low temperature preservation

1. Freezing and
Chilling
Revision: Chilling & Freezing

2. Freezing
Freezing is a longer-term method of preserving food.
Freezing uses two controls to prevent the growth of
bacteria
1. The liquid is converted to a solid (ice) and is therefore
unavailable to bacteria
2. The low temperatures reduce the rate of bacterial
growth
Together these controls allow food to be stored for long
periods. It is important to remember that the bacteria
have not been destroyed. On thawing they will become
active again

3. While freezing reduces the growth rate
of bacteria, it does not prevent enzyme
action. For this reason, fruits &
vegetables which are to be frozen are
blanched.
Blanching is a process which involves
dipping fruit of vegetables first into
boiling water, then ice cold water for a
few seconds. This deactivates the
enzymes and reduces the number of
bacteria present on the surface of the
food.
Blanching
Boiling water
Transferring
to ice water

4. Before freezing, anti-oxidants may
be added to foods to prevent
fats present in the products
from becoming rancid as this
chemical reaction is not
prevented by freezing.
During freezing the time factor is
critical. If freezing is slow, large
ice crystals will form, damaging
the cell walls of the food
Slow freezing =
large crystals
Quick freezing
= small crystals

5. Quick Freezing
Food quickly frozen from 0 > -18°C in approximately 12
minutes. Creates smaller ice crystals & less damage to
the food. The food retains its shape & texture with
this method.
There is little change in the nutritional value of foods & in
some cases the vitamin content could be higher than in
fresh foods. Vegetables are frozen very quickly after
harvesting, while fresh fruit & vegetables lose ascorbic
acid (vit. C) during transportation and storage

6. Freezers
Frozen food should be
transported home quickly, using
a cool box to prevent
defrosting. They can be stored
using the star markings on the
freezer, which are a guide to
the length of time they may be
stored
* = one week
** = one month
*** = 3 months
The freezer star ratings
are as follows
* Ice Box -6°C
* * Ice Box -12°C
* * * Ice Box -18°C
* * * * Freezer -18°C
Temperature
Length of storage

7. Immersion Freezing
In immersion freezing, food is placed in a refrigerant
prior to freezing.
Brine is often used for fish, and a sugar solution for
fruits.
This provides a layer which protects the food from the
dry atmosphere of the freezer

8. Cryogenic Freezing
Uses liquid nitrogen which is very cold (-196ºC) Food
passes through a tunnel where nitrogen gas is
sprayed downwards. A beefburger will be frozen in 1
minute at these extreme temperatures.
This produces small crystals, and little moisture loss.
This method is used when freezing prawns. The prawns
are first dipped in liquid nitrogen to freeze the
outside layer. This prevents the prawns sticking
together and from sticking to the freezer belts.

9. Plate Freezing
Ideal for thin, flat foods such as steak, fish fillets
or burgers.
The food is placed between two “plates” which make
contact with the food’s surface.
This speeds up the freezing process & freezing
occurs evenly throughout the food

10. Accelerated Freeze Drying (AFD)
This is a combination of freezing and drying.
Foods are quickly frozen which produces
small ice crystals in the cells. The food is
then placed in a vacuum under reduced
pressure and heated gently.
In these conditions the ice crystals change
directly to water vapour. This is known as
sublimation.
Products treated this way weigh very little,
are porous and look like fresh products.
They can be easily reconstituted with
water
Coffee, tea, shellfish and food for camping
expeditions are treated in this way

11. The market has been driven by several factors including consumer
demand for convenience foods, a rise in freezer and microwave oven
ownership, fewer families sitting down to eat together and a rise in
one and two-person households.
•The UK chilled ready meals sector is the most highly developed in
Europe in terms of the range of products available and total sales.
•Sales are dominated by the major supermarket chains, which
accounted for two-thirds of sales by value in 2001.
•As many as half of all new ready meal launches use international
or ethnic recipes.
•Low-fat and low-calorie only account for a small percentage of
new product developments. Most of these types are within
existing well-known brands such as Weight Watchers from Heinz.
•New product developments are likely to focus on healthiness,
although not necessarily low-fat/low-calorie. The emphasis is more
on wholesome fresh ingredients.
•The first chilled ready meal to go on sale in the UK was developed
by Northern Foods for Marks & Spencers in 1970 – Chilli Con
Carne.
Foods Standards Agency

12. Chilling
Chilling is a short term process of preservation. Chilled
foods are prepared foods which, for reasons of safety
or quality are designed to be stored at or below 4ºC for
their entire life (e.g. salads).
The optimum temperature for storage is 4ºC .
Cook-Chilled products are dishes which are cooked first,
rapidly chilled to below 4ºC within 90 minutes then
stored in a fridge. The product needs to be reheated
prior to eating.
Cook-chilled products can only be stored for 5 days & this
includes the day of manufacture
Collins Real World –Food Technology