Section 2: Study on the flat-bed dryer in the Mekong River Delta of Viet Nam pptx

Data on the crack of rice upon milling in March 2007, and July 2007 with three pairs of drying batches With Air reversal, and Without air reversal showed that: • Mechanical drying, wheth

Section 2 Study on the flat-bed dryer in the Mekong River Delta of

Viet nam

Section 2 Study on the flat-bed dryer in the Mekong River Delta

of Viet nam

ABSTRACT

The study, including experiments and survey on the flat-bed dryer, focused on the cracking of paddy grains, and on comparing the air reversal mode Results showed that, in both the 8-ton production-scale dryer and the 20-kg laboratory dryer, the effect of air reversal was very apparent in reducing the final moisture differential; however, its effect on the drying time or the drying rate was not statistically significant Mechanical drying, whether with or without air reversal, was superior to sun drying in terms of reducing rice crack However, compared

to shade control drying, drying (with or without air reversal) did decrease the head rice recovery and increase the crack; the causing factor was not apparent, most suspected reason was the drying rate The decrease in head rice recovery was inconsistent, slightly lower or higher in each specific pair of experiments with and without air reversal; this was not expected in line with data on the final moisture differential Testing of a 4-ton dryer at Long-

An equipped with the solar collector as supplementary heat source resulted with good grain quality and confirmed the good economic potential Major findings from the survey on the current status on the use of flat-bed dryers in 7 Provinces were: The trend for increased drying capacity, the role of local manufacturers and local extension workers, government support with interest reduction for dryer loans, the drying during the dry-season harvest, and especially the unbalance between drying costs and drying benefits

INTRODUCTION

Flat-bed dryers have been with the rice agriculture of the Mekong Delta of Viet Nam for a long time From the first flat-bed dryers in the 1980’s to about 6500 units in 2007 is quite a good progress But not all is optimistic Acceptance varies among provinces, even among districts or communes in the same province Finding the interrelated factors affecting the dryer acceptance is quite complex Within the context of the CARD Project 026/VIE-05 with focus on the cracking of paddy grains in the area, the study on the flat-bed dryer from 2006 to

2008 included the following objectives:

• Conduct experiments under laboratory controlled drying conditions and under actual production conditions to evaluate the effect of air reversal on the rice crack and other drying outputs

• Conduct experiments on the 4-ton flat-bed dryer, using solar energy as supplementary heat source

• Conduct a Participatory Rapid Rural Appraisal (PRRA) survey to update on the use of flat-bed dryer in the Mekong Delta

REVIEW OF RELEVANT INFORMATION

The following information is based on data by various Provinces presented during different seminars, on an integrated assessment study by the Ministry of Agriculture-Rural Development in collaboration with DANIDA in 2004, and on the first author’s working experience with flat-bed dryers in the past 25 years

Development of the flat-bed dryer

The Mekong Delta in Southern Viet Nam, with 2.7 million hectares of rice land, is producing about 50 % of Viet Nam total rice output With 16 million people or about less than 20 % of the total population, this region has accounted for more than 90 % of Vietnamese rice export

in the past decade Average farm size is about 1 ha per household, although in some reclaimed districts, 3 - 10 ha per household is not uncommon

newly-Rice drying became an issue in Mekong Delta in early 1980’s when a second crop was promoted, of which the harvest fell into the rainy season Different dryer models were tried

by various agencies; only one model was accepted by the production sector, namely the bed dryer (FBD) The first FBD was installed in Soc-Trang Province in 1982 by the University of Agriculture and Forestry (now renamed Nong-Lam University NLU) Farmers

flat-in Soc-Trang copied/ modified/ improved this FBD usflat-ing cheap local materials In 1990, there were about 300 FBD units in the Mekong Delta, half of which were in Soc-Trang Other Provinces began to adopt these dryers In 1997, a survey conducted by a Danida-assisted Project reported a total of 1500 FBD in all Mekong Delta, with 3 leading Provinces (Kien Giang, Soc-Trang, Can-Tho) accounted for 850 units; all remaining 10 Provinces shared the balance of 650 units

scheme and extension activities The Project terminated in 2001, and replaced by a Program managed by the Ministry of Agriculture, but still assisted by Danida, then with only extension activities The Program terminated in mid-2007 The number of FBD dryer rose rapidly, about 3000 units in 2002 and 6200 units in 2006 The dryers in the Mekong Delta account for more than 95 % of all dryers in Viet Nam

The technical development of the FBD in the past 25 years followed an interesting pattern First, a design was released by a research institution, NLU in this case Next, farmers/ mechanics copied/ modified/ improved the design Next, NLU monitored those modifications and came up with a major design change and improvement The cycle repeats

The landmarks for these major design releases by NLU have been:

1982: Conventional FBD with central air inlet to the plenum chamber, using

flat-grate rice husk furnace with precipitation chamber (Fig.1)

1994: Conventional FBD with side-duct plenum (Fig.2), rice husk furnace with

vortex and central-pipe precipitation chamber (Fig.3)

2001: Reversible FBD (Fig.5 & 6)

(expected):

2006: Automatic rice husk furnace (model NLU-IRRI-Hohenheim, Fig.4)

2007: Solar collector for FBD

Major modifications /improvements by farmer-mechanics have been:

1987: Rice husk furnace with inclined grate

2004: Drying bin for reversible dryer, with distributed central inlet

2006: Raking mechanism under the rice husk hopper for more uniform husk feeding

Figure 1 Conventional FBD with central air inlet to

the plenum chamber Figure 2 Conventional FBD with side-duct plenum

Figure 3 Rice husk furnace with vortex and

central-pipe precipitation chamber Figure 4 The automatic rice husk furnace for SRA-4 reversible flat-bed dryer

Drying Air UP

Drying Air DOWN

REVERSIBLE SRA DRYER

0.6m Floor: 25 sq.m / 8 ton

Figure 5 Principle of reversible-air dryer

Figure 6 The SRA-10 reversible air dryer (10 tons per batch)

NLU have taken the leading role in releasing efficient dryer fans, both for conventional and reversible dryers, with transfer of design and fabrication technology to 15 manufacturers in the Mekong Delta, among them 7 have built fan test ducts according to JIS Standards

Quality of paddy dried by the flat-bed dryer

The quality of dried paddy is judged by several criteria:

ƒ The paddy is not contaminated with black ashes from the furnace

ƒ The paddy final moisture content is uniform at the desired level for storage

The first criterion (no ash mixed with grain) has been met after some years, due to gaining of experience in furnace building, and competition among furnace builders, to deal with farmers’ first-visual reactions

The second criterion is difficult to meet due to the inherent principle of flat-bed drying A final moisture differential of 1.5 % (between the top and the bottom layer) is considered good, while in continuous-flow mixing-type dryer, 1.0 % is normal For flat-bed dryers, farmers rely on manual mixing Technically, a good high airflow rate at moderate temperature (below 44oC) helps reducing the differential The air-reversal principle introduced since 2002 also reduces the non-uniformity All these technical features should be re-evaluated / confirmed in this current CARD Project

