FT 305 Lecture Sildes

7/25/2019 FT 305 Lecture Sildes

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FT 305: UNIT OPERATIONS IN

FOOD PROCESSING IIThermal Processes in Food Processin

Main references: Food En ineerin O!era"ions# 2nd Edn. J.G Brennan et. al.

Chemical En ineerin # Vol 2. Coulson and Richardson Uni" O!era"ions o$ Chemical En ineerin # 2nd Edn, McCabe andSmith

Chemical En ineers %and&oo'# th Edn, !err" J.#. Princi!les o$ Food Processin # $. #eldman and R. #artel In"rod(c"ion "o Food En ineerin # Sin%h, R.!., and #eldman, $.R.

&'()*+. cademic !ress.


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Contents

$r"in%-$eh"dration E a/oration

Cr"stalli0ation 1rradiation ree0in%

Sol ent E3traction


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C%APTER ONE: DR)ING AND DE%)DRATION

In"rod(c"ion

$eh"dration can be defined as the com/lete remo al of moisture /resent in the foodb" e a/oration or sublimation b" usin% heat. $r"in% is the term a//lied 4hen theresultin% food material, after deh"dration, contains er" little moisture i.e. it becomesdr". Generall" it can be said that t4o terminolo%ies mean the same /rocess but one ismilder than the other.

Ad*an"a es1ncreased shelf5life b" reduction in the 4ater acti it" and en0"me inacti ation.Sa in% in 4ei%ht and olume 5 stora%e and trans/ortation are easier 4ith dr"materials.

Sa in% in stora%e facilities 5 no refri%eration needed.Con enience of foodstuff 5some foods are con enientl" ta6en dr".

Disad*an"a es!h"sical characteristics can chan%e &densit", sha/e, si0e etc+7r%anole/tic /ro/erties can chan%e &fla our, te3ture etc+8oss in nutritional alue 9 thermal labile constituents can de%rade.


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+ain +e"hods:,- %o" air dr.in : #ot air is blo4n o er the foodstuff and heat is transferred mainl" b" con ention.

/- Direc" con"ac" dr.in : he foodstuff is /laced on a heated surface and heat is transferred b" conduction.

;. $r"in% b" a//lication of ener%" from a radiatin%, micro4a e or dielectric source sothat the food tem/erature is raised and 4ater is lost b" e a/oration. Ver" small


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- Free1in dr.in :

he moisture in the food is first fro0en and then directl" sublimed to a/or, usuall"b" a//lication of heat under er" lo4 /ressure conditions. his method /reser esmost of the attributes of food, includin% heat sensiti e nutrients, colour andsha/e .1t also results in im/ro ed reh"dration characteristics and lo4 bul6 densit". he

main disad anta%es are hi%h ca/ital and runnin% costs.


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5- Osmo"ic deh.dra"ion :8imited amounts of moisture can be remo ed from a food material b" a//l"in% the

/rinci/les of osmosis 4hereb" a %i en sol ent &4ater in this case+ mo es from are%ion of its hi%h concentration to a lo4 concentration one thou%h a semi5/ermeable membrane. 1t is not a er" common dr"in% method a//lied inindustr".


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F(r"her ill(s"ra"ion on "he de*elo!men" o$ "he osmosis !rocess

>ote:? he semi5/ermeable membrane allo4s onl" sol ent molecules to /ass

? //lication of osmotic /ressure 2 re erses the mo ement, resultin% in

the Re erse 7smosis /rocess


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%OT AIR DR)ING

his is the most common method of dr"in% because of its sim/licit".

Material e3/osed to atmos/here 4ill %ain or lose 4ater de/endin% on the humidit" ofthe air. E entuall" e


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Dr.in s"a es

E (ili&ra"ion !eriod: 4A 67his is a er" short /eriod 4hen the s"stem is adAustin% to the dr"in% conditions. 1ts

trend is not clearl" defined and. 1t is not a si%nificant /art of the dr"in% c"cle.

