180 Ministry of Agriculture & Rural Development

Project Completion Report

MS14: PROJECT COMPLETION REPORT 026/05VIE
Investigation of rice kernel cracking and its control in the field and
during post-harvest processes in the Mekong Delta of Vietnam APPENDIX 8A

EXTENSION WORKERS MANUAL-PART 1

IMPROVING FARMER’S PROFITABILITY BY MINIMIZING
GRAIN LOSS AND MAINTAINING QUALITY THROUGH
CORRECT PROCEDURES DURING HARVESTING AND POST-
HARVEST OPERATIONS
181


3. Drying method, and
4. Milling

Harvesting loss is relevant only in the first two sections, while the grain cracking and head rice yield
are affected by activities during the harvesting as well as post-harvest. The main factors which contribute to the rice cracking and value losses are:
• Harvesting time- before and after grain maturity
• Harvesting methods- manual, reaper, combined harvester
• Drying methods- sun drying and mechanical drying
• Milling losses- small, medium, large rice milling plants and milling procedure.
182
Types of Losses
Harvesting
Time At the mature date G
After the mature date G+V
Method Manual cutting + Hand collecting + Mechanical Threshing
Reaper + Hand collecting + Mechanical Threshing G+V
Reaper + Combined-Collector (collecting + threshing)
Combined-Harvester
Transportation G

Drying
Field drying G+V
Sun drying G+V
Mechanical drying Correct G


183 SUMMARY OF POST-HARVEST LOSSES

Table 2 is the summary of total post-harvest losses (Table 2) based on the data collected in the past two
years from Mekong Delta region. In this table, the values are the overall estimation from different
varieties. The values presented are in the relative term between the cases. For instance, late harvesting
has 3.5% of losses in comparison to harvesting at correct time. The value loss at correct harvesting time
is considered to be 0%.

• From the milling survey data, an average milling quality system caused a value loss of about
4% in comparison to a good quality system which was considered to be 0% value losses.
• Experimental data of flat-bed drying gave that incorrect drying procedure resulted in 5% value
losses in comparison to correct case which was considered to be 0% value losses.
• Sun drying of panicle means that paddy after cutting was left in the field for sun drying (field
drying). The value losses of 8.7% and 4% of field drying and sun drying on yard, respectively,
were the relative values compared to correct drying procedure.

Table 2: Overall evaluation of total post – harvest losses (late harvesting, moderate milling quality)
Harvesting
time
Harvesting
method
Threshing Sun drying Drying Milling
Total

(G=2.9%)
Yes
(V=1.5%)
Incorrect (V=5%) 16.9
Yard(V=4%) 14.2
Correct(0%) 10.2
Late
(V=3.5%)
Combined-
Harvester
(G=1.2%
V = 1.5%)
No
(0%)
Incorrect (V=5%)
Average
quality
(4%)
15.2
Note: V= Value losses, G = Grain losses.
There are three cases of losses that can be highlighted in Table 2.
1. The highest total losses is 20.6% including late harvesting (3.5%), manual cutting (2.9%),
threshing (1.5%), field drying (8.7%), incorrect drying procedure (5%) and using average
milling quality (4%).
2. The common cases of total losses are in the range of 12.4 to 15.9% including correct (or late)
harvesting time, manual cutting, threshing, sun drying on yard, average milling quality applied.

184
grain can develop cracks during the late maturity stage due to rewetting. At the same time, during dry
season it is likely that the grains over-dry if not harvested at correct time. Thus we need to identify the
correct time for harvesting.

Estimating the correct time of harvesting

Recommended time of harvesting for each variety is estimated from the time of sowing in each season.
Table 3 presents the estimated days from sowing to harvest for various varieties in MRD.

185

Table 3. Growing time recommended by extension center

Variety Recommended
growing time (day)
Variety Recommended
growing time (day)
OM1490 87-92 OM2517 85-90
OM2718 90-95 IR50404 90-95
Jasmine 95-105 OM4498 90-95
AG24 85-90 While this is useful way to estimate correct harvesting time, each crop differs in the correct harvesting
186

From the experiments on 7 rice varieties in three seasons in the past 2 years (2006-2008), the optimal
harvesting time to maximise head rice yield for each variety in each season (dry/rainy) is presented in
Table 4. Most of the cases fell within the recommended day mentioned in Table 3. However, some
cases were outside of this range such as OM2517 (94 days in rainy season), AG24 (94 days in rainy
season) and OM2718 (88 days in rainy season).

Table 4: Optimal growing time for highest HRY (coloured numbers are the optimal growing time
for harvesting)

Variety Season
Growing time (day)
Dry
88
90 92 94 96 98
OM1490
Rainy
88 90
92
94 98 104
Rainy
88
90 92 94 96 98
OM2718

IR50404
Rainy
88 90
92
94 96 98

Yield loss

Correct timing of harvest is crucial in order to prevent crop loss. Grain losses may occur from rats,
birds, crop lodging, insects, and shattering. Timely harvesting ensures good grain quality and high
market value.

