Characteristics of selected functional properties of apple powders obtained by the foam-mat drying method Ewa Jakubczyk a, Ewa Gondek a, Krzysztof Tambor b a Department of Food Engineering and Process Management, Warsaw University of Life Sciences, Nowoursynowska 159C, 02-776 Warsaw, Poland, ewa_jakubczyk@sggw.pl, ewa_gondek@sggw.pl b Analytic Centre, Warsaw University of Life Sciences, Ciszewskiego, 02-786 Warsaw, Poland, krzysztof_tambor@sggw.pl ABSTRACT Foam-mat drying is an alternative method which enables the removal of water from high-sugar content juices and purees that are difficult to dry. The aim of this study was to investigate the effect of different foam-mat drying methods on some physical properties and aroma of apple puree powder. Apple puree was foamed with the addition of foaming agents. Foamed puree was dried as a layer using different techniques: convection drying, microwave drying, and freeze-drying. Also foamed puree with and without maltodextrin (6% or 15% w/w) was dried. Selected properties such as hygroscopicity, wettability, solubility, colour, water content, water activity, bulk density and the changes in retention of aroma compounds of apple powder were determined. The drying method affected the physical properties of apple powders. Foaming prior to drying allowed us to obtain less hygroscopic powders with low moisture content and water activity. The freeze-dried samples were characterized by lower wettability and solubility than powders obtained by air (convection) drying and microwave drying. The increase of maltodextrin addition from 6 to 15% resulted in a decrease of hygroscopicity, bulk and tapped density, and flowability of apple juice powders were also improved. The addition of maltodextrin to apple puree reduced the wetting time of freeze-dried powders but only high concentration of the carrier significantly improved wettability of powders The convection drying of foamed apple puree enabled us to obtain instant powder with higher retention of apple aroma. Keywords: food powders; foam-mat drying; physical properties. INTRODUCTION Fruit powders are convenient, easy to handle and can be used to prepare several products such as snacks, beverages, bakery goods and pastes [9]. The characteristic flavours, colours and water binding properties of powders make them an ideal ingredient in extruded cereal products, snacks, cakes, sauces and baby foods. Drying of fruit juices and pulps to powder is difficult because of the high content of low molecular weight sugars [6]. Fruit powder can be produced by adding maltodextrin to fruit pulp or juice and by drying the material in a spray dryer. Foam-mat drying is an alternative and relatively simple method which enables the removal of water from heat-sensitive, high-sugar content and viscous foods that are difficult to dry. The main advantages of foam-mat drying are lower temperatures and shorter drying times, when compared to non-foamed material [14]. The foam-drying process consists of several stages: whipping of liquid food into stable foam, drying of foamed material and grinding of the dried product to powder [8]. The drying of foamed materials facilitates high rates of drying due to the open structure of foam. The foam-mat dried materials are characterized by desirable properties such as favourable rehydration and retention of volatiles [11]. Most frequently, foamed juices and pulps have been dried by the air convection method but also microwave and freeze-drying have been applied. Only a few studies have been reported on physical properties of food powders obtained by the foam-mat drying method. The physical properties of fruit powders such as moisture content, apparent density, true density and particle porosity as well as wettability, dispersibility and solubility play an important role in quality control of powders during storage and also affect the susceptibility to reconstitution of fruit juices. The colour of many food products is one of the most important sensory attributes for product acceptance by consumers [1]. During drying, colour can be affected by the drying parameters (temperature, air velocity) and kind of drying method.