The preserved seed germination is well established by Seed Companies in using a safe drying temperature below 42 oC, and most importantly to dry the grain within 12 hours after harvest For commercial grain, the dried grain crack is a big issue A report (Phan Hieu Hien, 1998) based on surveys of a few rice mills in Can-Tho and Long-An showed a reduction of 5 to 7 %

of farmers’ profit due to more broken rice due to improper drying This high loss was due to the habit of field drying in the dry-season harvest, and estimated to be about 20 million US$ per harvest in the Mekong Delta However, data and estimates were based on a few interviews, and not on systematic testing Thus, in this CARD Project, the need is to confirm

or reject based on solid test data

MATERIALS AND METHODS

Testing

Testing of dryers followed standard procedures described in RNAM (1991) and ASABE (2006) Measurement equipment included different thermometers, moisture meter and drying oven, power meter etc

For the 8-ton dryers, the drying temperature was at 2 levels: a) Constant at 43 oC; and b) At

50 oC for the first hour, and afterwards constant at 43 oC In reality, due to the furnace configuration, the temperature rarely exceeded 50 oC, and was about 48 oC at most In all tests, the focus was to compare two drying modes: WITH air reversal, and WITHOUT air

reversal Some experiments also compared with sun drying on the cement drying yard with a 7-cm paddy layer, as popularly practiced by local farmers

The crack analysis and head rice analysis was first done at the Vinacontrol, an accredited agency in charge of certifying rice quality for export, and later at the Rice Quality Laboratory

of the NLU Chemical Technology Department, following procedures adopted by International Rice Research Institute and the University of Queensland Each treatment was analyzed by 3 samples, each consists of 50 grains taken at random; each paddy grain was hand-husked and examined under the magnifying glass for fissure The increase in crack or decrease in head rice of each treatment based were on the control shade drying (or further shade drying to 14 %MC)

The biggest problem for testing has been the input paddy We encountered severe difficulties

in securing batches of the same quantity or initial moisture content This was apparent with the 8-ton batches But even scale-down to 1-ton batch, the 3-factor experiments could not be run, due to different initial MC Finally, from the “lumpsum” conclusions with 8-ton dryers,

we had to concentrate on and be contented with 20-kg batches in paired experiments (block)

of Air reversal and No air reversal

For experiments on the use of solar heat for paddy drying, a 4-ton popular flat-bed models fabricated by a local mechanical shop was selected, and added with a solar collector designed

at the NLU Center for Agricultural Energy and Machinery

Survey

The objectives of the surveys were: (i) to update the role of flat-bed dryers in reducing harvest losses and in preserving rice quality; (ii) to identify operating factors of the flat-bed dryer which contribute to the reduction of rice crack; and (iii) to identify problems with the flat-bed dryer that the CARD Project could possibly help

post-The survey used the Participatory Rapid Rural Appraisal (PRRA) method, through interviewing different people class, from farmers to rice millers to governmental officials etc But it also relied heavily on both available data gathered in the past 10 years by various

agencies, and on personal experience of the people involved with the dryer at NLU over the past 20 years

Four Provinces were selected in 2006, namely Can-Tho City, Kien-Giang, Long-An, and Tien-Giang The first three Provinces have sites which had been selected by the CARD Project for all related experiments, demonstrations, and extension activities The fourth Province is adjacent to Long-An, and also planned as site for rice milling survey, so facts and data on the dryer would be relevant In 2007, we visited more Provinces such as Hau-Giang, An-Giang, Kien Giang, Soc-Trang…, with resulted with additional findings

RESULTS AND DISCUSSSION

TESTING

Experimental results on the 8-ton dryer, the laboratory dryer, the solar-assisted dryer, as well

as the survey results are presented in the following sections

The 8-ton dryer

Two 8-ton dryers were selected for experiments One was a NLU-designed air-reversible dryer installed at Tan-Phat-A Cooperative, Tan-Hiep District, Kien Giang Province in July

2006 (Figs 7&8) The other was an air-reversible dryer made by a local manufacturer installed at Tan-Thoi Cooperative in Can-Tho Province, with the design patterned on the SRA-8 of NLU; the difference was the under-plenum duct inside the drying bin, “ong gio chim” in Vietnamese (Fig.9), in order to distribute the airflow evenly

Figure 7 The 8-ton dryer at Tan-Phat-A

Cooperative, Kien Giang Figure 8 The 8-ton dryer with the air for downward direction

Figure 9: The 8-ton flat-bed dryer at Tan-Thoi Cooperative, Can-Tho Province

Experiments from Kien-Giang were under more control thus more results are reported here, while results at Can-Tho are supplementary Refer to Phan Hieu Hien (2006, 2007, 2008) for testing details

In Kien-Giang experiments were conducted in two wet seasons (July 2006, and July- August 2007), and two dry-seasons (March 2007, and March 2008) Major findings are as follow:

• The drying temperature is stable and can be kept within ± 3 oC, usually from the nominal value of 43 oC

• The effect of air reversal was very apparent in reducing the final moisture differential When operated correctly, this differential was less than 2.2 % with air reversal, but over 4.6% without air reversal More MC differential means more rice cracking during milling This explains why dryers installed since 2003 have been more and more of the reversible principle

• However the effect of air reversal on the drying time or the drying rate was not clear because of several other factors involved (Fig.10)

Air Re ve r sa l

No a ir r e ve r sa l

Data on the crack of rice upon milling in March 2007, and July 2007 with three pairs of drying batches (With Air reversal, and Without air reversal) showed that:

• Mechanical drying, whether with or without air reversal, was superior to sun drying in terms of less crack percentage or more head rice recovery About 3- 4 % less cracking, and about 4 % more head rice recovery were main data obtained from March 2007 experiments

• The grain cracking in Air-reversal batches were lower than No-air-reversal batches (Fig.11) This is a basic result

• However, the decrease in head rice recovery was inconsistent, slightly lower or higher in each specific pair (Fig.12) This is confirmed by the statistical comparison on the head rice recovery with t-test between batches of Air reversal and No air reversal, which did not show significant difference at 5% level This was not expected in line with the above data on Final MC differential The reason was probably due to the sample milling; the whitening time was only 1 minute, thus some slightly cracked kernels might not be broken during milling

• In both cases (Air reversal and No air reversal) drying did decrease the head rice recovery and increase the crack The causing factor was not apparent, due to so many factors involved in a large mass of 8 tons of grain: paddy non-uniformity, drying rate… Most suspected reason was the drying rate (Fig.13), data pointed to an optimum drying rate in the 1.0– 1.2 %/hr range, but this has to be confirmed by further elaborate experiments, or from laboratory scale experiments

Crack % INCREASE (Kien Giang 2007 wet-season)

Air reversal No air reversal

Figure 11 Crack% INCREASE, Kien-Giang,

wet-season 2007

Head rice, Kien Giang 2 0 0 7 Wet -s eas o n (AR = Air Revers al; NAR = No air revers al B2 = Bat ch No 2 )

0 10 30 50 70

Head Rice Before drying, % Head Rice After drying, %

Figure 12 Head rice Before and After drying

0 4 12 20 28 36

Grain Crack Increase, % Head Rice Decrease , %

Figure 13 The effect of the drying rate on the crack increase or the head rice recovery

The laboratory dryer

The effects of two factors were studied: Factor A was the final MC with two levels (14%

coded X14, and 17% coded X17) Factor B was the air reversal mode with two levels (Air Reversal AR, and No Air reversal NoAr) Each four treatments (or factor combinations)

were in one block of experiment that is, conducted at the same time This was possible thanks to 2 identical laboratory dryers running in parallel Each batch contains 20 kg of paddy Four replications (or 4 blocks) were made