Harvesting too early will result in a larger percentage of unfilled or immature grains, which will result
in a lower yield and in higher grain breakage during milling.

Harvesting too late will lead to excessive losses and increased breakage in rice. Grain cracking and head rice yield (HRY)

The following figures (Figure 2 and 3) show the mean of grain cracking and also that of head rice yield
among several varieties obtained in Mekong Delta during 4 seasons of experiments in 2006-2008. 187

51.06
52.3
50.73
47.99

16
20
24
-6 -4 -2 0 +2 +4 +6
Harvesting time (days from maturity)
Grain cracks (%)
0.40 0.40
1.20
2.80
10.80
4.00
5.20
0
2
4
6
8
10
12
-6 -4 -2 0 +2 +4 +6
Harvesting time (days from maturity)
Grain cracks (%)

Variety: 1490 (Rainy season 2006) Variety: 2718 (Rainy season 2006)

Figure 2: Effect of harvesting time on grain cracking.
The X axis indicates days of early or late harvesting compared to maturity date (day 0).

45.41
51.47

Do Not harvest late after the
maturity date in order to avoid
cracking and to increase HRY
188

Table 5 summarises the losses due to late harvesting for different varieties harvested in different
seasons of MRD in 2006-2008. The HRY is reduced by 5-28% if harvesting time was 6 days late.
Assuming 1% of HRY reduction equivalent to 0.5% of losses (50% value loss), then late harvesting
would cause a loss of 2.5-14%. The varieties AG24 and IR50404 had losses below 2.5%, whereas the
varieties OM1490, OM2517 and OM2718 had high losses of above 5% (6 days late harvesting). Theses
varieties need to be harvested earlier.

Table 5: Losses of Head Rice Yield (HRY %) due to late or early harvesting.
Variety Season
Losses of HRY (%) in comparison to optimal harvesting time
Dry/Rainy??
0
1.57 4.31 10.07 15.79 17.77
OM1490
Rainy/Dry??
1.69 0.97
0
3.33 7.65 11.55
Rainy
0
7.93 7.56 12.71 14.64 10.75
OM2718

4.27
IR50404
Rainy
-0.25 0.89
0
5.97 1.29 2.16
Note: 0 is the optimal day. After (before) this day is late (early) harvesting. From one cell to the adjacent cell the harvesting
time changed by two days.

The survey data shows that most of the farmers were late harvesting by 1 to 3 days. For varieties,
OM1490, OM2718, Jasmine and AG24 were harvested late by 80%, 90%, 55% and 50% of farmers,
respectively.

1. The harvesting time is one of the important factors to control the rice cracking and eventual
head rice recovery. There is a clear trend that a few days early harvesting (before maturity) is
better than the late harvesting.
2. The same effect persists in rainy or dry season (check further data – more figures have been
inserted to Figure 2 &3).
3. Varieties differ in their grain cracking and those with small cracking such as AG24 is
recommended. When head rice recovery is also considered, OM4498 had smaller loss even it
Field experiments indicated
that: 4-6 days late harvesting
increase cracking by up to
24%, and reduce HRY by up to
28%.
189

was harvested later than the optimum harvest time (Check data further). In one of the varieties
tested in this investigation, a proportion of cracked rice was as high as 24%.


Manual cutting Manual collecting Machine threshing 2. Harvesting by reaper, manual collecting followed by machine threshing

Cutting by reaper Manual collecting Machine threshing

A lot of labours are required for manual collecting after harvesting by reaper.
191
3. Machine harvesting with a reaper, followed by combined collection-threshing.
Cutting by reaper Combined collection and harvesting

Cutting and laying the crop on a windrow is done using a reaper, followed by collection and
threshing using a machine. 4. Combine harvesting.
The combine harvester combines all operations: cutting, handling, threshing and cleaning


The grain germinated
20 hours after
harvesting due to
wetting in the rain
(Spring-Summer)

+ Harvesting by reaper, field
drying, piling, threshing

- The machine is not heavy, high
capacity (1-6 ha/day); it can work
in the soft soil field.
- The strength of reaper is
moderate and rarely damaged
seriously. The spare parts are
available locally.
- High cleanness of rice.
- High mobility.
- Moderate investment.
- High amount of rice stem (2-3 times
compared to manual harvesting)
increased time and energy for piling
and threshing; it’s hard to harvest
lodged rice. This is the major
disadvantage of the reaper.
- High losses during harvesting and
piling over-matured rice.
- Grain quality is not as good as in the
case of manual harvesting if field

- Grain quality good.
- The machine consists of many parts
and elements which lead to complex
operation and maintenance.
- Degree of cleanness is not high when
grain moisture is high.
- The mobility is not high where the
chain is used.
The investment cost is high. Estimated harvesting losses
Harvesting losses consist of shattering and threshing losses. Table 7 shows each component and total
harvesting losses. The total harvesting losses can be as high as 4.4%. A threshing loss of 1.0% for
combine harvester was estimated by the manufacturer. On an average, mechanical harvesting reduces
harvesting losses.

