Fermentation Characteristics of Fermented Milk with Streptococcus thermophilus CICC 6063 and Lactobacillus helveticus CICC 6064 and Volatile Compound Dynamic Profiles during Fermentation and Storage

Abstract

The lactic acid bacteria Streptococcus thermophilus and Lactobacillus helveticus are commonly used as starter cultures in dairy product production. This study aimed to investigate the characteristics of fermented milk using different ratios of these strains and analyze the changes in volatile compounds during fermentation and storage. A 10:1 ratio of Streptococcus thermophilus CICC 6063 to Lactobacillus helveticus CICC 6064 showed optimal fermentation time (4.2 h), viable cell count (9.64 log10 colony-forming units/mL), and sensory evaluation score (79.1 points). In total, 56 volatile compounds were identified and quantified by solid-phase microextraction and gas chromatography-mass spectrometry (SPME-GC-MS), including aldehydes, ketones, acids, alcohols, esters, and others. Among these, according to VIP analysis, 2,3-butanedione, acetoin, 2,3-pentanedione, hexanoic acid, acetic acid, acetaldehyde, and butanoic acid were identified as discriminatory volatile metabolites for distinguishing between different time points. Throughout the fermentation and storage process, the levels of 2,3-pentanedione and acetoin exhibited synergistic dynamics. These findings enhance our understanding of the chemical and molecular characteristics of milk fermented with Streptococcus thermophilus and Lactobacillus helveticus, providing a basis for improving the flavor and odor of dairy products during fermentation and storage.

Keywords: Lactobacillus helveticus; Streptococcus thermophilus; fermentation characteristics; fermented milk; solid-phase microextraction and gas chromatography–mass spectrometry (SPME-GC-MS).

Figure 1

Characteristics of fermented milk with varying strain proportions: (A) 24 h pH monitoring results during milk fermentation by varying ratios of S. thermophilus CICC 6063 and L. helveticus CICC 6064 co-cultures, including 1:1, 5:1, 10:1, 100:1, and 1000:1 (S. thermophilus to L. helveticus) and commercial starter culture (CSC). (B) Variations in fermented milk pH during storage. (C) Variations in fermented milk titration acidity during storage. (D) Counting viable bacteria in fermented milk stored for 1 day (S: Streptococcus, L: Lactobacillus).

Figure 2

Rheological results of fermented milk prepared with different proportions of co-culture and commercial starter culture: (A) aacroscopic viscosity index (MVI) values; (B) elastic index (EI) values; (C) fluidity index (FI) values; (D) solid-liquid balance (SLB) values.

Figure 3

Electronic nose and sensory evaluation results of fermented milk prepared with different proportions of co-culture and commercial starter culture: (A) radar fingerprint chart of electronic nose; (B) principal component analysis (PCA); (C,D) scores by PLS-DA of E-nose data; (E) sensory scores of different sensory indexes; (F) total sensory scores of samples; (G) the average scores of sensory descriptors of 10:1 and CSC group.

Figure 4

Differences in volatile metabolome of 10:1 S. thermophilus to L. helveticus-fermented milk at different time points during fermentation and storage. (A) The principal component analysis (PCA) score plot revealed the volatile metabolomes. These time points included the initial point before fermentation (0 h (F)), two intermediate time points (2 h (F), 4 h (F)), the termination point of fermentation (0 d (S)), and different stages of refrigerated storage (1 d (S), 7 d (S), 14 d (S), and 21 d (S)). (B) Heatmap of identified volatile metabolites in each group. (C) VIP result diagram of PLS-DA.

Figure 5

Metabolic pathway prediction and concentration changes in discriminatory volatile metabolites in this study.