Reduccion de particulas

8/9/2019 Reduccion de particulas

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Size reduction


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Theory

Force for reduce the size of food

Compression forces

Impact forces

Shearing(or attrition) forces


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Stress-strain diagram for various foods


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Relationship beteen stress and strain force in size reduction

!hen stress (force) is applied to a food theresulting internal strains are first absorbed" tocause deformation of the tissues#

If the strain does not e$ceed a certain critical levelnamed the elastic stress limit (E)" the tissuesreturn to their original shape hen the stress isremoved" and the stored energy is released as heat

(elastic region(%&'))


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If the strain area e$ceeds the elastic stress limit"the food is permanently deformed# If the stress iscontinued" the strain reaches a yield point(Y)#

bove the yield point the food begins to flo (&

*) Finally" the breaking stress is e$ceeded at thebrea+ing point (*) and the

food fractures along a line of ea+ness# ,art of the stored energy is then released as

sound and heat#


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The size of the piece is reduced" there are feer linesof ea+ness available" and the brea+ing stress thatmust be e$ceeded increases#

!hen no lines of ea+ness remain" ne fissuresmust be created to reduce the particle size


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Force for size reduction in food

Friable or crystalline foods Compression force

Fibrous foods

Impact force Shearing force


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Size-reduction Equipment


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The size-reduction equipment

Using to reduce the size of food materials

fibrous foods (as meats, fruits and

vegetables) to smaller pieces or pulps

dry particulate foods to powders.


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Size reduction of fibrous foods

slicing and flaing equipment

dicing equipment

shredding equipment

pulping equipment


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Slicing and flaking equipment

used to slice the products including

cheeses, pizza toppings, cooed meats,

cucumber and tomato.

!eats are also cut using circular rotary

nives with a blade at right angles to the

path of the meat.


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The blade advances with the product on

the conveyor to ensure a square cut edge

regardless of the conveyor speed or cut

length which can be ad"usted.


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Dicing equipment

The products are first sliced and then cut

into strips by rotating blades.

The strips are fed to a second set of

rotating nives which operate at right

angles to the first set and cut the strips intocubes


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Shredding equipment

is a modified hammer mill in which nives

are used instead of hammers to produce a

cutting action.


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Size reduction of dry foods

&all mills

'isc millsammer mills

oller mills


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Ball mills

These have a slowly rotating, horizontal steel cylinder

which is half filled with steel

balls *.++ cm in diameter.

t low speeds, the small balls are used.

t higher speeds, the larger balls are used.

They are used to produce fine powders, such as food

colourants.


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Disc mills

0 /ingle-disc mills in which food passes through anad"ustable gap between a stationary casing and a grooved

disc, which rotates at high speed.

0 'ouble-disc mills which have two discs that rotate inopposite directions to produce greater shearing forces.


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1in-and-disc mills

which have intermeshing pins fi#ed either

to the single disc and casing or to double

discs. These improve the effectiveness of

milling by creating additional impact and

shearing forces.


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ammer mill


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Roller mills

Using to mill wheat.

Two or more steel rollers revolve towards

each other and pull particles of food throughthe 2nip3 (the space between the rollers).

The size of the nip is ad"ustable for different

foods and overload springs protect againstaccidental damage from metal or stones.


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1roperties and applications of selected size reduction equipment


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Energy for size reduction


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The energy required to reduce the size of solid foods is

calculated using one of three equations, as follows:

Kicks law

Rittingers law

Bonds law


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Kicks law

=

*

lnd

dKE K

K4 5 4ic3s constant,

d(m) 5 the average initial size of pieces,

d*(m) 5 the average size of ground particles.

d6d* 5 thesize reduction ratio (RR) and is used to

evaluate the relative performance of different types of

equipment. 7oarse grinding hasRRs below 89, whereas

in fine grinding, ratios can e#ceed ::9

the energy required to reduce the size of particles is proportionalto the ratio of the initial size of a typical dimension to the final

size of that dimension

;(


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Rittingers law

= *

ddKER

K 5 ittinger3s constant,d(m) 5 the average initial size of pieces,

d*(m) 5 the average size of ground particles.

the energy required for size reduction is proportional to the

change in surface area of the pieces of food

;(


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Bonds law

;(nde# (?:,:::8:,::: < g-

for hard foods /uch as sugar or grain)

d(m) 5 diameter of sieve aperture that allows 8:@ ofthe mass of the feed to pass

d*(m) 5 diameter of sieve aperture that allows 8:@ of

the mass of the ground material to pass.

-*

-::-::ddW

E =


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Kicks law gives reasonably good results for coarsegrinding in which there is a relatively small increase in surface

area per unit mass

Rittingers law gives better results with fine grindingwhere there is a much larger increase in surface area

Bonds law is intermediate between these two

!owever,equations Rittinger"s lawand #ond"s law were

developed from studies of hard materials $coal and limestone% and

deviation from predicted results is li&ely with many foods


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$ood is milled from A mm to :.::* mm using a : hp motor.=ould this motor be adequate to reduce the size of the

particles to :.:::8 mmB ssume ittinger3s equation and that

hp C?+.C =.

