APLICACIÓN DEL ALGORITMO D-H

Solid fat content is the measure in percent of the amount of solid fat present at a particular temperature, traditionally determined by NMR or dilatometry (Basiron, 1996). It is now also possible to construct it from DSC data. The DSC thermal profile could be integrated at different temperatures to determine the percentage of fat remaining solid at a particular temperature and this information can be used to compare fats which had undergone various thermal treatments (Lehrian et al, 1980).

Figure 1.13: Solid Fat Content o f crude stearin (Basiron, 1996).

100 -

2 5 3 5 Temperature C O

Palm oil is composed of the stearin and olein fractions as indicated previously in Figure 1.12. B y controlling the fractionation conditions, stearin or olein o f required specifications for a particular use could be produced and SFC curves plotted to characterise these fractions. For example, stearins obtained with various iodine values (TV) gave differing SFC curves with higher IV samples being softer and lower IV samples being harder (Figure 1.13).

The SFC is the more commonly used technique to characterise mixtures o f fats since it is very difficult to construct a phase diagram for mixtures o f fats (heat changes occur over a

Chapter 1: Introduction....53

broader range, overlapping of melting and eutectic points, etc.). The 'contours’ on the SFC graph can indicate behaviours such as eutectics, compound formations and sharp or broad- range melting. The detection of the eutectic effect is especially important in assessing the compatibility between two different fats. In a compound coating mixture containing lauric hard butter and cocoa butter, the eutectic phenomenon was observed when the SFC of the mixture at various concentrations of the butters was plotted (Laustsen, 1991). At the eutectic composition, the SFC of the mixture is at its lowest. This resulted in liquid fat being formed by the eutectic composition which migrated to the surfaces and recrystallised as bloom. Blooming is the effect that causes the loss o f gloss and texture on fat surfaces such as chocolates. Therefore, some edible fats need to be high in their SFC in order to remain stable during storage at ambient temperature and prevent liquid fat from separating out.

Figure 1.14: Incompatibility of the blend of cocoa butter (CB) and refined palm kernel oil (RPKO) (Kristott, 1999). c Ü c o o 3 "o 00 100 — I 10«C 2 6 .7 » C CB R P K O % Composition

A eutectic composition indicated by the minimum on the curves was also discovered for cocoa butter and palm kernel oil when the SFC diagram o f the blends was plotted (Kristott, 1999) (Figure 1.14). Similarly, this minimum was found for the blends of cocoa butter and fully hardened milk fat (Timms, 1984). Knowledge obtained from these diagrams can then

Chapter 1: Introduction....54

allow the manufacturers to manipulate the composition of fat mixtures and conditions of production. When a certain percentage of solid fat is required to crystallise at a particular temperature, additives can also be mixed into pure oils and DSC can be used to measure the SFC (Yap et al, 1989b). Again, such information would be very useful in choosing the right processes to achieve a certain outcome. For more details on this subject, refer to Section 1 o f Chapter 6.

1.3,5 Factors affecting the polymorphism o f fa tty acids and glycerides

The complexity of the fat system makes it very difficult to predict its physical properties. For example, in order to construct reliable phase diagrams or isosolid diagrams, the molecular composition, together with the polymorphs associated, need to be obtained. However, this is an extremely difficult task due to the complex mixtures of glycerides in the fat (Liversidge et al, 1981). Even though the individual glyceride can be extracted using a technique such as HPLC, for a system such as gelucire, whereby the glycerides are made more complex by the estérification with PEG, the poor understanding o f the phase changes has impeded the construction of such diagrams.

Besides the factors that could possibly affect the polymorphism o f the fatty acid component in the gelucire which have been mentioned before, such as the types of fatty acid mixture and additives present, preparation conditions also play an important role. Heating rate affected the extent of the production of the different polymorphs (Aronhime, 1988). It is also well known especially in the food industry that cooling rate can influence the hardness o f the fat, by causing differences in the crystal size, solid fat content or possibly polymorphism (deMan, 196 lb). The type o f change induced on the fat by the cooling rate is also influenced by the additional processing undergone by it such as interesterification and isomerization (deMan, 1961a). When the original fat was slow cooled, the product was softer than when rapidly cooled. However, after interesterification o f the fat, the slow cooled product was much harder whilst isomerization caused an even greater increase in the hardness. The increase in hardness after interesterification was attributed to the elevation in the high melting glycerides whilst for isomerization, it was attributed to the increase in the trans-form.

Chapter 1: Introduction....55

Interesterification has also produced smaller crystals after slow cooling and bigger crystals after rapid cooling but the reverse is valid for the original fat. Rivarola et al (1987) showed that for hydrogenated sunflowerseed oil, as the rate of cooling increased, the form (more unstable) tended to crystallise out. Rapidly cooling the oil produced small, thin needles (1-

10|lm long) whilst slow cooling produced long needles (20-30|im ). Similarly, with sunflower / cotton oil blends, a rapid cooling rate increased the proportion of the solution which crystallised at low temperature. Laine et al (1988) noted that there was more amorphousness in rapidly cooled triglycerides than slow-cooled ones. Further factors that may affect polymorphism are ageing and storage conditions, as will be elaborated below.