Kinetics of Heat Coagulation of Concentrated Milk Proteins at High Sucrose Contents

Viscosity of liquid and semisolid materials: Establishing correlations between instrumental analyses and sensory characteristics

Considering the importance of texture for food products, we aimed to evaluate viscosity of different liquid and semisolid materials through instrumental analyses (rheometer and texture analyzer), as well sensory descriptive analysis, and establishing correlations between all these analyses. Eight materials were used: water, strawberry yogurt, condensed milk, honey, UHT cream, creamy dairy dessert, petit suisse strawberry flavor (a traditional French cheese from Normandy region, and sold as an infant product), and dulce de leche (obtained through cooking of a can of condensed milk, during 15 min under pressure, resulting in a brownish color product and more consistent than condensed milk). All materials were submitted to rheological analysis, analysis on texture analyzer, and descriptive sensory analysis. All techniques of measurement discriminated the texture of samples. The visual viscosity, defined as a sensory attribute evaluated by visual observation, was negatively correlated to apparent viscosity measured through rheological analysis with shear rate at 10 s^-1 . Oral viscosity and body (both defined as sensory attributes evaluated by oral perception) were positively correlated with areas from graphs obtained in the texture analyzer, and with apparent viscosity measured through rheological analysis at shear rate of 10 s^-1 , although positive strong correlation was also found between body and apparent viscosity at higher shear rates (50 and 100 s^-1 ). The strong correlations enable application of these instrumental tests as indicators of the sensory texture of liquid and semisolid materials. PRACTICAL APPLICATIONS: Texture has a minor importance to liquid and semisolid materials in comparison to meat and crunchy products. However, the relevance of texture to these kinds of products has growing up recently. Therefore, measuring and understanding viscosity of liquid and semisolid materials, using different ways of evaluation, brings relevant information to the area. Moreover, establishing correlations between instrumental and sensory analyses may indicate which instrumental analysis and which analysis condition would be more adequate to correlate with sensory perception of texture, allowing a convergence for future studies and for discussion of results.

Lubricated squeezing flow viscometry for dulce de leche (milk sweet) Viscometría de extensión biaxial sin fricción de dulce de leche

Commercial samples of dulce de leche (milk sweet, caramel jam) of two types, made by two different manufacturers were squeezed between two parallel Teflon® 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 discern ible 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.

Kinetics of heat-induced browning in concentrated milk with sucrose as affected by pH and temperature / Cinética del pardeamiento inducido por calor en sistemas de leche concentrada con alto contenido de sacarosa. Influencia del pH y de la temperatura

A model system constituted by whole dry milk powder, sucrose, and distilled water was used to establish kinetic parameters of the heat-induced browning process (HIBP). Second-order central com posite design was chosen with pH and temperature as selected variables. Color response was the Kubelka-Munk index (K/S) calculated from reflectance colorimetric measures as a function of time using the self-backing reflectance transformation (SBRT) procedure. Results showed that HIBP pseudo- order reactions were variable between 0 and 0.6. Reaction rate coefficient (k) was strongly dependent on pH and temperature. In general, k increased as both pH and temperature increased but the influ ence of temperature was higher than that of pH. Q10 was dependent on temperature, ranging be tween 2.15 at 130-140 °C and 2.44 at 100-110 °C, whatever the pH. k values were analyzed with the Arrhenius expression. Values of Ea, calculated for different pH, were coincident (25.34 kcal/M).