Microbial and Biochemical Impact of Starter Culture in Valtellina Casera PDO Cheese Production

Valtellina Casera PDO, a semi-hard cheese from Lombardy, is prone to late blowing defect caused by Clostridium spp. This study evaluates the impact of two starter cultures, a commercial (containing Lactococcus lactis, Lc. cremoris, Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, and Lb. helveticus) and an experimental autochthonous culture (EAC) composed of St. thermophilus VC60 and Lc. lactis VC106, a bacteriocin producing strain effective against clostridia. Microbial populations and biochemical parameters were determined during cheese ripening at 8 °C and 12 °C. A total of 48 cheese samples, produced in three dairies in duplicate, were analysed at 70 and 180 days. Analytical evaluations included microbiota profiling (16S rRNA sequencing on Illumina MiSeq), LAB counts, proteolysis (soluble nitrogen at pH 4.4 by Kjeldahl and casein degradation by urea-PAGE), and lipolysis (free fatty acids by GC-FID). Microbiota analysis revealed a dominant Streptococcus population (95%) in EAC cheeses, contrasting with 80% in commercial starter samples, which had a higher abundance of Lactobacillus. Microbial changes affected metabolic outputs, emphasizing EAC’s ability to regulate enzymatic activity and modulate fermentation. The use of EAC resulted in a reduction of heterofermentative bacteria by 1 log CFU/g. Proteolytic activity increased with ripening time and temperature, with significant differences in soluble nitrogen observed between 70 and 180 days and between 8 °C and 12 °C. However, the two starter cultures exhibited similar index levels. Additionally, there was a notable decrease in propionic acid at 8 °C, indicating a decrease in Propionibacterium activity associated with cheese eyes. Moreover, EAC contributed to higher counts of rod-shaped lactic acid bacteria, particularly at 8 °C. This study emphasizes that selecting starter cultures influences microbial composition and biochemical traits, including fermentation processes, with potential and relevant effects for managing cheese defects and improving product quality during ripening.