Some Factors Affecting USED for CONCLUSIONS

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Some factors affecting sieving performance and efficiency KeShun Liu , Grain Chemistry and Utilization Laboratory, National Small Grains and Potato Germplasm Research Unit, USDA-ARS,

1691 S. 2700 West, Aberdeen, ID 83210, USA

Abstract

Sieving or screening has been the oldest yet most important unit operation for industrial separation of

solid particles or as a laboratory method in size analysis. A stack of sieves with decreasing mesh size

is usually used. Alternatively, particles can be sifted in a fine to coarse order by multiple sieving steps

with each step using a single sieve. The latter is referred to as reverse sieve method. This study

compared the two methods for sieving performance and efficiency using flours made from soft white

and hard white wheat, hulless barley and medium grain rice. Additional factors, including milling

method (impact vs. abrasive), flour moisture (7% vs. 11%), duration of sieving (60 vs. 120 min), and

tapping (percussion during sieving), were also investigated. Mass frequency and protein content of

oversize fractions were measured. Results show that all the variables and their interactions had

significant effects on sieving performance and efficiency. Among them, tapping was most important,

followed by sieving duration, sieving method, milling method, flour type, and flour moisture. When

other conditions were equal, the reverse sieve method always gave improved sieving efficiency over

the stacked sieve method. The observation can be attributed to the beneficial effect of oversized

particles on reducing sieve blinding by near or sub-sieve sized particles. Furthermore, the reverse

sieve method also expanded the difference in protein content among sieved fractions. Because of its

practical significance, this so far unreported effect would bear further confirmation of other sieving and

screening conditions.

Graphical abstract

This study has demonstrated how such factors as flour type and moisture, milling method, tapping,

sieving method, duration, and their interactions can affect sieving efficiency and performance. More

importantly, it has shown decisive effects of the reserve sieve method over the conventional stacked

sieve method on improving sieving efficiency and performance and enlarging the difference chemical

composition among sieved fractions.
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Figure options

Keywords

Size separation; Flour; Powder; Reverse sieving; Screening; Sieve blinding

1. Introduction

The size distribution of particulate matter is very important in determining its physicochemical

properties in a large number of processes of various industries (e.g. production of food powders,

chemicals, colorants, paints, and pharmaceuticals). The sieves/screens are the oldest and most

widely used working elements for the separation of solid particles by size. They are used both

industrially and in laboratories for the classification of particulate material. Often the term screening is

used to refer to a continuous sizing operation as distinct from sieving, which usually means a batch

process.

Although sieving/screening has played an important role in studying and processing particulate

materials, it has not received enough scientific attention[1]. Simplicity and familiarity of the process

may explain this curious situation. In reality, the sieving process is governed by multidisciplinary

principles, ranging from physics to applied fluid mechanics. Many factors have been identified to

affect this unit operation, including the size and shape of particles relative to the aperture of the sieve,

the mesh size of the sieve itself, the amount of material on the sieve surface, the direction of

movement of the sieve, the rate of movement of the material relative to the sieve surface,

etc.[1],[2],[3],[4],[5]and[6]. Furthermore, the interactions among variables are so complex that no

satisfactory method of evaluating and predicting the sieving process has yet been

developed[5]and[7]. This has led to the inefficient operation of industrial sieving equipment as well

as misleading and erroneous results of laboratory sieve analysis[1].

Among all the elements of the sieving operation, sieve blinding is considered as the most important

and direct controlling factor. Sieve blinding occurs when particles block up and lodge in the sieving
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mesh. It reduces the effective transfer area on the surface, resulting in reduction of sieving rates

(sieving performance or capacity) and the degree of sharpness of particle separation (sieving

efficiency)[1],[2],[4]and[8].

In cereal science, the subject of flour particle size has intrigued many investigators, mostly for its

effect on flour quality[9]. Flour is a blend of particles. Flours of different particle sizes differ in physical

properties and chemical composition[10]and[11]. These properties in turn affect flour performance in

final products[11]and[12]. Although flour particle size can be reduced by regrinding a sample, further

reduction of flour particle size by grinding is accompanied by an increased level of starch damage,

which negatively affects flour performance in many final products[13]. An alternative method is to

separate flours according to particle size through sieving or air classification. The fractioned flours are

characterized by not only the difference in chemical composition and physical

properties[10],[11]and[14]but also minimal starch damage[12]. However, fractionating flour by

sieving, although relatively simple, is limited by sieve blinding.