Table 7: Effect of harvesting methods on the harvesting losses

Harvesting method
Shattering
losses (%)
Threshing
losses (%)
Harvesting
losses (%)
Hand and heaped immediately 1.4 2.6-4.4


194

Grain cracking and head rice yield (HRY)

Effect of harvesting method
Harvesting method as such may not affect grain cracking and head rice recovery (Check), but manual
harvesting often results in delayed harvesting, and this can cause grain cracking and reduced head rice
recovery. Thus, when harvesting is to conducted to be manually, it is important to time the operation so
that crops can be harvested at correct time.
In addition, manual harvesting results in higher scattering losses in comparison to machine harvesting.
Data collected from some farmers who harvested manually are presented in Table 8.

Table 8: Scattering losses in the field of different farmers harvested manually

Farmer Rice
variety
Initial rice
moisture
(before
harvesting)
(% wb)
Moisture
content of
rice
during
threshing
(% wb)
Number
of kernels

18.9 19.4 318 380 3.4
Average 2.9±0.9

Effect of threshing method on grain cracking and head rice recovery
The threshing method applied can cause the cracking in the rice kernels and eventually reduce the head rice
recovery. The data collected in two provinces in Mekong Delta at the same time when experiments were
conducted are presented in Table 9. The results indicate that the grain cracking is not significantly affected by
the method of threshing. However, some reduction of head rice recovery was observed in the case of rice
threshed by machine.

Table 9: Effect of threshing method on rice cracking and head rice recovery

Grain cracking (%)
Brown rice White rice
Head rice recovery
(%)
Rice varieties
Hand Machine Hand Machine Hand Machine
OM2718/
OM 1490
4.1 3.9 3.0 1.8 49.9 46.7
An Giang 24 0.9 2.4 1.5 0.7 45.6 44.0

195 Ministry of Agriculture & Rural Development

Project Completion Report



196
3. EFFECT OF DRYING METHODS ON RICE
CRACKING AND HEAD RICE RECOVERY Importance of drying

Drying is the process of removing water from the grains to reach safe storage
moisture content (MC). Rice is harvested at grain moisture content between 20 and
30% (wet basis), depending on the season of harvest. Failure to dry the rice often
results in its deterioration due to micro-organisms and discolouration. Depending on
the desired storage period there are different recommended moisture contents for safe
storage (Table 10). Table 10: Safe storage period of rice at different moisture content.

Storage period Desired moisture content
(w/w)
Potential problems at higher
moisture content
2 to 3 weeks 14 - 18% Molds, discoloration,
respiration loss
8 to 12 months 13% or less Insect damage
More than 1 year 9 % or less Loss of viability



Field drying is practiced prior to threshing when hand or reaper is used for cutting the
rice. The drying rate will highly dependent on the weather condition and the stage of
harvesting. There are losses due to shattering of the grain and rice cracking due to
over drying or rewetting of the grain, if the rice is not threshed in time. The major
disadvantages of this method are:

 Rewetting from straw or soil which can cause rice
cracking and high proportion of broken rice during
milling

 Rewetting during night time due to high humidity
resulting in grain cracking and low milling yield

 Non-uniform drying due to uneven spreading of the
stack. Grains outside will dry faster than the inside
the stack. The experimental results presented in Table 11 indicate that there will be reduced
head rice yield due to field and sun drying. Table 11: Loss of head rice yield (HRY) in equivalent to losses of paddy (kg/100
kg yield) for different varieties in the Mekong Delta due to current practices of
field and sun drying

Variety OM1490 OM2718 Jasmine AG24
Loss of head rice yield (%)
7.32 8.97 10.45 7.87

Variety
In shade
drying
(controlled
sample)
Mechanical
drying (Flat-bed
dryer)
Sun drying
(Plastic, Ts =
42-48
o
C)
HRY reduction due
to sun drying (%)
OM1490
OM2517
Jasmine
Sticky rice
49.1
48.7
50.4
50.6
49.6
49.1
50.7
51.3
47.5
45.3
44.6

properly controlled. High temperature dryer may use above 60
o
C. High temperature
drying can cause rice cracking resulting from the moisture driven mechanical stress
due to faster drying from the exterior surface of the grain than from the interior.
199

Therefore, an additional intermittent tempering process is necessary to allow moisture
to equilibrate from interior to exterior and reduce the differential moisture.

