Mold in leather production

Mold growth in leather
Fungal deterioration is apparent from the growth on the leather of molds. Moulds are part of the group known as fungi - a group of unique organisms that sit uneasily between microbiology and botany. They are neither filamentous bacterium nor are they similar to green plants.
There are many strains of mold that are capable of growing on leather, but often the presence of one strain suppresses the appearance of others. Aspergillus niger, for example, usually results in the inhibition of other strains, although the others are quite capable of widespread growth in the absence of A. niger.
Like all living organisms they need certain basics to grow and multiply: a source of moisture and a source of nutrient. In wet-blue leather the source of moisture is clear.
In finished leather the moisture content is very much lower and the growth of mold is less, but when transporting leather between different climatic zones, or if storing leather in an inappropriate manner, moisture build up in certain areas, such as just inside any packaging, can readily lead to the growth of mold. Many of the process chemicals used in the manufacture of leather act as nutrients for mould growth, examples include ammonium salts, phosphates, surfactants, fatliquoring agents and other organic agents.
The appearance of the mold growth is related to the type of mold present. In the minds of many people mould is usually associated with green/grey growths. Tanners, however, may also be familiar with the red spots or red discolouration caused by Paecilomyces ehrlichii, P. aculeatum, P. purpurogenum and P. roseopurpureum.
In poorly controlled drying operations, where the humidity remains high or air circulation is poor, other types of growths may also be found, giving rise to other coloured appearance such as green, yellow-brown, dark-brown and grey. Strains associated with damage arising from poor drying control, include A. ochraceus, A. wentii, P. rugulosum, P. funiculosum, P. variotii and V. glaucum.
Tropical chamber mold resistance test
Test method
The leather samples were evaluated for their resistance to mold growth by exposure in a tropical chamber for 28 days.
The tropical chamber consists of an insulted cabinet that is maintained at a temperature of 27 to 30 degree Celsius and a humidity level of 95 to 100 %, and has been inoculated with the spores of fungal species that commonly attack leather, including Aspergillus and Penicillium species. This tropical chamber environment is highly conductive to fungal growth and this method is intended for the accelerated evaluation of mold resistance.
Several replicate test pieces measuring 7cm by 10 cm are cut from each leather sample and exposed in the tropical chamber. The leather pieces are inspected weekly and rated for fungal growth. The rating system is based on the percentage of the leather surface covered with mold. The scale is from 0 to 100: 0 mean 0% of the surface is covered with mold and 100 mean 100% of the surface is covered.
Interpretation of the mold resistance ratings:
 Long-term mold resistance : a rating of 0 to 20 in week 4
 Medium-term mold resistance: a rating of 0 to 20 in 3 week
 Short-term mold resistance: a rating of 0 to 20 in 2 week
 Limited mold resistance: a rating of 0 to 20 in 3 week
 No mold resistance: a rating greater than in 1 week