Parboiling: a process that deeply changes the properties of riceThermoanalytical Approach

Alberto Schiraldi, Marco Signorelli and Dimitrios Fessas*

DISTAM, Università di Milano. Via Caloria 2, 20133 Milano, Italy.

AbstractDSC, TGA and Texture Analysis (with Kramer Cell) investigations allowed a thorough comparison

between parboiled and white rice from RIBE cultivar cooked in excess water to various extent.

Parboiling implies a complete gelatinization of the rice starch and the formation of a homogeneous

structure of the kernels; the cooking process corresponds to the migration of water from the surface

toward the core through a practically isotropic mean. Cooking of white rice instead implies the

progress of starch gelatinization from the surface toward the core which therefore produces a

layered kernel structure in the partially cooked rice.

As a result the final texture of the cooked rice is significantly different in the two cases. In the case

of parboiled rice, a straight line correlation was found between the chewing load (simulated with

dynamometric tests) and the water content of the material, no matter the temperature-time operative

conditions selected for the parboiling process. This finding can have a practical applications as far

as it suggests that the mechanical (and other) properties of the parboiled rice can be tuned through

the control of the water uptake on cooking. This is not the case of white rice.

Keywords: Parboiling, rice, DSC, Termogravimetry, Texture Analysis, Thermal analysis

*Corresponding author. Tel.: +39 0250316637, Fax: +39 0250316632

e-mail address: Dimitrios.Fessas@unimi.it

1

Introduction

Rice is a starch product that is largely consumed also in western countries, where only

recently (when compared with pasta) has been proposed as a convenience product, namely,

precooked and dressed dish with a reasonable shelf life which can be consumed just after a quick

refresh treatment in warm water. Preparation of precooked rice dishes with white rice cannot be

easily standardized to meet the industrial needs since cooking time and behavior of the cooked

product during storage significantly change on passing from one to another starting product [1].

When white rice is boiled at 100°C, starch gelatinization starts from the outer region of the

kernels, where an early layer of starch gel is formed, and progresses toward the core being sustained

by the diffusion of water [2]. Volume and texture of the kernels are substantially modified and, after

a given cooking time, which can be referred to as the Optimal Cooking Time (OCT), attain the

desired level that is dictated mainly by sensorial requirements. For many types of white rice starch

gelatinization does not exceed 75-80% at OCT [2]. The OCT strongly depends on rice cultivar, rate

of starch gelatinization in the core of the kernels and diffusion and uptake of water. Furthermore

cooked rice that is kept at ambient or sub-ambient (e.g., 4-5 °C) temperature undergoes changes of

texture and sensorial properties which make the product less appealing for the consumer. These

changes are largely related to the starch retrogradation [3,4], namely, formation of amylose and

amylopectin crystal phases, the growth of which is enhanced in boiled rice because of its high

(larger than 50% w/w) moisture content. The refreshment of the product in warm (50 – 60°C) water

produces the “fusion” of amylopectin crystals, but is ineffective for amylose crystals and cannot

fully reverse the consequences of the water displacements that occur during the storage period.

Some water is indeed squeezed away from the regions where amylopectin and amylose crystals are

growing and is relatively free to migrate toward the kernel surface where its plasticizing role softens

the local texture that flakes off producing a chalky perception in the mouth of the consumer.

Parboiled rice, that was designed (although parboiling is a very ancient practice) to improve

the nutritional properties of this cereal (it retains some vitamins of the bran, like thiamin, and is

richer in lipids than white rice), is a promising candidate for many industrial preparations, like

convenience rice dishes, since it shows a reduced stickiness of the cooked kernels and a tunable (see

below) cooking behavior; and can experience industrial processes like cooking, freezing and

canning without significant loss of kernel integrity [5,6,7].

Parboiled rice is the final product of a four step treatment of a given raw material (paddy rice):

soaking, steaming, drying and milling. Each step can be performed in a different way at the

industrial level. For example, soaking is performed in hot water, even under vacuum or high

pressure (to speed up the process). The main scope of this treatment, that may take from few hours

2

to few days, is to wet the inner kernel and favor the migration of nutritionally valuable substances

(e.g., vitamins) from the hull to the kernel [8, 9].

