3rd Africa Rice Congress Theme 3: Rice processing and marketing Mini symposium:Improving the quality of rice produced in Africa Author: Graham-Acquaah

1. EFFECTS OF SOAKING AND
STEAMING REGIMES ON THE
QUALITY OF ARTISANAL PARBOILED
RICE
Manful J.T
Graham-Acquaah, S*
Tchatcha, D

2. Introduction
Parboiling rice
involves partially boiling rice in its husk
It is done to
– seal internal fissures in the grain and obtain higher
head rice yields during milling
– enhance the nutritional value of rice

3. How it’s done
Industrial parboiling
Artisanal parboiling
1. Soaking
1. Soaking
 Constant temperature
(60 -65ºC)
 For about 3hours
2. Steaming
 Pressure 4–6 kg/cm2
 For up to 20min
3. Drying
 Initial temperatures
(from ambient to >90ºC )
 for up to 16 hours
2. Steaming
 atmospheric pressure
 Duration is variable
3. Drying

4. Artisanal parboiling
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)
The effect of soaking and steaming regimes on
several other grain quality traits of parboiled
rice however remains largely unexplained

5. Objective
To 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

6. Methods
Experimental design
• Central Rotatable Composite Design (CRCD)
– 2 factors namely
• soaking temperature: 30-90ᵒC
• steaming time: 5-20mins
– 14 treatment combinations
• Rice variety used: NERICA1
Responses
Milling recoveries, appearance, pasting
properties, cooking properties, cooked grain texture

7. Methods
Plate 1: Soaking process
(300g/900ml water)
Plate 2: Steaming process

8. Results
Table 1: Effect of treatment combinations on some quality indices
Total milled
Cooking
Treatment
rice yield (%) time (mins)
Control (raw rice)
65.3
19
17.6ᵒC/12.5mins
72.3
21
30ᵒC/5mins
71.1
22
30ᵒC/20mins
72.4
24
60ᵒC/2mins
73.7
21
60ᵒC/12.5mins
72.5
21
60ᵒC/23mins
73.5
23
90ᵒC/5mins
74.5
23
90ᵒC/20mins
74.7
21
100ᵒC/12.5mins
74.4
22
Color
Index- B
3.5
13.9
10.5
13.9
13.1
12.3
13.8
10.5
13.3
13.3
Cooked grain
Hardness (g)
2833
3616
4094
3971
3795
3954
4255
4197
4592
4268

9. Estimated Response Surface
Raw rice HRY = 36.3%
Regression
Coefficient
23.40
X0(Constant)
X1(Soak temp)
X2(Steam time)
0.14***
-0.61*
X12
2.58×10-3
X22
X1X2
R-square
Lack-of-fit
0.04
1.60×10-3
91.37
2.18
***p<0.001; **
p<0.01; *p<0.05
68
HRY (%)
Term
58
48
38
28
30
40
50
60
70
80
90
5
8
11
14
17
20
Steaming time(mins)
Soaking temp(°C)
Fig 1: Regression coefficients, ANOVA of terms, R2 and lack of fit,
and surface plots for effects of factors on head rice yield (HRY)

10. Estimated Response Surface
Regression
Coefficient
4.48
X1(Soak temp)
-0.05***
X2(Steam time)
0.15
Term
X12
0.21×10-5
X22
X1X2
-2.68×10-3
-1.33×10-3
R-square
85.11
Lack-of-fit-
5.29
***p<0.001; **
Fig 2:
5.2
Chalkiness (%)
X0(Constant)
Raw rice chalkiness = 8.1%
4.2
3.2
2.2
1.2
30
20
17
14
11
40
50
60
70
80
8
5 Steaming time (mins)
90
Soaking temp(°C)
p<0.01; *p<0.05
Regression coefficients, ANOVA of terms, R2 and lack of fit,
and surface plots for effects of factors on chalkiness

11. 5000
90ᵒC/20min
4000
60ᵒC/1.9mins
60ᵒC/23mins
3000
2000
1000
0
8
56
104
152
200
248
296
344
392
440
488
536
584
632
680
728
Viscosity
(cP)
Raw
Time/s
Fig3: Pasting profile of selected treatments

12. Estimated Response Surface
Raw rice peak viscosity = 2565cP
Term
X0(Constant)
1183.69
7.69***
X1(Soak temp)
X2(Steam time)
-21.88***
X12
X22
X1X2
R-square
Lack-of-fit -
-0.12***
-0.24
1.11×10-3
99.03
7.90
***p<0.001; **
Fig 4:
p<0.01; *p<0.05
1390
Peak Visc (cP)
Regression
Coefficient
1190
990
790
590
20
17
14
11
390
8
30 40 50
5
60 70 80
90
Steaming time(mins)
Soaking temp(°C)
Regression coefficients, ANOVA of terms, R2 and lack of fit,
and surface plots for effects of factors on peak viscosity

13. Chalkiness ≤ 2.0
HRY ≥ 60%
Steaming time
Optimal region
20
15
10
5
20
30
40
50
60
70
80
Soaking temperature
90
100
Fig5: Overlaid contour plot showing optimal region for good head
rice yields (≥ 60%) and low chalkiness (2.0)

14. Table 5: Validation of results from response surface optimization
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

15. Conclusions
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
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