Potential of Selected Tropical Fruit Peels as Dietary Fiber in Functional Foods

Abstract

The peels of rambutan, durian, santol, longan, longong, Kaeo mango, and Chok Anan mango were evaluated for their potential to be used as dietary fiber for food enrichment. All dietary fiber samples prepared from selected fruit peels showed high content of total dietary fiber (52–84 g/100 g dry matter) and also exhibited the significant difference in dietary fiber quality. All dietary fiber samples were safe for consumption, which was ensured by the results of an acute toxicity test. In summary, peels of tropical fruits used in this study had a great potential to be used as low-caloric functional ingredients for dietary fiber enrichment.

Keywords:

• Fruit
• Peel
• Dietary fiber
• Functional property
• Antioxidant activity

INTRODUCTION

The fruit processing industry is one of the important industries of Thailand that generates a huge amount of by-products, including fruit peel. From the laboratory experiment, it is found that fruit peels represent 20–40% by weight of the total fruit. Generally, these products are used in animal feed and fertilizers although they can also be converted to obtain more valuable products in order to considerably reduce the cost of transportation and disposal. Therefore, there have been increasing interests in exploiting the possibility of using health-promoting components and other valuable ingredients of fruit peels in the preparation of functional food. The major compositions of fruit peels are non-starch polysaccharide and lignin, the important constituents in the plant cell wall. Both non-starch polysaccharide and lignin are generally defined as dietary fiber that neither is digested nor absorbed in the human small intestine.^[1,^] Therefore, attempts have been made to turn many fruit peels into dietary fiber, such as banana,^[2] mango,^[3,^4] citrus,^[5] and lime.^[6]

Dietary fiber has shown beneficial effects in the prevention of several diseases, such as cardiovascular diseases, diverticulosis, constipation, irritable colon, colon cancer, obesity, and diabetes.^[7] Some dietary fiber can retard starch digestion, absorb glucose, reduce glucose absorption, and control postprandial serum glucose level.^[8] Dietary fiber components are usually grouped into two major classes: water soluble dietary fiber (pectic polysaccharides, gums) and water insoluble dietary fiber (cellulose, lignin, some hemicellulose). The physiological properties of dietary fiber depend on chemical structure and mass fraction of its components. Soluble dietary fiber fractions undergo bacterial fermentation in the gastrointestinal tract and influence the metabolism of carbohydrates and fats, while insoluble dietary fibers shorten the gastrointestinal transit time, thus preventing constipation.^[7] Hemicellulose and pectins have a remarkable ability of binding heavy metal ions.^[9] Cellulose and lignin are able to bind heavy metals but in the smaller extent than hemicellulose and pectins.^[10] Dietary fiber from fruit has a better quality than other dietary fiber sources due to its high total and soluble fiber contents, water and oil holding capacities, and colonic fermentability as well as a lower phytic acid and caloric value content.^[5] With the continuing demand of consumers for a unique dietary fiber ingredient, by-products from fruit processing have attracted intense interest as a source of dietary fiber during the past decade.^[11] The residues of apple^[5,^11] and citrus fruit, including grapefruit,^[5] lemon,^[5,^12] and orange,^[5] from juice extraction and citrus peels, such as Valencia orange,^[13] Persa lime,^[13] Maxican lime,^[6,^14] lemon,^[15] and Sweet orange,^[15] were considered as fiber-rich plant foods since they had more than 50% of total dietary fiber,^[16,17] while those of pear and peach had comparably lower contents (35–36%).^[11] Moreover, citrus dietary fiber exhibited better quality than other fiber sources due to the presence of associated bioactive compounds, such as flavonoids, polyphenols, and carotene, with antioxidant activities.^[13,15] However, these kinds of fruit were of sub-tropical, while not very exhaustive information has been conveyed in the literature about peels of many tropical fruits and transformation of industrial fruit processing wastes into a cost-effective and valuable source for dietary fiber food enrichment.

