# Molecular Hydrogen: The Postharvest Use in Fruits, Vegetables and the Floriculture Industry
## 2 Application of H\({}_{2}\) to Produce  
There are several methods for the application of H\({}_{2}\) to food products. The easiest way would be to use H\({}_{2}\) as a gas. An example of this treatment is [PERSON] et al. [14], in the treatment of kiwifruit. H\({}_{2}\) can be readily commercially purchased, or it can be produced locally by the electro-hydrolysis of water--which also produces O\({}_{2}\). Any O\({}_{2}\) generated can be separated or alternatively used as a H\({}_{2}\)/O\({}_{2}\) gas mixture, referred to as oxy-hydrogen (HHO: 66% H\({}_{2}\)/33% O\({}_{2}\)) [5]. However, there are safety issues to be considered if H\({}_{2}\) is being used in the gaseous form, as exemplified by the disastrous explosion of the _Hindenburg_ airship [15]. Therefore, caution needs to be exercised if H\({}_{2}\) gas is used in a confined space.  
Alternatively, H\({}_{2}\) may be used in the form of a gas enriched solution, often referred to as hydrogen-rich water (HRW). This is widely used, an example being the work on heat stressed cucumber where both photosynthesis and the antioxidant capacity of the plants was altered [16]. Although probably of limited use in the treatment of food, a variation of this is the bubbling of a saline solution with hydrogen gas, to make hydrogen-rich saline (HRS), as used in a study to investigation hydrogen and radiation effects in mice cells [17]. HRW can be sprayed on foliage, or added to feed water, being poured straight onto the soil or growth medium. It is therefore relatively inexpensive and simple to use.  
A more advanced variation on HRW is hydrogen nanobubble water (HNW). One of the issues with HRW (and HRS by extension) is that the H\({}_{2}\) will rapidly return to the atmosphere, so depleting the solution of H\({}_{2}\). This means that HRW needs to be used as a fresh solution. It also means that the biological material will be exposed to a bolus effect, where there is a very high concentration on treatment that does not persist. It also means that it is often hard to know the exact concentration of H\({}_{2}\) that the material gas is exposed to. To some extent the use of HNW should mitigate some of these issues, as it is reported that the solution has a higher concentration of H\({}_{2}\) which remains in solution for longer. An example of its use is a study on how hydrogen affects the copper toxicity of _Daphnia magna_[18].  
H\({}_{2}\) can also be supplied to biological materials in the form of donor molecules, such as magnesium hydride (MgH\({}_{2}\)). This was used in the study of the vase life of carnations, for example [19]. Donor molecules are likely to supply H\({}_{2}\) gas to a biological material more slowly and over a longer period of time, so perhaps mitigating the need for repeated applications, which may be needed with HRW. Nanoparticles have been suggested for gas delivery, for example with nitric oxide (NO) [20]. Similar technologies are being developed for the storage and delivery of H\({}_{2}\) as well [21]. However, donors often give by-products that may be detrimental to human health. If that is the case, then the use of such donors for the food industry would not be possible.  
However, the application of H\({}_{2}\), regardless of the method, needs to have a thorough cost/benefit analysis, unless the need is to preserve food for sustaining a population, where cost may be of less consequence. If the amount (value) of food gained from treatment does not outweigh the cost of that treatment it is financially unsustainable. Having said that, the use of hydrogen has been mooted for a variety of industries, not least in supplying energy for methods of travel, be that car, train or airplane [22]. Therefore, there is a need for more cost-effective production and supply of H2 gas, and this will no doubt bring the cost of its use down. This would be of great benefit for the use of H2 in the food industry.