Selection of a galactose-positive mutant strain of Streptococcus thermophilus and its optimized production as a high-vitality starter culture.

Streptococcus thermophilus is a common starter in yogurt production and plays an important role in the dairy industry. In this study, a galactose-positive (Gal+) mutant strain, IMAU20246Y, was produced using the chemical mutagen N-methyl-N'-nitro-N-nitrosoguanidine (NTG) from wild-type S. thermophilus IMAU20246, which is known to have good fermentation characteristics. The sugar content of milk fermented by either the mutant or the wild type was determined using HPLC; metabolism of lactose and galactose was significantly increased in the mutant strain. In addition, we used response surface methodology to optimize components of the basic M17 medium for survival ratio of the mutant strain. Under these optimal conditions, the viable counts of mutant S. thermophilus IMAU20246Y reached 4.15 × 108 cfu/mL and, following freeze-drying in the medium, retained cell viability of up to 67.42%. These results are conducive to production of a high-vitality starter culture and development of "low sugar, high sweetness" dairy products.

Influence of Different Ratios of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus on Fermentation Characteristics of Yogurt.

Lactic acid bacteria (LAB) are industrially important bacteria that are widely used in the fermented food industry, especially in the manufacture of yogurt. The fermentation characteristics of LAB are an important factor affecting the physicochemical properties of yogurts. Here, different ratios of L. delbrueckii subsp. bulgaricus IMAU20312 and S. thermophilus IMAU80809 were compared with a commercial starter JD (control) for their effects on viable cell counts, pH values, titratable acidity (TA), viscosity and water holding capacity (WHC) of milk during fermentation. Sensory evaluation and flavour profiles were also determined at the end of fermentation. All samples had a viable cell count above 5.59 × 107 CFU/mL at the end of fermentation, and a significant increase in TA and decrease in pH were observed. Viscosity, WHC and the sensory evaluation results of one treatment ratio (A3) were closer to the commercial starter control than the others. A total of 63 volatile flavour compounds and 10 odour-active (OAVs) compounds were detected in all treatment ratios and the control according to the results from solid-phase micro-extraction-gas chromatography-mass spectrometry (SPME-GC-MS). Principal components analysis (PCA) also indicated that the flavour characteristics of the A3 treatment ratio were closer to the control. These results help us understand how the fermentation characteristics of yogurts are affected by the ratio of L. delbrueckii subsp. bulgaricus to S. thermophilus in starter cultures; this is useful for the development of value-added fermented dairy products.

Unveiling the genetic basis and metabolic rewiring behind the galactose-positive phenotype in a Streptococcus thermophilus mutant.

Streptococcus thermophilus (S. thermophilus) is a widely used starter culture in dairy fermentation, but most strains are galactose-negative and only metabolize glucose from lactose hydrolysis. In this study, we aimed to uncover the mechanisms underlying the acquisition of a stable galactose-positive (Gal+) phenotype in a mutant strain of S. thermophilus IMAU10636. By treating the wild-type strain with the mutagenic agent N-methyl-N-nitro-N-nitrosoguanidine, we successfully isolated a Gal+ mutant, S. thermophilus IMAU10636Y. Comparative enzyme activity assays revealed that the mutant exhibited higher ß-galactosidase and galactokinase activities, but lower glucokinase and pyruvate kinase activities compared to the wild-type. High-performance liquid chromatography analysis confirmed the mutant's enhanced ability to utilize lactose and galactose, leading to increased glucose secretion. Integrated genome and transcriptomics analyses provided deeper insights into the underlying genetic and metabolic mechanisms. We found that the metabolism regulatory network of the glycolysis / Leloir pathway was altered in the mutant, possibly due to the upregulation of the gene expression in the galR-galK intergenic region. This likely led to increased RNA polymerase binding and transcription of the gal operon, ultimately promoting the Gal+ phenotype. Additionally, we identified a mutation in the scrR gene, encoding a LacI family transcriptional repressor, which also contributed to the Gal+ phenotype. These findings offer new perspectives on the metabolic rewiring and regulatory mechanisms that enable S. thermophilus to acquire the ability to metabolize galactose. This knowledge can inform strategies for engineering and selecting Gal+ strains with desirable fermentation characteristics for dairy applications.

A wearable conductive hydrogel with triple network reinforcement inspired by bio-fibrous scaffolds for real-time quantitatively sensing compression force exerted on fruit surface.

INTRODUCTION: Mechanical stresses incurred during post-harvest fruit storage and transportation profoundly impact decay and losses. Currently, the monitoring of mechanical forces is primarily focused on vibrational forces experienced by containers and vehicles and impact forces affecting containers. However, the detection of compressive forces both among interior fruit and between fruit and packaging surfaces remains deficient. Hence, conformable materials capable of sensing compressive stresses are necessary. OBJECTIVES: In the present study, a triple-network-reinforced PSA/LiCl/CCN@AgNP conductive hydrogel was synthesized for compression force detection on fruit surfaces based on changes in intrinsic impedance under mechanical loading. METHODS: The conductive hydrogel was characterized in terms of its adhesion, mechanics, frost resistance, water retention, conductivity, mechanical force-sensing properties, and feasibility for monitoring mechanical forces. Then, a portable complex impedance recorder was developed to interface with the conductive hydrogel and its mechanical force sensing ability was evaluated. RESULTS: Beyond its inherent conductivity, the hydrogel exhibited notable pressure sensitivity within the strain range of 1 % to 80 %. The conductive hydrogel also demonstrated a commendable adhesion property, favorable tensile property (580 % elongation at break), substantial compressive strength and durability, and a long-term water retention capability. After exposure to -20 °C for 96 h, the hydrogel maintained its mechanical strength, affirming its anti-freezing property. In addition, a portable complex impedance recorder with sustained signal measurement stability was developed to quantitatively acquire the hydrogel resistance changes in response to compression forces. Finally, the effectiveness of the conductive hydrogel for sensing compression force on the surface of apple fruits was validated. CONCLUSION: The conductive hydrogel holds promise for applications in smart packaging, wherein it can detect crucial mechanical stress on fruit, convert it into electrical signals, and further transmit these signals to the cloud, thereby enabling the real-time sensing of mechanical forces experienced by fruits and enhancing post-harvest fruit loss management.

Seven new chemical constituents from the roots of Gentiana macrophylla pall.

Gentimilegenins A, B (1, 2), (6R, 8R)-6-hydroxy swerimuslactone A (3), (6R, 8S)-6-hydroxy swerimuslactone A (4), 4-hydroxy roburic acid methyl ester (5), (±) 3'-hydroxy gentioxepine (6), N-heptacosanoyl anthranilic acid (7a), N-nonacosanoyl anthranilic acid (7b), together with 40 known compounds were isolated from the roots of Gentiana macrophylla Pall. Their structures were elucidated on the basis of comprehensive analysis of HRESIMS, IR, 1D-, 2D-NMR and X-ray diffraction. The anti-inflammatory effects of selected compounds were also evaluated through the detection of their inhibitory effects on NO production in LPS-induced RAW264.7 macrophage cells.