisture

content to be evaporated during roasting process.

Boiling step is hence further increased to 3 min, which

markedly increases the moisture absorption by the

grains. The process for new waxy rice is developed

(Fig. 5-C). Even though the use of waxy rice grains

resultes in excellent textural properties, the higher

level of resultant moisture (11.98%–12.69%) is found

to cause early spoilage due to mould growth, colour

change and rancid odour. Any further roasting to

reduce moisture can result in grain breakage due to

moisture gradient within the grains. Therefore, a two-

stage roasting of the soaked grains with a tempering

period in between the two stages is carried out to

equilibrate the grain moisture. Amongst the different

combinations tried, the grains that are roasted for 10

min at 130 °C, tempered for 30 min and roasted again

for 4.5 min at the same temperature give higher head

rice yield and the optimum moisture level in the grains.

Sandahguri

Unlike Hurum, Komal chaul and Bhoja chaul,

Sandahguri is obtained as a coarsely ground powder

of parboiled rice (Fig. 2) involving a longer process.

Although no specific type or rice variety is used for

this product, Chokua rice variety is known to be

preferred over others. The traditional method of

processing Sandahguri is given in Fig. 6-A. Instead of

soaking for long duration, a double parboiling method is

traditionally employed. The grains are cooked in water

for 15–20 min until a few bubbles appear indicating

boiling. The vessel is then removed from fire and kept

overnight to hydrate, after which the grains are again

boiled afresh till a few husks split. The water is

drained out and the grains are allowed to sun-dry. The

dried grains are stored and further processed into

Sandahguri whenever needed. For that, the grains are

milled in a dheki and roasted in an iron pan without

sand. The roasting is done either directly or after

moisture treatment. Moisture treatment may be simple

washing and draining or longer soaking for 15–20 min

and draining. Roasting results in very slight puffing of

the rice, now called Korai. This may be consumed as

such or further powdered to obtain Sandahguri. The

moisture content before roasting influences the extent

of puffing of Korai, which in turn determines the

product quality of Sandahguri. High moisture gives

lower puffing and therefore is roasted at lower flame

for a longer time which gradually reduces the moisture

and develops desirable flavour and aroma in the Korai.

Lower moisture results in undesirably excessive

puffing. The puffing should not be extensive as that

gives an unsuitable end product quality. Roasting may

be carried out for 2–20 min depending on the rice used,

followed by pounding using a dheki to get lumpy or

dry powdery textured Sandahguri. Traditionally, the

powder is mixed with hot milk and stirred to a thick,

slightly sticky, cohesive, porridge like consistency and

consumed along with jaggery or sugar. A strong

roasted aroma from the slightly particulate mixture is a

desirable characteristic. Certain notable variations in the

traditional process are also available. Instead of

double boiling, many households use a standard dry-

heat parboiling technique with grains soaked for 4–6 d.

A practice of rubbing salt solution to the parboiled

rice and sun drying for 20–25 min prior to roasting is

also practiced by some for better puffing quality of

Korai.

Many variations in conditions of soaking, cooking,

moistening and roasting were tried to develop a basic

laboratory scale method for processing Sandahguri.

Based on sensory acceptance of five panellists, a

method using aged Chokua rice is developed (Fig. 6-

B). Soaking for 1 min in boiling water and keeping

overnight are enough for the necessary moisture

uptake. The developed method used single stage

pressure cooking (0.5 kg/cm2) for 10 min instead of

the traditional, time consuming open cooking

technique carried out twice. The milled parboiled rice

is moistened by soaking in water for 40 min prior to

roasting in an electrically operated roaster used as for

Bhoja chaul. Optimum roasting condition is 50 g rice

grains at 160 °C for 7.5 min at 39 r/min. The roasting

operation gives the Korai which is coarsely pulverized

to obtain Sandahguri.

Research possibilities on the products

Even though there is a tremendous scope for studying

the traditional parboiled rice products of Assam,

research has been scanty. The uniqueness of the products

Himjyoti DUTTA, et al. Rice Parboiling: A Review 197

has seldom been reported. Komal chaul has every

scope to be popularized as a convenience food for

defence personnel, for people in adventure quests or in

any places where there is a scarcity of fuel for cooking.

