Abstract
The demand for functional food products has led to an increased interest in nutrients such as minerals, vitamins, bioactive compounds, fibre and prebiotics to be present in food formulations. Amongst the prebiotics, Resistant starch (RS) has gained more attention in recent years, due to its acknowledged health benefits such as prevention and control of colon cancer, diabetes, and obesity. Banana, the world’s most favourite fruit, is one of the richest sources of RS at early stages of ripeness, when it is green (unripe). According to some estimates, more than 100 billion bananas are consumed globally each year, with an annual per capita consumption of 20 kg. Green banana pulp is a rich source of essential phytonutrients, phenolic compounds, vitamin B group, ascorbic acid and tocopherols, while the green banana peel is a rich source of minerals, bioactive compounds and dietary fibre (DF) such as pectin, cellulose, hemicelluloses and lignin. Considering the nutritional value of both pulp and peel of green bananas, the production of green banana flour (GBF), which can be obtained by proper drying techniques, provides a way to preserve the nutritional benefits and increase the shelf-life of banana nutrients. White bread is the most popular bread type in the world, however, there is a growing research on fortifying bread with an array of different DF and functional compounds to take advantage of bread as a carrier of health benefiting compounds. Very few studies available that considered the effect of the GBF on technological properties, nutritional aspects and volatile fingerprint. The physicochemical and thermal properties of GBF obtained from air oven drying (ODF) at three temperatures (50, 80 and 110 °C) and freeze-drying (FDF) were compared to white wheat flour (WF). Lightness and yellowness were negatively affected by the oven temperature increment. The FDF samples exhibited higher a* and L* values and had the closest browning index to WF (P-value < 0.05). Also, the ODF50 samples had the highest emulsion activity, whereas FDF had the highest emulsion stability (P-value < 0.05). The oil holding and water holding capacities of the FDF samples were significantly higher than all other samples (P-value < 0.05). A higher RS content was found in the FDF (46.72%) and ODF50 (44.58%) samples. Oven drying significantly increased the gelatinization temperature drastically for all GBF samples. Results from particle size separation indicated that drying at 50 °C generated smaller flour particles compared to ODF80 and ODF110 treatments. Freeze-dried flour samples had significantly higher bulk density, viscosity and firmness compared to the oven-dried samples and the reference sample, WF (P-value < 0.05), but the cohesiveness, consistency, compressibility and Hausner Ratio were not different from the ODF50 (P-value > 0.05). While the ODF110 presented the highest pasting temperature (81.23 °C) and breakdown viscosity (7118.67 cp) amongst the GBF samples, ODF50 were the only heat-treated samples that showed similar hold, final and setback viscosity values to those found in the FDF. In terms of mineral contents, all GBF samples had higher concentrations of K, Mg, Ca and Zn compared to the WF which makes GBF as a better source of these nutrients (P-value < 0.05). The overall results from both nutritional and technical aspects showed that amongst heat-treated GBF, ODF50 (ODF) was the best flour to be compared to FDF for fortification in bread. Three levels of FDF and ODF were used to substitute WF at three levels (10%, 20% and 30%) in bread formulation. At 30% fortification level, elasticity, loss modulus and complex viscosity of dough were significantly higher in the fortified samples compared to the 100% WF bread. At 20% fortification level, cohesion was significantly decreased in the fortified dough samples compared to control and FDF samples (P-value < 0.05). The use of GBF resulted in a denser, harder and chewier bread with increasing the fortification level. In terms of shelf life, the banana bread stored at -20 °C for one week had significantly lower firmness and water loss compared to 4 °C and 25 °C (P-value < 0.05). A significant decrease in energy caloric value and an increase in moisture and total dietary fibre at > 20% fortification level was observed. The ODF-fortified samples had higher browning index compared to control and FDF ones. The addition of both GBF types improved macro minerals (Mg, Ca, Na, K and P) without a significant change in micro minerals (Fe, Zn, and Mn). The use of FDF in bread resulted in a marked increase in both resistant and slow digestible starch content in F30 compared to ODF fortified samples at their comparable fortification levels. GC-MS-based chemical fingerprinting successfully detected more than 100 volatile compounds in the GBF fortified bread samples. Chemometrics methods used to compare the effect of GBF type in bread (FDF and ODF-fortified-bread), fortification level (10%, 20% and 30%) and bread part (crumb and crust) on the formation of volatile compounds. Furan (furfural, 2-furanmethanol), Strecker aldehydes (2- methyl butanal and 3-methylbutanal) and ketone (2-undecanone) were the most abundant volatiles in crust while alcohol (1-hexanol and 1-heptanol) and ester (ester butanoic acid ethyl) abundant in the breadcrumb. The level of fortification had a significant impact on the formation of 3-methyl-butanal (P-value < 0.05). Furthermore, bread made with freeze-dried GBF had more distinguished ‘banana-like’ flavour due to the presence of ethyl ester butanoic acid and 2-undecanone, while bread made with ODF represented more Maillard-related compounds which could signify a wood malty aroma impression. It can be concluded that fortification of bread with the GBF achieved from freeze drying had a more desirable results from technological and nutritional points of view. Although between 10% and 20% fortification level there was no clear difference, the 30% bread samples showed a high value nutritious bread with distinctive volatile flavour. Overall, the type of the drying method of GBF preparation had an impact on developing discriminant volatiles compared to bread part and fortification level.