Syzygium samarangense cv. ‘Giant Green’ is an underutilised fruit that can be found in Malaysia and other Asian countries. Since this fruit is not fully commercialised, the information about its potential health benefits is limited. Thus, this study was carried out to determine the polyphenolic contents (total phenolic and total flavonoid) and biological activities (antioxidant, alpha-glucosidase and antibacterial assay) of ‘Giant Green’ leaves and fruits at different maturity stages. The young, mature and old leaves, and unripe, half-ripened and ripened fruits were analysed. The results showed that the young leaves increased the TPC and TFC by 35% and 41%, over the old leaves. Similarly, TPC and TFC contents were 37% and 54% higher in unripe fruits compared to the ripened fruits. In addition, young leaves exhibited the strongest scavenging activity towards DPPH, NO and ABTS radicals with IC50 values increasing 1.6-fold, 1.7-fold and 2.3-fold, respectively, over the old leaves. However, in fruit samples, only unripe fruits were able to inhibit more than 50% of radicals. A comparable trend was observed in alpha-glucosidase inhibitory assay whereas young leaves and unripe fruits recorded 81% and 99% increases in IC50 values, respectively, from young leaves to old leaves and unripe fruits to ripened fruits. Identically, young leaves also showed a significant effect in antibacterial assay with an inhibition zone increase of 19%, 36%, 32%, and 31% in S. aureus, E. faecalis, S. typhimurium and E. coli, respectively, over the old leaves. However, only unripe fruits were most effective against all tested bacteria while half-ripened fruits were only effective against E. faecalis with a 1.1-fold increase in the inhibition zone compared to unripe fruits. Ripened fruits were resistant to all of the bacteria. These results suggest that the young leaves and unripe fruits of ‘Giant Green’ cultivar of S. samarangense could be a potential candidate for the management of some diseases coming from harmful free radicals or bacterial infection.

‘Giant Green’ is one of the Syzygium samarangense cultivars planted throughout Malaysia because it has great potential for benefitting human health. However, its variation in chemical compounds, especially in the leaves at different maturity stages, cannot be systematically discriminated. Hence, Fourier transform infrared spectroscopy (FTIR) and gas chromatography–mass spectrometry (GCMS) coupled with chemometric tools were applied to discriminate between the different stages of leaves, namely, young, mature, and old leaves. The chemical variability among the samples was evaluated by using principal component analysis (PCA) and hierarchical clustering analysis (HCA) techniques. For discrimination, partial least squares discrimination analysis (PLS-DA) was applied, and then partial least squares (PLS) was used to determine the correlation between biological activities (antioxidant and alpha-glucosidase inhibitory assay) and maturity stages of ‘Giant Green’ leaves. As a result, the PCA, HCA, and PLS-DA of the FTIR and GC-MS data showed the separation between clusters for the different maturity stages of the leaves. Additionally, the PLS result demonstrated that the young leaves showed a strong correlation between metabolite quantities and biological activities. The findings of this study revealed that FTIR and GC-MS coupled with chemometric analyses can be used as a rapid method for the discrimination of bioactive structural functions in relation to their biological activity.