objective measurement of retrogradation in cooked rice during storage

OBJECTIVE MEASUREMENT OF RETROGRADATION IN COOKED RICE DURING STORAGE

ISABEL LIMA and R.P. SINGH'

Department of Biological and Agricultural Engineering University of California, Davis

Davis, CA 95616

Accepted for Publication May 3, 1993

ABSTRACT

An Instron Universal testing machine andDifferentia1 Scanning Calorimetry were used to investigate textural changes in cooked rice during storage. Two cells were used with the Instron for hardness measurement (Ottawa Texture Measuring System and Back Extrusion cells) and one cell for adhesion measurement (adhesion cell). During storage of cooked rice, retrogradation of the starch led to an increase in hardness as well as a decrease in the adhesion of cooked rice. Storage time, temperature and variety significantly influenced the hardness and adhesion of cooked rice. A correlation analysis between methods showed that Instron measurements OTMS and BE correlated better with a correlation coeficient of 0.978 than the two Instron methods and DSC with a correlation coefficient of 0.752.

INTRODUCTION

In bland foods such as rice, texture plays an important role in consumer acceptance (Szczesniak 1971). Similar to other starch-based foods, cooked rice undergoes physicochemical changes (commonly referred to as staling) during storage. These changes are mainly related to a process called retrogradation, which leads to the hardening of the kernel, This process results in significant textural changes with time. Due to these changes, storage can shift rice from an acceptable range to unacceptable in terms of eating quality. However, cooked rice can be frozen to realize advantages of convenience and saving preparation time for cooking, if appropriate temperatures are utilized (Boggs et al. 1951). From a storage study done by Boggs et al. (1951), where cooked rice was stored for several periods of time up to 8 months, a sensory panel found that the frozen rice was fully equal to freshly cooked rice in every

'To whom correspondence should be sent.

Journal of Food Quality 16 (1993) 321-337. All Rights Reserved. Copyright 1993 by Food & Nutrition Press, Inc., Trumbull, Connecticut. 321

322 I. LIMA and R.P. SINGH

respect, even after 8 months of storage at -12C. Mitsuda er al. (1993) performed a three week storage study with cooked rice using two temperatures, refrigerator temperature and -2OC. Texture measurements showed that cooked rice stored at -2OC had only a slight change in its hardness and adhesion values, whereas cooked rice stored at the cold refrigerator temperatures had a considerable increase in hardness and lost its stickiness. It is then primordial to monitor and quantify physicochemical changes to be able to assess storage conditions that minimize them.

Several instruments are available for measuring cooked rice texture and extensive literature is available on instrumental and/or sensory evaluation of cooked rice texture (Blakeney 1979; Tsuji 1981; Okabe 1979; Juliano et al. 1981; Sowbhagya et al. 1987; Juliano 1982; Reyes and Jindal 1990). Some of the available instruments to determine cooked rice texture (either hardness and or adhesion) are the following: Instron food tester, General Foods Texturometer, tensipresser, Haake Consistometer, the parallel plate Plastometer, the Chopin- INRA Viscoelastograph, a sieving test, and a table balance. However, a standard method is currently not available. This study attempts to utilize two different techniques to evaluate the texture of cooked rice: a Differential Scanning Calorimeter (DSC) and the Instron food tester (using three different cells: Ottawa Texture Measuring System (OTMS), Back Extrusion (BE) and Adhesion cell). No research has been reported attempting to correlate these methods, combined to study starch retrogradation and changes in texture.

The specific objectives of this research were: (I) to study the dependence of cooked rice texture on storage temperature and (11) to determine the correlation between force values obtained from the Instron and enthalpy values measured with the DSC for different rice varieties.

MATERIALS AND METHODS

Rice Preparation

Samples of all milled rice varieties were sent to Foster Farms Chemistry Lab (Delhi, CA) for proximate analysis. Amylose content was determined by the Rice Research Center located at the United States Department of Agriculture, Agriculture Research Service, South Plains Area, Beaumont, Texas.

