Physiological characteristics of Bacillus strains originated from dairy products and their impacts on rheological properties of pasteurised yoghurt

Abstract

Due to the strong stress resistance, causing food spoilage and toxin production of spore-forming Bacillus species, their presence will be detrimental to the quality assurance of dairy products. Impacts of five Bacillus strains (Bacillus licheniformis H1, B. licheniformis H2, B. subtilis Z2, B. cereus Z6-1 and B. cereus Z6-2) originated from dairy products on the quality of pasteurised yoghurt were investigated. Physiological characteristics of the strains, including growth, acid production, proteolysis, amylase production, extracellular polysaccharide and biofilm formation, were analysed. Additionally, changes in Bacillus-contaminated pasteurised yoghurt during a 60-day incubation at 28°C were observed. Physiological characteristics of the five Bacillus strains were strongly strain-specific. Both B. cereus strains had the highest growth rate and acid production in Brian Heart Infusion broth and the strongest proteolytic ability in skimmed milk. Bacillus subtilis Z2 exhibited the highest amylase activity, while the highest yields of extracellular polysaccharides and biofilm were obtained from B. cereus Z6-2 and B. licheniformis H2. The acidic environment (about pH 4.2–4.4) was not suitable for the growth of Bacilli, as the low survival rate after inoculation and the slow growth. Yoghurt contaminated with B. licheniformis H1 showed strong proteolytic activity, resulting in a gel with relatively low elasticity. Yoghurt containing H2 exhibited a high amylase activity, leading to the formation of many interspersed holes. The Z2 strain had the strongest effect on the rheological properties of yoghurt, resulting in the lowest viscoelasticity. Protease from Bacillus strains mainly interferes with the thickness of the protein matrix in the yoghurt, while amylase mainly affects the size of holes in the gel. The residual Bacillus strains may adversely affect the rheological properties of pasteurised yoghurt, particularly in products with a long shelf life.