Effect of drying methods on the physical properties and microstructures of mango (Philippine ‘Carabao’ var.) powder

Abstract

Mango powders were obtained at water content below 0.05 kg water/kg dry solids using Refractance Window® (RW) drying, freeze drying (FD), drum drying (DD), and spray drying (SD). The spray-dried powder was produced with the aid of maltodextrin (DE = 10). The chosen drying methods provided wide variations in residence time, from seconds (in SD) to over 30 h (in FD), and in product temperatures, from 20 °C (in FD) to 105 °C (in DD). The colors of RW-dried mango powder and reconstituted mango puree were comparable to the freeze-dried products, but were significantly different from drum-dried (darker), and spray-dried (lighter) counterparts. The bulk densities of drum and RW-dried mango powders were higher than freeze-dried and spray-dried powders. There were no significant differences (P ⩽ 0.05) between RW and freeze-dried powders in terms of solubility and hygroscopicity. The glass transition temperature of RW-, freeze-, drum- and spray-dried mango powders were not significantly different (P ⩽ 0.05). The dried powders exhibited amorphous structures as evidenced by the X-ray diffractograms. The microstructure of RW-dried mango powder was smooth and flaky with uniform thickness. Particles of freeze-dried mango powder were more porous compared to the other three products. Drum-dried material exhibited irregular morphology with sharp edges, while spray-dried mango powder had a spherical shape. The study concludes that RW drying can produce mango powder with quality comparable to that obtained via freeze drying, and better than the drum and spray-dried mango powders.

Introduction

Mango (Mangifera indica L.) is one of the most appreciated fruits in the world. The 2005 world production of mango was estimated at 28.5 million metric tons, of which 85% was produced in the following 10 countries: India (37.9%), China (12.9%), Thailand (6.3%), Mexico (5.9%), Indonesia (5.2%), Pakistan (5.9%), Brazil (3.5%), Philippines (3.5%), Nigeria (2.6%), and Egypt (1.3%) (Evans, 2008). In the Philippines, mango ranks third among fruit crops next to banana and pineapple in terms of export volume and value, with a total of metric tons harvested in 2007. The Carabao variety popularly known as “Philippine Super Mango” accounts for 73% of the country’s production (BAS, 2009). This variety is acclaimed as one of the best in the world due to its sweetness and non-fibrous flesh.

Fresh mangoes are perishable and may deteriorate in a short period of time if improperly handled, resulting in large physical damage and quality loss, ranging from 5% to 87% (Serrano, 2005). Gonzalez-Aguilar et al. (2007) reported that 100% of untreated ripe mango fruits of the ‘Hadin’ variety showed fungal infection and severe decay damage by the end of 18 days of storage at 25 °C. In order to take advantage of the potential health benefits of mango and add value to the commodity with lesser handling and transport costs, there is a need to develop mango products in forms of mango powders that not only have desired functionality but also are stable over a longer storage time. Mango powder offers several advantages over other forms of processed mango products like puree, juice and concentrate. Besides having a much longer shelf life due to considerable reduction in water content, the transport cost is also significantly reduced. Mango powders may also offer the flexibility for innovative formulations and new markets. For example, mango powders can be used as a convenient replacement for juice concentrates or purees, and as shelf-stable ingredients for health drinks, baby foods, sauces, marinades, confections, yogurt, ice cream, nutrition bars, baked goods and cereals (Rajkumar et al., 2007). Development of high quality mango powder may match the increasing worldwide demand for more natural mango-flavored beverages either singly flavored or in multi-flavored products (FAO, 2007), and meet the great demand for natural fruit powders by the pharmaceutical and cosmetic industries.

Several drying technologies can be viable commercial options for manufacture of mango powders, including freeze drying, drum drying, spray drying and Refractance Window® drying. Each has its own advantages and limitations. The final product obtained from these methods may differ in physicochemical or nutritional properties and microstructures. Freeze drying, also known as lyophilization, is a drying process in which the food is first frozen then dried by direct sublimation of the ice under reduced pressure (Oetjen and Haseley, 2004, Barbosa-Cánovas, 1996). To carry out a successful freeze drying operation, the pressure in the drying chamber must be maintained at an absolute pressure of at least 620 Pa (Toledo, 2007). Freeze drying is generally considered as the best method for production of high quality dried products (Ratti, 2001). But, it suffers from high production costs, high energy consumptions, and low throughputs (Ratti, 2001, Hsu et al., 2003, Caparino, 2000).

