Interactions between polyphenols and macromolecules facilitate the extraction of low viscosity banana juice

Doctoral thesis, 2021

Bananas are an important staple food and cash crop throughout the African Great Lakes region where they contribute highly to livelihood and food security. However, bananas are perishable fruits that require effective processing techniques to reduce post-harvest losses. Processing
bananas into juice could be a better strategy for the mitigation of these losses and thus the improvement of the banana value chain. Low viscosity banana juice has been produced in East and Central Africa for many years by mechanical blending and extraction, but the technology has not been industrialized due to some processing challenges. Occasional juice failures and cloudy appearance are among the problems related to banana juice production. Extraction of banana juice using the mechanical method is limited to banana cultivars with high tannin content. Most other banana cultivars do not produce juice by this method. The presence of tannins favorable to juice extraction seems to be related to the cultivar and ripening process.
The challenges in the extraction of low viscosity banana juice have been associated with the ability of the tannins to interact with proteins, but some studies suggest the involvement not only of tannins and proteins but also of pectic polysaccharides. A better understanding of the mechanisms behind banana juice extraction could be a solution to the ongoing challenges. To accomplish this, the overall aims of this thesis were to identify the physical and chemical factors responsible for juice release and the potential interactions between protein and phenolic compounds. Phenolic and proteomic  characterizations of juice-producing and cooking cultivars were performed to investigate the potential link between the chemical composition of banana
pulps and their ability to release juice. The nature of the interactions at different stages ofmechanical juice extraction was studied by Fourier Transform Infrared Spectroscopy (FTIR).
Blending speed, extraction time and stage of ripeness were identified as significant factors affecting banana juice release with the optimum values of 2650 rpm, 162 s and stage 5, respectively. Chemical characterization revealed that juice-producing banana cultivars had a higher content of total phenolic, tannins and gallocatechin monomers than non-juice producing banana cultivars. There were no significant differences in protein content, molecular weight distribution and amino acid composition among the cultivars. The observed band's shifts of the FTIR spectra at the OH (3300.13-3270.94 cm-1), amide (1655.14-1641.89 cm-1) and polysaccharide (995.65-875.08 cm-1) regions suggest interactions between phenolic compounds,
proteins and pectins.
The results in the present thesis show that the high content of phenolic compounds in bananas may be the main factor for the ability of the banana pulps to release juice. In summary, the observed changes in the functional groups of the pulp occurring during mechanical juice extraction suggest that the mechanism behind banana juice extraction involves interactions of phenolic compounds with both proteins and pectic polysaccharides. Further studies on the characterization of the banana pulp functional groups using Nuclear Magnetic Resonance are needed to verify the mechanism behind banana juice release.