Sorbitan tristearate (abbreviation STS), also known as Span 65, a nonionic surfactant that can be used as an emulsifier and stabilizer in food with the European food additive number E492. Its main functions are to retard fat bloom in chocolates and prevent cloudy appearance in cooking oils.

How is it made?
Like sorbitan monostearate, this ingredient is also made from direct esterification of sorbitol with stearic acid, but different from the former, as with three stearic acid molecules attached to sorbitan molecule in the structure.

The final product is not a pure compound but a mixture of the partial esters of sorbitol and its anhydrides with edible stearic acid.  

Stearic acid
Commercial stearic acid may contain up to 48.7–50.0% palmitic acid.

The purity of food grade stearic acid depends on the raw material sources as well as the manufacturing processes.

Vegetable sourced stearic acid is the most used in the manufacturing process of sorbitan tristearate and other sorbitan esters of fatty acids.

Sorbitol
It is produced from d-glucose which is mainly produced from maize and tapioca.

http://www.chinafooding.com/Sorbitol