Impact of using yogurt bacteria co-cultured with probiotics on melatonin content and its derivatives, antioxidant activity, and quality characteristics in cow milk and plant-based milk yogurt

Abstract

Functional foods designed to improve sleep quality are gaining widespread attention, with melatonin being recognized as a key compound for regulating human circadian rhythms. In the current work, cow milk and plant-based milk yogurts were developed due to their health-promoting properties, which include contributions from yogurt bacteria, probiotics, and the inclusion of melatonin and antioxidants. Although there are many different studies on yogurt produced with probiotic bacteria, this study is the first to explore the effect of using yogurt bacteria co-cultured with probiotics on improving melatonin content and antioxidant activity. The objective of this study was to investigate melatonin, serotonin, and tryptophan contents; antioxidant activity; physical characteristics; and sensory evaluation of yogurt as affected by yogurt bacteria co‐cultured with different probiotics and to evaluate the effect of adding tryptophan on enhancing melatonin content, antioxidant properties, and quality aspects during yogurt fermentation. Four different types of yogurt (milk powder and plant-based; soybean, white sesame seed, and chickpea) were prepared using different combinations of yogurt bacteria cultures, including S. thermophilus, L. delbrueckii subsp. bulgaricus, and two probiotics (L. acidophilus and B. lactis). The cow milk yogurt (MPBY) and soy milk yogurt (SBBY) fermented by LAB co-culture with both probiotics had higher melatonin and tryptophan contents (MPBY: 3.64 and 5.48 ng/g dw, SBBY: 18.55 and 303.10 ng/g dw, respectively) than unfermented milk (MP: not detected of melatonin and tryptophan 5.00 ng/g dw, SB: melatonin 17.67 ng/g dw and tryptophan 159.07 ng/g dw). The strongest antioxidant activity was evaluated by DPPH, ABTS, and FRAP assays, which were observed in probiotic yogurt inoculated with L. acidophilus and B. lactis. The color (L*, a*, b*), syneresis (%), and yogurt texture were significantly different (p ≤ 0.05) by mixed yogurt bacteria cultures and probiotic addition. The FTIR spectrum indicated some new peaks of peptides at the wavelength 1538 cm⁻¹. Furthermore, to increase the content of melatonin and its derivative, the study revealed that adding tryptophan to both cow milk and soy milk at 0.5, 1.0, 1.5, and 2.0% (w/v). The melatonin content was highest in cow milk yogurt (MPBY+tryp 1.5%) and soy milk yogurt (SBBY+tryp 1.5%) compared to unfermented milk. Therefore, during storage, physicochemical properties and bioactive compounds of cow milk and soy milk yogurts were evaluated. The results also demonstrated a 28-day shelf life for the yogurts, during which their antioxidant activity increased throughout the storage. The highest syneresis level occurred after 21 days. The pH value decreased on day 7 compared to the initial day and remained constant until 28 days. Melatonin content of cow milk yogurt was highest on day 28 (1.36 ng/g dw) and day 14 for soy milk yogurt (4.1 ng/g dw). Soy milk yogurt consistently outperformed cow milk yogurt in probiotic counts and ACE inhibitory activity, reflecting its superior bioactive potential. The ACE inhibition of cow milk yogurt was 42.94% to 74.72%, and that of soy milk yogurt was 83.40% to 95.19%, gradually decreasing with time until the final storage. Soy milk yogurt exhibited LAB and probiotic counts of 8.0×10⁴ CFU/g and 8.5×10⁷ CFU/g, respectively, compared to 2.9×10⁴ CFU/g and 1.1×10⁵ CFU/g in cow milk yogurt. These findings underline the potential of soy milk yogurt as a more effective functional food, with its higher melatonin retention, antioxidant properties, and bioactive compounds contributing to its health benefits.

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