Consequently, the aim of this study was to evaluate the possibility of using peels of the seven tropical fruits, rambutan, durian, longan, Kaeo mango, Chok Anan mango, santol, and longong, which are by-products of the fruit processing in Thailand. All of these fruits are of high economic importance of Thailand in terms of their huge volume of processed fruit export. In addition, from the study conducted in 2009, it was found that these seven varieties of fruits had the largest volume of peels, which caused a severe environmental problem from their disposal. In this study, the chemical composition, dietary fiber composition, characterization of insoluble dietary fiber fraction, functional properties, and toxicity test of dietary fibers prepared from the peels of the seven tropical fruits were explored.

MATERIALS AND METHODS

Materials

The materials used in this study include peels from rambutan (Nephelium lappaccum Linn.), Kaeo mango variety (Manaifera indica Linn.), Chok Anan mango variety (Manaifera indica Linn.), longan (Euphoria longana L.), longong (Lansium domesticum Corr.), durian (Durio ziberhinus Merr.), and santol (Sandoricum koetjape (Burm. f.) Merr.). Rambutan peel was supplied by Malee Sampran Co., Ltd., Thailand; Chok Anan mango was supplied by Sam Roi Yod Co., Ltd, Thailand; Kaeo mango, longan, longong, and santol came from the fruit processed at the Food Technology Laboratory, Thailand Institute of Scientific and Technological Research (TISTR), Pathum Thani, Thailand. Durian peel was obtained from a local market in Thailand. Dietary fiber was prepared according to the following procedure: peels were blanched at 100°C for 3 min, dried at 50°C in a hot-air oven overnight, ground with an Udy cyclone mill (Udy Corporation, Fort Collins, CO, USA), sieved on a 0.5-mm screen, and kept at 4°C in sealed plastic containers for further analyses.

Chemical Composition Analysis

Moisture content was determined by using a moisture meter at 105°C. Ash, protein, lipid, and total starch content were analyzed according to AACC methods 08-01, 46-13, 30-25, and 76.13, respectively.^[18] Total dietary fiber (TDF), insoluble dietary fiber (IDF), and soluble dietary fiber (SDF) contents were determined by an enzymatic and gravimetric method using a TDF-100 kit obtained from Sigma Chemical Company(USA).^[19] Total sugar was determined by the method of Dubois et al.^[20]

Characterization of Dietary Fiber

The method employed was that described by Claye et al.^[21] The Soxhlet extraction was performed using diethyl ether. Two grams of extracted samples were extracted with cold and hot water to remove partially soluble polysaccharides and protein before enzyme treatment. The enzyme-treated fraction was then depectinated using ammonium oxalate solution prior to lignocellulose extraction by potassium hydroxide (under nitrogen purge). The lignocellulose was then extracted for crude cellulose using potassium permanganate and lignin buffer and also lignin by the Klason lignin method.

Water Holding Capacity (WHC) and Oil Holding Capacity (OHC)

To evaluate the WHC and OHC of the dietary fibers, the amount of water and oil released after centrifugation was quantified according to the modification of centrifugation method of Larrauri et al.^[3] The dietary fiber samples in the amount of 0.5 g were stirred into 10 ml of water or soybean oil and left at 30°C for 20 min. Then, the mixture was centrifuged at 3,000 × g for 20 min and the residue was weighed. WHC and OHC were then calculated as gram of water or oil per gram of dry sample, respectively.

Total Phenolic and Total Flavonoid Content Analysis

Total phenolic content was measured by the Folin-Ciocalteau method using gallic acid as a standard; results were expressed as gallic acid equivalent per 100 g dry basis.^[22] The total flavonoid content of seed extracts was determined according to Al-Farsi and Lee.^[23] The total flavonoid content was expressed as grams of catechin equivalent per 100 g of sample on a dry basis.

