Ultrasonic characterization of crystal dispersions
Open Access
- Author:
- Yucel, Umut
- Graduate Program:
- Food Science
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- March 04, 2010
- Committee Members:
- John N Coupland, Ph D, Thesis Advisor/Co-Advisor
John Neil Coupland, Thesis Advisor/Co-Advisor - Keywords:
- sensor
crystal dispersions
ultrasound - Abstract:
- The aim of this study was to investigate the applicability of ultrasonic techniques to the characterization of crystal dispersions. Longitudinal ultrasonic waves (2.25 MHz) were used to follow the changes in dispersions of sugar crystals (i.e., lactose or sucrose) in different liquid phases (i.e., water or vegetable oil). In general, it was found that the ultrasonic attenuation measurements were more sensitive to the amount and size of the dispersed crystals while velocity measurements were more sensitive to the changes in total concentration (dissolved or undissolved). Ultrasonic measurements were used to measure the compositions (e.g., concentration) of lactose solutions and aqueous suspensions of lactose crystals. The ultrasonic velocity in lactose-water mixtures (i.e., solution or dispersion) was independent of the state of lactose molecules (i.e., either dissolved or crystallized) and increased almost linearly with lactose concentration. In contrast to the ultrasonic velocity measurements, ultrasonic attenuation was relatively unaffected by changes of lactose concentration in solution, but increased with the amount of dispersed crystals. Similarly in lipid systems, attenuation was low in oil and increased with dispersed sucrose crystal concentration. Furthermore, it was also found in lipid systems that sucrose crystal aggregation (induced by adding water) increased ultrasonic attenuation. Since ultrasonic attenuation is more sensitive than velocity measurements to the presence of dispersed particles in a liquid, attenuation measurements were used to follow dynamic changes in the dispersed phase through dissolution and crystallization of lactose in aqueous environments. The dissolution of lactose crystals (d~50 ¦Ìm) in stirred aqueous solutions was investigated using continuous on-line ultrasonic attenuation measurements and at discrete intervals by off-line refractive index measurements. Upon addition of powdered sugar into water or an under-saturated solution, air pockets between and around the crystal agglomerates strongly attenuated the acoustic signal, and obscured any changes resulting from the dissolution process until all air was removed. In order to minimize air incorporation, water was added to suspensions of lactose crystals in saturated lactose solution to produce an under-saturated solution and cause crystal dissolution. In this case the rate constant for lactose dissolution evaluated from on-line attenuation measurements (~700 ¦Ìs-1) was in reasonable agreement with off-line measurements and literature values. The crystallization of lactose within gelatin gels was also monitored by ultrasonic attenuation measurements using a modified pulse-echo reflectometer as an immersion probe, and results were compared to turbidity and isothermal differential scanning calorimetry measurements. The gel environment prevented crystal sedimentation and hindered convection flow effects and secondary nucleation. The crystallization rate of lactose was increased with increasing degree of supersaturation, but relatively unaffected by gelatin concentration. The kinetic parameters for growth rate and induction time varied in a similar way with changes in lactose and gelatin concentration for all methods. Although it is difficult to make conclusions about the relative effects of gelatin-lactose interactions on lactose crystallization, ultrasonic measurements were able to differentiate between different crystallization conditions. Finally, the effects of dispersed crystal (de)aggregation on ultrasonic attenuation were studied using the model of sucrose crystals dispersed in oil (8-16 wt%). During the dispersing process, powder agglomerates are broken up and the viscosity of the suspension decreases (e.g., chocolate conching). It was found that the deagglomeration of sucrose crystals (i.e., dispersing the sugars in a liquid uniformly) in oil was much slower than in an aqueous environment due to the hydrophobic nature of the oil and the possible presence of small amounts of water at the crystal surfaces. Attenuation increased linearly with concentration, and also with increasing effective particle size. Additionally, when crystal aggregation was triggered by the addition of small amounts of water (¡Ü1 %), the ultrasonic noise also increased, which may be due to either uneven distribution of agglomerates or the inhomogenity of the agglomeration process (i.e., diverse particle size distribution and morphology). Finally, the stirrer was switched off and the crystals and aggregated crystals were allowed to sediment out of suspension. The subsequent sedimentation kinetics, as followed at a certain height from the bottom of the container through the ultrasonic beam path, was used to provide further information about the state of agglomerated sucrose crystals. It was found that the addition of 1% (vol. water/wt. sucrose) decreased the total sedimentation time to half that of the water-free samples. In addition, the agglomeration process was shown to be inhomogeneous yielding uneven sedimentation profiles, as the degree of inhomogeneity was confirmed by micrometer measurements.