Effect of ultrasound-assisted extraction of dietary fibre from the sweetest variety Queen Pineapple peel waste of Tripura (India)

doi:10.21203/rs.3.rs-1732871/v1

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Effect of ultrasound-assisted extraction of dietary fibre from the sweetest variety Queen Pineapple peel waste of Tripura (India)

https://doi.org/10.21203/rs.3.rs-1732871/v1

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The study aimed to assess the functional attributes of dietary fibre from the sweetest variety of Queen pineapple of Tripura Region (India) having high pleasant aroma and flavour. Queen pineapple peel waste was used to extract dietary fibre, which is one of the important bioactive compounds in the human diet and health. The ultrasound-assisted extraction method was compared to the traditional alkali extraction method for extracting dietary fibre from Queen pineapple peel waste. The extracted dietary fibre was further examined for its functional attributes. The parameters used for ultrasound-assisted extraction (UAE) of dietary fibre were optimised using response surface methodology (RSM). The results showed that Queen pineapple peel waste was having a significant amount of dietary fibre and had a high potential in food processing. The conventional alkaline extraction yielded 64.43% dietary fibre, whereas the UAE method yielded 86.67% after 22.35 min of sonication with a solid: liquid ratio of 1:27.5 g/mL and a 46.9% ultrasonic amplitude. UAE improved the functional attributes of dietary fibre by significantly increasing the water-holding and swelling capacities while decreasing the oil holding capacity. The cation exchange capacity, glucose adsorption capacity, and glucose dialysis retardation index were enhanced considerably by ultrasound-assisted extraction, whereas the emulsifying capacity decreased. These findings suggested that Queen pineapple waste extracted dietary fibre could be used as a potential hypoglycemic agent as a functional food ingredient in bread and bakery-based products, meat-based products, dairy-based beverages, and other products of food application.

Queen pineapple peel waste

ultrasound-assisted extraction

dietary fibre

functional properties

hypoglycemic agent

Bioactive compounds like dietary fibre from the Queen variety of pineapple waste of Tripura (India) were extracted using ultrasound-assisted extraction (UAE) and obtained a maximum extraction yield of 86.67% and characterized.
The extracted UAE dietary fibre evinced a honeycomb structure with a rough surface found on SEM images and revealed good thermal stability, and a typical polysaccharide structure with a semi-crystalline pattern, as compared to the alkali extracted dietary fibre and dried powder.
Glucose adsorption capacity (GAC) and glucose dialysis retardation index (GDRI) of the extracted dietary fibre revealed the hypoglycaemic activity and it can be incorporated in food products to act as a potential hypoglycemic agent for developing functional food.

The pineapple (Ananas comosus), is a tropical fruit belonging to the Bromeliaceae family and is well-known around the world for its high nutritional value and medicinal characteristics, which include antioxidant, anticancer, and hypoglycemic capabilities (Sepulveda et al., 2018). In 2019, over 26 million tonnes of pineapple were produced globally, according to FAO reports [FAO, Rome 2020.]. This fruit can be consumed raw or processed into concentrated juices, fruit salads, candy, and jams. Canned pineapple accounts for a considerable portion of the harvested pineapple. The leaves, stem, crown, peel, core, and pomace of pineapple generate thousands of tonnes of waste as an agricultural by-product during various processes, and these wastes typically account for half of the entire fruit weight. Thus, any growth in the production of pineapple leads to an increase in the generation of waste (Hadidi et al., 2020).

Pineapple waste has high levels of bioactive chemicals such as dietary fibre (DF), polysaccharides, mineral compounds, vitamins, enzymes, proteins, and other volatile compounds which are spoilable if not treated. Thus, causing not only a significant waste of generation but also contamination of the environment (Zhang et al., 2017; Luo et al., 2017). As a result, waste extraction and utilisation are crucial in both practical and economic terms. Dietary fibre (DF) is produced mostly from cereals, legumes, fruits, and vegetables, and is characterised as the edible component of plants or a carbohydrate polymer with favourable physiological qualities (Zhu et al., 2018).

DF is resistant to digestion and absorption in the human small intestine, but it can be partly or fermented in the large intestine (Chen et al., 2018). Recently, DF has gotten a lot of attention because of its inexpensive cost and good physiological benefits to human health, such as lowering the risk of diabetes, colon cancer, arteriosclerosis, obesity, and cardiovascular disease (Zheng et al., 2018). (Hu & Zhao, 2018). DF can be subdivided into two categories based on solubility: insoluble dietary fibre (IDF) and soluble dietary fibre (SDF) (Luo et al., 2018).

Adsorption of heavy metals, bile acids, and cholesterol, reduction of postprandial blood glucose levels, improvement of intestinal microbiota, and prevention of diabetes and other cardiovascular illnesses are all benefits of SDF. On the other hand, IDF includes cellulose, lignin, and insoluble hemicelluloses, having a great swelling capacity along with water-holding capacity which further increases fecal bulk and hence reduces the risk of colon cancer, constipation and obesity (Wang et al., 2015). Not only does DF have physico-chemical and functional features, but it also has a higher viscosity and a superior texture, making it easier to use in food preparation. As a result, finding DF from agricultural waste is a major technical study topic in the food industry's development (Hu & Zhao, 2018).

For the application of plant bioactive components, the extraction process is critical. The composition and structure of DF are altered during processing, causing both positive and detrimental effects on their physico-chemical and functional qualities (Al-Dhabi et al., 2020). Chemicals, enzymatic, thermal, fermentation, and mechanical methods have all been used to extract DF from pineapple waste in the past (Hu & Zhao, 2018), but some of these methods have several drawbacks, including long chemical reaction times, high solvent intake, excessive temperature requirements, and the insertion of a large number of ions throughout the reaction procedure.

