The freeze-thaw gelation of hen's egg yolk low density lipoprotein (1.d.l.) has been examined by viscometry, agarose gel filtration, electron spin resonance spectroscopy, electron microscopy and differential scanning calorimetry. The data indicate that the core lipids, the hydrocarbon chains of interfacial lipids of 1.d.l. and the sulphydryl, imino, amino and tyrosine groups in the side chains of 1.d.l. peptides are not directly involved in the gelation mechanisms. The gelation is caused by non-specific aggregation due to cross-linking of peptides and/or phospholipid polar head groups of adjacent 1.d.l. particles. During freezing, interparticle contacts are formed and strengthened as transformation of solvent water into ice increases the concentration of the particles. Salt and sugar additives inhibit large increases in concentrations of 1.d.l. during freezing and prevent gelation by helping to increase solvation of interfacial regions of the 1.d.l. particles. Agents that induce large viscosity increases in the 1.d.l. sols before freezing may act by producing a local conformational change in 1.d.l. protein and/or disrupting lipid-protein bonds.

The question of lipid release and destabilisation of egg yolk lipoproteins has been examined (a) as a function of temperature in the absence and presence of sucrose and (b) during the fatless sponge cake making. Accessibility of lipids to n-hexane was used as a criterion of lipoprotein destabilisation. Yolk plasma is more susceptible to lipoprotein destabilisation than whole yolk at high sucrose concentrations between 70 and 100°C. During baking, yolk lipoproteins are destabilised in a random manner with over 40% of the lipids being released towards the finishing stages of cake structure development. The lipid release does not become significant until llO"C, rises sharply above 110-120°C and appears to correlate with water loss. It is predominant in regions nearer the cake periphery and in the final stages of cake structure development.

Enzymatically modified yolk or yolk plasma and succinylated yolk plasma gave unacceptable Madeira cakes but good fatless sponges of reasonable cell structure although somewhat inferior to the control. Modified yolk plasma showed variable degrees of aggregation in response to various treatments but the overall integrity of lipoprotein structure was unaffected. It is thought that in addition to the macroscopic integrity of yolk lipoproteins, their interfacial molecular anatomy has a marked influence on baking performance in both Madeira cakes and fatless sponges.