Effects of different pretreatment methods on drying kinetics, three-dimensional deformation, quality characteristics and microstructure of dried apple slices

Apple (Malus domestica Borkh.), fruit of the domesticated tree Malus domestica (family Rosaceae), is one of the world's most popular fresh fruits (Mditshwa, Fawole, & Opara, 2018). Apples are rich in nutrients such as carbohydrates, vitamins and minerals, and they have physiological functions such as lowering blood pressure, lowering cholesterol and anti-oxidation (Saarenhovi et al., 2017; Sharabiani, Kaveh, Abdi, Szymanek, & Tana, 2021). Apples, like many other fruits and vegetables, have a high moisture content, which can range from 80%–85% (wet basis), leaving them prone to microbial infection and spoilage during storage. Drying, which is a typical method for limiting the spoilage of fruits and vegetables, can effectively reduce moisture content and extend shelf life of apples (Amanor-Atiemoh, Zhou, Wahia, Mustapha, & Zhou, 2020; Kaleta, Gornicki, Winiczenko, & Chojnacka, 2013).

Drying can be done in a variety of ways, including hot air drying, microwave drying, infrared drying and freeze drying, and so on (Baysal, Ozbalta, Gokbulut, Capar, & Gurlek, 2015). Hot air drying is the most commonly used drying method. The browning caused by enzyme and thermal reaction during the drying process has a significant impact on the color and quality of dried products (Aktas, Ulger, Daglioglu, & Hasturk, 2013). Moreover, the drying process is time-consuming and energy-intensive. Therefore, it is critical to understand how to preserve the characteristics of fruits and vegetables while drying and how to reduce the drying time appropriately. Pretreatment is an important way to produce high-quality dried products.

Numerous studies have shown that pretreatment can change not only the initial moisture content of agricultural products, but also the microstructure of the sample, affecting the color and nutritional value of the dried sample (Rani & Tripathy, 2019). Common pretreatment methods in the drying industry include blanching, ultrasound, ultra-high pressure, osmotic dehydration, etc. (Bi et al., 2015). Many researches have shown that blanching can significantly shorten the drying time, properly inactivate enzymes to prevent the sample from discoloration during the drying process (Wang et al., 2021; Wang, Min, Mujumdar, Mothibe, & Azam, 2012). It is also reported that over-blanching results in severe cellular disruption, which increases the loss of phytochemicals (Wang et al., 2018). Therefore, the blanching time needs to be effectively controlled to ensure product quality. Ultrasound pretreatment has been shown to shorten the drying time while also having a significant impact on the physical and chemical properties of the sample (Cao et al., 2020; Dehghannya, Kadkhodaei, Heshmati, & Ghanbarzadeh, 2019; Rodriguez et al., 2014). Furthermore, osmotic dehydration before drying can be used to preserve bioactive ingredients and inhibit browning to prevent negative changes during drying process (Castillo-Gironés et al., 2021; Kowalska et al., 2020). In addition, pectinase pretreatment is another effective pretreatment method whose use has a significant impact on the moisture state of agricultural products during drying, shortens the drying time, and affects the microstructure (Sahyoun, Mounir, & Allaf, 2017; Xiao et al., 2020). Most of the researchers have focused on the changes of drying time, physical and chemical properties of the dried samples after pretreatment, while few on the changes of appearance.

For dried samples, there would always be shrinkage, collapse, curl and other phenomena, influencing subsequent processing, storage and transportation, as well as consumer choice and consumption. At the moment, research on the deformation of dried samples focusses solely on the change of total volume, which is basically detected using vernier caliper and displacement methods (Oikonomopoulou, Krokida, & Karathanos, 2011). Some researchers used machine vision technology to determine the volume-based shrinkage changes of materials during drying (Chen & Martynenko, 2013; Sampson, Chang, Vasantha Rupasinghe, & Zaman, 2014), but most of them were based on two-dimensional(2D) image. However, sample deformation occurs in 3D space, and it is difficult for ordinary 2D images to accurately reflect this 3D deformation. In an attempt to resolve the challenges associated with 3D image deformation studies, Cai, Lu, Bai, Sun, and Xiao (2019) used Kinect sensor to obtain the depth images of potato slices. Despite the usefulness of the Kinect sensor, its application for the acquisition of 3D information on samples and law analysis of drying process is somewhat limited due to the sensor's low detection accuracy (Wasenmüller & Stricker, 2017). Besides, there are few information about the effect of pretreatment methods on the 3D deformation of agricultural products during drying.

The purpose of this research was to evaluate how blanching, ultrasound, pectinase treatment and osmotic dehydration affected the internal and external quality characteristics of dried apple slices, including 3D deformation, color, polyphenols and antioxidant properties. Furthermore, the drying characteristics and microstructure of apple slices were compared under different pretreatments, and the external quality of dried apple slices was explained from the 3D deformation direction, providing a reference for the quality evaluation of other agricultural products during drying.

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