438 Chapter 17
scraped off the surface with a bladed dasher.
Air is whipped in by the rotation of the
dasher. The main differences between the
two designs are the surface - area - to - mix -
volume - ratio (continuous barrels have the
greater surface - to - volume ratio, hence faster
freezing), the dasher tolerance for inclusions,
and the amount of overrun that can be
whipped into the product (continuous freez-
ers can achieve higher overrun). Batch freez-
ers typically freeze the base within eight to
10 minutes, whereas with continuous freez-
ers the base residence time is measured in
seconds; however, this depends on the freezer
design and desired overrun.
The important characteristics of the freez-
ing process are the creation of small ice crys-
tals (less than 30 μ m), the crystallization of
liquid milk fat and aggregation of fat clumps
to help stabilize air cells, and incorporation
of air to create overrun (Marshall et al.,
2003 ). An additional process that has been
developed after continuous freezing allows
for improvements to the texture of reduced
fat products. Low - temperature extrusion
( − 15 ° C; − 11 ° F) after classic ice cream freez-
ing ( − 5 ° C to − 6 ° C; 21 ° F to 23 ° F) greatly
increases the shear forces applied to the base.
This shearing occurs when a large fraction of
the fat has already crystallized, and results in
a destabilization of the fat which creates
smaller fat aggregates, smaller air cells, and
smaller ice crystals (Bollinger, Kornbrust
et al., 2000 ; Eisner, Wildmoser et al., 2005 ).
These changes positively affect the ability to
scoop cold ice cream (making it easier) and
reduce the melt - down of the ice cream,
increasing the perception of creaminess
(Eisner, Wildmoser et al. 2005 ). This process
is being used in reduced fat ice - cream to
create product with less fat but with the desir-
able creamy mouth feel.
The consistency of the frozen dessert as it
exits the freezer is referred to as either wet
or dry (or stiff). The ice content, mix viscos-
ity, size of the air cells, and amount of air allstabilizer systems, base is typically held for
a minimum of four hours. Bases frozen
without aging show evidence of improper
emulsifi cation, which is observed by the rate
of meltdown of the fi nished product and
churned fat in the freezer (Marshall et al.,
2003 ).
Flavoring
Base is fl avored immediately before freezing
if the fl avors are not added to the mix before
pasteurization. This can be done in agitated
batch tanks, or injected in - line using a pump
and variegate nozzle, with a metering system
(mass - fl ow or mag meters, or loss in weight
of the fl avor on a scale) to inject the proper
amount and a static mixer to thoroughly
blend the fl avor. In the case of batch freezers,
additional bulky ingredients or variegates can
be added to the freezer itself and folded in
just prior to discharge from the freezer. In the
case of continuous freezers, bulky ingredi-
ents cannot be added to the freezer and are
added after the ice cream exits the freezer by
means of a fruit feeder, which mixes nuts,
cookies, and other pieces into the base with
a dynamic mixer. Swirls and variegates are
added in - line by using variegating nozzles to
inject syrups into the fl owing ice cream.
Freezing
During freezing, aeration of the mix results
in a suspension of crystallized fat and water
in a concentrated sugar solution that also
contains emulsifi ers, proteins, and stabilizers.
The structure of the fat crystals contributes
to the foam stability, texture, and melt - down
properties (Goff, 2009 ). Two types of ice
cream freezers are used in commercial pro-
duction, batch and continuous. Both are
scraped surface heat exchangers: they employ
a barrel design in which water in the mix is
frozen on the surface of the barrel by means
of refrigerant in the barrel walls, and then