Ultrasound-assisted extraction (UAE) has become a standard technique for extracting polysaccharides and secondary metabolites from a variety of plants, owing to significant reductions in processing time and energy and solvent usage. Furthermore, when compared to traditional extraction procedures, UAE is more secure and affordable, with equivalent or higher extraction efficiency (Fan et al., 2015). The breakdown of the cell wall matrix, improved mass transfer of solvent within the solid particles, and accelerated release of desired chemicals from the matrix structure to the solvent are all caused by sonic cavitation of bubbles in UAE (Gayas et al.,2020). The particle size of dietary fibre was connected to its functional characteristics. In general, smaller particle size equals more surface area and greater yields, which can be obtained using an ultrasound-assisted extraction process that combines homogenization and hydrolysis to produce bioactive DF. Only a few research have been conducted on the use of ultrasound-assisted extraction to extract DF from pineapple waste to date. In this study, DF was extracted from pineapple waste using an ultrasound-assisted extraction method, and the extraction parameters were optimized using a Box–Behnken design based on single-factor experiments. The parameters of DF were then compared to those of the standard acid-alkali extraction method [oil adsorption capacity (OAC), water holding capacity (WHC), cation exchange capacity (CEC), glucose adsorption capacity (GAC)]. In addition, thermal, structural, and functional properties of DF were examined. These findings can be used as a technical guide for making efficient use of pineapple waste (Hadidi et al., 2020).

Diabetes mellitus is a frequent and widespread disease that affects people over the countries. Insulin insufficiency or insensitivity causes glucose to build up in the blood, resulting in a variety of symptoms (Gallagher et al., 2003). Plant extracts are a rich source of potentially beneficial dietary supplements for improving blood glucose management and reducing long-term problems in people with Type 2 diabetes (Chau et al., 2004). The current investigation was carried out to determine the glucose adsorption, glucose diffusion and glucose dialysis retardation index of the Queen pineapple peel waste and its extracted dietary fibre.

In this study, ultrasound-assisted extraction was used to extract the dietary fibre from Queen pineapple peel waste, and the conditions of extraction were adjusted using a Box-Behnken design using RSM. The functional properties viz., water holding capacity (WHC), oil holding capacity (OHC), swelling capacity (SC), cation exchange capacity (CEC), emulsion activity (EA) and emulsion stability (ES) of dietary fibre were compared between ultrasonic-assisted extraction and alkali extraction methods. Furthermore, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) was used to assess the potential structural features of dietary fibre. The thermal stability of dietary fibre was further examined using thermogravimetric analysis (TGA). The effect of extracted dietary fibre on hypoglycaemic activity through glucose adsorption capacity (GAC) and glucose dialysis retardation index (GDRI) were investigated. These findings could serve as a technical guide for making efficient use of highly demanding Queen pineapple in developing functional foods which can be further exploited in food industries.

2.1 Raw materials and Chemicals

Naturally ripened Queen variety of pineapples were collected from an agricultural farm located in Tripura (India) because the best Queen variety is grown in this region only (Das et al., 2021). All the chemicals were procured by Sigma-Aldrich Chemicals Pvt Ltd.

The pineapple peels were dried at 40°C till the equilibrium moisture content was obtained in an electric tray drier to avoid the changes in functional attributes. Grinding of the pineapple waste sample was done and sieving through a 100mesh size sieve to get the dried powder. Further, the dried powder was de-oiled using petroleum ether.

2.2 Extraction of Dietary Fiber

Dietary Fiber (DF) extraction from queen pineapple waste was done using the conventional alkaline extraction method according to Ma and Mu (2016) and Zhang et al., (2017) with simple modifications. The queen pineapple waste DF was extracted in the following order: the de-oiled dried powder was mixed with 0.5M NaOH (pH 13.69) followed by the agitation of 1000 rpm speed using a laboratory based magnetic stirrer (RQT-12TD, Remi, Maharashtra, India) at 50°C for 30min. Followed by neutralization with 0.5M HCl (pH 0.3). The precipitate was collected by adding 85% ethanol (2:1 v/v) (Standing time was 1h). Finally, the solution was centrifuged at 7000rpm for 30min to collect the pellet. The washing step was repeated three times. Then, the pellet was then dried at 40°C overnight. The extract was renamed alkaline extracted dietary fibre (AEDF) and stored in refrigeration conditions for further analysis.

Ultrasound-assisted extraction (UAE) method was used to extract the DF by the method of Begum and Deka (2019) and Zhang et al. (2017) with simple modifications. The treated dried powder (DP) was mixed with 0.5 M NaOH solution (pH 13.69) followed by the conventional alkaline extraction method. Further, the sample was sonicated at 20 kHz for different time ranges from 10–30 min with a shutdown temperature of 60℃ using a probe ultrasonicator (Q700-220 Digital Sonicator, Qsonica LLC, USA) with an amplitude of 20% – 50%. Then, neutralization of the sample was done using 0.5 M HCl (pH 0.3). The precipitate was collected by adding 85% ethanol (2:1, v/v) and allowed to stand for 1h. Finally, the sample was centrifuged at 7000 rpm for 20 min at 20°C and the pellet was collected, followed by washing and again centrifugation at 7000rpm for another 10 min, while decanting the supernatant. Then, the pellet was dried overnight at 40°C to obtain UAE extracted dietary fibre (UAE DF) and stored for further analysis.

