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In the early 1990s partially hydrogenated fats could be found in essentially all fat applications that involved some kind of challenge to the fat composition. Partially hydrogenated fats were the most important and versatile ingredient for fat technologists. In simple terms, this role was based on three distinct properties: (1) the high chemical stability against oxidation of the partially hydrogenated fats, corresponding to the significantly reduced levels of polyunsaturated fatty acids (PUFA) compared with native oils; (2) the possibility of manipulating the melting profile of fat compositions as a function of the degree of hydrogenation; (3) partially hydrogenated fats have favorable crystallization properties as they crystallize quickly and effectively deliver structure to fat phases. An additional benefit of the application of partially hydrogenated fats is the fact that the final fat functionality is more dependent on the hydrogenation process than on the actual native starting fat. This feature creates a fair amount of raw material flexibility with its known benefits - a phenomenon described as 'interchangeability'.

Mildly hydrogenated oils are preferred in frying applications because their semi-liquid nature permits convenient handling while their chemical composition, in particular the absence of linolenic acid (18 : 3), ensures longevity of the frying medium. The extremely steep melting behavior related to high levels of TFA qualifies partially hydrogenated fats as good cocoa butter substitutes and coating fats. Compared with alternative fats with a similar melting range, TFA-containing fats show a solidification behavior that is clearly superior in manufacturing processes under quiescent conditions (the term 'quiescent' indicates the absence of shear during crystallization). A number of other applications require long-term storage at ambient temperatures. Examples are bouillon cubes, cookies and the like. In these applications aspects such as chemical stability, crystallization behavior for manufacturing and melting profile are of key importance. While the first two aspects are self-evident, the melting profile in these applications is related to a compromise between the absence of solid fatty residue or waxy mouthfeel when consumed and the integrity of the product over its shelf life. For this type of commodity the storage conditions are in essence not controlled and robust designs are necessary. Last but not least, the application of partially hydrogenated fats in spread products such as margarine is highly favored by the ease of manufacturing and the good structuring and melting behavior. In this product area the substitution of TFA has been widely achieved in Europe as the elimination process started in the mid 1990s. Before starting a detailed product-specific discussion of the conversion from partially hydrogenated fats to VTF fats it is necessary to consider aspects such as crystallization behavior, product integrity or stability, and melting behavior explicitly. Detailed discussions on the different aspects of chemical stability can be found elsewhere (Allen and Hamilton, 1994; Chan, 1987).

15.1.3 Crystallization behavior

To what extent a fat composition is suited to supply the necessary structure in a certain product application depends on a combination of the structuring potential of the formulation and the manufacturing process. Without reiterating the discussion of fat structures elsewhere (e.g. Marangoni et al., 2006), a few basic comments on the structuring of fat phases must be made here. In a first rough approximation the structure or hardness of a semi-solid fat mixture is proportional to the amount of solid fat present (de Bruijne and Bot, 1999; Kloek, 1998). Taking a more refined view without going into any details, the number of continuous connections through the bulk of the mass and the strength of these connections drive the bulk rheological properties. The first aspect can be directly linked to the size and shape of the crystals present in the system. The rule of thumb supports the line of thought that smaller crystals are favorable. To get a comprehensive view of crystal-crystal interaction is far more complicated. In cases where only secondary bonds are considered, one might assume that the van de Waals adhesive forces between the various crystals are similar. They will, however, be very different if primary bonds come into play. Primary bonds are related to the so-called 'sintering'. As the term suggests this is the formation of solid bridges between the crystals of the primary network due to additional material crystallizing on the original fat crystal scaffolding (Johansson and Bergenstahl, 1995a). The presence of primary bonds delivers much harder structures but is accompanied by a dramatic increase in the brittleness of the semi-solid material. For products that are meant to be plastic, these primary bonds have to be avoided (Haighton, 1965). This is easily understood by appreciating that the slow relaxation of the sintered bonds involves re-crystallization of the bridging material.