The aim of this study was to investigate the effect of different foam-mat drying methods on some physical properties and aroma of apple puree powder. MATERIALS & METHODS Apple puree without additives was provided by a local manufacturer. Apple puree was foamed with 2% (w w) egg albumin (Fluka) and 0.5% methylcellulose (Methocel, 65HG, Fluka) using a kitchen mixer at maximum speed for 5 min. Foamed puree was dried as a 4-mm layer using different drying techniques. Foamed material was convection dried at 60 o C and at air velocity of 1.2 m/s. The drying was also carried out using a convection-microwave dryer at the output power of 180 W. The foamed puree was also frozen at - 40 o C and then dried in a freeze-drier (63 Pa, 25 o C). Additionally, foams with the addition of 6% and 15% (w w) maltodextrin DE10 as well as non-foamed apple puree without additives were freeze-dried. The dried product was ground in a laboratory mill and passed through a sieve having holes of 0.5 mm. Selected properties such as hygroscopicity, wettability, solubility, colour (Minolta spectrophotometer), water content, water activity, bulk density and the changes in retention of aroma compounds of apple powder were determined. The moisture content of powders was measured by the vacuum drying method (70 o C, 24 h). A hygroscope (Rotronic-DT) was used to measure water activity. Wettability was expressed as the time required for all the powder to become wetted. Hygroscopicity of the powders was determined according to Kurozawa et al. [12]. Hygroscopicity (H) of the apple puree powders was expressed as kg of water uptake/kg d.m. The bulk density (ρ b ) was calculated by dividing the mass of the powder by the volume occupied in the cylinder. For the tapped density (ρ t ), the cylinder was tapped 1250 times, using a STAV2003/Engelsmann AG tester. The particle density (ρ p ) of powders was measured using a helium pycnometer (Quantochrome Instruments). The solubility of the powder was assessed by adding 1 g of the material to 20 ml of distilled water. The mixture was agitated with a magnetic stirrer at 892 rpm [4]. The time required for the material to dissolve completely was recorded. Flowability and cohesiveness of the powder were expressed by Carr index (CI) and Hausner ratio (HR). ρ HR = t (1) ρ b CI = ( ρ ρ ) t ρ t b 100% (2) Manual solid-phase microextraction (SPME) coupled with gas chromatography/mass spectrometry (GC MS) was applied to determine the changes in retention of aroma compounds in apple powders according to the method described by Komes et. al. [10]. The peak area ratio was calculated by dividing the peak area of individual compounds by the peak area of 3-nonanone. RESULTS & DISCUSSION Table 1 presents the effects of the different drying methods and composition of material prior to the process on water content, water activity and hygroscopicity of apple powders. The non-foamed apple puree after freeze-drying was characterized by higher water content and water activity than powders obtained by different foam drying methods. The open-pore structure of foam may facilitate the dehydration process. An increase in maltodextrin concentration resulted in an increase in water content. It can be explained that it is difficult for water molecules to diffuse past the larger maltodextrin molecules [2]. The lowest moisture and water activity were observed for freeze-dried foamed apple puree. Table 1. Hygroscopicity, water activity and content of different apple powders Material Water content (%) Water activity Hygroscopicity (g/g d.m.) Non-foamed - freeze-dried 3.81±0.11 0.171±0.002 0.23±0.01 Foam - microwave-dried 3.10±0.09 0.122±0.001 0.17±0.02 Foam - convection-dried 2.31±0.14 0.106±0.004 0.17±0.01 Foam - freeze-dried 1.81±0.04 0.084±0.002 0.19±0.01 Foam + 6% maltodextrin, freeze-dried 1.92±0.08 0.097±0.003 0.14±0.01 Foam + 15% maltodextrin, freeze-dried 2.90±0.12 0.115±0.002 0.11±0.02
The addition of maltodextrin and structure of material before drying had a significant effect on the hygroscopicity of powders (Table 1). The highest water uptake during storage was observed for non-foamed freeze-dried apple puree. Foaming of apple resulted in lower hygroscopicity of powders. This physical feature did not differ significantly for powders obtained by convection drying and also by microwave technique as well as freeze-drying of foams. The addition of maltodextrin reduced the hygroscopicity of the powders. The 15% addition of carrier appeared to give better results than addition of 8% maltodextrin. A similar effect of maltodextrin addition was found by Quek et. al. [13] for spray-dried water melon juice. 