Characterization of the Distribution of Oil Uptake in French Fries
Clifford S. Todd , 1 * David M. Williams , 1 ** and Jing Guo 2 1 Analytical Sciences , T e Dow Chemical Company , Midland , MI 2 Food & Nutrition R&D , T e Dow Chemical Company , Midland , MI
*
CTodd2@dow.com ** Retired
Abstract: Methyl cellulose based coatings applied to food before deep- fat frying can reduce the amount of oil absorbed by the food during cooking as measured by bulk analysis techniques. However, information about the distribution of oil in the food, and how that is impacted by the coatings is lacking. A method is presented using osmium tetroxide to stain the oil and light microscopy to visualize its distribution. The method was applied to French fries and showed that the extent of oil ingress was reduced when a methyl cellulose coating was used.
Keywords: light microscopy, fat content, methyl cellulose coating, osmium staining, image analysis
Introduction
Deep-fat cooking is a common cooking technique used to quickly produce tasty and satisfying food, both in texture and flavor. Through the years concerns have grown regarding negative health consequences of fried foods. These concerns are generally associated with the addition of fat to the food during the frying process. Obesity rates in the USA and many other countries are rising, and some of the blame is directed toward the fat content of the public’s diet [ 1 ]. Because of this, food producers are coming under increasing pressure to lower the fat content of their foods. For deep-fried foods this requires an understanding of the mechanisms of oil uptake during deep frying and what to do about it. When room temperature food such as potato is submerged in hot oil, the water inside the food will boil. The resulting volume increase causes steam bubbles to leave the food, leading to the foaming and bubbling typically seen during frying. The continual escape of steam bubbles can establish small capillary channels between the interior and the food-oil interface. When the food is removed from the hot oil, the temperature in the food decreases to the point where steam still inside the food condenses back to liquid. The resulting volume decrease may lead to surface oil being drawn into the interior of the food, increasing its fat content [ 2 ].
Several variables can impact the amount of oil absorbed during frying [ 3 ]. T e temperature and the length
30
of time frying interact to impact the caloric increase during frying. At lower temperatures thorough cooking takes more time, generally leading to more oil uptake. Even assuming thorough cooking, the length of time in a hot oil bath at a given temperature is a variable controlled by the chef. Generally, cooking so long that steam bubbles diminish or cease leads to more oil uptake in the food. Controlling the temperature and time of cooking need to be balanced with their impact on the fl avor and texture of the food. Surface roughness of the food and viscosity of the oil can also impact ultimate fat uptake [ 4 , 5 ].
Other than optimizing the frying process, an approach to reduce the amount of oil absorbed by food during deep frying is to apply a coating designed for that purpose [ 6 , 7 ]. Some materials form a barrier that can reduce the ability of oil to penetrate into the food. The Dow Chemical Company has developed a suite of products under the brand name WELLENCE™ Smart Fry for this purpose [ 8 ]. They contain methyl cellulose, a material with the unusual property that the viscosity of formulations increases with
Figure 1 : Sample preparation. (a) Cooked French fries. (b) Cut fry sample. (c) Osmium-stained fry glued to stub. (d) Vibrotome blade part-way through cutting a section.
doi: 10.1017/S1551929517001201
www.microscopy-today.com • 2018 January
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