Accepted Manuscript Refractance Window drying of pomegranate juice: Quality retention and energy efficiency Vahid Baeghbali, Mehrdad Niakousari, Asgar Farahnaky PII:
S0023-6438(15)30235-8
DOI:
10.1016/j.lwt.2015.10.017
Reference:
YFSTL 5012
To appear in:
LWT - Food Science and Technology
Received Date: 1 March 2015 Revised Date:
28 September 2015
Accepted Date: 5 October 2015
Please cite this article as: Baeghbali, V., Niakousari, M., Farahnaky, A., Refractance Window drying of pomegranate juice: Quality retention and energy efficiency, LWT - Food Science and Technology (2015), doi: 10.1016/j.lwt.2015.10.017. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Refractance Window drying of pomegranate juice: Quality Retention and Energy
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Efficiency
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Vahid Baeghbalia, Mehrdad Niakousaria,b,*, Asgar Farahnakya,c
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a
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Iran.
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b
Advanced Technologies Research Institute, Shiraz University, Shiraz, Iran
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c
School of Biomedical Sciences and Graham Centre for Agricultural Innovation, Charles Sturt
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University, Wagga Wagga, New South Wales 2678, Australia
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Department of Food Science and Technology, Faculty of Agriculture, Shiraz University. Shiraz,
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*
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Agriculture, Shiraz University. Shiraz, Iran. Tel.: +987136138246; fax: +987132286110.
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Email:
[email protected]
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Corresponding author. Address: Department of Food Science and Technology, Faculty of
Email addresses:
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[email protected] (V. Baeghbali)
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[email protected] (M. Niakousari)
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[email protected],
[email protected] (A. Farahnaky)
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Abstract
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Refractance Window (RW)1 drying system utilizes circulating hot water as a means to carry
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thermal energy to materials to be dehydrated. Products are spread on a transparent plastic
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conveyer belt that moves over circulating water in a shallow trough. In this study the quality
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retention characteristics of pomegranate juice (PJ) concentrate dried in a continuous pilot scale
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RW2 drying system were evaluated against freeze drying and spray drying methods. Samples of
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PJ3 concentrate (Brix 64) mixed with Gum Arabic as a carrier (35%, dry basis) were dried by the
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RW drying, freeze drying and spray drying methods. Physicochemical properties including
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moisture content, solubility, bulk density and color parameters of samples dried using different
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methods were compared. Chemical analysis and color measurements of reconstituted samples
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showed that RW dryer can produce high-quality products with anthocyanins content,
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anthocyanins color and antioxidant activity equal or greater than those of the freeze dried and
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spray dried samples. Energy consumption of the RW dryer was about one third and 1/40 of
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those of spray drying and freeze drying systems, respectively.
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Keywords: Refractance Window drying; Pomegranate juice; Anthocyanins; Antioxidant
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activity; Energy efficiency.
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1
RWTM is a trademark used exclusively for Refractance Window® drying and evaporation. Their mention in this paper is solely for correctness and does not imply endorsement of the technology over other systems performing similar function. Unless otherwise stated, RW will refer to Refractance Window® or RWTM. 2 Refractance Window 3 Pomegranate Juice
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1. Introduction
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Refractance Window™ (RW) as a relatively new film drying technique is characterized by short
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time exposure of foods to relatively low temperatures (Magoon, 1986; Nindo, Feng, Shen, Tang,
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& Kang, 2003). For drying a similar amount of a product, the cost of the RW equipment is
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estimated about one-third of the cost of a freeze-dryer, whereas the energy consumption of a RW
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is less than half of a freeze-dryer (Nindo & Tang, 2007). RW drying system utilizes circulating
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water at 95 to 97 °C as a means to carry thermal energy to materials to be dehydrated. Pureed
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products are spread on a transparent Mylar plastic conveyer belt that moves over circulating
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water in a shallow trough and the unused thermal energy in the circulating water is recycled (Fig.