With regard to the sieving process, either for industrial separation of solid particles or as a laboratory

method in size analysis, a stack of sieves or screens of decreasing mesh size, also known as a sifter

cascade, is often used[6]. The sieve stack is typically mounted on a device that provides vibration or

shaking to achieve the movement of particles in relation to the sieve surface. For example, in flour

milling, breakage of particles is always followed by separation. A plansifter, a stack of sieves of

decreasing mesh size that separate particles by size, is the main equipment used for this separation

purpose. On a laboratory scale, standard ASAE procedure for particle size analysis of particulate

materials also requires use of a stack of sieves[15]. For simplicity, this common sieving process is

referred to as the stacked or cascaded sieve method. It features separation of particles in a coarse to

fine order by a single operation.

The subject of this study was prompted by a surprising observation during dry fractionation of barley

flour by sieving at the author's laboratory. It was found that in separating barley flour, when other

conditions were kept same, a reverse sieve process, that is, flour is sifted in a fine to coarse order by

multiple sieving steps with each step using a single sieve, gave a better sieving efficiency and

performance than the conventional stacked sieve method. Therefore, the objectives of the present

study were: (1) to make a systematic comparison between the stacked and reverse sieve methods for

separation of various types of flour, (2) to investigate some additional factors that govern sieving

performance using the two methods, and (3) to provide a scientific explanation for the observed

difference between the two methods. Since each year literally hundreds of millions of tons of

particulate material are subjected to industrial sieving/screening, an understanding of factors affecting

sieving efficiency and performance has great economic significance.

2. Materials and methods

2.1. Materials
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Seed samples of four cereal crops were used, including a hulless barley line (03HR3052), a soft white

wheat variety (Nick), a hard white wheat variety (Lochsa), and a medium grain rice variety (Bengal).

Seed samples were cleaned before milling.

2.2. Sample milling

Cleaned seed samples were milled into particulate material (whole grain flour) with a Cyclone sample

mill (UDY Corp, Forth Collins, CO), having an enclosure and a vacuum system. The Cyclone mill

employs impact milling action. A screen with 0.5 mm round openings was used. Approximately 30 g of

seed could be ground with each run.

To study the effect of milling methods on subsequent sieving, portions of seed samples were also

milled by two additional methods. One involved using the Cyclone mill, having a 0.8 mm screen,

instead of the 0.5 mm screen. The other one used a laboratory scale electrical seed scarifier

(Forsberg Inc., Thief River Falls, MN). The scarifier uses abrasive milling action. The apparatus

consists of a metal drum with its inner surface mounted with 40-grit sandpaper, a cylinder, and a

horizontal rotating steal propeller that is mounted at the center of a metal cylinder. The propeller was

driven by a 1/3 hp motor. The diameter of the drum was small enough to slide into the cylinder. The

drum was horizontally aligned into the cylinder with the propeller fixed at the center. The motor ran at

a fixed speed (1145 rpm) and was stopped after 3 min. For each run, 120 g of seed were put into the

drum. Scarified kernels, mixed with surface layer powder, were removed from the chamber and

brushed into a container. The mixture was sifted over an 18 mesh (1.00 mm opening) sieve. The

undersized particles were saved as milled flour. The abraded kernels that remained on the sieve were

repeatedly milled by going through several cycles of scarification. The flour for each cycle of

scarification was combined. The milling operation for each method was repeated when necessary to

produce large enough sample lots for sieving experiments.

2.3. Flour sieving

Milled flour samples were sifted with a series of five selected U.S. standard sieves (Nos. 60, 100, 200,

270, and 400, corresponding to sieve opening dimensions of 250, 150, 75, 53 and 38 m,

respectively) and a pan, fitted into a sieve shaker (DuraTap, Model DT168, Advantech Mfg. Co., New

Berlin), according to two procedures. In the stacked sieve procedure, the selected sieve series were

stacked with decreasing size of openings. One hundred g of milled sample was put on the top sieve of

the stack and shaken for 60 min. The mass of material retained on each sieve as well as on the pan

was determined, and the mass frequency (%) for the oversize on each sieve was calculated. In the

reverse sieve procedure, a milled sample was sifted with a single sieve, from fine to coarse order,

with oversize proceeding to the next sieving step. For each step, the single sieve was also mounted

on the shaker. The cumulative time of all 5 sieving steps for a single sample was also 60 min. The

time distribution for each sieve of 400, 270, 200, 100, and 60 mesh size was 22.5, 17.5, 12.5, 5, and

2.5 min, respectively.