Cons"an" Ra"e !eriod: 46 C7he surface of the 4et solid is saturated and it beha es li6e an o/en surface li


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Dr.in C.cle Anal.sis

8et subscri/t c denote the constant rate /eriod and f fallin% rate /eriod

&'+

@hereDPs Saturation /ressure of 4ater in the hot air

Pa !artial /ressure of 4ater in the hot air kg Mass transfer coefficient A #eat transfer area

he rate of heat transfer from the hot air to the solid can be established as follo4sD

&2+ @herehc Con ecti e heat transfer coefficient. a - s em/erature %radient

( )a s g c

P P Ak dt dw =

( ) sacc Ahdt dQ

=


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1t follo4s that, therefore, durin% the constant rate /eriodD

&;+

@here F8= is the latent heat of e a/oration of 4ater

&*+

& +

@here rea associated 4ith ' 6% of material

& +

d thic6ness of the material, s densit" of solid material

cc dt

dQ L

dt

dw =

( ) sacc

A Lh

dt dw

=

( ) sacc

A Lh

dt dW

= '

sd

1

d


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( ) sa sc

c d Lh

dt dW

Se/aratin% the ariablesD

( ) sac s

hdW Ld

dt

=

1nte%ratin%D

( )

=cc w

w sac

st

dW h

Ld dt

00

( ) ( )co

sac

sc W W h

Ld t =

he duration of the constant rate dr"in% /eriod is thus %i en b" e


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he moisture mo ement inside the material is b" an" or a combination of thefollo4in% 4a"s: 5

Ca/illar" mo ement: @ater mo es b" ca/illar" action in the interstitial s/aces 4ithin the material 4hichact as ca/illaries.

8i


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or materials de/endin% on the ca/illar" mo ement, the e


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E$$ec"s o$ %o" Air Dr.in

here are t4o main effects of hot air dr"in%, namel" the mi%ration of solubles andshrin6a%e.

Solubles migration:

s the dr"in% /rocess continues the 4ater from the bul6 of the material bein% dried isdri en to the surface and conse


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Shrin'a e:

@ith lo4 initial dr"in% rate the resultin% dr" material 4ill be of a tou%h te3ture, hi%hbul6 densit" and slo4 reconstitutabilit". #i%h initial dr"in% rate results in lo4 bul6densit", ra/id reconstitutabilit" and soft te3ture.

8o4 initial dr"in% rate

5 hi%h bul6 densit" 5 slo4 reconstitution 5 tou%h te3ture

5 lo4 bul6 densit" 5 ra/id reconstitution

5 soft te3ture

#i%h initial dr"in% rate


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%OT AIR DR)ING E8UIP+ENT

,- Ca&ine" drier

5 $irect combustion is the most efficient 4a" of heatin% the air but /roblems ofcontamination /uts it do4n in man" cases

5 #eat e3chan%ers can be used to a oid contamination but the efficienc" is lo4er 5 Steam heatin% is e en less efficient but due to its a ailabilit" a /lant sites it is often

used.5 ir elocities used ran%e from 25 m-s. oo hi%h a elocit" 4ould result in the

coolin%of the /roduct.


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lternati e la"out


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he hot air flo4 can be either co5current or counter5current to the direction of tra"s.

Co5current flo4 allo4s for hi%h initial dr"in% rates 4hile counter5current flo4 results inlo4 initial dr"in% rates. !roduct re


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Side ie4 for a tunnel drier


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37 Con*e.or &el" drier

Multi/le sta%e

Sin%le sta%e


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luidi0ed5bed drier details

Steam coils


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+ S/ra" drier

5 hese are used for fine /article s"stem &/o4ders+ 'I52II m5 he time ta6en for dr"in% is er" short &'5'I s+5 he /rocess is continuous .

$ue to e a/orati e coolin% the tem/erature of the /roduct is usuall" lo4. he

e


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8a"out details


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Prod(c" Reco*er.his is a er" difficult o/eration. Ksuall" 4i/ers can be used and sometimes air

brooms are used. or the fines in the air, C"clones, ilters etc, are used.

C.clone se!ara"or :hese use the /rinci/le of centrifu%ation to se/arate fine /articles from the air.he mi3ture enters tan%entiall" to the se/arator and is 4hirled inside the

se/arator c"linder. he /articles are thro4n out4ards because the" are denserthan air. he air is dis/laced to4ards the center of the c"linder and thus se/aratedout ia a different outlet. he /o4der falls under the action of %ra it" and iscollected se/aratel".


