chilling & freezing part 1

Question 1

what is chilling

Answer 1

chilling is a unit operation in which the temperature of foods are reduced to between -1 degree and 8 degree. This reduces the rate of biochemical and microbiological changes during storage and extend the shelf life of fresh and processed foods

Question 2

how does chilling help plant foods

Answer 2

it is important to know that the respiration of plant foods continue even after harvesting and during storage. Chilling helps remove the surrounding heat and respiration heat, which slows down the rate of metabolic reactions (eg ripening) and microbial growth, it also reduces transpiration as well as delay the decaying of plants and consequently extend the shelf life of foods

Question 3

how does chilling affect animal foods

Answer 3

if meat carcass is chilled less than 10 degrees before rigor mortis starts or complete, muscles excessively contracts due to chemical changes in their myofibrils, ceasing the toughening of meat and harden in the subsequent cooking.

Question 4

what is cold shortening

Answer 4

after slaughter, if meat carcass is chilled less than 10 degrees before rigor mortis starts or completes, muscles excessively contracts due to chemical changes In their myofibrils, causing the toughing of meat and harden in the subsequent cooking. this is called cold shortening

Question 5

what is cold shortening

Answer 5

after slaughter, if meat carcass is chilled less than 10 degrees before rigor mortis starts or completes, muscles excessively contracts due to chemical changes In their myofibrils, causing the toughing of meat and harden in the subsequent cooking. this is called cold shortening

Question 6

chilling vs freezing

Answer 6

both chilling & freezing are the means of preserving foods by lowering their temperature to significantly reduce or stop the rate at which detrimental changes occur in the foods microbiologically, physiologically, chemically and/or physically.

Question 7

differences between chilling and freezing

Answer 7

chilling
-low temperature(-1 to 8)
-no significant structural changes
-water is in the liquid state
-short term preservation

freezing
-long term preservation
-very low temperature(-18 to -20)
-water in the food undergoes phase change from liquid to solid
-water is in the solid state

Question 8

what is the pre treatment for veggies

Answer 8

blanching

Question 9

how is veggies pre treated

Answer 9

blanching by either steam or hot water prior to freezing to inactivate any specific enzymes which lead to quality deterioration for eg. enzymatic browning

Question 10

why does veggies and fruits need pre treatment prior to freezing

Answer 10

to maintain or minimize changes in sensory properties and nutritional values of frozen foods after thawing.

Question 11

use of sugar and/or ascorbic acid as a pre treatment

Answer 11

to inactivate specific enzymes leading to quality deterioration enzymatic browning
in general ascorbic acid slows down enzymatic browning at low temperatures

Question 12

functions of sugar

Answer 12

preserve the texture and shape of fruits
sugar binds to the water in fruits. thus, preventing water in the fruits from leaving their cellular structures. Thus, the texture of the fruits is protected against weakening during freezing, which keeps their original shape.

Question 13

function of sugar and ascorbic acid

Answer 13

sugar and ascorbic acid protects the surfaces of frozen fruits from contact with air which cases enzymatic browning (discoloration) due to oxidation.

Question 14

need pre treatment for berries?

Answer 14

no, due to high acidity present in berries that is high ascorbic acid content. hence they are stable during storage

Question 15

how to obtain freezing curve?

Answer 15

if the temperature of a for is monitored at the point that cools most slowly when heat is removed, freezing curve is obtained

Question 16

explain the 3 stages that happens in the freezing curve

Answer 16

at the PRE-COOLING STAGE
- the food is cooled to below its freezing point
- at point s, water remains liquid even though temperature is below cooling, and this is known as supercooling.
-then, the temperature rises rapidly to the freezing point when ice crystals start to form and latent heat of crystallization is released, hence resulting in an increase in the temperature of the food.

at the phase change period
-major part of the ice Is formed
-a lot of latent heat is removed from the food as ice forms. Thus, the temperature of the food remains relatively constant.
-as more ice is formed, it increases the solute concentration in the unfrozen liquor, leading to a gradual decrease in the freezing point

reduction to storage temperature
-the temperature of food falls to the temperature of the freezer.
- small amount of water remains unfrozen that is bound water. A proportion of the unfrozen water at the temperatures used in commercial freezing depends on the type and composition of food and the storage temperature.

Question 17

how does slow freezing affect the quality of frozen foods?

Answer 17

if foods are slowly frozen, ice crystals exclusively forms in the intercellular spaces. as ice crystals have a lower vapor pressure than regions within the cells, water moves from the cells to the growing crystals, resulting in a large ice crystal formation. cells are dehydrated, and the solute concentration of the cell increases. hence they lead to permanent damage to the cellular structure.
on thawing, a lot of cellular materials leak out of the damaged cells that is excessive drip loss. because the cells are permanently damaged, they cannot re-absorb it. thus, they do not regain their original shape and turgidity

Question 18

what derives the freezing time

Answer 18

freezing time is dependent on how fast heat move from the interior of the food to the surface aND IS REMOVED BY THE FREEZING MEDIUM.

Question 19

the rate of heat transfer

Answer 19

1 thermal conductivity of the food
2 the area of food available for heat transfer
3 the temperature difference between the food and the freezing medium that is the refrigerant
4 the packaging if food is pre packed before freezing
5 the distance that the heat must travel through the food
6 insulating effect of the boundary film of air surround the food

Question 20

why is it not possible to calculate an exact freezing time?

Answer 20

1 due to changes in density, thermal diffusivity, thermal conductivity and specific heat with a reduction in temperature of the food.
2

Question 21

assumptions involved in the estimation of freezing time using plank’s equation

Answer 21

1 heat transfer is one dimensional
2 the initial temperature of food is uniform
3 the freezing air temperature is constant
4 latent heat is removed at a constant temperature
5 food is initially at a uniform temperature equivalent to its initial food temperature
6 heat transfer inside the food is by conduction
7 properties of food above snd below the freezing are different but constant.

Question 22

what is f1 and f2 for slab, cylinder, sphere and cube shape?

Answer 22

f1- 2, 4, 6
f2- 8,16,24