Rheological characterization of ricotta cheeses by imperfect squeezing flow viscometry

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(R) container with a wide Teflon(R) plate, of 64 or 100 mm in diameter, to indu ce imperfect lubricated squeezing flow. The log force versus log specimen h eight relationship had a linear region from which the region of a dominant squeezing flow regime could be clearly identified. Its slope was on the ord er 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 (I and 2 mm) and at two given times after relaxation (60 and 120 s) were sensitive measures of consistency and could be used to dist inguish between the ricottas of the three types. The coefficient of variati on of these measures was 10% or less, while the differences between the ric ottas or the different brands were 20 to 100%. The apparent stresses, befor e and after relaxation, had a modest dependence on the upper plate diameter , but the latter had no effect on their sensitivity as measures of consiste ncy. Doubling the compression rate, from 0.1 to 0.2 mm. s(-1), had a relati vely small effect on the magnitude of the apparent stresses, probably a con sequence of the high yield stress.