*3.2. Feeding Aldehydes to Banana Pulps to Produce the Corresponding Acids*

When feeding C2–C6 branched and straight chain aldehydes to the banana pulp slices (at the "yellow with green tips" stage), the acids with the highest production were hexanoic acid, followed by butanoic acid, pentanoic acid, 2-methylpropanoic acid, propanoic acid, and 3-methylbutanoic acid. The acid that was produced in the lowest quantities was ethanoic acid (Figure 4a).

When feeding butanal to banana pulp homogenate samples from fruit from different ripening stages, butanoic acid produced more in over ripe fruit—"yellow with brown flecks" and "brown" stages—and less in the "yellow" stage fruit. Butanoic acid production in fruit at the "yellow with green tips" stage was even lower (Figure 4b).

To confirm the ALDH activity in pulp homogenates, fruit at the "yellow with brown flecks" stage was combined with butanal feeding experiments. Little butanoic acid was detected when the banana pulp homogenate was heated with a 600 W microwave oven for 1 min (Figure 4c). However, anaerobic conditions did not affect the butanoic acid production (Figure 4c). When adding 50 μM NAD<sup>+</sup> to the reaction system, butanoic acid production increased by two-fold in comparison to the addition of 10 μM NAD+, which was no different from non-NAD+ addition control (Figure 4c).

**Figure 4.** Aldehyde feeding experiments and production of corresponding carbolic acids in banana pulps. Exogenous aldehyde (50 μmol) on filter paper was fed into 10 g of banana pulp tissues. (**a**) Substrate specificity of C2–C6 branched and straight chain aldehydes at the "yellow with green tips" stage (banana pulp slices); (**b**) Effect of ripening stage (pulp homogenate); (**c**) Effects of enzyme deactivation by heat, anaerobic incubation, and feeding of NAD+ at different levels (pulp homogenate). Vertical/horizontal line at each column shows average ± SD (n = 3).
