**4. Applications to probiotic powders**

The wettability of probiotic powders is first a good indicator of surface cell hydrophobicity, and therefore the cell adhesion capacity through liquid contact angle measurements. Such a functional property is also important for powder product dispersibility and compatibility with coating materials.

Probiotic powders are heterogeneous and complex solid materials that generate high variability data of water contact angle measurements. Therefore, the determination of their wettability is a difficult task and needs a judicious choice of available techniques and methods. In this section, we first describe and discuss the different methodologies used for probiotic powders before illustrating their practical importance in food and agricultural areas.

#### **4.1 Surface wettability**

A first approach is to prepare a compact surface by compressing the probiotic powder sample under high pressure conditions for making pellets, and then measuring the liquid contact angle by the sessile drop method. This method has been used for the first time for determining the water angle contact of the cell surface of the probiotic multistrain sample [85]. Neither visible cracks nor chemical alteration has been observed before and during the measurement of the static contact angle. This method has provided similar results compared to the static advancing water contact angle technique used for the same sample [45]. The combination of the sessile drop method with the compressed disc preparation under well-defined press conditions provides reproducible data and appears as a very practical methodology for determining the probiotic surface contact angles.

A second approach is to prepare a multilayer or lawn of probiotics by passing through an appropriate filter a liquid dispersion of powder particles under vacuum. The filter on which the probiotic lawn adheres is then treated by a first step of moisture equilibration time for several minutes, followed by a second one of air drying. This is the most commonly used method for determining the liquid contact angle, and distinguishing hydrophobic microorganisms to hydrophilic ones [43]. It has some advantages in preparing smooth and homogenous microorganism layers, and is more convenient for fragile cells [86]. Such a procedure has been optimized to lower the variability in the results of contact angles by searching the best moisture equilibrium time and moisturizing medium [43]. Through this method, it has, for instance, recently been shown the contribution of two main probiotic components (*L. bulgaricus* and *S. thermophilus*) of a multistrain sample (vivomixx) for their respective surface hydrophilicity [45]. Some water contact angle data measured by this technique are available in the literature for some bacterial probiotics such as *L. acidophilus* (66–80°), *L. rhamnosus* (33–86°), *L. plantarum* (25–79°), and *S. thermophilus* (23°).

#### **4.2 Bulk wettability**

A third approach to assess the wettability of probiotic powder is the use of the capillary rise technique measuring the amount of liquid penetrating a bed column of sample over time, and applying a mathematical model that fits the penetrating rate profile of a liquid up a tube packed. Even though the capillary rise technique is by far the popular method for determining contact angles of powders or porous materials [69], it has rarely been applied to probiotic powders. Such a method has, for instance, been used for characterizing the wettability of a multistrain powder sample of probiotics, showing two regimes of the liquid penetrating rate profile [85]. The first one is a power law regime for the first few moments while the second one can be described using Darcy's law. The use of this modeling approach has led to the possibility of assessing the particle-packed bed permeability and porosity through an effective radius determined by the semi-empirical Kozeny-Carman approach [85]. These physical properties are interesting for predicting some performance of probiotic strain such as product stability and cell viability during the powder storage. In fact, high powder porosity or permeability promotes the presence of air (oxygen) and/or moisture in inter- and intra-particle pores, accelerates the oxidative process of cell membranes, and reduces rapidly the cell viability rate.

### **4.3 Practical use in food and agricultural areas**

Most ingredients used in product formulations are available in a powder form, owing to many practical advantages in transportation, handling, processing, formulation, packaging, and stability [20].

The wettability of powders plays important roles in product preparation, engineering, stability, and performance, since it is the first step of powder dispersion that is of crucial importance for the process of making a dispersion [87].
