## 4 Discussion
### Identification and Analysis of VOCs in Silage by HS-SPME-GC-MS  
#### 4.3.1 Esters  
The aroma compounds in different flavor types of silage vary somewhat, both in type and content. Abnormal silage exhibits considerably greater diversity and a higher concentration of volatile compounds than the normal silage. Esters were detected in both samples. Notably, 3-ethyl-3-pentyl methylphosphonofluidate is the most abundant VOC in abnormal feed and is exclusively detected in abnormal fermentation. It is a phosphate derivative where an alkyl group replaces the hydrogen atom in the phosphate molecule. Organophosphate esters (OPEs) are widely reported to exist in the environment [36] as well as in various plant and animal bodies [37]. They play an important role in controlling harmful bacterial or insect populations on corn plants by functioning as organophosphate pesticides. Typically, OPEs degrade rapidly through the action of biological enzymes or microorganisms [38]. Diisopropyl fluorophosphate (DFP)can be degraded and metabolized by enzymes isolated from environmental microbes [39]. Isothiocyanates (ITCs) and related derivatives, such as benzyl isothiamate (BITC) and isocyanates, are key flavor contributors to both types of silage feeds. These compounds are secondary metabolites found primarily in cruciferous plants and are typically the main substances responsible for their characteristic odor [40]. When plant tissues are damaged by cutting or chewing, benzoyl isothiamate hydrolyzes to produce degradation products such as isothiates, indoles, and thiocyanates. These compounds decompose slowly in the air, imparting a unique pungency and bitterness, which creates a distinctive sensory experience for animals consuming these plants [41]. As a naturally occurring organic isothiamate, benzoyl isothiamate is also believed to be a potential new antibacterial agent [42]. Research [43] has shown that certain isothiamate derivatives are plant chemicals found in the seeds of _Sinapis alba_ L. (white mustard) that inhibit harmful gut bacteria like _Clostridium_ and _Escherichia coli_ while also exhibiting antioxidant, anti-cancer, and anti-inflammatory properties [44]. Isothiocyanates are known to produce unpleasant odors. The characteristic pungent odor of the abnormal silage stored on the side of the silo may result from the frequent use of bleach or disinfectants for cleaning. Previous studies have reported that environments frequently cleaned with bleach exhibit higher levels of HNCO, organic isocyanates, HCN, and amides [45]. Within one day of cleaning with bleach, HNCO and other nitrogen-containing substances, including isocyanates, cyanogen chloride, and carbon chloride, are formed [46]. In industrial water, swimming pool disinfection, and other applications, use of chloroisothiocyanuric acid esters as disinfectants produces byproducts like cyanuric acid, which has a slightly bitter taste and decomposes into a strongly acetone-like cyanide odor, resembling almond flavor upon heating [47]. This compound is also utilized as a disinfectant in agriculture and silkworm rearing. Isothiocyanates play a crucial role as intermediate organic compounds in pesticide production and readily react with alcohol to form aminocarbonyl compounds, which are essential for producing various insecticides, fungicides, and herbicides. Consequently, residues of isothiamate derivatives, such as phenyl isothiamate, may accumulate in silage corn due to extensive use of aminocarbonyl insecticides during corn's growth stage. Additionally, isocyanates can be found in furniture. For example, the -NCO functional group has been detected in bedding covers and foam [48]. The most widely used and highest-yielding organic isocyanate is toluene diisocyanate (TDI), known for its strong and pungent odor, typically produced through spray-coating polyurethane. It remains unclear whether these sources directly release isocyanates or if they chemically or microbiologically degrade to form HNCO.