The solid state of fats is characterized by monotropic polymorphism; polymorphism is the ability to appear in different forms. The different crystal structures - a, P' and p - relate to different molecular packing arrangements. Each structure has its specific set of physical properties. The term 'monotropic' indicates that for a given composition only one of the three basic polymorphic forms is thermodynamically stable. More detailed descriptions of the polymorphism of fats can be found elsewhere (e.g. Sato, 1999, 2001). According to the basic principles of thermodynamics the molecular composition of a fat defines how much solid material can at best exist at any given temperature. If one ignores the fact that fats are complex multi-component mixtures, composed of numerous triacylglycerols (TAGs), knowledge of the physical properties for a given composition makes it possible to calculate the equilibrium solubility of each polymorph (Wesdorp, 1990). The solubility is straightforwardly - as usual this expression relates to a cumbersome exercise - converted into the so-called 'solid fat content' (SFC) at any given temperature. Typically, fat compositions are characterized according to their SFC. A few typical SFC lines are shown in Fig. 15.1. Owing to overriding kinetic influences, these are not a reflection of equilibrium states. The basic nature of the multi-component systems with possibly multiple solid phases in combination with a complicated crystallization behavior make the equilibration of these systems highly unlikely. The various TAGs that are supersaturated with respect to the system's temperature form multiple mixed crystals. How this process evolves is strongly dependent on the actual crystallization conditions. The main parameters by which to classify the process are: the supersaturation, the speed at which the supersaturation is generated and the shear the system is exposed to during the crystallization process. These parameters influence the essential processes of crystallization, i.e. nucleation and growth. The final size of the resulting crystals is strongly related to the management of these two processes. In instances of abundant nucleation, a large number of small crystals will evolve from the crystallization process. Nucleation increases with increasing supersaturation, as does crystal growth. However, the polymorphism of fats complicates this picture. When substantial supersaturation is

10 25 40

Temperature (°C)

Fig. 15.1 Solid fat content versus temperature lines for selected fats:----, partially hydrogenated rapeseed oil (slip melting point, 32 °C);---, partially hydroge-

nated rapeseed oil (slip melting point, 36 °C); •, interesterified fat based on palm oil and palm kernel fat; ♦, dry fractionated stearin of palm oil; ▲, partially hydrogenated palm oil (slip melting point, 44 °C); ■. fully hydrogenated palm oil (slip melting point, 58 °C).

10 25 40

Temperature (°C)

Fig. 15.1 Solid fat content versus temperature lines for selected fats:----, partially hydrogenated rapeseed oil (slip melting point, 32 °C);---, partially hydroge-

nated rapeseed oil (slip melting point, 36 °C); •, interesterified fat based on palm oil and palm kernel fat; ♦, dry fractionated stearin of palm oil; ▲, partially hydrogenated palm oil (slip melting point, 44 °C); ■. fully hydrogenated palm oil (slip melting point, 58 °C).

applied the crystallizing material behaves according to Ostwald's famous rule of stages (Ostwald, 1897). This rule implies that a less-stable polymorph appears as an intermediate state in the crystallization process. Obviously this is subject to the necessary condition that this less-stable polymorph is also supersaturated. Consequently, a high supersaturation crystallization process takes place in the following order: supersaturated liquid ^ crystall ization a ^ transition into P' or p.

In light of the fact that cocoa butter has a fairly simple structure - only three main TAGs account for 80% of the composition - and yet has six polymorphic forms, the above scheme is only a first approximation. The initial crystallization of the a polymorph can be controlled by the cooling process and occurs almost instantaneously once the solution is supersaturated with respect to the a form. In contrast to this, the solid-to-solid transition processes towards more stable polymorphic forms primarily depend on the fat composition. In the process description above, it is referred to as the p' structure, as this more commonly persists over time in typical fat mixtures. In pure TAGs the p structure is considered the most stable poly-morph due to the better crystal packing, but the p' form is often energetically more favorable for crystals containing many different TAGs. Even though the crystallization process outlined above is more complicated than a single-step process, it is preferably applied in industrial practice. This is because this detour via the metastable crystal form is the fastest -and sometimes the only - way to create solid fat in the preferred final polymorph. The main complication resulting from this process is the adjustment of the manufacturing processes to the kinetics of the polymorphic transition, which is primarily a function of fat composition. Depending on the molecular composition of TAGs and also other minor components such as partial glycerides, the time scale of the polymorphic transition can vary between tens and thousands of seconds. To avoid the development of primary bonds it is advisable to manufacture in such a way that the polymorphic transition is largely achieved before packaging of the product (Bot et al., 2003).

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