Influence of the matrix physical state

Carbohydrate matrices undergo various phase transitions during drying or during baking. These phenomena result primarily from the properties of the polysaccharides (see later in this chapter), but mono- and disaccharides also play a role. Levi and Karel (1995) measured the retention of n-propanol from a sucrose matrix and from the sucrose:raffinose system (raffinose is a tri-saccharide, made of a galactose, a glucose and a fructose unit). These two systems allowed them to compare matrices with different structures (crystallised and amorphous) and under different physical states (glassy and rubbery). They observed a concomitant sucrose crystallisation and propanol release. The sucrose :raffinose system which is less prone to crystallisation shows a rapid initial release of propanol above the glass transition temperature (Tg) attributed to the collapse of the matrix, followed by a slower diffusional release.

In more complex systems also made of polysaccharides, the small sugars may act as plasticisers on carbohydrate polymers. They thus lower the matrix glass transition temperature. As a result, the presence of mono- and disaccharides tends to favour the collapse of carbohydrate matrices used for flavour encapsulation. On the other hand, mono- and disaccharides still slow the diffusion of aroma compounds through the matrix. Consequently, in such matrices low molecular weight carbohydrates plug the molecular level holes between the long entangled polymer chains and thus slow the release of volatile compounds after the initial loss due to collapse (Levine et al. 1992).

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