Caracterización de las muestras de aguamiel

A colloidal dispersion is considered stable if the dispersion is able to resist aggrega- tion into larger entities that would then segregate from the medium (López-León et al., 2008).

A colloidal system to be considered as thermodynamically stable requires that the size and the size distribution of the system particles are not altered and cannot sediment or float. On the other hand, colloidal systems can also be classified as kinetically stable. These systems are stable for a period of time but will destabilize in the future.

Colloidal stability is a main issue in applications in food technology and engi- neering. In most cases in the food industry, stable dispersions are desired, as is the case of milk, fruit juices, and processed foodstuffs, such as butter, mayonnaise, and salad dressings; many times, the product shelf life is related to its colloidal stability (Jang et al., 2005). On the other hand, in some applications, such as wine clarifica- tion, aggregation is needed (Norde, 2003). Therefore, it is essential to understand the stability of colloidal systems and manipulate the state of the dispersions for specific applications (Cruz-Silva et al., 2007; Eastman, 2005).

Colloid systems can be classified as lyophilic and lyophobic. The first refers to systems that are thermodynamically stable, and the other is related to unstable sys- tems. Lyophobic particles tend to aggregate, because they try to minimize contact with the continuous phase.

18 Engineering Aspects of Milk and Dairy Products

There are many factors contributing to the instability of a colloidal system (Meyer et al., 2006; Zhang et al., 2008) which will be discussed in this section. First, the mechanisms of colloid formation will be presented.

There are two ways to form colloids. The first is related to breaking down large pieces to the size required, known as comminution, and the other refers to starting with a molecular dispersion and building the size by aggregation—that is, by condensation (Myers, 1999). Both colloid formation mechanisms are presented in Figure 1.17.

There are three basic mechanisms for the destabilization of colloidal systems: isothermic distillation, coalescence, and coagulation.

The basic principle of isothermic distillation is that smaller particles transfer mol- ecules to bigger particles. Hence, smaller particles become increasingly smaller, and bigger particles become increasingly larger, destabilizing the colloidal system. This process occurs as a function of a transference process from a region with higher chem- ical potential to a region with smaller potential, reducing the free Gibbs energy.

The difference in chemical potential is related to the Gibbs energy excess on the inter- face, and this energy excess is a consequence of the closeness between molecules in the smaller particles, which promotes repulsive forces and reduction in entropy. To avoid the particle increase, it is possible to add a surfactant in the solution to reduce the interfacial tension, as when a stabilizer is added in a food formulation, improving the stability.

Coalescence is the collision phenomenon between two particles, producing just one particle. This mechanism promotes the diminution of the interfacial area (Figure 1.18) and, consequently, the free Gibbs energy. Food emulsions often undergo coalescence (Akartuna et al., 2008).

(b) (a)

FIGuRE 1.17 Colloid formation: (a) comminution and (b) condensation.

(a) (b)

FIGuRE 1.18 The coalescence process: (a) particles present smaller radius and bigger inter- facial area and (b) particles have bigger radius and smaller interfacial area.

Physical Chemistry of Colloidal Systems Applied to Food Engineering 19

There are some strategies to stop the increase of the colloidal particles, such as using a surfactant to reduce the interfacial tension. In food systems, proteins are often used as an adsorbed layer to stabilize fat (Jang et al., 2005). There are other ways to avoid coalescence, such as diminishing the system temperature, because this action decreases particle movement and, consequently, the frequency of collisions; an increase in the system viscosity to reduce the speed of particles also results in a diminution of collisions.

Sherman (2007) studied the colloidal stability in ice cream and observed that the size of the oil globule, as well as the number of globules and variation in holding temperature, influences the coalescence process. The author found that globules of diameter greater than 0.95 m allow a sharp reduction in coalescence rate, because decreasing the interfacial area with increasing diameter of the globule leads to a more stable colloidal system.

Coagulation can be defined as the aggregation of particles that start moving together (Figure 1.19). This phenomenon occurs aiming to reduce the interfacial area, but this reduction is smaller than in the coalescence process.

Sometimes the coagulation phenomenon is desirable, as shown when a practical example in the dairy industry is considered. The milk stability is mainly attributed to the presence of casein. When rennin enzymes are added in the milk, the casein micelles are destroyed. Therefore, cheese formation is a coagulation process that results from the destabilization of a colloidal system, the milk. On the other hand, the acid coagulation of milk, as a result of removing calcium bound between casein micelles, causes destabilization of casein which aggregates and forms a curd, com- promising milk and yogurt shelf life (Shaker et al., 2000).

To avoid the coagulation process, similar procedures to those applied to avoid coalescence can be adopted.