In the manufacture of pasta, the drying controls the productivity. For traditional cool-temperature-dried pasta, because the drying takes between 40 and 50 hours, the productivity is low, the wastage is negligible, the quality is superior and the price is high. Through the use of high- and ultra-high-temperature drying, the drying-time has been reduced to between 5 and 12 hours. The productivity is now high and the price is low, but, potentially, at the expense of quality and wastage.
To guarantee high quality and low wastage, the drying must be controlled so that no stress damage (“checking”) occurs within the pasta. Consumers judge pasta quality in terms of its cooking characteristics – clear cooking water and no sediment in the cooking pot at the end of the correct amount of cooking, with a cooked pasta of uniform colour and firm (al dente) mouth feel – all of which are affected by stress damage. In order to control and minimize the accumulation of stress during the drying, one needs to understand how the drying causes the stress.
Recent work, within CSIRO’s Mathematical Modelling of Industrial Processes (MMIP) Group, has established that the drying of pasta involves four distinct stages: (i) the removal of surface moisture just after extrusion; (ii) saturated drying; (iii) migration of the moisture/water-vapour interface into the pasta; (iv) the removal of the remaining weakly bonded moisture by vapour transport as the pasta cures. In the open literature, the drying of pasta is normally described as involving only three phases; namely, (ii)-(iv) above. The relevance and importance of stage (i) has been overlooked, thereby ignoring the fact that, at the end of stage (i), a boundary layer between the moisture and the water-vapour had formed within the pasta.
Previous models failed to assist with the efficient manufacture of pasta, because the stresses involved with the drying were ignored. In addition, the models proposed failed to fully take account of the experimental evidence that pasta is a porous medium, and that the essential physics of stages (i) to (iii) of the drying was predominantly a surface tension phenomenon.
Recent mathematical modelling preformed within the MMIP Group has proved that the stresses involved create high internal pressures through the surface tension in the pores of the pasta and put the outer layers of the pasta under tension. Validation that very high internal pressures can and do occur within the pasta comes from the pressures at which the pasta is extruded. They range from 80 to 100 atmospheres.
The results of this modelling are currently being applied, with the assistance of funding from the New South Wales Government Waste Planning and Management Fund’s Waste Reduction Grant Program, in a “Waste Reduction in Pasta Manufacture” project with Green’s Foods at their Vetta plant in Leeton.