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To study the effect of sieving duration on sieving efficiency, the soft white wheat flour was also sieved

for 120 min instead of 60 min, under the two sieving procedures. For the reserve sieve method, the

time distribution for each sieve was doubled compared to the procedure having 60 min sieving time.

Furthermore, the sieve shaker had a concurrent tapping option. To study the effect of tapping, all

above sieving operations were performed with or without tapping.

2.4. Effect of sample moisture on milling and sieving

Two tempering methods were used to adjust moisture levels of the soft white wheat seed sample.

One method involved milling the sample at ambient moisture with the Cyclone mill (0.5 mm screen),

then adding a calculated amount of water to a half portion of the flour and allowing it to stabilize for

3 days in a refrigerator to raise the moisture to a higher level (about 11%). The other half portion

served as a control. The 2nd method involved adding calculated amount of water to the seed sample

and allowing it to stabilize for 3 days in the refrigerator. The moisture level of the kernel was raised toabout 11%, similar to that of tempered flour obtained by the first tempering procedure. The tempered

kernel sample was then milled with the Cyclone mill (0.5 mm screen) to produce another sample of

tempered flour. A half portion of this tempered flour sample was dried in a forced air oven at 45 C

until its moisture was reduced to the level of the initial seed sample (about 7%). The original and

tempered flours were subsequently sieved by the two sieving procedures for 60 min with tapping.

2.5. Chemical analysis

All original seed samples and moisture-adjusted samples were measured for moisture content. In

addition, the original soft white wheat seed sample and its sieved fractions were measured for protein

content. Moisture was determined according to an official method[16]. The protein content was

measured by a combustion method[16], using a protein analyzer (Model FT528, Leco Corp. St.

Joseph, MI) and calculated with a conversion factor of 5.75.

2.6. Data treatments and statistical analysis

All experiments were duplicated at the milling stage. Data were treated with the JMP software, version

5 (JMP, a Business unit of SAS, Cary, NC, USA) for calculating means and standard deviation, and

for analysis of variance (ANOVA) in order to determine the effects of different variables and their

interactions on sieving efficiency and performance. The Tukey's HSD (honestly significant difference)

test was also conducted for pair comparison.

3. Results and discussion

3.1. Effects of sieving method, sieving duration and tapping

In the first experiment of this study, the above three variables were investigated. Results show that

the mass frequency of each particle size category as a function of particle size, commonly known asparticles size distribution (PSD), for the soft white wheat flour varied greatly with changes of sieving
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variablesstacked (S) or reverse (R) sieve procedures, sieving duration (60 or 120 min), and tapping

(T) or no tapping (NT) (Fig. 1). All three factors and their interactions had significant effects on PSD of

the same flour and thus sieving efficiency (p< 0.05). Among them, tapping (or percussion), which ran

concurrently with shaking, was most effective in shifting particle size distribution toward finer sizes.

For example, without tapping, the mode of PSD curves was in the center of the size class of No. 60

100 mesh (250150 m opening) or > 60 mesh (> 250 m opening). The mode is the center of the

size class that contains most of the material (highest mass frequency). With tapping, the mode shifted

to the size classes of finer particles, 100200 meshes (15075 m) or 200270 meshes (7553 m).

Fig. 1. Particle size distribution of soft wheat (Nick cv.), obtained by sieving with combinations of varying

factors: sieving method (S, stacked sieve, vs. R, reverse sieve), sieving duration in min (60 vs. 120), and

tapping option (T, with tapping, vs. NT, no tapping).

Figure options

During sieving, particles are separated on a sieve containing uniform apertures which permits the

finer particles to pass through. Two types of movement of the sieving surface are needed, a)

horizontal movement which would tend to open up or loosen the packing of the larger particles in

contact with the sieving surface thus permitting more sub-mesh particles to pass, and b) a vertical

movement to agitate and mix the particles and then redeposit them at the sieving surface. In this

study, sieving was performed with a shaking device that provided both movements. However, the

horizontal movement has the disadvantage that in moving across the sieving surface some particles,

particularly these of near-mesh size, tend to block some of the sieve apertures, leading to sieve

blinding. Tapping action apparently reinforced the vertical movement, and at the same time helped in

dislodging particles that blocked apertures, and thus reduced the sieve blinding effect. This explains

why tapping had a profound effect on sieving efficiency as compared with the no-tapping option.

Without tapping, sieving duration caused little change in the mode, but narrowed the PSD curves (Fig.

