Browsing by Author "Retief, Liezel"

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    Analysis of vegetable oils, seeds and beans by TGA and NMR spectroscopy
    (Stellenbosch : University of Stellenbosch, 2011-03) Retief, Liezel; Koch, Klaus R.; McKenzie, Jean; University of Stellenbosch. Faculty of Sciences. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Due to the commercial, nutritional and health value of vegetable oils, seeds and beans, the analysis of their components is of much interest. In this dissertation oil-containing food products, specifically vegetable oils, seeds and beans, were investigated. Selected minor components of three locally produced vegetable oils, namely apricot kernel, avocado pear and macadamia nut oils were investigated using 31P NMR spectroscopy. These minor components, including 1,2 diacylglycerols, 1,3 diacylglycerols and free fatty acids, were identified in the 31P NMR spectra of each of the three vegetable oils for the first time. Two approaches were used for the quantification of the minor components present in the spectra. A calibration curve approach used known concentrations of standard minor components to establish calibration curves while a direct correlation approach calculated the unknown concentration of minor components in the vegetable oils using a known amount of standard compound within the analysis solution. These approaches aided in determining the concentration of minor components during storage studies in which vegetable oils were stored in five different ways: exposed to light, in a cupboard, in a cupboard wrapped in tin foil, at -8 °C and at 5 °C. It was found that determining the best storage condition for each oil was difficult since individual minor components were affected differently by the various storage conditions. However, in general the best storage conditions appeared to be 5 °C and -8 °C. The oil, protein and carbohydrate contents of sesame, sunflower, poppy, and pumpkin seeds, and soy, mung, black and kidney beans were analysed by thermogravimetric analysis and 13C NMR solid state NMR spectroscopy. It was shown that the first derivative of TGA data for seeds and beans can give valuable information about the carbohydrate, moisture, protein and fat content. This has not been previously demonstrated. For the seeds, the integration of a region between 270–480 ºC was equal to the sum of the oil and protein content and compared well to quantitative results obtained by other conventional methods. For beans the integration of a region between 180-590 ºC, gave a value which represented the sum of the oil, protein and carbohydrate content. 13C solid state NMR spectroscopy, including SPE-MAS, CP-MAS and variable contact time experiments, was carried out on these seeds and beans and gave valuable information on the solid-like and liquid-like components. To our knowledge these seeds and beans have never been previously analysed using this technique. 13C SPE-MAS NMR spectroscopy indicated that the seeds contained more liquid-like components than the beans. In turn the 13C CP-MAS NMR spectra indicated that beans had higher levels of solid-like components than the seeds. These conclusions correlated well with the quantities of liquid-like components and solid-like components that were determined by conventional methods and TGA. Preliminary studies using T1pH experiments on the components present in the seeds and beans led to a few observations. Most interesting is that a model using a two- phase fit in order to determine T1pH values appears to be more accurate than a one-phase model.
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    The development and refinement of a questionnaire on the investigation of students' experience of first-year chemistry practicals
    (University of KwaZulu-Natal, 2013) Retief, Liezel; Lutz, Marietjie; Potgieter, Marietjie
    This article reports on the development and refinement of a questionnaire for first-year Chemistry students. Three subscales were probed, namely affective attitudes, perceptions towards tutor effectiveness and outcomes achieved.
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    Major component determination of selected vegetable oils by means of qualitative and quantitative ¹³C NMR spectroscopy
    (Stellenbosch : Stellenbosch University, 2006-12) Retief, Liezel; Koch, K. R.; McKenzie, J. M.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: The aim of this study was to assign the 13C Nuclear Magnetic Resonance (NMR) spectra of the major fatty acid components of six locally produced vegetable oils (apricot kernel, avocado, grapeseed, macadamia nut, mango kernel and marula oils). A first attempt at the assignment of the spectra involved the use of a literature method which had been developed for the assignment of the 13C NMR spectrum of olive oil. As this method proved to be time consuming and did not allow for the complete assignment of the oils a new rapid and potentially generic graphical method for the full assignment of the 13C NMR spectra of the vegetable oils was developed. The method was shown to be especially useful for regions of the 13C NMR spectra where significant peak overlap occurs. The method has been validated by using it to assign the 13C NMR spectrum of olive oil which has been extensively reported in the literature. Concentration studies have also proved that the method is independent of the concentration of the oil used thus contributing to its general applicability. Quantification of the major fatty acid components of the six vegetable oils was also carried out using 13C NMR spectroscopy and the results obtained were compared with those obtained from gas-liquid chromatographic studies. This study showed that 13C NMR spectroscopy has use in quantifying the major fatty acid components of a variety of vegetable oils though is limited in that the minor fatty acid residues can not be detected reliably. Finally, due to the fact that resonances were detected in the 13C NMR spectra of the six vegetable oils indicating the presence of free fatty acids, a titration method was also developed for the six vegetable oils to determine the percentages of these free fatty acids. As the quantity of free fatty acids in a vegetable oil is often an indication of the quality of that oil, the development of such a technique has potential use in the quality control of the oils studied. Carrying out these titrations proved more difficult than expected, however a suitably reliable procedure has been developed and is documented here.