http://www.thermapure.com/pdf/dr_burge_paper.pdf
How Does Heat Affect Fungi?
By Dr. Harriet Burge
The effects of heat on fungi depend on many factors, including the genus, species
and strain of the fungus, the amount of available water, kinds of nutrients, and many
other environmental factors. Of course, temperature is also a crucial factor. Most of
the research on temperature relationships for the fungi has been done in the food
industry where heat is commonly used to prevent fungal growth. Much of this
research involves wet heat, which is more effective than dry heat.
Fungi can be divided into groups according to temperature requirements for optimal
growth. Note that these requirements are dependent on water availability and
nutrients, and are measured under carefully controlled conditions. These same terms
are used for bacteria, but the temperature ranges differ.
The groups are as follows:
1. Psychrophiles: with optima less than 10°C
2. Mesophiles: with optima in the room temperature range (18-22°C)
3. Thermophiles: with optima at or above 37°C
There are also categories defining the ability of the fungi to withstand different
temperature regimes. In this case, we have psychrotolerant and thermotolerant
fungi, indicating that growth can occur at either low or high temperatures, but is not
optimal. Thus, some mesophilic fungi may be able to grow or at least survive at
either low or high temperatures, depending on the genetics of the strain and other
environmental conditions. Another form of temperature tolerance lies in the spores,
which can often withstand temperature extremes, and germinate when conditions
return to normal. Finally, some fungi that are normally mesophilic have spores that
require heat to stimulate germination. Temperature tolerance is strongly tied to the
amount of water so that wet heat is much for effective at damaging spores than dry.
The effects of heat on fungi are related to the chemical reactions within the fungal
cells. For optimum growth, temperatures must be in a range that allows the most
efficient progression of the chemical reactions necessary for growth. As temperatures
progress above the optimum temperature, the chemical reactions occur less
efficiently, and growth slows. Eventually, the temperature can reach a point where
growth stops, and cell components begin to be actually damaged by the heat.
Enzymes are proteins that change structurally when heated to their limit of
tolerance. Likewise, membranes, which contain lipids, change in structure, and their
function of protecting and regulating the internal environment of the cell becomes
compromised.
Most fungi are mesophilic, and have growth optima within the temperature range
that people find comfortable. This is why so many fungi appear when moisture enters
our homes, schools, and work environments. Because of air conditioning and
heating, mesophilic fungi flourish in occupied environments in all climates. However,
the fungal species that are abundant outdoors may vary considerably from one
climate to another. In hot dry climates, fewer species of fungi are present, both
because of the lack of water and the high temperatures. Thus, thermophilic and
xerophilic (dry tolerant) fungi are likely to be more abundant than in cooler wetter
environments. In tropical and subtropical places where both heat and moisture are
present, thermophilic and thermotolerant fungi with mesophilic water requirements
tend to be abundant. The incidence of fungal infections (including sinus infections)
tends to be higher in these areas in part because the fungi that can withstand human
body temperatures are more abundant than in temperate climates. Finally,
continental climates that tend to swing from hot humid to cold dry conditions have a
few overwhelmingly dominant fungi (e.g., Cladosporium species) with other mostly
mesophilic fungi filling in the gaps. Of course, this is an over-simplification, especially
since, as mentioned many times, an array of factors are necessary for optimal
growth of all kinds of fungi. Also, fungi live in microenvironments that may have very
different temperature/water conditions than are represented by climate.
Table 1 lists some of the fungal temperature relationships that have been reported in
the literature.
Table 1:
Activity Temperature and Duration Notes
Hot dry air sterilization 170 °C (340 °F), 1 hour
160 °C (320 °F), 2 hours
150 °C (300 °F), 2.5 hours
140 °C (285 °F), 3 hours
Kills virtually all spores
Wood heat treatment for
rot resistance
200 °C (392 °F), 24 hours Changes structure and
color of the wood
Whole house treatment for
termites and/or fungi
71°C (160 °F), 4-6 hours
Penicillium spore death in
water
54.4 °C (130 °F), 30 minutes
Ascospores activation in
grape juice
70 °C (158 °F), 30 minutes This is higher than the
temperature which kills
Penicillium in water
Germination of
chlamydospores in
Cladosporium-related
species stimulated by moist
heat
75 °C (167 °F), 30 minutes This is higher than the
temperature which kills
Penicillium in water
The question sometimes arises about heating an entire structure as part of
remediation. In this process, an entire house (or part of a building) may be heated to
a temperature that should kill most of the spores remaining following structural
remediation and any invisible growth. Clearly, using heat will help the building dry
faster because it reduces the relative humidity and increases the vapor pressure of
the water in the building. Heat may also damage some plastics, some types of
electrical equipment, and other items; so, naturally, heat sensitive items will need to
be removed prior to heating the structure. There may also be spores that are
particularly resistant to heat, microclimates that do not attain high enough
temperatures, or spores that are stimulated to germinate. Also, spores that are left
may still remain allergenic. As we do not have expertise in this method of
remediation, and the science in this area is still in its infancy, we do not feel qualified
to provide “the answer” for if or when heat remediation is good or bad. Also, like
many aspects of fungal investigations it depends upon many situation specific
criteria. Finally, it is important to remember that spores are continually entering the
structure and the only permanent fix for fungal contamination is the maintenance of
dry conditions throughout any building.