The quick cooking product has been used during

historical wars fought by the soldiers in Assam owing

to the ease in their carriage and consumption. The

product can hence also be further analyzed for fuel

and cost efficiency. Sandahguri and flours of Hurum,

Komal chaul and Bhoja chaul also have tremendous

possibility to be used as green ingredients in food

systems due to their high hygroscopic and pasting

properties. The peculiarity in the products’ hygroscopic

behaviour is attributed to the status of starch in them

(Bhattacharya, 1985). Starch undergoes definite

breakdown and probable reconstruction after the severe

hydrothermal treatments applied during processing.

Characterization of molecular weight distribution of

starch in the products can be considered as one of the

important parameters (Zhu et al, 2011) that have never

been studied. Application of hot soaking instead of

prolonged soaking at room temperature for longer

duration has markedly reduced the time consumption

of the parboiling process. The physicochemical

phenomenon behind this has not been understood well

till now, which creates scope for possible research.

A B

Fig. 6. Traditional method (A) and developed laboratory-scale method (B) of Korai and Sandahguri making.

198 Rice Science, Vol. 21, No. 4, 2014

Again, certain rice varieties used for the Komal chaul

preparation are coloured and hence may be studied for

antioxidant and phytochemical status. Drying has been

considered as an important step for producing instant

rice. Further research trials on soaking and drying

stages of Komal chaul processing are hence

necessitated. The intermediate tempering stage in case

of Bhoja chaul as practiced by certain household

processors is interesting. Further systematic studies on

the process involving this step is required for a

superior standardization of the process. A standardized

method using new Chokua paddy rice grains for

making Sandahguri is still awaited. Engineering

aspects of the standardized processes may further be

studied after developing industrially feasible processes.

A scientific approach has to be made to understand the

peculiar characteristics of the products. Molecular

characterizations primarily relating to the end product

characteristics have to be carried out. Study of the

thermal behaviour is important as the products are

hydrothermally treated and further processed

involving temperature gradients. Besides further

development in processing methods, proper packaging

may increase their potential commercialization. The

products have been brought to the market as packaged

commodity by only a few local companies but value

addition in terms of uniform product quality and

extended shelf life is at a suboptimal level. Assam’s

waxy rice varieties are markedly high in lipid content.

The hydrothermal treatment employed results in quick

development of rancidity in the product, which is a

negative attribute. Measures or studies on controlling

rancidity in the products have however not been

reported in the available literatures. Again, the

hygroscopic nature of the products make them prone

to become unfavourably moist and the high starch

content makes them suitable media for fermentative

mould and bacteria. A great part of the modern

research in food science involves digestibility study of

the products. A study on the products’ glycaemic

status is hence important for its commercialization and

targeted use. Furthermore, there is a great need of

preserving the ethnic identity of the products, and an

anthropological approach seems to be the most

important need of the time. There is also need for

proper identification of waxy and low amylose rice

varieties that are traditionally grown and used by the

village processors. Our general observation suggested

that many such unidentified varieties are gradually

getting endangered due to the lack of scientific

preservation of their germplasms.

CONCLUSIONS

Parboiling is a traditional hydrothermal technique

used for value addition of rice. It brings about

significant changes in the rice grain properties.

Different parboiling techniques have been used to

make speciality rice products, the qualities of which

are highly dependent on amylose and proximate

compositions. Assam produces a wide range of rice

varieties from which different products are made. The

products are special due to the high hygroscopic

behaviour acquired during their processing. Hurum is

a unique expanded flaked rice product made from

waxy rice with high moisture absorption capacity.

Komal chaul and Bhoja chaul are other products that

require simple soaking and no cooking to attain

cooked rice texture. Sandahguri is another product

relished for its texture, colour and roasted aroma as

well as for its convenience. Although peculiar in

behaviour, the products have been studied only up to

limited levels. The thoroughly described traditional

techniques and newer processes developed in our

laboratory aims to popularize them for further in depth

analytical studies and commercialization. The inherent

characteristics of the products make them eligible for

GI registration by the state of Assam, India.

ACKNOWLEDGEMENT

One of the authors, Himjyoti DUTTA acknowledges

the Council of Scientific and Industrial Research, New

Delhi for awarding Senior Research Fellowship to

carry out this research.

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