FREEZING AND QUALITY OF FROZEN VEGETABLES - ISEKI-Food

FREEZING AND QUALITY OF FROZEN

VEGETABLES

Prof.ssa Paola PittiaFaculty of Bioscience and Technology for Food Agriculture &

environment

University of Teramo (ITALY)

[email protected]

Vegetables and modern eating habits

Important role of fruit and vegetables in the diet:

- nutrizional aspects (vitamins, mineral salts)

malattie da carenza Vit C (scorbuto)

- healthy quality (presence of bioactive compounds

with antioxidant activity)

The consumption is inversely correlated to the risk

of oxidative induced illnes

Positive effects on the delay and/or inhibition of

these illness are due to the consume of foods rich in

bioactive antioxidant compounds

A diet rich in vegetables decreases the tendency to become over-

weight

the World Health Organisation reccomends the consumption of at

least 400 - 500 g of vegetables/day

Vegetables (fruit and vegetables) modify their quality attributes and safety in a relatively short time after ripening and picking up due to:

- metabolic respiration

- spoilage

- enzymatic reactions

- chemical reactions

- physical processes (water loss, dehydration)

- microbial growth and production of toxins (moulds...)

- …….

Fresh vegetables: quality and stability

Prof.ssa Paola Pittia [email protected]

WHY?

In the past:

Preservation of food commodities of high nutritional importance due to:- Season

- Transport oversees....

- ….

Vegetables and processing


In the past:

Preservation of food commodities of high nutritional importance :- Seasons

- Transport oversees....

- ….

PROCESSED

VEGETABLES with a shelf-

life medium-to-long term,

at room/environmental

conditions

Marked changes of the

quality of the raw/fresh

product


HOW?

Dehydration, addition of water binding ingredients (salt,

sugars), acidification, fermentation, thermal treatments

WHY?

TODAY:

Preservation of food commodities, with high nutritional and healthy quality:- Consumption at time and place different from picking up

- Diversification

- Convenience (easy-to-use, easy-to-cook, easy-to-eat….)

- ….



HOW?TODAY:

Preservation of food commodities with high nutritional and healthy importance :

- Consumption at time and place different from picking up

- Diversification

- Convenience (easy-to-use, easy-to-cook, easy-to-eat….)

- ….


Mild technologies, Modified atmosphere

Use of low temperatures (chilling, freezing)

PROCESSED VEGETABLES

with a short-to-medium

shelf-life chilling or freezing

conditions

Limitated changes of the

quality attributes of the

raw/fresh product

In the past TODAY

1. Nutrizional value

2. Safety and Hygene

3. Shelf-life

4. Sensory quality (colour,

flavour, texture similar to

those of fresh/raw product)

5. ....

1. Safety and Hygene

2. Sensory quality (colour,

flavour, texture similar to

those of fresh/raw product)

3. Healthy quality

(antioxidant, anti-radical

activity)

4. Nutrizional value

5. Convenience

6. Shelf-life

7. ....

Quality of processed vegetables

Process that reduces the temperature of a food below

its freezing point (T< Tf); a portion of water in the

food undergoes a change in state to form ice

crystals (aw lowering)

Freezing

Preservation is achieved by:

- Low temperature (decrease of rate of reactions and processes)

- Reduced water activity due to ice formation & high

concentration of solutes in unfrozen water

- Pretreatments (blanching, in vegetables)


Preservation:

• Microbial growth inhibited – Pathogen growth is halted below -4oC

– Spoilage microorganisms don’t grow below -10oC.

• Chemical reaction rates are significantly reduced (every 10°C the reaction rate is halted).

Processing aid:– Freezing changes the texture and viscosity for further processing,

e.g. slicing meat products

Product definition:– Freezing defines some food products, e.g. ice-cream and frozen

desserts.

Freezing: AIMS

Hystory of freezing and frozen foods

– 1886: Birdseye, the man that had the idea to pack frozen

foods was born.

– 1922:Frozen food takes off

– 1946:Frozen food considered a luxury

– 1951:Sales start to grow

– 1955:The world famous fish finger

– 1963:A move to convenience food

- 1965:The era of the supermarket

– 1969:Peas and more peas

– 1970:Frozen food loses popularity

– 1974:Freezer sales revive the market

– 1983:Freezers and microwaves enter the kitchen

– 1990:Recession hits the country

– 1995:Food safety is big news

– 2000 and on: Frozen is the future

Hystory of freezing and frozen foods

The freezer of Birdseye

Birdseye realised that

freshly caught fish, when placed onto

the Arctic ice and exposed to the icy

wind and frigid temperatures, froze

solid almost immediately.