1). With tapping, a longer sieving time caused shifting of the mode toward finer sizes. As early as

1958, Whitby[17]studied a batch sieving process, using a standard Tyler Rotap sieve shaker, and

showed that by plotting the percentage of particles passing through a sieve vs. sieving time creates a

curve that could be divided into two distinct regions. The first region is during the early stage of

sieving when there are still many particles on the sieve that can pass the mesh size. This region is

characterized by a faster increase in mass frequency with time. Region 2 begins when residue on the

sieve consists entirely of near-mesh or larger particles. It is featured by slower increase in mass
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frequency with time, approaching a plateau. In this study, only two sieving durations were used. At

any combination of the other two factors (sieving method and tapping/no tapping option), 120 min

sieving duration was found to improve sieving efficiency significantly over the 60 min operation. It

should be pointed out that sieving time is closely related to sieve loading, a reduction in the latter

resulting in a reduction in the former.

Regarding the effect of sieving methods, at any combinations of duration and the tapping/no tapping

option, the curves of the two procedures (stacked or reverse sieving) had the same or slightly different

modes. However, the PSD curves of reverse sieving were significantly wider than curves of the

stacked sieve method. More importantly, the mass frequencies for fractions of finer particles,

particularly those passing 200 mesh (< 75 m) or 270 mesh (< 53 m), were much higher by reverse

sieving than those by the stacked sieve method, indicating significant improvement in sieving

efficiency by the reverse sieve method. This is in fact the most important finding of the present study,

since the phenomenon has been either unreported or non-emphasized in previous reports on particle

size separation by sieving/screening.

3.2. Protein content in sieved fractions

Several previous studies showed that flour fractions of different particles sizes sieved from the same

flour samples varied significantly in chemical composition[10]and[11]. In this study, a significant

difference in protein content of sieved fractions of the same soft wheat flour was also evident (Table

1). With regard to which specific fraction(s) having higher protein content than others obtained by the

same sieving operation, discrepancy existed among reports. Wang and Flores [10]analyzed the

chemical composition of flours from red and white hard wheat varieties in relation to particle sizes,

and concluded that the ranges between 3853 m and 5375 m had higher protein content than

smaller or larger particle fractions. In contrast, Toth et al. [11]claimed that protein content generally

increased in proportion to the decrease in particle size. In the present study, both smallest

(< 0.38 m) and largest (> 250 m) particle fractions had higher protein content than fractions of

medium particle sizes, just opposite to the finding of Wang and Flores [10]. This was true for fractions

obtained by any combinations of the three sieving variables. The discrepancy might be due to use of

different sieving methods and equipment and the number of sieved fractions obtained among the

studies. For example, Wang and Flores[10]used an Alpine air jet sieve. Nevertheless, the observeddifferences in chemical composition and flour performance among sieved fractions indicate a

possibility of obtaining different types of flour from a same initial material or producing a better baking

quality product from poor-quality, less-valuable wheat flour by fractionation according to particle sizes.

More importantly, this study shows that by choosing the reverse sieve procedure, not only the mass

frequency of finer particle classes was significantly improved but also the difference in protein content

among sieved fractions was expanded.

Table 1. Effect of sieving method, sieving duration, tapping option and their interactions on the protein content

of sieved fractions of soft wheat (Nick cv.).a

Protein content (%) of each sieved fraction
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Sieving

method

Sieving time

(h)

Tapping Mesh size no.

> 60

60100 100

200

200

270

270

400

< 400

m opening

> 250

250

150

15075 7558 5338 < 38

Stacked 60 No 13.46 a 11.67 b 11.97 b N/A N/A N/A

Reverse 60 No 12.65 bc 11.90 cd 11.25 d 11.89

cd

12.76 b 14.44 a

Stacked 120 No 15.14a 12.22 b 11.84 b N/A N/A N/A

Reverse 120 No 13.33 b 11.88 cd 11.15 d 11.87

cd

12.70

bc

14.81 a

Stacked 60 Yes 15.97 a 13.47 b 12.08 c 11.63

cd

10.77 d N/A

Reverse 60 Yes 15.67 a 13.40 bc 12.55 c 10.89 d 12.05

cd

14.21

ab

Stacked 120 Yes 16.11 a 13.89 b 13.43

bc

11.48 c 11.93 c N/A

Reverse 120 Yes 15.58 a 13.49 b 13.52 b 11.45 c 11.05 c 14.30

ab

Average 14.74 12.74 12.22 11.54 11.88 14.44

aMilled by an impact mill (Cyclone with a 0.5 mm screen); seed moisture level was 7.69%, protein content was

12.25%, dmb.Sieved fractions are described in U.S. standard mesh size No. and micrometers of sieve

opening dimentions.Row means with different letters differed significantly at p< 0.05.N/A, the volume of the

fraction was too low to measure its protein content.

