Abstract
The application of faba bean (Vicia faba L.) in wheat bread is increasing due to recent demand among consumers for sustainable plant-based high-protein foods. However, such applications are associated with major technological difficulties; thus, further research is needed to optimise the formulation of faba bean-enriched wheat breads. Moreover, faba bean contains a number of antinutritional compounds that reduce the availability or the digestibility of nutrients, and it is uncertain whether the bread making process is sufficient to effectively inactivate or denature these compounds. The risk of antinutrients withstanding the bread making process can be mitigated by pre-treating faba bean with soaking, dehulling or germination – processing methods which are capable of reducing or eliminating antinutritional compounds. The germination of faba bean can take up to 10 days to complete, which is not feasible in the food industry. Hence, there is a growing interest in the use of short-term germination (24-72 h), as well as the novel processing technology, pulsed electric field (PEF), to improve the efficiency of faba bean germination. PEF technology has also shown potential in enhancing the digestion of compounds such as starch and protein in raw plant materials and could thus be used to facilitate the application of faba bean in food product formulations. There are currently very few studies reporting the use of PEF technology for seed germination, particularly legumes, and its consequence on the digestibility, and thus further research is needed to understand this relationship. Furthermore, the use of a germinated and/or PEF-treated legume flour in a food product formulation has not yet been investigated. Therefore, the aim of this thesis was to investigate the use of PEF technology and short-term germination as pre-treatment processes for faba bean flour and to evaluate their effects on the quality (i.e., volume, specific volume, density, colour, and texture), nutritional composition, in vitro starch digestibility (IVSD) and in vitro protein digestibility (IVPD) of faba bean-enriched wheat bread. ‘Wheat bread’ was referred to as that which was made from refined wheat flour. This is the first study of its kind on the application of a germinated and/or PEF-treated legume flour in a food product formulation.
The optimal formulation of faba bean-enriched wheat bread was determined by substituting the wheat flour (WF; all-purpose) with 10, 20, 30 or 40% mass of whole faba bean flour (FBF) or 10 or 20% mass of faba bean protein-rich fraction (FBPI, ~55% protein content), and assessing the quality (volume, specific volume, density, colour, and texture), nutritional composition (total starch, free glucose, and protein contents), IVSD, and IVPD of the breads. Breads containing 10 or 20% mass of FBF, or 10% mass of FBPI, were similar in quality compared to the control (100% wheat bread). Higher levels of substitution (30 and 40% mass of FBF and 20% mass of FBPI) were associated with breads of lower volume and specific volume and higher density, brownness, and hardness (p < 0.05). Therefore, the optimal substitution of WF with FBF or FBPI in wheat bread was determined as 20 and 10% mass, respectively. However, the highest substitution levels (40% mass of FBF and 20% mass of FBPI) were more favourable in reducing the starch content and improving the protein content and IVPD of wheat bread; thus, a substitution level of 30% mass of FBF was selected for further analysis as it falls directly between 20 and 40%. Breads containing FBPI were not investigated further as FBPI is more refined and more expensive to produce than FBF and is therefore less suitable for use in bread making.
The effect of PEF technology on the short-term germination of faba bean was determined by subjecting pre-soaked (16 h) faba beans to PEF at 0, 0.3, 0.5 or 0.7 kV/cm electric field strength (pulse width of 20 μs; pulse frequency of 20 Hz; 1000 pulses), followed by germination at 20°C for 0, 24, 48 or 72 h. The radicle length, germination percentage, and germination rate of the beans were measured, and the nutritional composition (total starch, free glucose, and protein contents), IVSD, and IVPD of their derived flours were assessed. All PEF treatments had no marked effect on the germination of faba bean yet had a positive effect on its IVSD and IVPD (except for 0.3 kV/cm). Germination for 72 h caused a significant increase in the IVSD and IVPD of faba beans subjected to PEF at 0.5 and 0.7 kV/cm, only. Those treated with PEF at 0.5 kV/cm gave the highest values for IVSD and IVPD at 72 h of germination. Therefore, non-germinated and 72 h-germinated faba beans pre-treated with PEF at 0.5 kV/cm were investigated further.
The application of a germinated and/or PEF-treated faba bean flour in wheat bread was investigated by substituting the WF with 30% mass of faba bean flour prepared by either soaking the beans (SFBF) followed by germinating them at 20°C for 72 h (GFBF), PEF-treating them at 0.5 kV/cm (20 μs; 20 Hz; 1000 pulses; PFBF), or PEF-treating and germinating them (PGFBF). The incorporation of SFBF, GFBF, PFBF or PGFBF into wheat bread caused marked improvements in its nutritional composition (total starch and protein contents) and IVPD but had detrimental effects on its quality (i.e., significantly reduced its specific volume and increased its density, brownness, and hardness). The nutritional composition and IVPD of any faba bean-enriched breads were comparable, although those containing SFBF gave the highest values for IVPD. The pre-treatment of faba bean with PEF followed by germination had a synergistic effect (i.e., a combined effect that was greater than their individual effects) on the gastric IVPD of wheat bread, but not on its small intestinal IVPD. In addition, it led to fewer detrimental effects on the quality of wheat bread in comparison to soaking, germination, and PEF treatment alone. Therefore, it was concluded that the substitution of WF with 30% mass of PGFBF was more favourable in improving the nutritional composition and IVPD of wheat bread whilst retaining more of its quality than SFBF, GFBF, and PFBF.
These findings clearly demonstrated a positive effect of faba bean addition on the nutritional composition and IVPD of wheat bread. The pre-treatment of faba bean with PEF at 0.5 or 0.7 kV/cm followed by germination for 0 or 72 h is a promising means to improving its IVPD and IVSD and should be considered to promote its utilisation in the food industry. Overall, faba bean-enriched wheat bread is a high-protein plant-based food that may be appealing to individuals such as health-conscious consumers, athletes, and the elderly with higher protein requirements. Future research should focus on the consumer acceptance of faba bean-enriched wheat bread, the use of different PEF process parameters in pre-treating faba bean flour, and the functional properties of faba bean flours pre-treated with PEF and/or germination.