effects of thawing process on fish

1. MERITS AND
DEMERITS OF
THAWING OF FISH
Presented by: group 6

2. Thawing
• changing a product from frozen to unfrozen
• involves transferring heat to a frozen product
• ice crystals are converted back to water occurs at -1⁰C
Thawing time
• time required to melt all the ice in the frozen seafood

3. Thawing process
1.Tempering phase:
• temperature is increased until melting of ice is accelerated
• Short phase due to low specific heat capacity and high thermal conductivity
2. Latent zone phase
• The temperature of the product is almost constant
• majority of the supplied energy is used to melt the ice
3. Heating phase
• Product’s thermal capacity is comparatively low
• temperature increases rapidly as a result of further energy supply

4. Freeze-thaw cycle
• number of times freezing and thawing is done
• repeating thawing, refreezing and rethawing effects the quality

5. Methods of thawing
Conventional methods of thawing
• Refrigerator Thawing
• Cold Water Thawing
• Microwave Thawing
• Thawing at room temperature
Novel methods of thawing
• High pressure thawing
• Ohmic thawing
• Acoustic thawing

6. Merits of thawing of fish
• Yield increase
• Less cooking time
• Tenderness
• Textural quality

7. Yield increase
how much of the total raw material is converted to saleable
products with as high quality as required

8. Less cooking time
• properly thawed fish is properly cooked
• post processing become easy

9. Tenderness
• increases with freezing and thawing
• correlated to the length of frozen storage
mechanism
• breakdown of muscle fibres by enzymatic action during proteolysis
• loss of structural integrity caused by ice crystal formation
• formation of large, extracellular ice crystals disrupts the physical structure,
largely breaking myofibrils apart and resulting in tenderisation

10. Feasible processing operation
• thawed fish makes the processing operations feasible
• Cutting, grounding, slicing, filleting, canning, etc become easy if
the fish is thawed

11. Textural quality
• texture is close to that of fresh and unfrozen food
• may be changed by the freezing process and yet result in
acceptable product

12. Demerits of thawing
WEIGHT
LOSS
DRIP LOSS WATER
HOLDING
CAPACITY
COLOUR
CHANGES
LIPID
OXIDATIO
N
THAW
RIGOR

13. Weight loss
• ice crystal melts into water and leeches out
• Thus, effect the weight certainly lower than the original fish

14. Drip loss
• form of moisture migration
• soluble nutrients are lost
Mechanism
• water is removed from its original location in the product and
collected elsewhere in the form of ice crystals
• this water may or may not be reabsorbed into its original location
• If not reabsorbed it leaches out of in the form of drip loss

15. Water holding capacity
• the ability muscle tissue and foodstuff based on muscle tissue must
keep/withhold the original amount of water during a specified
treatment
• WHC is reduced as the water content increases
• important factor considering the juiciness and taste of fresh food
and for drip loss related to thawing
• reduced ability to absorb water is linked to hydration of proteins
and destruction of ultra structure of the cells

16. Lipid oxidation
• since chemical reactions can occur during frozen storage that
initiate primary lipid oxidation (peroxidation) in the meat
• This lead to radical secondary lipid oxidation upon thawing

17. Colour changes
• non-enzymatic and enzymatic browning
• mainly caused by a strong increase in lightness and decrease for
both redness and yellowness
• Changes in myofibrillar and sarcoplasmic proteins due to pressure
could induce meat surface changes and consequently colour
modifications

18. Thaw rigor
• Rigor after thawing
• Seafood frozen pre-rigor enters a very strong rigor mortis when
thawed at high temperatures, resulting in gaping (breaking of
connective tissue between the muscle segments) and loss of drip.

19. Precautions to prevent thaw rigor
• to extend the cold storage time
• least eight weeks at a temperature of -28⁰C or lower
• thawed slowly at room temperature

20. Controversial effect
• Microbial count
• Nutritional availability

21. Conclusion
• The quality of frozen food is closely related to freezing and thawing
processes. During thawing process foods are subject to damage by
chemical and physical changes and microbial attack. Quick thawing
at low temperature avoiding notable rise in temperature and
increased dehydration of food is desirable. Longer the thawing
treatment time, the more adverse effect will be. Nutritional quality
reduction due to leaching of soluble proteins, high energy
consumption and large quantities of loaded waste water are also
other disadvantages of conventional thawing