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Effect of Selected Drying Methods and Emerging Drying Intensification

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Effect of Selected Drying Methods and Emerging Drying Intensification processes Review Effect of Selected Drying Methods and Emerging Drying Intensification Technologies on the Quality of Dried Fruit: A Review Milivoj Radojˇcin 1,*, Ivan Pavkov 1 , Danijela Bursa´cKovaˇcevi´c 2 , Predrag Putnik 3 , Artur Wiktor 4 , Zoran Stamenkovi´c 1 , Krstan Kešelj 1 and Attila Gere 5 1 Faculty of Agriculture, University of Novi Sad, Trg Dositeja Obradovi´ca8, 21000 Novi Sad, Serbia; [email protected] (I.P.); [email protected] (Z.S.); [email protected] (K.K.) 2 Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia; [email protected] 3 Department of Food Technology, University North, Trg dr. Žarka Dolinara 1, 48000 Koprivnica, Croatia; [email protected] 4 Department of Food Engineering and Process Management, Faculty of Food Sciences, Warsaw University of Life Sciences, Nowoursynowska 159c, 02-776 Warsaw, Poland; [email protected] 5 Institute of Food Technology, Szent István University, Villányi Str. 29-31, H-1118 Budapest, Hungary; [email protected] * Correspondence: [email protected] Abstract: Drying is one of the oldest methods for food preservation that removes the water from fruit and makes it available for consumption throughout the year. Dried fruits can be produced by small- and large-scale processors, which makes them a very popular food among consumers and food manufacturers. The most frequent uses of drying technology include osmotic dehydration, vacuum drying, freeze-drying and different combinations of other drying technologies. However, drying may provoke undesirable changes with respect to physiochemical, sensory, nutritional and Citation: Radojˇcin,M.; Pavkov, I.; microbiological quality. Drying process energy efficiency and the quality of dried fruits are crucial Bursa´cKovaˇcevi´c,D.; Putnik, P.; factors in fruit drying. Recently, innovative technologies such as ultrasound, pulsed electric field and Wiktor, A.; Stamenkovi´c,Z.; Kešelj, K.; Gere, A. Effect of Selected Drying high pressure may be used as a pretreatment or in combination with traditional drying technologies Methods and Emerging Drying for process intensification. This could result in quality improvements of dried fruits and enhanced Intensification Technologies on the efficiency and capacity of the production process, with a positive impact on environmental and Quality of Dried Fruit: A Review. economic benefits. Processes 2021, 9, 132. https://doi. org/10.3390/pr9010132 Keywords: processing; drying; fruits; pretreatments; quality; emerging technologies Received: 17 December 2020 Accepted: 6 January 2021 Published: 9 January 2021 1. Introduction The term drying usually refers to the operation by which the moisture present in a Publisher’s Note: MDPI stays neu- material evaporates because of heat and matter exchange between the product and the tral with regard to jurisdictional clai- ms in published maps and institutio- working medium. Fresh fruits have high moisture contents as they are classified as highly nal affiliations. perishable commodities; therefore, storage at refrigerated temperatures and controlled humid conditions is required [1]. Fruits are rich sources of nutrients, including vitamins, minerals, dietary fibers, phenolics, carotenoids, etc., that are useful for human health. Drying is an alternative method for the preservation of the nutritional value of fruits, which Copyright: © 2021 by the authors. Li- increases their relative concentration, extends their shelf life, and minimizes packaging, censee MDPI, Basel, Switzerland. handling and transportation costs [2]. In addition, drying is an alternative to expensive This article is an open access article postharvest management and selling surpluses of fruits on the market. The drying of fruits distributed under the terms and con- by conventional methods, such as sun drying or open-air drying, can degrade quality ditions of the Creative Commons At- and food safety. Numerous disadvantages of these technologies led to the development tribution (CC BY) license (https:// of new technologies, such as oven drying, microwaving, vacuuming, as well as infrared, creativecommons.org/licenses/by/ freeze and different hybrid drying, which are being used successfully for different kinds of 4.0/). Processes 2021, 9, 132. https://doi.org/10.3390/pr9010132 https://www.mdpi.com/journal/processes Processes 2021, 9, 132 2 of 21 fruits [3–5]. Each drying technique depends on various factors, such as the required type of product, size, level of ripeness, structure, color, aroma, chemical composition, nutritional composition, together with expected final quality, availability of a dryer and costs. Sette et al. [6] investigated the