Lubricated squeezing flow viscometry for dulce de leche (milk sweet)

Commercial samples of dulce de leche (milk sweet, caramel jam) of two types, made by two different manufacturers were squeezed between two parallel Teflon(R) plates, 64 mm in diameter. The initial height of the samples was about 8 mm and they were compressed to 0.4 mm at rates of 0.1, 0.2, 0.3 and 0.4 mm/s. After the samples had reached the final height, they were allowed to relax for about 3 min. The force-height relationships, when plotted on logarithmic coordinates, had a clearly discernible linear region, with slope in the range of -1.1 to -1.5. The apparent stress at pre-selected heights was used as a measure of the product consistency. The experimental scatter of these stresses was in the order of the 10%, small enough to detect differences between the different brands and monitor rate effects. The latter did not conform to theoretical predictions based on the power law model, most probably because of the products' considerable apparent yield stress on the experimental time scale. An empirical formula to describe the rate effect was developed and found to be appropriate for all the products tested. The yield stress itself was manifested in the magnitude of the residual stress after relaxation. It could also be used as a measure of consistency which provided the same ranking of the samples as the apparent stress at a fixed height.