refrigeration (kylteknik)users.abo.fi/rzevenho/ref18-oh6.pdf · Åa 424519 refrigeration /...

6. Food cooling and freezing, ice making

Ron ZevenhovenÅbo Akademi University

Thermal and Flow Engineering Laboratory / Värme- och strömningstekniktel. 3223 ; [email protected]

Refrigeration (Kylteknik) course # 424519.0 v. 2018

ÅA 424519 Refrigeration / Kylteknik

22.11.2018Åbo Akademi Univ - Thermal and Flow Engineering Piispankatu 8, 20500 Turku 2/32

6.1 Thermal properties of foods


Food products /1

About 15-20% of world energy use is linked to the food chain

Refrigeration plays an important role in the cycle of energy use, food, pollution, health and quality of life, population growth, consumption

Cooling and freezing of food gives important alternatives for other food preservation methods like drying, salting, smoking, cooking, etc., which reduce nutricity, change taste, etc.

Special considerations in food freezing and cooling are, for example, that fruit and vegetables respire (sv: andas) and generate heat during storage; freezing occurs over a tempe-rature range (see Figure); rate of freezing affects quality; velocity of cold air affects moisture loss from products, etc.

Picture: ÇB98

22.11.2018 Åbo Akademi Univ - Thermal and Flow Engineering Piispankatu 8, 20500 Turku

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Food products /2

Food deteriorates fastest at 20 -60°C, especially under humid conditions

Cooling to 1-4°C can extendthe storage life of fresh, perishable foods (meat, fish, fruit, vegetables) with severaldays

Freezing to -18 ~ -35°C canextend storage life to months

Refrigeration slows downchemical/biological processes in food, lowering sugar content, vitamin content, colour changeand changing smell P

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Food products /3 Important for freezing is the rate: too

slow freezing gives large ice crystalsthat destroy the texture which gives loss of juices during thawing; fast freezing gives growth of many small ice crystals.

The respiration of fruits and vegetables after harvesting meansproduction of heat; cooling to nearthe freezing point as soon as possiblemaximises storage life,

except some for fruits and vegetables(like bananas) for which a lowest safestorage temperature is given in the table → to avoid chilling injury

Table: ÇB98


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Food products /4

Freezing injury on fruits and vegetables can result from sub-freezing temperatures (duringwinter, transport vehicles mayhave to be heated to avoid this)

Dehydration or moisture loss can be avoided by a storagetemperature as low as possible, humidity as high as possible, and avoiding high air velocities

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Food products: thermal data It must be noted that

– Food (thermal) properties changewith time and temperature

– Fruit and vegetables generate heat (and CO2 and water, consumingoxygen) during storage

Water content is very important:

Also, the (first) freezing point drops whilethe water content of food freezes

kJ/kg a334 h

heat eezingMelting/fr

kg/kg water fractionmass the a where

kJ/(kgK)0.84 a1.26 c

kJ/(kgK)0.84 a3.35 c

:foods of heat specific forequations s Siebel'

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freshp,

Table: ÇB98

!!!

Food storage conditions

22.11.2018Åbo Akademi Univ - Thermal and Flow Engineering Piispankatu 8, 20500 Turku 8

Table: HTW08



6.2 Fruits and vegetables


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Fruits and vegetables refrigeration For highly perishable products

like ripe tomatoes, leafyvegetables, peaches, strawberries, ..., fast pre-cooling beforetransport is necessary; this is less urgent for potatoes, carrots, apples, green tomatoes, ...

Heat (rate) calculations can be made assuming a mass-averagedtemperature for the fruit or vegetable:

Q/Δt = m· cp· (Tt=t+Δt–Tt=t)/Δt

Picture: ÇB98


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Respiration heat Depending on temperature, fruits and

vegetables give of respiration heat(as a result of glucose oxidation), which varies strongly from oneproduct to another:

see table → The respiration heat qresp (W/kg)

decreases with time, except for fruitsthat ripen during storage

The respiration heat rate to be removed from a storage equals

Q/Δt = Σmi· qresp,i for i products

.

.

Table: ÇB98


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Example: cooling of bananas A storage room contains 18 pallets

with each 24 boxes of bananas that are being cooled, see the data →

The specific heats of bananas and box material are 3.35 and 1.70 kJ/(kg· K), respectively

Calculate the required heat removal from the storage

Answer: mbananas = 8208 kg; mboxes = 994 kg Qresp = 8208 kg· 0.3 W/kg =2462 W

Energy given off for cooling at 0.2 K/h equals m· cp· ΔT/Δt = 1619 W for the bananas, 94 W for the boxes

Total heat removal = 4175 W

.

