Control of Powder Properties for Food Materials
by Fine Grinding with Various Pulverizers
BYDr. Satoshi Akiyama
ManagerNisshin Seifun Group Inc.
Dr. Carl K. Ishito
General Manager
Nisshin Engineering Inc.
* Powder & Bulk Solids 2006 (29th) Technical Session S21* Presented date: May 11, 2006 (Thu) 9:00-10:15AM* Place: Donald E. Stephens Convention Center, IL USA

Content• Background and objectives• A new approach to optimize blade geometry ofa mechanical mill using a computer simulation• Effects of types of mill on powder properties• How to control powder properties including
pasting property, color and flavor by finegrinding

BackgroundStress mechanisms in particle size reductiona) Pressure between two surfaces (compressive)b) Cutting between two surfacesc) Shear stressd) Impact with a solid surfacee) Impact with another particlef) Attrition (friction)(a) Pressure(b) Cutting & shear(c) Impact(d) Attrition

Shear(attrition)

Frequency

Pressure(compressive)

Impact0.11

10
100 1000
Diameter [µm]

Typical particle distributions of powder
ground by three kind of forces

BackgroundStress mechanisms in particle size reductiona) Pressure between two surfacesb) Cutting between two surfacesc) Shear stressd) Impact with a solid surfacee) Impact with another particlef) Attrition (friction)
Changes in powder properties:
particle size (surface area),
crystal structure etc.

Product quality
for food materials:
dissolution, absorption,stability etc.

Objectives
1 Development of a new approach to optimize
blade geometry of a mechanical impact millfor control of powder properties.
2 Control of powder properties for food materials
by fine grindingA new approach to optimize blade geometryof a mechanical impact mill
Numerical simulation:
Particle trajectories in a mechanical impact mill
Impact energy
Experiment:
Calcium carbonate particles groundin a mechanical mill with various rotor bladesParticle size distribution,
crystallinity changes

Grinding mechanisms of a mechanicalimpact milla) collision against walls(blades and stator)

StatorRotational
direction

Bladeb) collision with another particlec) attrition on wallsd) cutting by bladese) shear in a narrow gap between
blade tip and stator

Grinding zone

Bag filter
OrificeScrew feeder
BlowerManometer

Mechanical millTachometer

Experimental apparatus

Table Dimensions
D1=150mm, D2=136mm
W1=20mm, T=6mm
c=2mm,
H1=40mm

D1

Outlet(to Bag filter)

BladeD2zr
RotorcT

H1W1

θΒ >0

Stator(cylinder)

Blade

Air flowω

Inlet

Definition of blade angle

Cross-sectional diagram of a mechanical impact mill

Numerical simulations using Fluent Blade (u = r ω)θFlow field
Realizable k-ε turbulent model
Conditions:Rotation speed ω: 14,000 min-1
Inlet velocity uz: 12 m/s

Rear side
Front side

Particles Trajectories
Lagrangian dispersed phase model
Conditions and assumptions:

Periodic
Stator wall
(uz=0,ur=0,uθ=0)∂ur ∂z = 0, ∂uθ ∂z = 0, ∂u z ∂z = 0

OutletRotor discParticle diameter Dp: 100 µm
Particle density ρp: 1,400 kg/m3

BladeCoefficient of restitution: 1 (perfectly elastic)zr

Particle-particle interaction : negligible
Effect of particles on air properties: negligible

θuChange in particle size: negligible

Inlet

Number of particles: 100 (mono-sized )uz=12m/sInjected velocity v: 0 m/s

Rotor discCalculation regionω =14,000 min-1
Impact position
with stator wall

Pressure×10 4 Pa
1.00.50.0z-0.5r

θ
Inlet

(a) Velocity vectors andpressure contours(b) Typical particle
trajectories (Dp=100µm)

Calculated results for blade angle θB=0°vz

Impact positions
with stator wall(a) θB=0o(b) θB=15ov

Rebounding velocity
in z-direction(c) θB=30o(d) θB=45o

Typical trajectories of a particle with a diameterof 100µm for various blade angles

θBuω

100with bladewith stator
total

120

Impact velocity [m/s]Number of impacts [-]16080400010203040
Blade angle θB [deg.]

50vtvn80
6040200010 20 30 40
Blade angle θB [deg.]

Number of impacts and impact velocity
as a function of blade angle

50

The definition of impact energies
per particle mass

3Impact Energy [kJ/kg]

10Dp=100µm
EnEt

2

10

1 N t ni ⎛ 1 2 ⎞En =∑ ∑ ⎜ vn i , j ⎟N t i =1 j =1 ⎝ 2⎠(2)

1 N t ni ⎛ 1 2 ⎞Et =∑ ∑ ⎜ vt ⎟N t i =1 j =1 ⎝ 2 i , j ⎠(3)

where
Nt=total number of injected particles[-]
ni=total number of impacts for an ith particle [-]vn=normal component of impact velocity [m/s]vt=tangential component of impact velocity [m/s]

11001020304050vt

Blade angle θB [deg.]