The consequences of the parboiling treatment on the behavior of rice on cooking and storing

are important and deserve investigation. Studies so far reported in the literature [8, 9, 10, 11]

concern the physico-chemical changes that take place during each major step of the transformation

from the raw material to the parboiled rice and the final rice-based food. These changes are mainly

related to physical processes, like starch gelatinization and retrogradation, leaching of amylose (up

to 10% of the overall amylose mass), denaturation and aggregation of proteins, which can

significantly affect the sensorial and nutritional properties of the final product. For instance, the

opaque and white belly of white rice related to the random distribution of starch granules,

disappears in parboiled kernels that become yellowish, glassy and translucent. This is related with

the almost complete starch gelatinisation and protein denaturation which expand and occupy all the

air spaces in the endosperm. [12].

In the present work a systematic comparison has been performed between white and parboiled

rice (preconditioned with different industrial treatments) as for starch gelatinization, water uptake

and simulation of chewing through dynamical tests

Materials and Methods

Medium grain Ribe rice, a type of arborio (short-grain) rice grown the in Piedmont and

Lombardy regions of northern Italy, was used in this study and was purchased from industry.

Differential Scanning Calorimetry (DSC)

A Perkin Elmer DSC-6 was used to investigate starch gelatinization. The samples (30 mg)

were hosted in sealed (max. pressure 24 atm.) stainless steel pans, and a suitable amount of

distilled water was used as reference. Measures were carried out in the 20-105 °C range at 2.0 °C

min-1 scanning rate. Indium was used for calibration. The raw data were worked out with the

dedicated software IFESTOS [13, 14] to obtain the trend of the excess (with respect to the trend

prior gelatinization) heat capacity, / J K-1 g-1 (per gram of dry matter) and then evaluate

the enthalpy drop H through a straightforward integration of the corresponding trace. Each run

was repeated at least twice.

The rice samples were boiled in excess water for a given cooking time. The rice was then

quenched in ice water to block any further progress of the cooking. This material was finally

lyophilized and stored under vacuum at about 4°C. DSC samples were water suspension (60% w/w)

3

of powdered lyophilized rice. The actual dry mass of such samples was determined at the end of

the DSC run by weighing the pierced cell kept at 105°C for about 5 h.

Thermo-Gravimetry Analysis (TGA).

The TGA instrument was a SETARAM TG-DSC111 (Lyon, France) with the simultaneous

output of the thermal effect (heat flow-vs-T), TG trace, namely, mass loss-vs-T, and its time

derivative DTG . Measures were carried out in the 20-200 °C range at 2.0 °C min -1 scanning rate.

The typical sample mass was 30 mg. The reference cell was empty. Each run was repeated at least

twice. The ratio between the heat flux and the related mass loss rate was found equal to the enthalpy

of water evaporation in the whole temperature range. This check confirmed that the mass loss was

substantially related to water evaporation only. Possible losses of volatiles therefore were

neglectable in our case. All the TG traces were normalized to 100 mg water. Accordingly, the DTG

traces were expressed as milligrams of lost water per degree K (having fixed the scanning rate).

The typical sample of this investigation was a single kernel of boiled rice. In this case

however the rice used was taken just after the quenching in ice water (see above).

Dynamometry with Kramer Shear Cell

The texture analyzer, used for this test was a TA.HDi Texture Analyser (StableMicroSystems,

UK) The Kramer cell used have 10 sliding blades (HDP/KS10, StableMicroSystems, UK).

A 100 g lot of rice was previously cooked in boiling water for the desired cooking time and

then drained for a given time (6 min) and finally loaded into the Kramer cell. The progress of run is

shown in Figure 1 that reports the plot of the applied load versus the displacement. Three main

regions can be recognized: (i) just after an early onset a re-alignment of the kernels, followed (ii)

by a compression phase that goes through a maximum of the load (extrusion), and (iii) a final easy

sliding of the blades through the base grid (shear). The physical parameter used for the analysis is

the maximum load observed in each run. Three replicas of each run allowed an estimation of the

relevant experimental error.

4

0

5

1 0

1 5

2 0

2 5

3 0

3 5

4 0

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

S / m m

Load

/ kg

Figure 1. Load vs displacement record of a cooked rice sample extruded through a Kramer Cell.

