EFFECTS OF SOAKING AND STEAMING REGIMES ON THE

QUALITY OF ARTISANAL PARBOILED RICE

Manful J.TGraham-Acquaah, S*

Tchatcha, D

Parboiling rice involves partially boiling rice in its husk

Introduction

It is done to– seal internal fissures in the grain and obtain higher

head rice yields during milling– enhance the nutritional value of rice

How it’s done

Industrial parboiling1. Soaking

Constant temperature (60 -65ºC)

For about 3hours

2. SteamingPressure 4–6 kg/cm2 For up to 20min

3. Drying

Artisanal parboiling1. Soaking

Initial temperatures (from ambient to >90ºC ) for up to 16 hours

2. Steaming atmospheric pressure Duration is variable

3. Drying

Artisanal parboiling

The effect of soaking and steaming regimes on several other grain quality traits of parboiled rice however remains largely unexplained

Improving artisanal parboiling is a key component of many rice improvement programs in Africa

Soaking temperature and steaming time are two critical operations in artisanal parboiling. inappropriate soaking and steaming regimes during

artisanal parboiling result in lower head rice yield than raw-milled paddy (Manful et al., 2009)

ObjectiveTo characterize the effects of initial soaking temperature and steaming time on some physicochemical properties of artisanal parboiled rice using Response surface methodology (RSM)

• RSM is useful for modeling and analyses of factors Predicting and optimizing response variables

MethodsExperimental design• Central Rotatable Composite Design (CRCD)

– 2 factors namely• soaking temperature: 30-90ᵒC• steaming time: 5-20mins

– 14 treatment combinations

• Rice variety used: NERICA1Responses

Milling recoveries, appearance, pasting properties, cooking properties, cooked grain texture

Plate 1: Soaking process (300g/900ml water)

Plate 2: Steaming process

Methods

TreatmentTotal milled

rice yield (%)Cooking

time (mins)Color

Index- BCooked grain Hardness (g)

Control (raw rice) 65.3 19 3.5 283317.6ᵒC/12.5mins 72.3 21 13.9 361630ᵒC/5mins 71.1 22 10.5 409430ᵒC/20mins 72.4 24 13.9 397160ᵒC/2mins 73.7 21 13.1 379560ᵒC/12.5mins 72.5 21 12.3 395460ᵒC/23mins 73.5 23 13.8 425590ᵒC/5mins 74.5 23 10.5 419790ᵒC/20mins 74.7 21 13.3 4592100ᵒC/12.5mins 74.4 22 13.3 4268

ResultsTable 1: Effect of treatment combinations on some quality indices

Soaking temp(°C)

HR

Y (%

)

Steaming time(mins)

Estimated Response Surface

30 40 50 60 70 80 905

811

1417

20

28

38

48

58

68

Head Rice Yield0.0-6.06.0-12.012.0-18.018.0-24.024.0-30.030.0-36.036.0-42.042.0-48.048.0-54.054.0-60.060.0-66.066.0-72.0

TermRegressionCoefficient

X0(Constant) 23.40

X1(Soak temp) 0.14***

X2(Steam time) -0.61*

X12 2.58×10-3

X22 0.04

X1X2 1.60×10-3

R-square 91.37Lack-of-fit 2.18

Fig 1: Regression coefficients, ANOVA of terms, R2 and lack of fit, and surface plots for effects of factors on head rice yield (HRY)

***p<0.001; ** p<0.01; *p<0.05

Raw rice HRY = 36.3%

Cha

lkin

ess

(%)

Soaking temp(°C)Steaming time (mins)

Estimated Response Surface

30 40 50 60 70 80 905

811

1417

20

1.2

2.2

3.2

4.2

5.2

Chalkiness3.2-3.63.6-4.04.0-4.44.4-4.84.8-5.25.2-5.65.6-6.06.0-6.46.4-6.86.8-7.27.2-7.6

TermRegressionCoefficient

X0(Constant) 4.48

X1(Soak temp) -0.05***

X2(Steam time) 0.15

X12 0.21×10-5

X22 -2.68×10-3

X1X2 -1.33×10-3

R-square 85.11

Lack-of-fit- 5.29

Fig 2: Regression coefficients, ANOVA of terms, R2 and lack of fit, and surface plots for effects of factors on chalkiness

***p<0.001; ** p<0.01; *p<0.05

Raw rice chalkiness = 8.1%

8 68 1281882483083684284885486086687280

1000

2000

3000

4000

5000Raw

90ᵒC/20min

60ᵒC/1.9mins

60ᵒC/23mins

Time/s

Viscosity(cP)

Fig3: Pasting profile of selected treatments

Soaking temp(°C)Steaming time(mins)

Pea

k V

isc

(cP

)

Estimated Response Surface

30 40 50 60 70 80 905

811

1417

20

390

590

790

990

1190

1390

Peak Viscosity410.0-490.0490.0-570.0570.0-650.0650.0-730.0730.0-810.0810.0-890.0890.0-970.0970.0-1050.01050.0-1130.01130.0-1210.01210.0-1290.0

TermRegressionCoefficient

X0(Constant) 1183.69

X1(Soak temp) 7.69***

X2(Steam time) -21.88***

X12 -0.12***

X22 -0.24

X1X2 1.11×10-3

R-square 99.03Lack-of-fit - 7.90

Fig 4: Regression coefficients, ANOVA of terms, R2 and lack of fit, and surface plots for effects of factors on peak viscosity

***p<0.001; ** p<0.01; *p<0.05

Raw rice peak viscosity = 2565cP

Soaking temperature

Steaming time

1009080706050403020

20

15

10

5

Opt

imal reg

ion

Fig5: Overlaid contour plot showing optimal region for good head rice yields (≥ 60%) and low chalkiness (2.0)

HRY ≥ 60%Chalkiness ≤ 2.0

Validation run

Soaking temp (°C)

Steaming time (mins)

Head Rice Yield (%)

Chalkiness (%)

1 80.0 21.0 60.6 1.6

2 30.0 5.0 32.6 3.6

3 90.0 5.0 65.7 1.0

4 90.0 20.0 66.1 0.0

5 17.6 12.5 34.8 2.6

6 60.0 12.5 40.5 1.2

7 100.0 12.5 66.5 0.5

Table 5: Validation of results from response surface optimization

Conclusions

Parboiling treatments that increase head rice yield also influence on other physicochemical properties such as grain appearance, cooking properties, cooked rice texture and pasting properties.

The effect of parboiling on pasting properties suggest that parboiling could be optimized to alter the functional properties of rice in food formulations

To parboil to achieve good head rice yield and low chalkiness: Optimal range for initial soaking temperature = 80-95ᵒC Optimal range for steaming time = 22-

3mins

Thank you