In situ microstructure and rheological behavior of yeast biofilms from the juice processing industries

Abstract

The factors affecting the mechanical properties of biofilms formed by yeast species (Rhodotorula mucilaginosa, Candida krusei, C. kefyr and C. tropicalis) isolated from the juice processing industries have been investigated. Variables studied were: the food matrix (apple/pear juice), the sugar concentration (6/12 °Bx) and the hydrodynamic conditions (static/turbulent flow). A range of environmental cues were included as the mechanical properties of biofilms are complex. Yeast counts were significantly higher in turbulent flow compared with under static conditions. The thickness of the biofilm ranged from 38 to 148 μm, from static to turbulent flow. Yeast biofilms grown under turbulent flow conditions were viscoelastic with a predominant solid-like behavior and were structurally stronger than those grown under static conditions, indicating gel-type structures. Only the type of flow had a significant effect on $G_0^{\prime }$ and G*. Flow velocity and nutrient status modulated the biofilm thickness, the biomass and the mechanical properties. A better knowledge of the factors controlling biofilm formation will help in the development of control strategies.

Keywords:

• Yeast
• biofilms
• fruit juice
• rheology
• flow conditions
• stainless steel