mentioned features of not controlled methods pose a threat to
post-thaw function of avian semen, compared with programmable
freezers.
Examples of nonprogrammable freezing methods are, for
instance, the abovementioned “pellet method” and freezing in
static LN 2 vapor. Freezing straws in vapor can be done by placing
the straws on a typical (metal) straw rack in an insulated box
containing a level of LN 2 . The cooling rate can be varied by varying
the distance of the straws to the surface of LN 2 . This method has a
poor reproducibility as the boiling of LN 2 caused by the heat of the
metal rack causes turbulence, disturbing the built-up layer of LN 2
vapor. However, reproducibility can be very good when placing the
straws on a Styrofoam raft, which is then placed floating on the
surface of LN 2 , as proposed by Dong [58]. We have modified the
“raft” concept by making Styrofoam frames, which means that the
straws are exposed to the nitrogen vapor over almost their complete
length, while both ends of the straws rest on opposing sides of the
frame. This gives a very repeatable relation between the thickness of
the frame (i.e., the distance between the straws and the LN 2 surface) and the cooling rate. Cut out grooves are provided in the sides
of the frames to allow easy and regularly interspaced placing of the
straws using a “distribution block” for mini straws (IMV Technologies, L’Aigle, France). This freezing method results in highly
repeatable sigmoidal freezing curves [32], with moderate cooling
rates during the dissipation of heat of fusion followed by high
cooling rates after the dissipation of heat of fusion (cf. [57]. The
overall cooling rate can be set by choosing the appropriate
-100
-80
-60
-40
-20
0
20
0
30
60
90
120
Temp (ºC)
time (sec)
9 mm thick
24 mm thick
Fig. 2 Freezing curves recorded in 0.25-mL straws placed on a “Styrofoam rack”
floating on a surface of LN 2 . The overall cooling rate can be set by choosing the
appropriate Styrofoam thickness (see this figure). This method results in
reproducible sigmoidal freezing curves yielding good freezing results for avian
semen
Cryopreservation of Avian Semen
387
post-thaw function of avian semen, compared with programmable
freezers.
Examples of nonprogrammable freezing methods are, for
instance, the abovementioned “pellet method” and freezing in
static LN 2 vapor. Freezing straws in vapor can be done by placing
the straws on a typical (metal) straw rack in an insulated box
containing a level of LN 2 . The cooling rate can be varied by varying
the distance of the straws to the surface of LN 2 . This method has a
poor reproducibility as the boiling of LN 2 caused by the heat of the
metal rack causes turbulence, disturbing the built-up layer of LN 2
vapor. However, reproducibility can be very good when placing the
straws on a Styrofoam raft, which is then placed floating on the
surface of LN 2 , as proposed by Dong [58]. We have modified the
“raft” concept by making Styrofoam frames, which means that the
straws are exposed to the nitrogen vapor over almost their complete
length, while both ends of the straws rest on opposing sides of the
frame. This gives a very repeatable relation between the thickness of
the frame (i.e., the distance between the straws and the LN 2 surface) and the cooling rate. Cut out grooves are provided in the sides
of the frames to allow easy and regularly interspaced placing of the
straws using a “distribution block” for mini straws (IMV Technologies, L’Aigle, France). This freezing method results in highly
repeatable sigmoidal freezing curves [32], with moderate cooling
rates during the dissipation of heat of fusion followed by high
cooling rates after the dissipation of heat of fusion (cf. [57]. The
overall cooling rate can be set by choosing the appropriate
-100
-80
-60
-40
-20
0
20
0
30
60
90
120
Temp (ºC)
time (sec)
9 mm thick
24 mm thick
Fig. 2 Freezing curves recorded in 0.25-mL straws placed on a “Styrofoam rack”
floating on a surface of LN 2 . The overall cooling rate can be set by choosing the
appropriate Styrofoam thickness (see this figure). This method results in
reproducible sigmoidal freezing curves yielding good freezing results for avian
semen
Cryopreservation of Avian Semen
387