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1). The actual product temperature is usually between 70°C and 80°C (Abonyi, Feng et al.
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2002). Previous studies on RW technology have shown a high retention of product quality
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(color, vitamins and antioxidants) as compared to other conventional drying methods including
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freeze drying (Abonyi, Tang, & Edwards, 1999; Abonyi, Feng et al. 2002; Nindo & Tang, 2007).
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A study on the effect of RW drying, freeze drying, hot-air oven drying and natural convective
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drying on color characteristics of paprika showed that the freeze dried and RW dried paprika had
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better reflected color characteristics and there was no significant difference in browning index
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between freeze dried and RW dried samples (Topuz, Feng, & Kushad, 2009); however further
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studies on the influence of drying methods on carotenoids and capsaicinoids of paprika showed
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that natural convective drying method, due to an ongoing synthesis, resulted in higher
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carotenoids and capsaicinoids contents than those of other methods (Topuz, Dincer, Özdemir,
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Feng, & Kushad, 2011). Another study on drying of tomato juice using a batch Refractance
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window dryer and a laboratory freeze dryer, showed no significant difference between ascorbic
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acid content and color parameters of RW and freeze dried samples (Baeghbali, Niakosari, &
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Kiani, 2010). In a recent study haskap berry puree was dried using an RW dryer in industrial
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scale and the anthocyanin content of the product was determined using pH-differential method
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and HPLC analysis. The results showed that the RW dried haskap berries retained more than
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90% of anthocyanins (Celli, Khattab, Ghanem, & Brooks, 2016).
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The pomegranate (Punica granatum, Punicaceae) is a native seasonal fruit of Iran; however, its
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high nutritional value and appealing taste make it desirable to have a pomegranate product
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available throughout the year and drying is regarded as a suitable tool to achieve this goal
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(Yousefi, Emam-Djomeh, & Mousavi, 2011). Pomegranate juice (PJ) concentrate is currently
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being produced in industrial scale and used for production of various fruit juices, nectars, drinks
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and sauces. Dried PJ has better storability and a longer shelf life than PJ concentrate. Studies
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have shown the health benefits of phytochemicals in PJ, primarily polyphenols including
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anthocyanin pigments, flavonol glycosides, procyanidins, phenolic acids and ellagic acid
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derivatives (Negi & Jayaprakasha, 2003). Red color is the most important quality criteria for
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fruit juices containing anthocyanin but unfortunately, anthocyanins are susceptible to
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degradation (Somers & Evans, 1986). Various factors affect the stability of anthocyanins,
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including the temperature of processing, the chemical nature of anthocyanins, pH, ascorbic acid,
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hydrogen peroxide, sugars, light and metals (Turfan, Türkyılmaz, Yemis, & Özkan, 2011). The
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antioxidant capacity of commercial PJ is three times higher than those of red wine and green tea
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(Gil, Tomas-Barberan, Hess-Pierce, Holcroft, & Kader, 2000).
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The present study aims to investigate the potential to produce PJ powder using a pilot scale
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continuous RW dryer, a batch freeze dryer and a pilot scale spray dryer and to compare the
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powders obtained from the three methods in terms of physiochemical characteristics of the
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powders, including moisture content, solubility, yield, bulk density, color, anthocyanin color,
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total anthocyanins content, antioxidant activity and total phenolic compounds. The three drying
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systems were also analyzed for specific energy consumption and energy efficiency.