Drum drying is commonly used in production of low moisture baby foods and fruit powders (Kalogiannia et al., 2002, Moore, 2005). A drum dryer consists of two hollow cylinder drums rotating in opposite directions. The drums are heated with saturated high temperature (120–170 °C) steam inside the drums. Raw materials are spread in thin layers on the outer drum surface and dry rapidly. The product is scraped from the drum in the form of dried flakes (Kalogiannia et al., 2002, Saravacos and Kostaropoulos, 2002). A major likely drawback is undesirable cooked aromas and other severe quality losses in the final products caused by the high temperature used in the drying process (Nindo and Tang, 2007).

Spray drying is widely used in commercial production of milk powders, fruits and vegetables (Kim et al., 2009, Kha et al., 2010). This method has several advantages, including rapid drying, large throughput and continuous operation (Duffie and Marshall, 1953). During the drying process, the feed solution is sprayed in droplets in a stream of hot air (Saravacos and Kostaropoulos, 2002). The liquid droplets are dried in seconds as a result of the highly efficient heat and mass transfers (Toledo, 2007). The finished product can be made in the form of powder, granules or agglomerates (Nindo and Tang, 2007). Spray drying processes can be controlled to produce relatively free flowing and uniform spherical particles with distinct particle size distribution (Barbosa-Canovas et al., 2005, Duffie and Marshall, 1953). However, due to the relatively high temperatures involved in spray-drying processes, this drying technique may cause loses of certain quality and sensory attributes, especially vitamin C, β-carotene, flavors and aroma (Dziezak, 1988). In addition, it is difficult to directly spray dry sugar-rich materials such as mango, because they tend to stick to the walls of the dryer (Bhandari et al., 1997a, Bhandari et al., 1997b, Masters, 1985). Drying aids, such as maltodextrin, are widely added to the feed to increase glass transition temperature of the dried product and hence overcome the problem of stickiness during spray drying.

Refractance Window® (RW™) is a novel drying technique designed mainly to convert fruit puree into powder, flakes, or concentrates. The technology utilizes circulating hot water (95–97 °C) to transfer thermal energy to a thinly spread liquid material placed on a polyester conveyor belt that moves at a predetermined speed while in direct contact with hot water. During drying, the thermal energy from hot water is transmitted to foods through the plastic conveyor by conduction and radiation. Water vapor from foods is carried away by a flow of filtered air over the thin layer. This technology offers several benefits when applied to fruits and vegetables. For example, good retention of nutritional (vitamins), health-promoting (antioxidants) and sensory (color, aroma) attributes were reported for dried carrots, strawberries and squash (Nindo and Tang, 2007). The bright green color of pureed asparagus remained virtually unchanged when dried in the RW dryer, and was comparable to the quality of freeze-dried product (Abonyi et al., 2002). In addition, energy efficiency of RW drying method compares favorably with other conventional dryers (Nindo and Tang, 2007).

Studies were reported that compared the influence of different drying methods on various quality attributes of fruits and vegetables, including the color of dehydrated apple, banana, carrots and potatoes (Krokida et al., 2001), β-carotene and ascorbic acid retention in carrots and strawberry (Abonyi et al., 2002), antioxidants and color of yam flours (Hsu et al., 2003), asparagus (Nindo et al., 2003), and antioxidant activities in soybean (Niamnuy et al., 2011), encapsulated β-carotene (Desobry et al., 1997), and color and antioxidant of beetroots (Figiel, 2010). However, no studies have been conducted to evaluate the effect of drying methods on mango powders in terms of color, bulk density, porosity, hygroscopicity, solubility, and microstructures. Thus, the objective of this work was to investigate the influence of four drying methods (Refractance Window® drying, freeze drying, drum drying and spray drying) on the physical properties and microstructures of resulting mango powders to provide better understanding in selecting drying techniques that can be applied toward the manufacture of high quality mango powder.