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Title:Investigating the loss of crystal structure in carbohydrate materials
Author(s):Schwenk, Michelle P
Director of Research:Schmidt, Shelly J
Doctoral Committee Chair(s):Bohn, Dawn M
Doctoral Committee Member(s):Engeseth, Nicki J; Nehmer, Warren L
Department / Program:Food Science & Human Nutrition
Discipline:Food Science & Human Nutrition
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Twenty-five carbohydrate type materials were screened using differential scanning calorimetry (DSC) to characterize the heating rate dependence of their melting temperature. Three heating rates were used: 1, 5 and 25°C/min. Based on previous research, it was hypothesized that if the material had the same melting onset temperature (Tm onset) regardless of heating rate, the material was classified as a thermodynamic melting material. Five compounds (erythitol, maltitol, mannitol, sorbitol and xylitol) exhibited little to no change in Tm onset regardless of heating rate and were classified in the no/low heating rate dependency group. The eleven compounds (D and L arabinose, fructose, glucose anhydrous, glucose monohydrate, lactose anhydrous, lactose monohydrate, maltose, sucrose, trehalose and xylose) that exhibited a very large difference in Tm onset (greater than 10°C) between the lowest (1°C/min) and highest heating rate (25°C/min) were classified in the high heating rate dependency group and were considered to be apparent melting materials. The remaining compounds (citric acid, galactose, lactitol, lactulose, mannose, raffinose, ribose, and tagatose) exhibited a Tm onset difference of 2 to 10°C between the lowest and highest heating rates and were classified in the medium heating rate dependency group. Further investigation of the materials in the medium heating rate dependency group will need to be done in order to understand more conclusively their thermal behavior and to determine if they are apparent melting or thermodynamic melting materials. A shift in peak onset temperature with heating rate is a function of the activation energy of the process causing the shift. Therefore, each material can show a different shift. High activation energy results in a small shift, while low activation energy results in a large shift. D and L arabinose, fructose, galactose, glucose anhydrous, glucose monohydrate, ribose, xylose, xylitol and citric acid were tested to understand their thermal behavior using regular modulated differential scanning calorimetry (MDSC), quasi-isothermal MDSC and TGA. Using regular MSDC did not yield any meaningful signals outside of total heat flow. The equations used to calculate the regular MDSC signals could not be applied during the loss in crystal structure event, since the modulated heating rate cannot be controlled during the loss of crystal structure. The quasi-isothermal MDSC yielded important information in regards to the reversing heat capacity (Cp) signal. If the reversing Cp signal exhibited signs of recrystallization during melting and returned to the baseline at the end of the heating process, the material exhibited thermodynamic melting. If it did not, the material was could be explained to be apparent melting. An overlay of plots for Tm onset from DSC and Ti (onset of initial decomposition) from TGA showed that several compounds (L- arabinose, D- arabinose and galactose) exhibited a loss in weight that occurred before the onset of the loss in crystalline structure, even without taking thermal lag into consideration. Glucose monohydrate exhibited weight loss that occurs substantially before the onset of the loss of crystalline structure, but the early weight loss is attributed to the loss in hydrate water. Fructose loss of crystalline structure and onset of decomposition overlay showed both events occurring at a similar temperature. Xylose and ribose overlay weight loss occurred at a time slightly after the onset the loss of crystalline structure. In the case of ribose, Tm onset occurs under 100°C, and weight loss would not occur until after 100°C when the water is volatile. The onset of decomposition occurred significantly after the loss of crystalline structure in xylitol. The citric acid Tm onset occurred before the Ti. The difference between Tm onset from DSC and the Ti from TGA were similar numerically for the sugars and citric acid (<15°C), but very different for the polyol, xylitol (~125°C). Through comparison of Tm onset to Ti and through reversing Cp results from Quasi-isothermal MDSC, L- and D- arabinose, fructose, galactose, glucose anhydrous, glucose monohydrate, ribose, and xylose were found to be apparent melting materials, whereas xylitol was found to be a thermodynamic melting material. The results on citric acid were inconclusive. Fructose, galactose, glucose monohydrate, sucrose and xylose were each held at a temperature below their measured Tm onset value for various times. Visual inspection of the samples, reversing heat capacity (Cp), monosaccharides retention and the HMF and furfural of each sample were measured. Sugar retention was measured by HPLC. HMF and furfural were selected as decomposition indicator compounds and were detected by HPLC. Each of the sugars tested showed an increase in reversing Cp, a development of a yellow color within the sugar crystal, a decrease in the amount of sugar present, and an increase in HMF and furfural with increasing time at the isothermal temperature. The sugars showed the decomposition indicator compound (HMF or furfural) being identified after 20% of the stabilization of heat capacity in all cases. Each of these factors (visual appearance, reversing Cp, decomposition products) indicate decomposition occurs below the measured melting temperature, and the loss of crystalline structure is caused by the onset of thermal decomposition. Xylitol was held at its melting temperature (94°C) for 1000, 2000, and 3000 minutes. Holding xylitol at 94°C did not show any change in heat capacity, or loss in xylitol during any of the three holding times. Decomposition products were not detected validating that xylitol did not decompose. When comparing the apparent melting materials (xylose, glucose, fructose, sucrose, and galactose) to related thermodynamic materials (mannitol and xylitol), the major structural difference is the presence of a carbonyl group (aldehydes or ketones) compared to a hydroxyl group. Citric acid also contains a carbonyl group in the form of carboxylic acid, but the results for citric acid were inconclusive. The polyols (xylitol) have been reduced and only contain hydroxyl groups. The polyols were demonstrated to be thermodynamic melting materials.
Issue Date:2016-04-15
Rights Information:Copyright 2016 Michelle Schwenk
Date Available in IDEALS:2016-07-07
Date Deposited:2016-05

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