Table options

3.3. Effects of flour type, milling method, and sieving method

When sieving was carried out for 60 min with tapping (concurrently with shaking), the flour type,

milling method, and sieving method, and their interactions all had significant effects (p< 0.05) on

sieving efficiency (ANOVA data not shown). In general, for all types of flour, abrasive milling by the

electrical seed scarifier produced a flour having a PSD with the highest mass frequencies in the finer

size classes (those passing through 200 mesh or finer), while impact milling by Cyclone Mill with

0.8 mm opening screen gave a flour having PSD with the highest mass frequencies for the coarser

particle size classes (those retained on 100 mesh or coarser) (Table 2). Impact milling by Cyclone Mill

with 0.5 mm opening screen gave a flour having PSD with the highest mass frequencies in the

medium particle size classes (those passed through 100 mesh but retained on 270 mesh). Since the

shape of particles and the size of particulate material relative to that of sieves are among key factors

affecting sieving performance[2], the effect of the milling method on PSD of different flours can beattributed to its effect on shapes and sizes of resulting flour particles.

Table 2. Effect of flour type, milling method, sieving method and their interactions on sieving efficiency. a

Mass frequency (%) of each sieved fraction

Flour

type

Milling

method

Flour

moisture

Sieving

method

Mesh

size no.

> 60

60

100

100

200

200

270

270

400

< 400 Sieving

loss

m

opening

> 250

250

150

150

75

75

58

5338 < 38

Wheat

(soft)

Impact

(0.5 mm)

7.69 S 6.73 k 15.48

d

48.69

a

22.81

hi

0.11 k 0.07 i 6.10

Wheat Impact 7.69 R 6.97 k 14.22 29.75 29.66 10.07 j 5.07 f 4.27
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Flour

type

Milling

method

Flour

moisture

Sieving

method

Mesh

size no.

> 60

60

100

100

200

200

270

270

400

< 400 Sieving

loss

m

opening

> 250

250

150

150

75

75

58

5338 < 38

(soft) (0.5 mm) e d g

Wheat

(soft)

Impact

(0.8 mm)

7.69 S 16.97 d 11.79

g

15.19

g

28.86

g

23.3 h 0.29 i 3.60

Wheat

(soft)

Impact

(0.8 mm)

7.69 R 17.01 d 10.65

g

14.95

g

29.60

g

16.68 i 8.94 d 2.17

Wheat

(soft)

Abrasive 7.69 S 14.86 e 27.39

a

19.04

ef

33.12

f

2.11 k 0.25 i 3.73

Wheat

(soft)

Abrasive 7.69 R 14.04 e 3.32 i 7.34 j 36.85

e

33.37 f 3.80 g 1.26

Wheat

(hard)

Impact

(0.5 mm)

7.60 S 9.89 i 15.11

d

21.95

e

23.59

h

26.53

gh

0.26 i 2.68

Wheat(hard) Impact(0.5 mm) 7.60 R 10.08 h 13.97e 20.99e 16.15j 30.67 g 6.42 e 1.71

Wheat

(hard)

Impact

(0.8 mm)

8.22 S 24.28 c 17.19

c

19.46

ef

11.82

k

24.18 h 1.38 i 1.68

Wheat

(hard)

Impact

(0.8 mm)

8.22 R 24.53 c 15.98

d

18.81

f

10.66

k

10.04 j 18.06

b

1.92

Wheat

(hard)

Abrasive 9.01 S 4.67 l 2.80 i 6.37

k

67.59

a

15.42 i 0.44 i 2.71

Wheat

(hard)

Abrasive 9.01 R 5.27 l 2.29 i 7.37 j 39.52

d

40.22 d 3.38 g 1.95

Barley

(hulless)

Impact

(0.5 mm)

7.25 S 12.23 g 13.66

e

13.10

h

44.50

c

12.32

ij

1.14 i 2.56

Barley(hulless)

Impact(0.5 mm)

7.25 R 13.34 f 12.59f

13.32h

15.35jk

30.83 g 13.21c

1.38

Barley

(hulless)

Impact

(0.8 mm)

