Multiple Glass Transitionsand Freezing Events ofAqueous Citric Acid.

Calorimetric and optical cryo-microscopemeasurements of 10–64wt % citric acid (CA) solutions subjected to moderate (3 K/min) andslow (0.5 and 0.1 K/min) cooling/warming rates and also to quenching/moderatewarming between 320 and 133 K are presented. Depending on solutionconcentration and cooling rate, the obtained thermograms show onefreezing event and from one to three liquid–glass transitionsupon cooling and from one to six liquid–glass and reverse glass–liquidtransitions, one or two freezing events, and one melting event uponwarming of frozen/glassy CA/H2O. The multiple freezingevents and glass transitions pertain to the mother CA/H2O solution itself and two freeze-concentrated solution regions, FCS1and FCS2, of different concentrations. The FCS1and FCS2(or FCS22) are formed duringthe freezing of CA/H2O upon cooling and/or during the freezingupon warming of partly glassy or entirely glassy mother CA/H2O. The formation of two FCS1and FCS22regionsduring the freezing upon warming to our best knowledge has never beenreported before. Using an optical cryo-microscope, we are able toobserve the formation of a continuous ice framework (IF) and its morphologyand reciprocal distribution of IF/(FCS1+ FCS2). Our results provide a new look at the freezing and glass transitionbehavior of aqueous solutions and can be used for the optimizationof lyophilization and freezing of foods and biopharmaceutical formulations,among many other applications where freezing plays a crucial role. [ABSTRACT FROM AUTHOR]