Analysis of the reasons for reduced viscosity in ambient yoghurt

Ambient yoghurt is susceptible to microbial contamination. Decreased viscosity adversely affects the quality parameters of yoghurt. Microbial contamination is the significant cause of viscosity reduction. Ambient yoghurt contains various additives, among which colloidal substances and microbial cells are particularly resistant to separation and purification, posing significant challenges for the detection and identification of contaminating microorganisms. In a batch of Ambient yoghurt, only a small subset of samples exhibited reduced viscosity, while routine microbial tests showed no abnormalities. In order to expeditiously determine the underlying factors responsible for yoghurt viscosity reduction and overcome critical technological limitations in quality monitoring methodologies, a thorough microbial traceability analysis was performed. The addition of NaOH facilitated the separation of colloids and microbial cells, enabling subsequent microscopic analysis and sequencing to identify potential microbial contaminants. Additionally, a starch hydrolysis test was performed to analyse the microorganisms responsible for the viscosity reduction. The yoghurt with reduced viscosity exhibited lower colloid content compared with normal viscosity samples. Under staining and microscopic examination, fungal hyphae were observed, and ITS sequencing revealed the highest similarity to Geotrichum candidum. Cultivation tests of retained raw materials showed that only granulated sugar formed white, fluffy colonies on the culture plate, demonstrating strong starch-degrading activity. ITS sequencing analysis revealed the highest homology with G. candidum, confirming it as the source of contamination. After enhancing the sterilisation of granulated sugar, the produced yoghurt no longer exhibited reduced viscosity. Comprehensive analysis indicated that the viscosity reduction in yoghurt was likely caused by G. candidum contamination in the granulated sugar. This study established a simple and rapid method for separating colloids and microbial cells from yoghurt, and a universal method for extracting microbial genomic DNA, enabling rapid and efficient analysis of the causes of viscosity reduction in ambient yoghurt.