Doctoral Degrees (Food Science)

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Browsing Doctoral Degrees (Food Science) by Author "Germishuys, Zandré"

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    X-ray micro-computed tomography (μCT) evaluation of bubble structure to determine quality of dough and bread made from roasted wheat flour
    (Stellenbosch : Stellenbosch University, 2021-04) Germishuys, Zandré; Manley, Marena; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.
    ENGLISH ABSTRACT: Roasting of cereals has been shown to improve its sensory properties, increase its shelf life and inactivate proteolytic enzymes promoting increased loaf volume. The breadmaking process is a series of aeration stages which ultimately affects the final crumb structure of the bread. X-ray micro-computed tomography (μCT) is a non-invasive technique capable of producing high quality three-dimensional images enabling microstructural evaluation of food products. The aims of this study were to characterise white flour produced from roasted wheat differing in hardness and protein content using rheological and physicochemical analyses; to determine the optimal roasting conditions that would minimise the effect on the protein properties of the produced white flour; to evaluate the freeze drying of dough as a suitable sample preparation procedure to maintain the structure of the samples during X-ray μCT scanning for efficient analysis of the bubble structure of dough and foam properties of 10 g bread loaves produced from roasted wheat flour; and to evaluate the quality and shelf life of bread loaves prepared from roasted wheat flour by means of C-Cell and texture analysis. Hard, medium and soft textured wheat kernels were roasted for 140 s at 180°C using a forced convection continuous tumble (FCCT) roaster. This resulted in the largest reductions in hectolitre mass (7.36 hg/hl), flour yield (2.33%) and moisture content (2.87%) for the hard wheat. The largest increase in damaged starch (4.54%) and flour ash (0.06%) was observed for the hard and soft wheats, respectively. Roasting resulted in gluten protein changes as a gluten network could not be formed during dough mixing with the roasted flours. The use of composite flours (80% untreated flour and 20% flour from roasted wheat) displayed the largest increase in water absorption capacity (5.2%) for the medium textured wheat. Improved alveograph P/L ratios and higher levels of free starch was observed for the hard and medium textured wheat. The central composite design (CCD) showed no significant differences (p>0.05) for protein content, mixograph peak time and peak height for either the high or low protein roasted wheat. The roasting conditions chosen (based on trends observed) for X-ray μCT evaluation was 90°C and 86 Hz (ca. 130 s) as this combination maximised protein content and peak height and minimised peak time. To evaluate the bubble structure of dough and the foam structure of bread, 20- and 40 min proofed dough as well as 10 g bread samples, produced from roasted wheat flour based on the CCD, were subjected to X-ray μCT. The use of 10 g dough and bread samples enabled scanning at a much higher resolution. A finer crumb structure and softer texture was observed for the bread produced from roasted wheat flour due to decreased strut thicknesses. Lower mixograph peak heights and increased porosity suggested a weaker gluten strength for the roasted wheat samples. The roasting conditions used did not negatively impact the foam properties of the breads. C-Cell analysis showed a coarser crumb structure and a darker crumb colour for breads produced from roasted wheat flour although this did not negatively impact the breads texture. More importantly, texture analysis showed the use of flour produced from roasted wheat resulted in a softer bread (i.e., lower firmness) with an increased shelf life.