A systematic study of the residence time of flour in a vibrating apparatus used for thermal processing

Abstract

The dry heat treatment of flour is well established for the production of cake flour for high ratio cakes. This study investigates a new tubular apparatus in which flour is conveyed by vibrations through a helical pipe. Residence time distributions (RTDs) of flour were characterised for various processing conditions and the development of the residence time in extended operation was analysed. A method was developed to accurately determine the RTDs, which could be approximated by normal distributions. The width of the distributions is a critical factor for the accuracy of a thermal process and was identified for different processing conditions. The distributions were narrow, with variations of ± 1% at most. In some cases, the residence time increased over 3.5 h of machine run-time by 7.7%–13.9%. To explain this phenomenon, several hypotheses have been tested. The machine performance was constant with time and no influence of ambient temperature or humidity could be found. It was furthermore shown that changes in the bulk material passing through the apparatus were not the cause of the increase. However, electrostatic charging of the material was observed. Two things led to a reduction in residence time: i) cleaning the pipe with a cleaning pig and water and ii) time, during which the machine is not running. It was suggested that a thin layer of particles inside the pipe in combination with electrostatics effects could be the reason for the residence time increase. Frequent cleaning can therefore allow relatively uniform behaviour and control of residence time. This work investigates the potential application of a novel, vibrating device for the dry heat treatment of flour as a replacement for chlorination in the production of cake flour. Since chlorination was banned in the EU in the year 2000, there is an industrial interest for alternative treatments and equipment to produce flour for high ratio cakes.