He also learned that the fish, when

thawed and eaten, still had all its fresh

characteristics. He concluded that

quick freezing of certain items kept

large crystals from forming,

preventing damage to their cellular

structure.


Frozen foods: the market today

The annual global production of various frozen foods is about 50 million

tons (plus 20 million tons of ice creams and 30 million tons of fish), with a

remarkable growth of 10 % every year.

The EU is still behind the USA as regards the consumption of frozen foods per

inhabitant

TODAY, frozen foods include

-“raw” vegetables (ready-to cook)

- Fresh meat and fish (ready-to cook)

-Bread and bakery products (ready-to eat)

-Formulated, cooked foods (ready-to eat)

-Ice-creams, desserts

-In food industry freezing is also used to preserve and store half-

products (fruit pieces, purees, concentrated fruit juices, ....

Frozen foods consumption

Fikin, 2003


Water is the basis for freezing foods.

– In addition to water, foods contain a lot of soluble

materials which slow down the movement of water

molecules, and hence the freezing occurs at lower

temperature.

– 1 g of soluble materials will decrease the freezing point by

1oC.

– Freezing points (temperatures):

Fruits and vegetable = 0/-5oC

Meat and fish = -1/-2oC

Freezing theory


– During freezing, heat is conducted from the interior of a food to the surface and is removed by the freezing medium.

– Sensible heat is first removed to lower the temperature of a food to the freezing point.

- In addition to the sensible heat, most foods have high specific heat and latent heat due to a large proportion of water.

– Therefore, a substantial amount of energy is needed to remove latent heat, form ice crystals, and hence to freeze foods

Freezing theory

Freezing theory

T

E

M

P

E

R

A

T

U

R

E

TfC

D

EFB

A

SENSIBLE HEAT

LATENT HEATEutectic

temperature

Supercoolingtime

G

Pure water

FoodTm

The freezing curve: change of the temperature of

water and a food product during freezing

AB: The food is cooled to below its freezing point which, the exception of pure water, is always below 0oC.

At point B, the water remains liquid, although the temperature is below the freezing point. This phenomenon is known as supercooling and may be as much as10oC below the freezing point.

BC: The temperature rises rapidly to the freezing point as ice crystals begin to form and latent heat of crystallization is released

CD: Heat is removed from the food at the same rate as before, but it is latent heat being removed as ice forms and the temperature therefore remains almost constant. The freezing point is gradually depressed by the increase in solute concentration in the unfrozen liquor, and the temperature therefore falls slightly. It is during this stage that the major part of the ice is formed.

DE: Crystallization of water and solutes continues.

FG: The temperature of the ice-water mixture falls to the temperature of the freezer.

Eutectic point is the temperature where there is no further concentration of solutes due to freezing, hence the solution freezes. Maximum ice crystal formation is not possible until this temperature is reached.

Freezing curves

Freezing theory

supercooling

Freezing point

20,0

70,040,0

85,0

97,0

98,0

Percent frozen

water

99,5

99,8

99,9

T

Freezing time

A glass is defined as a non-equilibrium, metastable, amorphous, disordered solid of extremely

high viscosity (ie., 10 e10 to 10 e14 Pa.s), also a function of temperature and concentration.

A glass is formed when a liquid or an aqueous solution is cooled to a temperature that is

considerably lower than its melting temperature (Tm) and is defined glass transition

temperature (Tg).

– In the formation of a glass, the disordered liquid state is immobilized into a disordered glassy

solid, which has the rheological properties of a solid but no ordered crystalline structure.

Thus, a food product stored below its Tg

is a “glass” with high stability

Freezing theory: glass transition (Tg) and

amorphous state

+ solidification and cristallisation of

lipids

Glass transition values of foods Fellows, P. J. 2002. 2nd Edition. Food Processing Technology. Woodhead

Food T’g (oC) Food T’g (oC)

Banana -35 Ice cream -31 to -33

Peach -36 Cheddar cheese -24

Tomato -41 Cream cheese -33

Fresh sweetcorn -15 Cod muscle -11.7

Fresh potato -12 Mackerel muscle -12.4

Frozen pea -25 Beef muscle -12

Frozen spinach -17 Pineapple juice -37

It means that, at conventional storage temperatures (-15/-18°C) many foods

are above their glass transition and, thus, not clompletely stable

Two main steps

1. Nucleation:

– Association of molecules into a tiny ordered particles sufficient tosurvive and serve as a site for crystal growth.