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2. Materials and methods
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PJ concentrate (°Brix =64) was purchased from a commercial supplier (Green Farm, Neyriz,
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Iran); it was produced in industrial scale by evaporation of clarified fresh pomegranate juice. PJ
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concentrate was dried by a pilot scale continuous Refractance Window dryer, a batch freeze drier
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(Dena Vacuum Industry, Teheran, Iran) and a pilot scale spray drying unit (Maham Sanat,
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Neyshabur, Iran). In preliminary tests, it was established that the PJ concentrate could not be
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dried to a powder without supplementing the feed with certain amount of a drying aid
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(antiplasticizing agent); due to its high sugar and organic acid contents. Thus, the feed was
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supplemented with gum Arabic prior to drying process. Gum Arabic has high water solubility,
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relatively low viscosity (compared to other gums e.g. xanthan gum) and high stability in acidic
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solutions (Montenegro, Boiero, Valle, & Borsarelli, 2012). Based on the previous studies, the
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minimum amount of gum Arabic needed to yield a reasonable quantity of powder with the
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lowest particle stickiness, was 35% of the total soluble solid of the PJ concentrate (35% gum
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Arabic in the feed dry matter). In all drying methods, the total soluble solid of the feed under
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investigation was adjusted to 50% (oBrix = 50) by adding distilled water. The total soluble solids
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(°Brix) were determined at 25 °C using an Abbe refractometer (CETi, Belgium). All tests were
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conducted in triplicate. The experimental conditions of the drying methods are explained in the
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following sections.
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2.1. Dryers
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2.1.1. Refractance Window drying. A continuous pilot scale RW dryer with an effective length
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of 1.95 m was designed and fabricated in the Department of Food Science and Technology at
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Shiraz University. Because of operation of the exhaust fan, air at 28 °C and 30% relative
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humidity (RH) was forced over the bed at an average air velocity of 0.1 m/s to remove the
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moisture. Air velocity was measured using an AM-4201 anemometer (Lutron, Taipei, Taiwan).
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The hot water temperature was 91 °C and the cooling water temperature was 20 °C. The hot
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water and the cooling water flow rates were 2.5 m3/hr and 1 m3/hr, respectively. The Mylar belt
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speed was 3.9 mm/s. The thickness of the PJ concentrate applied to the polymer belt was set to
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be approximately 0.5 mm, using an adjustable blade. Flow rate of the feed was regulated at 0.5
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liter per hour using a peristaltic pump. Residence time of material on the drying belt was set to
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8.5 min by adjusting the belt speed. The feed (a mixture of PJ concentrate and gum Arabic) was
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dried to 5.3% moisture content (wb). Dried product was removed from the belt, using a doctor
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blade (Fig. 1 and Fig. 2).
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2.1.2. Spray drying. A pilot-scale mixed flow spray dryer (Maham Sanat, Neyshabur, Iran) was
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used in spray drying experiment. Inner diameter of the dryer chamber was 115 cm and its height
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was 165 cm. A two-fluid nozzle was used to atomize the sample pneumatically by high-velocity
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compressed air at 1 bar pressure. Feed rate was 0.75 liter per hour and the feed temperature was
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40 °C. The inlet and outlet air temperatures were 140 ± 1 °C and 75 ± 1 °C, respectively.
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Samples were dried to moisture content of 2.9 % (wb).
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2.1.3. Freeze drying. The PJ concentrate samples were quick-frozen at –80 °C. The freeze
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dryer (Dena Vacuum Industry, Tehran, Iran) was operated at an absolute pressure of 3.0 kPa.
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The heat plate temperature of the freeze dryer was 20 °C and the condenser temperature was –40
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°C. Initial weight of each sample was 100g and the drying time to reduce moisture content to
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8.5% (wb) was 24 h. Some researchers have chosen long periods of time (up to 8 days) for
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freeze drying in order to obtain powders with lower moisture content; e.g., Topaz et al., 2009 and
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2011. In this study freeze drying time was selected to be 24 h in order to have practical energy
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efficiency comparison with pilot scale RW and spray dryers. Freeze drying was included
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because it is frequently used as a high quality drying standard against other drying systems in
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producing dehydrated fruits and vegetables (Nindo & Tang, 2007).
