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K. S. PANNU, Consultant, 7852 109-A Street, Delta, BC V4C 4G6, Canada and F. Castaigne, Dept. of Food Science, Laval University, STA-Pavilion Comtois, Ste-Foy, QC G1K 7P4, Canada.

There is a growing demand for ready to eat (tray-pack) meals. Such meals often have small portions of two vegetable items. The nutritional quality of steam processed vegetables (without liquid carriers) is good because leaching losses are minimized. Thus, it is desirable to process vegetables in steam and then transfer the pre-weighed batches to the tray-pack.

To study the batch cooking of diced vegetables by steam injection and evaluate the influence of steam temperature, steam flow-rate and particle size on product center temperature.

A small lab-scale processor was developed for this study. The container used for the steam-injection heating of food particulates had a perforated bottom which permitted the free flow of steam through the food-bed. The flow-rate of steam through the processing chamber was controlled by a throttle valve located at the discharge end. Steam mass flow rate (SMFR) was determined by condensing steam and collecting the condensate. The experimental study was conducted with raw carrots and potatoes that were diced as cubes and slices (1 and 1.2 cm thick). Steam temperature at different locations and food center temperatures were monitored and recorded.

The data showed that temperatures throughout the bed were uniform. Carrot pieces cook faster. The 1-cm carrot cubes took 90s to attain 121C (T-initial=20C and T-steam=125C) vs. 130s for the potato. SMFR (6-fold increase) and steam temperatures (115 to 135C) do not influence the particle-heating rate index (f-value). There was finite surface resistance to heat transfer (Biot < 40).

The concept of processing pre-weighed batches of particulate foods by steam injection has a role to play in aseptic processing. Thus, there is need to explore the development of continuous systems that accomplish batch processing.