Surface roughness reduces drainage in vacuum and atmospheric fried apple and potato slices

Transcription

1 Surface roughness reduces drainage in vacuum and atmospheric fried apple and potato slices Verónica Dueik, Carolina Moreno and Pedro Bouchon a Department of Chemical and Bioprocess Engineering, Pontificia Universidad Católica de Chile, PO Box 306, , Chile. ABSTRACT The importance of microstructural changes has been recognized in understanding the mechanisms of oil uptake during frying of foods. Several studies have shown that oil is retained in the crust region and mainly penetrates during the post-frying cooling period. Thus, the equilibrium between drainage and suction is of paramount importance. The objective of this work was to relate surface microstructure of vacuum and atmospheric fried apple and potato slices to their surface-oil drainage capacity during post-frying. Apple and potato slices were vacuum and atmospheric fried until achieving a dehydration level of 2% (w.b) using a single equivalent thermal driving force of 70ºC. Slices were then drained for different periods of time (0, 3 and 9 min) before pressurization (vacuum frying) and cooling (atmospheric frying). Then the final oil content of the products was determined using Soxhlet extraction with petroleum ether. In order to understand the different drainage capacity of both products, surface roughness was evaluated using scanning laser microscopy and characterized by an area-scale fractal analysis. Vacuum fried products absorbed significantly lower oil than atmospheric fried ones (56.7 and 18% less oil in vacuum fried potatoes and apples, respectively). We observed large differences in the drainage capacity of both products. In fact, apples drained significantly more oil from their surface than potato. When comparing both products surfaces, apple showed smoother surfaces than potato which can be a direct cause of the higher drainage of the first ones. In fact, we found that surface roughness and drainage capacity are inversely related (R 2 = 0.949). For potato products, vacuum fried had a rougher surface than the atmospheric fried ones (29% rougher), which can explain the lower drainage of the first ones, along with the higher oil viscosity at lower temperatures. The higher roughness of vacuum fried potatoes can be a result of the pressure changes that suffer the tissue during depressurization and pressurization steps. In the case of apple, there were no significant differences in the surface roughness of either vacuum or atmospheric fried products. Certainly, surface roughness is a key factor determining its capacity to drain oil during the post frying period, but there might be other factors such as crust microstructural parameters that affect the final oil content of fried products. Keywords: vacuum frying, atmospheric frying, potato, apple, surface roughness, oil absorption. 1. INTRODUCTION Oil uptake is one of the most important quality parameters of fried food, which is incompatible with recent consumer trends towards healthier food and low fat products (Bouchon & Pyle, 2004). Despite this, oil consumption derived from salty snack products is extremely high. For instance, United States consumers eat over 6.5 kg of snacks annually and salty snacks account for slightly over half of total snack sales (Kuchler et al., 2004). In order to obtain a low fat product, it is critical to understand when, how and where oil absorption takes place, so that oil migration into the structure can be minimized. Several studies have revealed that most

