Identification And Formation Mechanism Of Predominant Contributors To Off-odors In Thermally Processed Muskmelon Juice

Taking two varieties of muskmelon(the representative of the heat-sensitive fruits)with a broad spectrum of representative-Xinjiang Hami China(CMJ)and the American netted cantaloupe(AMJ)as research object,this project elucidated the chemical nature resulting in the thermally generated off-note in muskmelon juice,identified the dominant precursor for the major key off-note,demonstrated the generation pathway for the off-note based on the integrated use of traditional sensory profile analysis and modern molecule sensory science and technology.The main results and conclusions are as follows:(1)Even though there was no difference in terms of volatile flavor components between the two muskmelon varieties used,totally different aroma profiles were observed.AMJ was characteristic by rich"fruity" and "sweet" resulted from mainly by acetates.butyric and propionic esters,while CMJ is featured by delicate "cucumber-like" and "fresh" note imparted by unsaturated C9 aldehydes and alcohols.(2)A consistent changing trend induced by high temperature and short time processing(HTST,135 0C/15 s)was observed for the two muskmelon varieties used despite that they have totally different aroma characteristics.Unpleasant "fermented" and "sulfurous" were the two dominant aroma attributes in thermally-treated muskmelon juice and the intensity of five original agreeable notes including "sweet","fruity","cucumber-like"."grassy" and "floral" decreased significantly.(3)Static headspace analysis(SHA)is in good agreement with solvent-assisted flavor evaporation(SAFE)in screening most of the off-notes in heated muskmelon juice.However they have different advantages and can complement with each other very well.SHA is efficient in isolating the highly volatile sulfur compounds,esters,and aldehydes with relative small molecule weight,while SAFE is favored in moderately volatiles with high polarity such as alcohols,ketones,phenols and especially acids.(4)A fundamental change in aroma-active compounds was observed after muskmelon juice was treated by HTST processing.Same off-note compositions were found in different thermally-processed melon cultivars.The aroma profile of heated juice is made of unpleasant newly formed compound with higher dilution factors and the number of the aroma-active compounds increased compared with that of fresh one.Dimethyl sulfide(onion,cooked corn),acetaldehyde(yogurt,fermented),3-(methylthio)propanal(cooked potato),methanethiol(rotten cabbage),2-methyl propanal(dark chocolate),1-octen-3-one and Z-1,5-octadiene-3-one(metallic)were characterized as the key off-notes in heated muskmelon juice.Compared with AMJ,the original aroma profile of the CMJ was more seriously destroyed and more off-notes were generated.The off-notes in heated AMJ were all detected in thermally-processed CMJ and all these common off-notes showed higher dilution factors in CMJ,demonstrating that CMJ is more thermal-sensitive than AMJ.(5)Sulfides,including dimethyl sulfide,methanethiol,dimethyl disulfide,dimethyl trisulfide and 3-(methylthio)propanal were the main contributors to "sulfurous" off-note attribute of heated muskmelon juice,while strecker aldehydes such as 2-methyl proponal,2-methyl butanal and 3-methyl butanal together with heterocyclics namely 2-acetyl proline,2-acetyl-2-thiozole mainly contributed to the "fermented"off-note.(6)Both sandwich enzyme-linked immunosorbent assay(S-ELISA)method and ion exchange chromatography verified the presence of SMM(Vitamin U)in muskmelon.Its content amounts to 7.24 ppm,2.59 times higher than that of tomato.The thermal degradation of SMM was demonstrated to be the main formation pathway for the key off-note,DMS in thermally processed muskmelon juice by using standard addition method and R-index based ranking test analysis.(7)Acidification of muskmelon juice can enhance the thermal stability of SMM,effectively suppress the formation of key off-note,DMS,in thermally processed muskmelon juice.Thermally-induced"sulfurous" off-note in muskmelon juice could be controlled by acidifying the muskmelon juice to pH = 2 and 3.