Characterization of water-solid interactions in crystalline ingredients and development of deliquescence measurement recommendations
There are five major mechanisms of water-solid interactions. The primary focus of this thesis was on two of these: deliquescence and hydrate formation. Many crystalline food ingredients are deliquescent compounds (e.g., NaCl, sucrose, and ascorbic acid) and some are both deliquescent and hydrate formers (e.g., glucose, thiamine HCl, citric acid). Deliquescence is the first order phase transformation of a crystalline solid to a solution above a critical relative humidity (RH) known as the deliquescence point (RH0). A crystalline hydrate is a pseudo-polymorph in which water is incorporated into the crystal structure, altering the molecular formula and the physical properties. To design stable formulations, it is important to know the deliquescence points of all ingredients present. Multiple approaches have been reported for determining the RH0; however, the discontinuity between methods may influence the reported values. The objective of this study was to provide a comprehensive comparison of methods used to measure the deliquescence points of single ingredients and blends. The effects of altering sample preparation and experimental parameters on the measured deliquescence points were determined using the following methods: water activity of saturated solutions, gravimetric moisture sorption isotherms, dynamic dewpoint sorption profiles, and static isopiestic methods. Significant differences (p<0.05) in measured deliquescence RHs were found between different methods. Advantages, disadvantages, precision, accuracy, run times, and costs of the different methods were summarized. When two or more deliquescent ingredients are in contact, the deliquescence RH of the blend, RH0mix, is always lower than the individual ingredient RH0s. The RH0mix has been be estimated using the Ross Equation, which assumes the presence of an ideal solution. However, when the compounds in a blend share a common ion, a diminished deliquescence lowering effect (higher than the Ross Equation predicted RH0mix) occurs. This can be attributed to the common-ion effect. The diminished deliquescence lowering effect was found in blends of organic and/or inorganic ingredients that shared a common ion (both anion and cation), and higher deviations between the Ross Equation predicted RH0mix and measured RH0mix increased with increasing number of ingredients. A new modified Ross Equation was developed to compensate for the common-ion effect, and RH0mix predictions using this equation correlate well with measured RH0mix values of blends with common ions. Deliquescence and hydrate formation are influenced by RH and temperature and the boundaries can be plotted on a RH-Temperature phase diagram. Each deliquescent hydrate forming ingredient has a minimum of three boundaries: the hydrate RH0, the anhydrous RH0, and the hydrate formation boundaries. As temperature increases, the hydrate formation RH boundary increases and the hydrate RH0 boundary decreases, eventually intersecting. The intersection is known as the peritectic temperature. Above the peritectic temperature the hydrate is no longer thermodynamically stable and an anhydrous RH0 boundary is found. It was also shown that the anhydrous RH 0 is a critical boundary below the peritectic temperature as this is the point at which hydrate formation rapidly increases. It was proven that the anhydrous form can deliquesce below the peritectic temperature and RH 0 of the hydrate. Upon deliquescence, rapid hydrate formation takes place slowing down sorption kinetics. This is the first study to report not only the use of water activity measurements to create RH-temperature phase diagrams, but also the RH-temperature phase diagrams of common food ingredients and the relationship of deliquescence to hydrate formation behavior.
Mauer, Purdue University.
Food Science|Physical chemistry
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