FREEZING AND QUALITY OF FROZEN VEGETABLES - ISEKI-Food
Transcript of 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)
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
Prof.ssa Paola Pittia [email protected]
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
Vegetables and processing
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….)
- ….
Vegetables and processing
Prof.ssa Paola Pittia [email protected]
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….)
- ….
Vegetables and processing
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)
Prof.ssa Paola Pittia [email protected]
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.
Prof.ssa Paola Pittia [email protected]
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, ....
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
Prof.ssa Paola Pittia [email protected]
– 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
Prof.ssa Paola Pittia [email protected]
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
Prof.ssa Paola Pittia [email protected]
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
Prof.ssa Paola Pittia [email protected]
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
Freezing: air freezing
Prof.ssa Paola Pittia [email protected]
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.
Freezing: air freezing
Prof.ssa Paola Pittia [email protected]
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
Freezing: air freezing
Prof.ssa Paola Pittia [email protected]
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
Prof.ssa Paola Pittia [email protected]
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
Prof.ssa Paola Pittia [email protected]
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
Prof.ssa Paola Pittia [email protected]
CONS
– Irreversible changes of the structural properties and texture
– Loss of nutrients due to exudates (upon thawing)
Freezing and quality of frozen vegetables
Prof.ssa Paola Pittia [email protected]
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
Prof.ssa Paola Pittia [email protected]
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
Prof.ssa Paola Pittia [email protected]
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…….
Freezing and quality of frozen vegetables
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
Freezing and quality of frozen vegetables
Prof.ssa Paola Pittia [email protected]
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...)
Freezing and quality of frozen vegetables
2. Process
Prof.ssa Paola Pittia [email protected]
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
Freezing and quality of frozen vegetables
2. Process: pretreatment
Prof.ssa Paola Pittia [email protected]
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)
Freezing and quality of frozen vegetables
2. Process: optimisation of process parameters
Process Product
Prof.ssa Paola Pittia [email protected]
Packaging
Thermal insulator
Light barrier
Modified atmopheres
Freezing and quality of frozen vegetables
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
Freezing and quality of frozen vegetables
2. Post-process management
Cold-chain
Traceability!!!!!
Freezing and quality of frozen vegetables
2. Post-process management
- High Pressure freezing
- Dehydro-feezing
- Uso of cryoprotectors
Anti-freezing proteins
Sugars (trehalose), salts
Freezing and quality of frozen vegetables
3. Process Innovation
Prof.ssa Paola Pittia [email protected]