2.3 Experimental design

The process variables of ultrasound-assisted extraction that affect the yield of dietary fibre viz., solvent-solute ratio, amplitude, and sonication time were optimized by response surface methodology (RSM) (Raj and Dash, 2020). In this process, Box-Behnken design (BBD) was used to design the experiments by implementing “Design Expert” software (Version 7, Stat-Ease, Inc. Minneapolis, MN USA). The following quadratic polynomial equation was used to fit the experimental data:

$$Y= {b}_{0} + {\sum }_{i}{b}_{i}{x}_{i}+ \sum \sum {b}_{ij}{x}_{i}{x}_{j}+ \sum {b}_{ii} {x}_{i}^{2}$$

Where, Y is the dependent variable, b₀ is the constant coefficient; b_i, b_ii, and b_ij are linear, quadratic and interaction coefficients, respectively; x_i and x_j are coded independent variables.

The values of R² and adjusted R² were checked for the model adequacies and the significance of the model was tested using analysis of variance (ANOVA).

The yield of the extracted dietary fibre was calculated using the following equation:

$$Y \left(\%\right) = \left(\frac{{W}_{t}}{{W}_{i}}\right) \times 100$$

Where w_t is the weight of extracted dietary fibre; w_i is the initial weight of pineapple waste dried powder.

2.4 Analysis of the chemical composition of extracted dietary fibre

The proximate analysis viz., moisture, crude protein, crude fat, crude fibre, and ash content of Queen pineapple peel waste dried powder was determined by A.O.A.C., 2016. The carbohydrate content was estimated by the difference from all the other constituents.

2.5 Fourier transform infrared spectroscopy (FTIR) analysis of extracted dietary fibre

The different functional groups present in pineapple waste dried powder and extracted dietary fibre (both UAEDF and AEDF) were determined by FT-IR (Nicolet Instruments 410 FTIR, Thermo Scientific, USA). Spectrum was analysed in the wavelength of 400–4000 cm^− 1 with 4 cm^− 1 resolution (Zhang et al., 2017).

2.6 Scanning electron microscope (SEM) analysis of extracted dietary fibre

The microstructure and surface morphology of pineapple peel waste dried powder, UAEDF and AEDF were studied by SEM (JSM-6390LV, JEOL, Japan), operated at 20kV, 500X and 1000X amplification.

2.7 XRD analysis of extracted dietary fibre

The crystallinity of pineapple peel waste dried powder, UAEDF and AEDF were determined using XRD (Bruker AXS, Bruker D8 FOCUS, Germany) functioned at 60 kV. The study was executed at a 2θ range of 5–80° and the degree of crystallinity was calculated as follows:

$$D \left(\%\right) = \frac{{A}_{c}\times 100}{{A}_{c}+{A}_{a}}$$

Where, D is the degree of crystallinity; A_c is the crystallized area and A_a is the amorphous area on the XRD diffractogram. (Ma and Mu, 2016).

2.8 Thermo-gravimetry analysis (TGA) of extracted dietary fibre

The thermal analysis of pineapple peel waste dried powder, UAEDF and AEDF were investigated by TGA (NETZCH TG 209F1 Libra, Germany) (Zhang et al., 2017; Wongkom and Jimtaisong, 2017). The TGA was performed under the controlled atmosphere of nitrogen and the temperature ranged from 30°C-600°C at a rate of 10°C/min.

2.9 Functional properties of the extracted dietary fibre

2.9.1 Water holding capacity (WHC) and oil holding capacity (OHC)

WHC and OHC of the extracted dietary fibre were determined according to the method described by Ma and Mu (2015). Dietary fibre (2g) was mixed with 20 ml of deionized water or vegetable oil (depending on WHC or OHC) and agitated for 30–35 min at 25°C. The slurry was centrifuged at 4000 rpm for 10 min and the supernatant was decanted. The WHC and OHC were expressed as the weight gained by the sample (g/g dry weight) and were determined by the following equation:

WHC or OHC (g/g) = (W_t – W_i) / W_i

Where W_t is the final weight of dietary fibre which contains water or oil and W_i is the initial weight of dietary fibre.

2.9.2 Swelling capacity (SC)

Swelling capacity (SC) was analysed by the method of Daou and Zhang (2012) with a simple modification. The sample (1g) was placed into a graduated glass cylinder and the initial volume occupied by the sample was recorded. Thereafter, 30 ml of phosphate buffers were added to maintain the pH at 6.6 and the samples were allowed to swell for 60 min. The final volumes were noted and expressed as mL of swollen sample per gram of dry initial sample.

2.9.3 Cation exchange capacity (CEC)

CEC was investigated by treating the samples with excess 2M HCl for 48 h and from this 0.5g of wet sample was taken and volume made to100 ml with 5% NaCl in buffers (pH 6.6) and then filtered. The sample was then incubated and shaken at 37⁰C for 60 min and then titrated. The final results were expressed as meq per g dry initial sample (Gorecka et al., 2000).

2.9.4 Emulsifying activity (EA) and emulsion stability (ES)

Sample (5g) was dispersed in deionized water (10 mL) and sonicated for 10 min with 5 mL refined oil. The emulsions were centrifuged (1100 g, 5 min) and the height of the emulsified layer and the total contents in the tube were determined (Abirami et al., 2014).

