| Lecithin is widely accepted as a desirable ingredient due to the functional food concept. A thorough understanding of the interactions between lecithin and starter cultures is necessary for the utilization of lecithin as a functional additive in fermented dairy products. Volatile compounds were isolated from 6 mo old 33% reduced-fat Cheddar cheeses containing deoiled soy lecithin by vacuum distillation-solvent extraction technique. Gas chromatography-olfactometry (GC-O) demonstrated that (E)-2-nonenal, (E,E)-2,4-nonadienal, (E,Z)-2,4-decadienal, and (E,E)-2,4-decadienal were detected at high odor intensities. Gas chromatography-mass spectrometry (GC-MS) revealed that (E,Z)-2,4-decadienal and (E,E)-2,4-decadienal were the key compounds responsible for the off-flavors in the cheeses. To elucidate the formation of the lecithin associated off-flavors in fermented dairy products, a lecithin-reduced fat milk (LM) system was used. Each LM system was inoculated with specific lactic acid bacteria (LAB) for 24 h prior to analysis. The interactions between lecithin and LAB were measured by performing a series of chemical, microbiological, sensory evaluation and aroma analysis. Three different lecithin types (deoiled soy, hydrogenated soy, and rice) and five strains of LAB were used in this study. LAB included in this study were Lactococcus lactis ssp. lactis D280, Lactococcus lactis ssp. cremoris S2, Streptococcus thermophilus , Lactobacillus helveticus LH212 and Lactobacillus delbrueckii spp. bulgaricus RP 160. It was found that lecithin did not alter growth or acid production of any LAB. Excluding hydrogenated soy lecithin, all LM systems exhibited typical off-flavors described as mayonnaise-like or bread-like. The panel agreed that the direct addition of 2,4-decadienal at about 340 ppb into fermented milk (no lecithin) resembled the aforementioned undesirable odors. Aroma analysis data supported the sensory data. It was found that the formation of 2,4-decadienal in the LM system (deoiled soy lecithin, D280) started early (4 h) and rapidly approached the maximum level by 14 h of incubation at 32°C. Amounts of 2,4-alkadienals found in LM systems (deoiled soy) could not be correlated with the acid production nor with the hydrogen peroxide production of the corresponding LAB. Nonetheless, the oxidative responses of LAB to added soy phospholipids in milk were proportionally determined by the initial culture load and the amount of soy lecithin. |
