Abstract: Objective: This study aimed to explore the formation mechanism and composition of unpleasant odorants in fermented walnut yogurt in order to improve its flavor quality. Methods: Comparative experiments and sensory evaluation were used to analyze the effects of different starter cultures and raw materials on the formation of the off-flavor of fermented milk, and the changes in the characteristic odor value of fermented walnut milk as well as acidity, organic acid, protein and fat contents, carbonyl value and the profile of volatile compounds were investigated during walnut milk fermentation, together with the correlation between them. The composition of volatile compounds in walnut yogurt was analyzed by headspace solid phase microextraction combined with gas chromatography-mass spectrometry (HS-SPME-GC-MS), and the key flavor compounds and unpleasant odorants were analyzed by calculating their relative odor activity values (ROAV) as well as using principal component analysis (PCA). Results: The fat content and the carbonyl value were both significantly related to the formation of off-odorants. The major volatile flavor compounds of walnut yogurt included three acids such as caproic acid, three ketones such as acetoin, seven alcohols such as n-hexanol, four aldehydes such as (E)-2-heptenal, butyl butyrate and other compounds. The major volatile compounds with ROAV greater than 1 included (E)-2-heptenal, acetoin, caproic acid, n-hexanol, n-octanol, 1-heptanol, benzaldehyde, 2-nonanone, 2-heptanone, styrene and phenethyl alcohol, and the key flavor compounds were acetoin, caproic acid, (E)-2-heptenal, and n-hexanol. Conclusion: Caproic acid, (E)-2-heptenal, n-hexanol, and benzaldehyde are the major unpleasant odorants of walnut yogurt, and they are mainly formed through lipoxygenase-catalyzed oxidative decomposition of large amounts of linoleic acid and linolenic acid derived from fat hydrolysis during the fermentation process, yielding C6–C9 alcohols, aldehydes, and acids.

Key words: lactic acid bacteria; walnut yogurt; volatile compounds; off-flavor; analysis