Characterization of Watermelon Rind Powder Using Foam Mat Drying With Different Types of Foaming Agents and
Temperatures
Nursabrina Munawar1,3, Najjah Azhar1, Nur Syahirah Rohaizad1, Norhayati Hussain1,2*
1 Department of Food Technology, Faculty of Food Science and Technology, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia
2 Halal Products Research Institute, Putra Infoport, 43400 UPM, Serdang, Selangor, Malaysia
3 Alliance of Research and Innovation for Food (ARIF), Universiti Teknologi MARA, Cawangan Negeri Sembilan, 72000 Kuala Pilah, Negeri Sembilan, Malaysia.
*Corresponding Author: [email protected] Accepted: 15 September 2020 | Published: 30 September 2020
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Abstract: Utilization of food waste as the new source for food product is of interest in current research field. Watermelon rind is one of the natural food wastes that can provide nutrients including carbohydrates, proteins, and lipids. This study aims to evaluate the best conditions of foam mat drying using three different temperatures (50, 60, 70 °C) and foaming agents (maltodextrin and egg albumin) at different concentrations (10, 15, 20%) to produce watermelon rind powder. The rind powder was characterised by its powder’s foam density, foam expansion, solubility index and water absorption. Drying with maltodextrin as foaming agents at 50°C showed significantly (p<0.05) high solubility (23.94%), low water absorption (0.21), Hausner ratio (1), Carr index (3.16%) with foam expansion of 12.04%, bulk and tapped density at 0.66 g cm-3and 0.68 g cm-3, respectively. Maltodextrin aid in increasing the solubility of rind powder due to its high-water solubility and low moisture content properties. This finding may assist fruit industry to apply the watermelon by-product into a useful food ingredient in bakery, confectionary and beverages thus benefits in waste management.
Keywords: Watermelon rind, Foam mat drying, Egg albumin, Maltodextrin
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1. Introduction
Watermelon (Citrullus lanatus) originated from southern Africa and has widely spread to other countries with warm climate. Its rapid growth is promoted by the warm climate and long growing season. The pulp and juice of watermelon are common for human consumption, while the rind and seeds, representing 30% of the whole fruit are the major solid wastes (Persistence Market Research, 2018). A research by Al-Sayed and Ahmed (2013), showed that the watermelon by-products are important sources of protein, dietary fibres, and natural antioxidants especially pectin, cellulose, citrulline, and other phytochemical compounds.
Watermelon rind possesses a good amount of total phenol contents (0.248 mg mL-1) and high free radical scavenging ability (hydroxyl radical scavenger).
Watermelon rind has antioxidant properties and acts as a natural source of citrulline, but it is often discarded as waste due to its bland taste. The rind has minerals that is often absent in other fruits such as lycopene and beta-carotene which are good for eye health (Figuerola et
al., 2005). The amount of fruit waste has increased over the past 10 years and is expected to increase further with the growing population and economic activity. Watermelon rind is one of the natural wastes that can provide nutrients such as carbohydrates, proteins, and lipids.
Therefore, it should be utilized as a material in production instead of left as environmental waste.
Water is the most important factor that contributes to the deterioration and microbial spoilage of food. Watermelon rind may deteriorate and spoil in a short period due to bad handling during processing. Transforming rind into powder by reducing the water content (water activity) is one of the alternatives to extend its shelf life. Fruit powders are more convenient and suitable for preparation of several products such as snacks, beverages, bakery goods, and pastes (Karim and Wai, 1999). Several of drying techniques can be applied during production such as spray drying, freeze-drying, drum drying, and foam mat drying. Foam mat drying is the simplest alternative that facilitates the removal of water from fruit juice and vegetable puree. This method allows the dehydration of heat-sensitive, high sugar content and viscous foods, which are difficult to dry and sticky under mild conditions without changing the quality of the end product (Khalil et al., 2002). Foam mat drying method is also applicable to a liquid food material. The demand for using foam mat drying has been increasing since application of drying liquids will render high-quality concentrate such as milk, fruit juices, coffee and tea on a commercial scale.
