BOUNDARIES OF THE CRYSTAL MEMORY EFFECT IN SATURATED TRIACYLGLYCEROLS
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One factor that determines the crystalline phases formed from liquid triacylglycerols is the “crystal memory”: fats that re-crystallize from a liquid obtained by melting a crystal may form the same crystal structure that they had as a solid before melting. The mechanism of this phenomenon is almost unknown, and no systematic research on pure triacylglycerols has been done. To advance the understanding of this effect, pure triacylglycerols and triacylglycerol mixtures were crystallized, tempered and subsequently melted. The liquid was then held at a combination of time and temperature before recrystallizing it. After testing many combinations, the [time + temperature] pairs that produced a recrystallization equal to a crystallization from a random liquid were found. These combinations define a time temperature boundary. Above the boundary, the memory is erased, since the liquid recrystallizes in a different form from the parent crystal present before re-crystallization. These experiments were done in a differential scanning calorimeter, given its precise temperature control. Two kinds of memory were identified: early formation of the α polymorph, as observed by Arnaud; and the formation of a stable form, β or β’, directly from the melt. The time-temperature combinations needed to erase each one of these memories were determined. For pure triacylglycerols, the times and temperatures required were shorter and lower than for their mixtures. This supports the hypothesis that the liquid structure of pure triacylglycerols is disrupted by thermal fluctuations. In blends, it is additionally necessary to homogenize the concentration of the mixture. The size of the domains of different concentration in the freshly molten liquid, which depends on the history of the crystallization, also influences the memory. The effect on the onset temperature is more difficult to predict.