Results and Discussion

Starch Gelatinization

White rice: Starch gelatinization in white rice is sustained by the water intake and progresses from

the kernel surface toward the core. The water intake produces the swelling of the starch granules

and the whole rice kernel that becomes softer. However if the gelatinization front does not attain the

internal regions of the kernel, the texture of this and the water adsorption are not homogeneous: a

whitish core is clearly visible within a surrounding opalescent matrix. Partial cooking therefore

leaves some fraction of starch mass ungelatinized. The DSC traces obtained (in the presence of

excess water) from partially cooked and lyophilized rice samples show the accomplishment of the

process: the shorter the cooking time, the larger the endothermic peak related to the residual starch

gelatinization (Figure 2). The minor low temperature peaks correspond to the fusion of amylopectin

crystals formed during the lyophilisation treatment. The underlying areas of the main endothermic

peak that corresponds to the enthalpy of the residual starch gelatinization, gelH(t), where t indicate

the cooking time, scaled with respect to the signal observed for a totally ungelatinized rice, gelH(t =

0), allow definition of the progress of the starch gelatinization , t with the cooking time

(Figure 3). This trend suggests that starch gelatinization in white rice progresses monotonically with

a rate that decreases with increasing . This not a first order kinetics process [16], although it can

be roughly described as such [17].

5

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

30 40 50 60 70 80 90 100

T / °C

Cpex

c / JK

-1g-1

0'

2'

6'

8'

12'

14'

0’

2’

6’

8’

14’

12’

14’

Figure 2. DSC traces of partially cooked white rice. Lettering refers to the cooking time in minutes.

Parboiled rice: DSC traces of parboiled rice did not show any endothermic signal even in excess

water. This suggests that the whole starch mass has undergone gelatinization during the parboiling

treatment.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 5 10 15 20

t (cooking) / min

Figure. 3. Progress of starch gelatinization with cooking time in excess water at 100°C. t = [gelH(t=0) - gelH(t)] / gelH(t=0). gelH(t=0) = 11.6 Jg-1

Water properties in rice kernels

6

White rice: Figure 4 shows the TGA traces (normalized to a 100g overall mass) obtained from

samples of white rice partially cooked in excess water at 100°C. The end value of each TGA trace

corresponds to the mass of the dry matter, while the drop from the starting level corresponds to the

water content, namely the mass of the water up taken on cooking (if one neglects the original

moisture content of uncooked kernels, which is the same for all the samples). It is therefore clear

that the water intake increases with increasing cooking time up to 70% of the overall sample mass.

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

110.0

0 50 100 150 200 250

T / °C

TG

/ m

g K

-1

2'6'8'14'17'

2’

6’ 8’

14’ 12’

Figure 4. TGA traces obtained from partially cooked white rice samples. Lettering refers to the cooking time in minutes.

A more interesting information comes from the inspection of the DTG traces (namely, the

time derivative of the TGA records), which are the trends of the corresponding dehydration rates.

When the DTG records are normalized to the water content, the traces reported in Figure 5 are

obtained. The relevant signals are broad peaks. This means that the dehydration rates goes through

a maximum, the position of which is related to the trapping strength experienced by water

molecules and to the release mechanism. [15].

7

0.0

0.5

1.0

1.5

2.0

2.5

20 40 60 80 100 120 140 160 180 200

T / °C

DT

G /

mg

K-1

2'6'8'14'17'

2’

6’ 8’

14’ 17’

Figure 5. DTG records from partially cooked white rice (lettering refers to the cooking time in minutes ) after normalization of the initial water content.

The dehydration rate is related to the migration of water from the inner regions of the kernel toward

its surface: the migration rate is strongly affected by the texture of the matrix, which, as said above,

changes with cooking time in white rice, namely it becomes less permeable to water with increasing

the progress of starch gelatinization. This affects both the water intake (during cooking) and the

water release during the TGA run. As a result, the DTG records of rice samples that have

experienced a longer cooking are shifted toward higher temperatures. This trend tends to a

“saturation” behavior for cooking time larger than 14 min.

Parboiled rice: The dehydration of cooked parboiled rice follows a rather different trend. Figure 6

reports the TGA traces obtained from partially cooked samples after normalization of the overall

mass to 100 g .

8

2’

6’ 8’

14’ 12’ 10’

Figure 6. TGA traces obtained from partially cooked parboliled rice samples. Lettering refers to the cooking time in minutes.

Figure 7 reports the same data in the form of DTG records normalized to the same water content to

allow a direct comparison with figure 5. In this case the rate of dehydration practically does not

depend on the cooking time, since after 6 min cooking the parboiled rice shows the same

“saturation” behavior as the white rice after 14 min cooking.

0.0

0.5

1.0

1.5

2.0

2.5

20 40 60 80 100 120 140 160 180 200

T / °C

DTG

/ m

g K

-1

2'

6'

8'

10'

14'

2’

Figure 7. DTG records from partially cooked parboiled rice after normalization of the initial water content. A part from the record after 2’ cooking time, all the other (6, 8, 10 and 14’) can be considered wholly overlapped to one another.

