Freezing botanical specimens

In response to Jonathan Farley <j.farley [at] kew__org>

The following are important points:

    1.  The moisture content of the materials when exposed to
        reduced temperature is important. If it is high enough to
        support condensed moisture--above the fiber saturation
        point--ice crystals will form in the materials and cause ice
        damage. If the moisture content is near the fiber saturation
        point the small amount of moisture it adsorbs during reduced
        temperature could reach the saturation point and ice
        crystals form. It is because of this that it is recommended
        that materials to be subjected to reduced temperatures for
        insect eradication, are dry and have reached equilibrium
        with an environment with parameters 20 deg. C and circa 50%
        RH. The water in  materials with a moisture content less
        that fiber saturation point, because it is bonded to sites
        in the material it is unable to freeze, thus materials do
        not dry in this closed environment.

    2.  If the bag is sealed and contains ambient room temp and RH,
        and excess air removed there is very little moisture present
        and during temperature reduction the amount available to the
        materials for adsorption is small compared to their(plant
        specimens) adsorptive ability.

    3.  Stress has to be clarified--during temperature reduction and
        increase in moisture content--material stress is reduced.
        Dry materials become more flexible.

    4.  The purpose of subjecting the specimens to a reduced
        temperature is to kill insect structures. The moisture in
        their protoplasm must freeze- ice crystals must form.
        Freezing the protoplasm causes condensation effect by
        withdrawal of water causing damage to proteins and DNA and
        an increase in pH, enzymes, etc. These along with the
        physical ice damage causing membrane leakage are the cause
        of death of insect structures- eggs, larvae, pupae  and
        adults. The initial blast freezing may prevent ice formation
        and protect the insect but it is likely that during increase
        in temperature ice crystals will form which will kill the
        insect. Has any research been done on the effect of the
        blast freezer on the insects? The blast freezer may be good
        but the simple chest freezer is also suitable for many
        materials.

Mary-Lou Florian
Conservation Scientist, Research Associate
Royal British Columbia Museum
Victoria BC, Canada


                                  ***
                  Conservation DistList Instance 18:41
                  Distributed: Tuesday, March 8, 2005
                       Message Id: cdl-18-41-002
                                  ***