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Effect of different banana cultivars on the physico-chemical properties of the flour and the quality characteristics of gluten-free bread.
Kongolo, Joelle Ilunga
Kongolo, Joelle Ilunga
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Abstract
Dessert bananas are the most important subtropical fruit grown in South Africa. However, approximately 20-50% of all bananas harvested are wasted due to post-harvest losses. The production of unripe banana flour (UBF) is one of the ways to valorise the banana crop and reduce waste. The clear benefits of UBF include a high total starch, high resistant starch and high dietary fibre content. In addition, UBF has been identified as a potential commercial ingredient in various food products with increased resistant starch and gluten-free products. Yet, the proportion of fruits processed into flour and the suitability of cultivars used for processing is still relatively unknown. The board aim of the study was to gain a better understanding of the differences between cultivars in terms of their physico-chemical properties. This was achieved by determining the general characteristics, the functional and thermal properties of UBF from 10 different cultivars grown at the Agricultural Research Council Tropical and Subtropical Crops (ARC-TSC). In addition, the potential use of UBF in gluten-free bread production was investigated using sourdough and hydrocolloids. In the first part of the study, the general, functional and thermal properties of UBFs produced from dessert banana cultivars (n = 10) cultivated in South Africa were analysed and juxtaposed to wheat and maize flour. The functional properties varied significantly (p ≤ 0.05) between banana cultivars, and from wheat and maize flours to various extents. Selected properties of UBFs, wheat and maize flours, ranged respectively for: flour colour index (63.16–76.42, 77.34 and 80.96), paste viscosity (405.5–556.6, 124.7 and 115.6 RVU), gelatinization temperature (64.67–71.21, 71.11 and 69.95 °C), gel firmness (7.24–11.44 × 10− 2 N, 3.49 × 10− 2N and 6.56 × 10− 2 N) and resistant starch content (19.9– 47.4, 2.8 and 2.2% w/w). Multivariate analysis (principle component analysis) showed that the UBFs from different varieties were distinguished from each other based on the pasting temperature. The UBFs were distinguished from both wheat and maize flour based on breakdown and peak paste viscosities. The breakdown viscosity was the most positively related measure to the resistant starch content with a linear regression (R2= 0.898) indicating a significant role played by granule structure in resistance to enzymatic hydrolysis. The present research demonstrates that the selection of appropriate dessert banana variety is important when replacing staple flours (wheat and maize) with UBF as a functional ingredient. In the second part of the study, the development of gluten-free bread containing UBF was investigated. The basic formulation containing 60% UBF (Gros Michel cultivar) and 40% white rice flour. Hydrocolloids (guar gum and xanthan gum) and sourdough (fermentation using Lactobacillus Plantarum strain R and A) were added to the gluten-free bread formulation, individually or in combination. The rheological behaviour (Mixolab analysis) of the formulations, the baking characteristics (Volume, height, mass after baking, specific volume, moisture, colour and texture) and sensory evaluation (using a 9-point hedonic scale) of baked bread was assessed. During the development of gluten-free bread containing UBF, the formulation required the addition of more water compared to the wheat formulation. The addition of hydrocolloids (individually or in combination) resulted in the improvement of rheological properties of the formulations. This in turn, after baking, resulted in bread with improved volume (ranging from 18% - 49% increase) and better crumb with more uniform gas distribution. The addition of sourdough improved the quality characteristics of gluten-free bread. The baked bread resulted in improved properties mainly due to the effect of acidification of dough rheological properties (Moroni et al., 2009, Naqash et al., 2017). However, it was noticed that a high amount of sourdough addition resulted in less acceptable bread, particularly in terms of aroma and taste. The sourdough used in combination with hydrocolloids resulted in the best baking performance of the bread, compared to the standard wheat formulation (3.07g/cm3 and 3.59g/cm3 for the gluten-free bread and standard wheat bread respectively). However, when subjected to consumer acceptability test, a clear preference for gluten free-bread containing sourdough only (without hydrocolloids) was observed. This is the first study that investigates the characteristics of UBF from cultivars grown on the same orchard plot in South Africa. These cultivars were subjected to identical environmental conditions, thus reducing variabilities based on environmental factors. This study provides new knowledge that characteristics of flour from banana cultivars from the same genomic group are different independently of the growing conditions the plant was subjected to. Thus it would be important to select the banana cultivar for a specific food application based on the required properties to obtain the best performance. In addition, it was shown that UBF (Gros Michel) can be used successfully as the main ingredient for the development of gluten-free bread, with or without the use of hydrocolloids and sourdough. However, the best baking performance is obtained when sourdough is used in combination with hydrocolloids. Thus further research into the synergistic effect of the addition of sourdough and hydrocolloids on gluten-free bread containing UBF is being investigated to determine whether or not the improvement observed is dose-dependent.
Description
Submitted in partial fulfilment of the requirements for the degree, Magister Technologiae in the Department of Biotechnology and Food Technology, Faculty of Science at the Tshwane University of Technology.
Date
2019-05-01
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Tshwane University of Technology
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Keywords
Subtropical fruit, Banana, Post-harvest, Agricultural Research Council Tropical and Subtropical Crops, South Africa
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CC0 1.0 Universal