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After drying, the products were vacuum packed in polyethylene bags, sealed using a DZ-400
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vacuum packing machine (Wenzhou Zhonghuan Packaging Machine Co., Ltd, China) and stored
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at 4 °C for further analysis.
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2.2. Moisture content determination
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Moisture content of the dried PJ samples was determined using oven method (103±1°C)
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according to the approved method of Association of Official Analytical Chemists (AOAC,
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2002).
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2.3. Solubility
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Solubility was determined according to Eastman and Moore (1984) with some modifications.
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First, 100 mL of distilled water was transferred into a blender jar. The powder sample (1 g, dry
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basis) was carefully added to the blender operating at 15,000 rpm for 5 min. The solution was
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poured into a tube and centrifuged at 3,000× g for 5 min. An aliquot of 25 ml of the supernatant
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was then transferred to a pre-weighed petri dishes and immediately oven-dried at 105°C for 5 h.
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The solubility (%) was calculated as the weight difference (Cano-Chauca, Stringheta, Ramos, &
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Cal-Vidal, 2005).
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2.4. Yield
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Weight of the dry material of the produced powders and weight of the feeds consumed were used
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to determine the spray dryer and RW dryer yields according to the Equation (1).
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Yield=
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where P is the rate of powder production (g/min), SP is the percent of total solids of the powder,
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L is the feed flow rate (g/min), and SF is the percent of total solids of the feed (Chegini &
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Ghobadian, 2007). The freeze drying was performed in a batch system therefore its yield was
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calculated using the same equation where P is the weight of dried product (g), SP is the
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percentage of total solids of the powder, L is the weight of sample before drying (g), and SF is
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the percentage of total solids of the sample.
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2.5. Bulk density
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To determine the (tapped) bulk density, 20 g of each powder was weighed into a 100 mL
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graduated cylinder then gently dropped 10 times on a rubber mat from a height of 15 cm. The
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bulk density was calculated by dividing the mass of the powder by the volume of the cylinder
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occupied by the powder (Goula, Karapantsios, Achilias, & Adamopoulos, 2008).
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2.6. Pomegranate juice reconstitution
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Dehydrated samples were rehydrated at room temperature with deionized water. The amount of
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water used to reconstitute the PJ powders was calculated based on the moisture content of the
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dried samples to reach a soluble solid content of 20% (g solid/g water) in the mixture.
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According to the Codex general standard for fruit juices and nectars, minimum pomegranate
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P × SP × 100 L × SF
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soluble solids level (Brix) for reconstituted PJ is 12.0% (Codex Standard 247, 2005). In the
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present study pomegranate soluble solids in reconstituted samples were adjusted to 13.0%. Since
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the dried samples contained 35% gum Arabic (in dry matter), a reconstituted sample with 20%
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soluble solids, was consisted of 13% pomegranate soluble solids (and 7% gum Arabic). The
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control sample was prepared by adding deionized water and gum Arabic to PJ concentrate to
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reach a soluble solid content of 20% in the mixture similar to the reconstituted samples.
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2.7. Chemical analysis
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In this study, the influences of different drying methods on pH, titratable acidity, anthocyanin
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color, total anthocyanins content, antioxidant activity and total phenolic compounds of
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reconstituted PJs were investigated.
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The pH was determined using a pH/mV/temperature meter (Keison, UK). Titratable acidity was
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determined according to the AOAC (2002), and expressed as percentage of the citric acid.
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Total anthocyanins content of samples was determined by pH differential method using two
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buffer systems: potassium chloride buffer, pH 1.0 (0.025 M) and sodium acetate buffer, pH 4.5
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(0.4 M) (Cam, Hisil, & Durmaz, 2009). Briefly, 0.4 ml of PJ sample was mixed with 3.6 mL of
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corresponding buffers and read against water as a blank at 510nm (A510) and 700nm (A700). The
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Equation (2) was used to calculate absorbance (A):
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A = (A510-A700)pH1.0-(A510-A700)pH4.5
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Total anthocyanin content (TAC) of samples (mg cyanidin-3-glucoside/100 mL of PJ) was
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evaluated using equation (3):
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TAC= (A×MW×DF×100)/MA
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(2)
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where A: absorbance; MW: molecular weight (449.2); DF: dilution factor (10); MA: molar
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absorptivity of cyanidin-3-glucoside (26,900).
