Doctoral Degrees (Food Science)

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Browsing Doctoral Degrees (Food Science) by browse.metadata.advisor "Du Plessis, Anton"

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    Characterisation and quantification of microstructure, physicochemical and functional properties of oven and forced convection continuous tumble roasted cereal grains
    (Stellenbosch : Stellenbosch University, 2017-03) Schoeman, Letitia; Manley, Marena; Du Plessis, Anton; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.
    ENGLISH ABSTRACT: The use of roasted cereal grains in foods can improve organoleptic properties, enhance shelf life, ease incorporation into ready-to-eat products and increase antioxidant activity. Structural, physicochemical and functional changes will inevitably occur during roasting and this study aimed to investigate these properties. X-ray micro-computed tomography (μCT) was used as a non-destructive technique for characterising and quantifying microstructural changes in individual cereal grains induced by conventional oven and forced convection continuous tumble (FCCT) roasting at 180°C for 140 s. X-ray μCT uses differences in X-ray attenuation arising from differences in density within a sample. X-ray tomograms of the raw and roasted wheat and maize kernels were obtained, using a General Electric Phoenix model V|Tome|X L240 X-ray μCT system with a source voltage of 60 kV and an electron current set at 240 μA. Analysis of the whole kernel and regions-of-interest (ROIs) was performed with VGStudio Max 2.2 three-dimensional (3D) software. Qualitative results were depicted as two-dimensional (2D) transmission images and 3D volumes. Internal structural changes were observed as a loss of endosperm integrity, detected as a decrease in attenuation. For both cereal grains oven roasting was associated with a larger increase in kernel volume (wheat=4.47%; maize=10.76%) than FCCT roasting (wheat=1.57%; maize=3.41%), as well as larger relative density decreases (wheat=2.76%; maize=6.33%) in comparison to FCCT roasting (wheat=0.55%; maize=1.92%). During FCCT roasting the material density (excluding air) remained unaffected. Structural changes can strongly influence physicochemical and functional properties. Kernel hardness and hectolitre mass (HLM) can be helpful to assess the milling yield, which was not affected (P>0.05) by either roasting methods. Scanning electron microscopy (SEM) illustrated the starch-protein morphology, where both roasting methods resulted in a partially disintegrated protein network and swollen and/or ruptured starch granules in the oven-roasted samples. Quantification of these structural differences included crystallinity determinations using X-ray diffraction (XRD), thermal properties using differential scanning calorimetry (DSC) and pasting properties employing a Rapid Visco Analyser (RVA). The reduction in crystallinity for both FCCT (wheat=0.12%; maize=0.45%) and oven (wheat=0.16%; maize=1.83%) roasting was consistent with the decrease in gelatinisation enthalpy (ΔH). Only partial gelatinisation occurred in both the oven (wheat=17.16%; maize=25.27%) and FCCT-roasted (wheat=10.14%; maize=16.23%) samples. Oven roasting caused more adverse changes in most of the measured properties, being a more destructive process. With FCCT roasting the samples are continuously moving, resulting in an even heat transfer in comparison to oven roasting where the samples are stationary. FCCT roasting maintained more of the favourable characteristics, i.e. microstructure, hardness and milling yield and will thus result in a more acceptable final product. This study demonstrated the capability of X-ray μCT in combination with image analysis as a non-invasive technique to study microstructural changes in cereal grains, induced by roasting, both in a qualitative and quantitative manner and at a relatively high spatial resolution of 12 micron. The results integrate qualitative and quantitative information that could be useful for understanding structure-property relationships in terms of further processing and utilisation, e.g. the development of value-added products with improved digestibility and viscosity or delayed staling phenomenon.
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    Maize endosperm texture characterisation using the rapid visco analyser (RVA), X-ray micro-computed tomography (μCT) and micro-near infrared (microNIR) spectroscopy
    (Stellenbosch : Stellenbosch University, 2015-04) Guelpa, Anina; Manley, Marena; Geladi, Paul; Du Plessis, Anton; Stellenbosch University. Faculty of Agrisciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Maize kernels consists of two types of endosperm, a harder vitreous endosperm and a softer floury endosperm, and the ratio of the vitreous and floury endosperm present mainly determines the hardness of the kernel. Maize (Zea mays L.) is a staple food in many countries, including South Africa, and is industrially processed into maize meal using dry-milling. For optimal yield and higher quality products, hard kernels are favoured by the milling industry. Despite many maize hardness methods available, a standardised method is still lacking, furthermore, no dedicated maize milling quality method exists. Using an industrial guideline (chop percentage), a sample set of different maize hybrids was ranked based on milling performance. Unsupervised inspection (using principal component analysis (PCA) and Spearman’s rank correlation coefficients) identified seven conventional methods (hectoliter mass (HLM), hundred kernel mass (HKM), protein content, particle size index (PSI c/f), percentage vitreous endosperm (%VE) as determined using near infrared (NIR) hyperspectral imaging (HSI) and NIR absorbance at 2230 nm (NIR @ 2230 nm)) as being important descriptors of maize milling quality. Additionally, Rapid Visco Analyser (RVA) viscograms were used for building prediction models, using locally weighted partial least squares (LW-PLS). Hardness properties were predicted in the same order or better than the laboratory error of the reference method, irrespective of RVA profile being used. Classification of hard and soft maize hybrids was achieved, based on density measurements as determined using an X-ray micro-computed tomography (µCT) density calibration constructed from polymers with known densities. Receiver operating classification (ROC) curve threshold values of 1.48 g.cm-3 , 1.67 g.cm-3 and 1.30 g.cm-3 were determined for the entire kernel (EKD), vitreous (VED) and floury endosperm densities (FED), respectively at a maximum of 100% sensitivity and specificity. Classification based on milling quality of maize hybrids, using X-ray µCT derived density and volume measurements obtained from low resolution (80 µm) µCT scans, were achieved with good classification accuracies. For EKD and vitreous-to-floury endosperm ratio (V:F) measurements, 93% and 92% accurate classifications were respectively obtained, using ROC curve. Furthermore, it was established that milling quality could not be described without the inclusion of density measurements (using PCA and Spearman’s rank correlation coefficients). X-ray µCT derived density measurements (EKD) were used as reference values to build NIR spectroscopy prediction models. NIR spectra were acquired using a miniature NIR spectrophotometer, i.e. a microNIR with a wavelength range of 908 – 1680 nm. Prediction statistics for EKD for the larger sample set (where each kernel was scanned both germ-up and germ-down) was: R2 V = 0.60, RMSEP = 0.03 g.cm-3 , RPD = 1.67 and for the smaller sample set (where each kernel was scanned only germ-down): R2 V = 0.32, RMSEP = 0.03 g.cm-3 , RPD = 1.67. The results from the larger sample set indicated that reasonable predictions can be made at the fast NIR scan rate that would be suitable for breeders as a rough screening method.