Influence of power ultrasound application on drying kinetics of apple and its antioxidant and microstructural properties
Introduction
Apples constitute a major part of fruit production and there is a growing tendency to its consumption in the world, in the form of fresh fruit, juice or dried products including snack preparations, integral breakfast foods and other varieties (Biedrzycka and Amarowicz, 2008). Apples also ranked the second for total concentration of phenolic compounds, and perhaps more importantly, apples had the highest portion of free phenolics when compared to other fruits (Boyer and Liu, 2004), being the Granny Smith variety, one of the polyphenol-richest apple cultivars (66.2–211.9 mg/100 g fresh weight) with flavonoids (catechin and proanthocyanidins) as the major class of apple polyphenols (Vrhovsek et al., 2004, Biedrzycka and Amarowicz, 2008). Nowadays, consumers demand for high quality products which keep the fresh-like characteristics of flavor, texture, color but more important the nutritional content, along with an equitable or extended shelf life. Unfortunately, apple processing, either juice obtaining or drying leads to a significant loss of the phenolic content and also its antioxidant activity (Van der Sluis et al., 2002).
Recently, an increase of the concerning about the health’s benefits of apple consumption has encouraged the research of the effects of processing on product attributes in order to minimize the quality degradation. Several processing methods have been studied in the recent years in order to assess its influence on the quality loss of the final product. With this aim, microwave heating (Picouet et al., 2009), high pressure (Landl et al., 2010), cold-break (Le Bourvellec et al., 2011), straight pressing (Van der Sluis et al., 2002), among others, have been used for apple juice or puree obtaining. Apple drying has been widely addressed in literature. Convective drying is the most frequently used dehydration operation in food and chemical industry, is used to assure the food stability, minimizing the microbiological and physicochemical activity by means of the reduction of water activity (Krokida et al., 2003); however, drying causes changes in the nutritional value, physical properties and microstructure of fruits and vegetables and their products (Chen et al., 2011). Heras-Ramírez et al., 2012, Vega-Gálvez et al., 2012, among others, have studied the effects of drying conditions, such as temperature, air velocity and drying time, on the degradation of thermolabile compounds, such as polyphenols and flavonoids, and its antioxidant activity in apples. Other authors have reported those changes in different vegetables: orange peel (Garau et al., 2007), quinoa seeds (Miranda et al., 2010), carrots (Eim et al., 2013), tomatoes (Mechlouch et al., 2012), and garlic (Calín-Sánchez et al., 2013).
Not only the nutritional quality is affected by the drying process, but also the microstructure is modified by water removal. The microstructural analysis of foodstuffs has been widely described in literature by Ramírez et al., 2011, Vega-Gálvez et al., 2012, Eim et al., 2012 among others, as an effort to relate microstructural changes with macroscopic alterations during the drying process.
The ultrasound application may overcome some of the limitations of convective drying by increasing the drying rate at lower temperatures and so, the mass transfer phenomena (García-Pérez et al., 2007). The mechanical energy provided by the application of power ultrasound contributes to the reduction of both the internal and external resistances to the mass transfer, being the water transfer mainly improved by alternating expansion and compression cycles (sponge effect). Besides, high-intensity airborne ultrasound causes microstreamings at the interfaces that reduce the diffusion boundary layer, increase mass transfer, and accelerate diffusion (Gallego-Juárez et al., 2007). García-Pérez et al. (2009) reported this effect when different acoustic power densities (0–37 kW/m3) were tested during a convective drying (40 °C, 1 m/s) of carrot cubes and lemon peel slabs. Besides, there is a limited heating effect of ultrasound on gas systems, which is relevant considering the preservation of thermolabile compounds during drying.
Therefore, the main objective of this study is to evaluate the influence of ultrasound application on the convective drying of apple at different temperatures. For this purpose, the drying curves have been studied by using a diffusion model and the total polyphenol and flavonoid contents, the antioxidant activity losses, and the microstructural changes due to the ultrasound application have been evaluated.
Section snippets
Sample preparation
The Granny Smith apple used in this study was purchased in a local market. Fruits were washed, peeled, cored and cut into cubes (0.01 m edge). The initial average moisture (W0) obtained by using the AOAC method No. 934.06 (AOAC, 2006) was of 5.64 ± 0.30 kg water/kg d.m. After cutting, cubes were processed immediately.
Drying experiments
Drying experiments were carried out in a convective drier assisted by power ultrasound, which has already been described in a previous work by Cárcel et al. (2011). The equipment
Drying kinetics
Drying kinetics were studied until a final moisture content of 0.33 ± 0.02 kg water/kg d.m. Fig. 2 shows the experimental drying curves (dots) for the different drying air temperatures (30 °C, 50 °C and 70 °C) using the conventional hot air drying method (AIR), and the ones assisted by power ultrasound (AIR + US1 and AIR + US2). It could be observed in this figure that for a drying temperature of 30 °C (Fig. 2A) the drying time for AIR samples was ca. 9.92 h, being the effect of the US application evident,
Conclusions
In the convective drying of apples assisted by power ultrasound, the drying rate was affected by both the drying temperature and the ultrasonic power applied. The ultrasound application induced an increase of both the effective diffusion and the mass transfer coefficients, although, this increment was more notorious at low temperatures than higher ones. The proposed diffusional model taking into account the influence of drying temperature and ultrasonic power allowed a satisfactory prediction
Acknowledgments
The authors want to acknowledge the Spanish Government (MICINN), and European Regional Development Fund (FEDER), the European Social Fund (FSE), the Generalitat Valenciana and the Govern de les Illes Balears for the financial support (DPI2009-14549-C04-02, DPI2012-37466-C03-02, DPI2012-37466-C03-03, AGL 2012-34627, PROMETEO/2010/062, Project 57/2011).
References (39)
- et al.
Choosing an appropriate drying model for intermittent and continuous drying of bananas
J. Food Eng.
(2007) - et al.
Production of antioxidant high dietary fiber powder from carrot peels
LWT – Food Sci. Technol.
(2008) - et al.
Drying kinetics and quality changes during drying of red pepper
LWT – Food Sci. Technol.
(2008) - et al.
Effect of air-drying temperature on physico-chemical properties of dietary fibre and antioxidant capacity of orange (Citrus aurantium v. Canoneta) by-products
Food Chem.
(2007) - et al.
Power ultrasound mass transfer enhancement on food drying
Food Bioprod. Process
(2007) - et al.
Drying kinetics of some vegetables
J. Food Eng.
(2003) - et al.
Effect of high pressure processing on the quality of acidified Granny Smith apple purée product
Innovative Food Sci. Emer. Technol.
(2010) - et al.
Phenolic and polysaccharidic composition of applesauce is close to that of apple flesh
J. Food Compos. Anal.
(2011) - et al.
Modelling shrinkage during convective drying of food materials: a review
J. Food Eng.
(2004) - et al.
Impact of air-drying temperature on nutritional properties, total phenolic content and antioxidant capacity of quinoa seeds (Chenopodium quinoa Willd.)
Ind. Crops Prod.
(2010)