Three types of rice, long, medium, and short grain, were used. A 500 g sample of rice was cooked in 2.1 times the amount of distilled water in an automatic rice cooker. The overall average cooking time was 26.2 min. Cooked rice samples were placed in disposable cups with lids modified by punching out holes. These cups were inserted in polyethylene bags and vacuum sealed. To determine moisture content, duplicate samples of cooked rice were

RETROGRADATION IN COOKED RICE 323

dried in a vacuum oven at 70C for 48 h. Samples were then placed in a desiccator and allowed to cool to room temperature before final weight was measured on a microbalance. No additional weight change was observed for additional drying time. Rice containers were placed into storage incubators (-4C, -2C, +3C, 12C) for a 10 day storage period. Temperature fluctuations of & 0.6C were monitored in the storage incubators. For each day of analysis the containers with the frozen samples were defrosted in a 26C water bath, after which they were analyzed.

DSC Sample Preparation and Analysis

Differential Scanning Calorimetry (DSC) was performed using a Perkin Elmer DSC-7 with a Perkin Elmer PC series thermal data analysis unit.

Cooked rice samples were compressed until a homogeneous rice paste was formed and a very small sample ( - 15 mg) was placed in a pan. A droplet of water was added to the rice sample and the pan was sealed with a pan lid. A second pan was sealed empty as the "reference" pan. The rice samples were analyzed at a constant heating rate of 10C/min over a temperature range of 20C to 80C ("sample" scan). This procedure was repeated for the same sample pan to obtain a "baseline" scan, following the same parameters for the "sample" scan. Sample pans were punched after the trial for drying purposes to determine the dry weight.

Data analysis included the subtraction of the baseline scan from the sample scan. The area under each endotherm peak was computed by instrument software to give an enthalpy value (AH) in Joules per gram (J/g). Calculated values of enthalpy were averaged from three replicates, higher values of enthalpy corresponding to higher rates of retrogradation (Chang and Liu 1991).

Instron Analysis Procedures

Data from all Instron measurements were collected with an IBM-PC" computer, controlled by LabTech Notebook" software and analyzed using Lotus 123" software. Voltage readings were converted into force values.

OTMS Cell. For measurement of hardness, a 17 g sample of cooked rice was gently placed in the OTMS cell. This cell consists of a perforated bottom plate with 6 mm diameter holes for the extrusion of the rice and a square shaped plunger that travels through the cell while extruding the rice sample. The cross head speed was set at 100 mmlmin until it reached 0.4 mm from the bottom plate, after which the plunger automatically returned to the initial position. Hardness was defined as the maximum force in Newtons required to compress the 17 g of rice through the perforated plate.


BE cell. A cell similar to the one from Reyes and Jindal (1 990) was used with a 12 g sample of cooked rice to measure hardness. It consists of a 80 mm long aluminum cylinder with an internal diameter of 16.3 mm and a stainless steel spherical plunger with a 12.7 mm diameter. The control setting was set to position the plunger at 7 mm below the top of the cylinder cell with a down stroke of 67.0 mm ending at a position 6.4 mm from the bottom of the cylinder. Cross head speed was set at 50 mm/min. Hardness was defined as the extrusion energy in N*m from the value of the area under the curve.

Adhesion cell. For measurement of adhesion, the OTMS plunger and a flat aluminum plate (6.9 cm x 6.9 cm) were used. A 17 g sample was placed on a flat anvil and pressed at a cross head speed of 50 mm/min to a clearance of 0.4 mm with the OTMS plate for a period of 10 s, after which the plunger was raised at 5 mm/min. Data were collected during the upward stroke of the plunger. The adhesion was defined as the force required to pull the plunger away from the rice sample on the base plate. Adhesion energy was obtained from the area under the force-displacement curve in N*m.

Consistent placement of the rice sample in the Instron cell was found to be the most sensitive step as different compaction levels caused discrepancies in the replicates. For the adhesion test, preliminary tests indicated that packing the rice on the platform yielded higher adhesion values than with no packing. These results were also found when studying cooked rice stickiness using the adhesion cell (Mossman ef al. 1983). For this reason, in this study the mode for all three Instron cells was standardized. For the OTMS cell, the sample was packed during 10 s using a 145 g steel cube (for better distribution and compaction inside the OTMS cell). In the BE method, the sample was gently packed with the help of a steel rod. For the adhesion test, samples were placed in a squared pile with minimum packing. In all tests, the stickier cooked rice samples were the ones presenting more obstacles to fill and pack.

