Optimization of foaming characteristics of bael (Aegle marmelos) fruit pulp using Glycerol Mono Stearate (GMS)

Introduction

Bael (Aegle marmelos), a member of the family Rutaceae, is an indigenous fruit of India and is known by several vernacular and traditional names, including Bengal quince, Bilva, Indian quince, Bel, Belwa, Sriphal, Stone apple, Golden apple, Holy fruit, and Maredo (Sharma et al., 2007) [1] . It is a subtropical, deciduous fruit tree that produces globose fruits characterized by a hard, woody shell, which ranges in color from grey to yellowish. The fruit encloses a soft, yellow to orange-colored, mucilaginous pulp containing numerous seeds. These seeds are densely covered with fine fibrous hairs and are deeply embedded within the thick, sticky, aromatic pulp, which contributes to the distinctive characteristics of the bael fruit (Kaushik et al., 2002) [2] . Virtually every part of the Bael tree possesses considerable utility and has been valued for its nutritional, medicinal, and functional properties. The fruit is particularly rich in sugars, riboflavin, and essential minerals. Traditionally, ripe Bael fruit has been regarded as a tonic and restorative, with laxative properties, and is considered beneficial for maintaining heart and brain health. Mature fruits are especially valued for their digestive properties and have traditionally been used in the management of digestive disorders, including diarrhea and dysentery. Ripe Bael fruits can be processed into a variety of value-added products, such as ready-to-serve nectar, squash, and toffees, whereas unripe fruits are commonly utilized for the preparation of preserves, candies, and dried products in powdered form. The dried and powdered fruit pulp can also serve as a valuable ingredient in various pharmaceutical preparations. Mature bael pulp contains carotenoids and vitamins, while its protein content and secondary metabolites undergo considerable changes during fruit development, particularly from the green stage through maturation up to November (Kaur and Kalia, 2017) [3] .

Owing to its high mucilage content and the presence of important secondary metabolites, including coumarin and marmelosin, Bael has been considered beneficial in the management of diabetes (Prajapat et al., 2012) [4] . Furthermore, Bael has been reported to possess potential therapeutic value against several disorders, including cancer, cardiovascular diseases, and ulcers (Maity et al., 2009) [5] . In foam-mat drying, food liquids and pastes are initially whipped to form a stable foam through the incorporation of suitable foaming agents and are subsequently dried as thin, continuous layers, commonly referred to as mats. The incorporation of air or gas into the material substantially increases its exposed surface area, thereby facilitating rapid moisture removal during drying. The resulting dried product generally possesses a porous structure, which contributes to improved quality, rapid reconstitution, and desirable instant properties (Lewicki, 2006) [6] . Foam-mat drying offers the advantage of being performed at relatively lower temperatures, while the foaming process considerably reduces drying time. In addition, the technique is relatively economical when compared with the drying of non-foamed materials under similar drying conditions. The characteristics of the foam, particularly its expansion and stability, play a critical role in moisture migration during drying and consequently influence the quality of the final product. Foams capable of maintaining their structure without collapse for at least 1 h are generally considered mechanically or thermally stable throughout the drying process (Bates, 1964; Ratti and Kudra, 2006) [7,8] . Thermally stable foams are able to retain their porous structure during drying, thereby contributing to enhanced reconstitution characteristics of foam-mat-dried products. Foam expansion is commonly evaluated in terms of expansion ratio or foam density, whereas foam stability can be assessed through the measurement of drainage volume. Several researchers have investigated the application of foam-mat drying using different foaming agents and stabilizers. Falade et al. (2003) [9] employed glyceryl monostearate (GMS) and egg albumin for the foam-mat drying of cowpea. Similarly, Muthukumaran et al. (2008) [10] investigated the foam-mat drying of egg white using a freeze dryer and evaluated different foam stabilizers, including methylcellulose (MC), propylene glycol alginate, and xanthan gum. Their study reported that the incorporation of 0.125% xanthan gum provided sufficient foam stability, demonstrating its effectiveness as a stabilizing agent during the foam-mat drying process.

Systematic investigations on the drying of Bael (Aegle marmelos) fruit pulp remain relatively limited, particularly with respect to foam-mat drying and the optimization of foam formation. Although the foaming characteristics and drying behavior of several tropical fruits, including pineapple, passion fruit, guava, banana, papaya, mango, star fruit, and apple, have been investigated, the available literature does not appear to report systematic studies on the foaming characteristics of Bael fruit pulp. Therefore, the present investigation was undertaken to systematically evaluate the influence of whipping time and foaming-agent concentration on the foam characteristics of Bael fruit pulp, with particular emphasis on foam density, foam expansion, and foam stability. The foaming conditions were optimized by identifying the combination of whipping time and foaming-agent concentration that resulted in maximum foam expansion (corresponding to minimum foam density) along with maximum foam stability. The optimized foam characteristics were intended to provide a suitable basis for the subsequent foam-mat drying of Bael fruit pulp and the development of a high-quality, porous, and readily reconstitutable dried product.

