Section 1 - Physical grain characteristics of paddy/milled rice and its grades and standards

Contents - Previous - Next

The anatomy and physical properties of the rice grain
Paddy and milled rice grading
Laboratory test methods for paddy and milled rice grading
NFA standard specification for milled rice (Second revision) TRED SQAD No. 2; 1980

 

The anatomy and physical properties of the rice grain

by B. Belsnio

I. INTRODUCTION

A combined knowledge of the physical properties and anatomical composition of the rice grain is a prerequisite in gaining a closer understanding of what happens to the grain in the different postharvest operations. The understanding of the anatomy of the rice grain will clarify the reasons why rice kernels break so easily on mechanical impact during the physical operations of threshing and milling, and under thermal stress during drying. methods on the surface tissue of the grain kernel and the kernel itself, will indicate the importance of the correct adjustment of hulling machines in order to prevent breakage, and ensure higher milling recovery.

II. ANATOMY OF THE PADDY GRAIN

The anatomy of the rice grain consists of a brown rice kernel enclosed by the husk. The brown rice grain consists or a bran layer, a germ and the starchy center of the grain.

THE HULL

The most visible part or a rough rice grain is the husk. This is also known as the hull (figure 1). This is formed from the two leaves of the spikelet namely the palea covering the ventral part of the seed and the lemma covering the dorsal portion. Both parts are longitudinally joined together by an interlocking fold. This fold is a weak point in the hull and easily breaks up when a twisting force is applied to the grain. The upper end of the two hull sections transfer into the apiculus sections and finally ends in the pointed awn.

At the lower part, where the grain is fixed on the panicle is a tiny leaf-shaped part called the sterile lemma and then the rachilla. Normally the panicle breaks off during threshing, however a small part of the pedicel frequently remains attached to the grain. The husk is formed mostly of cellulosic and fubrous tissue and is covered with very hard glass-like spines or trichomes. The present of this make the husk abrasive and very hard thus, they give the grain a good protection against insects, microrganism, moisture and gases

The caloric value to the hulls is rather high and ranges from 3000 to 3500 kcal/kg making hulls an important source of energy in agriculture.

However, the most disturbing presence in rice hull is high proportion of silica which causes considerable damage to processing equipment through excessive wear of machine parts and interconnecting transfer facilities.

THE PERICARP

When the hull is removed, a thin fibrous laver can be seen (Figure 2). This is called the pericarp, frequently known as the "silverskin". The layer is usually translucent or greyish in color. When the pericarp is not translucent, but reddish in color the gain is referred to as red rice. It is considered as an integral part of the brown rice kernel (caryopsis) but is easily removed in the or whitening process. The main function of this layer is to serve as an additional protective layer against molds and quality deterioration through oxidation and enzymes due to the movements of oxygen, carbon dioxide and water vapor.

The pericarp actually consists or 3 layers namely epicarp, mesocarp & cross layer. Immediately under the pericarp layer is the testa or sometimes called tegmen layer which is only a few cells in thickness but with less fibrous than the pericrap layer. This layer is rich in oil and protein but its starch content is very low.

Sometimes this layer is considered as part of the seed coat but because of its oil content, it is normally considered as the outermost layer of the bran.

THE BRAN

Immediately under the testa or tegmen layer is the bran layer or aleurone layer (Figure 2). This part is the main constiturent removed in the whitening stage during milling. It has a very low starch content but has a high percentage of oil, protein, vitamins and minerals. Because of its high oil, content, the bran is easily affected by oxidation when the oxygen in the air comes in contact with oil.

In the milling process, the higher milling degree indicates a greater percentage of bran removed. Table 1 shows the degree of milling as determined by the quantity of the outer layer removed from the brown rice kernels. When rice is fully milled the vitamins (a complex), protein, mineral, and oil contents are lessened. This explains why persons with beri-beri (Vitamin A deficiency) are advised to eat brown rice. This also probably explains why persons who eat well milled rice are prone to be protein difficient or even malnourished. Thus, it is not surprising that some dieticians recommend the eating of regularly milled or even undermilled rice.

In the processing Industry, the vitamins in the grain can be retained by parboiling before milling. This allows the movement of nutrients from the bran layer to the inner part of the grain thus, making the vitamins available in the milled rice.