The total thickness of the paddy layer in the AR batches was 0.51 m while that of NoAr batches was 0.31 m Paddy was sampled at three layers –Bottom, Middle, and Top layer— in

3 specific trays, with other buffer trays in-between

In each block of experiment, the dependent variables were: the drying rate (shown by the drying curve), the uniformity of the final MC (shown by the MC of the bottom, middle, and top layers), the head rice recovery, and the grain crack Data in one typical block are graph in Fig.14, 15, 16&17 Results are statistically analyzed as a RCBD (randomized complete block design) with data compiled in Table 1

From the results and the statistical analysis, the following remarks can be derived:

a The final MC differential:

The effect of both the reversal mode and the final MC was statistically significant at 5% alpha level Air reversal yielded less final MC differential than No air reversal (Table 1, Fig.14) Also, drying stop at 14% MC gave less final MC differential than at 17% MC

treatment, that is between factor combinations For example in Table 1, treatment NoArX14 and AR_X17 had similar MC differential

Figure 14 Moisture non-uniformity

AR X17-Bottom

AR X17-Middle

AR X17-Top Layer

Figure 15 Drying curves down to 17% MC of the Top, Middle, and Bottom layers

AR = Air Reversal; NoAr = No Air reversal

X14 = Average Final MC 14% X17 = Average Final MC 17%

The effect of both the reversal mode and the final MC was not statistically significant at 5%

alpha level However, at 10% alpha level, drying down to 14% MC was significantly at slower rate than down to 17% MC (Table 1, Fig.15 &16)

-6.38

-8.56

-12.92

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0

compared to shade drying, % (decrease =

Table 1 Data from experiments with the laboratory dryers AR = Air Reversal; NoAr = No Air reversal;

X14 = Final MC 14%; X17 = Final MC 17%

DRYING RATE,

%/hr

FINAL MC DIFFERENTIAL, %

% DECREASE IN HEAD RICE RECOVERY

MC

B = Reversal mode

A = Final

MC

B = Reversal mode

A = Final

MC

B =Reversal mode

Statistical Analysis Results (at 5% significance level):

Interaction AB:

Interaction AB: Yes Interaction AB: Yes Interaction AB: Yes

c Decrease in head rice recovery and Rice crack

Drying, whether with air reversal or not, did decrease the head rice recovery and increase the rice crack compared to control shade drying The reason was suspected as too high drying rate (over 1.3 %/hr) But the regression analysis and graphing (Fig.18) showed no definite trend More crack and lower head rice recovery at 17% as expected because paddy was milled with the laboratory mill at that MC which was not the optimum

0 5 10

Figure 18 Effect of the Drying rate on Rice crack

In theory, more head rice recovery corresponds to less rice cracks; and more head rice and less crack correspond to more MC differential The less final differential with Air Reversal compared to No Air reversal was expected to go with less decrease in head rice and less crack, but data showed the contrary (Table 1)

In summary even with the laboratory experiments, the experimental data were still hard to analyze for balanced and good-looking results, with the possible reason traced to the variability between the individual grains

The 4-ton solar-assisted dryer

A 4-ton popular flat-bed models (named SDG-4 dryer) fabricated by a local mechanical shop was selected This is a collapsible unit, which consists of the following components:

i.) A two-stage axial fan, a design transferred by NLU, powered by a 15 HP Chinese diesel engine

ii.) A coal furnace, with coal consumption adjustable within 5 to 12 kg/hr

iii.) A drying bin, with the grain floor size 4.50 m *3.27 m made from bamboo slat and nylon net The bin is supported on 7 metal legs, thus can be easily installed on rough land The airflow can be upwards (Fig.19), or downward (Fig.20) with a covering tarpaulin

iv.) A solar collector (designed at NLU) consisting of 2 cylindrical plastic collector (Fig.19

&20) Each cylinder is φ1.0 m * 27 m long Inside the transparent plastic layer is the

which also received heat from the coal furnace The collector used cheap materials such

as bamboo slats and plastic wires, and was installed on the open ground instead on the rooftop, thus the investment cost was significantly reduced compared to the steel-frame collector of the macaroni dryer (Phan Hieu Hien et.al, 2007)

The solar collector and the coal furnace can be used separately or in combination Tests were done at Long-An Province in March 2007, the driest month of the year

Figure 19 The SRA-4B dryer with the upward

airflow

Figure 20 The SRA-4B dryer with the downward

airflow, using solar heat

Five drying batches were tested in March 2007: Batch 1 with heat from coal only; Batches 2

& 3 with heat from solar energy only, Batches 4 & 5 with heat combined from both coal and solar energy

Results are summarized as follow:

• The capacity was 3.8 – 4.1 ton per batch of 7- 12 hours, with moisture reduction (average

± standard deviation) from 23.8 ±1.7 % MC down to 14.2 ± 0.8 %

• The drying temperature could be adjusted within 38- 44 o

C using coal With solar heat, the drying temperature could reach 38 oC with good sunshine (over 800 W/m2 radiation),

or only 36 oC in cloudy weather (about 500 W/m2 radiation, which is also typical in the wet-season) With less sunshine, the 12-hr drying time as in Batch 3 was expected

• The combination of solar and coal heat is handy in ensuring to finish one batch within the day The harvest season in one village or commune usually lasts less than 25 days, thus can not allow the “luxury” of 2-day drying batch

• The head rice recovery in all batches were comparable to “shade” drying; or even slightly better with 2 batches with solar energy, possibly to slightly lower drying temperature

The contribution of solar energy is analyzed using data of Batch 4 and Batch 5, both with combined heat from coal and solar energy; the following can be drawn:

ƒ Solar energy could contribute to a cost saving of 43– 78 % from reducing the coal consumption

ƒ The saving translated into US$ 3– 5 per batch, or US$ 0.7– 1.3 per ton) # # #

ƒ For estimation, assume that in one year, the dryer is used for 100 batches or 400 tons, of which ½ totally use solar energy, and ½ use supplementary solar energy with 50 % saving, or US$1.6 and US$0.8 per ton respectively Thus the total saving would be US$480 per year

ƒ Compared to the additional investment for the solar collector of about US$ 560, with the replacement of the plastic sheet costing about US$120 after every 7 months, the payback period is about 2 years

ƒ For stand-alone dryer owner, he/she might not be able to dry for 100 batches per year In contrast, dryer owner-cum-rice miller can surpass that quantity easily Thus the solar collector would aim practically more for the rice milling compound

In the Mekong Delta of Viet Nam, farmers currently use the flat-bed dryer mostly for paddy harvested in the wet-season For the dry-season harvest, people mainly rely on the pavement natural sun drying to save the cost of fuel for drying Thus paddy crack in the dry-season harvest is even more severe, as repeatedly warned by research and extension agencies without many results, obviously due to the very low drying cost under sunshine

Thus, from test data, solar energy has been used to dry paddy at a production scale; early attempts in the 1980’s dealt with 50- 300 kg/ batch lasting 2 days The test has proved the

quality of the dried paddy Economically, it could refute the popular saying that “Solar energy is free but not cheap” with the fact that the owner can recover the additional

investment for the solar collector in about 2 years Environmentally, solar energy is clean The problem remains to introduce the solar heat to the integrated rice mill with dryers The test has proved the function of the solar collector in saving fuel cost, especially in the dry-season harvest

