## 4 Stimulation Process of UV-C
### Moderate and High UV-C Dose Stimulation  
The application of a high dose during treatment is indicated to have a severe effect on the physiological constituent of plant crops and potentially affect the mechanical attributes during storage. Firmness is an important quality indicating parameter that indicates the degree of softness as the crops undergo a given storage duration. Studies have not yet sufficiently shown the effect of the mechanical changes as a result of high UV-C dose. Nevertheless, some research works depicted the negative impact of high UV-C doses on the physiological constituents that are discussed in this section. The correlation between the physiological change and firmness demonstrated by a previous study indicates that softening of the fruit during storage is a result of cell wall damage that is caused by the generation of two major degrading enzymes in a cell wall, namely polygalacturonase (PG) and pectin methylesterase (PME) [126]. An experiment conducted on the physiological damage of cells of certain vegetable crops at a high UV-C dose was reported to cause cell membrane damage. A research work conducted on spinach exposed to a high UV-C dose from 7.94 to 11.35 kJ\(\cdot\)m\({}^{-2}\) 13 days of storage duration and 8 \({}^{\circ}\)C of storage temperature depicted the reduction of antioxidant chemicals up to 75% [127]. Likewise, in other experimental work on pepper exposed to the UV-C dose of 7 kJ\(\cdot\)m\({}^{2}\), 18 days of storage duration, and 10 \({}^{\circ}\)C storage temperature resulted in lower antioxidant chemicals but with fewer changes [128]. The latest research work on satsuma mandarin fruit resulted in a significant decrease of DPPH (antioxidant) and phenolic compounds when exposed to a high UV-C dose of 10 kJ\(\cdot\)m\({}^{-2}\)[129]. An experimental study on Mangosteen treated over the range of 6-40 kJ\(\cdot\)m\({}^{-2}\) at 25 \({}^{\circ}\)C for seven days resulted in a higher defense response and decreased weight loss at 13 kJ\(\cdot\)m\({}^{-2}\) than the lower and the higher UV-C dose, which showed no significant difference from the control sample [130]. However, UV-C irradiated persimmon and cucumber at 12.9 kJ\(\cdot\)m\({}^{-2}\) at varied exposure times and storage periods were found to be ineffective for the enhancement of phytochemicals [131]. Fresh-cut lotus treated with UV-C from 0.3 to 12 kJ\(\cdot\)m\({}^{-2}\) indicated the moderate UV-C intensity from 1.5 to 3 kJ\(\cdot\)m\({}^{-2}\) was significant in inducing phenolic content [132]. A comparable result was noted in a tomato treated at a UV-C dose of 4 kJ\(\cdot\)m\({}^{-2}\) and storage temperature of 13 \({}^{\circ}\)C, elucidating an increase in phenolic content throughout the storage period significantly higher than the control sample [133]. In other research, using a similar dose of UV-C, the total phenolic content of fresh-cut strawberries was enhanced significantly [134]. The UV-C stimulation at a mild dose is predominantly reported to induce phytochemicals and antioxidants for the role of defense response. However, the reduction in these defense-triggering chemicals when treated at a high dose is due to the depleting activity of UV-C, resulting in membrane damage that gives rise to the change in the chemical compositions, hence lowering the antioxidant activities of the crop and lowering the firmness. The physiological change in terms of weight loss and firmness in relation to the phenolic content was excellently indicated by a recent research article conducted on UV-C exposed bitter gourd sample, maintaining two-fold lower weight loss than the control sample at prolonged days of storage [135]. The softening of tissues during the prolonged storage period is attributed to the gradual alteration in cell wall composition and cell separation [85]. Besides, a moderate dose could also result in higher softening. For instance, a tomato fruit treated at a moderate UV-C dose of 3.7 kJ\(\cdot\)m\({}^{-2}\) resulted in higher weight loss compared to untreated ones [136].