Introduction
l-Glutamine (l-Gln, Fig. 1), a derivative of l-glutamate (l-Glu), is
considered as a conditionally essential amino acid and a promising
new drug [1]. It is the most abundant amino acid in blood, and is a
carrier or nitrogen transport. l-Gln plays an important role in regulating
the synthesis of protein, nucleic acid and lycoprotein. Medical
studies have indicated that a shortage of l-Gln could cause many
diseases [2]. l-Gln is mainly produced by microbiological fermentation.
The manufacturing process of an amino acid by fermentation
comprises fermentation, crude isolation and purification processes.
Final purification is performed to ensure the required quality for the
intended use. The final product is obtained as a crystalline powder.
In the final purification step, l-Gln is often refined through
crystallization from solution. But the solubility of l-Gln in water is
barely affected by temperature [3]. In addition to this, the higher
temperature favors l-Gln degradation [4]. Consequently, cooling
crystallization for which the precondition is to heating the system
is not applicable for harvesting l-Gln.
l-Gln is stable around the isoelectric point (pH 5.65), but if the
pH shifts from the isoelectric point to either acid or alkaline conditions,
l-Gln is easily hydrolyzed to l-glutamic acid and ammonia