THE EMBRYO

The embryo is located at central bottom portion of the grain, where the grain has been attached to the panicle of the rice plant (Figure 2). This is the living organism in the grain which develops into a new plant. The embryo respires by taking in oxygen in the air, consumes food which comes from the starch in the grain itself while simultaneously releasing moisture and heat. This explains why grains during storage have the tendency to decrease in weight as a result of the loss in moisture and dry matter content in the endosperm. During milling, the embryo is removed resulting in an indented shape at one end of the milled rice grain.

THE ENDOSPERM

When the husk, the pericarp, the bran and the embryo are removed, what remains is the endosperm. It mainly consists of starch with only a small concentration of protein and hardly any minerals, vitamins or oil. Because of its high percentage of carbohydrates, its energy value is high. In the central core of the grain the starchy cells are somewhat hexagonal in shape, but between the centre and outside they are elongated with the long walls radiating outwards from the centre (Figure 2)

III. PHYSICAL PROPERTIES/ CHARACTERISTICS

1. Length

The length of the paddy grain is variable, even within a variety, because of variation in the length of the awn and the pedicel. It is for this reason that the type of paddy is not determined by the length of the paddy grain but by the length of the brown rice kernel. In threshing, it is important to watch at what point the paddy grain will break off its panicle, because the pedicel should not be a part of the grain, of else it will have a reduced milled rice recovery through the increase of "husk" production.

2. Husk-surface

The husk surface is rather rough and abrasive through its high silica content. It is for this reason that rubber-rolls of the hullers wear so fast, that precleaning machines have many parts which ferquently need to be replaced, augers used for paddy transport become very sharp, elevator discharge spouts, especially bends in spouts and elevator cups wear so quickly, and that parts of the husk aspirators in direct contact with the husk have to be repaired or replaced very often.

In rice mill equipment, the rough surface of the paddy grain compared with the smooth surface of the brown rice, plays an important role in the determination of specific design criteria.

3. Free space between the Husk and the brown Rice kernel.

When the grain is dried, there is a distinct space between the rice hull and the kernel inside. With the weak point in the interlocking fold and the space between the rice hull and grain kernel, a rubber roll huller or any dehulling machine can dehul the grain with minimum (or even without) abrasion to the pericarp and other internal parts of the grain. This enables the dehulling to be done with minimal pressure against the grain, thus minimizing breakage and losses.

4. Tight Interlocking fold of the husk

The husk sections consising of the lemma and the palea, are tightly seamed together through a double fold. This requires a level of force to open these folds in the process of dehusking which made the design of hullers rather difficult in order to avoid unnecesary breakage of the grain.

It is only when the rice is parboiled that dehusking will practically cause no problems because as a result of the hot water soaking and steaming process, the two husk sections open without releasing the brown rice kernel.

5. The Awn

The awn sometimes is very long on certain varieties, so that special machine is required in order to break off and remove the awns prior to the dehusking of the paddy. However, awners are expensive, energy consuming, slightly increase the amount of breakage and ultimately result in a less profitable, less efficient conversion of paddy into milled rice.

6. The Pericarp

When damaged, it allows oxygen to penetrate into the bran layer which leads to an increase of the free fatty acid (FFA) content of the oil in the bran. The oxidation process makes the bran rancid and will ultimately result in serious quality deterioration of the brown rice kennel. Its mainly the abrasive disc huller which damages the pericarp. But, this is no disadvantage if it is immediately converted into milled rice. However if brown rice is produced for storage (as in Japan) or for shipment as cargo rice to riceimporting countries or to central whitening plants, the use of rubber roll hullers is a must in order to avoid or at least reduce oxidative and enzymatic deterioration of the bran tissues.

7. The Longitudinal Starchy Cells

It is unfortunate that the outermost starch cells of the endosperm are elongated in shape, and from the processing point of view it is even more unfortunate that these long cells are positioned with the long side directed towards the center of the grain. This give the grain the potential to react to thermal stresses resulting in fissures and ultimately in cracks throughout the grain. It can easily break under the impact of force either when it is threshed, conveyed, cleaned or dehusked. And lastly, this characteristic gives the grain the potential to break when incorrect drying procedures are followed. This aspect has made it extremely difficult to design drying systems that would enhance an optimum milled rice recovery through minimum breakage.

Other physical properties that are related to the physical composition of the grain are:

8. Angle of Repose

Paddy forms a complete cone when it is vertically unloaded on a flat surface. The angle of the side of this cone-shaped mass of grain, measured after the flow of grain has completely stopped, is the angle of repose. This angle defers from each type of grain and depends much on the smoothness of the surface of the grain.