Drying aids such as maltodextrin, tricalcium phosphate, and silicon dioxide are used to help in the powder’s free flowing and non-sticky characteristics (Jaya et al., 2006). Maltodextrin is a soluble fibre that can produce free flowing and high stability of powder with high sugar content by reducing stickiness and agglomeration problems. Egg albumin is a foaming agent that been used extensively due to the functional properties of gelation and foam formation. It readily adsorbs on the air-liquid interface by forming a cohesive viscoelastic film by intermolecular interactions. The presence of egg albumin as a foaming agent also helps in pore formation, which increases the surface area for drying. Hence, maltodextrin and egg albumin were chosen as foaming agents to dry watermelon rind. The objectives of this study were to determine the optimum conditions of foam mat drying using different temperatures (50, 60, 70 °C) and different foaming agents (maltodextrin and egg albumin) at different concentrations (10, 15, 20 %) and evaluate their effects on watermelon rind powders (WRP) flowability and properties
2. Materials and Methods
Puree preparation
Red watermelon was purchased from a local fruit and vegetable wholesaler in Sri Serdang, Selangor during July-August seasons and kept as a whole fruit at room temperature. Each watermelon can provide 300 to 400 g rind. The egg was purchased from a hypermarket in Selangor and kept at room temperature. The watermelons were washed and peeled to separate the flesh and rind. The rind was blended using a blender for 5 min. The foaming agents (10, 15 and 20%/100g) was added to the rind purees and whipped using a mixer for 5 min. The rind puree without any foaming agents (RP) was used as a control and kept in a chiller for further analysis.
Foam expansion
Foam expansion was done to determine the increase in the egg albumin-rind powder (EARP) and maltodextrin-rind powder (MRP) purees volume (Peleg at al., 2003). The foam expansion was calculated using the following equation:
Foam expansion (%) =(V₁ -V₀ )/(V₀ ) ×100 Where
V0 is volume of unfoam puree V1 is volume of foam puree
Foam density
The EARP and MRP purees were transferred into a standard measuring cylinder (Peleg et al., 2003). The foam density was calculated according to the following equation:
Foam density = Weight of foam (g)/Volume of foam, (cm3)
Foam stability
The stability of EARP and MRP were determined by placing each puree in a beaker for 3 h at room temperature. The volume reduction was measured every 30 min (Peleg et al., 2003).
Drying preparation
The RP, EARP and MRP were subjected to a drying process at different temperatures (50, 60 and 70℃). The purees were spread uniformly (15 x 15) cm in a stainless-steel tray and subjected to a smoke dryer. After 8 h of the drying process, dried watermelon rind was removed from the smoke dryer and placed at room temperature for about 1.5 h to attain thermal equilibrium. The dried watermelon rind was scraped from the tray, ground for 15 min, packed and sealed in a zipper bag for further analysis.
Powder analysis Bulk density
The bulk density of each RP, EARP and MRP was determined manually by pouring 20 g of powder into a 100 mL measuring glass cylinder. The bulk density was calculated according to the following equation (Cleiton et al., 2014):
Bulk density = Mass of powder (g)/Volume of powder (cm3)
Tap density
The tap density of each RP, EARP and MRP was measured by placing 20 g of powder in a 100 mL measuring glass cylinder. The tapped volume was measured after the powder was gently dropped 100 times onto a rubber mat from a height of 15 cm. The final volume was recorded (Cleiton et al., 2014). The tap density was calculated according to the following equation:
Tap density = Mass of powder (g)/Final tapped volume (cm3)
Flowability analysis
Hausner ration (HR) and Carr index (CI)
The CI and the HR were used to determine the powder flowing properties (Cleiton et al., 2014). The CI and the HR were calculated from the bulk density and tapped density based on following equations:
CI = (ρT - ρB)/ρT × 100 HR = ρB/ρT
Where;
CI is Carr index
HR is Hausner ratio ρT is tap density ρB is bulk density
Solubility index
About 2.5 g of powder was added to 30 mL distilled water at 30°C and stirred intermittently for 30 min. The mixture was centrifuged for 10 min at 10,000 rpm. The supernatant was carefully transferred to a Petri dish and oven-dried overnight. The number of dried solids referred as percentage of the total dry solids in the original sample, which indicates solubility index (Bag et al., 2011). The solubility index was calculated using the equation:
Solubility index (%) = W1/W0
Where;
W1 is weight of dry solid after centrifugation in g W0 is weight of initial dry sample in g
Water absorption index
The water absorption index was measured by the amount of wet solid remaining after centrifugation (Grabowski et al., 2006).
Statistical analysis
The normality test, analysis of variance test and comparison of means were analysed using Minitab software
3. Result and Discussion
Foaming properties
The foam densities of all EARP and MRP of different concentrations were high (Table 1).
