FOOD FREEZING FOOD FREEZING. Food Freezing. Food Freezing

FOOD FR EEZING

FOOD FREEZING FREEZING (PEMBEKUAN): ➡

Unit operasi yang menurunkan suhu bahan pangan hingga di bawah titik bekunya.

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Tujuan: pengawetan dengan menurunkan suhu dan Aw.

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Penurunan suhu (beku): • menurunkan aktivitas mikroba dan kerusakannya, • Menurunkan reaksi biokimia (enzim dan oksidasi).

Bahan Bacaan: Sing, R.P. dan D.R. Heldman. 2009. Introduction to Food Engineering 4th Ed. Elsivier Pub. Co. New York, USA. Fellows, P.J. 2010. Food Processing Technology 3rd Ed. Woodhead Pub. Ltd. Cambridge, England.

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Penurunan Aw: • mengurangi air untuk aktivitas biologis.

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Mutu produk tetap tidak berubah (sensori dan nutrisi).

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Semua jenis bahan pangan dibekukan.

Jurusan Teknologi Hasil Pertanian

Fakultas Pertanian Universitas Lampung



7.1 Freez w

Insulated

Food Freezing

outside Food Freefrom zing environment

■ Figure 7

Insulated from outside environment

A.Indirect Contact Systems SISTEM PEMBEKUAN: • Untuk membekukan bahan pangan, panas sensibel dan panas laten harus dikeluarkan. • Suhu bahan pangan turun dan perubahan fase cair (air) menjadi padat (es).

2. Direct Contact System (kontak langsung).

Plate

1. Plate Freezer. •

•

1. Indirect Contact System (kontak tidak langsung).

indirect-con

Plate ■ Figure 7.1

Schem indirect-contact freezing

Food

Bahan pangan dan refrigerant dipisahkan oleh pembatas.

• Sekitar 10% air tetap cair.

• Sistem pembekuan:

w

Food

Refrigerant

Bahan pangan di letakan pada perbukaan yang dialiri oleh refrigrant. Single dan double plates.

•

Batch system operation.

•

Fleksibel jenis produk.

•

Pembekuan cepat.

Refrigerant Pressure

w

■ Figure 7

w

Pressure

plate freezin

Plate ■ Figure 7.2

Schem plate freezing system.

Food

Food

Plate

Package Pressure




FakultasPackage Pertanian Universitas Lampung

Pressure

between product and refrigerant will include both the plate and package material. The heat transfer through the barrier (plate and package) can be enhanced by using pressure to reduce resistance to heat

Food Freezing

Plate F reezer

2. Air Blast Freezer •

Menngunakan udara dingin (beku) sebagai media pendingin.

•

Udara beku dialirkan dalam ruangan dan kontak dengan bahan pangan dalam kemasan yang disusun dalam rack.

•

Waktu pembekuan lama: perbedaan suhu rendah, kontak bahan dan udara beku tidak maksimal, transfer panas rendah.

•

Sistem Batch dan continuous.

•

Pada sistem kontinyu, bahan pangan begerak mengikuti konveyor di dalam freezer room.





Food Freezing

Air Blast Freezer

3. Liquid Food Freezer

Batch System

Continuous System



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Pindah panas paling efisien pada produk cair adalah sebelum dikemas.

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Scraped-surface Freezer. •

Digunakan untuk pembuatan ice cream.

•

Sistem Batch dan Continuous.

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Sekitar 60-80% panas laten dikeluarkan, menghasilkan frozen slurry.

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Dilengkapi dengan air injection system (sistem penginjeksi udara).

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Frozen slurry masuk ke dalam proses hardening. Jurusan Teknologi Hasil Pertanian


in Scrap ed-surface Freezer Making Ice Cream

the Factory

p c r a p e r blade

\

(b)

