# Molecular Hydrogen: The Postharvest Use in Fruits, Vegetables and the Floriculture Industry
## 3 Application of H2 to Postharvest Produce  
As outlined above, there are various ways in which plant materials can be treated with molecular hydrogen. What can be gleaned from below is that treatment of fruit and vegetables has been relatively widely studied, using a range of plant species (Table 1). [PERSON] et al. [23] treated kiwifruit with HRW and found that ripening was delayed and that the fruits stayed firmer for longer, with 80% HRW having the best results. Pectin solubilization was lower in the fruit and there was less oxidative stress in the cells, therefore less lipid peroxidation, whilst antioxidant superoxide dismutase (SOD) activity was increased. The inner mitochondrial membrane also maintained a better integrity. In a more recent paper H2 was used as a fumigation treatment of kiwifruits [14]. This increased endogenous H2 and delayed fruit softening. As with bananas [30], H2 effects were found to be mediated by ethylene metabolism, with ethylene synthesis being inhibited by H2, and concomitant decreases in 1-aminocyclopropene-1-carboxylate (ACC) [14]. Enzymes involved in this metabolism were appropriately altered too, i.e., ACC synthase and ACC oxidase. [PERSON] et al. [24] also studied kiwifruit and found that treatment with HRW significantly delayed the increase in soluble solid content, weight loss, and the total microbial \begin{table}
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**Biological Material Treated** & **Manner of Treatment** & **Effects Seen/Comments** & **Reference(s)** \\ \hline Kiwifruit & HRW & Delayed ripening and senescence & [23] \\ \hline Kiwifruit & H2 fumigation & Better fruit, mediated by ethylene metabolism & [14] \\ \hline Kiwifruit & (HRW) (and with slightly acidic electrolyzed water (SAEW)) & Reduced loss of antioxidants such as flavonoids, and delayed chlorophyll loss. Reduced oxidative stress markers. & [24] \\ \hline _Rosa sterilis_ & HRW & Better fruit, mediated by ROS and energy metabolism & [25] \\ \hline Okras & HRW & Delayed fruit softening, better cell wall maintenance & [26] \\ \hline Litchi (Lychee) & HRW & Reduced pericarp browning, lower oxidative stress indicators & [27] \\ \hline Chinese water & HRW & Less tissue yellowing, reduced oxidative stress, effects on the phenylpropanol pathway & [28] \\ \hline Tomato & HRW or H2 fumigation & Reduced nitrite accumulation, with relevant enzymes affected & [29] \\ \hline Banana & HRW & Delayed ripening, effects mediated by ethylene metabolism & [30] \\ \hline Tomato & HRW & Altered defense responses, increased polyphenol oxidase (PPO) activity and NO & [31] \\ \hline Strawberry & Gas in packaging & Better storage, lower fruit oxidation & [32] \\ \hline _Hypiszgus marmoreus_ & HRW & Better storage mediated by antioxidants & [33] \\ \hline \hline \end{tabular}
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Table 1: Some of the examples of molecular hydrogen treatment of postharvest fruit and vegetables. HRW: hydrogen-rich water; NO: nitric oxide; PPO: polyphenol oxidase; ROS: reactive oxygen species; SAEW: slightly acidic electrolyzed water. load of the samples when compared with the controls. It also allowed the maintenance of chlorophyll, color, and firmness and improved the levels of ascorbic acid, total phenols and flavonoids during refrigerated storage. Interestingly, the authors also found that the aforementioned positive effects were even more pronounced when HRW was combined with a slightly electrolyzed water treatment. Further studies into using H\({}_{2}\) gas as a fresh food preservative demonstrated that incorporating 4% of reducing hydrogen gas into the packaging (RAP) of strawberries can protect the color, texture, and nutritional parameters of the fruits when compared with conventional, modified atmospheric packaging (MAP). The results describe that through the addition of H\({}_{2}\) into the packaging headspace, oxidation of fruits is diminished, thereby preserving anthocyanin and phenolic content of strawberries. RAP also extended both the best before and expiration date periods longer than MAP.