The potential of water hyacinth as a source of vegetable tannins

Introduction
Depending on the type of leather being produced, hides and skins are normally tanned using either a mineral or a vegetable tanning process. Chrome tanning accounts for more than 90% of the world’s leather production, but there is concern over the environmental impact of this tanning method. Vegetable tanning is a more environmentally friendly process, but the vegetable tanning extracts are not always widely available in many countries. Vegetable tannins are phenolic compounds that bind to collagen, resulting in a product that has been tanned and resistant to bacterial attack. Tannins are obtained from plants.
Water hyacinth is a problem plant that grows on lakes and in rivers, and is spreading rapidly in many water resources around the world. Various studies have been carried out globally investigating possible uses for water hyacinth. The aim of this study was to determine the tannin content of water hyacinth, and to investigate the plant as a potential source of vegetable tannins to be used in the tanning industry.

Experimental
Fresh water hyacinth plants were collected, and the roots removed. The plants were then separated into the leaves and the stems. This material was cut up into smaller pieces and placed outside to dry. After drying for 7 days, the leaves and stems were then dried in an oven at 104°C for 1 hour.
The dried leaves and stems were individually ground in a high- speed grinder to maximize the surface area available to extract the vegetable tannins.
The tannin content of the leaves and the stems was analysed, and it was found that the leaves had a higher tannin concentration compared to the stems (Table 1).

Based on this information it was decided to carry out the tanning investigation using only leaf extracts.
Fresh hides were processed through to the pickling stage using conventional processing methods. The hides were then tanned for three days in different batches of water hyacinth leaves based on the weight of the hide (10% leaves, 20% leaves, 5% leaves+5% Quebracho and 10% Quebracho as the control). The different tannages were then retanned and fatliquored using a standard dyehouse process for the four different tannages.
After toggle drying, the different leathers were tested for their physical properties.

Results
The physical test results of the leathers produced is shown in Table 2.

Table 1. Tannin concentration found in the water hyacinth plant’s leaves and stems.

In all tests carried out, the vegetable tanned leathers tanned with the water hyacinth leaf extract produced results that were lower than the leathers tanned with the Quebracho. However, except for the shrinkage temperature and the tensile strength, the leathers tanned with the water hyacinth leaf extract did meet the minimum test specifications.

Conclusion
Most of the recommended parameters for leather product manufacturing such as tear strength, elongation at break, grain crack and grain burst were achieved in the hides tanned with water hyacinth leaves. However, the tensile strength and the shrinkage temperature were both below the minimum standards.
The authors concluded that water hyacinth has limited potential as a vegetable tanning agent. However, it is of the opinion of the reviewer of this article that the researchers made an important error in this study. They used offers of 10% and 20% of the dried water hyacinth leaf extract, but they perhaps failed to realize that the tannin content of the leaves is only 4.1%. They were therefore not offering 10% or 20% of tannins, but a very much lower percentage than this. The reviewer feels that if a much higher percentage of leaf extract was offered during the tanning process, that much better results would have been achieved. It must be remembered that the Quebracho has a tannin content of greater than 40%, so the offer of the water hyacinth leaves needs to be at least 10 times that of the Quebracho in order to carry out a proper comparison of the tanning ability of the water hyacinth leaves. Further investigations may well show that water hyacinth leaves do indeed have good potential as a vegetable tanning agent.

This article is a summary of the paper “Potential of Water Hyacinth Leaves Extract as a Leather Tanning Agent” Journal of the American Leather Chemists Association (JALCA) Vol 117, 391 - 399 (2022).