Oil uptake during potato slice frying

This work was carried out to determine and quantify the mechanism whereby oil was transferred into potato slices during frying. At the same time processing variables were identified and examined to determine the effect of each variable on oil uptake. Samples of potatoes of the variety Record UK were sliced, using a commercial slicer, and fried in "Sizzle", a 50:50 blend of hydrogenated soya and palm oils. The moisture loss / oil uptake was examined at 145°C, 165°C, 185°C by frying samples for 0–5 minutes at 30 second intervals. It was found that oil uptake, a temperature independent effect in the range examined, was related to moisture loss and that moisture loss was a diffusion controlled process. Mass transfer data were analysed to determine the diffusion coefficients at each of the three frying temperatures. Thermal changes in the frying medium and in the core of the slices were measured to examine the heat transfer process during frying and its relationship to mass transfer. Attempts were made to determine the process of thermal transfer and thermal diffusivities were calculated using the unsteady state heat transfer equations. The thermal changes occurring were not able to be characterised in any simplified form. The processing variables of frying time (0–5 minutes), temperature (145°C–185°C), slice thickness (40–70 thousandths of an inch) and initial solids content (20–24.4%) were used to examine the effect of each variable on the yield of product and its oil content. Use of equations included allow oil uptake to be modified by determined amounts by controlling initial solids content or slice thickness. Frying process modifications are suggested to exploit the determined effects of pre-fry drying and salt blanching to lower oil uptake. Pre-fry drying using hot air or microwave processing will reduce the final oil content by known amounts depending on the drying operation time. A freeze drying process will increase oil uptake also by a known amount. The effect of blanching on oil uptake was measured using time (1–4 minutes), temperature (60–80 °C) ionic component (sodium and calcium cations) and ionic concentration (0–2.0 M) as variables. Salt blanching in solutions of sodium or calcium chloride will reverse the trend in oil uptake increase identified in hot water blanching. Throughout the course of the work structural changes were examined by microscopic analysis of sections taken using a cryostatic ultramicrotome. Sections were differentiated using vapour stains such as iodine and osmic acid. To locate oil in the prepared slices a technique was developed in the laboratory involving frying in oil containing 0.5g litre-1 of the lipo-selective stain oil Red O. Photographs and photomicrographs are included in this report.