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C"clone se/arator details


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!roduct and air mo ement in the c"clone se/arator


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C%APTER T9O: E APORATION

In"rod(c"ion

E a/oration is one of the main methods used for concentration of a


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Fac"ors In$l(encin ;i (id 6oilin Poin"he dri in% force for heat transfer in the heat e3chan%er of an e a/orator is the

tem/erature difference bet4een the heatin% medium and the li


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Raoult s la4D

Ao

A A X P P =

@here ! is the /artial /ressure of the solution and ! o is the a/our /ressure of the

/ure sol ent F = at the same /ressure. X A is the mole fraction of the sol ent in thesolution.

1t follo4s that, therefore, under similar /ressure conditions a solution 4ill boil at atem/erature hi%her than that of the corres/ondin% /ure sol ent. he ma%nitude ofthis Boilin% !oint Rise &B!R+ 4ill de/end on the amount of solutes dissol ed in thesolution, in other 4ord it 4ill de/end on the concentration of the solution.

t a hi%her concentrations the B!R 4ill be /ro/ortionall" hi%her, as %o erned b" themole fraction &L + of the solute in the solution. his /henomenon is e3/lained b" the$ hrin% s rule, 4hich states that a linear relationshi/ e3ists bet4een the

tem/eratures at 4hich t4o solutions e3ert the same a/or /ressure. he rule is oftenused to com/are a /ure li


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A ".!ical D


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!"drostatic #ead

t an" /oint belo4 its free surface, the li


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The in$l(ence o$ "he "em!era"(re di$$erence:

he ma%nitude of the tem/erature difference cannot be aried Aust arbitraril" becauseof the effect of film boilin% or /ool boilin%D

@hen boilin% be%ins a/our bubbles are formed at the hottest surface &closest to the

heatin% medium+. hese bubbles raise b" con ecti e action, creatin% s/ace for colder/ortions of li


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his results in bubble coalescence, a situation that creates lar%er bubbles. heselar%er bubbles collide and coalesce and e en lar%er bubbles are formed. E entuall"a a/our film is created bet4een the heatin% surface and the bul6 of the li


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Gra/hical isuali0ation: >ucleate T ransition T 8eidenfrost /oint T ilm boilin%

&a+:>ucleate boilin%, &b+:Ma3 heat flu35 < ma3 or h ma3, &c5d+:


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A ".!ical e=am!le sho>in "he *ario(s &oilin re imes
http://upload.wikimedia.org/wikipedia/en/4/49/Boiling_Curve.jpg


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/- er"ical "(&e &Steam %oes outside the tubes+

Va/our


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3- Film e*a!ora"ors:


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CA;CU;ATIONS

ssum/tion: ll the heat ener%" %i en out b" the heatin% medium &normall" steam+ is

utili0ed in affectin% the e a/oration /rocess i.e ne%li%ible heat losses.

#eat %i en out b" the heatin% medium is %i en b" the ourier heat transfer ratee


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or a sin%le effect e a/orator s"mboli0ed belo4D

%ea" 4ener .7 &alance:

>e%lectin% heat losses, assumin% onl" condensation latent heat of steam isutili0ed.

8atent heat of a/ori0ation-condensation. Mass fraction of solids in the feedCp era%e s/ecific heat ca/acit" of feed

( ) 11100 V t t C F V f p +=

= = x


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11 V P F +=

( ) p f X P FX solidsi 1: =

( ) ( ) ( ) P X V X F Solvent ii p f += 11: 1

+a"erial 4+ass7 &alance:

7 erall:

Com/onent:


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+U;TIP;E EFFECT OPERATION

1n order to ma3imi0e the use of steam a ailable, the a/our /roduced from the firsteffect can be used as the heatin% medium in the second effect, /ro ided the boilin%

/oint in this second effect is reduced so that an ade


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ario(s e*a!ora"ors : is(als

Sin%le5effect hori0ontal tube e a/orator


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Multi/le5effect & uadru/le5effect+allin% film e a/orator s"stem

Multi/le5effect & ri/le5effect+ertical tube e a/orator s"stem


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Multi/le5effect ertical tube e a/oration s"stem


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C%APTER T%REE: CR)STA;;I ATION

In"rod(c"ion

Cr"stalli0ation is mainl" used as a se/aration /rocess in most food industries. 1tcan be defined as the se/aration of a solid cr"stalline /hase from a li


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Consider the eei h"

T e m ! e r a

" ( r e

B C D

c

SolutionSolution N sucrose

Solution N ice

1ce N sucrose

a

b

c

rom the solution re%ion, to enter the su/ersaturation re%ion & sucrose N solution + eithere a/oration of the sol ent W a X,coolin% of the solution W b X or both methods can be a//liedsimultaneousl" & adiabatic e a/oration + Wc X.


