The addition of organic solvents to α-amino acids in aqueous solution could be an effective method in crystallization. We reviewed the available data on the solubility of α-amino acids in water, water–ethanol mixtures, and ethanol at 298.15 K and 0.1 MPa. The solubility of l-alanine, l-proline, l-arginine, l-cysteine, and l-lysine in water and ethanol mixtures and the solubility of l-alanine, l-proline, l-arginine, l-cysteine, l-lysine, l-asparagine, l-glutamine, l-histidine, and l-leucine in pure ethanol systems were measured and are published here for the first time. The impact on the solubility of amino acids that can convert in solution, l-glutamic acid and l-cysteine, was studied. At lower concentrations, only the ninhydrin method and the ultraperfomance liquid chromatography (UPLC) method yield reliable results. In the case of α-amino acids that convert in solution, only the UPLC method was able to discern between the different α-amino acids and yields reliable results. Our results demonstrate that α-amino acids with similar physical structures have similar changes in solubility in mixed water/ethanol mixtures. The solubility of l-tryptophan increased at moderate ethanol concentrations.

Amino acids are important in human and animal diet, as well as being potential feedstocks for chemical production. Amino acids can be obtained from protein after hydrolysis. In addition, several agro-industrial residues already contain a mixture of free amino acids. The objective of this study was to develop a method for amino acids separation, starting from mixtures containing amino acids, and using antisolvent precipitation with ethanol. Protamylasse, rubber seed protein hydrolysates, and grass juice were used in the experiments, representing existing and potential agro-industrial residues. Our results show that in a water–ethanol system, some amino acids had lower solubility in mixtures than as a single component, thereby facilitating precipitation. A sufficiently high total amino acid concentration in the mixture is needed to achieve precipitation; therefore, a concentration step is sometimes required. Ethanol precipitation can be applied as a pretreatment to separate mixtures into groups of amino acids or a polishing step to increase purity.

The addition of organic solvents, such as ethanol, to molecules in solution is an effective process for crystallization and is used in industrial settings (i.e. pharmaceutical production, downstream processing, etc.). In this study, we use solubility data of all proteinogenic α-amino acids in binary ethanol/water systems to model their excess solubility. We use the empirical and regressive models of Gude and NRTL and the predictive Jouyban-Acree model. Based on the results, we hypothesize that amino acids that are spherical and lack a reactive side chain show little or no excess solubility. Being rod-like and/or having a reactive side chain leads to a positive excess solubility in a mixed solvent of ethanol and water. The empirical and regressed models, NRTL and Gude, fit the data well and the predictive Jouyban-Acree model, not originally intended to be used for small molecules, is less accurate but offers insights into the thermodynamic properties of the amino acids.