Useful Principles to Predict the Performance of Polymeric Flavour Delivery Systems

Abstract

In this chapter, we review semi-empirical principles of chemical engineering that are particularly useful to understand the phase behaviour of glassy polymers applied to encapsulate flavours and fragrances. Hildebrand's solubility parameter is used to outline how the polarity of a polymer increases its cohesive energy density, its density, its glass transition temperature and how it reduces its permeability towards apolar molecules such as gaseous oxygen and most flavouring ingredients. Flory's model of polymer solutions is coupled to a generalised Freundlich adsorption model to highlight the limiting phase behaviour of polar polymer glasses at low partial pressures of water and of hydrogels at the saturation vapour pressure of water.This approach suggests that sigmoidal vapour sorption phenomenology is the macroscopic fingerprint of glassy materials. Below the glass transition point, water sorption is therefore assumed to take place on the pre-existing sites formed by an excess free volume, which characterises glassy polymers. This excess free volume needs typically to be minimised in controlled release applications to minimise the diffusion of oxygen and flavour molecules. Mass transport is further described with Fick's laws of diffusion to highlight the effects of polymer cross-linking or plasticisation, as induced by temperature or partial pressure of water. Diffusion-controlled kinetics, scaling with the square root of time, is distinguished from zero-order kinetics observed when mass transport is polymer relaxation-controlled. INTRODUCTIONThe encapsulation of flavours was initially motivated by the need to protect volatile food components from evaporation and oxidation by locking them in a solid carrier able to dissolve rapidly in water (Schultz et al., 1956). Nowadays, more emphasis is laid on slowing aqueous dissolution down and on triggered release systems activated typically by heat and mechanical stress. Controlled flavour release thus provides new solutions to the food industry and enables the development of innovative consumer goods. In this context, the primary objective of flavour delivery systems is to maximise flavour perception by generating impact and persistence when needed.Flavours are traditionally encapsulated by spray-drying an aqueous polymer solution, and the resulting glassy powders have a fine granulometry, favouring quick aqueous dissolution. The selective retention of flavour molecules as water evaporation proceeds during