αvvn

Normal impact energy En and tangential impact
energy Et by the computer simulation

(104)

Intensity [a.u.]

Calcium carbonate

Estimation of change incrystallinity:I0FeedI

Intensity Ratio = I I 00deg.
15deg.
30deg.
45deg.

20304050

2θ [deg.]

X-ray diffraction (XRD) patterns for
calcium carbonate particles ground inthe impact mill with various blade angles

Intensity ratio I/I0 [-]Median diameter Dp50 [µm]1510500 10 20 30 40 50

0.4
0.3
0.2
0.1
0.0

0 10 20 30 40 50

Blade angle θB [deg.]

Blade angle θB [deg.](a) Particle size(b) Crystallinity

Changes in particle size and crystallinity of calcium
carbonate particles ground in the impact mill withvarious blade angles

Another application of the optimizing approach-effects of blade angle on properties ofpotato starch particlesMaterial: Potato starch
Mill: the mechanical impact millwith different blade angles
Measurement
Particle size
Crystallinity
X-ray diffraction (XRD)
Pasting properties:
Rapid Visco Analyser (RVA)RVA (Rapid Visco Analyser, Newport Scientific Pty. Ltd.)
SwollenBroken

water

Viscosity
measurement
PaddlePasting state in the RVA
100

10000

Viscosity [cP]

8000

80

Pasting Temp.
6000

60
Peak viscosity

4000

40

2000
00400

800

1200

201600Time [s]Typical RVA pasting curve

Temperature [oC]

Temperature
Canister
Heating/cooling

Parameters for pasting properties
Pasting temperature:
Temperature where viscosity
first increases
Peak viscosity:
Maximum viscosity

Cumulative undersize [%]10080(i) Feed25µm(ii) Ball mill

25µm

60
D50
Feed (37.0µm)
0º (31.5µm)
15º (27.6µm)
30º (25.5µm)
45º (32.0µm)
Ball mill (59.3µm)

4020(iii) θB=0° 25µm(iv) θB=15° 25µm(v) θB=30° 25µm(vi) θB=45°25µm

010100Particle diameter Dp [µm](a) Particle size distributions(b) SEM Photos

Potato starch particles ground in the impact millwith various blade angles and a ball millIntensity [a.u.]θB=45ºIntensity [a.u.]

Ball-milled

ICFeed material
Ball-milled
Ia30º

1015º

2030
2θ [deg.]

400º
Feed

Crystalline index (Wakelin et al.)C=

1020302θ [deg.](a) X-ray diffraction patterns

40∫ I s − I a d ⋅ 2θ
∫ I c − I a d ⋅ 2θ

(1)

where
Ic: intensity of feed particles (crystalline)
Ia: intensity of ball-milled particles (amorphous)
Is: intensity of pulverized particles

X-ray diffraction (XRD) patterns and the definitionof crystalline index for potato starch particles

Crystalline index [-]

1.2

θB

1.0u
ω

0.8
0.6
0.4
0.2
0.001020304050

Blade angle θB [deg.]

Changes in crystallinity for potato starch particles
ground in the impact mill with various blade angles

10000

100

6000

60

4000
40

2000
00400

800

1200

Time [s](a) RVA pasting curve

20Temperature [oC]

Viscosity [cP]

800o15o30o45o

Peak viscosity [cP]

8000

8000

6000

4000

010203040

Blade angle θB [deg.](b) Peak viscosity

Pasting properties of potato starch particles groundin the impact mill with various blade angles

50Median diameter Dp50 [µm]

Pasting temperature [ C]o60
5856545250

40353025
200 10 20 30 40 50

0 10 20 30 40 50

Blade angle θB [deg.]

Blade angle θB [deg.](a) Pasting temperature(b) Particle size

Changes in properties of potato starch particles groundin the impact mill with various blade angles

Effects of types of mill on powder properties
Material: Potato starchMills: a mechanical impact mill, a jet mill, a ball millMeasurement:Particle size, crystallinity, pasting properties

Mechanical mill:Blade MillJet mill:Super Jet MillBladeStator

Rotational
direction

Pulverized powder

Rotor

Chilling jacket

StatorPulverizing zone
Raw material

Structure of a mechanical impact mill (Blade Mill)10Frequency [wt%]Wheat bran

86Optimized
D50=9.6µm
D100=124µm

Conventional
D50=35.2µm
D100=352µm

4200.1

110100Particle diameter Dp [µm]1000Comparison of particle size distributions for wheat bran
ground in optimized and conventional mechanical mills

100µm

Optimized mill

100µm

Conventional mill

SEM photos of wheat bran powder
ground in optimized andconventional mechanical mills

Specific energy consumption w [kWh/kg]

100
Wheat bran
Optimized
Conventional
Jet mill

1080%

10.1
10100
Median particle diameter Dp50 [µm]Comparison of specific energy consumption in the optimized
mechanical mill, a conventional mechanical mill and a jet millGrinding zone
(1st stage classifying)