Materials and Methods

According to the method described by Singh and Chaurasiya (2014) [11] , the mature bael fruits were thoroughly washed under running tap water to remove any dust, dirt and other unwanted materials. The cleaned fruits were then sliced in half and the pulp was extracted using a bael fruit cutter cum scooper unit. Then, it was mixed with water in a 1:1 (w/v ratio) (Panda et al., 2014) [12] . Then the mixture was weighted by weighing balance. The foaming agent glycerolmono-stearate at four different concentrations was added in weighted samples and whipped at four different whipping time.

Foaming Trials

To determine the effect of foaming agent, and whipping time on foam density (FD), foam expansion and foam stability. I optimal design was used. 100 mL prepared sample of bael fruit pulp mixed with different concentrations of GMS (3, 4, 5 and 6 % w/w) and whipped at different whipping times (4, 6, 8 and 10 min).

Foam Expansion

The expansion of the prepared foam was determined by air incorporated into the sample during whipping. It was calculated using the difference in volume of pulp before and after foaming as per the following equation: (Hardy et al., 2017) 13v

where, V f = Volume of foam, cm 3 V pi = Initial volume of pulp, cm 3

Foam Density

The density of the prepared foam was determined by weighing 50 mL of the mixture in a 50 mL measuring cylinder. The foam transferring was carried out without destroying the foam structure and trapping the air voids was avoided while filling the cylinder (Karim and Wai, 1999b) International Journal of Advanced Biochemistry Research https://www.biochemjournal.com [14] . The determinations were done in triplicate for each batch of preparation and the average values were taken. Foam density will be calculated as

where, M f = Mass of foam V f = Volume of foam

Foam stability

The stability of the bael fruit pulp foam was determined by estimating the drainage volume. 100 ml of the bael fruit foam was gently transferred into a 250 ml beaker and left undisturbed for 3 h. The drainage volume was measured by gently tilting the beaker and pouring the liquid collected in the bottom of the beaker into a measuring flask. Foam stability will be calculated as (Hardy et al., 2017) [13] .

where, V fi = Initial foam volume t = Time interval V cf = Change in foam volume

Experimental Design and Statistical Analysis

The ranges of experimental parameters were selected based on preliminary trials. The independent variables considered were: GMS (3-6 g/100 g of pulp) and whipping time (4-10 min). RSM was applied to the experimental data using a commercial statistical package (Design Expert version 8.0.7.1, Statease Inc, Minneapolis, MN) for the generation of response surface plots and optimization of process variables (Patil et al. 2014) [16] . The I-Optimal design was used for designing the experiments on foaming using three factors with four levels. A total of 16 runs of different combinations were conducted in this study. The statistical significance of the terms in the regression equation was examined by analysis of variance (ANOVA). In this study, the optimization of the foaming process was aimed at identifying the optimum levels of three factors which would give minimum foam density, maximum foam expansion and foam stability. Predictive models were used to graphically represent systems. Contour plots of the response variables were utilized applying super imposing surface methodology to select optimum combinations of GMS and whipping time for the production of bael fruit pulp foam. The details of the experimental design used in the present study are given in Table 1.

Optimization of foamed bael fruits

Numerical and graphical optimization was carried out for the independent variables to obtain the foam with minimum foam density (i.e., maximum foam expansion) and maximum foam stability using Design-Expert software. Kusum et al. (2022) Optimized egg powder by foam-mat tray drying using response surface methodology. A conventional graphical method was applied to obtain minimum foam density. Predictive models were used to graphically represent systems. Contour plots of the response variables were utilized applying superimposing surface methodology to select optimum combinations of GMS concentration and whipping time for the production of bael fruit pulp foam.

Results and Discussion

The observations for FE, FD and FS with different combinations of the independent variables are presented in Table 1. Analysis of variance showed that the models are highly significant (p<0.001) for both responses. The lack of fit was insignificant in the case of FE, FD and FS, indicating that these models are sufficiently accurate for predicting those responses. The coefficient of determination (R 2 ) values of both responses was 0.9664, 0.9945 and 0.9387 % indicating high proportion of variability was explained by the data and that the RSM models were adequate. The complete experimental results are given in Table 1. FD is commonly used to evaluate whipping properties. The more air incorporated during whipping, the lower the foam density; the more air present in the foam, the higher the whipping ability (Falade et al., 2003) [9] . The density of the foam varied from 0.38 to 0.69 gcm -3 (Table 1). ANOVA indicated that the FD was highly significant at a 0.1% level on linear terms of GMS and whipping time. The effect of GMS and whipping time on foam density is shown in figure 2.

Acknowledgement

AcknowledgementTechnical support from AICRP on PHET, CIPHET, Ludhiana and Department of Processing and Food Engineering, College of Technology and Engineering, MPUAT, Udaipur, Rajasthan, India is gratefully acknowledged.

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