Mechanical Damage to Food Particles During Processing in a Scraped Surface Heat Exchanger

Abstract

The influence of processing conditions on particle damage in a commercial-scale scraped surface heat exchanger (SSHE) was studied using a model system of potato cubes in carboxymethyl cellulose solution. In order to separate thermal degradation effects from mechanical damage, the potato cubes were pre-cooked, for a controlled time, before being passed through the SSHE isothermally. Particle attrition occurred within the SSHE and was found to lead to a severe reduction in the size of soft particles (e.g. following a long cooking period). Experimental evidence suggested that the particle damage was due primarily to particle-blade collisions. Two processing variables were found to have a direct bearing on the degree of particle attrition: the rotor speed, which determined the collision frequency, and the mean particle residence time, which governed the total number of collisions experienced per particle. An increase in the fluid viscosity reduced particle attrition by decreasing the mean particle residence time rather than offering any protective effect per se. Selection of the appropriate operating conditions for particle processing in a SSHE should take account of mechanical damage as well as thermal treatment considerations. Least damage to a soft particle will occur with a low rotor speed and a low particle residence time. The latter may be achieved by an increase in the viscosity of the carrier medium.