A novel and facile method is developed for rapid estimation of lactic acid bacterial concentration in fermented milk. Growth of bacteria in a liquid changes physicochemical property of the medium and its behavior at solid-liquid interface. Wettability determines characteristic of solid-liquid interface. Nano-rod, helical tetragonal and L-shaped morphologies were designed and fabricated. Hydrophobicity and zeta potential were measured for dried surfaces of 5 dairy bacterial strains. Relationship between microbial population and changes in solid-liquid interface was studied by wettability and surface free energy measurements. Due to hydrophobic surface property of conventional dairy strains, they strongly affect solid-liquid and liquid-vapor surface tensions when dispersed in a liquid, which are dependent on the bacterial concentration. Response surface methodology results showed that type and concentration of bacteria, droplet volume and solid-surface morphology affect wettability significantly. Higher hydrophobicity resulted in higher ∆θ (absolute value of the difference between the pure physiological saline and the bacterial suspension contact angles) dependence on the bacterial concentration. Probiotic bacteria concentration in fermented milk was estimated using the proposed method. A direct relationship was obtained between milk contact angle and bacterial concentration. Results show that this physical method can be applied for rapid estimation of bacterial concentration.
Keywords: Contact angle; Hydrophobicity; Microbial surface; Probiotics; Surface free energy; Zeta potential.
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