Sc ra pe d- surface Syst em

69

Jacket

7.1 Freezing Systems

Chapter 4

74 Refrigerant

Dasher

Figure 4.9 (a) External view of an industrial ice cream freezer; (b) internal view (the Ice crystals mixed lower cylinder is the jacketed barrel) Figure 4.8 Cross-sections of a factory ice cream freezer (a) parallel and (b) perpendicu(Kindly provided by WCB Ice Cream) lar to the axis of the barrel ,,,,,,,,,,,,,,,,,,,,

on the inside with a thin layer of chromium. Nickel gives good heat transfer, and can withstand high pressures. The chromium coating provides resistance to wear from the scraping, and chemical resistance to the cleaning agents used between batches. There are two types of dasher, open and closed, which are used for different types of product (Figure 4.10). Open dashers have an open cage supporting the scraper blades, within which is a passively rotating whipper. The dasher occupies 20-30% of the volume of the barrel. Closed dashers have a solid core,Jurusan and occupy approximately 80% of the Teknologi Hasil Pertanian Pertanian Universitas Lampung volume. Open dashers giveFakultas lower shear and longer residence times than blades removing ice crystals from theand barrelthroughput. wall Figure closed4.12 onesScraper for the same outlet temperature Open dashers are generally used for ice cream production. The longer residence time helps to achieve good aeration. Closed dashers are used awhen dramatic reduction inisheat transfer. The small ice crystals that are a low throughput required ( e .g for sauces and ripples) or for ice scraped off are dispersed into the mix by the beating of the dasher. cream that needs to retain its shape after extrusion, because, as we will There are large temperature gradients inside the barrel, both radial see below, the higher shear increases the amount of partially coalesced (from colder at the wall to warmer at the centre) and axial (from fat and makes the ice cream stiffer. warmer near the inlet and colder towards the outlet). Therefore, when ice crystals are dispersed into the warmer mix at the centre of the barrel, they melt and cool it down. Near the inlet the crystals all melt, but about one-third of the way through the barrel the mix becomes cold enough for the ice crystals to survive. Heat must be extracted from the mix both to cool it down (the sensible heat) and to freeze water into ice (the latent heat). We can estimate their relative contributions as follows. Consider a 1 kg mass (m)of a 20% sucrose solution as a simple model for ice cream. This enters the factory freezer at +4.0 "C ( T I ) and , leaves at - 5.0 "C ( T J .The specific heat capacity (c), i.e. the amount of heat required to change the temperature of 1 kg by 1 "C, of a 20% sucrose solution is 3.5 kJ kg-' K-l. (Although this varies with temperature it is very nearly constant between 4 and - 5 "C.) The sensible heat removed (AQs) is therefore

w ■ Figure 7.7

A fluidized-bed freezing system. (Courtesy of Frigoscandia Contractin Inc.) Jurusan Teknologi Hasil Pertanian


H ow Ice Cream Is M ad e

+

AQs=m~(T1-T2)=l.Ox3.5x 1 0 3 ~ ( 4 . 0 - - 5 . 0 ) = 3 . 2 ~lo4 J

(4.1)

The latent heat of fusion of ice ( L ) is 330 kJ kg-'. A 20% sucrose solution has a freezing point of - 1.5 "C and at - 5 "C the ice content is

Food Freezing 4. Immersion Freezer ➡

Produk yang akan dibekukan direndam dalam larutan pembeku (brine = larutan garam).

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Proses pembekuan cepat (IQF = Individual Quict Freezing). Feed

Exit

w ■ Figure 7.8

Schematic illustration of a immersion freezing system.

Liquified gas

Freezer conveyor



7.1.2.2 Immersion



By immersion of the food product in liquid refrigerant, the product surface is reduced to a very low temperature. Assuming the product objects are relatively small, the freezing process is accomplished

od Freezing

Immer si on Fre ezer

Immersion Freezer

quick freezing (IQF) tesy of Frigoscandia

Colloidal Dispersions, Freezing and Rheology

21



A commercial version of the immersion IQF freezing system is shown in Figure 7.9. In this illustration, the product freezing compartment is filled with refrigerant vapors while the product is conveyed through the system. In addition, the product is exposed to a spray of liquid MEC HANISM FOOD FREEZING refrigerant, which absorbs OF thermal energy from the product while changing phase from liquid to vapor. One of the major disadvantages of immersion-type freezing systems is the cost of the refrigerant. Since Colloidal Dispersions,Susunan Freezing andmolekul Rheology air dalam bentuk cair21dan padat (es) the refrigerant changes from liquid to vapor while the product freezes, it becomes difficult to recover the vapors leaving the freezing compartment. These refrigerants are expensive, and the overall efficiency of the freezing system is a function of the ability to recover and reuse the vapors produced in the freezing compartments.