The angle of repose is also directly dependent on the moisture content of the grain. At a moisture content level of 20%, the angle of repose for paddy will be greater than for dry paddy at 14% MC.

This properties is important in the construction of bulk storage facilities and the calculation of the dimensions of intermediate holding bins of a given capacity.

9. Angle of Friction

The angle of friction refers to the angle measure from the horizontal at which paddy grain will start moving downwards over a smooth wooden surface with gravity discharging the paddy grain. This differs for each type of grain and characteristic of the surface, since it depends much on the smoothness. Also, the moisture content of grain has an impact on the angle of friction the angle for wet grain is greater compared to dry grain.

This angle of friction is important in the construction of self-unloading holding bins and bulk storage facilities. It also plays a role in the construction of grain discharge spouts.

10. Bulk Density

The bulk density refers to the ratio between weight and volume of grains. It is normally expressed in kg per HL, Ibs per cuft or kg/cu m.

The density data are important in the calculation of the dimension of bulk storage facilities and intermediate holdings bins of given capacity. It also indicates the purity-degree of the grains since the presence of light foreign matter reduces the grain density.

11. Grain Dimensions

The dimension of the paddy grain and milled rice kernel play an important role in the determination of grain standards and throughout the processing cycle. This grain dimension is classified according or in relation to the following.

1. The type of paddy-classified according to the length of the whole brown rice grain.

  1. Extra long -paddy with 80% of the whole brown rice kernels having a length of 7.5 mm. or more.
  2. Long -paddy with 80% of whole brown rice kernels having a lenght of 6.5 mm. or more but shorter than 7.5 mm.
  3. Medium -paddy having 80% of the whole brown rice kernels with a length between 5.5 mm. to 6.5 mm.
  4. Short -paddy with 80 % of the whole brown rice kernels shorter than 5.5 mm.

2. The sub-type of paddy The sub-type of paddy grain refers to the ratio of length and width of the whole brown rice kernel

  1. Slender-paddy of which the brown rice grain has a length/width ratio of 3.0 or more.
  2. Bold -paddy of which the brown rice grain has a length/width ratio of 2.0 or more but smaller than 3.0.
  3. Round -paddy of which the brown rice grain has a length/width ratio smaller than 2.00.

3. The type of Milled Rice-milled rice is classified according to the length of the whole grain.

  1. Extra long - milld rice of which 80 % of the whole milled rice kernels have a length of 7.0 mm. or more.
  2. Long -milled rice of which 80 % of the whole milled rice kernels have a length of 6.0 mm. or more but shorter than 7.0 mm.
  3. Medium -milled rice of which 80% of the whole milled rice kernels have a length of 5.0 mm. or more but shorter than 6.0 mm.
  4. Short -milled rice of which 80 % of the whole milled rice kernels are shorter than 5.0 mm.

4. The sub-type of milled rice -This refers to the length/width ratio of the whole milled rice grain. The 3 sub-types for milled rice is defined in the same manner as that for paddy as slender, bold and round.

5. Brokens in milled Rice -The definition of brokers is generally based on the length of the rice particle and is referred to in units 118th of the length of the whole unbroken milled rice grain. These are categorized as:

  1. Head Rice -is a milled rice particle with a length of 618 or more of the length of the whole unbroken milled rice kernel.
  2. Large Broken -is a milled rice particle with length of 618 or more of the length of the whole length of the whole unbroken milled rice kernel.
  3. Small Broken -is a milled rice particle which will not pass through a perforated sieve with a round perforation of 1.4 mm. but the length of the grain is shorter than 3/8 of the whole unbroken mill rice kernel.
  4. Brewer's Rice-is a milled rice particle which will pass through a sieve with a round perforation of 1.4 mm.

TABLE 1 Degree of milling based on the removed outer layer of the brown rice kernels

DEGREE PERCENT REMOVED FROM BROWN RICE KERNELS %
Under milled 3 - 4
Medium milled 5 - 6
Fully milled 7 - 8
Over milled 8

 

References:

1. H. TH. Van Ruiten, Physical properties of paddy and milled rice. Grain post-harvest processing technology.

2. E.V. Aronilo, D.D. de Padua and Michael Grainham, Rice post harvest technology.

Figure 1. The Rice Grain

Figure 2. Structure of the Rice Grain

Figure 3. Parts of a Spikelet.

