ERT 426 FOOD ENGINEERING

CHAPTER 7:

FREEZING PROCESSES

Low Temperature Food Processing

and Preservation

Low Temperature Food Preservation

Freezing and cold storage is one of the oldest

methods of preservation

Low temperature can retard chemical reactions as

well as the activity of food enzymes

It can also stop the growth/slow down activity of

microorganism

Low Temp. Preservation: Benefits

• Preservation of food without any adverse effects

on the nutritional values and flavour, colour and

textural characteristics

• Control of the rate of chemical/enzymatic

changes in food due to spoilage organism and

endogenous enzymes as in aging of beef

• Control of the growth & metabolic activity of

starter cultures of desirable food microorganism as

practiced in cheese ripening & aging of wines

Low Temperature Preservation: Benefits

Enhanced ease and efficiency of unit operation

such as peeling for canning, as also cutting and

slicing of bread

Reduced loss in flavour & associated changes

during extraction of juice from fruits

Ease of precipitation of waxes from edible oils

Increased solubility of carbon dioxide in water

used for aerated drinks

Low Temperature Preservation:

Methods

• Low temp. processing & preservation methods –

refrigeration & freezing

• Refrigeration & freezing are distinguish based on

temp. of operation

• Refrigeration means storage at temp. above

freezing of water in the food (16o C to -2.2 o C)

• Refrigerators usually operate 4-7 o C

• Pure water freezes at 0o C and most food will not

begin until -2.2 o C/still lower temp is reached

Low Temperature Preservation:

Methods

Refrigerated/cool storage will preserve

perishable/fresh foods for days or weeks

depending on the type of food

Freezing/frozen storage refer to storage at temp

where the food is maintained in solid frozen

condition, generally at -18o C . Frozen storage will

preserve foods for month or even years

Microbial Activity at Low Temperature

• Food spoilage microorganism grow at temp > 10o

C while some food poisoning organism grow slowly at 3o C

• Psychotropic microorganism grow slowly between 4.4 0 C and – 9.4o C provided the food is not solidly frozen

• Complete death of all microorganisms does not occur merely due to low temp. and when food is thawed there can be rapid multiplication of microorganism

Factors Influencing Changes Occurred in Food

Factors of Importance in Refrigerated Storage

Refrigeration

Refrigeration and Cool Storage

Gentlest method of food preservation, no adverse

effect on taste, texture and nutritive value

Refrigeration (perishable/fresh) should start

immediately after harvest/slaughter and should be

maintained throughout transportation, warehousing

and storage prior to ultimate use

Refrigeration and Cool Storage (cont)

• Refrigeration influence effect agricultural and marketing practices and sets to economic climate of the food industry

• Control of prices during different seasons is also possible

• Refrigeration

• Is the removal of heat out of a body

• Process: cold nitrogen gas volatilizing of liquid nitrogen is allowed to pass over the food product to quick cooling

Factor influencing changes that occur in

food

Growing conditions & varieties of plant

Feeding practices of animals

Harvest & slaughter

Sanitation

Damage to tissues

Mixture of foods in the storage

Too low temperature which

Factor of Importance in Refrigerated

Storage

• Control of low temperature

• Relative humidity and air circulation

• Composition of atmosphere in the storage chamber

• Food variability (to type of food)

• Fruits and vegetable will produce heat due to

respiration

• Amount of heat expressed in term of British thermal unit

• Most food store best at refrigerated temp when air

humidity is in the range 80-95%

Freezing Characteristics of Foods

Factors Affecting the Quality of Frozen Food

Freezing

Freezing and Frozen Storage

Proper freezing preserve food without major

changes in size, shape, texture and flavour of a

variety of foods of plant and animal origin

Frozen foods require only a single thawing

operation before eating

Freezing Characteristics of Foods

Freezing (unless properly controlled) can disrupt

food texture, break emulsions, denature proteins

and cause undesirable physical and chemical

changes

Freezing temp. depend on their different

compositions and solid content

Factor Affecting the Quality of Frozen

Food

5 factors are importance in the maintenance of the

quality of foods in frozen storage

Solute concentration effects

Ice-Crystal Size

Rate of freezing

Final Temp

Intermittent thawing

Solute Concentration Effects

Maintaining the quality of most food

Food must be frozen to a solid/nearly so to

maintain good quality during frozen storage

A partially unfrozen are will deteriorate with

respect to texture, colour, flavour and other

properties

Ice- Crystal Size

Freeze rapidly it forms crystal of ice

Rate of freezing is slow, the ice crystal size is large

and clusters are also formed leading to physical

rupture of cells

Rate of Freezing

Rapid/instantaneous freezing produces ice crystals

of small size and also minimizes concentration

effects of solutes by decreasing the time of contact

between solutes and food tissues and other

constituent

Final Temp

Maintain of final temp to an accuracy of ± 1.0o C

is important

Intermittent Thawing

Quick final thawing is better than slow thawing

Freezing Systems

3 methods of freezing are employed in commercial

practice;