9

Furthermore the texture of the parboiled kernels was substantially isotropic (section image analysis,

data not shown). This is a further confirmation of the DTG data i.e. water that migrates from the

surface on cooking is adsorbed practically with the same strength in any region of the kernel.

Texture Analysis (Kramer Shear Cell)

White rice: The load - vs- displacement records observed for partially cooked white rice are

reported in Figure 8. The maximum load is attained at about 65 mm displacement for all the

samples and decreases with increasing cooking time (namely, the rice becomes softer).

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70 80 90 100 110

S / mm

F /

kg

Riso Ribe

12'

14'

16'

Figure 8. Load -vs - displacement records obtained from partially cooked (12, 14 and 16 min from upper to bottom) white rice forced to trespass the grid of a Kramer cell in a Texture Analyzer. Three replicas for each cooking time are reported.

Parboiled Rice: The load - vs- displacement records observed for partially cooked parboiled rice

are reported in Figure 9. The maximum load was attained at about 55 mm displacement for all the

samples and decreases with increasing cooking time. It has to be noticed that the maximum load

observed for parboiled rice are about twice larger than for white rice.

10

Figure 9. Load -vs - displacement records obtained from partially cooked (12, 14 and 16 min from upper to bottom) parboiled rice forced to trespass the grid of a Kramer cell in a Texture Analyzer. Three replicas for each cooking time are reported.

In order to understand whether the parboiling operation can affect the properties of the final

product, a separate investigation was undertaken in a pilot scale parboiling device. After a given

soaking treatment, the raw rice was divided in 9 lots, each planned for a specific steaming

treatment. Three steaming temperatures (100°C < T1 < T2 < T3) and three durations (1min < t1 < t2 <

t3) were designed so as to prepare significantly different types of parboiled rice. Their were labeled

according to the following matrix,

t1 t2 t3

T1 lot 1 lot 4 lot 7T2 lot 2 lot 5 lot 8T3 lot 3 lot 6 lot 9

The nine lots were boiled in excess water for 12 minutes. Samples of them were used for the same

kinds of investigation reported above, namely, dehydration in a TGA run, to assess the water intake

on cooking, and extrusion through a Kramer cell, to quantify the stiffness of the product. The

cooking time, 12 minutes, was selected in order to have samples of comparable load maximum in

the extrusion test. A straight line correlation was found between the maximum load observed in the

extrusion test and the water intake on cooking (Figure 10).

11

0

10

20

30

40

50

60

0 10 20 30 40 50

Dry matter % (w/w)

Loa

d / k

g

lot 1

lot 2

lot 3

lot 4

lot 5

lot 6

lot 7

lot 8

lot 9

Figure 10. Maximum load observed in the extrusion test with a Kramer cell versus dry matter per 100 g overall mass of cooked (12 minutes) parboiled rice samples. Lettering refers to the elements of the sample matrix (see text).

This finding suggested that the different steaming conditions can affect the rate of water intake on

cooking, but do not produce substantial differences in the nature of the final texture (in terms of

Kramer analysis) which would simply be related to the water content of the boiled kernels. To

confirm this expectation , the cooking time was adjusted so as to attain the same water intake for all

the 9 lots. The same load maximum was found in the extrusion tests of the relevant samples (see

Figure 11).

30

35

40

45

50

55

60

20 25 30 35 40 45 50

Dry matter % (w/w)

Loa

d / k

g

lot 1

lot 4

lot 5

lot 6

lot 9

lot 1

lot 1

Figure 11. Maximum load observed in the extrusion test with a Kramer cell versus dry matter per 100 g overall mass of parboiled rice samples cooked for different time (Lot 1 9.5 min, Lot 4 10 min, Lot 5 10.5 min, Lot 6 12 min, Lot 9 12.5 min) so as to achieve the same water intake. Extra data were also collected for lot1 for 9.5 and 12 min cooking.

12

Conclusions

A systematic comparison between white and parboiled rice highlighted the different behavior

versus the cooking time as for starch gelatinization, water uptake and chewing load, which is

supposed to correspond to intrinsically different textures.

The different steaming conditions can affect the rate of water intake on cooking of parboiled rice,

but do not produce substantial differences in the nature of the final texture (in terms of Kramer

analysis) which is linearly correlated only to the water content of the boiled kernels. This is not the

case of white rice.

The possibility to use rice, that experience different parboiling procedures, to prepare cooked rice

samples with the same water content and with the same initial hardness, simplifies the design of

future investigations about the shelf life of the product and convenience rice based dishes,

discriminating the other sensorial variables that may depend on the parboiling process parameters.