Data Analysis

Analysis of variance of the experimental data was performed using ANOVA software package adapted to a Macintosh” computer. A split-plot model design was developed to fit the data. Rice variety, storage temperature and storage time were the parameters influencing the variables: hardness, adhesion, and enthalpy. Further, at each main effect (rice variety, storage temperature, and storage time), different treatment levels were compared using Duncan multiple range test at 5% level of significance.

A correlation analysis was developed for all three Instron methods as well as the DSC in order to compare the results obtained from each one of these cells.


RESULTS AND DISCUSSION

Starch content of raw milled rice for long, medium and short grain rice varieties was 81, 80, and 80%, respectively (Table 1). These values are within the commonly reported range of 80-90%.

TABLE 1. PROXIMATE ANALYSIS OF THE THREE MILLED RICE SAMPLES

Sample Long grain Medium grain Short grain

Moisture (wb) (8) 11.3 11.2 11.3

Carbohydrate (8) 80.54 80.48 79.88

Amylose (%) 25.4 17.4 0.0

Moisture content (wet basis) for all three varieties of raw milled rice ranged between 11.2 and 11.3% (Table 1). Sample variances for cooked rice moisture content (dry basis) ranged from207.5% f 2.04,205.9% f 10.6, and 213.1 % f 7.0 for long, medium, and short grain rice, respectively.

During the storage period, an increase in hardness and decrease in adhesion were observed for all rice varieties as a result of rice retrogradation. Initial average hardness values measured from OTMS, ranged from 83.19 f 8.44 N, 48.76 f 5.89 N, and 24.23 2.55 N, for long, medium, and short grain rice, respectively. The most significant changes in enthalpy occurred between days 0 and 2, and generally only a small increase took place thereafter (Fig. la). Hardness values in Fig. 2a for long grain rice present the same trend. Taylor (1991) observed that the retrogradation peak developed much faster in the beginning of the recrystallization and after that only a slight increase in retrogradation was present in cooked rice stored up to 28 days.

Analysis of variance showed that all effects (storage time, temperature and rice variety) were significant, at 5% significance level (Tables 2 and 3). Duncan Multiple Range tests showed that within each effect, hardness, enthalpy and adhesion values taken from different treatment levels were significantly different, at 5% significance level (Tables 4, 5 and 6).

Minimum and maximum average hardness values at the end of the storage period were 133.12 f 4.32 N and 326.87 f 11.87 N , 62.98 &- 0.88 N and 102.71 f 3.04 N, and 19.03 f 2.25 N and 95.65 f 2.75 N, for long, medium, and short grain rice varieties, respectively. These values reflect the differences in hardness between different varieties and the influence of the storage temperature (Fig. 2a, 2b, and 2c). All measurement methods used in


-c t 3 C --o- - 2 c ' " 1 --Q- +12c - .4c

a 2 4 6 a 10

Time, days

FIG. la . ENTHALPY VALUES (AH, J/g) FOR LONG GRAIN RICE MEASURED WITH DSC

I -c + 3 c --c-- -2c -0- +12c - -4c

6

5 m 'i if

3

2

1

a 0 2 4 6 8 10

Tlme, days

FIG. lb. ENTHALPY VALUES (AH,J/g) FORMEDIUM GRAIN RICE MEASURED WITH DSC

0 2 4 6 8 1 0

Time,. days

FIG. l c . ENTHALPY VALUES (AH, J lg ) FOR SHORT GRAIN RICE MEASURED WITH DSC

TABL

E 2.