2 of the oil is confined to the surface region of the fried product and restricted to a depth of a few cells (Keller et al., 1986; Lamberg et al., 1990). In relation to oil absorption kinetics, it has been suggested that most of the oil is absorbed after removing the product from the oil bath, since the vigorous escape of water vapor would impair oil penetration during most of the immersion period. As a result, oil absorption would result from the competition between drainage and suction into the porous crust once the food is removed from the oil (Bouchon et al., 2003). Oil adherence and drainage also play an important role, since they determine the available oil fraction that can be pulled into the pores. Rubnov & Saguy (1997) and Pedreschi et al. (2000) used fractal geometry analysis to study the surface topography of potato products, confirming the significant role of crust roughness in oil absorption. Using a similar approach, Moreno et al. (2010) examined the relationship between surface roughness and oil uptake in fried formulated products, which were either based on potato-flakes or wheat-gluten. They determined that products with higher surface roughness absorbed more oil. However, this relationship was restricted to products of similar nature (either gluten or potato-flake based products categories) and could not be extended when comparing different product categories. A promising technology that has shown potential to produce low-fat snacks is vacuum frying (Dueik et al., 2010; Dueik & Bouchon 2010; Mariscal & Bouchon 2008). Vacuum frying is a deep-frying process, which is carried out in a closed system, under pressures well below atmospheric levels (preferably lower than 7,000 Pa), making possible to substantially reduce the boiling-point of water, and therefore, the frying temperature. The low temperatures employed and minimal exposure to oxygen account for most of its benefits, which include nutrient preservation (Da Silva & Moreira, 2008), oil quality protection (Shyu et al., 1998) and reduction of toxic compound generation (Granda et al., 2004). In relation to oil absorption, the mechanism of oil uptake is not fully understood, but several authors have found that vacuum fried products may absorb between 25 and 55% less oil compared to atmospheric fried products (Garayo & Moreira, 2002; Dueik & Bouchon, 2010). The objective of this work was to understand the relationship between surface microstructure of vacuum and atmospheric fried apple and potato slices, and surface-oil drainage capacity during post-frying. 2. MATERIALS & METHODS 2.1 Sample preparation Apples (Malus domestica cv. Granny Smith) and potatoes (Solanum tuberosum cv. Yagana) were purchased from a local supermarket and stored at 7ºC and 85-95% relative humidity. They were washed and cut into 2-mm slices using a mandolin slicer (Danesco International Inc., USA) from which 3.8 cm discs were extracted. Solids content was determined for each sample. 2.2 Frying experiments Atmospheric and vacuum frying experiments were carried out in the same equipment using an equivalent thermal driving force of 70 C, maintaining a constant difference between oil temperature and the boiling point of water at the working pressure (Mariscal & Bouchon, 2008). The vessel was filled with 3 l of high-oleic acid sunflower oil (Camilo Ferrón Chile S.A, Chile), which was preheated at 160ºC for 1 h prior to frying and discarded after 3 h of frying Vacuum frying experiments Once the oil reached frying temperature, ten slices (~20 g) were placed in the frying basket in order to minimize the drop in temperature. The slices were then covered with a grid to prevent them from floating. The vessel lid was fastened and the vessel depressurized. When the pressure inside the vessel reached 6,502 Pa the basket was immersed in the frying oil for increasing frying times until it reached the bubble-end point. The time required to reach said point varied depending on the frying temperature. After each frying time, the basket was lifted out and left to stand for 0, 3 or 9 minutes (equilibrium

3 condition, according to preliminary experiments) and the vessel was pressurized. The samples were then removed from the fryer and allowed to cool to room temperature Atmospheric frying experiments Once the oil reached the frying temperature, ten slices were placed in the frying basket and covered with a grid. The basket was immersed in the frying oil for increasing frying times until it reached the bubbleend point. The time required to reach said point varied depending on the frying temperature. The basket was lifted out and left to stand for 0, 3 or 9 minutes (equilibrium condition, according to preliminary experiments). The samples were then removed from the fryer and allowed to cool to room temperature. 2.3 Analytical methods Oil content Total oil content of ground was determined gravimetrically by Soxhlet extraction with petroleum ether (AOAC, 1995) Solids content Each extracted oil-free sample was placed in a Petri dish, dried in a forced air oven at 105ºC to constant weight, and cooled in a desiccator (AOAC, 1995). Solids content of raw slices was determined using the same procedure Surface roughness The surface topography of samples was measured using a UBM (Solarius Development, Sunnyvale, California, USA) scanning laser microscope (SLM). A triangulation laser height sensor (Keyence LC- 2210, Osaka, Japan) was mounted in the UBM and two positioning tables allowed moving the sample under the sensor at a speed of 0.5 mm/s. The sensor measured the heights, z (µm), at 10 µm intervals in the x and y directions, covering regions of 2.5 x 2.5mm 2. All products were superficially defatted before surface measurement by immersing them in petroleum-ether o C for 30 s after frying. The measured surface topographies were characterized by using an area-scale fractal analysis, as described by Moreno et al. (2010). Briefly, the apparent area of the measured surface is calculated by covering it in a patchwork fashion with triangular patches, whose size represents the scale of observation. The virtual tiling exercise is repeated with patches of progressively smaller areas to determine the apparent area as a function of the scale of observation. The apparent area at each scale is normalized by dividing it by the projected area of the tiled region to determine a relative area, which is used as surface roughness descriptor. A higher relative area at the same scale of observation means a rougher surface. Area-scale fractal analysis was conducted with SFRAX software ( Oil viscosity Oil viscosity was measured over a temperature range of 20 to 170 o C every 10 o C, using a in a Brookfield LVDV I + Versión 5.1 serie AT78847 sensor S 62. The results obtained for oil viscosity correspond to the average of 6 different measurements over that range, at each temperature Statistical Analysis One way variance analyses were carried out to establish significant differences between values obtained for the same sample. The differences of means between samples were resolved by confidence intervals using a Bonferroni test in the Statgraphics 5.0 program (Manugistics Inc., Rockville, MA). The level of significance was set for P 0.05.