 

Paddy and milled rice grading

BY: REBECCA L. SAMPANG

INTRODUCTION

Rice is the staple food of the Philippines contributing 74% of total food consumption and per capita food consumption? More than 30% of all agricultural lands and more than 50% of the food cropland is devoted to rice. In 1986, the country produced 2.67 metric tons of paddy per hectare.

Under these accelerated production conditions, paddy and rice grading has become a major aspect in marketing both for local and international trade, quality assurance and in varietal improvement programs since the 70s. The National Food Authority is the agency entrusted with grain market stabilization, market development and industry regulation.

IMPORTANCE OF GRAIN GRADING

Grain grading is a set of standard procedures and methods in quality determination which is essential in marketing, quality assurance operations and in the varietal improvement program of the country and other research projects involving paddy and milled rice.

GRADES AND MARKETING

Grading is necessary in the development of quality standards that define the relationship between grades and prices in the assessment of the value of grains. Official standards are important in the marketing process because they furnish the means of describing variations in quality and condition. They also provide a basis for merchandising contracts, for quoting prices, for loans on product in storage and for sorting and blending by producers to meet market requirements. Grading then provides for an orderly marketing and trading system.

When grades and prices are defined, the farmers become virtually interested in producing better crops because with grading they are assured that their return are based on the quality of their produce. This is supportive of the quality assurance program of the agency.

GRADES AND QUALITY ASSURANCE

With better crops procured, quality assurance in the other post-harvest operations becomes more manageable.

With grades and quality standards, quality evaluation or quality assessment operations aimed at preventing quality deterioration and reducing postharvest losses becomes more uniform.

Grading is conducted at regular intervals in the various stages of post-harvest operations as a means of quality monitoring. It becomes a basis for comparison between grain quality particularly of stocks before and after long storage, thus as basis for remedial measures to be undertaken.

Quality has become one of the dominant factors for consideration in the rice industry and the first step towards the achievement of quality rice is grading. Setting-up modern post-harvest facilities alone cannot solve the quality problem completely. Grading is particularly desirable prior to milling as it offers the following advantages: 1) immature grains are separated 2) more precise adjustment of the huller is possible, which minimizes breakages, and 3) independent milling of graded lots is possible.

GRADING AND IMPROVEMENT PROGRAMS

Since 1955, the country has developed a varietal improvement program through the Philippine Seed Board which has recommended a total of 96 high yielding varieties for cultivation. It has been estimated that about 75% of the total riceland area are now planted to improved varieties.

Paddy and milled rice grading is a component of the varietal improvement program as different varieties have different intrinsic and acquired characteristics. This responsibility was entrusted to the National Food Authority since it joined the Board in 1979. Some quality standards set by NFA has since become the basis for comparison among varieties with respect to acquired characteristics such as headrice and brokers, chalky and immature kernels and brown rice and milling recovery. In the past 10 years, 26 lowland and 6 upland varieties were approved and are now widely cultivated.

PADDY AND MILLED RICE GRADING IN THE PHILIPPINES

Paddy and milled rice grading in the Philippines is patterned after the model rice grading system introduced by FAO through the Inter-Government Group on Rice. This grading system includes standard definition of terms.

PADDY GRADING

The present NFA grading system is of two types: Field grading during procurement and laboratory grading after procurement.

Field grading during procurement is a very important phase of the agency's post-production operations. At this stage, all grains being procured from individual farmers, corporations or cooperatives are inspected and analyzed to establish the quality of the commodity. By doing so, the commercial value of the commodity is assessed as well as its fitness for processing, storage or distribution.

During procurement of paddy, moisture content and purity are quality factors which form part of the basis of payment and are therefore properly determined before the grain is procured.

