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Week 5: Low-Temperature Processing - Coggle Diagram
Week 5: Low-Temperature Processing
Chilling: Food temperature is reduced to temperature above freezing point. Usually in range between -1 to 8°C.
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Objectives:
To slow down deterioration changes and microbial growth
To extend shelf life of perishable foods
retards deteriorative activity. The reaction rate is usually agreed to be lowered by half by reducing the temperature by 10⁰C.
Chilling have minimal effects on the sensory characteristic and nutritional properties of foods.
use of mechanical refrigeration system: enables the transfer of heat from the cooling chamber to location where it can be discarded rapidly.
Ammonia is one of the common refrigerant used in industrial plant. It offers an exceptionally high latent heat of vaporization and is non corrosive to iron and steel.
Rate of heat transfer mainly due to convection to surface of the food and by conduction within the food. Medium of heat exchange is usually air whereby heat is extracted from food and gives up to the refrigerant.
Freezing: product temperature is lowered to -18oC. Food is reduced to below the freezing point and proportion of water undergoes a change of state from water to ice.
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Stages in Freezing Process :check:
Removal of Heat (Pre-freezing)
Conversion of Water to Ice (freezing zone)
Further cooling of Frozen Food (Post-freezing)
Temperature of food drops sharply and rapidly to the ultimate temperature for storage
A proportion of water may still remain unfrozen which is dependent on the composition of food/storage temperature.
Temperature remains almost unchanged until most of the water turns to ice (0 – 5oC) i.e. short plateau.
Stage whereby crystallization of water occurs
Latent heat of freezing is removed
Temperature of food drops rapidly to just below freezing point of water (0oC)
Initial freezing temperature varies with product and is dependent on the water content
Sensible heat is being removed from the product
Thermodynamics of Freezing: Ice Crystal Formation
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Nucleation ~ Initiation of freezing and involves the formation of small nuclei which will act as “seeds” for growth of ice crystal.
Homogenous Nucleation
Happen only in pure water
Random accumulation of a sufficient number of water molecules
Heterogeneous Nucleation
Formation of nucleus around suspended particles or at a cell wall
Occurs mostly in food and usually take place during super cooling
Crystal Growth ~ generally more rapid than nucleation. The rate of crystal growth is dependent on:
Rate of heat removal (esp. at end of freezing whereby solutes become more concentrated)
Temperature of freezing medium
Diffusion rate of water molecules from unfrozen solution to crystal surface
Estimation of Freezing Time: time required to reduce the initial product temperature to a pre-determined final temperature at its thermal centre :check:
Factors
Size/shape of product (distance that heats need to travel)
Thermal conductivity of food material
Area of food surface available for heat transfer
Temperature difference between food and freezing medium
Insulating effect of boundary film of air that surrounds the food
Type of food packaging material in case of packaged food
Freezing System: products are exposed to cooling medium for sufficient time to remove both the sensible and latent heat of fusion from the products
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Removal of sensible and latent heat of fusion from product will cause an reduction of production temperature and conversion of water from liquid to solid (ice). Generally, approx. 10% of water remains in liquid state at the storage temperature of foods
2 types
Direct contact systems
Examples: Air blast freezing, cryogenic freezing, immersion, fluidised bed freezing, individual-quick-freezing (IQF)
Indirect contact systems:
Examples: Plate freezing, scraped surface freezing
Rate of movement of ice front:
Slow freezing (< 0.2 cm/h): e.g. Still air freezing, cold store;
Quick freezing (0.5 – 3 cm/h): e.g. Air blast, plate freezing;
Rapid freezing (5 – 10 cm/h): E.g. Fluidised bed freezing;
Ultra-rapid freezing (10 – 100 cm/h): E.g. Cryogenic freezing
Effects of Freezing Rate :check:
Slow Freezing: food contains few large ice crystals
Few nuclei form so ice crystals can grow extensively
Maximum dislocation of water (drip loss)
Ice crystals can be found mainly in extracellular locations because it is the extracellular fluid that initiates crystallisation
Affects food quality significantly
Shrunken appearance of cells in frozen food
Fast Freezing: food contains numerous small ice crystals
Many uniform nuclei formed so ice crystals is much smaller and each can grow to a limited extend
Minimum dislocation of water (less drip loss)
Ice crystals can be found mainly in intercellular because extracellular spaces are small
Food quality is more superior than slow freezing
Frozen food appearance similar to original unfrozen appearance
Effects of Freezing:check:
Changes During Freezing
Ice Crystals Damage
Animal & plant tissues have different resistances to freezing damage. Animal structure tends to be more fibrous and flexible than plant/fruit cells so instead of breaking it is separated resulting in lesser damage
Slow freezing results in large extracellular ice crystals that can cause physical rupture and separation of cells
disrupt emulsion/ starch retrogradation from slow freezing
Minimal effects on pigment, flavours or nutritional components.
Concentration effects : Concentration of solutes (e.g. minerals / salt) in remaining water can cause deterioration (e.g. denature protein/ break emulsion)
Objectives: to lower food temperature to slow down biochemical, enzymatic activity and microbial growth. It also results in reduced water activity. Furthermore, it has significant physical effect on food.
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Due to the presence of substances dissolves in water it does not freeze at one temperature. Generally, it freezes over a range of temperatures.
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Selection of Freezing Equipment :check:
Factors
Rate of freezing required
Size/shape of food/packaging
Packaging material
Batch/continuous operation
Scale of production
Product range
Capital/operating cost
Freeze Drying: drying of an already frozen product in a vacuum below the triple point of water (611 Pa and 0.01oC) so that the ice formed will turn directly into vapor without passing through the water stage.
Effects of Freeze Drying
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Dried product has light and porous structure which facilitates rehydration (4-6 times higher than air-dried).
hygroscopic and prone to oxidation.
Potential denaturation of proteins due to pH changes and concentration of solutes during freezing
Freeze Drying Process
Freezing : Most critical as would affect final quality
Sublimation drying (primary drying phase):
Desorption drying (secondary drying phase):
Remove unfrozen water molecules remaining after primary drying.
Temperature is this stage is raised higher than in the primary drying to vaporize the water molecules. Sometimes pressure is also lowered.
Pressure is lowered through usage of high vacuum and heat is applied to provide energy needed for sublimation of ice.
Remove about 95% of the water present in the food but a very slow step ranging from few hours to two days.
Condensation of the sublimed water vapor also happens in this stage
Freeze foods rapidly (e.g. blast freezer or cryogenic freezing) to avoid the formation of large ice crystals,