Antioxidant Activity Measurement

The antioxidant activity of dietary fibers from the fruit peels was measured following the method of Teow et al.^[24] in terms of hydrogen donating or radical scavenging ability using the stable radical, 1,1-diphenyl-β-picrylhydrazyl (DPPH). The 1 g of the dietary fiber sample was sonicated with 10 mL of methanol for 30 min. The extract was filtered through No. 1 Whatman paper. The diluted sample (20 μL) was pipetted into 180 μl of DPPH solution to initiate the reaction. The absorbance was read every minute at 515 for 60 min using the microplate reader equipped with a Magellan reader software (Tecan, USA). Methanol was used as a blank. Trolox (0, 100, 200, 300, 400, and 500 μM) was used as a standard. The EC₅₀ value, defined as the concentration of antioxidant in the reactive system necessary to decrease the initial DPPH concentration by 50%, was calculated from the results.

Acute Oral Toxicity Test

The acute oral toxicity test was conducted according to the test guideline No. 420: Acute oral toxicity^[25] at the Pharmaceutical and Natural Products Department, Thailand Institute of Scientific and Technological Research (Pathum Thani, Thailand). The dietary fiber (DF) sample was dispersed in 1% CMC solution (w/v) for injection by syringe. After fasting for 16 h, the sample was administered orally to male (n = 5) and female (n = 5) rats in both treatment and control groups at a dose of 2000 mg/kg body weight, and the animals were kept under observation for 14 days. On day 15, all survival rats were euthanized by CO₂ asphyxiation and subjected to necropsy. The mean of body weight gain of the rats of the treatment group was calculated in comparison to those of the control group using Student’s t-test.

TABLE 1 Chemical composition of dietary fibers from the peels of different fruit varieties

Dietary fiber	Protein	Fat	Ash	Starch
Rambutan peel	6.36 ± 0.17^a	0.89 ± 0.07^a	3.11 ± 0.03^a	0.48 ± 0.05^a
Durian peel	5.48 ± 0.10^b	0.82 ± 0.01^a	3.58 ± 0.15^a	2.55 ± 0.17^b
Santol peel	5.89 ± 0.20^c	4.09 ± 0.16^b	2.74 ± 0.07^ab	0.93 ± 0.11^c
Longan peel	7.12 ± 0.25^d	0.43 ± 0.04^c	5.74 ± 0.15^c	0.56 ± 0.01^a
Longong peel	11.50 ± 0.09^e	12.01 ± 0.12^d	4.58 ± 0.08^d	0.59 ± 0.00^a
Kaeo mango peel	4.74 ± 0.12^f	2.00 ± 0.01^e	2.86 ± 0.07^ab	1.14 ± 0.07^d
Chok Anan mango peel	5.87 ± 0.14^c	3.36 ± 0.04^f	2.55 ± 0.00^b	0.84 ± 0.03^c

Values are given in g/100 g of dry matter and are means ± standard deviations of triplicate measurements. Treatments followed by the same superscript letter in a column were not significantly different (P > 0.05).

Statistical Analysis

All analyses were performed in triplicate. Data were subjected to the analysis of variance (ANOVA) test followed by the Duncan multiple range test to compare means at the 5% significance level.

RESULTS AND DISCUSSION

Chemical Composition Analysis

The chemical compositions of the DF samples from different kinds of fruits in dry weight are summarized in Table 1. DFs from all fruit peel samples had low protein contents, ranging between 4.74 and 7.12 g/100 g DM in Kaeo mango peel DF and longan peel DF, respectively, except for longong peel DF having a significantly higher content. Fat content of fruit peel DF was low, between 0.82 g/100 g DM in durian peel DF and 4.09 g/100 g DM in santol peel DF, except for longong peel DF exhibiting the highest content (12.01 g/100 g DM). The low fat content represents an advantage of the products with a reduced caloric value. Ash content ranged between 2.74 g/100 g DM in santol peel DF and 5.74 g/100 g DM in longan peel DF. Fat, oil, and protein contents of Kaeo and Chok Anan mango were comparable to values previously reported for Hayden mango.^[3] Starch content of fruit DF was very low compared to that in banana peel (11.1–37.6 g/100 g dry matter).^[2]