– It can be:

- Homogenous (pure water)

- Heterogeneous (most foods)

- Dynamic (spontaneous)

2. Crystal growth:

– The enlargement of the nucleus by the orderly addition ofmolecules. Crystal growth can occur at temperatures just belowmelting point while nucleation starts at lower temperature(supercooling).

– Heat transfer is most responsible for limiting the rate ofcrystallization due to the large amount of latent heat needed.

Freezing theory: cristallisation

Freezing rate and ice dimension

- slow temperature decrease

- low nucleation

- high cristal growth

few crystals, big dimensions

- quick temperature decrease

- high nucleation

- low cristal growth

many crystals, small dimensions

Freezing theory

Freezing theory

Effects on cell structure

characteristics

As high is the amount of frozen water, higher is the solute concentration in the unfrozen phase.

This leads to:

• – pH () denaturation effects on proteins

• – Viscosity ()

• – Surface tension (

• – Redox potential of the unfrozen liquor

• Cristallisation of solutes (es. lactose, salts)

Freezing: chemical effects

The effects become evident

- during long time storage

- as due to slow freezing

To favour the formation of intracellular- small size ice cristals, the technology of freezing could rely on:

1. Low freezing temperatures (costly approach) (process)

2. Enhanced surface heat transfer coefficients by increased refrigerating medium velocity and boundary layer turbulence, involvement of surface phase-change effects and less packaging) (equipment)

3. reduced size of the refrigerated objects (by freezing small products individually or appropriate cutting the large ones into minor pieces) (product)

Freezing: process aspects


Freezing Types:

1. Air Freezing- Sharp Freezing

- Air Blast Freezing

- Fluidized-bed Freezing

- Spiral Freezers

2. Indirect contact freezing

- Plate Freezing

3. Direct contact freezing

- Liquid Immersion Freezing

- Cryogenic Freezing

Freezing: equipment and process aspects


Sharp Freezing:

• Very cold room (-15oC to -29oC)

• Slow freezing (3-72 hours or more depending on the conditions and the size of a product)

• Lacks efficient design characteristics

• Non constant final quality.

• Dehydration may occur

• Uncommon in modern freezing operations (home applications)

Freezing: air freezing


Blast Freezing:

• Temperature: (-30oC to -40oC)

• Air velocity: 1.5 to 6.0 m.s-1

• Moderate rapid rate

• Vigorous circulation of cold air by means of large fans or by refrigerated coils

• Continuous vs. Batch operations

• Economical and capable of accommodating foods of a variety of sizes and shapes



Spiral belt Freezing:

Modified air-blast freezers in which a continuous flexible mesh belt is formed into spiral tiers.

- Spiral freezers require relatively small floor-space and have high capacity (for example a 50-75 cm belt in a 32-tier spiral processes up to 3000kgh-1).

• Other advantages: automatic loading and unloading, low maintenance costs and flexibility for different products.

• Used for a wide range of foods: pizzas, cakes, pies, ice cream, whole fish and chicken portions, raw and cooked meat patties, fish fillets, chicken parts, pizza and a variety of packaged products.



Fluidized-bed Freezing:

• Modification of air-blast freezing

• High air flow velocities: 2-5 m/s

• Bed depth: 2-13 cm

• Both are determined by food size and shape.

• Higher heat transfer coefficients, shorter freezing times, higher production rates (l0,000kgh-1) and less dehydration of unpackaged food than blast freezing.

• Limited to particulate foods



Plate Freezing:

• Advantages:– Good use of floor space

– Low operating cost

– Little dehydration of food

– High rates of heat transfer

– Food package keeps dimensions

• Disadvantages:– High capital costs

– Size limitations

– Package must be uniform of thickness

– Moderate low rate

Freezing: indirect contact freezing


http://foodtechinfo.com/foodpro/gas_techn

ologies/chill-freeze_-_plate/

Liquid Immersion Freezing:

• Direct immersion freezing

• Packaged or unpackaged foods frozen by immersion in or by spraying with a freezant that remains liquid throughout the process.

• Freezants: propylene, glycol, glycerol, sodium chloride, and mixtures of salt and sugars

• Used for canned citrus juice concentrate, poultry, fish and shrimp

• Rapid freezing

• Easily adapted to continuous operations

• Difficult to find freezing media with suitable properties

Freezing: Direct contact


Cryogenic agents may be sprayed on food or food may be imersed in cryogen.

