FORMATION OF ORDERED STRUCTURES IN MILK SUGAR SOLUTION BILAYER SYSTEMS/

Ordered structures as observed in bilayer fruit (bottom)/yoghurt (top) products are described. Systematic studies were performed on model sy stems, where the yoghurt layer was modelled by milk and the fruit laye r by sugar solutions or sweetened agar gels (5% to 40% (w/w) sugar). T he structures formed were regularly spaced and grew vertically from th e interface, only upwards in agar gel/milk or fruit/yoghurt systems, b ut also downwards in sugar solutions/milk systems. The mechanism previ ously proposed for the development of similar structures in other mult icomponent bilayer systems was used to explain this behaviour. The sta rting point for the development of the fingers was a water/sugar diffu sive transport between the two phases, due to an osmotic pressure grad ient. With protein present ill the upper phase, these mass transfers c aused a density inversion at the boundary that led to a convective flo w, the upper phase tending to sink and the lower phase tending to floa t. Once the instability appeared, the fingers started to grow and cont inued to do so, owing to exchange of water and sugar between the upwar d- and downward-growing fingers.