Air freezing

Indirect contact freezing

Plate freezer

Air-blast freezer

Direct contact freezing

Air blast

Immersion freezing

Air Freezing

Cold air is used with diff. velocities

Cold air between -15 to -23 o C

Quick freezing is done by air blast freezing

Freezing air at -18 to -34 o C is blown across the

foods/fluidized bed freezer

Indirect Contact Freezing

Food/food package does not come into contact

with the refrigerant

The food/food package is brought into contact with

a cold surface maintained at temp. in the range of -

18o C to -45 o C by a refrigerant

Direct Freezing

Food is immersed in the refrigerant/sprayed with

the refrigerant

Effect of Freezing on Constituent of

Food

• Not affect nutritive value

• Enzyme activity is only retarded by freezing temp.

• Hence control of enzyme activity is achieved by

heat treatment (blanching) prior to freezing &

storage in the case of vegetable and fruit

• Enzyme activity is simulated in the super cooled

water and the reaction rate is greater than in

crystallized water at the same temp.

Hence frozen storage even at -9o C permit severe

damage to the quality of the food both in the lost of

nutrients and appearance

Long term storage at -6o C yields unacceptable

food

Fats and oils undergo oxidative deterioration in

frozen foods

Freezing may destabilize emulsions of oil-in-

water/water-in-oil

Plank’s Equation

Pham’s Method

Prediction of Freezing Time of Finite-Shaped Object

Freezing Time

Freezing Time

It is a key calculation in designing a freezing process

Three distinct period at any point of food during freezing – prefreezing, phase change & postfreezing

Frozen temperature of food

Water at below 0°C

Fruits and vegetable at -18°C

Food with higher fat content i.e ice cream at -25°C

Prefreezing period

•Temp decrease to freezing

point Super cooling (

Freezing Time

Freezing involves removal of both sensible and

latent heat.

Freezing of pure water exhibits sharp transitions

between the different freezing periods, whereas

with foods, the transitions are more gradual.

At the endpoint temperature for freezing foods, the

frozen food may still have some water present as a

liquid; in fact, up to 10 % water may be in liquid

state for foods frozen to -18°C.

Plank’s Equation

First and most popular equation in predicting freezing time

This equation describes only the phase change period of the freezing process

The limitations to Plank’s equation are related primarily to assignment of quantitative values to the components of the equation

Even with these limitations, the ease of using Plank’s equation has made it the most popular method for predicting freezing time

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Example 7.1

A spherical food product is being frozen in an air-

blast freezer. The initial product temperature is 10°C

and the cold air -40°C. The product has a 7 cm

diameter with density of 1000 kg/m 3 , the initial

freezing temperature is1.25°C, the thermal

conductivity of the frozen product is 1.2 W/(m K), and

the latent heat of fusion is 250 kJ/kg. Compute the

freezing time.

Solution to Example 7.1

Pham’s Method

Pham’s method can be used for finite-size objects of irregular shapes by approximating them to be similar to an ellipsoid.

Another advantage is that it is easy to use, yet it provides answers with reasonable accuracy

Assumption in developing this method:

The environmental conditions are constant.

The initial temperature, Ti, is constant.

The value for the final temperature, T c, is fixed.

The convective heat transfer at the surface of an object is described by Newton’s law of cooling

Ef for infinite slab = 1

Ef for infinite cylinder = 2

Ef for infinite sphere =3

Example 7.2

Recalculate the freezing time in Example 1, using

Pham’s method with the following additional

information. Final center temperature is 18°C, density

of unfrozen product is 1000 kg/m3 , density of

frozen product is 950 kg/m3 ,moisture content of the

product is 75%, specific heat of unfrozen product is

3.6 kJ/(kgK), and specific heat of frozen product is

1.8 kJ/(kgK).

Solution to Example 7.2

Solution to Example 7.2 (cont)

Solution to Example 7.2 (cont)

Prediction of Freezing Time of Finite-

shaped Object

Pham’s method can also be used to calculate

freezing times of other shapes such as a finite

cylinder, infinite rectangular rod, and rectangular

brick, which are commonly encountered in freezing

foods.

Pham’s equation, may be used for this purpose using

an appropriate value of the shape factor, Ef. In

order to calculate Ef, two dimensional ratios are

required, β 1 and β 2.

Example 7.3

Lean beef in the shape of a large slab with 1 m

length, 0.6 m width and 0.25 m thickness is to be

frozen in an air-blast freezer with a Biot Number of

2.5. Calculate the shape factor from the given

dimensions.

Solution to Example 7.3

Solution to Example 7.3 (cont)