Investigating the impact of pre-harvesting sprouting on maize hardness using near infrared (NIR) hyperspectral imaging and X-ray micro-computed tomography (μCT)
Thesis (MSc (Food Sc))--Stellenbosch University, 2016.
ENGLISH ABSTRACT: Maize forms an integral part of the human energy intake in South Africa and its quality should always be maintained. A quality attribute important to the dry milling industry is maize hardness. In the milling industry, hard maize hybrids are described as kernels with a higher ratio of vitreous endosperm in comparison to floury endosperm. Certain circumstances such as agricultural conditions (e.g. pre-harvest germination), post-harvest conditions and improper treatments (e.g. no irrigation during periods of insufficient rain or improper drying procedures) will have a negative impact on maize hardness. This study focused on the impact of pre-germination on maize hardness, subsequently observations on the rate of the pre-germination process on three hardness variants (hard, intermediate and soft) were noted. Six maize hybrids with varying hardness levels were pre-germinated (from 0 – 22 h) bi-hourly and three imaging techniques (Scanning electron microscopy (SEM), near-infrared (NIR) h y p e r s p e c t r a l imaging and X -ray m i c r o computed tomography (X-ray μCT)) used to investigate the impact of the process on hardness. From the NIR hyperspectral imaging technique, principal component analysis (PCA) score plots and score images were employed in the investigations due to the amount of data obtained. Three types of endosperm (vitreous, intermediate or floury) were observed in the first principal component (PC1) of the score images. PCA classification plots revealed the three types of endosperm present within a maize kernel. Classification plots (both score plots and images) allowed for the isolation of the vitreous endosperm by removing the two clusters representing the floury and intermediate endosperm. PCA score images depicted decreasing trend in the content of the vitreous endosperm as pre-germination took place. Another observation from the PCA score images was that, the soft maize hybrids showed traces of vitreous endosperm after 8 h of pregermination. The intermediate hybrids showed a decrease in the endosperm content at 12 h and the hard hybrid was noted to be at 18 h. Pixel count (obtained from the PCA score plots) indicated a decreasing trend in all hybrids investigated. The graphs plotted from pixel counts of hard hybrids (i.e. H2 and H3), intermediate (H7) and soft (H9) depicted decreasing curvilinear plots. The rest of the hybrids’ graphs (H6 and H5) depicted a decreasing linear trend. X-ray μCT indicated fissures and shrinkage stress cracks occurring as a results of either or both of the pre-germination and drying processes. Larger fissures were assumed to have developed due to pre-germination and shrinkage stress cracks from the drying process. These (i.e. fissures and shrinkage stress cracks) were observed in all the 10 h and 22 h images, on 2D slices and 3D volumes. The side orientation 2D image slices depicted the intermediate and soft maize hybrids’ endosperm integrity as having deteriorated more than the hard hybrid at 10 h and 22 h incubation time period. The top orientation of the 2D slice images indicated the hard and intermediate deteriorated more than the soft hybrids. It was noted that the fissures and shrinkage stress cracks developed throughout the maize kernel. SEM was used to validate results obtained from the X-ray μCT imaging system, fissures could also be observed on SEM images. Starch and protein matrix deterioration was also observed. iii Stellenbosch University https://scholar.sun.ac.za Starch granules and protein matrix developed numerous pores on the surfaces indicating the extent of deterioration. Single kernel analysis using 2D slices at T8 = 21 h, crevices started to develop and propagated until the end of the pre-germination process at T19 = 143 h. Fissure measurement indicated an increment in all measured areas of the maize kernels, i.e. from the left side of the maize kernel fissures propagated from 1.51 mm at 99 h to 4.22 mm at 143 h, on the right side from 1.86 mm at 99 h to 3.65 mm at 143 h. At T15 = 123 h, a horizontal fissure was observed and measured to be 1.62 mm long while at T19 = 143 h it had propagated to 4.30 mm. The fissures were noted to be pathways used to transport hydrolytic enzymes and monomers (obtained from starch and protein hydrolysis) to the germ where the growing embryo utilises them as a source of nutrition. Volumes of the vitreous endosperm were also determined and a decreasing trend was noticed. At T1 = 0 h the content of the vitreous endosperm was 64.7 mm3 and at T19 = 143 h the content had decreased to 50.5 mm3. Endosperm deterioration due to pre-germination should thus be of great concern to the milling industry as it influences the desired end product.
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