The role of cell surface hydrophobicity in the adhesion to stainless steel (SS) of 11 wild yeast strains isolated from the ultrafiltration membranes of an apple juice processing plant was investigated. The isolated yeasts belonged to four species: Candida krusei (5 isolates), Candida tropicalis (2 isolates), Kluyveromyces marxianus (3 isolates) and Rhodotorula mucilaginosa (1 isolate). Surface hydrophobicity was measured by the microbial adhesion to solvents method. Yeast cells and surfaces were incubated in apple juice and temporal measurements of the numbers of adherent cells were made. Ten isolates showed moderate to high hydrophobicity and 1 strain was hydrophilic. The hydrophobicity expressed by the yeast surfaces correlated positively with the rate of adhesion of each strain. These results indicated that cell surface hydrophobicity governs the initial attachment of the studied yeast strains to SS surfaces common to apple juice processing plants.

The aim of the present work was to investigate the in situ rheological behavior of yeast biofilms growing on stainless steel under static and turbulent flow. The species used (Rhodototula mucilaginosa, Candida krusei, Candida kefyr and Candida tropicalis) were isolated from a clarified apple juice industry. The flow conditions impacted biofilm composition over time, with a predominance of C. krusei under static and turbulent flow. Likewise, structural variations occurred, with a tighter appearance under dynamic flow. Under turbulent flow there was an increase of 112 μm in biofilm thickness at 11 weeks (p < 0.001) and cell morphology was governed by hyphal structures and rounded cells. Using the in situ growth method introduced here, yeast biofilms were determined to be viscoelastic materials with a predominantly solid-like behavior, and neither this nor the G’₀ values were significantly affected by the flow conditions or the growth time, and at large deformations their weak structure collapsed beyond a critical strain of about 1.5–5%. The present work could represent a starting point for developing in situ measurements of yeast rheology and contribute to a thin body of knowledge about fungal biofilm formation.

Membrane separation systems represent a hot spot for biofilm formation in juice industries. Sodium hypo chlorite (NaOCl) has been traditionally the disinfectant of choice; however, its effectiveness over well-established biofilms is limited. In this work the study of biofilm formation on ultrafiltration membranes was proposed. The effectiveness of cleaning and disinfection procedures commonly used in juice industry was tested on the removal and killing of cells. The species used (Rhodotorula mucilaginosa, Candida krusei, Candida kefyr and Candida tropicalis) were isolated from ultrafiltration modules of a clarified apple juice industry. Industrial concentrations of NaOCl (200 mg CL.L-1) showed to be effective against planktonic cultures with > 4 log reductions, whereas their overall efficiency against adhered cells was smaller. Recovery of viable cell counts to initial numbers was evidenced regardless of the time of colonization. The topography of the surface showed to have an impact on the efficiency of the disinfectant, presenting membranes smaller log reductions than stainless steel (similar to 1.09-1.53 log CFU). At 200 mg Cl.L-1 only membrane's cross flow recovery was reached with no long-term effect over the attached cells. The overall results demonstrated the recalcitrance of these biofilms to typical cleaning and disinfection process which may confer them with a selective advantage.

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.