3.4. Aminophilic reaction for removal of orange Monascus pigments
The tert-butanol phases after three times of butanol–ionic liquid
two-phase extraction were combined, which was subjected
to air stripping off solvent and to achieve oil-like slurry. The
oil-like slurry was re-dissolved with cyclohexane and dried again
for obtaining crude Monascus pigments. In this process, the recovery
yield of Monascus pigments was approximately 50% based on
absorbance at 410 nm (detailed in Fig. S2). The crude Monascus pigments
was dissolved in ethanol aqueous solution (70%, V/V, pH = 2)
to carry out aminophilic reaction. After complete aminophilic reaction,
addition of cyclohexane into the reaction solution separated
into two phases, where the upper one (cyclohexane) was yellow
while the lower one (ethanol aqueous solution) was red1
(Fig. 4A). The visible spectrum of reaction solution, the cyclohexane
phase, and the ethanol solution phase was determined, respectively
(Fig. 4B), in which pigment absorbance of the cyclohexane phase was
normalized to 24 ml. The pigment spectrum of the cyclohexane
phase exhibited the character of yellow Monascus pigments with
an apex at 395 nm while that of the ethanol solution phase exhibited
the characteristic apex at 510 nm of red Monascus pigments [26].
Those results were further confirmed by TLC analysis (Fig. 4C).
Thus, yellow Monascus pigments was achieved in the cyclohexane
phase by conversion of hydrophobic orange Monascus pigments into
hydrophilic red ones via aminophilic reaction. Further purification of
the yellow Monascus pigments was carried out using silica gel
column chromatography (detailed in Fig. S3) and yellow Monascus pigments could be crystallized in ethanol aqueous solution.