References

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[3] AU: Yanai, K.; Miura, M.; Nakamura, R.; Nishinomiya, T.; Harada, T.; Kobayashi, S. (2001) Effect of freezing, storage and thawing conditions on quality of frozen cooked rice. Journal of the Japanese Society for Food Science and Technology (Japan), 48(10), 777-786.

[4] Hoover, R. (1995). Starch retrogradation. Food reviews international, 11(2), 331-346

[5] Strandt, T.; Vorwerck, K.; Muenzing, K. (1995). Contributions to evaluation of quality of parboiled rice and on parboiling effects. Getreide Mehl und Brot (Germany), 49(4), 237-242.

[6] Ong, M.H.; Blanshard, J.M.V. (1995). Texture determinants in cooked, parboiled rice. I: Rice starch amylose and the fine structure of amylopectin. Journal of Cereal Science, 21(3), 251-260.

[7] Ong, M.H.; Blanshard, J.M.V. (1995). Texture determinants of cooked, parboiled rice. II: Physicochemical properties and leaching behaviour of rice. Journal of Cereal Science, 21(3), 261-269.

[8] Marshall, W.E.; Wadsworth, J.L.; Verma, L.R.; Velupillai, L. (1993). Determining the degree of gelatinization in parboiled rice: comparison of a subjective and an objective method. Cereal chemistry, 70(2), 226-230.

[9] Kim, G.S.; Noh, Y.H.; Lee, H.B. (1994). The chemical changes of lipid components of rice(rough rice, brown rice, polished rice and parboiled rice) during storage. Journal of Agricultural Science Chungbuk University (Korea Republic), 11(2), 83-93.

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[10] Bello M., Baeza R., Tolaba M P. (2006). Quality characteristics of milled and cooked rice affected by hydrothermal treatment. Journal of Food Engineering, 72(2), 124-133.

[11] Derycke, V; Veraverbeke, W S; Vandeputte, G E; Man, W de; Hoseney, R C; Delcour, J A. (2005). Impact of proteins on pasting and cooking properties of nonparboiled and parboiled rice. Cereal Chemistry, 82(4), 468-474

[12] Raghavendra Rao, S.N; Juliano, B.O. (1970). Effect of parboiling on some physico chemical properties of rice. J. Agric Food chem, 18, 289-294.

[13] Fessas D. and Schiraldi A. (2000). Starch Gelatinization Kinetics in Bread Dough: DSC investigations on «simulated» baking processes. J. of Thermal Analysis and Calorimetry, 61, 411-423

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14

Figure captions

Figure 1. Load vs displacement record of a cooked rice sample extruded through a Kramer Cell.

Figure 2. DSC traces of partially cooked white rice. Lettering refers to the cooking time in minutes.

Figure. 3. Progress of starch gelatinization with cooking time in excess water at 100°C. t = [gelH(t=0) - gelH(t)] / gelH(t=0). gelH(t=0) = 11.6 Jg-1

Figure 4. TGA traces obtained from partially cooked white rice samples. Lettering refers to the cooking time in minutes.

Figure 5. DTG records from partially cooked white rice (lettering refers to the cooking time in minutes ) after normalization of the initial water content.

Figure 6. TGA traces obtained from partially cooked parboliled rice samples. Lettering refers to the cooking time in minutes.

Figure 7. DTG records from partially cooked parboiled rice after normalization of the initial water content. A part from the record after 2’ cooking time, all the other (6, 8, 10 and 14’) can be considered wholly overlapped to one another.

Figure 8. Load -vs - displacement records obtained from partially cooked (12, 14 and 16 min from upper to bottom) white rice forced to trespass the grid of a Kramer cell in a Texture Analyzer. Three replicas for each cooking time are reported.

Figure 9. Load -vs - displacement records obtained from partially cooked (12, 14 and 16 min from upper to bottom) parboiled rice forced to trespass the grid of a Kramer cell in a Texture Analyzer. Three replicas for each cooking time are reported.

Figure 10. Maximum load observed in the extrusion test with a Kramer cell versus dry matter per 100 g overall mass of cooked (12 minutes) parboiled rice samples. Lettering refers to the elements of the sample matrix (see text).

Figure 11. Maximum load observed in the extrusion test with a Kramer cell versus dry matter per 100 g overall mass of parboiled rice samples cooked for different time (Lot 1 9.5 min, Lot 4 10 min, Lot 5 10.5 min, Lot 6 12 min, Lot 9 12.5 min) so as to achieve the same water intake. Extra data were also collected for lot1 for 9.5 and 12 min cooking.

15