Magnetic resonance measurements of French fries to determine spatially resolved oil and water content Bryce MacMillan a , Heather Hickey a , Ben Newling a , Manoharan Ramesh b , Bruce Balcom a, * a MRI Centre, Department of Physics, University of New Brunswick, Fredericton, NB, Canada E3B 5A3 b Potato Processing Technology Centre, McCain Foods Limited, Florenceville, NB, Canada E7L 3K5 article info Article history: Received 28 February 2008 Accepted 29 April 2008 Keywords: Conical SPRITE French fry Fried crust Moisture content MRI Oil content Spin–spin relaxation T 2 weighting abstract Deep-fat frying is a popular cooking method, particularly for potato based snack products. While fats and oils are known to add unique flavour, texture and overall palatability to foods, the absorption and distri- bution of oil is not well established. This work examines the absorption of oil in fried potato products with a range of nuclear magnetic resonance and magnetic resonance imaging techniques. We investigate the oil and water contents in French fries based upon the distribution of relaxation times. Relaxation times from different oil and water environments overlap precluding a simple analysis. We develop several techniques for isolating the oil signal from the water signal, and present images mapping the oil absorption as a func- tion of the degree of cooking. Lastly, we present an image derived T 2 map. Spatially resolving the relaxation times permits quantification of the oil and water contributions in the crust and in the core. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Immersion frying, also known as deep-fat frying, is one of the oldest and most popular methods for cooking potatoes. French fries and potato chips are favourite deep fried potato products, with fries accounting for approximately one quarter of all vegetable consumption in the US. Despite increasing health consciousness, fried food remains a multibillion dollar international industry with potato chip sales recently reaching 33% of total US snack sales (Garayo & Moreira, 2002). Fats and oils add unique flavour, texture and overall palatability to foods. As such, the amount of fat ab- sorbed during frying is a critically important parameter in the quality of fried food products. Quantification of the absorbed oil is the subject of this paper. Immersion frying is a complex process of heat and mass transfer involving oil, water, and water vapour. Frying produces a dehy- drated exterior region (the crust), cooks the interior material (the core) and alters both the colour and texture of the food being cooked. In the case of French fries, the crust forms when the potato surface reaches approximately 103 °C, after which the crust thick- ness increases at a rate proportional to the square root of time (Costa, Oliveria, & Boutcheva, 2001). Water evaporation from the surface is initially spatially uniform, but with time the thickening crust impedes the evaporation process. Pressure builds beneath the crust until fractures form, at which point water vapour is re- leased, and the evaporative process resumes at a constant rate ( Mittelman Ashkenazi, Mizrahi, & Berk, 1984). This is accompanied by a corresponding shrinkage of the potato material. When the internal temperature reaches approximately 57 °C, gelatinization – the break up and partial dissolution of the starch granule in the presence of water – begins to occur (Lund, 1984). This creates the texture that we associate with cooked potatoes. The bulk of oil absorption into fried potato products occurs not during the frying process, but only after removal from the hot oil (Bouchon, Hollins, Pearson, Pyle, & Tobin, 2001; Ufheil & Escher, 1996; Vitrac, Trysrtam, & Raoult-Wack, 2000). Absorption is a sur- face phenomenon, dependent on adhesion of oil to the potato sur- face. The vacuum produced by condensation of steam at the evaporation front during cooling drives the capillary flow of oil be- tween intercellular spaces. Oil is located predominantly in the crust, while the core remains virtually oil-free (Bouchon & Aguil- era, 2001; Bouchon et al., 2001). The moisture content in raw potatoes is approximately 80% (w/w), in fries it is approximately 50% and in chips, which may be considered analogous to a potato crust, 1–2% (Vitrac et al., 2000). The oil content in chips is approximately 35% (Ufheil & Escher, 1996) and in fries approximately 15% (Lamberg, Hallstroem, & Olsson, 1990). The principal factors influencing oil uptake during frying are the oil temperature and the duration of frying. An increase in frying time leads to an increase in oil absorption. At higher temperatures 0963-9969/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2008.04.011 * Corresponding author. E-mail address: bjb@unb.ca (B. Balcom). Food Research International 41 (2008) 676–681 Contents lists available at ScienceDirect Food Research International journal homepage: www.elsevier.com/locate/foodres