Effect of Pretreatments on Physical and Thermal Properties of Bael (Aegle marmelos Correa) Fruit Pulp During Storage

Abstract

The study was performed to evaluate the effect of pretreatment on various physical and thermal properties of Bael pulp. The fruit pulp of Bael fruit was extracted and TSS of the extracted pulp was raised to 25°Brix by adding 65°Brix sugar syrup. The pH of pulp was set at 3.0 and 3.5, which was heated at 80– 85°C for 15, 20 and 25 minutes and kept at refrigerated conditions for 80 days. The product was analyzed for TSS, pH, titratable acidity, colour values, thermal properties and yeast and mold counts during storage. The TSS, pH, titratable acidity, colour–L*, a*, b*, thermal conductivity and specific heat ranged between 18–25°Brix, 2.6–3.5, 0.15–0.35%, 20.32–56.87, 2.95–20.28, 23.58–64.01, 0.37– 0.76 w⁄m°C, 1.73–2.50 J⁄g°C respectively. The microbial load in terms of yeast and mold counts was also under the prescribed limits for 60 days under the refrigerated conditions. Minimum colour change and maximum sensory score was observed at 3 pH and 15 min heating under refrigerated storage. The zero or first order models were well fitted for the responses of the bael pulp (3 pH, 15 min) stored under refrigerated conditions.

Keywords: Bael; pulp; storage; pretreatments; physical and thermal properties.

Introduction

The bael fruit (Aegle marmelos Correa), occupies an important place among the indigenous fruits of India. The bael fruit is mentioned in Vedas, Ramayana and also in Buddhist & Jain literature. Its nutritional and medicinal properties make the tree one of the most valuable, cheap and good source of nutrients, vitamins and qualities to cure dirrhorea, dysentery and other stomach ailments. The marmelosin (C₁₃H₁₂O₃) content is found in the bael fruit which is known as panacea of stomach ailments. The unripe fruits are astringent, digestive, stomachic and are generally prescribed for treating diarrhea and dysentery.

The ripe fruit of bael is sweet aromatic, nutritious and very palatable being highly esteemed and eaten by all classes of people. The fruit has excellent aroma which is not destroyed even during processing, thus there is untapped potentiality for processing bael into various products. These products being highly nutritive and therapeutically important can be very easily popularized in internal as well as international markets [1]. Enzymatic extraction of Bael has been reported been reported by Singh et al. [2]. Moisture desorption isotherm of bael pulp and adsorption isotherm of pulp powder were determined at 20, 30, 40 and 50°C [3]. The characterization of bael fruit hydrolysate treated with commercial pectinase enzyme was investigated by Charoensiddhi and Anprung [4]. The antioxidant potential of bael fruit pulp extracts showed good antioxidant power [5].

Bael is usually processed into products like preserves, refreshing beverages, powder, leather, squash, nectars, toffee, jam, syrup. For all preparations, pulp is the prerequisite therefore is required to be stored to supply throughout the year. During storage, fruit pulps undergo various changes in their quality parameters. Fruits pulps, when used as raw material for industries, are exposed to various treatments like adjustment of pH, heating and cooling processes. Therefore, thermal properties of foods play an important role in heat transfer processes and are required to be studied during storage. These properties allow to predict the speed of the penetration of heat inside the food [6]. Physical and thermal properties of bael pulp during storage are required to be explored to predict the shelf life. Therefore, the objective of this work was to examine the physico–chemical and thermal properties changes of bael fruit pulp as a function of storage time and pretreatment conditions.

Materials and Methods

Materials

Fully ripe fresh bael fruits of Kagazi variety, without any visual defects, were procured from Agricultural farm of R.B.S. College, Bichpuri, Agra (India). The bael fruits were broken by hammering and the crude mass was scooped out with the help of stainless steel spoon. The scooped crude mass was homogenized by blending manually. This crude mass was used for extraction of commercial quality pulp. The fruit pulp was extracted as per the method given by Roy [7]. The extracted pulp had total soluble solids 18° Brix and pH 4.3.

Processing of pulp and experimental design

The TSS of the extracted pulp was raised to 25°Brix by adding 65°Brix sugar syrup [7]. The pulp samples were adjusted at two pH levels (3.0 and 3.5) and heated for three durations (15, 20 and 25 minutes) at 80–85°C and stored under refrigeration conditions. The pH was adjusted with citric acid solution. Each replicate consisted of 200 g of pulp and was analyzed for TSS, pH, titratable acidity, colour values, thermal properties and yeast and mold counts during storage.

Control samples were also adjusted for pH (3.0 and 3.5) to observe the effect of heating on the measured responses.

Measurement of responses

Total soluble solids

Total soluble solid (°Bx) was determined with a hand refractometer (ERMA, Tokyo, Japan, Range 0–32%). The refractometer was calibrated with distilled water. The pulp tissues, 10g were homogenized using a kitchen blender with 40 mL of distilled water. The mixture was filtered through cotton wool. A drop of the filtrate was then placed on the prism glass of the refractometer to obtain the degree Brix. The readings were multiplied by dilution factor to obtain the original percent total soluble solid content of the bael pulp.

pH

The pH determination was carried out by using the remainder of the filtrate from total soluble solid determination using digital pH meter (Labtronics, India; Model: LT–10) after calibrating with the buffers of pH 4, and 7.

