Analysis of the heat transfer coefficient during potato frying

The objective of this work was to study the dependence of the heat transfer coefficient (h) on the water loss rate of potato during frying. An indirec t method was used where a metal piece with the same geometry of the potato pieces was placed on top of various potato samples at different frying time s, and its temperature was recorded for 20-30 s. Another method consisted o f direct recording of the temperature within a potato slice, close to the s urface. Water loss rate was estimated by image analysis of bubbles. After i mmersion in hot oil, the potato temperature increases and water starts vapo urising, leaving the surface in the form of bubbles that flow through the o il. The water loss rate increases until complete drying of the potato surfa ce and then decreases till the end of. frying. The h value showed the same behaviour increasing up by two times in relation to the values measured in the absence of bubbling, with maximum values depending on the oil temperatu re and potato geometry (443-750 W m(-1) K-1). The percentage of heat transf erred to the potato that is used for water evaporation showed an increase w ith time up to complete surface drying. (C) 1999 Published by Elsevier Scie nce Ltd. All rights reserved.