EXAMPLE1

'iven( d() * mm ) * + (

-.m ,

d/) (/ mm ) (/+(-.m

E() ( hp + $0120 W3hp%

0 E/) 4 When d/)5 mm ) 5 +(-.m

6ssume rate of throughout no change


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=

*

--dd

KE R

7romRittinger"s equation

= .-:A

-

.-:::-*.:

-)6C.C?+.)(-:(

DD m+m+KhpWhp R

..::8E.: mWKR=Therefore,

To produce particles of 5 mm

= .-:A-

.-::::8.:

-.).::8E.:( DD* m+m+mWE

.-+-*D,--* hpWE ==

Therefore the motor is unsuitable and an increase in power of 28 is required


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/ugar is ground from crystals of which it is acceptable that 8:@pass a +:: m sieve (U/ /tandard /ieve Fo.D+), down to a size inwhich it is acceptable that 8:@ passes a 88 m (Fo.C:) sieve, anda +-horsepower motor is found "ust sufficient for the required

throughput. >f the requirements are changed such that the grinding

is only down to 8:@ through a *+ m (Fo.*:) sieve but thethroughput is to be increased by +:@ would the e#isting motor

have sufficient power to operate the grinderB ssume &ondGs

equation.

EXAMPLE

!i"en #

(st condition E() 2 hp , rate of throughout ) 9 &g3s

d()2 m ) 2+(-*m , d/)55 m ) 55+(

-*m

/nd condition E/) 4 , rate of throughout ) (29 &g3s

d()2 m ) 2+(-* m , d/)(/2 m ) (/2+(-*m


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(st condition Wm+m+

W9

hpC8EE.A8

.:+::

::

.:88

::.+AA

=

=

Wm+m+

W9

E*DA.??C

.:+::

::

.:*+

::

+.AA

* =

=

/nd condition

-*

-::-::

ddW

E=6ssume #onds equation

*

* $E/3(29% ) 110/(.* W

$2 hp39% *(505;; W


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Size Determination


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Sieve Analysis

Involves :

- Passing the material being sized through

openings of a particular standard size in a screen.

- he particle-size distribution is then reportedas the !eight percentage retained on each of a

series of standard sieves of decreasing size and the

percentage passed of the finest size.


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Sieving is a gravity-driven process. usually a stac" of

sieves are used !hen fraction of various sizes areto be produce from a mi#ture of particle size


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he sha"er may be in the form of an eccentricdrive

!hich a screens a gyratory or oscillating motion or

vibrator !hich gives the screens small-amplitude$

high frequency$ up and do!n motion


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St d d i i


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Standard sieve size

Sieves may be designated by the opening size$

%S sieve mesh or yler sieve mesh

he yler mesh designation refer to the number ofopening per inch.

he %S-sieve mesh designation is the metrication

he t!o mesh designations have equivalent openingsize although the sieve number designations are not

e#actly the same.


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Standard %S-sieve size


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&igure ' ( Schematic of relatie percentage frequenc!

distri$ution cure)

#he percentage frequenc! cure graph


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The probability curve graph

1lot opening sieve diameter against probability

percentage

The diameter at :.+ or +:@ probability is particle size


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&igure * ( Schematic of the pro$a$ilit! cure

#he pro$a$ilit! cure graph


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%alculate method

Method 1

=pi

>i

-

Particle size =

Method '

Particle size =

.

log.(

log

Wt

dia+Wt


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Example

The mass fraction of a sample of milled corn retained on each


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of a series of sieves. Calculate a mean particle diameter whichshould be specified for this mixture.

U.S. Micron Wt.

X (%) % accumulateSieve Size grams

& '$'&( ).& ).& ).&

+ $'+( '. '., ,.+

) )$&+( ./ .// ).+,

)& )$)/) )/., )/.& '.,&

( +,) )+ )+.( (.&&

'( /, ) ).) &.+

,( ,( )).& )).' .

( / + +.(/ +.&,

( ) &.& &.& /.'

)(( )( '., '.,, /.

),( )(' '. '., /+.//

(( ' (./ (./) //./(

( ' (.) (.)( )((.((

Pan ' ( (.(( )((.((

Sum. /+./ /+./ )((.((


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#he percentage frequenc! cure graph


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#he accumulatie percentage cure graph


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M th d '


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+)S) Micron ,t)log dia ,t"log dia

Siee Size grams

A D,DA: .A '.& .&,

8 *,D8: D.* '.' )(.+(

* ,A8: C.E '. .,+(

A ,E E.? '.(& /.&'

*: 8? 8 ./ .&,&

D: +E? + ., ,).&(

?: ?*: .A .&' '(.,'(

+: *EC 8 .,' )/.+

C: ** A.A .'& ).',

:: +: D.? .)& .'//

?: :D D.* .()' &.,,)

*:: CD :.E ).+&' ).&

*C: +D :. )., (.)

1an DC : ).&+ (.(((

/um. E8.E *CC.

.C*.ADD-:log.(

log E.E8--.*CC

- mWt

dia+Wt=vg ==

=

Method '

Reduccion de particulas

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