SURVEY

Background data

The 4 Provinces under study have similar data in terms of climate and other agricultural featuresm typical of the Mekong Delta All have the average monthly temperature of 27- 28 oC, with the average maximum of 29 oC in April and minimum of 25 oC in January But the temperature difference between daytime and night time is more pronounced, say between 25 and 36 oC in hot months, or 23 and 33 oC in cooler months

The rainy season in the region occurs from May to October, the remaining months are dry season and there is not four seasons such as the Spring, Summer, Winter like in Northern Provinces The annual rainfall is 1 400 mm in Long-An, and higher in Can-Tho and Kien-Giang, 1600 and 1800 mm respectively

The average annual relative humidity is 80- 82 % This just says that is typical tropical humid climate, and not specific enough about its significance in post-harvest Earlier compilarion (Phan Hieu Hien, 1998) showed that in a typical day of March (dry season) and

of August (rainy season) in of the Mekong Delta, the relative humidity during the night time (21h00 PM to 7h00 AM) is very high, over 90% This is totally different with Australia, where the Rh is below 70 % even in night time The implication is the moisture re-absorption

of the grain during storage

Specific data pertaining to each Province are shown in Table 2

Table 2 Selected data of the 4 Provinces under survey

% of wet-season paddy dried by machines ≈15

(10- 20)

Source: General Statistics Office, Ha-Noi, Viet Nam, http://www.gso.gov.vn/ (2005)

Danida ASPS Report (2004)

# Mr Con, Office of Long An Rural Development (2006)

## Mr Viet, Post-harvest Advisor at Tien-Giang Province(2006)

Post-harvest and drying status

a) The number of flat-bed dryers in the 4 Provinces is listed in Table 3 Long-An and

Tien-Giang are more backward in terms of dryer development

b) The flat-bed dryer was first installed in these Provinces in the early 1990’s These were

“first-generation” conventional flat-bed dryer with central air inlet to the plenum

chamber, using flat-grate rice husk furnace with precipitation chamber (Fig.1) Later,

“second-generation” flat-bed dryer with side-duct plenum (Fig.2) and improved rice husk

was installed between 1995 and 1997 in these Provinces Last, the “third-generation”

reversible dryer (the principle is shown in Fig.5), with its advantage of saving labor and

land space, was introduced first at Long-An in 2000, and at Tien-Giang and Kien-Giang

in 2002 There are now about 400 reversible dryers in the Mekong Delta, among which

30 units are from original design and installed by NLU, which include about 15 units in

the 4 Provinces under study

c) The percentage of mechanically dried paddy is not evenly distributed within each

Province For example, Kien-Giang with an average of 24%, yet in many villages, only

3 % of the paddy harvested in the wet-season is mechanically dried

d) The percentage of mechanically dried paddy might not proportional to the number of

dryers, but also depends on the weather That is why in Can-Tho, different sources quote

e) Mechanical drying not only reduces post-harvest losses, but also preserves grain quality

This fact is widely recognized now by farmers, rice millers, governmental officials, which

is a different view compared to about 10 years ago

f) Despite the above salient advantage, the majority still practice sun drying For example,

in Can-Tho, while the installed drying capacity can meet 25 % of the harvest, yet

only 15 % is dried by machine One source even says that 90 % are sun drying,

consisting of 40% on earthen yard, 40 % on cement yard, and 10 % on roadside

g) The reason lies with the drying cost, while the quality factor does not account much under

the present agricultural production and trading system Our data gathered from Long-An

resulted in Table 3

Table 3 Drying cost under different settings

SRA-4 (reversible, 4-ton/batch) dryer, with rice husk furnace 98 6.1

SRA-8 (reversible, 8-ton/batch) dryer, with rice husk furnace 79 4.9

SDG-4 (reversible, 4-ton/batch) dryer, rice husk furnace #1 80 5.0

SDG-4 (reversible, 4-ton/batch) dryer, COAL furnace 130 8.1

Sun drying, in the wet-season, normal (moderate) weather 140 8.8

Sun drying, in the wet-season, ADVERSE weather 210 13.1

Note: #1: SDG-4 = A “lower-cost” reversible 4 ton/batch dryer, made by a local manufacturer in Dong-Thap

Province (described in solar-assisted experiments)

From Table 3, the following remarks can be made:

• In the dry season, the mechanical drying cost of the SRA-4 dryer (8.1 US$/ton) is still

higher than the manual sun drying cost

• In the wet season, the mechanical drying cost is lower than sun drying cost Thus a

charged drying fee of 5 % of paddy value, or about 8 US$/ton, would enable the dryer

owner to recover the investment after 2- 4 years, depending on the investment

• From the farmer’s (paddy owners) standpoint, they would not spend more than sun

drying in the normal weather, and surely the paid fee is cheaper than sun drying in the

worst, adverse weather This has not yet taken into account the cost of paddy

deterioration, as reflected in the sale price drop of 10- 20 %, or 16- 32 US$/ton

• SRA-8 dryer, with rice husk furnace, and SDG-4 dryer with rice husk furnace are alternatives to further reduce the drying cost

• However, the SDG-4 dryer with coal furnace is not recommended, since the drying cost is so high that makes the operation unprofitable for the dryer owner to recover the investment

• The drying cost does not include yet the transportation cost charged to the grain owner, which is about 0.6- 0.7 US$/ton or about 10 % of the proper drying cost

h) The Danida-assisted Project in Can-Tho and Soc-Trang Provinces in 2001-2006, and its expanded Program in the whole Mekong Delta has done a good job in promoting the dryer various people in the rice sector, with a strong and well organized network of people and facilities Extension in drying, which used to be a limiting factor in promoting dryers in the 1990’s, appeared to be well fulfilled in 2000’s If resistance to using dryers persists, then it should be traced to other factors

Further surveys in 2007 resulted with the following additional findings:

• There is a trend in increase the drying capacity to 12- 20 ton/batch; this is reflected in the number of 10- 16 ton dryers in the last two years compared to 5 years ago with 4- 8 tons/batch; and in different requests for 20 tons/batch dryer in 2007

• The role of local manufacturers and local extension workers: Provinces with rapid dryer development such as An-Giang and Tien-Giang have strong manufacturers providing reliable and efficient dryers to farmers as well as “instant” after-sale service Extension workers with solid knowledge in dryer construction and operation are important in spreading a new design

• Government support, specifically with interest reduction for dryer loans, is another key factor in promoting the dryer

• In the dry-season harvest, there are now places where mechanical drying is popular with a percentage ranging from 30- 90 % such as Giong-Rieng District of Kien-Giang Province, Ke-Sach and My-Tu Districts of Soc-Trang Province, Go-Cong and Cho-Gao Districts of Tien-Giang Province… Farmers just sell wet rice In Soc-Trang the division of labor is more and more clear-cut Ten years ago among the following jobs: paddy growing, paddy buying, paddy drying, rice milling, there might be a person doing 2 jobs But

nowadays, these jobs are more and more specialized, in which the farmer-grower no longer dry his/her own paddy

• The cooperative as an administrative or business unit in paddy drying has not shown an established role or image in the Mekong Delta More social study might be needed to find out the reason

