The use of water under subcritical conditions (subW) for the processing of okara, a by-product of
soybean beverage production, has promoted advancements in sustainable resource management.
A key advantage of using water as a solvent is the elimination of the need for additional chemicals.
Industrial okara (Frías Nutrición S.A.U., Burgos, Spain) exhibits a moisture content of 82% w/w
and a protein content of 32% w/w. The subW process is environmentally friendly and eliminates
the need for a drying step, thus reducing energy costs. Various stages have been proposed to
fractionate the valuable bioactive compounds of this by-product: (1) extraction of isoflavones at
120 °C in a batch reactor and (2) extraction and hydrolysis of proteins from the residual solid at
higher temperatures in a semi-continuous reactor. More than 1000 μg isoflavones/g dry okara was
obtained after 30 min at 120 °C, with the genistein family being the most abundant group. The
residual solid was processed in a 2-liter semicontinuous extractor without drying. A total of 625
grams of solid were placed in baskets horizontally. Water flowed at 0.82 L/min, with a residence
time of 2.7 minutes. Two sequential temperatures were tested, 180ºC and 270ºC. At 270ºC,
approximately 80% of the protein was hydrolyzed. Very low concentrations of degradation
products were observed, owing to the short residence time. A medium productivity values of 1.2
g protein/Lh was achieved in a semi-continuous plant. SubW treatment is a promising green
chemistry approach for the sustainable valorization of by-products.