application of wet and dry infusion as a pretreatment to air and freeze-drying on the physical properties of raspberries. Freeze-dried pretreated samples exhibited higher firmness and lower deformability as compared to air-dried ones. Moreover, the highest volume reduction was developed after air-drying, while freeze-dried samples showed 11% shrinkage. Color is a very sensitive parameter in terms of the influence of drying methods. Krokida et al. [7] stated that color parameters L*, a* and b* of dried banana, apple, potato and carrot were significantly affected by convective, vacuum and microwave drying techniques. On the other hand, the same samples preserved their original color after freeze and osmotic drying. Fruits are commonly subjected to various chemical and/or physical pretreatments prior to thermal drying to shorten the drying time, reduce the energy consumption and preserve the quality of products. By modifying the properties of fruit tissue, pretreatments could increase the drying rate, inhibit the bio-enzymes, and minimize possible deterioration reactions during drying and subsequent storage [8]. Therefore, each product needs to be dried by using appropriate pre- and post-processing steps, such as osmotic dehydration, blanching, soaking, or by the use of innovative approaches, e.g., ultrasound (US), pulsed electric field (PEF), high hydrostatic pressure (HHP), cold plasma (CP) or other treatments to add satisfactory value after drying [9,10]. This review presents the effects of different drying technologies and following treatments and/or pretreatments on dried fruits by providing the most important quality aspects. 2. Drying of Fruits There are various studies on the topic of fruit drying where researchers have discussed the drying of fruit under different methods. The most commonly reported method is convective drying [11]. The other methods used for fruit drying are osmotic and osmo- convective [12–15], vacuum [16,17], solar drying [18,19], microwave [20,21] and freeze- drying [22–24]. 2.1. Convectional Hot Air Drying Dehydration/drying is still the most commonly used method due to economic benefits. Widely used equipment components are chambers, belts or tunnel dryers. Having a closed atmosphere with regulated airflow and temperature makes this method more advantageous than solar drying. In addition, convective drying is a quite effective and simple method; nevertheless, it is energetically inefficient. Kalra and Bhardwaj [25] found that convective drying in comparison to solar drying is faster and more efficient for mangos, papayas and apricots. On the contrary, the disadvantages of the convective drying method and heating can cause progressive physical, mechanical, chemical and nutritional changes in products. In particular, Stamenkovic et al. [26] revealed losses of 32–40% for total phenols, 3–25% for flavonoids and 44–60% for anthocyanins in dried raspberries as compared to controls (fresh samples). Moreover, in comparison to fresh samples, L-ascorbic acid content was significantly reduced during convective drying (0.94–97.93%), whereas the losses observed by freeze-drying were around 2.36%. The main drawbacks also include the length of the drying during the last stage and the slow heating of the material. Cavusoglu [27] studied the effects of high air temperatures on the drying kinetics and quality of tomato. It was reported that the treatment of raw tomatoes with air temperatures 150 ◦C, 130 ◦C and 100 ◦C within short intervals could reduce drying time without degradation of product quality. To remediate this, hot air drying can be assisted with other methods (e.g., microwave, osmotic or infrared) or different pretreatments (e.g., US, PEF or CP), which can yield better quality of product. Processes 2021, 9, 132 3 of 21 2.2. Osmotic Drying Osmotic drying is used to partially remove water from biological tissues by immersion in a highly concentrated osmotic solution. The driving force for the transport of moisture from the tissues into the solution is provided by the higher osmotic pressure of the highly concentrated solution. Moisture diffusion is accompanied by simultaneous diffusion of the dissolved substance from the osmotic solution into the tissue. Since the cell membrane responsible for the transport of matter is not absolutely selective, other solutions that are present in the cells can also reach the osmotic solution [28]. Osmotic drying is most often applied as a pretreatment to another process to reduce the moisture content of a product, improve its quality during storage, and reduce the total amount of energy for other subsequent processes to osmotic drying. Due to the non-selective nature of a cell membrane, components such as sugar, acids, minerals, and vitamins can be diffused in miniscule quantities from the plant material
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