Dietary Fiber Composition

Table 2 shows total dietary fiber (TDF), insoluble dietary fiber (IDF), and soluble dietary fiber (SDF) contents of DFs from peels of different kinds of fruits. The DFs had TDF contents of more than 50 g/100 g dry matter; according to Femenia et al.^[16] and Larrauri.,^[17] these products could be considered as rich sources of DF. The TDF contents of the DFs obtained in this study were higher than those found in the DF concentrates from the processing by-products of fruits and greens (35.8–38.8 g/100 g dry matter) reported by Grigelmo-Miguel and Martin-Belloso,^[11] but they were similar to those of lemon peel DF (66–70.4 g/100 g dry matter) reported by Ubando-Rivera et al.^[6] and apple pomace DF reported by Figuerola et al.^[5] (60.7 g/100 g dry matter). This implies that these products might be of interest for the food industry, considering its potential application as a functional ingredient in high-fiber dietetic products. In all cases, IDF was the predominant fraction. Longan peel DFs exhibited the highest IDF contents, followed by DFs from durian peel, longong peel, Chok Anan mango peel, santol peel, rambutan peel, and Kaeo mango peel, respectively. The IDF contents in this study was comparable to those found in apple fiber (48.7 g/100 g dry matter) and tomato fiber (57.6 g/100 g dry matter) reported by Claye et al.^[21] Consequently, it was possible that all DFs from fruit peels in this study possessed pronounced effects on intestinal regulation and stool volume, which were related to the consumption of IDF.^[11] SDF ranged between 10.15 g/100 g dry matter for rambutan peel DF and 32.82 g/100 g dry matter for Chok Anan mango DF. Chok Anan mango, Kaeo mango, and santol DF had high SDF contents that were greater than the previously described values for DFs from the residues of banana peel (12–18 g/100 g dry matter),^[26] juice extraction of grape fruit (4–6 g/100 g dry matter), lemon (6–9 g/100 g dry matter), orange (10 g/100 g dry matter),^[5] and DF concentrates from the processing by-products of apple (13.8 g/100 g dry matter), pear (14.1 g/100 g dry matter), orange (13.6 g/100 g dry matter), peach (9.71 g/100 g dry matter), artichoke (10.4 g/100 g dry matter), and asparagus (10 g/100 g dry matter).^[11] Among the samples in this study, longan peel DF had the highest IDF/SDF ratios. The lowest ratio was shown by the Kaeo mango peel DF due to the highest SDF content. The ratios of all DFs in this study, except that of longan peel DF, were relatively low when compared with lemon (9.9:1), apple (12.9:1), grape fruit (5.9:1), and orange (5.3:1) as reported by Figuerola et al.^[5] and close to the well-balanced values according to the recommendation of Spiller^[27] (1.0-2.3:1) in order to obtain the physiological effect associated with both soluble and insoluble fractions.

TABLE 2 Composition of dietary fibers from the peels of different fruit varieties

Sample	TDF	IDF	SDF	IDF/SDF	Insoluble pectin	Hemicellulose	Cellulose	Lignin
Rambutan peel	52.97 ± 1.00^a	42.74 ± 0.96^a	10.22 ± 0.12^a	4.0:1	5.52 ± 0.39^a	8.83 ± 0.27^a	14.61 ± 1.40^a	5.22 ± 0.55^a
Durian peel	79.18 ± 3.46^b	65.13 ± 2.21^b	13.05 ± 1.21^b	4.8:1	4.11 ± 0.08^b	18.51 ± 1.16^b	38.05 ± 1.35^b	2.36 ± 0.24^b
Santol peel	70.74 ± 0.47^c	49.98 ± 0.14^c	20.76 ± 0.34^c	2.4:1	3.87 ± 0.53^b	17.98 ± 1.29^b	15.13 ± 1.27^a	4.50 ± 0.76^c
Longan peel	83.86 ± 1.06^d	73.30 ± 1.81^d	10.55 ± 0.76^a	6.9:1	1.76 ± 0.02^c	2.09 ± 0.24^c	45.51 ± 0.86^c	18.66 ± 0.16^d
Longong peel	70.76 ± 0.43^c	56.63 ± 0.92^e	15.16 ± 1.28^d	3.4:1	9.79 ± 0.59^d	11.31 ± 0.76^d	26.09 ± 0.95^d	4.67 ± 0.17^c
Kaeo mango peel	67.84 ± 0.68^e	38.30 ± 0.85^f	29.54 ± 0.75^e	1.3:1	2.14 ± 1.01^c	11.08 ± 0.95^d	13.83 ± 0.75^a	2.39 ± 0.32^b
Chok Anan mango peel	84.53 ± 1.20^d	50.69 ± 0.23^c	33.84 ± 0.98^f	1.5:1	1.94 ± 0.07^c	13.72 ± 0.44^e	13.20 ± 0.20^a	1.93 ± 0.55^b

