## 3 Results and Discussion
### Comparison of Analyte Profiles from Different Packaging Systems
Acetoin is reported to be a VOC of raw meat and can form via a number of pathways; decarboxylation of \(\alpha\)-acetolactate, synthesis from aspartate in the presence of \(\alpha\)-ketoglutarate [49], or production via a multistep enzymatic pathway in which _Leuconococ gasicomittatum_ initially converts aspartate to oxaloacetate, which is then converted to acetoin via oxaloacetate decarboxylase [50]. Thus, acetoin can reasonably be assumed as a microbial by-product. As acetoin can be converted to 2,3-butanediol in the presence of 2,3-butanediol dehydrogenase [50], the presence of 2,3-butanediol was also expected and was observed in both MAP and CP samples (Table 1). 2-Vinyloxyethanol was also detected in all three packaging systems. This is not a known volatile released from raw beef during storage and hence is assumed to be a contamination from meat packaging, which contains polymerised forms of vinyl alcohol [51]. Ethylene oxide, on the other hand, observed only in meat from MAP packaging, is assumed to have been formed from endogenous ethylene as a result of microbial growth. A high number of hydrocarbons was observed; Table 1 notes if they have been previously reported. Hydrocarbons are associated with oxidative pathways in meat and thus the number observed is not unexpected. The relative abundance of the volatiles CS\({}_{2}\) and acetoin (see Figure 2) and 2-vinyloxyethanol (see Figure S5) varied significantly at certain timepoints among the three different packaging systems. The difference in the analyte abundance between the packaging systems is likely due to the different internal environments and duration of meat storage inside the package prior to sampling and analysis. Other factors affecting reaction rate may include endogenous microbial activity [52], and chemical reactions between the components present in the food matrix. These compounds are therefore indicative of the extent of spoilage of the raw beef in each packaging system, and their presence, absence, and concentration could be used as an indication of raw meat quality and freshness. Overall, 14, 15, and 17 VOCs were detected in raw beef meat in MAP, CP, and VP, respectively, over the 7-day sampling period. Many compounds were unique to a single packaging system. It is clear that the packaging environment plays a significant role in the volatile profile of the packaged raw beef. Investigation of the biological pathways and organisms responsible for the production of volatile compounds from raw beef was outside the scope of this study and the exact reagents, biological entities, and sources  
Figure 2: The average area, expressed as a percentage of all analytes, of (**a**) acetoin and (**b**) carbon disulphide (CS\({}_{2}\)); cling-wrapped packaging (CP), modified atmospheric packaging (MAP), and vacuum packaging (VP); \(n=3\pm\) S.E. of the majority of the analytes, as well as the differences in their presence, absence, or concentration in a different packaging environment, are still unknown. The knowledge from this qualitative, comparative study allows a better understanding of release profiles in different packaging systems, which is necessary in order to design an on-pack sensor for freshness detection.