Source: ÇB98



6.3 Meat


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Meat refrigeration Meat carcasses must be

rapidly cooled to ~1.7°C after slaughter

Temperature, humidity and air motion must be controlled to preventshrinkage, toughening and colour changes

Evaporative cooling removes2% of the 70% moisturemass

Meat can be stored less thana week at a ~ -2 °C

Freezing occurs between -1 and -4°C

↑ Typical beef carcass chilling and storage at 0°C

Figure &Table: ÇB98

Meat refrigeration


Pictures: HTW08


Meat products storage; Poultry Frozen meat can be stored for

several months: see table →Note that pork can be stored shorter due to its different fat structure

Poultry must be chilled rapidly to below 4°C as to control microbial growth; as a result of this air chilling (with cold air) has been replaced by the much faster immersionchilling using tanks of slush ice (risk ofSalmonella contamination !)

Table & picture: ÇB98


Poultry (continued)

As poultry meat is very perishable, the lowest possible storage temperature →maximises shelf life

Freezing of poultry should be done as fast as possible, at -23 ~ -40°C, 1 ~ 5 m/s air;

← freezing time (to -4°C) vs.temperature; and thermal propertydata →

Table & pictures:

ÇB98


Example: chicken in a box A box of 50 kg chicken at 6°C is to be

cooled to -18°C in a freezer; determinethe heat to be removed. Specific heat for chicken is 3.32 and 1.77 kJ/(kg· K) above and below freezing, respectively, and the freezing heat is 247 kJ/kg, at -2.8°C. For the box with mass 1.5 kg the specificheat is 1.4 kJ/(kg· K)

Answer: chicken +6 →-2.8°C: Q = m· cp· ΔT = -1461 kJ; freezing chicken at -2.8°C: Q = m· Δhfreeze = -12350 kJ; coolingfrozen chicken → -18°C: Q = m· cp· ΔT = -1345 kJ; cooling box +6 → -18°C: Q = m· cp· ΔT = -50 kJ

Total heat to be removed = 15206 kJ (of which 81 % is the freezing heat; the box can actually be ignored.....)

Source: ÇB98


Food properties

Source: ÇB98



6.4 Equipment


Hydrocooling

Picture: D03

Hydrocoolingsystems removefield heat from fresh crop usingchilled water

Especially usefulfor tree fruitsand other low-moistureproducts like peas, corn, carrots

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Forced-air cooling, cold rooms

Pictures: D03

Forced-air coolingsystems are simple, cheap and can be sanitary; can be quite large

Used for coolingberries, high moisturevegetables like melons, grapes, spinach, ...

Dehydration is a risk


Hydro-aircooling, Evaporative cooling

Developed for being ableto apply hydrocooling to vegetables stacked on pallets or in fiberboard

Pictures: D03

An evaporative coolerallows for lowering the temperature to near the wet bulb temperature at high humidity


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Blanching Blanching is a method of

stopping and destroyingenzymatic and micro-organism activity by spraying with water at 70-105°C Especially useful before

packing in cans or freezingCan also be used for meat

productsHowever, water-soluble

flavours can be lost; changeof taste and loss of nutritional value Pictures:

D03


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Tunnel freezers

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Tunnel freezers are suitablefor rapid freezing of small products like shrimps, berries, potato products; some units can handlecomplete meat carcasses

Primarily used for packedproducts

Temperatures down to - 40°C, air speeds up to 8 m/s

Picture: D03


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Spiral freezers Spiral freezers are very flexible for

packaged an unpackaged products Almost anything can be handled Very good contacting and heat

transfer

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Contact freezer


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Alternatively, wrapped products canbe frozen by immersion in a cold liquid,(NaCl, CaCl2, glycol brine), or spraying liquid N2 or CO2



6.5 Ice making


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Ice making /1 Block ice manufacture →

– Water in molds, immersed in a brine which is kept cold by a refrigerant. Gives ice blocks of25 ~ more than 150 kg. Can take up to 24 hours

Shell ice manufacture →– Freezing a falling film on the

inside and outside of a steel tube; after 8~15 min the 3-20 mm ice layer is removed by hot gas defrost

Flake ice manufacture– Thin layers of ice are frozen on

tubes or drums and are scrapedoff at 1.5 – 3 mm thickness

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Ice making /2

Tube ice manufacturing →– Freezing a falling film on the

outside or inside of a tube, and discharging ice cylinders by usinghot discharge gas

Plate ice manufacture– An ice layer is built on a flat

surface; after 12-45 min. a 6-20 mm thick plate is produced, dischargedusing a hot gas

Slush, slurry ice →– Ice from a brine or seawater

solution which forms ice crystalswhen supercooled

– Can be pumped

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Ice use...


B&W pictures: HTW08

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Sources #6

CB98: Y.A. Çengel, M.A. Boles “Thermodynamics. An Engineering Approach”, McGraw-Hill (1998) Chapter 3.10

C02:Y.A. Çengel “Heat transfer. A practical approach” 2nd ed. McGraw-Hill (2002) pp. 238-250

D03: İ. Dinçer “Refrigeration systems and applications” Wiley (2003)

HTW08: G.F. Hundy, A.R. Trott, T.C. Welsh “Refrigeration and air conditioning 4th ed. Butterworth-Heinemann (2008)

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refrigeration (kylteknik)users.abo.fi/rzevenho/ref18-oh6.pdf · Åa 424519 refrigeration /...

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