– Most common refrigerants - not fluorocarbons

– Heat content -

• – Liquid nitrogen: 48% of the total freezing capacity (enthalpy) is taken up by the latent heat of vaporization needed to form the gas; 52% of the enthalpy is available in the cold gas

• – Carbon dioxide: freezing capacity (85%) is available from the subliming solid

Freezing: cryogenic freezing

PROS

– Maintenance of the nutritional and most of the sensory attributes of the fresh/raw product (flavour, taste)

– Shelf-life (fTemperature)

– Convenience

Freezing and quality of frozen vegetables


CONS

– Irreversible changes of the structural properties and texture

– Loss of nutrients due to exudates (upon thawing)



Start/evolution of....

Chemical

reactions

Enzymatic

reactions

Lipid oxidation

Condensation/

polymerisation

Activity of

hydrolytic and

oxidative enzymes

Quality of frozen vegetables during storage

Physical

processes

Ice cristal growth

Water migration

•Loss of color

• Loss of nutrients

• Flavor changes

• Color changes

•Flavor changes,

rancidity

• Loss of nutrients

• Color changes

•Texture changes

•Mechanical damage of

tissues

•Exudates upon thawing

•Dehydration

•Freeze burn

•Especially due to

temperature fluctuation

Upon thawing ....

Microbial growth

Freezing and storage under frozen state could cause the death for a

limited number of microorganisms

Pretreatments (blanching) could decrease the microbial counts but

not totally

Quality of frozen vegetables during storage

Carefully inspect any frozen products which have

accidentally thawed by the freezer going

of or the freezer door being left open

Shelf-life of frozen vegs


Aspects to be considered to high quality frozen products

1. Raw material (fruit and vegs) quality

2. Process/technology- Optimisation of process conditions

- Pretreatments

- Packaging

- Innovation

3. Post- processing management

- Cold chain

Freezing and quality of frozen

vegetables


Selection of fruit and vegs to be frozen

- Choice of varieties and cultivars with uniform ripening, high nutritional and healthy value

- High microbial and safety quality

- Industry specifications: - Resistance to be mechanically picked up and be processed (cleaning,

cutting, calibration) in high-rate-industrial plants

- Color (no changes due to low temperatures)

- Texture: lignin, fibers, cellulose, starch and other polysaccharides have a protective effect choice of fruit and vegs with high content of these components.

Examples:

1. peach " Pavie " is characterized by more thick cell membranes and less active pectolytic enzymes. Both these aspects make this peach cultivar good for freezing

2. Potatoes for freezing: choice of the varieties with high solid content…….


1. Raw materials

Resistance to processing

High resistence :

melon, kiwi, bluberry, chestnut, carrot, artichock, onion, haricot, potatoes, peas et poireau

Medium resistence pear, peach, apricot, haricot vert, fève, fennel, pepper, parsil, celeri et courge

Low resistence: strawberries, mûre, blackberry, asparagus, brocoli, cauliflower, champignon, aubergine, tomatoes, spinach, courgette



Pretreatments

Enzymatic Inactivation

BLANCHING

Treatment aimed to inactivate enzymes whom activity in fruit and vegs during

freezing and in frozen state could impair the quality of the product (=

peroxidase, lipoxigenase, pectinesterase, ...)

It is carried out by mainly applying heat (water, water vapour) at temperatures

below 100°C for short times

Innovative technologies: High pressure treatments, radiofrequency...)


2. Process


BLANCHING

Determination of the BEST time-temperature conditions for each

veg, to

-Inactivate enzymes (max residual activity POD: 5%)

-Low/no effects on quality of the product before freezing and/or

during frozen state


2. Process: pretreatment


Freezing Rate

Quick freezing

Individual quick freezing, IQF

Cryogenic fluids

Individual quick freezing of foods by

hydrofluidisation

and pumpable ice slurries

Determination of process parameters (product-

specific; variety-specific):

T freezing

T end freezing

Glass transition Temperature (Tg)


2. Process: optimisation of process parameters

Process Product


Packaging

Thermal insulator

Light barrier

Modified atmopheres


2. Process: post-treatment

Post-treatment

Cold-chain

Check and control of the continuity of the low

temperatures during the different phases of

production-storage-distribution and sell by

proper instruments


2. Post-process management

Cold-chain

Traceability!!!!!


2. Post-process management

- High Pressure freezing

- Dehydro-feezing

- Uso of cryoprotectors

Anti-freezing proteins

Sugars (trehalose), salts


3. Process Innovation


Documents

FREEZING AND QUALITY OF FROZEN VEGETABLES - ISEKI-Food