Baobab (Adansonia digitata L.), which belongs to the Malvaceae family, is an indigenous African tree widespread in arid savannah regions of Madagascar, mainland Africa, the Arabian peninsula, and Australia, and it was once classified as the “lost crop” of Africa.
Focusing on Nigeria, an area of application for these polysaccharides could be the flour and baking industry. This would strengthen the use of indigenous flours and increase the income of local farmers. Nigeria’s agricultural policies aim at accelerating industrialisation through research and development into utilisation and value addition of under-utilised agricultural commodities, such as baobab. This can lead to sustainable economic growth and employment, improved agricultural production and a new income source for small scale food producers or rural population who harvest and sell such commodities in local markets. During the past decade, the Nigerian Government initiated a policy on 10% cassava flour inclusion in bread, as part of efforts to boost the utilisation and create markets for farmers and small/medium scale processors. The program drove the demand for cassava, increasing productivity by approximately 10 million tonnes in six years, making, temporarily, Nigeria a top world producer. Similarly, baobab polysaccharides can be used as a natural improver in wheat-cassava breads and subsequently increase the market demand for baobab.
Polysaccharides have been isolated from different varieties of baobab fruits and leaves (Adansonia digitata L.) under a range of extraction conditions (at pH 6.0 or 2.0). The polysaccharides were examined by means of sugar composition analysis, NMR spectroscopy, size exclusion chromatography (SEC) and dilute solution viscometry. It was found that fruit polysaccharides are xylogalacturonans, whereas leaf polysaccharides are homogalacturonans and rhamnogalacturonans.
The extracted polysaccharides from the fruits and leaves of the baobab tree were assessed for their emulsifying capacity. Emulsions were formed at acidic pH (pH 2.0 and ϕ = 0.1) and were investigated by means of droplet size distribution analysis, ζ-potential measurements, and interfacial composition analysis. Despite the macromolecular differences of baobab polysaccharides all emulsions were formulated at concentrations resulting in comparable zero shear viscosity of the continuous phase allowing structure vs. function relationships to be made. Emulsions made with fruit polysaccharides formed finer droplets and exhibited good long term stability than those formulated with leaf polysaccharides. Stability was achieved by formation of thick interfacial layers, as evidenced by the large polysaccharide interfacial loading (Γpolysaccharide 13.8 or 20.7 mg m−2) that created an effective steric barrier against droplet growth, whereas proteins did not seem to play central role. Fruit polysaccharide emulsification performance at acidic pH was attributed to their compact conformation at the interface that is linked to their structure. Conversely, the presence of rhamnogalacturonan segments in leaf polysaccharides resulted in chain desorption and poor emulsion stability. Overall, it has been shown that baobab fruit polysaccharides could be suitable emulsifiers for a range of technological applications that require low pH stability.
The structure and rheology of baobab polysaccharides were characterised by using linkage analysis and rheometry. The results demonstrate that leaf polysaccharides are comprised mainly of an RG-I-type backbone, with two predominant domains: one branched at O-4 of the →2)-Rhap-(1→residues with typical neutral arabinan, galactan and type II arabinogalactan side-chains; the other branched at O-3 of the →4)-GalpA-(1→backbone to single GlcpA-(1→residues, similar to that found in polysaccharides extracted from several other members of the Malvaceae family. On the other hand, the fruit polysaccharide was mostly xylogalacturonan, with co-extracted α-glucan. Regarding the rheological characterisation leaf polysaccharides did not show unusual rheological behaviour in response to changes in pH or temperature. In contrast, the relaxation dynamics of fruit polysaccharides after constructing master curves of viscoelasticity revealed a strong dependency of rheological behaviour on pH. More specific, polysaccharide dispersions present liquid-like viscoelasticity at acidic pH, whereas, at neutral pH a weak gel network formation was observed that destabilised rapidly under the influence of flow fields. These unique rheological characteristics may point to new directions in food and pharmaceutical formulation.
This entry is adapted from 10.3390/su13179915