Effects of freezing on vegetable/fruit quality : Ruud van der Sman

Transcription

1 Effects of freezing on vegetable/fruit quality : Ruud van der Sman

2 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 2

3 Advantages of freezing Long term food preservation method (~ 1 year) Low storage temperature (T ~ -20 o C) => 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 3

4 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 freezing via freeze-concentration Potential loss of texture (softening, drip loss, cracking) => further focus on this quality loss 4

5 Impact of freezing on quality Mechanical stresses due to ice expansion (freeze-cracking) Cell/texture disruption due to ice crystal growth Soft fruits are especially vulnerable Drip loss after thawing Frost formation, clumping (Voda et. al, 2012) Freezing of carrots 5

6 Physics of freezing: steps Displayed in state diagram (for sucrose=model for fruit): Nucleation = start of ice formation ; generates latent heat => T rise Ice growth: T decrease + freeze concentration ; until storage temperature Coarsening of ice crystals ; T>T glass still slow diffusion / chemistry 6

7 Nucleation Start of ice formation often not directly at freezing line => supercooling (T<freezing point) Energy barrier must be taken inherently random process For pure water nucleation at T=-40 o C Happens earlier if impurities present or shocks In fruit & vegetables sufficient impurities present Nucleation often few degrees below freezing Final number of ice crystals partly depends on nucleation 7

8 Ice crystal growth After nucleation: initial fast growth Release 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) (Sman, IJHMT,2016) T i =-2.8 o C ; T e =-3.6 o C T i =-2.8 o C ; T e =-4.4 o C T i =-2.8 o C ; T e =-6.0 o C 8

9 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 Frozen starch gels (Charoenrein,2007) 9

10 Coarsening 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 10

11 Texture + plant tissue 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) 11

12 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) 12

13 Effect processing on texture - 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) For fruit, NO blanching Avoid the loss of turgor! Texture softening: - Loss of turgor - Separation of cells (loss of pectin) - Softer Cell Wall Material (CWM) 13

14 Functionality of blanching Inactivate enzymes (freeze-concentration would enhance their activity) Enhance colour Enhancement of texture (Long-time Low-temperature) (Pectin solubilisation, crosslinking with Ca 2+ ) Loss of turgor 14

15 Production Chain Approach for optimum - 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 15

16 Critical factors in concept map (1) Acclimation 16

17 Critical factors (2) 17

18 Critical factors (3) Brining 18

19 Novel processing (Osmotic) dehydration (Dehydrofreezing) Ultrasound High-pressure freezing 19

20 (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 temperature Less ice will be formed (energy/time/freezing rate) Smaller ice crystals => improved texture 20

21 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 21

22 High-pressure freezing Instantaneous nucleation of ice at pressure release Specialized equipment required, no large volume Expensive Batch process!! Vegetables & fruit processing is continuous in industry 22

23 Useful alternatives + intensification (1) Alternative for fruit 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? 24

24 Useful alternatives + intensification (2) Alternative for vegetable 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) 25

25 Conclusions Minimization of textural damage requires a chain approach: tune every processing step Novel processing often difficult to combine with conventional process, except for air-dehydrofreezing For fruit possible alternative with process intensification: osmotic dehydration + immersion freezing + ultrasound 26