Tanning is a process in which the leather-making protein is completely stimulating against heat, enzymatic biodegradation, thermo mechanical stress by converting the fibrous protein of raw hide or skin into a stable material and making the leather to suitable for a wide variety of end applications [1]. In tanning processes tanning materials are able to crosslink with reactive site of fibrous protein, and it involves the conversion of putrefiable skin or hides to a non-putrefiable material by employing various techniques [2]. There are a wide variety of tanning agents that are available at the market. Some of the tanning agents that have been used currently include vegetable, alum, chrome, oil, aldehyde, and synthetic tannage.
Leather making is a lengthy process and involves the use of many different chemical and mechanical processes viz., soaking, liming, deliming, pickling, tanning, post tanning and finishing. Among the processes, tanning is considered as one of the important processes that protect the leather against microbial degradation and tanning with vegetable materials is the oldest process in the leather sector. Although chrome tanning has gained importance in leather manufacture ad about 90% of leather is manufactured using chrome tanning systems, its advantage is over shadowed by its negative impact on the environment, which in turn because of the low uptake of Cr (III) salts which released in tannery effluent. In general, leather making plant materials are the interest of tanners and a prominent theme for researchers in the area.
Vegetable tanning is the most eco-friendly method as compared to chemical tanning process and it discharge minimum pollutants to the environment. Vegetable tannins are polyphenols with molecular weight ranging 500 − 20,000 Daltons, which present in vegetable. The type, the maturity and the sources of vegetable tanning materials have an impact on the quality of leather regardless of tanning and post-tanning processes. Vegetable tanning uses vegetable materials (from leaves, fruits, seeds ad arks) to process hides/skins into waterproof, non- putrefiable, soft and supple [1]. As a result, many researchers have been focused on the development of a greener leather tanning processes using new plant as a natural source materials (vegetable tannins). Vegetable tannins are plant-based polyphenols that are water-soluble and capable of reacting with collagen. Up to know, a very limited number of plant species are exploited for tanning applications. Recently attempts have been made to explore the use of new plant materials for tanning. Recently, in an attempt of the use of natural materials in tanning process, one of the co-authors in this manuscript have recently established the use of mekmeko (Rumex abyssinicus) for preservation, tanning and dyeing in leather manufacture [4–7]. Our extensive search indicates that researches is being undertaken to establish vegetable tanning using new plant materials. However, there is no reported literature indicated on the use of Sodom apple as a tanning agent. Thus, in this study, an attempt was made to evaluate the potential of Sodom apple for tanning with satisfactory hydrothermal stability and strength characteristics. Sodom apple (Solanum incanum) commonly known as bitter garden egg is one of the 1,500 Solanum species in the world that belongs to the family Solanaceae. It is a shrub, growing 1–3 m high with spines on the stem, leaves, stalks and calyces, and with velvet hairs on the leaves. The leaves are simple, ovate, elliptic, 2.5–12 cm long and 2.5–8 cm wide, alternate, flowers often borne in the leaf axilles, sometimes solitary or in few flowered clusters [8]. The fruits are small berries of 2–3 cm in diameter and yellowish orange or brown in color when ripe. It is also spherical, green, often striped with white, turning yellow to orange-brown when ripe [9]. It has different vernacular name in different ethnic language of Ethiopia for instance, “Embuay’ in Amharic. This plant has been producing fruits twice a year and it is very easy to grow it especially in hot areas. Extracting the tannin from this plant and applying it for leather tanning process is economically and environmentally feasible and important.