A

NA

LYSI

S O

F V

AR

IAN

CE

FOR

HA

RD

NES

S O

F C

OO

KED

RIC

E

Sou

rce

Var

iety

df

Sum

of

Squ

ares

M

ean

Squ

are

F-V

alue

P

-Val

ue

Err

or

Term

2

254229.673

127114.836

222.188

,004

5 V

arie

ty *

B

lock

B

lock

1

17.181

17.181

,030

,8784

Var

iety

*

Blo

ck

Var

iety

*

Blo

ck

2 1144.21 1

572.106

1.846

.1696

Res

idua

l

Tim

e 4

50895.660

12723.915

28.719

,0001

Tim

e '

Var

iety

B

lock

Ti

me

* V

arie

ty

8 26022.138

3252.767

7.342

.0013

Tim

e '

Var

iety

*

Blo

ck

Tim

e *

Var

iety

*

Blo

ck

12

531 6.597

443.050

1.430

,1881

Res

idua

l Te

mp.

3

38845.663

12948.554

41.785

,0001

Res

idua

l Te

mp.

*

Var

iety

6

58253.512

9708.919

31.331

.0001

Res

idua

l Te

mp.

*

Tim

e 12

15661.530

1305.128

4.212

,000

2 R

esid

ual

Tem

p. ' V

arie

ty *

Ti

me

24

24355.950

1014.831

3.275

,0003

Res

idua

l R

esid

ual

45

13944.861

309.886

Dep

ende

nt: O

TMS

Sou

rce

Var

iety

Blo

ck

Var

iety

*

Blo

ck

Tim

e Ti

me

* V

arie

ty

Tim

e *

Var

iety

*

Tem

p.

Tem

p. ' V

arie

ty

Tem

p. *

Tim

e

Tem

p. *

V

arie

ty

Res

idua

l D

epen

dent

: BE

df

Sum

of

Squ

ares

M

ean

Squ

are

F-V

alue

P

-Val

ue

Err

or

Term

2

77.907

38.953

344.463

.0029

Var

iety

*

Blo

ck

1 2 4 8 B

lock

12

3 6 12

* Ti

me

24

45

,073

.226

23.040

8.602

1.257

13.394

11.779

6.106

4.845

3.350

,073

,1

13

5.760

1.075

,105

4.465

1.963

.509

.202

,074

,644

1.519

54.993

10.266

1.407

59.978

26.374

6.836

2.712

,506

3 ,2299

,0001

,0003

,1981

,0001

,0001

,0001

,001 9

Var

iety

' B

lock

R

esid

ual

Tim

e *

Var

iety

' B

lock

Ti

me

* V

arie

ty

Blo

ck

Res

idua

l R

esid

ual

Res

idua

l R

esid

ual

Res

idua

l

a: E 4

n

b a 5 z 2 E 0

M

u

N

-4

Dep

ende

nt v

aria

bles

OTMS a

nd

BE

, are

In

stro

n c

ells

for

mea

suri

ng h

ardn

ess

TABL

E 3.

A

NA

LYSI

S O

F V

AR

IAN

CE

FOR

EN

THA

LPY

AN

D A

DH

ESIO

N O

F C

OO

KED

RIC

E

Sou

rce

Var

iety

df

S

um o

f S

auar

es

Mea

n S

auar

e F-

Val

ue

P-V

alue

E

rror

Te

rm

2 22

1.03

5 11

0.51

6 18

3.12

1 ,0

054

Var

iety

*

Blo

ck

Blo

ck

1 3.

056

3.05

6 5.

063

,153

3 V

arie

ty '

Blo

ck

Var

iety

*

Blo

ck

2 1.

207

,60

4

1.16

8 ,3

203

Res

idua

l Ti

me

4 43

6.13

1 10

9.03

3 27

7.59

1 ,0

00

1

Tim

e V

arie

ty

* B

lock

Ti

me

* V

arie

ty

8 64

.130

8.

016

20.4

09

,000

1 Ti

me

* V

arie

ty '

Blo

ck

Tim

e *

Var

iety

*

Blo

ck

12

4.

713

.393

,7

60

,686

5 R

esid

ual

Tem

p.

3 60

.498

20

.166

39

.018

,0

00

1

Tem

p.

* V

arie

ty

6 57

.708

9.

618

18.6

09

,000

1 R

esid

ual

Res

idua

l Te

mp.

' T

ime

12

39

.585

3.

299

6.38

3 .0

00

1

Res

idua

l Te

mp.

