RHEOLOGICAL CHARACTERIZATION OF RICOTTA CHEESES BY IMPERFECT SQUEEZING FLOW VISCOMETRY 1

Samples of three commercial ricotta cheeses made from whole milk, part skim milk and fat free, were compressed in a 140 mm diameter shallow Teflon® container with a wide Teflon® plate, of 64 or 100 mm in diameter, to induce imperfect lubricated squeezing flow. The log force versus log specimen height relationship had a linear region from which the region of a dominant squeezing flow regime could be clearly identified. Its slope was on the order of ?1.0 to ?0.8 in the whole milk ricotta, ?1.6 to ?0.9 in the part skim milk and ?1.7 to ?1.2 in the fat free cheese. The apparent stresses at two selected heights (1 and 2 mm) and at two given times after relaxation (60 and 120 s) were sensitive measures of consistency and could be used to distinguish between the ricottas of the three types. The coefficient of variation of these measures was 10% or less, while the differences between the ricottas or the different brands were 20 to 100%. The apparent stresses, before and after relaxation, had a modest dependence on the upper plate diameter, but the latter had no effect on their sensitivity as measures of consistency. Doubling the compression rate, from 0.1 to 0.2 mm. s^?1, had a relatively small effect on the magnitude of the apparent stresses, probably a consequence of the high yield stress.     

ESTIMATION OF THE MECHANICAL PROPERTIES OF INDIVIDUAL BRITTLE PARTICLES FROM THEIR BULK COMPRESSIBILITY1

Two puffed cereal particles were compressed both individually and in cells of different diameters and heights. The degree of jaggedness of their force-displacement curves were expressed in terms of their apparent fractal dimension determined from the Richardson plot and by the box counting method (Kolmogorov's dimension). Plots of the degree of jaggedness versus cell diameter and height produced approximately planar relationships in a three dimensional space. Their extrapolation to the individual particle characteristic dimensions gave jaggedness values in agreement with those determined experimentally on the individual particles. This demonstrates that it is possible, at least in certain products, to assess the “brittleness” of individual particles from their bulk compressibility pattern. The same procedure could also be used to estimate the particle “stiffness” expressed in terms of the force at a preselected displacement level (e.g., 15 or 25%).     

INSTRUMENTAL AND SENSORY DETECTION OF SIMULTANEOUS BRITTLENESS LOSS AND MOISTURE TOUGHENING IN THREE PUFFED CEREALS1

The jaggedness of the compressive force-displacement relationship of cheese balls®, cocoa puffs® and peanut butter crunch® particles at various moisture contents was determined in terms of their apparent fractal dimension, and the mean magnitude of the power spectrum of their normalized residuals. The plots of these measures versus moisture contents had a characteristic sigmoid shape very similar to that of the corresponding sensory crispness and crunchiness vs moisture content relationships. This close similarity suggests that the jaggedness of the force-displacement relationship is a manifestation of the same failure events that produce the crispness or crunchiness sensation. (In the context of the tested products the two terms, crispness and crunchiness, were found to be synonymous.) In all three products moisture toughening could be clearly detected instrumentally but only in two there was an unambiguous corresponding increase in the sensory hardness scores. This indicates that at least in certain cereal products moisture toughening can be perceived simultaneously with brittleness loss.