Problems to be addressed from 2007

The above facts and analysis point to a single major problem in drying at the Provinces under study, which is: The unbalanced between drying cost and drying benefits

While the drying cost is real and quantified, the drying benefits might not be so Better quality rice due to mechanical drying may not be bought by traders with a price higher enough to compensate for the drying cost Different possible reasons are:

ƒ The output grain was not good due to improper dryer operation

ƒ Even with proper dryer operation, the output grain was not really good, because farmers only brought paddy to the dryer as a last recourse when paddy was about to deteriorate after days of rain

ƒ The good-quality dried rice was mixed with the bad sun-dried rice, for convenience in transporting in a same boat-load

ƒ The quality of the mechanically-dried grain was not yet appreciated enough by the market A few percentage point more of head rice recovery might not command a paddy price superior enough to compensate for the drying cost

ƒ Even in case the mechanically-dried grain obtained higher price, its effects did not benefit the farmer growing rice, because the rice miller got practically all advantages from the head rice recovery Farmers own paddy, while rice millers and traders own white rice ! Thus the drying problem in 2007 differs from that of 1997: It is no longer (or much less in scope) of quantity post-harvest losses, it is more on quality post-harvest losses

Possible measures for the drying problem from 2007

Given the status in drying as above-analyzed, the proposed activities could be grouped into 3 aspects:

a) Technology: There is the need to improve the dryer design so that the quality (in terms of

more head rice recovery, or less crack) is ensured and less dependent on the manual judgment of the operator This is not easy, due to the age-old constraint on the drying cost The theme of the current CARD Project seems to be in line with these requirements

b) Extension: In light of the “new” recognition on quality, the extension activities should be

geared more on dryer utilization to preserve quality, not just on reducing quantitative post-harvest losses Not just to build dryers to save the crop, but to demonstrate the effect

of more head rice and less rice crack through milling In another words, farmers not only see the dried paddy, but also see the white whole grain Thus a stand-alone dryer does not help much in the 2007 period

c) Policy: The above factors (technology and extension) are just necessary conditions, but

not sufficient A system to induce the dryer utilization for quality should be enhanced From the end of the chain, the market should be created for quality rice, with clear distinction in price Next, the benefit from high-priced rice should be distributed rightly

to the due contribution of both farmers and rice millers/ traders The policy should gear

to encouraging the adoption of these practices through financial measures

The policy affecting the whole rice system is complicated and not easy to alter in a few months But as far as the CARD Project is concerned, an integrated system from paddy supply to drying to milling, which involves farmers, should be established as demonstration sites at various Provinces

CONCLUSIONS

ƒ Testing of two 8-ton reversible dryer in Kien-Giang and Can-Tho Provinces; analysis of the rice cracks in the dry and the wet season harvests, in the actual production Testing under laboratory controlled drying conditions with 20-kg dryers in two modes of air reversal, and two final moisture contents In both test sets, air reversal reduced the final

MC differential but not the drying time The effect of air reversal on the head rice recovery and rice crack was not consistent and involved interactions with the final MC and possibly with other factors

ƒ Testing of a 4-ton dryer at Long-An equipped with the solar collector as supplementary heat source showed good grain quality and good economic potential

ƒ Rapid survey on the current status on the use of flat-bed dryers in 7 Provinces Among major findings are: The trend for increased drying capacity, the role of local manufacturers and local extension workers, the government support with interest reduction for dryer loans, and the drying during the dry-season harvest

One main proposal from these activities: To study on ways to integrate the dryer in the whole chain of rice post-harvest, so that the benefit and paddy drying reflect back to increase farmers’ income by their active participation

Kamaruddin A 2003 Fish drying using solar energy Proceedings of the Regional Seminar

and Workshop on Drying Technology ASEAN Sub Committee on Non-Conventional Energy Research

Ministry of Agriculture-Rural Development, and Danida ASPS 2004 Study on the current status and need assessment for post-harvest equipment in the Mekong Delta (Compiled from Reports and 12 Provinces) Internal Report (in Vietnamese)

Phan Hieu Hien 1987 Grain dryer for the summer-autumn crop in Southern Vietnam

Journal of Agricultural Science and Technology (In Vietnamese), No 6-1987, Ministry of Agriculture, Ha-Noi

Phan Hieu Hien 1998 Grain dryers and rice quality in the Mekong Delta of Viet Nam: Development process and perspective (In Vietnamese) Paper presented at the 15th Science and Technology Conference of the Mekong Delta , Ca Mau City 24 & 25 –9 –1998

Phan Hieu Hien, Nguyen Hung Tam, Nguyen Van Xuan 2003 The reversible air dryer SRA: One step to increase the mechanization of post-harvest operations Proceedings of the

International Conference on Crop Harvesting and Processing, 9-11 February 2003 (Louisville, Kentucky USA) ASAE Publication Number 701P1103e

Phan Hieu Hien 2006, 2007, 2008 Flat-bed dryer Sub-Component Reports to CARD Project Nong-Lam University (unpublished),

Phan Hieu Hien, Le Quang Vinh, Tran Thi Thanh Thuy 2007 The Solar Macaroni Dryer

Proceedings of the International Conference on Crop Harvesting and Processing, 11-14 February 2007 (Louisville, Kentucky USA) ASABE Publication Number 701P0307e

RNAM (Regional Network for Agricultural Machinery 1991 RNAM Test codes and procedures for farm machinery: Part 16 (Batch Dryer)

Effects of high temperature fluidized bed drying and tempering

on kernel cracking and milling quality of Vietnamese rice

varieties

Truong Thuc Tuyen1, 2, Vinh Truong2, Shu Fukai1, Bhesh Bhandari1*

1The University of Queensland, School of Land, Crop & Food Sciences, St Lucia, QLD 4072,

Australia 2

Nong Lam University, Ho Chi Minh City, Vietnam

*

Correspondence: Tel: +61-7-33469192; Fax: +61-7-33651177; Email:

b.bhandari@uq.edu.au

Abstract: Studies on the effects of high temperature fluidized bed drying and tempering on

physical properties and milling quality of two long-grain freshly harvested Vietnamese rice varieties, A10 (32±1 % wet basis moisture) and OM2717 (24.5±0.5 % wet basis moisture), were undertaken Rice samples were fluidized bed dried at 80 oC and 90 oC for 2.5 and 3.0 min, then tempered at 75 oC and 86 oC for up to 1 h, followed by final drying to below 14% moisture (wet basis) at 35 oC by thin layer drying method Head rice yield significantly improved with extended tempering time to 40 min Head rice yield tended to increase with decreasing cracked (fissured) kernels The hardness and stiffness of sound fluidized bed dried rice kernels (in the range of 30-55 N and 162-168 N/mm, respectively) were higher than that

of conventionally dried ones (thin layer dried at 35 oC) The color of milled rice was significantly (P<0.05) affected by high temperature fluidized bed drying, but the absolute change in the value was very small

