Integrierte Vermeidung und Verminderung der Umweltverschmutzung

Chapter 2
The operating principle of a countercurrent freezer, e.g a belt freezer or a spiral freezer, is the
same as that of a blast freezer, except the food being frozen is transported through a
countercurrent of cold air or liquid nitrogen. This reduces the evaporation of water from the
food.
In fluidised bed freezers, the food is fluidised with air of -25 to - 40 ºC by passing the air
vertically upwards through a perforated tray or conveyor belt and through a bed of food
2 to 20 cm thick. The shape and size of the food pieces determine the thickness of the fluidised
bed and the air velocity for fluidising. In this system, food comes into more extensive contact
with the air than in blast freezers, so that all surfaces are frozen simultaneously and uniformly.
This results in faster freezing and less dehydration, which also results in less frequent
defrosting. Fluidised bed freezing is restricted to small particulate foods, e.g. peas, sweet maize
kernels, shrimps, and strawberries.
In cooled surface freezers, vertical or horizontal hollow plates are cooled with a refrigerant of
about -40 ºC. The food is put on the surface in thin layers. Sometimes the plates are pressed
slightly together. This improves the contact between the food and the freezing plates. One
advantage of such freezers is that little dehydration of the food takes place, which reduces the
frequency of defrosting. A special form is the scraped surface freezer. This consists of a
freezing cylinder containing rotating knives which remove the frozen material from the freezing
surface. This type of equipment is used, e.g. in ice-cream production.
In immersion freezers, packaged food is passed through a bath of refrigerated glycol, brine,
glycerol or calcium chloride solution on a submerged mesh conveyor. High freezing rates are
obtained with this method. It is used, e.g. for freezing concentrated orange juice in cans and for
pre-freezing film wrapped poultry prior to blast freezing.
In cryogenic freezing, the food is in direct contact with the refrigerant, which can be solid or
liquid carbon dioxide or liquid nitrogen. The refrigerant evaporates or sublimates away
removing the heat from the food and causing rapid freezing. Liquid nitrogen and carbon dioxide
refrigerants are colourless, odourless and inert.
Cryo-extraction is a method of subtractive enrichment which consists of removing water from
bunches of grapes to increase the sugar content of the must. The objective of cryo-extraction is
to eliminate some water present in the raw material by selective freezing. The grapes are placed
in a coldroom at a temperature midway between the freezing temperature of the richest grapes
and that of the least rich grapes. The duration of the treatment of the grape crop is
from 12 to 20 hours at temperatures of between -3 and -6 ºC. The technique is performed only
on grapes harvested manually and carried out in low, perforated crates, to achieve a slow and
homogeneous lowering of the temperature over all the grapes. The pressing makes it possible to
select the ripest juices, i.e. those which flow out first. The maximum pressure used for pressing
is 4 bar.
Finally, concentration by cold is a technique applied only for wines. It consists of eliminating a
part of the water from a wine to increase its alcohol strength. This technique is regulated. The
wine is cooled below the temperature where ice crystal forms, but the alcohol does not freeze.
The crystals are prevented from congealing together by constant mixing. When a sufficient
quantity of water has thus solidified, the liquid is separated from the crystals by centrifuges or
pressure operated extractors.
2.1.7.3 Freeze-drying/lyophilisation (G.3)
2.1.7.3.1 Objective
Freeze-drying or lyophilisation, is the process of removing water from a product by sublimation
and desorption. The aim of the process is to preserve sensitive material that cannot be dried by
evaporation. Beware of the risk of degradation of specific components at high temperatures,
which would result in a loss of taste or other quality aspects.
RHC/EIPPCB/FDM_BREF_FINAL January 2006 47