At the buying stations, farmers' produce are sampled randomly by means of a grain probe and the gathered sample is then inspected. Purity, damaged and discolored kernels are determined by ocular inspection by quality assurance officers, while moisture content is determined by means of a calibrated moisture meter. The produce then is graded according to the following NFA buying specifications:

Paddy with lower purity (below 95%) and/or high MC (adove 14%) may be accepted or bought at a reduced price subject to weight adjustments. using the Equivalent Net Weight (ENW) (Table 2) for paddy, provided they do not exceed the lower limit set for purity (85%) and the upper limit set for moisture content (26%) and that they pass all other quality specifications. Paddy with MC above (26%) and that they pass all other quality specifications. Paddy with MC above 26% and purity below (85%) are not accepted

The laboratory grading based on the national standard considers the following factors for quality assessment:

1. Moisture Content

High moisture directly reduces the grade of rice; if the grain molds or spoils, the grade is lowered even more. This is a critical factor as it has a bearing on the keeping properties of grain during storage and also on the milling quality and yield. A tested and reliable moisture meter is used for moisture determination. For every region, only one brand of moisture meter is used to minimize variations of moisture reading.

2. Impurities and Foreign Matter

All materials other than paddy or rice kernels are called foreign matter or impurities. This includes soil, stones, weed seeds, fragments of rice stalk, dust, husk, and dead insects. The presence of impurities and foreign matter could result in grain deterioration in storage. It also affects the quality of milled rice and accelerates the wear and tear of the milling machine. Its presence likewise lowers the milling recovery of the grains.

3. Damaged and Yellow Kernels

Damaged kernels are defined as kernels and pieces of kernels of paddy/rice which are distinctly discolored or damaged by water, insects, heat or any other means. It is difficult to assess damaged and yellow kernels in paddy unless dehulling and whitening is done. The presence of these grains adversely affects the quality of milled products derived from such lots.

4. Immature and chalky Grains

Immature grains have a husk weight ranging from 30 to 40% of the grain weight. Hence, the presence of excessive amounts of immature grains in paddy lowers milling yield and increases the husk production. Furthermore, immature grains are predominantly chalky and brittle, breaking easily in the process of hulling and whitening, thus reducing both headrice and total milling yield.

5. Red Rice

Red rice is rice with discolored pericarp. The red seed coat (pericarp), usually in the form of a firmaly adhering bran, detrats from the appearance and market value of the commercial product. The bran of red rice adheres to the endosperm so firmly that it is difficult to remove in milling without causing excessive breakage of the gnain.

6. Classification of Grains (Varietal Purity)

The physical dimensions of the grains are important in many ways. Grouping of varieties are made on the basis of sizes: long, medium, and short and in the process, utmost care must be taken to see to it that they are of unifrom sizes. Admixture of different sizes adversely affects the milling quality and yield. Proper segregation of grain according to sizes is absolutely necessary. In some cases, rice is reprocessed several times to obtain the desired quality standards. Once the grains are mixed, it will be extremely expensive to improve the quality through milling.

For NFA Grade Specifications for Paddy, please refer to Table II Appendix B.

MILLED RICE GRADING

Grading of milled rice either for foreign or domestic consumption, is usually done after milling. Periodic inspection, however, is conducted during milling to check if the resulting product meets the required standard specifications. This makes possible the immediate restoration of a good process and prevent production of too many defective grain or rejects.

The quality parameters considered in milled rice grading are practically the same as in paddy grading with some additional parameters.

1. Head Rice Yield

When rough rice is milled, kernel breakages naturally occur resulting in different kernel lengths, hence, it is necessary to determine the variation. The length differences have been grouped into head rice, brokers and brewers with certain length limits.

Head Rice - a kernel or a piece of kernel with its length equal to or greater than 8/10 of the average length of the unbroken kernel.

2. Brokens

Brokens are still subdivided into the following types: Big Brokens, Medium Brokens, Small Brokens and Brewers.

Big Brokens - pieces of kernels smaller than 8/10 but not less than 5/10th of the average length of the unbroken kernel.

Medium Brokens-pieces of kernels smaller than 5110th but not less than 2/10th of the average length of the unbroken kernel.

Small Brokens - pieces of kernels smaller than 2/10th of the average length of the unbroken kernel.

3. Brewers

These are small pieces or particles of kernels that pass through a sieve having round perforations 1.4 mm in diameter. This is also known as "binlid" or "chips".

4. Paddy Kernels

Dehulling is not usually 100% efficient resulting in the presence of paddy kernels in the milled rice output.

Paddy - unhulled grain of Oryza saliva, which means grain with glumes enclosing the kernels. It is also known as "palay" of "rough rice" or "rice grain".

5. Red Streaked Kernels/Red Rice

As in red rice, these are kernels with discolored pericarp, whole or broken but having red streaks of the total length of which is one half or more of the length of the kernel as differentiated from red rice which have 25% of their surface red.