7.77 S 25.40 bc 12.98

f

10.65

i

37.86

e

10.46 j 0.37 i 2.29

Barley

(hulless)

Impact

(0.8 mm)

7.77 R 26.69 ab 11.21

g

10.59

i

4.99 l 23.77 h 21.65

a

1.10

Barley

(hulless)

Abrasive 8.35 S 0.88 m 1.34 l 5.98

k

52.73

b

36.45 e 0.39 i 2.24

Barley

(hulless)

Abrasive 8.35 R 1.06 m 1.47 l 8.36 j 39.35

d

43.08 c 4.68 f 2.01

Rice

(medium

grain)

Impact

(0.5 mm)

8.07 S 8.56 j 21.01

bc

41.91

b

24.74

h

1.45 k 0.13 i 2.21

Rice

(medium

grain)

Impact

(0.5 mm)

8.07 R 8.75 j 18.16

c

33.13

c

17.78

ij

16.70 i 3.66

gh

1.83

Rice(medium

grain)

Impact(0.8 mm)

9.01 S 27.86 a 23.45b

29.16d

17.31ij

0.00 k 0.00 i 2.22

Rice

(medium

grain)

Impact

(0.8 mm)

9.01 R 28.23 a 21.91

b

23.96

e

10.55

k

10.25 j 3.39

gh

1.70

Rice

(medium

grain)

Abrasive 8.51 S 9.47 i 4.93

h

9.11

ij

19.99

i

55.31 a 0.18 i 1.01

Rice

(medium

grain)

Abrasive 8.51 R 10.91 h 3.92

h

8.24 j 13.16

jk

48.75 b 13.85

c

1.18

Average 13.69 12.37 18.23 27.02 21.75 4.63 2.31


10/14

aAll samples were sieved for a total of 60 min with tapping.Impact (0.5 mm), by the Cyclone mill with 0.5 mm

screen; impact (0.8 mm), by the Cyclone mill with 0.8 nm screen; abrasive, milled by the electric seed scarifier.

S, stacked sieve procedure; R, reverse sieve procedure.Sieved fractions are described in U.S. standard mesh

size No. and micrometers of sieve opening dimensions.Column means with different letters differed

significantly at p< 0.05.

Table options

Different types of flour exhibited different PSD curves when two other variables (milling method and

sieving method) were kept the same. There were strong interactions of flour type with the other two

variables. In particular, hard wheat flour was easier to sieve than soft wheat flour, and the mode of its

PSD curves was in the finer particle size class than that of soft white wheat curves. This finding is

supported by a common observation that hard wheat flour flows and bolts more easily than soft wheat

flour[18].

Again, for any type of flour, and by any milling method, the reverse sieve method had a significant

effect in broadening and shifting PSD curves toward the finer mesh size, compared with the stacked

sieve method (Table 2), similar to the finding with soft wheat flour shown in Fig. 1. This implies that

the reverse sieve method could separate out more of the finer particles, particularly those finer than

270 mesh (< 53 m openings), from the same particulate material than the stacked sieve method. The

latter method is typically used in various processing industries and particle analysis laboratories. In

addition, the reverse sieve method generally gave lower sieving loss than the stacked method (Table

2). Sieving loss is the difference between the total mass put on the sieve and the sum of all sieved

fraction masses. It results mainly from sieving blinding and attachment of fine particulates to the sieve

surface.

3.4. Effects of flour moisture and sieving method

Both kernel moisture (right before milling) and flour moisture affected sieving performance (Table 3).

The moisture of soft wheat flour at an ambient temperature and moisture condition was about 7%.

This level of the control sample was relatively lower than typical flour moisture. The reason was that

the material was maintained at the author's laboratory during the winter season in Idaho, where and

when indoor heating was common. When the flour moisture was raised to about 11%, more fine

particles were sifted through, compared with the control flour. A similar observation was found with

the flour sample obtained by milling tempered wheat kernel (about 11%). Interestingly, when this

sample was dried to bring its moisture back to the control sample level, its PSD curve shifted toward

coarse particle size classes but could not match the same PSD curve of the control flour, indicating

complex interactions of milling and sieving.