The high-density value may be caused by the air that was incorporated into the purees during whipping (Gladvin et al., 2017). The amount of foaming agent added significantly influence the foam density of both MRP and EARP. An increase in foaming agent concentration produces rind powder with higher foam density. The 20% MRP and 20% EARP have the highest foam density of (0.99±0.01 g cm-3) and (1.01±0.02 g cm-3), respectively. The cause may be due to the increase in interfacial tension and surface tension of the liquid to form an interfacial film (Khalil et al., 2002). High amount of foaming agent promotes sudden bubble collapse and mechanical deformation in food system, producing food with high amount of air. The powders with low foam density and high foam expansion referred as stable and good quality powders (Morgan et al., 1961).
Table 1: Foam characteristics of watermelon puree added with 3 different concentration of foaming agents
Foaming agent Concentrations (%) Foam density (gcm3) Foam Expansion (%)
Maltodextrin 10 0.92±0.001c 4.67±1.155c
15 0.94±0.002b 8.00±0.000b
20 0.97±0.001
a
12.00±0.000
a
Egg albumin 10 0.93±0.001c 12.67±1.155c
15 0.97±0.010b 15.33±1.155b
20 1.01±0.002a 18.00±0.000d
Data are mean ± standard deviation (n=3)
a-d Different letter in the same column indicate significant difference at p<0.05
The results obtained shown that concentration and type of foaming agents significantly affect the foaming expansion for each powder. Based on Table 1, the MRP with 20% maltodextrin has the highest foam expansion of (12.04 ± 0.05) % compared to 10% and 15% MRP but is still significantly lower than EARP with 20% egg albumin which has the highest foam expansion of (18.07 ± 0.12) %. The high foam volume of EARP was attributed to the presence of proteins in the albumin. Previous reports stated that egg albumin contains a mixture of proteins with individual and specific function (Alleoni and Antunes, 2004). Foam formation is facilitated by globulins while the foam stability is maintained by ovomucin- lysozyme complex (Cotterill & Winter, 1955; Stadelman and Cotterill, 1994; Alleoni and Antunes, 2004; Lomakina and Míková, 2006). The foaming aptitude of proteins present in egg albumin is stronger when they interact with one another; the interaction between lysozyme and globulin promotes foam formation (Johnson and Zabik, 1981). These proteins denatured at the interphase and formed a stable interfacial film during whipping. The formation of a viscous and cohesive film through intermolecular interactions can produce effective foams. All of the MRP have significantly lower foam expansion values than all of the EARP because maltodextrin is one of the units in dextrose, which has low foaming expansion. The stability of foam is depending on liquid drains from the foam (Rajkumar et al., 2007). The stable structure of the foam is necessary for rapid drying and eases the removal of the dried material from trays. The foams that can maintain their foaming conditions for at least 1 h throughout the entire drying process without or little bubble collapsing suggest a strong mechanical and thermal stability (Ratti and Kudra, 2006;
Sangamithra et al., 2015). The volume of MRP and EARP showed no significant changes and remained stable for more than 3 h, which indicate high stability. Maltodextrin does not exhibit surface activities but it is highly hydrophilic and stabilize aqueous phase properties.
The addition of egg albumin has strengthened the bubble walls due to its high foam stability and preventing the foam structure from collapse (Lomakina and Míková, 2006).
Powder properties
The bulk density is determined to predict the quality of particulates of RP (Figuerola et al., 2005). Based on the results in Figure 1, the bulk and tapped density of control RP is insignificantly different (0.91 g cm-3) at all drying temperatures, indicating that temperature alone does not influence the bulk density of RP. Both MRP and EARP have significantly lower bulk and tapped density values than the control, suggesting that addition of foaming agents influence changes in the powder’s density. The increase in temperature and foaming agent concentration caused a significant reduction of density for both MRP and EARP. The same trend was observed on a study of different foam mat drying temperature on Nigella sativa beverage powder done by Affandi et al. (2017) but an opposite result was reported by Shaari et al. (2017) on Ananas comosus powder. A rapid moisture removal at higher temperature and denaturation of proteins in the foaming agents at higher temperature may attribute to the reduction in the powder density (Thuwapanichayanan et al., 2012). Both MRP and EARP have the lowest bulk and tapped density at 70°C. The high temperature facilitates the increase of evaporation rate, which leads to structural breakage and fragmentation, resulting in lower density powders. Therefore, dried MRP20 and EARP20 at 50 °C are good bulk and tapped density powders.