*

Var

iety

* T

ime

24

36

.996

1.

541

2.98

3 .0008

Res

idua

l R

esid

ual

45

23

.258

.5

17

Dep

ende

nt: D

SC

Sou

rce

df

Sum

of

Sau

ares

M

ean

Sau

are

F-V

alue

P

-Val

ue

Err

or

Term

V

arie

ty

2 ,0

25

.0

12

29.2

52

,033

1 V

arie

ty

* B

lock

B

lock

1

7.17

8E-5

7.

178E

-5

,168

.7

214

Var

iety

*

Blo

ck

Var

iety

*

Blo

ck

2 .0

01

4.26

4E-4

16

.277

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001

Res

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me

4 ,0

16

-0

04

33.2

85

,00

01

Ti

me

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lock

Ti

me

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8 ,0

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1.51

7E-4

1.

255

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8 Ti

me

* V

arie

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+

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12

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209E

-4

4.61

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6

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35

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ime

24

.003

1.12

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4.

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1 R

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ual

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ende

nt: A

dhes

ion

Res

idua

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5

.00

1

2.61

9E-5

w

h)

00

!-

Dep

ende

nt v

aria

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DSC

, is

the

met

hod

for

enth

alpy

det

erm

inat

ion

an

d A

dhes

ion

is th

e ce

ll to

mea

sure

adh

esio

n, r

espe

ctiv

ely


TABLE 4. DUNCAN NEW MULITIPLE RANGE TEST FOR THE TEMPERATURE EFFECT

ON ADHESION USING THE ADHESION CELL'

vs. Diff. Crit. diff.

t 2 t l ,008 ,003 S 13 ,014 ,003 S t 4 .017 ,003 S

t l t 3 ,006 ,003 S

t 4 ,010 ,003 S t 3 t 4 ,004 ,003 S

S = Significantly different at this level.

' t l = -4c, t2 = -2c, t3 = + 3 c , t4 = +12c.

TABLE 5 . DUNCAN NEW MULTIPLE RANGE TEST FOR THE STORAGE TIME EFFECT

ON ENTHALPY UTILIZING THE DSC'


d l d2 2.601 .394 s d3 3.461 .413 s d4 4.540 .426 S d5 5.591 ,430 S

d2 d3 ,861 ,394 s d4 1.940 .413 s d5 2.990 .426 S

d3 d4 1.079 ,394 s d5 2.1 29 .413 s

d4 d5 1.050 ,394 s S = Significantly different at this level.

y'dl = day zero, d2 = day one, d3 = day two, d4 = day four, d5 = day ten

TABLE 6 . DUNCAN NEW MULTIPLE RANGE TEST FOR THE RICE VARIETY EFFECT ON

HARDNESS UTILIZING THE BE CELL'


v 3 v 2 ,465 ,324 S v l I .a94 ,324 S

v 2 V l 1.429 ,324 S

S = Significantly different at this level.

I l v l = long grain rice, v 2 = medium grain rice, v3 = short grain rice.


400

- +12c - 4 c

300

z g 200 O Y

100

0 0 2 4 6 8

Time. days

FIG. 2a. HARDNESS VALUES EXPRESSED IN FORCE (N) FOR LONG GRAIN RICE MEASURED WITH OTMS CELL

0 2 4 6 8 10

Time, days

FIG. 2b. HARDNESS VALUES EXPRESSED IN FORCE (N) FOR MEDIUM GRAIN RICE MEASURED WITH OTMS CELL

FIG. 2c.

---o- r 3 C - -2c

-+ tTac - - 4 c

20 t 0 2 4 6 8 10

Time, daya

HARDNESS VALUES EXPRESSED IN FORCE (N) FOR SHORT MEASURED WITH OTMS CELL

GRAIN RICE

RETROGRADATION IN COOKED RICE 33 1

the study (Instron OTMS, BE and DSC) consistently demonstrated that long grain rice was the most retrograded rice variety upon storage. Retrogradation is a function of time, temperature, and concentration of amylose among several factors (Young 1984).