INTRODUCTION

On account of Vietnam being a major exporter of rice, the quality of rice has become a central issue for Vietnamese farmers, particularly for wet-season rice production, when the moisture content of paddy at harvest can be as high as 35% wet basis [1] It is important to dry rice as quickly as possible after harvesting to prevent spoilage and maintain grain quality The conventional drying technique such as flat bed drying can often take up to 8 hrs or even longer to dry the paddy to safe moisture level of less than 14% wet basis Being a slow drying batch system, flat bed drying is not able to cope with the drying of large volume of rice harvested within the short period of time High temperature drying can allow faster drying of paddy, thereby reducing the time taken for drying as well as requiring reduced space However, since high temperature drying can result in to moisture differential in the kernel causing stress-cracking, intermittent tempering step is required to allow moisture to equilibrate [2, 3, 4] The fluidized bed integrated with a tempering system can serve as a compact drier The high temperature fluidized bed drying technique has been established as

an effective method for drying high moisture rice grain, which can easily deteriorate in the tropical humid environment [5, 6, 7] Using this technique, good grain mixing benefits from a gas-solid contact in which the rice grains are suspended by upward-moving drying air The water on the surface of grain is removed quickly, and uniform outlet moisture content after fluidized drying is achieved High temperature fluidized bed drying is applicable in the first stage of drying, when the moisture content of the paddy needs to be reduced to 18 % or less (wet basis) The paddy can then continue to be dried by ambient air in storage bins or in flat bed drier [4, 5, 7]

There are a number of reports of the use of high temperature (over 100 oC) fluidized bed drying of paddy [8, 9] Normally, it is recommended that the temperature used in hot air

rice A lower range of drying temperatures (40-90 C) has also been investigated by

Sutherland and Ghaly [7], and Tirawanichakul et al [8] They reported an improvement in

head rice yield when a drying temperature above 80 oC was used, provided that the outlet moisture content of rice after fluidized drying was above 18 % (wet basis) The employment

of a high drying temperature accompanied with high initial moisture content in the paddy, can result in partial gelatinization during fluidized drying It has been shown that head rice yield can be maintained at the level of gently dried samples, when the drying temperature used in the fluidized bed is below 70 oC, as the moisture gradient is insufficient to induce the development of fissures inside the rice kernels [8] Some researchers have suggested the need for tempering the grain for 25-30 min if high temperature drying is to be used [9, 10] The introduction of ambient air ventilation at the end of the tempering period in each drying stage provides a higher head rice yield and maintains whiteness, while giving a high drying capacity and reduced energy consumption [10]

There have been many research reports on the potential and use of high temperature fluidized bed drying, while the effects of fluidized bed drying on the incidence of fissured kernels and mechanical strength of rice and its field application have not been investigated As tempering plays a key role in the high temperature fluidized drying technique, it is important that studies

be undertaken of the changes in cracking behaviour and milling quality of rice in response to the use of this drying technique This study aimed to enhance the knowledge on the effects of high temperature fluidized bed drying and tempering of high moisture paddy on percentage

of fissured kernels, mechanical strength, head rice yield and whiteness of Vietnamese rice varieties

MATERIALS AND METHODS

Fluidized bed dryer

A batch lab-scale dryer (HPFD150) developed at the Nong Lam University, Vietnam, was used in this study This dryer consists of three main components: (i) a cylinder shaped drying chamber 40 cm in height and 15 cm in diameter; (ii) two 5 kW electric heating units; and (iii)

a centrifugal fan driven by a 0.75 kW motor The inlet drying temperature range of 20-100 oC was monitored by a temperature controller (Hanyoung Electronics Inc., Model DX7, Seoul, Korea) The outlet drying temperature was monitored by a Daewon thermocouple

Sample preparation

Long-grain rice cultivars A10 and OM2717 were collected from the fields of local farmers in Tien Giang Province and Ho Chi Minh City, Vietnam, respectively, in 2007 Freshly harvested paddy (25-33 % wet basis) was immediately transported to the laboratory and kept

in cold storage conditions maintained at 5 oC Rice lots were allowed to equilibrate at room temperature before being subjected to drying

Fluidized bed drying and tempering procedures

Approximately 200 g of paddy (thickness of bed was about 2 cm) was dried in the fluidized bed dryer at two levels of temperature (80 and 90 oC) for two time periods (2.5 and 3.0 min) The dried samples were immediately transferred to sealed glass jars and then tempered in incubators set at 75 oC and 86 oC, which were grain temperatures after fluidized bed drying at

80 oC and 90 oC, respectively Before incubation the glass jars used had been previously warmed to set temperatures and stored in foam boxes to prevent loss of heat when taken out The tempering durations used were 0, 30, 40, and 60 min

After tempering, the samples were thin layer dried at 35 oC to the moisture content suitable for storage (below 14 % wet basis) Finally, the dried rice samples were sealed in plastic bags

percentage of fissured kernels, mechanical strength of rice kernels and color Approximately

200 g of paddy was also dried using the thin layer drying technique at 35 oC for 16 hrs down

to 14 % moisture content (wet basis) This was used as the control sample All treatments were undertaken in triplicate

Moisture content determination

The moisture content before and after tempering, and final moisture content after thin layer drying for each drying run were determined by drying (in duplicate) 5-10 g of rough rice in

an oven at 130 oC for 24 hrs [11]

Percentage of fissured kernels

Fissured kernel counting was carried out on 50 manually dehulled brown rice kernels for each sample using visual observation with the assistance of a light box The percentage of fissured kernels was calculated from the number of fissured kernels in each sample of 50 kernels tested Two replications for each sample were assessed

Three-point bending test

The mechanical strength in terms of hardness and stiffness of 50 individual sound rice kernels was measured by the three-point bending test, using a test device developed at the University

of Queensland, Australia This test device was attached to the Universal Texture Analyzer

TA-XTplus (Micro Stable Systems Co., UK) (Figure 1a) This special attachment includes a

probe and a sample holder plate with grooves of five different sizes Each groove was 2.0 mm deep and 9.0 mm long The width of the grooves was 2.0, 2.5, 3.0, 3.5, and 4.0 mm The dimensions of a stainless steel probe made for testing purposes were 1mm thick, 32 mm wide, and 111 mm long The end of the probe was blunt to reduce the cutting effect that can potentially lead to test errors

The bending test was performed in a compression test mode The pre-test, test, and post test

deformation curve obtained during the bending test for an individual rice kernel The hardness (N: the maximum force used to break the kernel) and apparent stiffness modulus (N/mm: the slope of the force-distance curve) for 50 sound brown rice kernels per treatment, were extracted by means of Texture Exponent software (Micro Stable Systems Co., UK)

Head rice yield

Approximately 100 g of paddy was dehusked and milled using a laboratory milling system (Satake Co Ltd, Japan) for 60 seconds Whole kernels were manually separated from broken kernels, to determine head rice yield as defined by the ratio of the mass of unbroken kernel to the total mass of paddy used The head rice is composed of grains which maintain 75% of their original length after milling

Colour determination

A sample of milled rice for each treatment was placed in a clear Petri dish and then colour was measured by a Minolta Chroma Meter CR-200 (Minolta Co., Japan) in CIE 1976 L*, a*, b* colour space Parameters L*, +a*, -a*, +b* and -b* describe the brightness, red, green, yellow, and blue colours, respectively The total colour difference, ∆E*, was also calculated

Microstructure of fluidized rice kernels and degree of gelatinization

To investigate the possibility of an occurrence of partial gelatinization of rice during fluidized bed drying, the microstructure of fluidized rice kernels and the degree of gelatinization were examined The dried white rice in Vietnam was imported to Australia for further analyses It should be noted that only the A10 rice variety was used for these measurements The rice samples were kept in sealed plastic bags and stored at 4 oC to minimize any physicochemical changes Before measurement, the rice samples were allowed to equilibrate to room temperature Rice flour was prepared by grinding milled rice with an IKA A10 analytical mill (IKA®, Germany), with screening through a 0.25 mm screen