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Solubilit" coefficient &C+ is defined b" the follo4in% e3/ression:5

solvent pureinsoluteof Solubilitysolutionimpureinsoluteof Solubility

C =

Both bein% at the same tem/erature.

N(clea"ion

Cr"stalli0ation or cr"stal formation does not necessaril" start 4hen the solutionreaches its saturation /oint. Ksuall" the immediate effect is to /roduce asu/ersaturated solution. 1n man" cases small seed cr"stals ma" be added andsometimes e en small


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he rate of nucleation is related to the su/ersaturatin% in the follo4in% %ra/hD

Supersaturation (S)

Rate of nucleation

S

@here the su/ersaturation &S+ is defined as follo4s:5

solutionsaturatedainsolventinsoluteof ionConcentratsolventinsoluteof ionConcentrat

S =

Both bein% at the same tem/erature.


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he cur e sho4s that the nucleation rate is er" lo4 at lo4 S alues but it increasesra/idl" be"ond a certain FS= alue, at FS =. he reason for this beha iour is that it canbe sho4n that the solubilit" of small cr"stals is hi%her than that of lar%e ones. s a

conse


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SE;ECTED APP;ICATIONS

,- 9in"erisa"ion o$ oils

Most edible e%etable oils contain some %l"cerides 4ith meltin% /oints hi%henou%h to solidif" at refri%erator or 4inter tem/eratures &sa" C+. his is usuall"the case /articularl" 4ith cottonseed oil and hi%hl" h"dro%enated so"bean oil &'I1odine alue+. his solidification results in turbidit" in the oils. 1t thus im/airs its/ourin% characteristics and s/oils its a//earance .Cr"stalli0ation b" coolin% can be a//lied and later on se/aration of the %l"ceride

cr"stals is done b" filtration. he oils become F4interised= and sta" li


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3- Prod(c"ion o$ sal" and s( ar

hese t4o common domestic /roducts are mainl" /roduced b" cr"stalli0ation ofthe res/ecti e solutions usin% e a/orati e concentration. Su%ar &sucrose+ ismostl" /roduced b" Fseedin%= the cane &or beet+ Auice s"ru/ 4ith finel" %roundsu%ar cr"stals because to attain the reaCl+ is relati el" stable in this ran%e of tem/eratures and su/ersaturationcan be attained b" boilin% alone.

he rate of coolin% of the su/ersaturated solution controls the si0e and sha/e of

the cr"stals /roduced. ast coolin% results in small , relati el" uniform cr"stals.Slo4 coolin% results in lar%e irre%ular cr"stals.

E8UIP+ENT


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'. 6a"ch cr.s"alli1ers

2 C "i ( ( C " lli1


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2. Con"in(o(s Cr.s"alli1ers

a 9 E a/oration s/ace

b 9 8i


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eed !roduct

Con"in(o(s Cr.s"alli1ers

is(als


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is(als


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C%APTER FOUR IRRADIATION


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C%APTER FOUR: IRRADIATION

1rradiation is a method of food /reser ation b" use of accelerated electrons, L5ra"s, 5ra"s and other forms of radiation. he ra"s cause ioni0ation, 4hich in turnfacilitates certain chemical chan%es in the food. 1n most cases it is the 4atercom/onent of the food that is most affected b" irradiation. 1f electrons are usedthen the" are referred to as leptrons &li%ht /articles+. 7ne of the /rinci/alad anta%es of this method is that the food can be treated after /ac6a%in%.


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ood irradiation is a /rocessin% techni


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he follo4in% terms are used based on the s/ecific aim and t"/e of the /rocessD

Radicidation is the /rocess of radiation used to 6ill e%etati e bacteria. he treatmentis intended to destro" or%anisms of /ublic health im/ortance. he /roduct ma" notnecessaril" attain commercial sterilit".

Radurisation is the /rocess in 4hich treatment is meant to increase the shelf5life b"%eneral reduction in the le el of e%etati e bacteria.