70 16 60 14 Wetting time (second) 50 40 30 20 12 10 8 6 4 Solubility (second) 10 2 0 Non-foamedfreeze dried Foam-Microw ave dried Foam - Convection dried Foam-Freeze dried Foam-Freeze dried+ 6M Foam-Freeze dried+ 15M 0 Drying method Figure 1. Effect of different drying methods of apple puree on wettability and solubility of powders. The drying method affected the physical properties of apple powders. The wettability of powders obtained by the foam-freeze-dried method was lower than that observed after convection drying (Fig. 1). The addition of maltodextrin to apple puree reduced the wetting time of freeze-dried powders but only high concentration of carrier significantly improved wettability of the product. The application of microwave drying proved advantageous to wettability of powders, since the wetting time was reduced by about 60% in comparison with the foam-freeze-drying method. The powder obtained by drying of non-foamed apple puree was characterized by better wettability than foam-dried materials. The instant properties of powders involve the ability of a powder to dissolve in water. The solubility of nonfoamed freeze-dried apple puree was two times lower than observed for other powders (Fig. 1). The statistical analysis showed that the solubility of convection-dried and microwave-dried puree did not differ. The application of freeze-drying led to the production of powders less soluble in water. The addition of maltodextrin resulted in reduction of the reconstitution capacity of apple powders. Goula & Adamopoulos [4] stated that maltodextrin incorporation into juice led to an increase in powder moisture content and a decrease in powder rehydration capacity. Adding maltodextrin to apple puree reduced the water-holding capacity of material after drying. Loose bulk density results ranging from 0.17 to 0.43 g/cm 3 are presented in Table 2. The highest bulk density was obtained for freeze-dried non-foamed puree, but using the same drying method for foamed material with maltodextrin, density was reduced two-fold. The density of powders is related to moisture content and structure and size of particles. The apple powder produced without foaming prior to freeze-drying contained more water. The higher the powder moisture content, the more particles tend to stick together, leaving more interspaces between them and consequently resulting in a larger bulk volume [3]. Decreasing moisture content decreased bulk density of apple powders. The same trend was also observed for tapped density, which varied from 0.58 to 0.43 g/cm 3. The freeze-drying of foamed puree produced powder with lower bulk and tapped density than achieved by microwave drying. The increase in maltodextrin concentration decreased bulk and tapped density of freeze-dried puree. This may be the result of maltodextrin properties as a carrier because the presence of maltodextrin minimizes the sticking of thermoplastic particles [4].
Flowability and cohesiveness of powders can be evaluated in terms of the Carr index (CI) and Hausner ratio (HR) [7]. The Hausner ratio of apple powders varied from 1.32 to 1.43 and the Carr index was in the range of 24.0-30.0, and in accordance with the classification of powders [7], most apple puree powders can be classified as materials with intermediate flowability and fair flowability. The foam-freeze-dried apple puree without maltodextrin addition was a powder with high cohesiveness (Hausner ratio above 1.4). Table 2. Selected physical properties of apple puree powders Material Loose bulk density Tapped density Particle density HR CI (%) (g/cm 3 ) (g/cm 3 ) (g/cm 3 ) Non-foamed - freeze-dried 0.43 0.58 1.54 1.35 25.8 Foam - microwave-dried 0.39 0.51 1.47 1.32 24.0 Foam - convection-dried 0.39 0.54 1.43 1.37 27.2 Foam - freeze-dried 0.32 0.46 1.49 1.43 30.0 Foam + 6% maltodextrin, freeze-dried 0.24 0.32 1.39 1.35 26.0 Foam + 15% maltodextrin, freeze-dried 0.17 0.24 1.42 1.39 28.0 Table 3. Colorimetric results of apple puree powders Material L* a* b* Non-foamed - freeze-dried 79.57 +2.48 +14.81 Foam - microwave-dried 74.23 +2.68 +16.19 Foam - convection-dried 76.14 +2.54 +16.29 Foam - freeze-dried 78.18 +2.39 +15.63 Foam + 6% maltodextrin, freeze-dried 79.97 +1.72 +14.11 Foam + 15% maltodextrin, freeze-dried 80.36 +1.77 +14.24 Colour is one of the most important quality parameters in dried materials. The colour parameters represented by L* (lightness), +a* (red colour), and +b* (yellow colour) are given in Table 3. The lowest value of L* observed for microwave-dried apple puree indicated that the colour of material was darker than observed for apple puree dried by different methods. The freeze-drying method increased the lightness of apple powders. An increase of maltodextrin concentration caused an increase of lightness and a* as well as b* values, powders became brighter and their yellowness and redness were reduced. This is in agreement with results obtained by Grabowski et al. [5] for sweet potato puree powders. Table 4. Peak area ratio in apple puree before and after drying Compounds Apple puree (before drying) Foamed puree convection-dried Peak area ratio Foamed puree freeze-dried 2-Methyl-1-butanol 0.455 0.063 0.035 Propyl propionate 0.162 - Butyl acetate 3.870 0.314 0.049 1-Hexanol 1.310 0.086 0.152 2-Methylbutyl acetate 3.760 0.422 0.070 n-propyl butanoate 0.845 0.084 0.017 Propyl 2-methylbutanoate 0.311 0.053 0.025 Benzaldehyde 1.220 0.304 0.268 n-butyl butanoate 0.331 0.073 0.030 Hexyl acetate 2.760 0.451 0.202 Hexyl hexanoate 0.314 0.165 0.172 α-farnesene 1.150 0.115 0.097 Table 4 presents compounds of aroma in raw apple puree and after freeze-drying and convection drying. The retention of some original compounds after drying was related to the applied dehydration technique. The