The EA was calculated as follows:

$$EA\left(\%\right) = \frac{Height of the emulsified layer }{Height of total content }\times 100$$

ES was determined by heating the emulsion at 80°C for 30 min and then followed centrifugation at 1100 g for 5 min. The emulsion stability was calculated as follows:

$$ES \left(\%\right) = \frac{Height of the emulsified layer after heating}{Height of emulsified layer before heating }\times 100$$

2.10 Hypoglycemic activity

2.10.1 Glucose adsorption capacity (GAC)

The GAC was analyzed following the method of Peerajit et al. (2012). Each sample of AEDF, UAEDF and pineapple waste dried powder (1%) were added to 25ml of glucose solution of 1, 10, 50, 100, and 200 mM concentration. The mixture was agitated using a magnetic stirrer and incubated in a water bath at 37°C for 6 h at 300 rpm. Further, centrifugation at 4,000×g for 20 min was done and the glucose content in the supernatant was determined. Glucose adsorption was calculated using the following formula:

$$GAC (mmol/g)= (G1-G6)/ \left(WS x Vi\right)$$

where G₁ is initial glucose concentration in mmol/L; G₆ is glucose concentration after 6 h in mmol/L; V_i is the volume of supernatant in ml; W_S is the weight of dietary fibre in g.

2.10.2 Glucose dialysis retardation index (GDRI)

GDRI analysis is one of the methods to predict the effect of samples on the delay of glucose absorption in the gastrointestinal tract. The GDRI measurement of glucose retardation was performed using the method described by Nsor-Atindana et al. (2012) with slight modification. Each sample (0.5g) was added to 25mL of glucose solution (50mmol/L) which was then added to the dialysis membrane (12,000 Da). Each dialysis bag was dialyzed against 100mL of distilled water (DW) at 37°C under a shaker water bath. The glucose content in the dialysate was measured using the glucose assay kit (Sigma GAGO-20) to determine the GDRI as a function of time. The volume of 1mL of glucose diffused from the treated sample (pineapple DP, AEDF, UAE DF) and control was taken at 30, 60, 120, and 180 min time intervals; the absorbance of each sample was measured using a spectrophotometer. A control test was also conducted in the same manner but without addition of sample. The GDRI was calculated using the following equation:

$$GDRI = 100 – \left[\right({G}_{ts}/{G}_{c}\left) x 100\right]$$

Where, G_ts is the total glucose diffused from the treated sample; G_c is the total glucose diffused from the control sample.

2.11 Statistical analysis

SPSS 16.0 was used for the statistical analysis of data. Evaluation of the means was done by investigation of variance (ANOVA) with Duncan’s multiple range tests to identify the significant differences (p < 0.05).

3.1. Optimization of UAE method

Optimization of a single factor was conducted using extraction yield (%) of DF as the dependent index to identify the initial ranges of the extraction independent variables (Table 1). As shown in Fig. 1(a,b,c) the extraction yield of DF initially raised to its maximum level as the time was increased from 10 to 30 min, and then declined as the amplitude was increased from 20 to 50%, implying that increasing amplitude reduces particle size and enhances the specific surface area of the sample, thus speeding up the mass transfer of intracellular substances. Excessive amplitude, on the other hand, may cause the degeneration of DF. As a result, the optimal amplitude was determined to be 46.90%. The effect of ultrasonic time on DF extraction yield (Fig. 1 [a,b,c]) revealed that as the ultrasonic time was increased from 10 to 30 minutes, the extraction yield of DF increased relatively quickly, indicating that a longer ultrasonic time could increase the particle size of extracted DF and increase the mass transfer rate of DF. As a result, in this study, the optimum ultrasonic time was set at 22.35 minutes. According to Fig, the extraction yield of DF increased significantly as the solid: liquid ratio (dried powder (DP): 0.5% NaOH solution) increased from 10–25%, after which it declined somewhat.

Table 1

Ranges of independent variables with their corresponding levels
Coded variable levels	Independent variables	Levels
Coded variable levels	Independent variables	-1	0	+ 1
A	Sonication time (min)	10	20	30
B	Amplitude (%)	20	35	50
C	Solid:liquid (g/mL)	10	20	30

The influence of a constant hydrolysis temperature has the potential to affect the mass transfer rate and the density of the solvent simultaneously. The activity of the alkali solution and molecular movement might be increased at a high enough temperature, speeding up the dissolving of DF. Higher hydrolysis temperatures, on the other hand, may limit yield activity and promote DF degradation. As a result, a constant hydrolysis temperature of 60° C was adopted. Hu and Zhao (2018).

3.2 Determination of significance factors

For the extraction, a BBD design with three independent variables was created, with the yield of DF as the response. The BBD design for DF extraction utilizing ultrasonication is shown in Table 2. The model's fit was expressed by the R² value (0.93), and the statistical significance was determined by the F-test. The analysis of variance (ANOVA) regression model was determined to be significant (p > 0.05), indicating that the model is legitimate shown in Table 3. The DF extraction yield is explained by the quadratic regression equation:

Table 2

Box-Behnken design for extraction of dietary fiber (DF) using ultrasound-assisted extraction (UAE)
Run	Sonication Time (min)	Amplitude (%)	Solid: Liquid (g/mL)	Yield of dietary fiber (%)
Run	A	B	C	Yield of dietary fiber (%)
1	20	50	10	71
2	10	20	20	63.8
3	20	35	20	76
4	20	35	20	76
5	30	35	30	85
6	30	35	10	82.8
7	20	20	10	69.17
8	20	35	20	77.5
9	30	20	20	83
10	30	50	20	87
11	20	35	20	75.95
12	10	35	30	66.67
13	10	50	20	68
14	20	35	20	77
15	10	35	10	64
16	20	50	30	74
17	20	20	30	70.5