4 3. RESULTS AND DISCUSSION Table 1 shows the final oil content (g oil/ g dry defatted-solids) of vacuum and atmospheric fried potato and apple slices at bubble-end point (2% moisture w.b). Vacuum fried apples reduced oil uptake in only 16% compared to atmospheric fried apples, whereas vacuum fried potatoes reduced oil absorption in 49.7%, compared to their atmospheric counterparts. As can be seen, atmospheric and vacuum fried apples absorbed more than 2 and 3 times oil than atmospheric and vacuum fried potatoes, respectively. This is most likely due to the high porosity of apples. Table 1: Initial moisture (d.b.) and final oil content (d.b.) of potato and apple slices after vacuum () and atmospheric () frying, when using a thermal driving force of 70 o C (T = 108 o C and T = 170 o C). Data are means ± standard deviation (n=3). Product Initial moisture content Frying conditions Final oil content (g water / g dry solids) (g oil / g dry defatted-solids) Potato Apple Surface drainage and oil uptake The balance between oil in the crust and residual surface oil is a result of the competition between capillary suction into the crust region and drainage along the surface of the product. Table 2 shows the oil absorption reduction obtained after allowing oil drainage during 9 min, before pressurization, in vacuum fried products, or before cooling, in atmospheric fried. As can be observed, in both products, vacuum fried snacks drained a significantly lower amount of oil compared to atmospheric fried ones. This can be a direct consequence of the lower oil viscosity at the higher temperatures (see Table 3), which may facilitate oil drainage. However, the available area for oil deposition and surface roughness may also play an important role. Table 2: Oil uptake reduction in vacuum and atmospheric fried, after 9-min drainage when using a thermal driving force of 70 o C (T = 108 o C and T = 170 o C). Values reported are means + standard deviations (n=3). Product Frying conditions Oil uptake reduction when including a drainage step (%) Potato Apple

