It can be produced by a reaction between methyl 2-keto-D-gluconate and sulphuric acid.

Generally, the manufacturing process has 5 steps:

Producing calcium 2-keto-D-gluconate: food-grade starch hydrolysate fermentation by Pseudomonas fluorescens with calcium carbonate.
Acidify the above fermentation broth to obtain 2-keto-D-gluconic acid (2-KG).
Esterification 2-KG with methanol under acid conditions to yield methyl 2-keto-D-gluconate.
The synthesis of sodium erythorbate: heating the above suspension with sodium bicarbonate or sodium carbonate.
The reaction between sodium erythorbate and sulphuric acid.
Structure
Erythorbic acid chemical structure
 
Properties
Appearance    White to slightly yellow crystalline solid which darkens gradually on exposure to light. 
Other names    D-Isoascorbic acid; D-Araboascorbic acid
CAS number    89-65-6
Chemical formula    C6H8O6
Molecular weight    176.13
PKa    It is a diprotic acid having pKa’s 11.34 and 4.04.
Solubility   
In water: 40 g in 100 mL water at 25 °C.
In organic solvents: Soluble in alcohol, pyridine; moderately soluble in acetone; slightly soluble in glycerol