A cross-section area of rice kernels was placed on a carbon adhesive tap on top of a SEM

mount, and then coated with platinum to approximately 5 nm thickness The microstructure

image of the rice kernel was captured using a Jeol model 6300 field emission scanning

electron microscope (SEM) (Jeol Ltd., Japan) operated at 15 kV and a magnification range of

250 to 7000 times

The rice flour gelatinization thermal profiles were assessed by DSC (Differential Scanning

Calorimetry) (Pyris-1 DSC, Perkin Elmer, Norwalk, CT) Rice flour (7-8 mg) and water (1:4

w/w rice flour/water) were accurately weighed into aluminum DSC pans and hermetically

sealed The sample was left to stand for 1 h at room temperature (25 oC) before DSC

scanning Indium was used for the calibration of DSC and an empty pan was used as a

reference All samples were heated from 25 oC to 98 oC at a rate of 5 oC/ min The onset (To),

peak (Tp), and final (Tc) gelatinization temperatures, and gelatinization enthalpy (∆H, J/g dry

flour), were determined with Pyris-1 software The degree of gelatinization was calculated as

following:

%100

*1

where DG is degree of gelatinization (%), ∆H is transition enthalpy of fluidized bed drying

rice samples, and ∆Hr is transition enthalpy of reference sample

Statistical analysis

A multilevel factorial design, comprising two drying temperatures (80 and 90 oC), two drying

durations (2.5 and 3.0 min), and four tempering durations (0, 30, 40, and 60 min), was used in

the experiment The data was analyzed using the statistical package MINITAB® Release 14

(Minitab Co., USA) The Analysis of Variance of GLM (General Linear Model) and the DOE

(Design of Experiment) procedures were used Treatment means were considered

significantly different at P <0.05 Pearson’s correlation coefficient was used when correlation coefficients were calculated

RESULTS AND DISCUSSION

Reduction of moisture content

For A10 the initial moisture content was 32±1 % while for OM2717 the initial content was at 24.5±0.5 % (wet basis) The change in moisture content during fluidized bed drying and subsequent tempering for both rice varieties is presented in Figure 2 The percentage of moisture removed from the paddy after drying and tempering was found to be in the range of 7.7-12.0 % Increasing the drying temperature to 90 oC was associated with an increase in the percentage of moisture removed

Within the initial 2.5 min of fluidized bed drying, the moisture for the A10 variety dropped

by 8.7-9.4 % at 80 oC and by 11.0-12.0 % at 90 oC, while for the variety OM2717 it dropped

by 7.7-8.6 % at 80 oC and by 9.7-11 % at 90 oC Extending the drying time to 3.0 min removed further 1 % moisture in both drying temperatures for A10 rice However, for the variety OM2717, there was only 0.1-0.3% further moisture reduction It is probable that the amount of moisture that can be removed during fluidized drying is dependent on the initial moisture content of paddy An extension of the drying time to 3.0 min did not remove further moisture since the diffusion of moisture inside the kernel is a time dependent phenomenon If the drying process is allowed to proceed further, moisture removal will continue from outer surface of the grain which will result in physical stress due to an increased moisture differential between the interior and exterior layers of the rice kernels The tempering step is therefore necessary to allow moisture to diffuse from the interior to the exterior, prior to further drying

Fissured kernels, mechanical strength, head rice yield and colour of milled rice

Tables 1 and 2 list the percentage of fissured kernels, head rice yield, and discolouration in terms of total colour difference (∆E*) and yellowness (b*) of A10 and OM2717 varieties, respectively Table 3 and Figure 3 present the mechanical strength (hardness of stiffness) of A10 and OM2717 Drying temperature, drying time and tempering time, all had a very significant effect on the grain quality indices measured in this study (P <0.05)

Fissured kernels

The results (Table 1 and 2) clearly showed that the longer the drying time, the higher the level of damage to rice kernels In preliminary studies it was found that a drying time longer than 3.0 min caused a marked reduction in moisture content to below 17.5 % (wet basis) but induced a higher level of fissuring in the grain (results are not presented here) As a consequence of moisture gradients existed, a longer exposure of grain to high temperature causes strain on the grain due to the rapid removal of moisture from the surface layers of the grain, while it takes time for moisture to diffuse from the interior layers A large moisture differential will cause increased fissuring in the kernels Based on this result, the drying time for the rice at these high temperature conditions should not be more than 2.5 min, assuming that the level of moisture removal would be insufficient under a shorter drying time (Figure 2)

Without tempering, the percentage of fissured kernels was 22-42 % for the variety A10, while

it was 77-84 % for the variety OM2717 Tempering significantly decreased the levels of fissured kernels This clearly demonstrates the benefit of tempering the paddy for an optimal time, if a high drying temperature is used There was a continuous decrease in the number of fissured kernels as the tempering duration was increased, particularly for OM2717 For A10,

a tempering time of between 30 and 40 min was required to minimize the level of fissuring

Effects on mechanical properties

The hardness and stiffness of the rice were two mechanical parameters which were measured

in this study In general, both hardness and stiffness of OM2717 increased with increasing tempering time (Figure 3), particularly with drying temperature 90 oC The stiffness of A10, however, tended to return to the original values (before tempering) after tempered for 60 min Generally, the mechanical strength (both harness and stiffness) of the rice kernels was higher

with high temperature drying compared to low temperature drying at 35 oC (Table 3) Hardness and stiffness of sound rice kernels of both varieties were higher at the 90 oC drying temperature than at 80 oC The improvement in the mechanical strength of rice kernels may

be attributed to the partial gelatinization of the starch on the surface of the rice kernels The gelatinization of starch makes the surface become dense, and some of the surface cracks can become fused and disappear during this process

The mechanical strength of the kernels of both varieties did not show much difference As shown in Table 3, the average hardness of both varieties ranged from 33 to 53 N, while the apparent stiffness modulus was in range of 162-186 N/mm Probably this range of strength is sufficient to resist breakage during milling However, it should be noted that the measurement was conducted on individual sound kernels which had no chalky parts or fissures Therefore, the result would not totally reflect the head rice yield or percentage of fissured kernels The presence of cracks or fissures in the rice kernels are the principal factors contributing to breakage

Head rice yield

It is clear that tempering improved the head rice yield in both rice varieties A tempering duration to 40 min improved the head rice yield for both drying temperatures (80 and 90 oC) with a drying time of 2.5 min There was a clear trend of increasing head rice yield with

tempering This indicated that 30-40 min tempering time to be optimal for both varieties It should be noted that the tempering temperatures used in this study (75 and 86oC) were above the glass transition temperature of rice [12] This means that the rice was in rubbery state during tempering, resulting in the relaxation of differential stresses induced throughout rice kernel as a result of rapid moisture removal

On account of the difference in initial moisture content, the outlet moisture contents after fluidized drying of the variety OM2717 in the drying conditions of the study (17-18 % at 80 o

