From the early 20th century, acid-hydrolysis method was developed
to shorten the lengthy brewing process. The proteins in soybeans
and wheat are hydrolyzed by heating in hydrochloric acid
for about 12–16 h until practically all the amino acids are liberated
(Yong & Wood, 1974). Nevertheless, some side products such as
levulinic acid (LV), 3-chloro-propane-1,2-diol (3-MCPD), formic
acid and sulfur compounds are generated in the chemical process,
which cause unnatural flavors. According to JAS (Japanese
Agricultural Standards (JAS) Association, 1978) and CNS (