Radapperti$ation is the /rocess of radiation treatment a//lied to canned foods.Ksuall" a commerciall" sterile /roduct is /roduced.

Radiation disinfestations is a treatment b" radioacti e ioni0ation to /re ent insect/ests from multi/l"in% . he" die out after one %eneration.

Sprout in#ibition in stored e%etables and %ro4th inhibition in mushrooms can also beachie ed throu%h irradiation.

7ther irradiation /rocesses are not necessaril" used for /reser ation but are a//lied toeffect desired chan%es in the food to im/ro e some of its


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he most common radioacti e source used in the food industr" is Cobalt 9 I & I Co+but sometimes Caecium 9 ';H & ';H Cs+ can also be used.

Cobalt has a half5life of .; "ears 4hile Caecium has ;I "ears.

T.!es o$ radia"ion4o factors of consideration: &i+ Ener%", and &ii+ $oseor electron beams the ener%" is measured in terms of electron olts &eV+

, e ? ,- = ,0 , H-his is a er" small unit and hence multi/les such as F 'e = and F+e = are used in

/ractical situations.

or L5 and 5ra"s the ener%" used can be 4or6ed out from the !lanc6=s e


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+eas(remen"s

Most of the radiation ener%" is not absorbed b" the food in the same 4a" as heatused for heat treatment remains mostl" as heat. his means that /racticalmeasurements of ener%" of radiation are chemical and-or o!"ical since calorimetric chan%es in the irradiated food are er" small .

73idation of errous sul/hate, under certain conditions, is linear 4ith the dose andhence can be used as a measure of the same under laborator" conditions.

1n industrial a//lications, a isible chan%e is more con enient. Colour chan%es oncertain materials under incident radiation can %i e an a//ro3imation of the dose.

n electrical readout can be obtained from an ioni0ation chamber . he amount ofioni0ation /roduced is related to the dose if the medium is not full of ions b" itself. he

radiation used in such cases must be of an electroma%netic 4a e and not of a /article beam.

or /article beams li6e an electron beam, the fact that electrons loose their ener%"4hen the" /enetrate a material, the attenuation of the beam is lo%arithmicall" relatedto the amount of incident radiation.


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6eam a""en(a"ion

1ntensit"

$e/th of/enetration


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he maintenance of a uniform distribution is another /roblem. !ro/er en%ineerin%arran%ements and %eometr" ha e to be a//lied to obtain a uniform distribution ofradiation in the food /roduct. Shields must be /ro ided for /ersonnel 4or6in% in suchfacilities and minimum standard radiations /ermitted must not be e3ceeded.

Geome"rical arran emen"s :

More details


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More details


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C%APTER FI E: FREE ING


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C%APTER FI E: FREE ING

In"rod(c"ion

ree0in% is another /reser ation method for foods. he earliest a//lication 4as onmeat for e3/ortation /ur/oses. Canned meat has been used for this /ur/ose but the/roduct is not that much desirable as com/ared to normal fro0en /roducts.

#o4e er, the


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formation+ and the stable /hase 4ill be the cr"stalline /hase if the coolin% medium isat a lo4er tem/erature & m Y I oC+ as com/ared to the material tem/erature.

he material s concentration 4ill increase as a result of 4ater cr"stalli0in% out. his

results in the de/ression of the free0in% /oint of the material &colli%ati e /ro/erties+. time 4ill come 4hen e


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Free1in "ime

1n the free0in% /rocess, it is er" im/ortant to be able to Aud%e the time re


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he tem/erature histor at the thermal centre of a material under%oin% free0in% canbe /resented as %i en in the follo4in% dia%ramD

he fall in tem/erature after the nominal free0in% /oint is


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(

4o dimensionless %rou/s are fre


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oooo K F

erf

F F

1

4

1

4

1exp

4

=

( ) dx x xerf x

=

0

2

exp2

)(

@hereD

he so called O error function P

he reci/rocal of this e


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1f the ourier >umber, ! o, is lar%eD 2! o K o D a//ro3imate but tolerable e


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1f the Uo >umber is lar%e heat remo ed durin% /hase chan%e is lar%e relati e toheat remo e durin% coolin% of fro0en material conditions of heat transfera//roaches stead" state i.e. the free0in% front is mo in% slo4l" and the %uasi static assum/tion can be made.