6. Milling Degree

Milling degree is determined by the degree of bran removal and are of the following types:

Undermilled - Rice grain from which the hull, a part of the germ and all or part of the outer bran layers but not the inner bran layers have been removed.

Regular Milled - Rice grain from which the hull, the germ, the outer bran layers and the greater part of the inner bran layers have been removed, but parts of the lengthwise streaks of the bran layers may still be present on more than 10% but not exceeding 30% of the kernels.

Well Milled - Rice grain from which the hull, the germ, the outer bran layers and the greater part of the inner bran layers have been removed, but parts of the lengthwise streaks of the bran layers may still be present on not more than 10% of the kernels.

7. Damaged and Yellow Kernels

8. Chalky and Immature Kernels

9. Foreign Matter

10. Contrasting Type (Varietal Purity)

11. Moisture Content

For NFA Grade Specifications for Milled Rice, please refer to Table 4.

TABLE 1: NFA VARIETY CODES

1.1 PALAY

For Procurement Purposes:

VARIETY (max. %) MC YELLOW AND DAMAGED
( % )
CHALKY
(Max %)
RED RICK
(Max %)
XQP 14 0 - 3 10 1
PAO 14.1 - 26 0 - 3 10 1
PA7 14 3.1 - 7 25 5
PA9 14.1 - 26 3.1 - 7 25 5
SDP 14 - 26 6.1-55Y - -
    1.1 - 25D    

 

For Reclassification Purposes:

VARIETY (max. %) MC YELLOW AND DAMAGED
( % )
CHALKY
(max %)
RED RICE
(Max %)
PA1 14 7.1 - 15 25 5
PA2 14 25.1 - 30 25 5
PA3 14 30.1 - 50 25 5
PA4 14 Greater than 50 25 5

 

1.2 MILLED RICE

VARIETY (Max. %) MC

AND

YELLOW
(Max %)
DAMAGED
( % )
CHALKY
(Max %)
RED RICE
WM0/RM0 14 3 10 1
WM1/RM1 14 3.1 - 7 25 5
WM2/RM2 14 7.1 - 15 25 5
WM3/RM3 14 Greater than 15 25 5

 

TABLE 2 EQUIVALENT NET WET WEIGHT FACTOR FOR PALAY

%MC: 14% : 14.1-:14.6-:15.1-:15.6-:16.1-:17.1-:
  18.1 -:19.1 -:20.1 -:21.1-:22.1 -:23.1 -:
  24.1 -:25.1 -:
Purity 14.5%:15%:15.5%:16%:17%
  : 18%: 19% :20% :21%: 22%
  : 23%: 24%: 25%: 26%:
95-100% :1.00:0.98:0.97:0.96:0.95:0.93
  :0.92:0.90:0.88:0.87:0.85:0.84
  :0.82:0.81 :0.79:
90-94.9% :0.97:0.95:0.94:0.93:0.92:0.91
  :0.89:0.87:0.86:0.84:0.83:0.81
  :0.80:0.79:0.77:
85-89.9% :0.92:0.90:0.89:0.88:0.87:0.84
  :0.82:0.81 :0.79:0.73:0.77:0.75
  :0.74:0.73:

 

INSTRUCTIONS FOR THE USE OF THE TABLE 2:

  1. Determine the Gross Weight (GW) of the Palay
  2. Determine the Wet Weight (NW) of the palay by subtracting the weight of container from the gross weight.
  3. Determine the percent (5) Moisture Content and the percent purity of the palay
  4. Based on the % moisture content and the % purity, determine the the Equivalent Net Weight Factor (ENWF).
  5. Multiply the Net Weight to the Weight Factor to get the Equivalent Net Weight (ENW).
  6. Peso Value-Equivalent Net Weight x Buying price:

N.8 This table shall not used for liquidation or other puposes except for palay procurement only.

