Co-Cultivation of Yoghurt Bacteria with Probiotics Increased Melatonin Content and Enhanced the Antioxidant Activity of Soy Milk Yoghurt.

Research background: Functional foods that improve sleep quality are attracting increasing attention. Melatonin is a key component that regulates circadian rhythms in humans. Soy milk yoghurt contains melatonin and antioxidants and has beneficial health properties. Many authors have investigated soy milk yoghurt produced by probiotic bacteria. This is the first study to explore the effect of using yoghurt bacteria co-cultivated with probiotics to improve melatonin content and antioxidant activity. This study investigates the melatonin, serotonin and tryptophan contents, antioxidant activity, physical and sensory properties of soy milk yoghurt bacteria co-cultured with different probiotics.

Experimental approach: Soy milk was fermented with four combinations of yoghurt bacteria and probiotics, namely Lactobacillus bulgaricus and Streptococcus thermophilus (control, SB-YC), Bifidobacterium lactis, Lactobacillus acidophilus, L. bulgaricus and S. thermophilus (SB-BY), B. lactis, L. acidophilus and S. thermophilus (SB-BT) and L. acidophilus, L. bulgaricus and S. thermophilus (SB-LA). Melatonin, serotonin and tryptophan content in yoghurt samples were determined using liquid chromatograph coupled with a mass spectrometer (LC-MS/MS). Apart from that, antioxidant activity and quality characteristics including syneresis, texture profile, colour and sensory properties were evaluated.

Results and conclusions: The highest melatonin mass fractions were found in soy milk yoghurt fermented with SB-BY (21.20 ng/g) and SB-YC (23.51 ng/g), while the highest tryptophan mass fraction was found in SB-LA (397.18 ng/g). Fermentation with different bacterial culture combinations resulted in various antioxidant activities. The yoghurt fermented with SB-LA had the strongest antioxidant activity, as indicated by DPPH IC₅₀ (10.69 mg/mL), ABTS IC₅₀ (0.51 mg/mL), and FRAP expressed as FeSO₄ (2577.86 μg/g) assays. The addition of a mixture of yoghurt bacteria and probiotics improved the colour values (L*, a* and b*), syneresis and texture profiles of the soy milk yoghurt. Sensory evaluation showed that yoghurt fermented with S. thermophilus, L. acidophilus and B. lactis received a favourable overall liking score. The successful co-culture of probiotics (B. lactis and L. acidophilus) with yoghurt bacteria produced soy milk yoghurt enriched with melatonin, tryptophan and antioxidants while maintaining acceptable quality characteristics.

Novelty and scientific contribution: The co-cultivation of yoghurt bacteria and probiotics (Bifidobacterium lactis, Lactobacillus acidophilus; SB-BY) or L. acidophilus (SB-LA) improved melatonin production, antioxidant activity and the overall yoghurt quality and provided valuable insights for the development of functional foods that can promote sleep and health.