There are various types of dryers are used.

Bin drying

This is a low temperature drying method. Paddy with moisture
content already below 18% can be slowly dried in storage bins
using aeration with slightly pre-heated air. Since there is not
mixing of the grain involved, the grains close to the air entry
point will dry faster than the interior. Some cracking of grain
may occur at the entry point if the temperature is a bit high and
the humidity of the air is very low. This type of drying method is
not a common practice in Mekong River Delta. Flat bed drying (FBD)

This is the most common dryer found in Mekong
delta. Flat bed driers are also called fixed-bed dryer.
In this dryer the heated air is forced through the bulk
grain. The grain is kept stationary in a holding bin

• Monitor moisture content and temperature hourly. Stop drying when the
moisture content in the middle of the bulk (half grain depth) reaches the
desired final moisture content.
• For seeds don’t exceed 43°C drying air temperature.
• Increasing the temperature reduces drying time but increases uneven drying
and can cause rice cracking
• Increasing the airflow shortens drying and reduces the moisture content but
increases energy cost.
• Mixing during drying reduces the moisture gradient.

Schematic diagram of furnace Rice husk furnace with cylindrical ash collector.

Automatic rice husk furnace.

Due to the large depth of the grain and drying passing though one end, there is a difference in
moisture content between the bottom and the top bed. Mixing reduces this difference, but the
mixing can not be efficient and is labour intensive. In order to simplify, a reversible air flow
is used after certain period of drying.


Batch Number Reversible

Non-
reversible
Drying time 6.00 6.00
Air reversal time, minute 15
Labour personnel for air reversal 2
Initial moisture content (% wb) 23.9 20.4
Final moisture content (% wb) 14.9 16.1
Top layer moisture (% wb) 13.9 18.2
Middle layer moisture (% wb) 16.2 16.4
Bottom layer moisture (% wb) 14.8 13.6
Differential moisture Top-Bottom (%) 0.83 4.64
Differential moisture Middle-Top (%) 2.24
Rice husk consumption: kg/batch 171.2 215.2
Rice husk consumption: kg/hr 28.5 35.9
Diesel consumption (for fan), L/h 1.70 1.75
Initial Paddy layer, mm 517.8 507.8
Brown rice cracks before drying (%) 12.0 21.0
Brown rice cracks after drying (%) 13.8 23.8
Cracks during sun drying- cement yard, 7 cm layer (%) 17.8 26.8
Head Rice Recovery (%)
Head Rice %, after drying 59.39 56.21
Head Rice %, Sun drying 55.58 52.12
Difference (Sun & Mechanical) % -3.81 -4.09

High temperature compact dryer

In large capacity, fluidised bed and column continuous dryer can be used. Use of high

Sun drying, in the wet-season, ADVERSE weather 210
# dryer uses the typical fan design developed by NLU
203

4.

LOSSES DURING RICE MILLING

Rice milling is the process of removing the husk and bran layer to produce white rice.

The process is undertaken as:
• A one step milling process where the husk and the bran are removed in one
pass and white rice is produced directly from the paddy.

• A two-step process where the husk and the bran are removed separately, and
the brown rice is further processed to get white rice.

• A multistage process where rice passes through a number of different
operations and machines from paddy to white rice.

The head rice yield (%) is the important criteria used to evaluate the milling quality of
rice. The head rice means the whole grain unbroken rice which has at least 75% of the
original length of the brown rice. In an ideal condition the head rice recovery is
expected to be above 59%. Rice cracking occurred prior to milling due to improper
harvesting and subsequent drying practices, presence of immature grain, chalkiness in
the grain and milling conditions are the main contributors of low head rice yield.
The head rice recovery will not only depend on the initial rice quality (existing cracks or
weaker grain), but also on the efficiency of the milling operation.

There exist three systems of rice mills in Mekong Delta (Kien Giang and Tien Giang

14. Tank
15. Whitener 1
16. Cyclone
17. Scale
18. Sieve

19. Polisher
20. Whitener 2
21. Tank
22. Cyclone
23. Tank
24. Whitener 3Figure 3: Schematic of traditional milling system

The head rice is usually low for smaller mills due to poor efficiency of the milling systems
caused by lack of maintenance and incorrect operations. Head rice recovery in small scale
mills can be as low as 33%. Large rice mill had the highest head rice recovery of 55%. In
literatures, the head rice recovery and total rice recovery have been achieved as high as 60%
and 70%. Therefore, there is still scope of improving the head rice recovery even in large
scale mills, let alone a poor performer small scale rice mills. Table 17: Head rice recovery data collected by surveying the millers (Kien Giang
and Tien Giang provinces)
Scale
Grain moisture
(%)
Average head rice