Table 3

ANOVA for response surface quadratic model for the yield of dietary fibre
Source	Sum of Squares	df	Mean Square	F Value	p-value Prob > F	Significant level
Model	811.39	9	90.15	193.51	< 0.0001	***
A	709.33	1	709.33	1522.54	< 0.0001	***
B	22.88	1	22.88	49.12	0.0002	***
C	10.58	1	10.58	22.71	0.002	***
AB	1.00E-02	1	1.00E-02	0.021	0.8877	*
AC	0.055	1	0.055	0.12	0.7407	*
BC	0.7	1	0.7	1.5	0.2608	**
A²	6.11	1	6.11	13.12	0.0085	***
B²	21.22	1	21.22	45.55	0.0003	***
C²	39.88	1	39.88	85.6	< 0.0001	***
Residual	3.26	7	0.47	-	-	-
Lack of Fit	1.21	3	0.4	0.79	0.5611	not significant
Pure Error	2.05	4	0.51
Cor Total	814.65	16
R² = 0.93
A = Sonication time (min), B = Solid: liquid (g/mL), C = Ultrasound amplitude (%)
* Not significant (p > 0.05), Significant different (p < 0.05), * Extremely significant different (p < 0.01)

$$Yield = +76.49 +9.42 \text{*} \text{A}+1.69 \text{*} \text{B}+1.15\text{*} \text{C}-0.05\text{*} \text{A} \text{*} \text{B} -0.12\text{*} \text{A} \text{*} \text{C} +0.42 \text{*} \text{B} \text{*} \text{C}+1.20 \text{*}\text{A}2 -2.25 \text{*} \text{B}2 -3.08 \text{*} \text{C}2$$

Where, A = sonication time (min); B = amplitude (%); C = solid:liquid (g/mL)

3.3 Optimum conditions for extraction

The optimum conditions for UAE DF was shown in Table 4. The maximum yield of dietary fibre was obtained at 22.35 min of sonication time with solid:solvent ratio of 27.5 mL/g, and ultrasonic amplitude 46.90%. The optimum conditions for extraction finally povide the value for the predicted extraction yield of DF which was 86.67%. This predicted value matched well with the experimental value with desirability of 0.993.

Table 4

Validation of optimum extraction conditions
Factors (independent variables)	Optimum conditions	Predicted value, dietary fiber yield (%)	Experimental value, dietary fiber yield (%)	Relative deviation (%)
Sonication time (min)	22.35	86.23	86.67	0.22
Amplitude (%)	46.9
Solid: Liquid (g/mL)	27.5
Desirability = 0.993

3.4 Chemical composition of pineapple waste and extracted dietary fiber

The chemical composition of queen pineapple waste DP and extracted AEDF, UAE DF are shown in Table 5 on the drybasis (DB). Total Dietary Fiber (TDF) content of UAE DF (86.67g/100g) was significantly higher as compared to queen pineapple waste DP and AEDF. Moreover, it was found to be higher than that of yellow passion fruit waste (71.79g/100g), lemon peel (70.76g/100g) and mango processing waste (74g/100g) (Begum and Deka, 2020). Less amount of fat was found to be present in ultrasound-assisted extracted DF (0.23g/100g). Protein content of UAE DF was found to be less than that of AEDF and DP as a result of the protein denaturation by the high intensity ultrasound reported by Begum and Deka (2020).

Table 5

Chemical composition of pineapple waste dried powder (DP), ultrasound-assisted extracted dietary fiber (UAEDF), alkaline extracted dietary fiber (AEDF)
Chemical composition (g/100g)	Pineapple waste DP	AEDF	UAEDF
Moisture content	15.43 ± 0.12	10.83 ± 0.10	10.04 ± 0.07
Crude protein	3.3 ± 0.13	1.6± 0.12	1.21 ± 0.04
Crude fat	1.84 ± 0.03	0.55 ± 0.03	0.52 ± 0.59
Ash	4.4 ± 0.05	11.23 ± 0.11	10.16 ± 0.18
Crude fiber	58.32±0.41	-	-
Carbohydrate	16.71±0.23	-	-
TDF	-	64.43±0.41	86.67±0.25
IDF	-	62.89±0.21	82.34±0.28
SDF	-	1.54±0.38	4.33±0.18
(Values represent mean ± SD; n = 3. Mean with different superscript letters in the same row represent a significant difference at p < 0.05)
*AEDF- alkali extracted dietary fiber UAEDF- ultrasound-assisted extracted dietary fiber; TDF-total DF, IDF- Insoluble dietary fiber, SDF- Soluble dietary fiber

3.5 Fourier Transform Infrared Spectroscopy (FTIR) analysis of extracted DF

FTIR spectroscopy is an analytical method for characteristic organic functional groups in polysaccharides which detected within the wavelength of 400–4000 cm^− 1. The functional groups and bonding configurations of FT-IR spectra of queen pineapple waste DP, UAEDF and AEDF were shown in Fig. 2. There was exception in some characteristic bands in queen pineapple waste DP and their extracted DF but the general spectral profiles of AEDF and UAEDF were similar. The strong and broad adsorption at 3398 cm^− 1 corelates to OH group stretching as a result of the inter or intra molecular hydrogen bands. This OH vibration appears mainly from hemicellulose (xylose, mannose, arabinose, galactose) and pectin (galactouronic acid) (Zhang et al., 2017). The weak adsorption bands were observed at 2996 and 2918 cm^− 1 originated from CH stretching which indicates the occurrence of characteristic structure of methylene polysaccharide (Zhang et al., 2017). The carbonyl group from GalA units (CH, CH₂, CH₃) pectin and methoxy group of glucose uronic acid hemicellulose can be studied from the band stretching of 1200–1800 cm^− 1 (Zhang et al., 2017). Band peaks at 1650 cm ^− 1 and 1421 cm^− 1 were corresponding for aromatic hydrocarbons of lignin and CH₂ stretching of cellulose respectively. Moreover, CO stretch bond was represented by the peak at 1066 cm^− 1 which indicates the collapse of oligosaccharides from DF (Bockuviene and Sereikaite, 2019). The presence of these band stretching and peaks confirmed the typical functional groups of polysaccharides of extracted dietary fiber from pineapple waste DP along with the AEDF and UAE DF.