5 As shown in Table 2, apples drained the highest amount of oil whereas potatoes drained the lowest amount. In order to understand the link between surface roughness and oil drainage, we studied their surface topography using a scanning laser microscope (SLM). This methodology, developed by Brown et al. (1993), uses the relative areas that are calculated from the virtual tiling exercise. As can be observed, when the size of the triangular patch, or scale of observation decreases, the relative area increases. An increase in the relative area at a certain area-scale is associated with a higher degree of irregular geometry and roughness. In potato products, vacuum fried surface was 29% rougher than that of atmospheric fried, whereas vacuum and atmospheric fried apple slices showed no significant differences between them. When comparing both products, apple showed smoother surfaces than potato (lower relative area at the same scale of observation), which can be directly linked to the higher oil drainage in apple. In fact, we found that surface roughness and drainage capacity were inversely related (R 2 = 0.949). For potato products, the rougher surface of vacuum fried ones, together with the higher oil viscosity at the vacuum frying temperature (Table 3), may explain the lower drainage capacity of these products. The higher roughness of vacuum-fried potato can be due to the changes that are induced during the depressurization and pressurization steps. In the case of apple, there were no significant differences in surface roughness between vacuum and atmospheric fried. In addition, vacuum fried drained only 14% less oil than atmospheric fried products. Table 3: High-oleic sunflower oil viscosity at different temperatures. Values reported are means + standard deviations (n=3). Temperature Viscosity (cp) CONCLUSION The development of healthier low fat snacks can be achieved if oil uptake mechanisms are adequately understood. As oil absorption is a surface-related phenomena occurring mainly during cooling, the removal of surface oil before its suction, allowed us to reduce the amount of oil taken up by the product in 5.7 and 15.6% for vacuum fried potatoes and apples, respectively. Reduction obtained for atmospheric fried products was about 10.8 and 18.1%, respectively. The lower drainage capacity of vacuum fried products can be attributed to their rougher surfaces and higher viscosity of the frying oil at a lower temperature. Furthermore, apples had smoother surface than potatoes, so oil drainage may be easier in those products. These findings confirm that oil absorption is a surface related phenomena occurring during cooling, in the case of atmospheric frying, or during pressurization/cooling, in the case of vacuum frying. ACKNOWLEDGEMENTS Authors appreciate financial support from FONDECYT Project No

6 REFERENCES A.O.A.C. (1995). Official methods of analysis, 16th ed., Washington: Association of Official Analytical Chemists. Bouchon, P.; Aguilera, J.M.; Pyle, D.L. (2003). Structure Oil-Absorption Relationships During Deep- Fat Frying. Journal of Food Science, 68: Bouchon, P.; Pyle, D.L (2004). Studying Oil Absorption in Restructured Potato Chips. Journal of Food Science, 69: E115 E122. Brown, C.; Charles, P.; Johnsen, W.; Chesters, S. (1993). Fractal Analysis of topographic data by the patchwork method. Wear, 161: Da Silva P.; Moreira R. (2008). Vacuum frying of high-quality fruit and vegetable-based snacks. Lebensmittel- Wissenschaft und Technologie, 41: Dueik, V.; Bouchon, P. (2010). Vacuum frying as a route to produce novel snacks with desired quality attributes according to new health trends. Journal of Food Science, Accepted manuscript. Dueik, V.; Robert, P.; Bouchon, P. (2010). Vacuum frying reduces oil uptake and improves the quality parameters of carrot crisps. Food Chemistry, 119: Garayo J.; Moreira R. (2002). Vacuum frying of potato. Journal of Food Engineering, 55: Granda, C.; Moreira, R.; Tichy, S. (2004). Reduction of acrylamide formation in potato by low-temperature vacuum frying. Journal of Food Science, 69: IUPAC (1972). Manual of symbols and terminology. In: Colloid and Surface Chemistry Pure Applied Chemistry, 31: Keller, CH.; Escher, F.; Solms, J. (1986). A method for localising fat distribution in deep-fat fried potato products. Lebensmittel-Wissenschaft und Technologie,19: Kuchler, F.; Tegene, A.; Harris J.M. (2004). Taxing snack foods: what to expect for diet and tax revenues. Agricultural Information Bulletin No , USDA Economic Research Service. Lamberg, I.; Hallstrom B.; Olsson H. (1990). Fat uptake in a potato drying/frying process. Lebensmittel- Wissenschaft und Technologie, 23: Moreno, M.C.; Brown, C.A.; Bouchon, P. (2010) Effect of food surface roughness on oil uptake of deep-fat fried food. Journal of Food Engineering, 101: Pedreschi, F.; Aguilera, J.M.; Pyle, D.L. (2001). Textural Characterization and Kinetics of Potato Strips During Frying. Journal of Food Science, 66: Rubnov, M.; Saguy, I.S. (1997). Fractal Analysis and Crust Water Diffusivity of a Restructured Potato Product During Deep-Fat Frying. Journal of Food Science, 62: Shyu, S.; Hau, L.; Hwang, L. (1998). Effect of Vacuum Frying on the Oxidative Stability of Oils. Journal of the American Oils Chemical Society, 75: