Influence of temperature on heat transfer coefficient during moderate vacuum deep-fat frying |
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Authors: | Jorge Mir-Bel,Rosa OriaMarí a L. Salvador |
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Affiliation: | Plant Foods Research Group, University of Zaragoza, Miguel Servet 177, 50013 Zaragoza, Spain |
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Abstract: | The objective of this study is to analyze the influence of temperature and reduced pressure on the convective heat transfer coefficient, h, during frying of products with different area/volume ratio. h was determined from surface temperature and moisture loss experimental data during frying of potato cylinders and “churros”, at different oil temperatures (100, 120 and 140 °C) and moderate vacuum (19.5–25.9 kPa). The results obtained during vacuum frying were compared with those obtained at atmospheric pressure, both for the same oil temperature (140 °C) and for the same thermal gradient (40 °C). During frying, h changes considerably, reaching a maximum between 700–1600 Wm−2 K−1 in vacuum frying and 800–2000 Wm−2 K−1 in atmospheric frying. To quantify the effect of oil temperature, pressure and size of the product on h, a parameter called “bubbling efficiency”, BE, was defined. BE relates the bubble departure radius and the area/volume ratio of the product. An equation (the derivative of the Gompertz function) was proposed to estimate the mean heat convective coefficients for each frying condition as a function of BE (R2 = 0.957). The relation between h and BE shows a maximum corresponding to an optimal bubbling pattern. Most of the vacuum frying settings are outside this optimum, being affected by the insulation effect of bubbles covering the surface. |
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Keywords: | &ldquo Churros&rdquo Heat transfer Moisture loss Potato Vacuum frying |
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