The influence of the rice variety is related to the amount of amylose present in the rice starch granules. Amylose content of raw milled rice for long, medium and short grain rice varieties was 25.4 f 0.7, 17.4 f 0.3 and 0 % , respectively. Having a higher amylose content, long grain rice retrograded the most, followed by medium and short grain rice varieties. However, amylopectin may contribute to retrogradation but at a much lower level. Teo and Seow (1992) used pulsed nuclear magnetic resonance (NMR) to study the kinetics of retrogradation of various starches stored at different temperatures. A short induction period during the first days of storage was observed for waxy rice starch (less than 1% amylose) but not for nonwaxy starch. Waxy starch was rated with the least tendency to retrograde due to the greater difficulty of amylopectin to undergo nucleation (Teo and Seow 1992). Similar results were obtained from Chang and Liu (1991), when using the DSC to study rice starch retrogradation. Again high amylose samples were more retrograded.

Besides its influence on retrogradation, and although not a dominant factor in cooking quality, amylose content was the dominant factor correlated to the eating quality of both warm and cold cooked rice (Juliano et af. 1965).

Adhesion in this study was measured as the force resisting separation, which was maximum for freshly cooked rice for a specific variety, and decreased during storage (Fig. 3a, 3b and 3c). Average initial adhesion values ranged between 0.028 f 0.0058 N*m, 0.051 f 0.0023 N*m and 0.048 k 0.0058 N*m for respectively long, medium and short grain rice varieties. Upon storage, the decrease in adhesion values (percentage from initial value, averaged for all temperatures) was the highest for long grain variety (96.0%), followed by medium grain rice (71 .O%) and short grain rice (48.1 %). Parallel to the hardening of the kernel, retrogradation lead to a decrease in the adhesion and again long grain showed the highest change. In terms of appearance, grains turned dry and flaky with separate kernels.

Parallel to texture evaluation, DSC measurements were taken to monitor cooked rice retrogradation. Final hardness and enthalpy values were dependent upon temperature of storage and cooked rice stored under lower temperatures retrograded more, therefore becoming harder than cooked rice stored under higher temperatures.

Lower temperatures of storage increased retrogradation rates (Fig. la, lb , and lc). It is known that starch retrogradation is favored by low storage temperatures, up to the glass transition temperature (Taylor 1991). Below this temperature, retrogradation is significantly reduced (Taylor 1991). A glass transition temperature of -5C for long grain rice, as reported by Taylor (1991),


OTMS

Back Extrusion

DSC

Adhesion

explains the maximum retrogradation rates observed for this variety when stored at -4C (Fig. la). However, for short grain rice, and considering a higher glass transition temperature for this variety, retrogradation levels where considerably reduced at the -4C temperature (Fig. lc). On the other hand, at -2C storage temperature, short grain rice was highly retrograded (Fig. lc). These observations demonstrate how storage temperature influences textural changes of the stored cooked rice and how the rice variety determines the extent of changes.

To correlate the retrogradation process with the textural changes taking place during storage, a correlation study was performed comparing Instron methods and DSC. To investigate how well the three methods correlate, values of enthalpy change (AHf, J/g) from the DSC, and Instron values for hardness from OTMS (N) and BE (Nom), and adhesion values (Nom) were compared (Table 7).

Instron OTMS and BE cells were the most correlated methods for hardness determination with a correlation coefficient of 0.978. DSC versus OTMS and BE cells, gave correlation coefficients of 0.716 and 0.788, respectively (Table 7). Negative correlation coefficients were obtained between adhesion measurements and hardness values because these two physical properties are inversely related.

An averaged correlation coefficient of 0.752 was obtained between Instron and DSC methods showing lower correlation than between OTMS and BE cells, 0.978. Figures 2a and 4a display hardness values for long grain rice measured by OTMS and BE, respectively. These two plots show very similar trends for all storage temperatures. The same trend was observed for the other two varieties Fig. 2b and 4b, and Fig. 2c and 4c).