Mechanism of Food Freezing Perubahan suhu pada proses mencair

Chapter 2

7.2 FROZEN-FOOD PROPERTIES The freezing process has a dramatic influence on the thermal properties of the food product. Because of the significant amount of water in most foods and the influence of phase change on properties of water, the properties of the food product change in a proportional manner. As the water within the product changes from liquid to solid, the Jurusan Teknologi Hasil Pertanian

density, thermal conductivity, heat content (enthalpy), and apparFigure 2.5 Arrangement of H,O molecules in (a) solid ice and (b) liquid water; oxygen Fakultas Pertanian Universitas Lampung atoms are grey and hydrogen are black ent specific heat of the product change gradually as the temperature decreases below the initial freezing point for water in the food. 0 "C,ice would begin to form. This would release the latent heat, so that

Time (minutes) Jurusan Teknologi Hasil Pertanian


Mechanism of Food Freezing

Mec ha nism of Food Fre e zing

Mekanisme Pembekuan

2 retpahCPerubahan suhu pada proses membeku Time (minutes)

10

X 20

• Suhu awal 6oC, turun (mengeluarkan panas sensibel) menjadi -2oC, baru terbentuk inti kristal = Supercooling. • Suhu naik menjadi 0oC,

I

30 30

40

I

53

I

Time )setun(minutes) im( emiT

Temperature as a function of time for (a) ice melting and (b) water freezing Jurusan Teknologi Hasil Pertanian


‣ karena pada saat terbentuk kristal, terbentuk pula dua permukaan (interface) es dan cair. Terjadi perbedaan energi dua permukaan yang menyebabkan penyerapan energi. ‣ Suhu konstan (0oC), terjadi pengeluaran panas laten (Panas laten dikeluarkan 1kg air = panas menaikan suhu 1 kg air dari 0oC menjadi 80oC) hingga semua air menjadi es. • Suhu turun kembali, karena semakin semakin banyak energi dikeluarkan (mengeluarkan panas sensibel). Jurusan Teknologi Hasil Pertanian


7.3 Freezing Time 515 molecules at the crystal surface switch from the ordered lattice into the liquid one at a time. The reverse process occurs when an ice crystal Ice Crea m Sa ndwiches 35 03 Food grows: individual water molecules from the Fre liquidejoin onto the ice Mec ha nism of zing crystal lattice. However, if there are crystals in the beaker, the Diagram pembekuan air no danicebahan pangan supercooled water does not have a lattice to join onto and cannot freeze ■ Figure 7.15 Freezing diagram of water one molecule at a time. So before freezing can occur, tiny embryonic Removal of and a food material. ice crystals (‘nuclei’) In the liquid state, nuclei form when sensiblehave heat to form. )setunim( emiT the random motion of the water molecules brings a small number of gnizeetogether rf retaw )b( in dnaagncrystal-like itlem eci )a( r oarrangement. f emit fo n o i t c n u fWhen a sa eruatarnucleus epmeT molecules is Water formed, an interface is created between the nucleus and the water. There Removal of Removal ofalatent heat because below 0°C ice has a is an energy gain in forming nucleus 0 sensible heat ht otni than ecittawater. l deredHowever, ro eht mothere rf hctiis wsalso ecafan rusenergy latsyrccost, eht tdue a selto ucelom lowereenergy Supercooling latsyrc of ecithe n anew nehinterface. w srucco The ssecnet orp energy esreverEeof hTforming .emit a at aspherieno diuqil the creation eci ehtoforadius tno nior j isdithe uqilsum eht of mothe rf senergy elucelogain m redue taw to lauchange dividni of :sworg cal nucleus Food , r e v e w o H . e c i t t a l latsyrc e h t , r e k a e b e h t n i s l a t s y r c e c i o n e r a e r e h t f i water to ice and the energy cost due to formation of the interface ezeer2.5). f tonnac dna otno nioj o t ecittal a evah ton seod retaw deloocrepus (equation cinoyrbme ynit ,rucco nac gnizeerf erofeb oS .emit a t a elucelom eno nehw mrof ielcun ,etatsHeat diuremoval qil eht (kJ/kg) nI .mrof o t evah )’ielcun‘( slatsyrc eci fo rebmun llams a sgnirb selucelom retaw eht fo noitom modnar eht n a nehW .tneprecooling megnarra as eksensible il-latsyrheat c a isniremoved. rehtegot selucelom si suelcudecreases temperature during e r e h T . r e t a w e h t d n a s u e l c u n e h t n e e w t e b d e t a e r c s i e afreice tni n a ,demrof However, the temperature at which initial nucleation occurs cand I

I

Temperature (°C)

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FOOD FREEZING FOOD FREEZING. Food Freezing. Food Freezing

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