# Comparative Analysis of Volatile Flavor Compounds in Seven Mustard Pastes via HS-SPME-GC-MS
## 2 Results and Discussion
### Degradation Products of AITC and Method for Stabilizing AITC  
According to the qualitative results, AITC was determined as the primary characteristic odorant in MPs; it could be produced by the degradation of sinigrin or introduced by adding flavor compounds or food flavorings. AITC is known to undergo various chemical reactions, such as hydrolysis, oxidation, thermal degradation, etc., resulting in the formation of hydrogen sulfide, allyl mercaptan, diallyl monosulfide, diallyl disulfide, carbon disulfide, allyl thiocyanate, diallyl thiourea, etc. [16; 23], many of which were also detected in this study, leading to variations in the aroma of MP products. There are two approaches that can be employed to avoid unwanted reactions of AITC. The first one is to adjust the pH value of the MP products. ITCs are more stable under acidic conditions, and keeping the pH value of MP products in the acidic range can delay the changes of ITCs to some extent. Citric acid was used in five (MP 1, MP 2, MP 3, MP 4, MP 6) of the seven samples  
Figure 3: Formation mechanisms of allyl isothiamate and allyl thiocyanate.  
analyzed, and MP 5 contained sodium diacetate. However, acidic conditions are also favorable for the degradation of thioglucosides to nitrile in the raw materials used. Hence, it is crucial to choose a suitable pH value. The second approach involves encapsulating AITC in microcapsules or nanoemulsions using encapsulation techniques, which can also stabilize AITC [24; 25].  
In addition to the above two methods mentioned, AITC could be replaced by other ITCs, which are more stable and have similar odor characteristics to AITC. More than 200 thioglucosides have been found in plants [26], which means that over 200 ITCs could be formed. From the point of view of flavor chemistry, most of the ITCs exhibited mustard-like characteristics, so structurally stable aroma compounds with mustard characteristics can be designed and synthesized based on the relationship between the molecular backbone and the odor characteristics in combination with the structure of detected thioglucosides. Other than AITC, fourteen types of ITC compounds were approved for use as flavor compounds in Chinese food additive use standard GB2760-2014, including six saturated ITCs (isopropyl isothi cyanate, butyl isothi cyanate, 2-butyl isothi cyanate, pentyl isothi cyanate, isopentyl isothi cyanate), three unsaturated ITCs (3-butenyl isothi cyanate, 4-pentenyl isothi cyanate, 5-hexenyl isothi cyanate), two aromatic ITCs (benzyl isothi cyanate, phenethyl isothi cyanate), and three saturated ITCs containing methylthio groups (3-(methylthio)propyl isothi cyanate, 5-(methylthio)pentyl isothi cyanate, 6-(methylthio)hexyl isothi cyanate). Most of them have been synthesized; their odor characteristics were evaluated, and the results showed that they had mustardy, pungent, and irritating aromas [27]. Among the synthesized compounds, isopropyl isothi cyanate has almost the same aroma characteristics as AITC, and it is more stable. However, from the volatiles detected in the seven MPs in this study, it could be seen that the above flavor compounds were not as common as AITC. Therefore, in order to improve the aroma quality of MP products, from the point of view of flavor preparation, part or all of AITC should be substituted by other ITCs which have been approved by the Flavor and Extract Manufacturers Association.