Table 3. Effect of sample moisture, sieving method and their interactions on sieving efficiency of soft wheat

(Nick cv.).a


Sample

treatment

Final flour

moisture

Sieving

method

Mesh size

no. > 60

60

100

100

200

200

270

270

400

< 400 Sieving
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m

opening

> 250

250

150

150

75

7558 5338 < 38 loss

Control 6.75 S 6.04 c 9.80 a 30.74

a

38.53

b

10.39 g 0.42 e 4.08

Control 6.75 R 6.07c 7.18 b 12.81

b

32.53

c

34.75 d 5.15 d 1.52

Tempering

flour

10.68 S 11.02 a 7.44 b 11.35

b

7.36 e 59.75 b 0.46 e 2.61

Tempering

flour

10.68 R 11.64 a 6.93 b 11.59

b

7.36 e 29.54 e 31.66

b

1.29

Milling

tempered kernel

(MTK)

10.60 S 7.09 b 7.38 b 11.47

b

7.72 e 63.84 a 0.41 e 2.11

Milling

tempered kernel(MTK)

10.60 R 7.37 b 6.76 b 11.05

b

7.47 e 23.69 f 41.93

a

1.73

MTK and then

drying the flour

6.73 S 5.53 c 7.73 c 11.74

b

47.03

a

25.08 f 0.24 e 2.66

MTK and then

drying the flour

6.73 R 5.41 c 6.61 c 11.54

b

21.76

d

41.98 c 11.59

c

1.12

Average 7.52 7.48 14.04 21.22 36.13 11.48 2.14

Range 6.24 3.19 19.69 39.67 53.45 41.69 2.97

Standard

deviation

2.45 1.01 6.77 16.25 18.30 16.33 0.97

Relative S.D.

(%)

32.64 13.49 48.23 76.60 50.66 142.23 45.35

aMilled by the Cyclone mill with a 0.5 mm screen (impact milling). Sieving was carried out for 60 min, with

tapping. S, stacked seive procedure; R, reverse sieve procedure. Sieved fractions are described in the U.S.

standard mesh size No. and micrometers of sieve opening dimensions.Column means with different letters

differed significantly at p< 0.05.

Table options

The moisture level of a particulate material affects such physical properties as adhesion and

stickiness, which in turn influence freedom of particle movement during the sieving process [6]. For a

given sieving condition and given particulate material, there will be a moisture level that allows

maximum freedom of particle movement. In the current study, only two levels of moisture in the soft

wheat flour were studied. Increasing moisture level from about 7% to 11% apparently promoted

particle movement. Neel and Hoseney[18]studied the effects of wheat flour characteristics on sifting

efficiency, including flour moisture, but no actual experimental data on flour moisture effect was given.

Referring back toTable 3, under any moisture treatments, the reverse sieve method was moreefficient in getting particles to pass through finer mesh sieves than the stacked sieve method. Again,

the sieving loss was less by the reverse sieve method than by the stacked method.

Overall, based on the results discussed so far (Fig. 1,Table 2andTable 3), regardless the observed

effects of other variables on sieving efficiency, which included flour type, milling method, sieving

duration, and tapping or no tapping, under a given sieving condition (a combination of other

variables), the reverse sieve method always gave better results than the stacked sieve method with

respect to increase in sieving performance and efficiency and decrease in sieving loss.

3.5. Significance and scientific explanation for the sieving method effect
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12/14

This study shows that, when other conditions are kept same, the reverse sieve procedure improved

sieving efficiency and performance over the stacked sieve method. The significance of this finding is

that by choosing the reverse sieve procedure, not only the mass frequency of finer particle classes is

dramatically improved but also the difference in chemical composition among sieved fractions is

significantly increased.

There are many other variables that have been previously shown to affect sieving performance and

efficiency. Among them, particle size relative to sieve aperture and the sieve aperture size itself are

most important and relevant to the present study since they influence sieve blinding, which is the most

important direct factor governing the sieving process. Roberts and Beddow[2]showed that the level

of sieve blinding is largely dependent upon mesh aperture. Blinding increases sharply when the mesh

aperture decreases below about 100 m.

With regard to the size of particles relative to that of sieving apertures, initially, near-mesh sized

particles were easily identified to cause aperture blocking[3]. Then, Apling[4]demonstrated that

particles as small as one-third the size of the apertures can have, under certain conditions, a blinding

capability. An undersize particle may, depending on conditions, have a measurable probability of

blinding an aperture by virtue of its own irregular shape and, also, that of the aperture. Fine particles

may also become trapped in an aperture when two or more attempt passage simultaneously. In other

words, although near-mesh particles can easily clog sieve openings, sub-mesh particles, either singly

or in combination with others, can also cause the blinding of apertures. Because of this finding,

Allen[6]recommended that, for a dry sieving operation, the fines be removed prior to the sieve

analysis. This is done by pre-sieving, usually by hands, on the finest sieve to be used in the

subsequent analysis. If this is not done, the fines have to pass through the whole stack of sieves, thus

promoting sieve blinding and increasing the risk of high powder loss. Note that the reverse sieve

method used in this study differs from the pre-sieving procedure recommended by Allen [6]in that, for

the subsequent sieving analysis, the former continues in the fine to coarse order whereas the latter is

followed by sieving in the coarse to fine order.

