Comparison between atmospheric and vacuum frying of apple slices

Abstract

Vacuum deep-fat frying is a new technology that can be used to improve quality attributes of fried food because of the low temperatures employed and minimal exposure to oxygen. In this paper atmospheric and vacuum frying of apple slices were compared, in terms of oil uptake, moisture loss and color development. In addition, some apple slices were pre-dried (up to 64% w.b.) before vacuum frying to determine the overall effect. To carry out appropriate comparisons between both technologies equivalent thermal driving forces were used in both processes (ΔT = 40, 50, 60 °C), keeping a constant difference between the oil temperature and the boiling point of water at the working pressure. Vacuum frying was shown to be a promising technique that can be used to reduce oil content in fried apple slices while preserving the color of the product. Particularly, drying prior to vacuum frying was shown to give the best results. For instance, when using a driving force of ΔT = 60 °C, pre-dried vacuum fried slices absorbed less than 50% of the oil absorbed by atmospheric fried ones. Interestingly, a strong relationship between water loss and oil content was observed in both technologies, allowing the extension of observations that have been made for atmospheric frying.

Introduction

Deep-fat frying is one of the oldest and most common unit operations used in the preparation of foods, since it results in products with a unique flavor-texture combination (Varela, 1988). However, numerous studies have revealed that excess consumption of fat, a main component in deep-fat fried food, is a key dietary contributor to coronary heart disease and perhaps cancer of the breast, colon, and prostate (Browner, Westenhouse, & Tice, 1991). Despite recent consumer trends towards healthier food, consumption of oils and fats is still high. For instance, in the United States consumers eat four or more snacks a day and consume more than 6.5 billion pounds of snack food annually. As such, salty snacks account for slightly over half of total snack sales and are consequently a large part of the American diet (Mintel, 2006).

Key growth categories are those that offer a wide range of product alternatives, adhering to convenience, sensory and health trends. In terms of health, interest in salty snack products that are organic or all natural, low-calorie, low-fat, low-carbohydrate, low-sodium or offer some health-promoting benefit, are in greater demand by consumers. Although consumers are interested in healthier snack products, they are not willing to sacrifice flavor. Intense and full-flavor snacks remain an important trend in the salty snack market.

Deep-fat frying is defined as a process where a food is cooked by immersing it in an edible oil or fat heated above the boiling point of water. The process is traditionally carried out under atmospheric conditions and the frying temperature is usually near to 180 °C (Dobraszczyk, Ainsworth, Ibanoglu, & Bouchon, 2006). It is a complex unit operation involving high temperatures, significant microstructural changes both to the surface and the body of the chip, and simultaneous heat and mass transfer, resulting in flows in opposite directions of water vapor (bubbles) and oil at the surface of the piece (Bouchon, Hollins, Person, Pyle, & Tobin, 2001). In fact, numerous studies have shown that oil uptake during deep-fat frying is confined to the surface region of the fried product and restricted to a depth of a few cells (Farkas et al., 1992, Keller et al., 1986, Saguy et al., 1997) and that oil uptake is essentially a surface-related phenomenon resulting from the competition between drainage and suction into the porous crust once the fried potato is removed from the oil and begins to cool (Bouchon et al., 2003, Gamble et al., 1987, Moreira et al., 1997, Ufheil and Escher, 1996).

Based on the previous mechanisms many oil reduction techniques have been proposed and developed. Post-frying treatments such as hot air drying (Nonaka, Sayre, & Weaver, 1977) and super-heated steam drying (Kochhar, 1999) are two processes which have shown to reduce significantly oil absorption after frying. Also different pre-frying treatments have shown to be effective in reducing the absorption. Lowering the moisture content of the food prior to frying using microwave, hot-air treatment and baking has resulted in a significant reduction in oil content in different products (Gamble et al., 1987, Krokida et al., 2001, Lisinska and Leszczynski, 1991, Moyano et al., 2002). In addition, much attention has been given to the use of hydrocolloids such as methylcellulose, hydroxypropyl methylcellulose, long fibre cellulose and corn zein, to reduce surface permeability and inhibit oil uptake (Bouchon & Pyle, 2004). In terms of quality improvement, besides oil uptake reduction, the blanching step prior to frying has shown to improve color and texture (Fan et al., 2005, Lisinska and Leszczynski, 1991, Pedreschi and Moyano, 2005, Shyu and Hwang, 2001).

Vacuum frying is a new technology that might be an option for the production of novel snacks, such as fruits and vegetables, with lower oil content and desired quality attributes. It is defined as the frying process carried out under pressures well below atmospheric levels, therefore lowering the boiling point of water, making possible to reduce substantially the frying temperature (Garayo & Moreira, 2002). In the literature, only few works are found in relation to this topic since research in this field is initiating. At present, vacuum frying has shown to have some advantages that include: (1) reduction of the oil content in the fried product, (2) preservation of natural color and flavors and (3) reduction of adverse effects on oil quality (Garayo and Moreira, 2002, Shyu et al., 2005). Granda, Moreira, and Tichy (2004) have concluded that vacuum frying can also diminish acrylamide (an animal carcinogen compound) content in potato crisps. Overall, benefits are mainly derived from the low temperatures employed and minimal exposure to oxygen. In terms of the raw materials under study, most researches have been oriented towards potatoes and only few works based in other foods are found (Shyu and Hwang, 2001, Shyu et al., 2005).

In relation to the effect of frying conditions, Garayo and Moreira (2002) observed that oil absorption in potato crisps was related to the moisture lost during the process and that color was not significantly affected by the oil temperature (118, 132 and 144 °C) and vacuum pressure used (16.661, 9.888 and 3.115 kPa). On the other hand, Shyu and Hwang (2001), when studying the effect of processing conditions on the quality of vacuum fried apple chips using a single vacuum pressure condition (3.115 kPa), and three different oil temperatures (90, 100, 110 °C), concluded that oil content increased for increasing frying times and temperatures.

In order to compare atmospheric and vacuum frying it would be interesting to study the effect of equivalent driving forces in both processes on the main quality attributes of the product (e.g. oil content and color development). This could be accomplished by maintaining the same thermal driving force, that is, by keeping a constant difference between the oil temperature and the boiling point of water at the working pressure (atmospheric or vacuum pressure). In addition, vacuum deep-fat frying could be combined with other pre-treatments such as drying, to determine if there could be any synergism. In accordance, the main objective of this paper is to compare atmospheric, vacuum and pre-drying vacuum frying of apple slices, when using equivalent thermal driving forces, on total oil intake, moisture loss and color development, to understand the differences between both technologies and how they determine main quality attributes.