Product stability

The stability or integrity of products always has to be seen in the light of the challenge the product is potentially exposed to. For the fat-based products discussed here, the main challenge is typically the fluctuation of the storage temperature. Other minor challenges are of a mechanical nature due to handling and transportation. Another driver for product disintegration is obviously gravity. As discussed above the products also have the potential to change because they are not found in their true equilibrium state. This is particularly true as fat crystal structures tend to change over time even in the most controlled storage conditions. These changes are mainly due to relaxation processes in the network and Ostwald ripening (Bot and Pelan, 2000). Ostwald ripening describes the phenomenon where large crystals actually grow larger at the expense of small crystals. This process is driven by a difference in chemical potential that results from the higher surface energy contribution in the small crystals. Additionally, a typical industrial crystallization process, with high driving forces, results mostly in kinetically frozen non-equilibrium states (Bot et al., 2003). This is possible because the speed at which the mixed solid system approaches its equilibrium state is extremely low.

At a macroscopic level there are principally five types of instabilities. These are: changes in the product hardness; product clumping for powders; oil exudation (meaning the separation of liquid oil out of the semi-solid mass); coalescence of droplets or change of the even distribution of the dispersed particles for emulsions and suspensions; and product inhomogeneity.

The hardness of a product might increase or decrease through a temperature challenge but change also occurs under well-controlled storage conditions. The reduction in hardness is accounted for by the coarsening of crystals. This is stimulated by temperature fluctuations since they involve a change in solubility. At higher temperatures some of the original solid material dissolves into the liquid oil and is re-deposited onto the solid material once the temperature is reduced again. Depending on whether the re-deposited solid forms solid bridges on the original scaffolding or just induces growth of the existing crystals, this process results in increased or reduced hardness respectively. Furthermore, either parts of or the whole crystal structure can undergo a polymorphic transition. This is either accompanied by a steep loss of product hardness or, in cases of a distinct segregation of specific TAGs into a certain conformation, by the occurrence of large crystals or distinct crystal agglomerates. A well-known example of this is the development of tropical graininess related to the separate crystallization of TAGs of the palmitic-oleic-palmitic type (Watanabe, 1992) derived from palm oil or its fractions in spherulitic crystals. The diameter of these particles can be as big as 2 mm. Another problem is the so-called 'sandiness' caused by large needle-like crystals that evolve from re-crystallization of TAGs made up from stearic and elaidic acid. The resulting particles are truly a product defect because their sizes are clearly above the threshold of oral perception, approximately 30 microns.

The process of clumping together of particles of a free-flowing powder is clearly related to temperature fluctuations. Adjacent particles partially melt and become greasy at elevated temperatures, forming a joint liquid layer. On cooling, this joint layer reverts to a solid or semi-solid structure gluing the particles together. Through this process a free-flowing powder can be easily converted into a solid brick.

The three other defects are strongly related to the coarsening of the crystalline network. The capacity of the fat crystal network to hold oil is based on capillary forces and adhesion similar to the function of a sponge. Smaller crystals generate much more surface and thus a finer sponge. On coarsening, it is less capable of holding oil. In most fat-based water-in-oil emulsion products, the main mechanism to stabilize the oil-water interface is Pickering stabilization (Pickering, 1907). This means that solid particles, in our case fat crystals, wet and cover the oil-water interface and thus prevent the coalescence of the water droplets (Johansson and Bergenstahl, 1995b; Johansson et al., 1995a, 1995b; Rousseau et al., 2003). Upon either crystal coarsening or depletion of the crystals due to higher temperatures the protective coverage of the interface might become incomplete or crystals start bridging the individual droplets and thus permit coalescence. This becomes a macroscopical problem only when excessive coalescence changes the product appearance. However, relatively limited progression of coalescence might also have detrimental effects on product quality. This is so because small droplets of diameters below 7 |im suppress microbiological growth through confinement (Verrips and Zaalberg, 1980; Verrips et al., 1980). At larger droplet sizes, microbiological stability can usually only be ensured by use of preservatives such as potassium sorbate. Consequently, the microbiological stability of emulsion products free of preservatives relies on the maintenance of the small droplets.

Lastly, inhomogeneous distribution of solid material is most likely to occur in systems that are best described as viscous liquids. Here also the coarsening or partial dissolution of the fat crystal network is the main cause of the defective distribution of the particles. In such cases the fat scaffolding turns out not to be strong enough to immobilize the dispersed phase. This can yield either sedimentation or creaming of the dispersed phase.

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