Standish[5]examined the effect of oversized particles (another possible case relative to near-mesh

size) on sieve blinding, and found that although the blinding effect was particularly notable when only

the material of the near-mesh size was sieved, the effect was minimized when oversized material was

also present. To understand the mechanism by which the presence of the oversize material enhanced

the sieving rates, Standish[5]took high speed films during sieving, then examined at low play back

speeds, and found that improved sieving efficiency was due to the oversize particles nudging the

embedded near-mesh particles through the effect of simultaneously increasing the number of

particles passing and at the same time freeing the apertures for other particles to pass through.

It turned out that the ability of near-mesh and sub-mesh sized particles to blind sieves and the

beneficial effect of oversized particles on reducing sieve blinding by near-mesh and sub-mesh

particles, observed by Standish[5], can provide a satisfactory explanation for the observed difference

in sieving efficiency and performance between the two sieving procedures in the present study. In the

stacked sieve method, particles are sieved in a coarse to fine order. During sieving, smaller particles
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pass through top sieves and are retained at one of the bottom finer sieves, depending on their size

relative to apertures of a particular bottom sieve. As each layer of the cascading sieves goes

downward, the mesh aperture size becomes smaller, the difference between the particle size and

sieve aperture size decreases. The net result is that, for finer bottom sieves, the blinding effect,

caused by both near-mesh and sub-mesh particles, is maximized while the beneficial effect (nudging

effect) of oversized particles is minimized. In contrast, in the reverse sieve procedure, particles are

sieved in a fine to coarse order. For the first few steps of sieving with finer sieves, the difference

between particle size and sieve apertures is large. The net result is that blinding effect by near-mesh

and sub-mesh particles is now minimized by the presence of oversized particles. Therefore, the

sieving performance and efficiency were improved while the sieving loss was generally reduced, as

compared with the stacked sieve method (Fig. 1,Table 2andTable 3).

It should be pointed out that although the reverse sieve method is advantageous over the stacked

method with respect to improvement in sieving efficiency and performance and reduction in sieving

loss, there is a limitation. This is because in the usual design of equipment, the sieve has to serve as

dual role, as a go-no-go gauge and as a support for a powder material. The use of the sieve surface

as a powder support puts an added strain on the sieve surface. It also imposes greater strength

requirements on the structure of the sieving surface. In the reverse sieve method, for the sieves with

finer apertures, over-loading with large particles will impose further strength requirements and cause

wear and breakage of the sieves much more easily.

Finally, because the equipment, analytical procedure and basic concepts are so deceptively simple,

sieving is probably the most widely used and abused method of particle size analysis and separation

of particulate materials. However, in reality it is governed by many interactive variables and

multidisciplinary principles. Without careful consideration of various factors, generation of misleading

and highly erroneous results or operations at inefficient conditions could occur. For the same reason,

comparisons for results of particle size analysis and for properties of sieved products obtained by

different producers should be made with caution.

4. Conclusions

This study has demonstrated how factors, such as flour type, milling method, moisture content,

tapping, sieving method, sieving duration, and their interactions can affect sieving efficiency and

performance. Among them, tapping was most important, followed by sieving duration, sieving method,

milling method, flour type, and flour moisture. It has also shown the decisive effect of the reserve

sieve method over the conventional stacked sieve method on improving sieving rates and final

fraction mass and minimizing sieve loss under all conditions of this study. The observed difference in

sieving efficiency and performance and in sieving loss between the two sieving methods can be

attributed to the beneficial effect of oversized particles, since during sieving, the presence of

oversized particle can reduce sieve blinding caused by near or sub-sieve sized particles.

Furthermore, by choosing the reverse sieve procedure, especially with tapping, not only the mass

frequency of finer particle classes was significantly improved but also the difference in protein content
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among sieved fractions was enlarged. Because of its practical significance, this so far unreported

effect would bear further confirmation of other sieving and screening conditions in general.

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Some Factors Affecting USED for CONCLUSIONS