There is no rational explanation for its sudden occurrence or why only a small number of hides are affected; no problems with putrefaction, fatliquoring is good, tannage is well penetrated. The problem then goes away of its own accord without making any process changes – very strange. The answer – vertical fibre!
Not only can vertical fibre defect be responsible for weakness problems, on some types of leather, shrunken grain leather in particular, it manifests itself as an unusual pattern on the back of the leather. For obvious reasons, this is known as the ‘cauliflower effect’.
The cause
Leather gets its strength from its unique and complex fibre structure – innumerable collagen fibrils twisted around each other to form fibril bundles, which in turn twist around each other to form fibres. These fibres then twist together to form fibre bundles and the fibre bundles then interweave in a three dimensional manner (Figure 1), forming a fibre structure so cohesive it takes great forces to pull it apart – unless something goes wrong in the processing of the hide which disrupts the structure, or it has vertical fibre defect.
A high angle of fibre weave can be process induced if the hides became particularly swollen in the lime and were not subsequently fully depleted in delime. If this is the case, very large numbers of hides are likely to be affected from the production.
True vertical fibre is a genetic problem in the animal. Some breeds of cattle, particularly Herefords, can carry the gene for vertical fibre which causes a vertical orientation of the upper corium fibres (Figure 2).
When the hide is split, the normal fibre structure of the lower corium is split away leaving only the vertical fibres. Because they do not interweave like normal fibres, they are easily pulled apart and the leather consequently has relatively little strength; anything up to a 50% reduction in strength can be experienced.
It is normally the butt area of the hide that is affected, particularly in the kidney area. However, in severely affected hides the defect can extend further up the back of the hide.
The gene that carries the defect is recessive and, for that reason, the problem is not so prevalent in UK hides because Hereford cattle are often crossed with dairy breeds thus diluting the gene. However, in countries such as North America and Australia where there are large herds of mainly Hereford beef cattle, the problem is much more prevalent. It is estimated that the incidence of VFD in different Hereford herds around the world is as follows:
UK 2%
USA 13%
Australia 23%
The so called ‘cauliflower effect’ in vertical fibred hides is sometimes seen in leather produced with a very astringent tannage, eg a shrunken grain leather, particularly on hides that have been split in the lime (Figure 3). Once the hide has been split, the normally orientated fibres in the lower part of the corium are removed leaving the vertical fibres with no anchorage and are thus more able to move apart.
The distortion to the collagen fibres that takes place in the astringent tannage causes the vertical fibres to separate around any points of weakness such as blood vessels. Consequently, the network of the blood vessels produces the cauliflower-like pattern where the fibres have separated from around them.
Being a naturally occurring problem, prevention from the tannery’s perspective is difficult, if not impossible, especially since affected hides are not easily identified until processing is complete. However, there are things that can be done to minimise its impact:
Hide selection – This is understandably difficult to achieve, but avoiding Hereford hides is one means of avoiding affected hides. Herefords are characterised by their red/brown colour with white face and underparts and white tail switch (Figure 4). There is also a black version of the breed, but the incidence of vertical fibre defect in these is not known.
Swelling – any process that is likely to disrupt the fibre structure by swelling the collagen fibres will exacerbate the effects of vertical fibre defect by forcing the fibres apart, eg excessive swelling in lime and astringent tannages.
Mechanical action – excessive drumming or high drum speeds will increase disruption of the fibre structure.
Final sort – Although it is not easy to identify affected leather without the aid of a microscope, sometimes the lack of interweaving of the corium fibres in affected hides allows greater penetration of process chemicals so affected hides can sometimes be identified by better dye penetration than other hides.
Selective breeding – modern DNA profiling should enable cattle breeders to selectively breed animals without the defect thus eventually eliminating it. However, market forces will need to be strong for this to be considered as the costs would be high.