Values are given in g/100 g of dry matter and are means ± standard deviations of triplicate measurements. Treatments followed by the same superscript letter in a column were not significantly different (P > 0.05).

TABLE 3 Water holding capacity (WHC), oil holding capacity (OHC), total phenolic content, total flavonoid content, and effective concentration (EC₅₀) of dietary fibers from the peels of different fruit varieties

Dietary fiber	WHC g water/g fiber (dry matter)	OHC g oil/ g fiber (dry matter)	Total phenolic content mg GAE/g fiber (dry basis)	Total flavonoid content mg of catechin equivalent/g fiber (dry basis)	DPPH EC50 (μg/ml)
Rambutan peel	4.87 ± 0.20^a	1.75 ± 0.07^a	278.00 ± 2.26^a	27.38 ± 0.19^a	28.36 ± 1.14^a
Durian peel	11.14 ± 0.27^b	3.20 ± 0.30^b	19.34 ± 0.73^b	2.77 ± 0.01^b	2829.11 ± 28.01^b
Santol peel	7.00 ± 0.60^c	1.58 ± 0.34^c	121.77 ± 6.01^c	24.67 ± 0.60^c	92.14 ± 4.11^c
Longan peel	3.63 ± 1.62^d	2.27 ± 0.66^d	36.49 ± 1.27^d	5.59 ± 0.28^d	188.31 ± 6.24^d
Longong peel	7.18 ± 0.19^c	1.73 ± 0.04^a	30.46 ± 1.24^e	7.41 ± 0.19^e	1469.26 ± 19.12^e
Kaeo mango peel	5.90 ± 0.55^e	1.26 ± 0.05^e	125.40 ±4.04^f	6.07 ± 0.12^f	54.05 ± 2.11^f
Chok Anan mango peel	6.94 ± 0.31^c	2.06 ± 0.41^d	105.55 ± 6.93^g	13.22 ± 0.05^g	82.55 ± 2.41^g
Trolox					109.48 ± 3.32^h

Values are means ± standard deviations of triplicate measurements. Treatments followed by the same superscript letter in a column were not significantly different (P > 0.05).

Characterization of Dietary Fiber

There is sufficient nutritional evidence that TDF values alone cannot predict the actual physiological properties of DF.^[28] Therefore, this study isolated and fractionated IDFs into the major components as shown in Table 2. The highest proportion in the IDF fraction corresponded to cellulose, ranging from 13.20 g/100 g dry basis for Kaeo mango peel to 45.51 g/100 g dry basis for longan peel. Insoluble pectin removed from the DF fractions varied from 1.76 g/100 g dry basis for longan peel to 9.79 g/100 g dry basis for longong peel. As already noticed, the hemicellulose contents were as follows: durian peel DF > santol peel DF > Chok Anan mango peel DF > longong peel DF = Kaeo mango peel DF > rambutan peel DF > longan peel DF. Lignin content was the highest in longan peel DF (18.66 g/100 g dry matter) while values of other peel DFs varied in a small range of 1.93–5.22 g/100 g dry matter.