3.6 SEM analysis of the extracted DF

The morphologies of Pineapple waste DP, UAE DF and AEDF were shown in Fig. 3(a,b,c). Pineapple waste DP had a block shape with compact texture. The surface of AEDF were found to have fibrous network like structure. However, the surface of UAE DF was much looser than that of AEDF and had porous structures. The difference in the microstructures inferred that the ultrasonication treatment interrupt the solute with the solvent which leads to a honeycomb structure of UAE DF. Begum and Deka (2019) reported that a well alienated fibrous network structure was perceived in case of AEDF. But a distinctive honeycomb arrangement found in case of UAE DF. The obtained results could be accompanied to the ultrasonication treatment which leads to the formation of large pores in the outer cell wall and hence untying the complex bond between the microfibrils of DF.

3.7 X-RD analysis of the extracted DF

X-RD analysis was conducted to evaluate the changes in crystallinity and amorphous region of DF and thus distinguish the cluster region of the AEDF and UAE DF. X-RD derives the patterns of both the AEDF and UAE DF in comparison with pineapple waste DP were shown in Fig. 4. The changes in crystallinity of pineapple waste DP, UAEDF and AEDF was evaluated from X-ray diffraction chromatogram. The analysis of both patterns of AEDF and UAE DF revealed characteristics major peaks at 22.53℃, 35.17℃, and non-crystalline minor peaks at 15.19℃,60.09°C. These results inferred that AEDF and UAE DF show the co-existence of crystalline and amorphous region, which leads to a typical cellulose crystallinity structure (Zhang et al., 2017). Similarity in the outline of UAE DF and AEDF specifies that ultrasonication changed the type of crystallinity. Though, the degree of crystallinity of UAE DF (30.21%) was significantly (p ˂ 0.05) higher than that of AEDF (28.84%) and pineapple waste DP (13.57%). ultrasonic treatment the of DF makes more loosen tissue. According to the findings, ultrasonication treatment substantially disrupted the crystal structure of UAE DF, which makes the strong ordered crystalline structure into less ordered crystalline or even disordered amorphous structure. Thus, characteristics amorphous structure was found in UAE DF. Furthermore, the interaction between the DF molecules weakens. As a result of the ultrasonication treatment, the UAE DF tissue became looser. Ultrasonic treatment may also assist to improve the water holding, oil holding, swelling capacity of DF in addition to lowering the degree of polymerization. Wang et al. (2016) showed a similar effect in the case of ultrasound-treated DF.

3.8. Thermo-gravimetric analysis (TGA) of extracted dietary fibre

The thermal stability of a polymer is one of the important parameters during the thermal processing of food unit operation such as baking. TGA was performed to evaluate the changes in the thermal behaviours and thermal stability of pineapple waste dried powder, UAEDF and AEDF. TGA was done at a temperature of 0-700℃ range (Fig. 5). Results evinced significant changes in both dietary fibre at temperature ranges (40–200℃, 200–325℃ and 325–450℃ up to 630℃) and dietary fibre was observed to have different degradation kinetics at these temperature ranges. Pineapple waste dried powder was observed to have temperature degradation at 35–100℃, 100–200℃, 200–295℃ and further up to 625℃. The results revealed that the evaporation of absorbed water took place during the first temperature range of 40–200℃ (Ma and Mu, 2016). Studies have shown that hemicellulose is rapidly degraded between 210 and 350℃ (Moriana et al., 2011; Yang et al., 2007), while the pyrolysis of cellulose started at 315℃ and continued up to 400℃ (Liu et. al., 2021). Due to the presence of different functional groups in dietary fibre, such as lignin, cellulose, and hemicellulose variable thermal breakdown temperatures have been observed. This result suggested that UAEDF had more absorbed water than AEDF. Depolymerization of hemicellulose and cellulose degradation as indicated by the temperature ranges (Begum and Deka, 2019). The polysaccharide pyrolytic degradation of hemicelluloses and pectic polysaccharides contained in the dietary fibre can be related to the relative strength intensity of the peaks (Carrier et al., 2011). The thermal degradation of char especially when the pyrolytic temperature ascended to the 2nd temperature range (200–315°C), the changes in mass were very slow. As per the temperature range investigation (40–200℃, 200–325℃ and 325–450℃ up to 630℃), the weight loss of UAEDF was less than AEDF, indicating that UAEDF had greater thermal stability than AEDF and pineapple waste dried powder.

3.9. Functional properties of extracted dietary fibre

The differences in the physicochemical and functional properties of extracted dietary fibre evinced (Table 6) in the WHC, OHC and SC of UAEDF were 12.72 g/g, 5.22 g/g and 7.38 mL/g samples, respectively. The capacity of a material to hold moisture content including hydrodynamic, physically entrapped, and linked moisture under centrifugation or compression condition represents the water holding capacity (WHC) (Alfredo et al., 2009). It was found to have significantly higher value when compared to pineapple waste and AEDF with UAEDF. The initial WHC of AEDF and pineapple waste dried powder was 8.64 ± 0.09 and 6.38 ± 0.07 g/g, respectively which is more than the WHC of dietary fibres from the banana pseudo stem (4.71 ± 0.31 g/g), malt bagasse (3.68 ± 0.08 g/g), oat hull (2.13 ± 0.11 g/g), passion fibre (7.2 g/g) and rice hull (2.58 ± 0.28 g/g) (Wang et al., 2015) but lower than that of chia fibre (15.41 g/g) (Alfredo et al., 2009). Moreover, the WHC of UAEDF (12.72 ± 0.167 g/g) was significantly greater (p < 0.05) than that of AEDF (8.64 ± 0.09g/g). Reduced particle size, porosity, surface, and microstructure of dietary fibre also led to the increase in WHC. Water interacts with dietary fibre through two mechanisms: (i) water is held in capillary structure because of surface tension strength and (ii) water is held in hydrogen bond and dipole form (Begum and Deka, 2019).

