Oil Uptake in French Fries Results Figure 3 : Higher-magnifi cation image of the methyl cellulose coated sample.
Frying . A commercial deep-fat fryer was used. Prior to the frying experiments, the fryer was preheated to ~375 °F (190 °C). Batter-coated potato strips were placed in a basket and submerged to par-fry for 30 seconds. The frying basket was shaken a couple times after about 15–20 seconds. The basket was removed from the oil and shaken to drain away excess oil from the fries. The par-fried potatoes were then transferred to a blast freezer for 10 minutes, after which they were covered with plastic wrap or aluminum foil. Once fries had been frozen overnight, the fries were placed into a frying basket and finish-fried for about 2 minutes at ~365 °F (185 °C). The frying basket was removed from the oil and shaken to remove excess oil. The fries were frozen again prior to bulk oil analysis and microscopy. The cooking temperature and time were carefully controlled so that comparison between coated and uncoated fries was valid.
Oil Content . Oil content of French fries was determined on dried samples using Soxhlet extraction method (AOAC, 2003.65).
Microscopy . An osmium tetroxide staining method
was adapted [ 16 ]. Cross sections measuring approximately 4–5 mm thick of the uncoated-cooked and coated-cooked French fries ( Figure 1 a and 1 b) were placed in 1% aqueous osmium tetroxide for fi ve minutes, then rinsed in running tap water for 20 minutes. T e staining “fi xed” or stabilized the oil. Each sample was then mounted on sample stubs using a cyanoacrylate adhesive ( Figure 1c ) and sectioned to 150-250 µm thickness using a Series 1000 Vibratome vibrating microtome ( Figure 1d ). T e Vibratome sample trough was fi lled with distilled water to facilitate sectioning and easy removal of the cut sections. T e cut sections were transferred to glass slides and imaged using a Wild M-5 photomicroscope equipped with indirect refl ected illumination. An Oil Red O staining method was found to work well [ 16 ]. Image Analysis . ImageJ soſt ware was used to convert color images to black-and-white, to apply thresholds, and to measure the resulting amount of stained area.
2018 January •
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Cooking oils typically contain unsaturated fats, meaning they contain carbon-carbon double bonds. Osmium tetroxide preferentially binds to material with carbon-carbon double bonds, imparting a dark color in visible light. In contrast, the main components of potato are water, carbohydrates, and proteins, which do not contain C-C double bonds. Therefore, the amount and distribution of oil in a French fry can be visualized by the dark staining. Figure 2a shows a cross section through an uncoated French fry showing the distribution of oil. The depth of penetration of oil from the sample exterior is variable, generally about half to one millimeter. Figure 2b shows oil distribution across a sample that had a thin WELLENCE™ Smart Fry coating formulated to reduce oil absorption. The depth and intensity of stained material decreased as a result of the coating. Higher magnification images provide more detailed information about the oil distribution in the fries ( Figure 3 ).
A semi-quantitative characterization of oil reduction can be measured from these images using image analysis. A threshold of the images from zero to 110/256 is shown in Figures 2 c and 2 d (zero is black and 256 is white), and the area of stained potato was measured. T en the interior of the binary image was fi lled in, and the area of the entire fry was measured. Results indicate that the uncoated fry cross section was about 57% stained, whereas the coated fry was about 36% stained, representing a 37% decrease in oil uptake. T e exact amount of stained area in these images is a function of the threshold chosen, graphically shown in Figure 4 . A higher threshold results in a larger fraction of the fry highlighted for both the coated and uncoated sample. T e ratio of stained area for the coated sample to the stained area of the uncoated sample represents the reduction in oil uptake attributable to the coating. T is ratio is also impacted by the threshold chosen. Reduction in stained oil area between 35% and 50% resulted from reasonable choices of threshold ( Figure 4 ). T ese image analysis results are roughly consistent with the
Figure 4 : Area stained in cross section as a function of threshold chosen for image analysis. Also shown is the reduction in stained area attributable to the methyl cellulose coating.
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