Microstructural, Physiological and Processing Characteristics of Improved Sweetpotato (Ipomoea batatas Lam.) Varieties

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November, 2017
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Sweetpotato has great potential to significantly impact Ghana’s food and nutritional security. It however remains largely under-utilized due to knowledge gaps about important quality traits and the major factors influencing stability. This study was carried out to characterize selected sweetpotato varieties for important quality attributes and their stability under natural controlled factors. Six improved high-yielding and disease-resistant varieties were studied for fresh roots tissue microstructure, storage stability and wound healing response. Influence of harvest maturity on flour pasting properties and selected nutrient components was also investigated using Rapid Viscosity Analysis (RVA) and Near Infra-red Reflectance Spectroscopy (NIRS) respectively. Starch granule morphology by light microscopy and starch pasting properties using RVA were also determined. The varieties were analyzed for fried crispy chips quality and stability at different stages of crop maturity using Instrumental Texture analysis and Sensory evaluation. Differences were observed in storage parenchyma tissue cell shapes, relative sizes and arrangement for the varieties studied. Storage stability of fresh roots varied significantly by variety and type of primary packaging (p<0.001 and p<0.05, respectively) over a 41- day storage period. Faara was the most stable variety in storage, with minimum average weight loss (7.80%) while Apomuden was the most susceptible to dehydration, with the highest average weight loss (28.02%). Size of sweetpotato roots also had an influence on storage characteristics, with larger-sized roots (330 - 602g) showing better resistance to weight loss than smaller roots (140 - 250g) during a 64-day storage period. Physiological response to wounding assessed through microscopy was found to vary among varieties, and the efficiency of tissue repair in wounded roots had a direct bearing on shelf-life of healthy undamaged roots. Key nutrient components of flour were influenced by harvest maturity and fresh roots storage. At harvest time protein contents ranged from 3.0 to 7.25% and starch contents ranged from 53.93 to 79.40%. Total soluble sugars ranged from 3.92 to 25.02% at harvest and 11.29 to 44.44% after storage. For protein, zinc and iron contents, samples harvested at earlier maturity had higher values. In all the varieties, there were losses in starch content during storage ranging from 1.95 - 23-73% of the total starch. Faara and Hi-starch had the lowest reduction or degradation of starch during storage, with Hi-starch showing the best stability across all maturity stages. Sugars increased concomitantly with the loss of starch during storage. Flour Peak viscosity and Stability ratio were significantly influenced by both maturity and storage (p<0.001), with storage resulting in a lowering of Peak viscosities in all the varieties. Stability ratio reduced at advanced maturity and also with storage, indicating better stability when processed at earlier maturity right after harvest. Among the RVA indices, flour Peak viscosity was the most affected by activity of amylase enzymes. Normal enzyme action accounted for decreases of 13.33% - 45.05% in Peak viscosity when compared with enzyme-inhibited flour pasting profiles. Starch granule shapes were heterogeneous and approximate size distributions varied from 2-15μm to 8-40μm, with Apomuden having the smallest granule sizes. Starch Peak Viscosity ranged from 4077 - 5260 centipoise, Pasting temperature 77.95 - 82.45oC, Setback ratio 1.25 - 1.61 and Stability ratio 0.52 - 0.73. Starches with larger granules had relatively higher Peak viscosities, lower Stability and Setback ratios. The effects of both variety and harvest maturity on starch pasting properties were significant (p<0.001). Instrumental hardness of fried crispy chips was influenced significantly by harvest maturity (p<0.05) and increased with increasing harvest age. Low sweetness and low flavour intensities were associated with higher Overall Acceptability. Chips from varieties with relatively larger starch granule sizes had higher Acceptability scores. These results are relevant in helping to achieve optimal quality and consistency for sweetpotato fresh produce markets as well as processed products for industry. Various recommendations are also made for breeding programmes and other stakeholders.
A thesis Submitted to the Department of Food Science and Technology, Kwame Nkrumah University of Science and Technology, Kumasi in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Science and Technology.