Quality Standards for palay: 14% and 95% purity

TABLE 3 GRADE REQUIREMENTS FOR PADDY

GRADING FACTORS GRADE PREMIUM :1:2:3
Purity (Min%) 98.00: 95.00: 90.00: 85.00
Foreign Matter (Max %) 2.00: 5.00: 10.00: 15.00
a) Weed Seeds and other Crop seeds (Max %) 0.10: 0.10: 0.25: 0.50
b) Other Foreign Matters 1.90: 4.90: 9.75: 14.50
Defectives:  
Chalky & Immature Kernels (Max. %) 2.00: 5.00: 10.00: 15.00
Damaged Kernels (Max. %) 0.25: 1.00: 3.00: 5.00
Contrasting Types (Max %) 3.00: 6.00: 10.00: 18.00
Red Kernels (Max %) 1.00: 3.00: 5.00: 10.00
Discolored Kernels (Max %) 0.50: 2.00: 4.00: 8.00
Moisture Content (Max %) 14.00: 14.00: 14.00: 14.00

TABLE 4 GRADE REQUIREMENTS FOR MILLED RICE

GRADING FACTORS GRADE PREMIUM: 1: 2: 3
Head Rice (Min %) 95.00:80.00:65.00:50.00
Big Brokens (Max %) 3.00: 10.00: 10.00:20.00
Broken Other than Big Brokens (Max %) 1.90: 9.75:24.00:29.00
Brewers (Max %) 0.10: 0.25: 0.50: 1.00
Defectives:  
Damaged Kernels (Max %) - : 0.25:0.50: 2.00
Discolored Kernels (Max %) 0.50: 2.00: 4.00: 8.00
Chalky & Immature Kernels (Max %) 2.00: 5.00: 10.00: 15.00
Contrasting Types (Max %) 3.00: 6.00: 10.00: 18.00
Red Kernels (Max %) - : 0.25: 0.50: 2.00
Red Streaked Kernels (Max %) 1.00: 3.00: 5.00: 10.00
Foreign Matter (Max%) - : 0.10: 0.20: 0.50
Paddy (Max. no. per 1000 grams) 1 : 8 :10 :15
Moisture Content (Max %) 14.00: 14.00: 14.00: 14.00

 

Laboratory test methods for paddy and milled rice grading

BY Wenifreda C. Fajardo

INTRODUCTION

Laboratory tests and analyses used in grading paddy and milled rice are based on the physical characteristics of grains and involve ocular or visual inspection of the commodity at hand. The tests are carried out with the use of several laboratory equipment following the procedures for laboratory test methods for Paddy and Milled rice analysis.

The fallowing should be carried out in conducting these tests.

TEST METHODS

1. Preparation of working sample:

1.1 Pass the representative sample at least three times through a laboratory mixer or divider to ensure homogenous mixing.

1.2 Prepare the following working samples for three trials for each test.

1.2.1 For Paddy Grading

3 - 150 gms. - for moisture content determination
3 - 100 gms. - for purity determination
3 - 100 gms.-for immature, chalky, discolored, damaged and red kernels
3 - 250 gms.- for potential milling recovery

1.2.2 For Milled Rice Grading

3 -150 gms. - For moisture content determination
3 - 100 gms. - for headrice, brokers and brewers
3 - 100 gms. - for discolored, damaged, red, red streaked, chalky, immature and foreign matter

2. Moisture Content Determination:

2.1 Determine the moisture content from the 150 gram sample using a properly calibrated moisture tester appropriate for the grain being tested, or use standard air-oven method or any other method which gives equivalent results. (The weight of the working sample may vary depending upon the requirement of the moisture tester.)

PADDY GRADING

3. Purity Determination:

3.1. Using the 100-gram paddy working sample, separate the weed seeds using a sieve. pass the remaining paddy twice through a laboratory aspirator to separate other impurities such as chaffs, hulls, stones, stalks, weed seeds, etc. Gather and weigh all the impurities except the weed seeds. Sorting out of weed seeds and other impurities can also be carried out manually by handpicking.

3.2 Calculate the following percentage using the following formula:

4. Determination of contrasting type and type classification

4.1 Contrasting type

4.1.1 Dehull the pure paddy sample obtained from the purity determination test by means of laboratory husker or laboratory huller.

4.1.2 Separate all whole brown rice kernels and weigh

4.1.3 Separate the kernels whose size and shape differ distinctly from the majority of the lot and weigh them

4.2 type Classification:

4.2.1 Pick out 10 whole kernels at random form the dominant size in 4.1.3

4.2.2 Measure the individual length of the kernels and compute for average length.

4.2.3 Determine the type classification by referring to the standard specification of Rough Rice (Palay)

5. Determination of Defective Grains:

5.1 Dehull the second 100 gram paddy working sample and weigh the resulting brown rice

5.2 Segregate the following components on a sorting board

5.2.1 Immature kernels

5.2.2 Chalky kernels

5.2.3 Discolored kernels

5.2.4 Damaged kernels

5.2.5 Red kernels

5.3 Weigh each component and calculate the percentages using the following formula:

(Chalky and immature kernels are combined and treated as one component)

