Effects of freezing on vegetable/fruit quality

02-02-2018: Ruud van der Sman

Overview

Pro and Cons of freezing

(Bio)physics + chemistry of freezing plant tissue

Textural quality issues

Processing chain for frozen foods: functionality

Conceptual map: processing steps + quality

Novel processing: additional functionality

Solution strategies: chain approach

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Advantages of freezing

Long term food preservation method (~ 1 year)

Low storage temperature (T ~ -20oC) =>low microbial, metabolic and enzymatic activity

Better quality retention than canning/drying

If frozen directly after harvest, nutritional quality can be better than freshly cooked

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Disadvantages of freezing

Specialized transport/storage required

High energy usage during freezing & storage(but food loss has higher impact on sustainability)

Dehydration / frosting / clumping during storage

Color and nutrients degradation during freezingvia freeze-concentration

Potential loss of texture(softening, drip loss, cracking)

=> further focus on this quality loss

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Impact of freezing on quality

Mechanical stresses due to ice expansion(freeze-cracking)

Cell/texture disruption due to ice crystal growthSoft fruits are especially vulnerable

Drip loss after thawing

Frost formation, clumping

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(Voda et. al, 2012)

Freezing of carrots

Physics of freezing: steps

Displayed in state diagram (for sucrose=model for fruit):

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Nucleation = start of ice formation ; generates latent heat => T rise

Ice growth: T decrease + freeze concentration ; until storage temperature

Coarsening of ice crystals ; T>Tglass still slow diffusion / chemistry

Nucleation

Start of ice formation often not directly at freezing line=> supercooling (T<freezing point)

Energy barrier must be takeninherently random process

For pure water nucleation at T=-40oC

Happens earlier if impurities present or shocks

In fruit & vegetables sufficient “impurities” presentNucleation often few degrees below freezing

Final number of ice crystals partly depends on nucleation

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Ti=-2.8oC ; Te=-3.6oC Ti=-2.8oC ; Te=-4.4oC Ti=-2.8oC ; Te=-6.0oC

Ice crystal growth

After nucleation: initial fast growthRelease of latent heat, T increases till freezing line

Subsequently T follows freezing line

Ice morphology depends on:freezing rate + initial solute concentration

Freezing rate depends on size, coolant temperature,heat transfer coefficient (type of coolant, fluid flow)

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(Sman,

IJHMT,2016)

Freeze-concentration

Ice rejects solute: freeze concentration

Substrate concentration increases =>higher enzyme activity (PPO: browning)

Salt/pH concentration increases: protein aggregation ; cell membrane damage

Enhancement starch retrogradation

Irreversible compaction of cell wall material

9Frozen starch gels (Charoenrein,2007)

Coarsening

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• System tries to minimize surface area of ice crystals:- Large crystals grow at expense of smaller ones- Crystals become rounder

• Larger compaction of cell wall materials

Texture + plant tissue

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Texture / Rigidity of fresh fruit & vegetables via:

- Turgor

- Rigidity of cell walls

Solutes in vacuole ; encapsulated by membrane

impermeable to solutes, but not to water

Attracts water from apoplast (extracellular)

Cell swelling

Stretching of cell wall

Enhanced pressure like inflated balloon

= turgor pressure

Loss of texture rigidity:

- Cell membrane integrity is lost

- Softening of cell wall (blanching)

Damage to texture via freezing

Solute-concentration damage (freeze-concentration)high salt concentration: impermeability cell membrane

Dehydration damage:At slow freezing ice forms extracellular,Cell shrinks due to extraction of water, if membrane intact !!=> Membrane buckles and ruptures

Mechanical damage:- Spearing of membrane by ice crystals (like needles)- Freeze cracking (expansion of ice during fast freezing)- Irreversible deformation of cell wall (compaction)

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Effect processing on texture

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- Blanching:

Loss of turgor

Solubilization of pectin

Relaxation of CWM (cell walls)

Creation of extracellular water

- Freezing:

Growth of ice crystals

Compression of cells

Extra crosslinks in CWM

- Thawing:

Partial reswelling of CWM

Melt water in extracellular space

(=> drip loss)

Texture softening:

- Loss of turgor

- Separation of cells (loss of pectin)

- Softer Cell Wall Material (CWM)For fruit, NO blanching

Avoid the loss of turgor !

Functionality of blanching

Inactivate enzymes(freeze-concentration would enhance their activity)

Enhance colour

Enhancement of texture (Long-time Low-temperature)(Pectin solubilisation, crosslinking with Ca2+)

Loss of turgor

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Production Chain Approach for optimum

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- Consider effects of all steps in processing chain

on final product quality => concept map

- Alternative processing steps

- Combine them: process intensification

- Evaluate alternative processing:

Ultrasound, Dehydrofreezing,

High-Pressure Freezing

Critical factors in concept map (1)

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Acclimation

Critical factors (2)

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Critical factors (3)

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Brining

Novel processing

(Osmotic) dehydration (Dehydrofreezing)

Ultrasound

High-pressure freezing

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(Osmotic) dehydration before freezing

Either via immersion in sugar/salt solution,or air drying

For osmotic dehydration, before blanching !(no blanching for fruits)

Water loss, solute (sucrose/salt) gain(HMW solutes, less penetration, lower osmotic pressure):

Change of freezing and glass temperatureLess ice will be formed (energy/time/freezing rate)

Smaller ice crystals => improved texture

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Ultrasound

Promotes nucleation: allows good control(cavitation bubble=nucleation site)Ice is fragmented=secondary nucleation site

Requires immersion freezing (in fluid)

Acoustic streaming: enhancement of heat transfer ?

Generates heat too => optimum in power / time

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High-pressure freezing

Instantaneous nucleation of ice at pressure release

Specialized equipment required, no large volumeExpensive

Batch process !!

Vegetables & fruit processing is continuous in industry

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Useful alternatives + intensification (1)

Osmotic dehydration + vacuum impregnation(additional ingredient in brine, p.e. ascorbic acid)

Use same fluid for immersion freezing

Can be combined with ultrasound for nucleation

Crust of solute should be washed away ?

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Alternative for fruit

Useful alternatives + intensification (2)

Cold acclimation after harvest => natural anti-freeze

Water blanching + vacuum impregnation(additional ingredient in brine, p.e. ascorbic acid), or

Hot air blanching + drying (for dehydration)

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Alternative for vegetable

Conclusions

Minimization of textural damagerequires a chain approach: tune every processing step

Novel processing often difficult to combine withconventional process, except for air-dehydrofreezing

For fruit possible alternative with process intensification:osmotic dehydration + immersion freezing + ultrasound

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