## 3 Results and Discussion
### Identification of Key Parameters at Different Dehydration Degree
The content of this compound decreased rapidly during the dehydration process and, at the end of the dehydration (T5), its content had decreased to 27.33% and 0.20% in volatile phenols and AADs, respectively. Unlike most of the volatiles, the content of 2,4-di-_tert_-butylphenol in juice (ug/L) still decreased rapidly even under the concentration effect of dehydration (Table S1). This compound has also been detected by some authors in wine [49; 50], distilled spirits [51], and other fruits [52] or fruit wines [53]. In addition to fruits, 2,4-di-_tert_-butylphenol is widely produced in microorganisms [54], medicinal plants [55], rice [46], and even some animals [56]. As a lipophilic volatile phenol, 2,4-di-_tert_-butylphenol possesses strong antioxidant [55; 56] and antifungal activities [57], and it may be generated by the synthesis of phenylalanine through the shikimic acid pathway or by the degradation of eugenol or isouegenol [55]. [PERSON] et al. (2018) have reported the powerful inhibitory effect of 2,4-di-_tert_-butylphenol against fruit (i.e., peach and lychee) fungal pathogens [54]. In a subsequent study, the same team found that 2,4-di-_tert_-butylphenol could induce the production of antioxidant and disease-resistant enzymes in lychee fruit and destroy the structure of fungal cells, effectively inducing postharvest disease resistance in lychee fruits and resisting fungal attack [57]. [PERSON] et al. (2020) have speculated that the primary biological function of the highly toxic secondary metabolite 2,4-di-_tert_-butylphenol may be to act as an endocide (endogenous biocide) involved in the endocidal regulation of its producers [56; 58]. Therefore, we inferred that the high content of 2,4-di-_tert_-butylphenol appearing at T1 might have been related to the induction of fungal pathogens caused by the rainfall and relatively high humidity at the beginning of dehydration (Figure S1, T1), and then gradually degraded under dry weather conditions without fungal infection. Another important metabolic marker with notable rose and honey aroma was 2-phenethyl acetate. As mentioned before, both the free and bound forms of this compound were improved dramatically at the end of dehydration (T5; Figure 5). With the content (ug/L) exceeding its threshold, this volatile might be a key sensorial marker of the final wine made from highly dehydrated grapes. It has been well-demonstrated that 2-phenethyl acetate is a key contributor to the nut, rose, and honey notes of Vidal icewine, and was detected in both grapes and the corresponding wines [31; 59; 60]. [PERSON] et al. (2019) have  
Figure 8: Hierarchical cluster heatmap of free volatiles expressed as ng/berry with VIP \(>\) 1.0 in OPLS\(-\)DA. T1, T2, T3, T4, and T5 represent the sampling points at different dehydration degrees. The corresponding compounds for the abbreviations are listed in Tables S1 and S2. found that 2-phenethyl acetate fluctuated during the late-harvest freeze-thaw cycle in Vidal grapes, and was present in higher levels in on-vine than off-vine samples at the end of harvest [31]. They hypothesized that biosynthesis and degradation occur simultaneously on this compound during late-harvest. A significant improvement in 2-phenethyl acetate was also observed at the end of the on-vine non-destructive dehydration in our study (Figure 5), indicating that the synthesis of this compound also took place in grapes at this period.