Table 2: Bulk and tapped density of watermelon rind powder (WRP) Temperature
(˚C)
Type of powders Concentration (%)
Bulk density (g cm-3)
Tap density (g cm-3)
50 Control
MRP 10
0.91±0.00c 0.77±0.05a
0.91±0.00c 0.77±0.05a
15 0.71±0.00b 0.71±0.00a,b
20 0.66±0.01c,d 0.68±0.01b,c
EARP 10 0.75±0.02a 0.78±0.03a
15 0.74±0.03b 0.74±0.03a
60 Control
20 0.66±0.01c
0.91±0.00b
0.68±0.01a 0.91±0.00b
MRP 10 0.67±0.04c 0.67±0.04b,c
15 0.65±0.01d,e 0.65±0.01b,c
20 0.64±0.02e 0.64±0.02b,c
EARP 10 0.71±0.00c,d 0.76±0.02a
15 0.65±0.01d,e 0.65±0.01b
70 Control
20 0.63±0.02e
0.91±0.00a
0.65±0.02b,c 0.91±0.00a
MRP 10 0.69±0.04e 0.69±0.04b,c
15 0.65±0.01f 0.65±0.01b,c
20 0.63±0.02f 0.63±0.02c
EARP 10 0.65±0.01e 0.66±0.03b
15 0.61±0.00f 0.61±0.01c,d
20 0.57±0.00g 0.40±0.30d
Data are mean ± standard deviation (n=3)
a-g Different letter in the same column indicate significant difference at p<0.05
Flowability properties
The rind flowability properties were evaluated using Hausner ratio (HR) and Carr index (CI).
The HR is a straightforward method to assess powder flowability based on inter-particulate friction while the powder bridge strength and stability are measured using CI (Saw et al., 2015). Based on Table 3, the HR and CI values for all the controls at each temperature are not significantly different. This suggest that without addition of foaming agents, temperature alone does not give significant effect on the powder’s flowability. Even so, a lower temperature (50 °C) did not give a significant difference in the HR values of all MRP and EARP at all concentrations. At a higher temperature (70 °C), it can be seen from the means that a higher concentration of maltodextrin and egg albumin caused a decrease in the HR values of MRP and EARP. A low HR and CI value suggests better flowability properties, where HR of less than 1.11 and CI less of 10% is considered as “excellent” flow (Rajkumar,2007). The difference in foaming agent concentration significantly affect the CI values of MRP and EARP, with temperature having little influence on the CI values. For MRP, the MRP with 20% maltodextrin have the lowest CI values of 3.16, 3.10 and 3.11% at 50, 60 and 70°C, respectively. As for EARP, the EARP with 20% egg albumin have the lowest CI values of 3.15, 3.14 and 2.96% at 50, 60 and 70°C, respectively. However, at higher temperature of 70°C, the influence of foaming agent concentration seemed to be reduced, leaving an insignificant difference of CI values between EARP at 15 and 20% egg albumin. Based on the means of HR and CI, all of the WRP were considered as having an
“excellent” powder flowability.
Table 3: Hausner ratio (HR) and Carr Index (CI) of watermelon rind powder with different foaming agent concentration
Temperature (˚C) Type of powders Concentration (%) HR CI
50 Control
MRP 10
0.00±0.00c 1.00±0.00a,b
0.00±0.00f 3.88±0.01a
15 1.00±0.00a,b 3.48±0.03b
20 1.00±0.00a,b 3.16±0.02e
EARP 10 1.00±0.00a 3.88±0.01a
15 1.00±0.00a 3.75±0.05a
60 Control
20 0.97±0.05a,b
0.00±0.00c
3.15±0.02d 0.00±0.00f
MRP 10 1.00±0.00a,b 3.48±0.04b,c
15 0.95±0.08a,b 3.44±0.01b,c
20 1.07±0.11a 3.10±0.02e
EARP 10 1.00±0.00a 3.56±0.05b
15 0.97±0.05a,b 3.40±0.05c
70 Control
20 0.92±0.00b
0.00±0.00c
3.14±0.02d 0.00±0.00f
MRP 10 1.00±0.00a,b 3.43±0.00c
15 1.00±0.00a,b 3.38±0.00b
20 0.89±0.10b 3.11±0.02e
EARP 10 1.00±0.00a 3.32±0.05c
15 1.00±0.00a 3.06±0.04d,e
20 0.92±0.00b 2.96±0.12e
Data are mean ± standard deviation (n=3)
a-e Different letter in the same column indicate significant difference at p<0.05
The solubility of powder shows the maximum quantity of the powder that can be completely dissolved and remain homogeneous with solvent such as water (Gong et al., 2007). The results showed that solubility index and water absorption index of rind powder were significantly influenced by the foaming agent concentration. The solubility of control RP dried at different temperature maintained at a range of 20.09-20.45 %, indicating the insignificant effect of drying temperature towards powder solubility. The dried MRP showed better solubility compared to control and EARP, with significantly higher solubility values ranging from 23.94-24.38 % at 20% of maltodextrin concentration for all drying temperatures. The high solubility may be caused by high amount of fibre and proteins of the rind itself, enhanced by maltodextrin which has high water solubility as well as the low moisture content in the powder (Chegini and Ghobadian, 2005; Desai and Park, 2005; Ersus and Yurdagel, 2007; Shaari et al., 2017). A low moisture content reduces powder stickiness, increasing the surface are for water binding (Chegini and Ghobadian, 2005).
