Modeling water loss and oil uptake during vacuum frying of pre-treated potato slices

Abstract

The objective of this research was to determine the kinetics of water loss and oil uptake during frying of pre-treated potato slices under vacuum and atmospheric pressure. Potato slices (diameter: 30 mm; width: 3 mm) were pre-treated in the following ways: (i) raw potato slices “control”; (ii) control slices were blanched in hot water at 85 °C for 3.5 min; (iii) blanched slices were dried in hot air until reaching a moisture content of ∼0.6 g water/g dry basis. The slices were fried under vacuum (5.37 kPa, absolute pressure, at 120, 130 and 140 °C) and atmospheric conditions (at 180 °C). Two models based on the Fick's law were used to describe water loss: (i) with a constant effective diffusive coefficient; and (ii) with a variable effective diffusive coefficient. Oil uptake data were fitted to an empirical model, with a linear behavior for short times whereas the model was time independent for long times. The variable diffusivity model better fitted experimental water loss, giving values of effective diffusivity between 4.73 × 10⁻⁹ and 1.80 × 10⁻⁸ m²/s. The proposed model for the study of the kinetics of oil uptake fitted the experimental data properly. Control and blanched vacuum fried potato chips increased their final oil contents to 57.1% and 75.4% respectively, when compared with those fried at atmospheric pressure. However, the oil absorption of dried vacuum fried potato chips diminished by ∼30%.

Introduction

Since the demand of healthier foodstuffs with high quality attributes by the consumer is a major issue in our society, one of the principal research fields in food engineering is the developing of alternative mild cooking methods. Generally, fried potato producers control the quality of their products through the appearance and flavor, attributes which are highly linked to some physical properties of the products. Specifically, French fry and potato chip industries take as main quality parameters texture, color, and oil content. Vacuum frying is an excellent alternative to conventional frying, since when frying below atmospheric pressure significant advantages such as healthier and high quality products are reached (Granda, Moreira, & Tichy, 2004). Due to the lowering of pressure, the boiling points of both the oil and the moisture in the foods are lowered, which can reduce oil content in the fried product, can preserve natural color and flavors of the product due to the low temperature and oxygen content during the process, and has less adverse effects on oil quality (Fan et al., 2005a, Fan et al., 2005b, Garayo and Moreira, 2002; Shyu, Haw, & Hwang, 2005).

Several factors have been reported that strongly affect oil absorption in potatoes fried at atmospheric pressure. Some of the most important are oil quality and composition, product shape, temperature and frying time, moisture content, initial porosity, and pre- and post-treatments (Bouchon and Aguilera, 2001, Saguy and Pinthus, 1995). Gamble, Rice, and Selman (1987) determined that the major oil fraction is suctioned by the microstructure of the potato piece when this is removed from the fryer during the cooling period, suggesting that the oil absorption is strictly linked to moisture loss. Oil absorption is mainly a surface phenomena and most of the oil is absorbed by the fried product in the post-frying period (Ufheil and Escher, 1996, Durán et al., 2007). However, oil absorption during vacuum frying follows transport mechanisms more complexly than those elucidated in conventional frying and is currently the subject of intensive study. Garayo and Moreira (2002) established the kinetics of oil absorption in potato slices fried under different vacuum conditions (different pressures and temperatures), reporting that oil uptake in final potato chips (moisture content of ∼1.9 g water/100 g wet basis) was affected significantly by the vacuum level and the frying temperature. Oil uptake increased with frying time and when the slices were fried at atmospheric pressure. Furthermore Fan et al. (2005a) reported that the rate of fat absorption of vacuum fried carrot slices increased with increasing frying oil temperature and vacuum. These results are in agreement with those found by Shyu et al. (2005) in carrot slices which have undergone different pre-treatments before being fried at only one vacuum condition. However, Tan and Mittal (2006) found that donuts fried in vacuum conditions absorbed considerably more oil (14.5–35.8 g oil/100 g dry basis; vacuum levels of 3, 6 and 9 kPa) than the corresponding donuts fried at atmospheric conditions (13.1 g oil/100 g dry basis).

There is little information on modeling, both empirical and phenomenological, for moisture loss and oil uptake during vacuum frying. However, there are several reports proposing different models to quantify changes in physical properties in some foodstuffs during conventional frying (at atmospheric pressure) which could be used as a first approximation to predict the kinetics of change of those physical properties during vacuum frying. It is worth mentioning that vacuum frying could generate an important hydrodynamic gradient which could affect significantly the product microstructure and consequently, its physicochemical and transport properties.

Several researchers have applied Fick's law of diffusion to model the water loss in fried products with successful agreement between experimental data and calculated values (Moreira et al., 1999, Moyano and Berna, 2002, Pedreschi et al., 2005). The applicability of this model in water diffusion in solid materials defines an effective coefficient which is a global transport property that includes all the participating water transport mechanisms. However, additionally to diffusion, water could be transported through hydrodynamic gradients and capillary flow according to the material structure (Saravacos & Maroulis, 2001) and, probably, these factors have not been completely represented by an effective coefficient of diffusion (Ni and Datta, 1999, Roca et al., 2008). For these reasons and since the diffusional theory does not consider some important structural aspects related to the product dehydration such as shrinkage (Barbosa-Cánovas and Vega-Mercado, 1996), the effective coefficient of diffusion for water could be assumed as a general property of mass transport where all the contributions of the mechanisms of water transport are included (McMinn & Magee, 1999). However, in order to avoid oversimplifying this process, it could be considered as the use of a variable coefficient of diffusion which considers the changes in material physical properties during frying (Moyano & Berna, 2002).

On the other hand, several models have been postulated to predict oil absorption during frying. Ni and Datta (1999) proposed a multiphase model in a porous media to study oil uptake kinetics in fried potato slices. Additionally, Krokida, Oreopoulou, and Maroulis (2000) used first order kinetics to model oil content in French fries where the velocity constant relied on the major process variables such as oil temperature, product width and oil type.

The aim of this study was to establish the kinetics of water loss and oil uptake during frying of pre-treated potato slices at vacuum and atmospheric pressure. Two models based on Fick's law of diffusion were used to describe water loss and an empirical model was used to describe oil uptake during frying.