Table 6

Functional properties of pineapple waste dried powder (DP), ultrasound-assisted extracted dietary fiber (UAEDF), alkaline extracted dietary fiber (AEDF)
Functional properties	Pineapple waste DP	AEDF	UAEDF
WHC(g/g)	6.38±0.07	8.64±0.09	12.72±0.167
OHC (g/g)	1.18±0.17	3.53±0.012	5.22±0.10
SC (mL/g)	1.68±0.28	4.23±0.10	7.38±0.28
EA (%)	18.06±1.78	31.89±1.88	41.89 ± 1.42
ES (%)	8.5±0.07	17.8±1.3	29.5 ± 2.1
CEC (mM/g)	1.27 ±0.15	3.98±0.14	6.34±0.62
(Values represent mean ± SD; n = 3. Mean with different superscript letters in the same row represent a significant difference at p < 0.05,)
*DP- Dried powder, AEDF- Alkali extracted dietary fiber UAEDF- Ultrasound-assisted extracted dietary fiber;
*WHC-Water holding capacity; OHC-Oil holding capacity; SC- Swelling capacity; EA- Emulsion activity; ES- Emulsion stability, CEC- Cation exchange capacity

The OHC of AEDF and UAEDF were 3.53 ± 0.12 g/g and 5.22 ± 0.10 g/g, respectively. The increase in OHC of UAEDF can be attributed to the comparatively loose texture due to the ultrasonication treatment as shown by the SEM analysis. A comparative study of the SC of UAEDF from pineapple waste dried powder with the SC of AEDF revealed that the SC of UAEDF (7.38 ± 0.28 mL/g) was higher than that of AEDF (4.23 ± 0.10 mL/g).

The emulsion activity (EA) value of extracted dietary fibre was significantly reduced (p < 0.05) using the UAE method. The stability index of EA values was less than 50% (Table 6), which indicated that UAEDF is not a good emulsifier and our results are in line with several authors who have reported low EA values of dietary fibre (Abdul-Hamid and Luan, 2000; Daou and Zhang, 2011). A bad emulsion's stability index is just 50%, while a good emulsion is larger than 94% (Wang & Kinsella, 1976). Lower protein content in both AEDF and UAEDF samples may have contributed to the lower EA values in this study (Table 6). The cation exchange capacity (CEC) of the Queen pineapple waste extracted dietary fibre was significantly (p < 0.05) enhanced by the UAE method. The presence of phenol groups in insoluble dietary fibre (lignin) determines the CEC of fibre (Liu et al., 2021). The UAEDF has a greater CEC value which could be due to its higher insoluble dietary fibre content (Table 6). The CEC appears to be connected to the fibre fluid permeability, which may be enhanced due to a more porous surface and less ordered fibre structure which raised the CEC of the dietary fibre (Fig. 3a,b,c). The increased fluid permeability into dietary fibre was also demonstrated by the higher WHC and SC values of UAEDF (Table 6). The variation in findings of functional properties of extracted dietary fibre results can be attributed to the different extraction methods (UAE and AE) from the Queen pineapple waste sample.

3.10. Glucose adsorption capacity

The glucose adsorption capacity of pineapple waste, AEDF and UAEDF at different glucose concentrations (1mM, 10mM, 50Mm,100mM and 200mM) have been shown in Table 7. All the extracted dietary fibre could adsorb glucose effectively and the glucose adsorption capacity was directly proportional to the glucose concentration (Fig. 6).

Table 7

Glucose adsorption capacity (GAC mmol/g)
Glucose Conc (mM)	Pineapple waste DP	AEDF	UAEDF
1	3.58 ± 0.74	5.35 ± 0.17	11.12 ± 0.96
10	6.83 ± 1.05	8.84 ± 0.16	20.94 ± 0.12
50	13.69 ± 0.34	16.48 ± 0.10	40.38 ± 1.29
100	20.84 ± 0.17	25.84 ± 0.17	59.87 ± 0.45
200	27.47 ± 0.28	33.09 ± 1.04	80.52 ± 0.13
Values are mean ± SD of triplicate determinations.
*DP- Dried powder, AEDF- Alkali extracted dietary fiber, UAEDF- Ultrasound-assisted extracted dietary fiber

The UAE approach considerably enhanced the GAC of Queen pineapple peel waste extracted dietary fibre, which could be attributable to the increased number of cavities or voids on the surface of the dietary fibre. The ability of dietary fibre to bind to glucose is thought to be one of the most essential aspects in determining its ability to adsorb lipophilic components. Increasing the porosity and specific surface area of dietary fibre can increase the number of glucose molecules entrapped within the fibre network, resulting in a higher GAC (Chau et al., 2007; Zheng et al., 2021). There are several similarities in the effects of extracted dietary fibre on glucose. The increased GAC of UAEDF could be related to more fibre binding sites being exposed.

3.11 Determination of GDRI

GDRI is an in vitro method for analysing the effect of samples on the delay in glucose absorption in the gastrointestinal tract. Analysis of variations in glucose diffusion in dialysate with the addition of pineapple waste dried powder, AEDF, UAEDF along with the control has been presented in Table 8. With the increase of time from 30 to 180 min, the concentration of glucose in the dialysis membrane with the samples (pineapple dried powder, AEDF and UAEDF) elevated from 15.16-168.53 µg/mL, 13.38-142.38 µg/mL, 12.86-135.93 µg/mL, respectively. In comparison with the control, the pineapple dried powder, AEDF, and UAEDF significantly decreased the amounts of diffused glucose in dialysate throughout experiment (30 to 180 min).

