About fungi
Moulds are fungi. Fungi are a major category of living things.
Fungi have the following characteristics:
They are mostly made up of filaments or threads called hyphae (with
the notable exception of yeast, which is mostly made up of single, round
cells).
Fungi have a cell nucleus (unlike bacteria).
Fungi have a cell wall, like plant cells. This contains chitin – a stiff
material.
Fungi do NOT have any way of making their own food – unlike green plants.
Instead they feed by “soaking up” food around them. Sometimes they make
enzymes that digest the food around them, so they can “soak it up.”
Fungi often produce millions of tiny cells called spores, which can be
carried long distances by wind or insects, before they germinate into new
fungi. These are not seeds.
All
the hyphae together make up a body called the mycelium. The mycelium
can be soft and loose, like a mould, or close packed and hard, like a mushroom,
or like the shelf fungus growing on the side of a tree. The mycelium
often pushes a small part of itself into the air, to make a structure for
growing spores.
Here is a photo showing mould spores, several hyphae and several structures
making spores. On the left is a structure from Penicillium,
and to the right, one from Trichoderma.
About moulds
Moulds are a special group of fungi. They are often found growing on
leather. We are all familiar with the mould that grows on damp bread,
or on oranges. Moulds also grow on wood, and even on the human body.
In fact, moulds are everywhere.
They start out
as a single spore. It is so tiny that it can only be seen through
a microscope. Spores are blown by the wind or carried by insects.
When a spore lands on a suitable surface, such as a piece of leather, it
germinates. The spore breaks open to send out a tiny thread – the
first hypha. This hypha grows down into the leather, soaking up food
as it goes, and dividing into many different hyphae.
Eventually, the leather becomes filled with mould hyphae. After
that comes time to reproduce. The mould sends up a thread into the air,
which divides many times, till it finally produces many, many spores. With
thousands of these structures, millions of spores are grown, ready to be
carried to a new site by wind or insects.
By looking at
the spore bearing structures under a microscope, scientists can often work
out what type of mould is present. Without other tests, this is never 100%
accurate, but at least it gives a good idea of the type of mould we are
dealing with.
The sketch shows :
To the left – spore bearing structure from Aspergillus
To the right – spore bearing structure from Penicillium
| Mould damage to leather
When mould grows into leather, usually it does not cause major damage.
The mould soaks up its food from the parts of the leather that do not contribute
to strength. In fact, until the mould starts to grow new spores, it is
effectively invisible – you cannot tell it is there. |
The biggest problem with moulds in leather is caused by a change in
colour - staining. Often they stain the leather in bright colours,
and leave spots and grain marks that will make finished leather look inferior.
This causes downgrading of the finished product, and can cost a lot of
money.
Once moulds are inside the leather, simply cleaning the surface will
not solve the problem, since the moulds will grow right out again.
Moulds can also consume some important additives used in leather making,
such as fat liquors, tannins and so on.
How to slow down mould growth
If we understand the things moulds need to grow, we can change these
things, and slow down growth. What do moulds need to grow.
-
Water. Mould will not grow on dry materials. If finished
leather is dried and kept dry, there will be no mould. However, allowing
leather to get wet will encourage mould growth. Leather in the wet
blue state is very prone to mould growth.
-
Acidity. Mould grows best when it is in a slightly acid
environment. Pickled pelts are too acid for fast mould growth.
Very alkaline conditions do not support much mould growth.
-
Warmth. Mould likes it best between 25 and 30 deg. C.
That is why it is worse in summer, or in the tropics. Keeping leather
stored in cool conditions will not stop mould, but will slow its growth
a little.
-
Oxygen. Without oxygen, moulds cannot grow. If wet blues
or pickled pelts are baled and pressed into a tightly packed state, mould
will only grow on the outside. There is too little oxygen inside
the pack.
-
Nutrients. Moulds soak up the soluble parts of leather as
food. Unfortunately, we cannot remove these to control mould growth.
Some leathers (like vegetable tanned leathers) have more nutrients, and
support especially vigorous mould growth.
-
Poisons. Many chemicals are toxic to moulds. We add
fungicides to leather as the primary method of controlling them.
Tannery cleanliness is also important. If we keep everything clean,
there will be fewer mould spores to infect the new leather.
The number of mould spores in your plant can be estimated by carrying
out regular spore counts. This involves exposing petri dishes with nutrient
agar to the air. Spores that land on the agar will grow into visible
mould colonies which can be counted. The number of mould spores varies
with the seasons, and there are more when the climate is warm and wet.
If there are excessive mould spores around for the time of year, it is
time to look at plant cleanliness. If this problem is not taken care
of, it may result in mouldy leather.
Adding fungicides
All fungicides are different, and it is important to make sure that
you use your fungicide correctly.
For example : the blend of ortho phenyl phenol and para chloro
meta cresol is best added under alkaline conditions. By contrast,
TCMTB is best added in acid conditions.
Some fungicides need to be pre-diluted and added slowly.
Some fungicides are fat loving. If added into liquors containing
lots of grease, then these will be depleted from solution.
Find out what your fungicide requires, to get the best performance.
Resistance, true or false?
A scare that has alarmed tanners on many occasions is the risk of “developed
resistance”. According to this belief, if the tanner uses just one
fungicide, eventually the moulds become resistant to it, and he will get
failure. The tannery may be told to alternate fungicides to avoid
this risk.
The real problem with this belief is that there is a severe lack of
evidence to support it. The conditions required to cause resistance
to develop do not readily occur in a tannery. Hence developed resistance
is seriously unlikely, and the need to alternate fungicides is almost certainly
not there.
A more common cause of failure against mould is a new species of mould
appearing. For example, TCMTB is a fungicide that is very effective
against most mould species. However, some species of Trichoderma
are very resistant to TCMTB. They grow naturally on wood. This
means that, if a tannery uses wood with this mould on it, they may suddenly
get a mould outbreak, in spite of using TCMTB. Not the development
of resistance – just a new species of mould.
Our advise is not to worry about developing resistance. If good,
broad spectrum fungicides are used, and used properly, then this risk is
very low indeed.
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