The ability of dried food to absorb water is evaluated by calculating water solubility index, whereby the values obtained are related with the food hydration capacity (Barbosa-Cánovas and Juliano, 2005). The results in Table 4 indicates that both temperature and foaming agent concentration significantly affect the water absorption index of RP. For the control, even without addition of foaming agent, the water absorption index is significantly different for RP dried at 50, 60 and 70°C. The increase in temperature and concentration of foaming agent results in significant reduction of water absorption index of both MRP and EARP. The same trend is observed by Affandi et al. (2017) for the production of Nigella sativa beverage powder using foam mat drying at different temperature. The MRP with 20% of maltodextrin and EARP with 20% egg albumin, both dried at 60°C has the lowest value of water absorption index which are 0.03 and 2.25 %, respectively. The significantly low values indicate better rehydration capacity. The free hydroxyls present in the foaming agents
facilitate the binding of water molecules from surrounding environment, influencing the water absorption index of powder (Harmayani et al., 2011; Shaari et al., 2017).
Table 4: Solubility and water absorption index of watermelon rind powder with different concentration of foaming agents
Temperature (˚C)
Foaming agent Concentration (%)
Solubility index (%)
Water absorption index
50 Control - 20.09±0.01a 3.53±0.02c
Maltodextrin 10 21.11±0.07a 0.23±0.00e
15 14.92±12.06a 0.23±0.01e
20 23.94±0.06a 0.21±0.00e
Egg albumin 10 17.30±0.54e 3.58±0.18a
15 18.69±0.04c,d 3.09±0.01b
20 20.56±0.12a,b 2.41±0.00e,f
60 Control - 20.17±0.16a 3.70±0.02a
Maltodextrin 10 21.23±0.11a 0.13±0.00f
15 22.29±0.18a 0.06±0.00h
20 23.83±0.13a 0.03±0.00i
Egg albumin 10 18.14±0.18d,e 2.70±0.02d
15 19.16±0.12c 2.43±0.00e
20 21.11±0.72a,b 2.25±0.06f
70 Control - 20.45±0.40a 3.62±0.09b
Maltodextrin 10 20.28±0.01a 0.27±0.00d
15 21.66±0.26a 0.11±0.01g
20 24.38±0.34a 0.09±0.01g
Egg albumin 10 18.65±0.14c,d 3.04±0.05b,c
15 20.20±0.10b 2.63±0.01d
20 21.41±0.44a 2.90±0.01c
Data are mean ± standard deviation (n=3)
a-f Different letter in the sama column indicate significant difference at p<0.05
Conclusion
The concentration of foaming agent added to RP give significant influence on the powder’s foam density, foam expansion, solubility index and water absorption index. The interaction between drying temperature and foaming agent concentration greatly affect the powder’s bulk density, tapped density, Hausner ratio and Carr index while temperature alone give insignificant changes to the characteristics. MRP20 and EARP20 have better foaming properties due to lower foam density, higher foam expansion and better foam stability than control. The MRP20 and EARP20 dried at 50°C have better bulk and tapped density compared to control. All WRP were considered as ‘excellent’ flowability based on low CI and HR values. Based on results, dried MRP has better solubility and water absorption index compared to control and dried EARP. However, the best would be dried MRP20 at 50°C as it
has foam density of 0.99 g cm-3, foam expansion of 12.04%, bulk and tapped density at 0.66 g cm-3and 0.68 g cm-3, respectively; Hausner ratio of 1 and 3,16% of Carr index; solubility and water absorption index of 23.94 and 0.21%, respectively.
Acknowledgement
This research was conducted at Faculty of Food Science and Technology, Universiti Putra Malaysia.
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