convection drying method prevented loss of some apple puree volatiles and increased the retention of their aroma in comparison with freeze-drying. Esters and alcohols including butyl acetate, hexyl acetate, hexyl hexanoate, 2-methylbutyl acetate, butyl butanoate, butanol and 1-hexanol were found in apple powder. Butyl acetate, 2-methylbutyl acetate and hexylacetate were the dominant fruity smelling compounds in apple puree. The retention of these esters was several times higher in apple powders obtained by the convection method than by freeze-drying. α-farnesene is considered to be responsible for the green odour of some varieties of apples [15] and the loss of this compound was lower in convection-dried apple puree. CONCLUSIONS The different drying methods of apple puree affected the physical and physicochemical properties as well as aroma retention of powders. Foaming prior to drying enabled us to obtain powders with low moisture content and water activity. Foam-mat drying was also a method that improved the properties of powders during storage due to lower hygroscopicity. The open-pore structure facilitated the dehydration process during the foam-mat method. The different methods applied to foamed materials influenced the wettability of powders. Microwave drying reduced the wetting time of powders. Also convection-dried material showed lower wettability than freezedried. The solubility of non-foamed dried apple puree was lower than observed for other powders. The application of the freeze-drying method instead of microwave technique led to the production of powders less soluble in water. Additionally, the foam-freeze-dried apple puree (without maltodextrin) was a more cohesive powder than materials produced by the convection method. The increase of maltodextrin addition from 6 to 15% resulted in a decrease of hygroscopicity as well as bulk and tapped density, and flowability of apple juice powders was improved. The addition of maltodextrin to apple puree reduced the wetting time of freeze-dried powders but only high concentration of carrier significantly improved wettability of the product. The convection drying of foamed apple puree enabled us to obtain instant powder with higher retention of apple aroma. REFERENCES [1] Abadio F.D.B., Domingues A.M., Borges S.V. & Oliveira V.M. 2004. Physical Properties of Powdered Pineapple (Ananas Comosus) Juice-Effect of Malt Dextrin Concentration and Atomization Speed. Journal of Food Engineering, 64, 285-287. [2] Adhikari B., Howes T., Bhandari B.R. & Truong V. 2004. Effect of Addition of Maltodextrin on Drying Kinetics and Stickiness of Sugar and Acid-Rich Foods during Convective Drying: Experiments and Modeling. Journal of Food Engineering, 62, 53-68. [3] Goula A.M. & Adamopoulos K.G.A. 2005. Spray Drying of Tomato Pulp in Dehumidified Air: II. The Effect on Powder Properties. Journal of Food Engineering, 66, 35-42. [4] Goula A. M. & Adamopoulos K.G.A 2010. A New Technique for Spray Drying Orange Juice Concentrate. Innovative Food Science and Emerging Technologies, 11, 342-351. [5] Grabowski J.A., Truong V. & Daubert C.R. 2008. Nutritional and Rheological Characterization of Spray Dried Sweet Potato Powder. Lebensmittel Wissenschaft und Technologie, 41, 206-216. [6] Jaya S. & Das H., 2004. Effect of Maltodextrin, Glycerol Monostearate and Tricalcium Phosphate on Vacuum Dried Mango Powder Properties. Journal of Food Engineering, 63, 125-134. [7] Jinapong N., Suphantharika M. & Jamnong P. 2008. Production of Instant Soymilk Powders by Ultrafiltration, Spray Drying and Fluidized Bed Agglomeration. Journal of Food Engineering, 84, 194-205. [8] Karim A.A. & Wai C.C. 1999. Foam-mat Drying of Starfruit (Averrhoa Carambola L.) Puree. Stability and Air Drying Characteristics. Food Chemistry, 64, 337-343. [9] Khalil K.E., Mostafa M.F., Saleh Y.G. & Nagib, A.I. 2002. Production of Mango Powder by Foam Drying of the Juice. Egyptian Journal of Food Science, 30(1), 23-41. [10] Komes D., Lovrić T. & Ganić K.K. 2007. Aroma of Dehydrated Pear Products. Lebensmittel Wissenschaft und Technologie, 40, 1578-1586. [11] Kudra T. & Ratti C. 2006. Foam-mat Drying: Energy and Cost Analyses. Canadian Biosystems Engineering, 48(3), 27-32.
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