6. Determination of Potential Milling Recovery:

6.1 Aspirate more than 250 gram working sample and weigh 250 grams (capacity of laboratory mill) of the pure paddy.

6.2 Dehull the pure paddy and mill the resulting brown rice to well milled rice using a laboratory testing mill

6.3 Weigh the resulting milled rice and calculate the percentage milling recovery

MILLED RICE GRADING

7. Determination of headrice, big brokers, other brokers and brewers

7.1 Prepare the working sample for milled rice analysis following the procedures for the preparation of working sample in 1.

7.2 From the first 100 grams milled rice working sample, separate the brewers by using 1.4 mm sieve.

7.3 Separate the broken kernels by means of an indented plate, mechanical grader or by handpicking and weigh the remaining headrice.

7.4 From the separated broken kernels, segregate the big brokers.

7.5 Weigh each component and calculate the percentages using the following formula:

8. Determination of Contrasting Type and Type Classification:

8.1 Contrasting Type:

8.1.1 Form the headrice obtained in # 7.2, separate the kernels whose size and shape differs distinctively form the majority of the lot

8.1.2 Weigh such kernels and calculate percent contrasting type using the formula:

8.2 Type classification:

8.2.1 Pick 10 whole kernels at random from the dominant size in 8.1.1

8.2.2 Measure the individual length of the kernels and compute for the average length.

8.2.3 From the average length, determine the type classification by referring to standard specification for milled rice.

9. Determination of Foreign matter and Defective Milled Rice:

9.1 From the second 100 grams milled rice sample, count the number of paddy grains

9.2 From the remaining portion, segregate the following:

9.2.1 Discolored kernels

9.2.2 Damaged kernels

9.2.3 Red kernels

9.2.4 Red Streaked kernels

9.2.5 Chalky kernels

9.2.6 Immature kernels

9.2.7 Foreign matter

9.3 Weigh each component and calculate the percentage using the following formulas: (Chalky and immature kernels are combined and treated as one component)

10. Determination of Milling Degree:

10.1 Examine the bran layers or bran streaks from the head rice obtained by using a microscope or a magnifying glass.

1 0.2 Milling degree can be determined from the bran removal or by comparison with a standard sample. The classes of milled rice according to the degree of milling is as defined in the Standard Specification for milled rice.

11. Grading

Based on the result of analysis, the grade shall be evaluated against the Standard grade requirements. The grade designation is set according to the grade of the lowest quality characteristic.

GRADING OF PALAY AND MILLED RICE

EXAM PLES:

PALAY GRADING

To illustrate how the grade is determined, consider a sample of palay with the following characteristics:

Parameters % Grade
Purity 99.00 Meets premium
Foreign Matters 1.00 -do
a) Weed seeds and other crop seeds 0.09 -do
b) Other foreign matter 0.91 -do
Chalky and Immature kernels 6.50 Meets grade 2
Damaged Kernels 1.50 -do
contrasting types 3.50 Meets grade 1
Red kernels 0.20 Meets premium
Discolored kernels 2.50 Meets grade 2
Moisture Content 13.00  

Upon evaluation of each quality parameter (characteristics) the lowest grade obtained is chalky and immature kernels, therefore, the sample would be designated as Grade 2

MILLED RICE GRADING

To illustrate how the grade of milled rice is determined, consider a sample of milled rice with the following characteristics:

Parameters % Grade
Headrice 70.00 Meets grade 2
Big brokers 8.00 Meets grade 2
Broken Other than Big Brokens 20.00 Meets grade 2
Brewers 0.20 Meets grade 1
Defectiveness:    
Damaged Kernels 0.30 Meets grade 2
Discolored kernels 2.50 Meets grade 2
Chalky and immature kernels 6.50 Meets grade 2
Contrasting type 5.00 Meets grade 1
Red kernels 0.25 -do-
Red Streaked kernels 4.50 Meets Grade 2
Foreign matter 0.15 Meets Grade 2
Moisture Content 14.00  

Since the lowest grade obtained is grade 2, the sample would be designated as Grade 2.

Flow diagram

Flow diagram - continue

Figure 4. Interpretation of Brokens in Milled Rice


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