Emulsion chemical instability

Lipid peroxides are also able to react with other components of parenteral nutrition admixtures (trace elements), causing a drop in pH with the subsequent potential for physical-chemical instability [29]. Table 11 shows the peroxide value and the pH drop in a pure lipid emulsion and a lipid-containing AlO admixture stored in EVA bags under different conditions of temperature and light exposure in the presence and absence of trace elements. 

Instability of an emulsion may be physical or chemical in nature. Chemical instability, which results in an alteration in the chemical structure of the lipid molecules due to oxidation or hydrolysis (McClements, 1999), will not be considered in this chapter for more information, the reader is referred to Chapters 11 and 12. Physical instability results in an alteration in the spatial distribution or structural organization of the globules (i.e., the dispersed phase of the emulsion). A number of important mechanisms responsible for the physical instability of emulsions, as depicted in Figure 5.1, can be divided into two categories gravitational separation and droplet aggregation. 

In most cases, physical instabilities are consequences of previous chemical instabilities. Physical instabilities can arise principally from changes in uniformity of suspensions or emulsions, difficulties related to dissolution of ingredients, and volume changes [6], For instance, some cases where physical stability has been affected are cloudiness, flocculence, film formation, separation of phases, precipitation, crystal formation, droplets of fog forming on the inside of container, and swelling of the container [8],... 

Chemical Instability Molecular species present in an emulsion can be subject to chemical or biochemical reactions that alter their perceived quality. One of the most important types of chemical changes in food emulsions is the result... 

A stable emulsion is considered to be one in which the dispersed droplets retain their initial character and remain uniformly distributed throughout the continuous phase for the desired shelf life. There should be no phase changes or microbial contamination on storage, and the emulsion should maintain elegance with respect to odor, color, and consistency. Instabilities of both chemical and physical origins can occur in emulsion formulations. Chemical instabilities, such as the development of rancidity in natural oils due to oxidation by atmospheric oxygen, the depolymerization of macromolecular emulsifiers by hydrolysis, or... 

The physical instability of emulsions involves creaming, flocculation, coalescence, or breaking, whereas the chemical instability can be a result of hydrolysis of the stabilizing moieties. In order to assess the stability of the emulsion, heating and freezing cycles and centrifugation and steam sterilization can be employed. 

Physico-chemical instability of emulsions (e.g. creams), can be predicted with a test where the temperature is changed with short cycles. 

Thermal Stress. The Arrhenius equation states that a 10°C increase in the temperature doubles the rate of most chemical reactions. However, this approach is generally only useful to predict a product s shelf life if the instability of the emulsion is due to a chemical degradation process. Furthermore, this degradation must be identical in mechanism but different in rate at the investigated temperatures. Thus, the instability of... 

The term emulsion stability is broadly used to describe the ability of an emulsion to resist changes in its properties with time. The properties of an emulsion may evolve over time due to a variety of physical, chemical, or biochemical processes. From a technological standpoint, it is important to identify the dominant processes occurring in the system of interest because effective strategies can then be rationally designed to overcome the problem. A number of the most important physical mechanisms responsible for the instability of emulsions are shown schematically in Figure 5. 

The breakdown of the stable emulsions and subsequent separation to oil and water (demulsification) are important in nuclear, petroleum, and environmental technologies. The emulsion stability is primarily induced by the use of surfactants and is enhanced by reduced size and narrow size distribution of the emulsion droplets. Disruption to low interfacial activity (hence instability) can be achieved by using demulsification agents, which are, however, costly and environmentally undesirable, as they are irrecoverable. Demulsification can also be achieved by electric and/or centrifugal fields, or by chemical treatment of the emulsion. 

Multiple emulsions may be interesting ways for releasing bioactive compounds in a controlled rate, useful in cosmetic, pharmacy, agricultural, and industrial chemicals nevertheless, their commercial applications have been limited due to their thermodynamic instability and unexpected fast release of encapsulated bioactive molecules (Yoshida et al., 1999 Beer et al., 2013). 

Macro- and miniemulsions are thermodynamically unstable. If not stabilized, the droplets tend to fiocculate, coalesce, sediment or cream [2-4]. Other instabilities, such as Ostwald ripening and phase inversion, are also known. At worst, an emulsion will break, i.e. the two phases will separate completely. A product becoming unstable will lose its quality within a short period of time and thus cannot be commercialized. Therefore, even in natural emulsion-based products, amphiphilic molecules are found (e.g. lecithin and proteins in egg yolk and milk and artificial surfactants and emulsifiers in cosmetics and chemical products. They adsorb at the droplets interfaces and stabilize them against flocculation and coalescence. Adsorption and stabilization mechanisms depend on the molecular structure of a surfactant or an emulsifier as depicted in Figure 20.1. Stabilization mechanisms are summarized in... 

The large number of components (sometimes > 50 different components) and the underlying meta-stabile emulsion system are obvious reasons for incompatibility (as described above) and instability. The occurrence of incompatibihty and instability is often invisible. Physico-chemical reactions with negative effects on the stability are [58] ... 

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