C and 14-16 % at 90 oC), were lower than those for the variety A10 (21.6-22.8 % at 80 oC and 19.4-21.5 % at 90 oC) While there was a large reduction of moisture content of A10 during fluidized drying, the head rice yield after 30 min of tempering, was higher than the head rice yield of the reference sample (Table 1) For the variety OM2717, it was only the treatment of 80 oC fluidized drying for 2.5 min, followed by tempering for 60 min (outlet moisture content 17-18 %) that could maintain a head rice yield similar to the reference sample (Table 2) The head rice yield of all remaining treatments was lower than the reference value, particularly for 90 oC fluidized bed drying for 3.0 min for which the outlet moisture content was below 17% (wet basis)

The high head rice yield for variety A10 can be attributed to the high initial moisture content resulting in partial gelatinization, while the low value of head rice yield of OM2717 may be related to the low outlet moisture content This is consistent with observations of Tumambing and Bulaong [13] who reported that the moisture content of paddy after fluidized drying up to

100 oC, should not be lower than 17% (wet basis) Under these circumstances, it is thought that there is a ‘critical outlet moisture content’ for fluidized bed drying at 80 and 90 oC As used in this study, in order to maintain head rice yield, the outlet moisture content should not

be lower than 18% (wet basis) before exposure to high temperature tempering for at least 40

The head rice yield is correlated very well with level of kernel fissuring With increasing tempering time, head rice yield tends to increase with decreasing percentage of fissured kernels This indicates that tempering has two simultaneous effects: one is to allow moisture

to equilibrate (diffusing from interior to exterior parts of the grain); the second is to relax the molecules thereby making the structure more rigid or dense [14, 15, 16] Both these effects would reduce the level of fissuring in the kernels, thereby improving head rice yield

It should be noted that not all the fissured kernels result in broken grain during the milling process While the HRY values for various drying times and tempering times from 30-60 min remained unchanged for the variety OM2717, the amount of fissured kernels continued to decrease with increased tempering time The identical values of HRY may be explained by the fact that some of fissured kernels which were not broken during milling, contributed to the head rice component It is also possible that grains with cracks close to two ends of the kernel may be more resistant to breakage during milling than kernels which have cracks at the middle It is therefore believed that the level of kernel fissuring can be a better indicator

of rice cracking than head rice yield

Colour of milled rice

The colour parameters measured for the two rice varieties are presented in Tables 1 and 2 The colour of the milled rice was measured with the expectation that high temperature drying and tempering can cause the discolouration of the rice The results in this study indicated that the yellowness value (b*) was higher for the samples dried at 90 oC than those dried at 80 oC Yellowness (b*) also increased with drying time, particularly for the variety OM2717 The total colour difference (∆E*) which is the combination of all principal colour parameters, also changed as the tempering time increased This is supposedly due to the Maillard non-enzymatic browning reaction at higher temperatures The intensity of discolouration increases

with higher drying temperature and longer drying time, particularly the yellowness in response to the effect of temperature accelerating the rate of the Maillard reaction

In addition, the results also showed that tempering time impacted on the whiteness of rice because the tempering at high temperature (75 oC and 86 oC) accompanied with long duration

up to 60 min, as used in this study, will also accelerate the discolouration of rice However, the visual appearance of the grain of both varieties was still at a level regarded as acceptable for commercial purposes (photographs are not presented here) The possibility of further colour development during storage after the drying/tempering treatments was not investigated

in this study

Microstructure of fluidized rice kernels and degree of gelatinization

Figure 4 presents the microstructure and cracking of cross-sectional areas of reference rice kernels (35oC thin layer drying for 16 h) As can be seen in Figure 4(a), the fissures existed between and inside endosperm cells A close-up view (Figure 4b) shows the cracks with starch granules in a polygonal shape Figure 5 depicts the microstructure of rice kernels subjected to the most severe heating conditions used in this study (drying/tempering regime:

90oC for 3 min/86oC for 60 min) at different magnifications The partial gelatinization caused

in response to the combination of a high temperature heat treatment and intermediate initial moisture content for freshly harvested rice paddy, was confirmed with the occurrence of a very smooth region in the outer layer of rice kernel (Figure 5a) As kernels swell, the starch granules expand, fuse and lose their sharp polygonal shapes (Figure 5b) It is hypothesized that the gel network created during gelatinization can heal the fissures within the rice kernel

by filling the void between adjacent fissure traces Consequently, kernel integrity may be improved through a partial gelatinization process resulting in higher head rice yield

Gelatinization properties of fluidized bed drying rice samples are characterized in Table 4

temperature and tempering time In contrast, gelatinization enthalpy declined with higher drying temperature and increased tempering duration The increasing trend in gelatinization temperature suggests that the internal molecular structure changed in response to high temperature heating for a prolonged time Degree of gelatinization of fluidized bed drying rice samples at 80 oC is in range of 10 – 28.3% while that at 90 oC is higher (13.7 – 42.18%) This result indicates that partial gelatinization occurs during fluidized bed drying and continues with an extended high temperature tempering period However, complete gelatinization is not possible during prolonged tempering due to the fact that the moisture content of rice after it is fluidized (around 18% wet basis) is insufficient for the occurrence of total gelatinization

CONCLUSIONS

For the drying and tempering conditions investigated, a fluidized bed drying temperature of

80 oC and 90 oC for 2.5 and 3.0 min, removed between 8.7-12 % of the moisture in wet paddy for variety A10 (initial moisture content 32±1 % wet basis) and variety OM2717 (24.5±0.5 % wet basis)

For both rice varieties, the tempering step significantly reduced the level of kernel fissuring and improved the head rice yield Resistance to breakage during milling might have been improved by the fusion of starch at outer layers of kernels due to partial gelatinisation at high temperature and over a prolonged heating time as confirmed by SEM photographs Consequently, the head rice yield was even higher than for the reference sample due to partial gelatinization In addition to the contribution due to partial gelatinization, the enhancement of kernel integrity with respect to hardness and stiffness during tempering, were identified in this study These findings assisted in improving our understanding of the role of tempering

In reference to the application of the fluidized technique for grain drying, it can be seen that fluidized bed drying can significantly reduce the drying time used when compared with the use of flat bed driers which are commonly practiced in Vietnam The actual drying time involved with the use of flat bed driers ranges from 8-10 hrs for wet paddy, if farmers want to

reduce the grain moisture content to a safe level (14% wet basis) If the paddy needs to be

dried to 15-16 % moisture, the fluidized bed drying system can be used as a compact drier The fluidized bed drying technique evaluated in this study is strongly recommended for drying paddy in Vietnam during the wet season to maintain rice quality as the use of this drying technique has shown beneficial effect especially on head rice yield

ACKNOWLEDGEMENTS

The authors would like to thank the Collaboration for Agriculture and Rural Development (CARD) Program, Vietnam for providing funding support for this study CARD is an AusAID funded project for Vietnam to promote agriculture and rural development through the application and adaptation of research, technology, skills and management practices with

a focus on smallholders

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2 Steffe, J F., & Singh, R P Theoretical and practical aspects of rough rice tempering

Transactions of the ASAE, 1980, 23, 3

3 Cnossen, A G., Jimenez, M J., & Sienbenmorgen, T J Rice fissuring response to

high drying and tempering temperatures Journal of Food Engineering 2003, 59,

61-69

4 Kunze, O R., & Calderwood, D L Rough-rice drying-Moisture adsorption and