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 of
mechanical 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.