#ere the assum/tion made is that the free0in% front is mo in% er" slo4l" to thee3tent that it can be assumed to be stationar" and the fro0en material thic6ness O " Pcan be assumed to be constant . his allo4s the a//lication of normal conductione


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. e ate a ee0es co / ete at t e o a ee0 % /o t so t at t e e s a

clear latent heat of fusion 8 released at a definite tem/erature I

2. he thermal conducti it" of the fro0en material has a constant alue 6F

;. here is a constant heat transfer coefficient h at all cooled surfaces.

*. here is a constant coolin% medium tem/erature 2.

. he densit" of the material is the same in the fro0en and unfro0en states anddoes not chan%e 4ith tem/erature.

. he


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+= $ %i

&K F oo

1

@here , the Biot >umber &another dimensionless number+.

$ and G are sha/e factors . he" de/end on the %eometr" of the material under%oin%free0in%. Ksuall" the" are obtained from free0in% charts. or an arbitrar" sha/e the/rinci/le of inscribin% the bod" into a 6no4n %eometr" fi%ure can be used for /racticalestimates. F$= is calculated from the follo4in% e


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Charts for ariation of OGP 4ith free0in% bod" %eometr"


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Free1in E (i!men" and +e"hods


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he food material can be brou%ht into contact 4ith a solid refri%erated surface orimmersed into a refri%erated li


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&ii+ 1ndirect contact


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ood /ac6a%es

!late

Refri%erant

!ac6a%es must not be under5filled or o erfilled for heat transfer reasons. 1f there isunder5fillin% then the free0in% time for that /articular /ac6a%e is drasticall" increased.

/- Immersion Free1inh i l i di// d di l" i h f 0i % di id i l f


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he material is di//ed directl" in the free0in% medium. o a oid material transfer ore3chan%e bet4een the medium and the food, /ro/er /ac6a%in% is re


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ir elocities are limited because if hi%h alues are used then the air %ets heated u/.#i%her elocities are %ood for transfer /ur/oses but there is no /oint to increasethe heat transfer and also increase the medium tem/erature at the same time. com/romise has to be reached. luidised bed free0ers are er" efficient. he

turbulent motion ma6es it /ossible to free0e indi idual items. $iced e%etables arean e3am/le of food best fro0en b" this method.

- 6oilin li (id


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8iitro%en &>2+ is the most /o/ular in this case. 8iitro%en is calledCr"o%en hence the term cr"o%enic free0in%.

Boilin% a lo4 boilin% /oint li


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In"rod(c"ionSol ent e3traction, in 4hich one com/onent is remo ed or reco ered from ami3ture of li


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he se/aration of /roduct F &= from a feed stream F A= in ol es three sta%es, firstl"mi3in% 4ith the sol ent F S = 4ith 4hich in theor", F = should be com/letel" immisciblethou%h this is ne er the caseD secondl", the settlin% sta%e in 4hich the e="rac"

/hase &the e3it sol ent stream+ is se/arated from the ra$$ina"e &the e3it treated feedstream+D and thirdl", sol ent reco er" and rec"cle.

here are four 4a"s in 4hich this o/eration ma" be carried outD

4a7 Sin le con"ac"


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4a7 Sin le con ac

1n this s"stem, the efficienc" of mass transfer de/ends u/on the e


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his is a hi%hl" efficient arran%ement, usuall" in ol in% 2 to sta%es.

resh sol ent S is introduced into the last sta%e 4here it meets a relati el" de/letedraffinate R/- he e3tract E3 is sent to the second sta%e 4here, bein% lo4 in solute, it

acts as a sol ent for that sta%e. he e3tract E/ from the second sta%e is fed to thefirst sta%e 4here it meets the fresh feed F, 4hich is rich in solute. he e3tract E, ista6en for se/aration and sol ent reco er".

4d7 Con"in(o(s co(n"er c(rren" con"ac"


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1n this mode the sol ent is introduced at the bottom of a column as a dis/ersion ofdro/s that are allo4ed to rise throu%h the feed. he feed constitutes the continuous/hase and it is fed at the to/ of the column. he column ma" be /ac6ed 4ith ceramic

rin%s for e3am/le, or some other de ice such as rotar" discs or /lates.

FT 305 Lecture Sildes

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Transcript of FT 305 Lecture Sildes