Freezing of Foods

Question 1

Name the two steps of ice crystal formation

Answer 1

• Nucleation
• Crystal growth

Question 2

What occurs during heterogeneous nucleation

Answer 2

crystals grow on inhomogeneities in the surrounds e.g. scratches on a dish, cell organelles. Once crystals are grown crystal growth can occur

Question 3

Why is heterogenous nucleation a problem

Answer 3

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

Question 4

What is homogenous nucleation

Answer 4

Formation of small ice embryos in the interior of a substance

Question 5

Why is homogenous nucleation hard to achieve

Answer 5

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.

Question 6

How can homogenous nucleation be achieved

Answer 6

Supercooling

Question 7

Define: Supercooling

Answer 7

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.

Question 8

How does the degree of supercooling affect the radius of the ice crystal

Answer 8

Crystal radius depends on the degree of supercooling. With a large degree of supercooling, small critical radius ice crystals can form throughout the food.

Question 9

What is the affect of adding a solute on freezing temperature

Answer 9

• 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

Question 10

Define: Refrigerant

Answer 10

Something that is cold

Question 11

Define: Primary refrigerant

Answer 11

Substances which are brought into direct contact with food, or contained within the mechanics of a system. E.g. water

Question 12

Define: Secondary refrigerant

Answer 12

Substance which is made cold by indirect contact with primary refrigerant. E.g Air (or brine) chilled by conact with a primary refrigerant.

Question 13

What must occur in order for something to freeze

Answer 13

Temperature must fall from ambient temperature through the solid:liquid phase boundary

Question 14

What factors act agains the removal of heat from a substance

Answer 14

• Insulating boundary layer at surface of food - Food freezes from outside in - Surface transfer co-efficient - Thermal conductivity

Question 15

How does the insulating boundary layer affect how food freezes. How can this be limited

Answer 15

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

Question 16

Define: Surface transfer coefficient

Answer 16

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

Question 17

Define: Surface transfer co-efficient

Answer 17

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

Question 18

What is the driving force for heat flow

Answer 18

Temperature difference between the food and the freezing medium

Question 19

What is the driving force for heat flow

Answer 19

Temperature difference between the food and the freezing medium

Question 20

What are the effects of blanching

Answer 20

• Reduced enzyme activity - Some reduction in microbial numbers - Some softening of soft tissues - Some enzymes are more resistant than others and regenerate

Question 21

Describe the mechanics and uses of a blast chiller

Answer 21

• 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

Question 22

What is the purpose of having two unit chillers

Answer 22

• 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

Question 23

Describe the mechanics and uses of a spiral freezer?

Answer 23

Add image - Conveyer blent travel around the stack with food becoming progressively colder. - Air used as a secondary refridgerant

Question 24

Describe the mechanics and uses of a fluid bed freezer

Answer 24

• 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

Question 25

Describe the mechanics and uses of a plate freezer

Answer 25

• 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

Question 26

Describe the mechanics and uses of cryogenic freezing

Answer 26

• Liquid nitrogen sprayed onto product on a conveyer belt (-195.8⁰C)
• Large temperature difference results in rapid heat flow
• High surface transfer co-efficent - good product quality
• High degree of supercooling

Question 27

What product defects can occur as a result of in-proper freezing

Answer 27

• Cold shortening - Heterogenous nucleation - Fat rancidity - Freezer burn

Question 28

What is the equation for predicting freezing time

Answer 28

t =𝐿𝜌/Δ𝜃

• tf is freezing time
• L is latent heat of fusion
• ρ is density of the frozen layer
• Δθ 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

Question 29

What are the assumptions made of the freezing time predictive equation

Answer 29

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

Question 30

What else can the freezing time predictive equation be used for

Answer 30

Estimating thawing time

Question 31

How does freezing preserve food

Answer 31

• Reduces the Aw - Reduces the rate of enzyme reactions - Reduces the rate of microbial growth

Question 32

What is occuring during stage 1 of this graph

Answer 32

Specific heat is lost to freezing temperature

Question 33

What is occurring during stage 2 of this graph

Answer 33

• 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.

Question 34

What is occuring during stage 3 of this graph

Answer 34

Product is in solid phase. Product cooling takes place

Question 35

Define:

Specific Heat

Answer 35

The amount of heat per unit mass required to raise the temperature by one degree Celsius.