Titratable acidity

Titratable acidity was analyzed using standard method [8]. Fruit pulp, 10 g was homogenized with 40 mL of distilled water using kitchen blender, the mixture was filtered through cotton wool. Five mL of the filtrate with one to two drops of phenolphthalein (1%) as indicator was titrated using 0.1 N NaOH to an endpoint. The results were expressed as percentage of citric acid per 100 g fresh weight.

Colour measurement

The colour of pulp was measured by Hunter colour measuring system (Hunter Associates Laboratory Inc., Reston, VA; Model: Miniscan XE plus). The colour was measured in terms of L*, a* and b* values, where L* is the lightness (0 = black, 100 = white), a* for the red–purple (positive values) to the bluish–green (negative values) and b* indicates the yellowness (positive values) and blueness (negative values).

Measurement of Thermal Properties

Thermal properties such as thermal conductivity, and specific heat were determined using a thermal properties analyzer (Decagon Devices Incorporation, USA, Model: KD2). It was operating based on the line heat source method and the values were obtained directly from the digital read–out.

Microbial load

Microbial load in terms of yeasts and molds were enumerated on acidified potato dextrose agar, which was acidified to pH 3.5 ± 0.1 by addition of sterile 10% tartaric acid. Plates were inverted and incubated at 25°C±1 for 3 – 7 days [9]. Colonies were counted after proper incubation using colony counter (Analab Systems, New Delhi, Model: LM–10) then results were reported as colony forming units (cfu) per gram.

Sensory characteristics

Sensory analysis was conducted for all the samples. Twelve panelists were asked to assess the bael pulp and mark on a Hedonic Rating Test (1 - Dislike extremely, 5 – Neither like nor dislike and 9 – Like extremely) in accordance with their opinion for taste, texture, color and overall acceptability.

Kinetics of change in responses during storage

The loss in food quality during storage may be described with zero and first order equations [10]. The experimental data obtained from the storage study was fitted to these equation. The degradation kinetics of colour and ascorbic acid with storage was also established by Kumar et al. and Rai et al. [11,12] respectively.

Changes in total soluble solids, pH, acidity, colour (L*, a* and b*), thermal conductivity and specific heats as a result of storage was investingated using the following zero and first order kinetics as shown in equation 1 and 2 respectively.

C = C ± k t …(1)

lnC = lnC ± k t …(2)

Where C is the measured value of response, C₀ is the initial value of the corresponding response, t is the storage time and K₀, K₁ are the reaction rate constants for zero and first order respectively. Whereas (+) and (−) signs represent the increase and decrease in the corresponding quality response of food respectively.

The goodness of fit of the selected mathematical models to the experimental data was evaluated with the correlation coefficient (R²), the reduced chi–square (χ²) and the root mean square error (RMSE). R² is a measure of the amount of variation around the mean explained by the model. Reduced chi–square (χ²) is the mean square of the deviations between experimental and predicted values for the models and was used to determine the goodness of fit. The RMSE gives the deviation between the predicted and experimental data and it is required to reach zero value [13]. The goodness of fit will be better, if R² values are higher and χ² and RMSE values are lower [14].

The reduced chi–square (χ²) and the root mean square error (RMSE) were calculated using following expressions:

$\begin{array}{ccc}{\chi }^2& =& \frac{\sum \begin{array}{cc}\underset{i=1}{\overset{N}{}}& (MR\begin{array}{cc}{}_{Exp,i}-M{R}_{\mathrm{Pr}e,i}{)}^2& \end{array}\end{array}}{(N-Z)}\end{array}$
$RMSE\begin{array}{cc}=& \sqrt{\frac{1}{N}{\sum }_{i=1}^N{(M{R}_{\mathrm{Pr}e,i}-M{R}_{Exp,i})}^2}\end{array}$

Where MR_Exp,i is the i^th experimental moisture ratio, MR_Pre,i is the i^th predicted moisture ratio, N is the number of observations and z is the number of constants. In this study, the regression analysis was performed using MS Office – 2007, Redmond, Washington.

Results and discussion

The samples were analyzed for total soluble solids (TSS), pH, acidity, colour, microbial loads, thermal conductivity and specific heat to study the storage stability of the product processed at various pH and heating temperatures. The total soluble solids, pH, acidity, colour-L*, a*, b*, microbial loads, thermal conductivity and specific heats for fresh sample at 3pH were 25°Brix, 3, 0.15%, 56.87, 13.71, 64.01, nil, 0.372 W⁄m°C, 1.73 kJ⁄kg.°C, and at 3.5 pH were 25°Brix, 3.5, 0.15%, 47.61, 14.50, 44.65, nil, 0.37 W⁄m°C., 1.75 kJ⁄kg.°C respectively.

Total soluble solids, pH and titratable acidity

Total soluble solid indicates concentration of sugar and the soluble constituents in the pulp, which may be changed or degraded with storage time due to interaction of pulp constituents with storage period. The TSS should be maintained during the storage to restrict water activity, which may affect the growth of micro–organism. The total soluble solids reduced from 25 to 22°Brix for heat processed samples against 18°Brix for control samples (Figure.1). Minimum reduction of 25°Brix to 23°Brix was observed for samples kept at 3 pH with 15min and 20 min heating under refrigerated conditions. F–values for variation due to treatments and storage were 13.35 and 24.22 against F_Critical values of 2.18 and 2.11 respectively (Table 1), indicate that the variation is significant (p<0.05). It can also be observed that no reduction was observed till 20 days storage in almost all the processing conditions. The change is TSS may be due conversion of sugar and other components to acid and different metabolites by the action of yeast and moulds during the storage. Similar decrease in TSS for steeped aonla fruits was also observed by [15].