Table 8

Glucose dialysis retardation index (GDRI)
Glucose in dialysate (µmol)
Sample	30 min	60 min	120 min	180 min
Pineapple waste DP	15.16	40.87	139.37	168.53
AEDF	13.38	26.28	105.29	142.38
UAE DF	12.86	30.32	110.28	135.93
Control	15.72	51.28	148.76	190.6
DP- Dried powder, AEDF- Alkaline extracted dietary fiber, UAEDF- Ultrasound-assisted extracted dietary fiber
Control means dialysis bag without sample containing glucose solution with distilled water only

The retardation index in glucose diffusion among different samples is expressed by the GDRI value at a different time (30, 60, 120, 180 min) and shown in Table 8. At 30 min GDRI of the pineapple peel waste dried powder, AEDF, and UAEDF were found to be 3.53%, 14.87%, and 18.13%, respectively. With the increase in time from 30 to 180min the GDRI values of pineapple peel waste dried powder, AEDF, and UAEDF were 3.53–11.58%, 14.87–25.3%,18.13–28.68%, respectively. The results showed that the UAEDF effectively delayed the diffusion of glucose out of the dialysis membrane than AEDF and pineapple peel waste dried powder. Similar GDRI values were observed in citrus peel extracted dietary fibre (Abirami et al., 2014) and millet bran dietary fibre (Zheng et al., 2021).

Results of the present investigation have evidenced that the Queen pineapple peel waste is high in dietary fibre and can be utilized as a dietary supplement. The optimized parameters of dietary fibre extraction by ultrasonic method revealed a solid:liquid ratio of 1:27.5 g/mL, the amplitude of 46.90%, extraction period of 22.35 min at constant hydrolysis temperature (60°C). Ultrasonic extraction evinced a better yield (86.67%) of dietary fibre compared to the alkaline extraction (64.43%). The former also revealed better functional properties than the latter. FT-IR, XRD, SEM, and TGA of the extracted UAEDF results evinced a typical polysaccharide structure with all the functional groups, semi-crystalline pattern, honeycomb structure with a rough surface, and good thermal stability as compared to the AEDF and dried powder, respectively. Increased surface porosity and semi-crystallinity of the dietary fibre after ultrasound-assisted extraction are likely the cause of the improved cation exchange capacity and glucose adsorption capacity. The lower emulsifying capacity value was found due to the fibre’s decreased protein content. As a result, changes in UAEDF physicochemical and functional properties were attributed to a more porous surface structure and exposure of fibre binding sites due to semi-crystallinity, and chemical composition alterations. When comparing the dietary fibre with various raw materials it was observed that the UAE extracted dietary fibre from Queen pineapple peel is also at par concerning various properties as mentioned above. Therefore, it may be a noble dietary fibre for developing different food products e.g., bakery products (viz., whole grain bread, noodles, biscuits, and steamed bread.), meat products (viz., sausages, meatballs, surimi, meat emulsion), dairy-based products (viz., yogurt, milk-based beverages) and also as an additive for product development (Yang et al., 2017).

Author’s contribution:

P.D. did the conceptualization; P.D. and S.C.D. formulated the methodology and carried out the formal analysis and investigation; S.C.D. Supervised the whole work and made the availability of resources; Original draft preparation by P.D. and S.C.D. and both the authors reviewed the manuscript.

Funding declaration:

There is no external funding.

Conflict of interest:

There is no conflict of interest among the authors.

Data availability:

The data generated during the analysis / current study are available in the main manuscript file, in the tables file and in the figures file.

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No competing interests reported.

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Effect of ultrasound-assisted extraction of dietary fibre from the sweetest variety Queen Pineapple peel waste of Tripura (India)

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Abstract

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Highlights

1. Introduction

2. Materials And Methods

2.1 Raw materials and Chemicals

2.2 Extraction of Dietary Fiber

2.3 Experimental design

2.4 Analysis of the chemical composition of extracted dietary fibre

2.5 Fourier transform infrared spectroscopy (FTIR) analysis of extracted dietary fibre

2.6 Scanning electron microscope (SEM) analysis of extracted dietary fibre

2.7 XRD analysis of extracted dietary fibre

2.8 Thermo-gravimetry analysis (TGA) of extracted dietary fibre

2.9 Functional properties of the extracted dietary fibre

2.9.1 Water holding capacity (WHC) and oil holding capacity (OHC)

2.9.2 Swelling capacity (SC)

2.9.3 Cation exchange capacity (CEC)

2.9.4 Emulsifying activity (EA) and emulsion stability (ES)

2.10 Hypoglycemic activity

2.10.1 Glucose adsorption capacity (GAC)

2.10.2 Glucose dialysis retardation index (GDRI)

3. Results And Discussion

3.1. Optimization of UAE method

3.2 Determination of significance factors

3.3 Optimum conditions for extraction

3.4 Chemical composition of pineapple waste and extracted dietary fiber

3.5 Fourier Transform Infrared Spectroscopy (FTIR) analysis of extracted DF

3.6 SEM analysis of the extracted DF

3.7 X-RD analysis of the extracted DF

3.8. Thermo-gravimetric analysis (TGA) of extracted dietary fibre

3.9. Functional properties of extracted dietary fibre

3.10. Glucose adsorption capacity

4. Conclusion

Declarations

References

Additional Declarations

Supplementary Files

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