## 3 Results  
### Changes in Volatile Compounds Profile Induced by DBD Plasma  
Table 1 shows the main volatile compounds of pineapple juice identified in our study and their relative abundance (% \(w/w\)) under each treatment condition. The pineapple juice was characterized by several esters (71.4%), two thioesters (14.8%), and other minor compound groups (acids, aldehydes, furans, and hydrocarbons) (13.8%). Methyl hexanoate (53.2%), methyl 3-(methylthio)propanoate (12.5%), ethyl hexanoate (10.0%), and methyl 2-methyl butanoate (7.8%) were the main compounds in the untreated pineapple juice. Mass balance calculations demonstrated that DBD plasma application considerably reduced the concentration of esters, particularly esters with high carbon numbers, such as methyl hexanoate, ethyl hexanoate, and methyl octanoate. Mass balance calculations with the ester compounds indicated that plasma induced the abstraction of methyl units from the esters, continuously decreasing its number of carbons. Figure 1 shows a putative chemical pathway for plasma-induced changes in ester compounds. The abstraction of methyl units occurred mainly with esters with more than six carbons, while compounds with lower carbon numbers showed to be less affected by methyl abstraction. Thus, the final product showed an increase in the mass fraction of ethyl acetate, methyl butanoate, and methyl 2-methyl butanoate. Methyl group abstraction has been previously reported in some cold plasma applications, such as in branched furans and pyrazines [20] and starch [38]. The abundance of thioesters did not change significantly during cold plasma application. However, due to the decrease in ester content, the mass fraction of thioesters in the volatile profiles increased. After plasma processing, the ratio between methyl 3-(methylthio)propanoate and ethyl 3-(methylthio)propanoate decreased. Mass balance calculations with the thioesters evidenced that part of the methyl radicals abstracted from the esters reacted with the methyl thioester producing ethyl thioesters (Figure 1). The reaction could also be observed with ester compounds, where the ratio between methyl and ethyl esters also decreased. This grafting reaction was observed mainly at high plasma generation frequencies (1000 Hz). The reactive oxygen species formed in the pineapple juice due to plasma treatment were evaluated and presented in Table 2. The characterization used the available methods used to evaluate radical scavenging capacity, and as such, the results represent the amount of reactive species absorbed by the juice. The methods calculate the scavenging capacity of the solution during the treatment and do not allow the determination of the concentration of the reactive species. Thus, the scavenging capacities were normalized between 0 and 1, with 1 representing the highest amount of reactive species absorbed by the juice. Plasma treatment at 50 Hz induced the formation of the highest amount of hydroxyl radical in the juice while inducing a low generation of superoxide anion. The treatment at 500 Hz presented the highest amounts of hydrogen peroxide and superoxide anion and a high hydroxyl radical content. Plasma at 1000 Hz presented high hydrogen peroxide content and the lowest content of hydroxyl radical and superoxide anion. The greatest changes observed in the volatile profile of pineapple juice occurred in the treatment carried out at 500 Hz, which presented the highest content of reactive oxygen radicals. Although no hydrolysis and oxidation products were observed, these reactive species may have induced the formation of free radicals in the volatile compounds that resulted in the abstraction of the methyl group observed in the reaction pathway presented in Figure 1.