Freezing of Foods Flashcards
Name the two steps of ice crystal formation
- Nucleation
- Crystal growth
What occurs during heterogeneous nucleation
crystals grow on inhomogeneities in the surrounds e.g. scratches on a dish, cell organelles. Once crystals are grown crystal growth can occur
Why is heterogenous nucleation a problem
Crystals grown between cells, e.g potato cells will draw water from the surrounding tissues, deforming the cells and rupturing the structure. This results in low product quality
What is homogenous nucleation
Formation of small ice embryos in the interior of a substance
Why is homogenous nucleation hard to achieve
For spontaneous nucleation to occur we need a negative free energy but because of the Laplace equation, pressure is always acting on the ice embryos which allows them to melt again.
How can homogenous nucleation be achieved
Supercooling
Define: Supercooling
A process which occurs when heat is removed from a product so rapidly that the molecules do not have enough time to align themselves in the ordered structure of a solid. The food is cooled below its freezing point without becoming a solid.
How does the degree of supercooling affect the radius of the ice crystal
Crystal radius depends on the degree of supercooling. With a large degree of supercooling, small critical radius ice crystals can form throughout the food.
What is the affect of adding a solute on freezing temperature
- Freezing point depression
- As water freezes, the concentration of the remaining solution increases. As solute concentration concentration increases the freezing temperature will continue to decrease. This continues until the eutectic temperature is met. The product does not have a single freezing point
Define: Refrigerant
Something that is cold
Define: Primary refrigerant
Substances which are brought into direct contact with food, or contained within the mechanics of a system. E.g. water
Define: Secondary refrigerant
Substance which is made cold by indirect contact with primary refrigerant. E.g Air (or brine) chilled by conact with a primary refrigerant.
What must occur in order for something to freeze
Temperature must fall from ambient temperature through the solid:liquid phase boundary
What factors act agains the removal of heat from a substance
- Insulating boundary layer at surface of food - Food freezes from outside in - Surface transfer co-efficient - Thermal conductivity
How does the insulating boundary layer affect how food freezes. How can this be limited
The insulating boundary layer is a thin layer of still air that surrounds food, insualting it and preventing heat loss Boundary layer can be reduced by agitation
Define: Surface transfer coefficient
Measure of how quickly heat can be removed from the surface A high surface co-efficients allows heat to be removed from the surface quickly
Define: Surface transfer co-efficient
Measure of how quickly heat can be removed from the surface A high surface co-efficients allows heat to be removed from the surface quickly
What is the driving force for heat flow
Temperature difference between the food and the freezing medium
What is the driving force for heat flow
Temperature difference between the food and the freezing medium
What are the effects of blanching
- Reduced enzyme activity - Some reduction in microbial numbers - Some softening of soft tissues - Some enzymes are more resistant than others and regenerate
Describe the mechanics and uses of a blast chiller
- I actually dont understnd the mecahics of blast chiller and am really hoping seac sees this and fixes this flashcard with the answer :)
- Good for irregularly sized items
- Product can be stored for a long time
What is the purpose of having two unit chillers
- Evaporation coil may cause condensation of any water vapor that is in the atmosphere.
- This builds up a layer of ice and the unit chiller become ineffective.
- Multiple chillers can be used at once so that one can defrost whilst another is running
Describe the mechanics and uses of a spiral freezer?
Add image - Conveyer blent travel around the stack with food becoming progressively colder. - Air used as a secondary refridgerant
Describe the mechanics and uses of a fluid bed freezer
- High velocity of cold air which is blown vertically upwards in the cmaber. - Food becomes suspendied in the cold air. - High air velocity reduced boundary layer and increases freezing time . - Small particles
Describe the mechanics and uses of a plate freezer
- Evaporation coil is replaced with a box
- The box forms 2 contact surfaces against which food can be placed
- Item placed between the plates, as the plates get cold they suck the heat from the food
- High surface heat transfer co-efficient as the product is in contact with the plate
- Burgers, fish fingers, regularly shaped items
Describe the mechanics and uses of cryogenic freezing
- Liquid nitrogen sprayed onto product on a conveyer belt (-195.8β°C)
- Large temperature difference results in rapid heat flow
- High surface transfer co-efficent - good product quality
- High degree of supercooling
What product defects can occur as a result of in-proper freezing
- Cold shortening - Heterogenous nucleation - Fat rancidity - Freezer burn
What is the equation for predicting freezing time
t =πΏπ/Ξπ
- tf is freezing time
- L is latent heat of fusion
- Ο is density of the frozen layer
- ΞΞΈ is the temperature difference between the food and the freezing medium
- x is the minimum distance from the surface to the centre of the food
- h is the surface heat transfer coefficient
- k is the thermal conductivity of the frozen layer
- P and R are shape constants
What are the assumptions made of the freezing time predictive equation
Only one freezing temperature and food starts at that freezing temperature, but is not frozen. Regular specific shape No change of temperature i.e. only latent heat lost
What else can the freezing time predictive equation be used for
Estimating thawing time
How does freezing preserve food
- Reduces the Aw - Reduces the rate of enzyme reactions - Reduces the rate of microbial growth
What is occuring during stage 1 of this graph
Specific heat is lost to freezing temperature
What is occurring during stage 2 of this graph
- Latent heat is removed. This is a long process due to the high latent heat of freezing compared with the specific heat capacity (e.g. 333,000 J/Kg vs. 4,800 J/Kg for water)
- Latent heat is the energy required not just to lower the temperature but to change the state of matter.
What is occuring during stage 3 of this graph
Product is in solid phase. Product cooling takes place
Define:
Specific Heat
The amount of heat per unit mass required to raise the temperature by one degree Celsius.
