MODELO DE MADUREZ DE PROCESOS DE NEGOCIOS DE FISHER

L iq u id Q . _{s o l i d u s} M i x t u r e o f s o l i d s o l u t i o n S x ( 0 ) a n ( t Sy( 0 ' ) X _y

composition

Figure 3.2 Schematic phase diagram of two pure triglycerides (X and Y). X

and Y are assumed to have stable polymorphs 6 and B', respectively and have partial miscibility in the solid state to give solid solutions Sx(B) and Sy(B')»

changing with temperature (see also Chapter 1 section 3.4). It should be noticed that the preceding discussion has not taken into account the existence of metastable forms of fat X ( a and 6') and fat Y (a-form) since the situation at equilibrium is assumed. Obviously, the phase behaviour of mixture of X and Y is dependent on time as well.

1.4.3 Solid Fat Content (SFC)

Most of natural fats and fat blends used in food industry are semisolid in the temperature range of practical interest. In such case the percent fatty solids at various temperature is very important to their quality control. Solid Fat Index (SFI) has been widely used in the determination of fatty solids. Essentially, SFI is a calculated value based on the measurement of thermal expansion and contraction of fat as it changes from solid to liquid or vice versa.

The determination of fatty solids in term of SFI is time consuming involving a dilatometric technique. Attempts have been made to replace this method by using the techniques such as pulse-NMR or DSC. NMR discriminates between proton in the solid phase of a sample and protons of the liquid phase and the results obtained are a more correct measure of the percent solids in the sample than SFI, which measure thermal expansion, a secondary effect of the solid-liquid composition (Applewhite, 1985). The use of NMR in solid measurement is established by the American Oil Chemist’s Society as a "recommended practise" and referred to as the Solid Fat Content (SFC).

The measurement of fatty solids by DSC is the measurement of the heat of fusion of a frozen, completely solid sample of fat. The area under the melting curve is measured, then, selected partial areas up to the temperature of interest are measured as a percentage of the whole (Miller et al, 1969; Applewhite, 1985). The precision of the DSC method in a measurement of solid fat contents is poorer than SFI but the time used for the measurement is much less (Applewhite, 1985).

deMan (1961 a, b) reported the difference in solid fat content (as determined by dilatometry) and penetration hardness in slowly cooled and fast cooled milk fat. Timms

Chapter 3 ...D S C Studies /111

(1980) also successfully used SFC in the study of phase behaviour and polymorphism of milk fat. In this study, SFC as calculated from the DSC data will be introduced into the characterisation of gelucire systems.

2. Experimental Methods

The methods and conditions described here were applied throughout this work (including in later Chapters) unless otherwise stated.

2.1 Sample Preparation

One of the following methods were used in sample preparations.

Method A and Method B or slow and fast cooling : These two methods were the same thermal treatments as those employed in Chapter 2. A sample of gelucire (5-7mg) was accurately weighed to 0.01 mg with an AD-4 Autobalance (Perkin Elmer, Beaconsfield) in an aluminium DSC-pan which was then covered with the sample pan cover and gently

crimped. Each sample pan was then placed in a small glass bottle, put in a

programmable oven and subjected to thermal treatment described earlier in Chapter 2.

Method C or solvent crystallisation : One hundred mg of 10%w/w gelucire in

chloroform solution was poured into a petridish (diameter = 9.2cm, height = 0.8cm). Solvent was allowed to evaporate at room temperature until the first sign of crystallisation occurred, after which the petridish cover was placed to retard solvent evaporation. After 24 hours when the bottom of the petridish was covered with thin plate of crystallised fat, a spatula was used to gently break up the aggregates, and hence further facilitated solvent evaporation. Crystallised sample was then allowed to dry under vacuum (200mBar) at room temperature for 48 hours more.

Method E or ambient cooling : This method was designed to produce samples which could be related as closely as possible to the samples used in the studies on dissolution and mechanical properties. Forty grams of sample was put in a 250ml beaker, placed in a programmable oven and heated at 3°C/min to 75°C. The temperature was held for 1 hour at 75°C before the beaker was taken out of the oven. Approximately 5g of the molten sample was immediately poured into a glass bottle (diameter = 2.0cm, height = 3.2cm), which had been previously left at room temperature, and allowed to cool. By inserting a temperature probe into the bottle, it was found that the melt was cooled from 75°C to 25°C in 2-5 minutes depending on the type of gelucires used. The cooling was non-linear fashion, being faster at higher temperature, thus the cooling rate could not be calculated.

Untreated sample : Samples which were used as received from the manufacturer were

labelled as untreated samples.

Reheated sample : Any samples, e.g. A, B, C, E or untreated samples whether aged

or freshly prepared, which were heated (or scanned) in the DSC sample cell at 2°C/min to 75°C, left at that temperature for 5 minutes then cooled to 25°C at the rate of 200°C/min and left at 25°C for ten minutes were assigned as reheated samples. These samples were usually rescanned to obtain another DSC curves.

2.2 Storage of Samples

Unless otherwise mentioned, samples were stored in one of two cabinets (57x27x24 cm and 32x37x32cm) over silica gel at room temperature («18° to 24°C). During the first 60 days of storage, silica gel was dehydrated occasionally. The relative humidity in the cabinets were found to be 0-10%.

For the study on the effect of moisture, a desiccator containing a saturated sodium chloride solution, which brought the relative humidity to 79-82%, was used to store samples and labelled as 80% relative humidity. Samples labelled as 0% relative humidity were stored in the cabinets described above.