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Anthocyanin color (AC) was determined by spectrophotometry (Alper, Savas, & Acar, 2005)
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using equation (4):
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AC= [(Abs533-a-Abs700)-(Abs533-b-Abs700)]×DF
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where Abs533-a is the reading in a sample without bisulfite application, and Abs533-b is the reading
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in sample with bisulfite application and DF is dilution factor.
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Antioxidant activity was determined according to the method of Cam, Hisil, & Durmaz (2009);
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0.1 mL of samples was mixed with 0.9 mL of 100 mM Tris–HCl buffer (pH 7.4) to which 1 mL
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of DPPH (0.500 µM in ethanol) was added. The control sample was prepared in similar way by
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adding 0.1 mL of water instead of sample. The mixtures were shaken vigorously and left to
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stand for 30 min. Absorbance of the resulting solution was measured at 517 nm by a Unico UV-
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2100 UV–vis spectrophotometer (South Brunswick, USA). The reaction mixture without DPPH
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was used for the background correction. The Equation (5) was applied to assess the antioxidant
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activity:
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Antioxidant activity (%)=[1-(ASample/AControl)]×100
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where ASample is PJ sample absorbance in 517nm and AControl is control sample absorbance at
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517nm (Cam, Hisil, & Durmaz 2009).
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The Folin–Ciocalteu assay was used for the determination of total phenol content in PJs (Vinson,
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Dabbagh, Mamdouh, & Jang, 1995).
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2.8. Color measurement
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(5)
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The color of dried samples and reconstituted PJ samples (L, a, and b) was measured using digital
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imaging and Photoshop software (Adobe Systems Inc., San Jose, California, USA) (Afshari-
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Jouybari & Farahnaky, 2011). Color difference (∆E) was evaluated based on the Equation (6):
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∆E = [(Li-L0)2 + (ai-a0)2 + (bi-b0)2]1/2
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where Li, ai and bi are color measurement values for reconstituted sample and L0, a0 and b0 are
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color measurement values for the control sample.
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2.9. Energy efficiency assessment
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1 kg of PJ concentrate (with 35% gum Arabic in dry matter and Brix=50), was used in three
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replica tests to evaluate the energy consumption of each dryer (Pilot scale RW, spray dryer and
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freeze dryer). Total energy consumption of each test was measured after water temperature of
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the RW dryer reached 91 °C, inlet air temperature of the spray dryer reached 140 ± 1 °C and
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condenser temperature of the freeze dryer reached –40 °C. The measurement was done using a
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digital single phase kWh meter with 0.01 kWh accuracy connected to the main power cable of
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each dryer so the measurements included energy consumption of all parts of the dryer (heaters,
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pumps, fans, etc.). Energy efficiency (EE) of each drying experiment was evaluated by the ratio
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of the energy needed for dehydration of 1 kg PJ sample (QT) to the measured energy
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consumption (QM). Basic energy efficiency calculation was performed using the following
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equations:
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QT=(mc∆T)+(mLv)
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EE=QT/QM
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where m is mass and c is specific heat of the sample; ∆T is the temperature difference; Lv is the
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latent heat of vaporization in RW and spray dryer and the latent heat of sublimation in the freeze
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dryer; QM is the measured energy consumption by digital kWh meter and EE is energy
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efficiency.
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2.10. Statistical analysis
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All experiments were conducted in triplicate and an analysis of variance was performed. The
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least significant difference at p freeze dried > hot air dried. No
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significant difference in powder solubility for powders obtained from RW dryer and spray dryer
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was observed (p