Abstract Gas chromatography-mass spectrometry analysis revealed that lipid oxidation products such as (E,E)-2,4-nonadienal, (E,Z)-2,4-decadienal, and (E,E)-2,4-decadienal were formed in a deoiled soy lecithin-reduced fat milk system during 24 h of fermentation by Lactococcus lactis at 32°C. In addition, initial numbers of L. lactis (10 2or 10 6cfu/mL milk) and soy lecithin content (0.2 or 2.0 g/100 mL milk) affected the formation of these 2,4-alkadienals in fermented lecithin-milk system. These 2,4-alkadienals were detected in reduced fat milk containing 0.2 g soy lecithin/100 mL milk (the lowest concentration investigated) and initial culture numbers of 10 2cfu/mL milk. Other lactic acid bacteria, Lactococcus cremoris, Streptococcus thermophilus, Lactobacillus helveticus, or Lactobacillus bulgaricus ssp. were subsequently evaluated in a deoiled soy lecithin-reduced fat milk system at one lecithin concentration (2.0 g lecithin/100 mL milk, 102 cfu/mL milk) and also produced 2,4 alkadienals. Sensory evaluation confirmed the presence of typical lecithin associated off-flavors (mayonnaise/bread) in all fermented milks containing lecithin across the concentration used in this study (0.2–2.0 g/100 mL milk).