Grinding nozzle
OutletInlet
Comp.airClassifying ring

Classifying zone
(2st stage classifying)

Structure of a jet mill (Super Jet Mill)

Classifying ring

Inner wall structure (calculation region)
Classifying ring(a) Velocity vectors

Classifying ring(b) Typical particle
trajectories (Dp=10µm)

Calculated results in the Super jet millConventional jet millSuper Jet Mill

Relation between particle size distribution ofproduct and mixture ratio ( color toner)

Frequency [vol.%]

302520Feed (36.5µm)
Jet mill (15.3µm)
Mechanical mill (15.1µm)
Ball mill (59.3µm)

151050110100
Particle diameter Dp [µm]1000Particle size distributions of potato starch
particles ground in various mills

(a) Feed(b) Jet mill(c) Mechanical mill(d) Ball mill

SEM photos of potato starch particles
ground in various mills

Intensity [a.u.]

Feed
Jet mill
Mechanical mill

Ball mill

510

15

20

25

30

35

2θ [deg.]

X-ray diffraction (XRD) patterns for potato
starch particles ground in various mills

100

Viscosity [cP]

80
Feed

T0=61.1oC，∆H=13.3J/g

4000

60

Jet mill

T0=56.1oC，∆H=11.2J/g

Mechanical mill

T0=50.7oC，∆H=4.1J/g

40

2000

Temperature [oC]

Temp.

6000

20Ball mill
T0=49.2oC，∆H=3.5J/g

00400

800
Time [s]1200

0

RVA (Rapid Visco Analyser） profile ofpotato starch particles ground in various mills

How to control color of powder by grinding
Material: Green tea
Method:Grinding temperature and particle size
Type of mills: mechanical mill, jet mill, disc attrition millMeasurement(color):
A tristimulus colorimeter CR-310 (Minolta)

Raw material(leaves of green tea)

Ground productCIE(International Committee on Illumination)
L* a* b* color coordinates
White (100)

L*Yellow

a*+60
Green-60+60

b* Red-60BlueBlack (0)Chroma: C* = (a*)2 + (b*)2
L* : Lightness of a color( ranges from black at 0 to white at 100 )C* : Chroma ( color saturation or purity )

3476

32
Chroma C*Lightness L*

72

D508µm
12µm
15µm

68

30

D508µm
12µm
15µm

2826642460-20 0 20 40 60 80
Outlet temperature To [oC]22-20 0 20 40 60 80
Outlet temperature To [oC]

Effect of grinding temperature on color ofgreen tea powder for various particle sizesFrequency [vol.%]2520

Jet mill (3.6µm)
Mechanical mill (7.9µm)
Disc attrition mill (19.8µm)

1510500.1

110100Particle diameter Dp [µm]1000Particle size distributions of green tea powder
ground in different types of mill

20µm(a) Jet mill

20µm(b) Mechanical mill

20µm(c) Disc attrition mill

SEM photos of green tea powder
ground in various mills

72
Jet mill
Mechanical millDisc attrition millLightness L*706866646260110100Median particle diameter D50 [µm]Effect of particle size on color of green
tea powderHow to control flavor of powder by grinding
Material: Buckwheat, green tea
Method:Grinding temperature and particle size
Type of mills;mechanical mill, jet mill, disc attrition millMeasurement (flavor):
GC-MS(Gas Chromatograph-Mass Spectrometer) system:
GC-17A&QP-5000 (Shimazu, Japan)(a) Flowers(b) Husks(c) Grains

Buckwheat(d) FlourAbundance

Mechanical mill
Buckwheat flour
Dp50=30µm

To = 10 oC
To = 50 oC

4681012Time [min]1416Example of Gas chromatogram for buckwheat
flour ground in the mechanical mill

Stone mill(Disc attrition mill)Mechanical mill
(To = 50oC)

Buckwheat
Dp50=30µm

Mechanical mill
(To = 10oC)
01.00.5
Flavor intensity ratio

Effect of grinding temperature on flavor of
buckwheat flour ground in the mechanical mill

1.5

Flavor intensity

108 µm12µm

864200204060

Outlet temperature T0 [oC]

Effect of grinding temperature and particle sizeon flavor intensity of green tea powder

Stone mill(Dp50=13µm)

Green tea

Mechanical mill
(Dp50=15µm, To = 45oC)

Mechanical mill
(Dp50=15µm, To = 20oC)

Jet mill(Dp50=11µm, To = 16oC)

00.2

0.4 0.6 0.8 1.0
Flavor intensity ratio

1.2

1.4

Effect of type of mill on flavor of green tea powder

Conclusions(1) The simulated impact energies are useful parameters
for determining the optimum blade angle of themechanical mill.
(2) The optimum blade angle for efficient changes incrystallinity of particles and pasting properties isdifferent from that for size reduction.
(3) The particle size has a significant effect on color ofpowder. The grinding temperature and particle size
influence flavor of powder.
(4) Types of mill strongly influence pasting properties ofpotato starch.