OTMS Back Extrusion DSC Adhesion

1

0.978 1

0.716 0.788 1

-0.702 -0.776 -0.862 1

TABLE 7. CORRELATION MATRIX OF INSTRON AND DSC TECHNIQUES


- r12C - 4 c

0 2 4 6 8 1 0

Tlme, days

FIG. 3a. ADHESION VALUES EXPRESSED IN ADHESION ENERGY (Nmrn) FOR LONG GRAIN RICE MEASURED WITH ADHESION CELL

0.06 ---e +3c - -2c

- + 1 2 c - - 4 c

0.03

Tlme, days

ADHESION VALUES EXPRESSED IN ADHESION ENERGY (Nmrn) FOR MEDIUM GRAIN RICE MEASURED WITH ADHESION CELL

0 06

0 05

E

i

1: 003

004

g 0 - 2 0.02 U

0.01

0.00 0 2 4 6 8

Tlme. days

FIG. 3c. ADHESION VALUES EXPRESSED IN ADHESION ENERGY (Norn) FOR SHORT GRAIN RICE MEASURED WITH ADHESION CELL.


-+3c --2c - r12C -C -4C

2 4 6 8 10

Tlme, days

FIG. 4a. HARDNESS VALUES EXPRESSED IN EXTRUSION ENERGY (Nmm) FOR LONG GRAIN RICE MEASURED WITH BE CELL

E l 5 iz

P h

f 10

g e -

0 5 - +3c - .2c

1 -+12c - 4 c l " " . I . ,

0 2 4 6 8 10

T h e . days

FIG. 4b. HARDNESS VALUES EXPRESSED IN EXTRUSION ENERGY (Nmm) FOR MEDIUM GRAIN RICE MEASURED WITH BE CELL

L"

- +3c - .2c - r12C - 4 c E i

E $ 1.0

h

0 - g

0 5

00 0 2 4 6 8 10

Tlms, days

FIG. 4c. HARDNESS VALUES EXPRESSED IN EXTRUSION ENERGY (N*m) FOR SHORT GRAIN RICE MEASURED WITH BE CELL


The correlation results between DSC and Instron indicate that the changes in the rheological properties (such as hardness and adhesion) and enthalpy changes expressed by DSC, do not observe retrogradation at the same level. Comparing retrogradation rates, starches are ranked differently by the rheological and DSC techniques; Roulet et al. (1990) while studying retrogradation, performed DSC and rheology measurements in eight different starches, including rice starch. Roulet et af. (1990) stated that rheology follows the stiffening effects of the global macromolecular modifications with time, whereas DSC enthalpy measurements follow the starch crystallization more closely. This observation helps to explain how Instron and DSC had a lower correlation than OTMS and BE methods within Instron.

The occurrence of variations in data was expected and methods to reduce excessive variation were observed. The variations in the data reported in this study can be the result of sample handling procedures. Prior to experimental trials, all weighed samples were kept inside closed containers. Additional improvements may be possible by stricter control of both room temperature and humidity and minimizing the time elapsed from removal of samples from storage to testing of the samples.

A batch of cooked rice can present moisture gradients from the edges to the center of the cooking pan. Mixing the batch after cooking is advisable and can help to remove the possible variation existing within the same batch of cooked rice. Together with the previous precautions, the use of replications and a large sample can be sufficient to reduce the variability of the measurements. The characteristics of the material being measured, the basic precision of the method, and the care with which the method is applied, are factors that influence the precision of the method.

CONCLUSIONS

Storage of cooked rice leads to a general increase in hardness and a decrease in adhesion for all varieties. The rate of change was dependent on the storage temperature, time and rice variety. Upon storage, textural changes related to the starch retrogradation resulted in a firmer texture with greater separation of individual kernels, and a dry appearance. Minimum retrogradation in cooked rice was observed at a storage temperature of -4C for short grain rice. Medium and long grain rice varieties require temperatures below -4C for retrogradation levels to be considerably reduced.

Correlation coefficients between methods showed that both OTMS and BE Instron cells correlate better than Instron and DSC. It was concluded that this observation was related to the fact that rheology follows the stiffening effects of the global macromolecular modifications with time, whereas DSC enthalpy

336 1. LIMA and R.P. SINGH

measurements follow the starch crystallization more closely. Possible significant sources of variation in the data were determined to be the cooking step, the packing step in the Instron cells, and variations wihin the rice sample itself.

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