Water Holding Capacity (WHC) and Oil Holding Capacity (OHC)

WHC and OHC are important properties of DF from both a physiological and technological point of view. The capacity of different types of DF to take up and hold water has been used to explain their faecal bulking properties.^[29] Durian peel DF showed the highest WHC of all samples studied (P ≤ 0.05) with more than 11 g water/g fiber (Table 3), which was higher than peach DF concentrate (9.2–9.3 g water/g fiber),^[30] apple DF (6 g water/g fiber),^[22] and fiber-rice cocoa product (4.76 g water/g fiber).^[31] The low WHC values of some DF samples in this study, especially the mango samples, could be attributed to the loss of some soluble polysaccharide in supernatant fraction, which was clearly observed. OHC is another functional property of some ingredients used in formulated food.^[32] Ingredients with high OHC are useful as emulsifiers for high fat food products. OHC values were in the range of 1.26–3.20 g water/g fiber and durian peel DF exhibited the highest value of all samples. The values of OHC of the fruit peel DF samples in this study were higher than those of peach DF (1 g oil/g fiber) reported by Grigelmo-Miguel et al.^[30] and orange fiber DF concentrate (1.2 g oil/g fiber) reported by Grigelmo-Miguel and Martin-Belloso.^[11] The high OHC of the DFs from fruit peels, especially durian peel, suggested their potential use as a fiber-rich ingredient in food stuff requiring oil retention and cholesterol absorption.

Attempts have been made to correlate WHC and OHC with each of the measured chemical constituents of DFs as concluded in Table 4. The best fit for WHC was found for hemicellulose with a correlation of r = 0.85, and it was negatively correlated with lignin. No significant correlation between WHC and SDF was observed as suggested by Marin et al.^[33] in different varieties of citrus. However, these results were in agreement with previous studies of chia fiber,^[34] which showed no linear correlation between WHC and SDF. It might be due to the loss of some soluble components in the supernatant discarded, especially Kaeo and Chok Anan mango samples. On the other hand, the significant correlation was found for the insoluble fractions retained in the centrifugal tube, hemicellulose and lignin. These findings were in good agreement with that of Pejic et al.,^[35] who found the decrease in water retention after hemicellulose removal and increase in water retention after lignin removal in hemp fibers. The best fit for OHC was found for IDF and cellulose content. No correlation between OHC and lignin was observed as reported by Sosulski and Cadden.^[36] However, Lopez et al.^[37] found that the insoluble fiber fraction contributed to the increase in OHC levels more than the soluble fiber fraction that was in accordance with this study. Moreover, OHC is not only dependent on the charge density and hydrophilic nature of the particles but it relates to the nature of the surface and the density or thickness of particles, which present a capacity to adsorb and bind components of an oily nature also.^[37]

Total Phenolic Contents

There is increasing evidence that consumption of several phenolic compounds presented in natural food may lower the risk of serious health disorders because of the antioxidant activity of these compounds.^[38] The contents of total phenolic compounds determined using the Folin-Ciocalteu method expressed as gallic acid equivalents are shown in Table 3. It was found that rambutan peel DF contained the highest amount (P ≤ 0.05) followed by Kaeo mango peel DF, santol peel DF, and Chok Anan mango peel DF, respectively, while the DF of durian peel, longan, and longong exhibited significantly lower contents. The high phenolic contents in the DF from the rambutan peel, the santol peel, and the mango peel is noticeably higher than the values found in the red grape seed (14.3–22.28 mg GAE/100 g dry weight),^[39] pomace of red grape (26.3 mg GAE/100 g dry weight),^[40] and other dried fruits, such as apple (91.6 mg GAE/100 g dry weight), blueberry (28.32 mg GAE/100 g dry weight), strawberry (11.6 mg GAE/100 g dry weight), and prune (10.3 mg GAE/100 g dry weight), reported by Ishiwata et al.^[41] Flavonoid is another phytochemical that contributes to health.^[42] Flavonoid is abundant in rambutan peel DF and santol peel DF, while other peel DF samples have significantly lower content (Table 3). The total flavonoid contents of rambutan peel DF and santol peel DF were slightly less than those of citrus peel (32.7–49.2 mg of catechin equivalent/g fiber dry basis).

TABLE 4 Linear correlation results (Pearson correlation coefficients) determined to investigate potential relationships between variables

	Insoluble pectin	Hemicellulose	Cellulose	Lignin	TDF	IDF	SDF	WHC	OHC
Insoluble pectin	1.00
Hemicellulose	0.12	1.00
Cellulose	−0.03	−0.35	1.00
Lignin	−0.22	−0.80	0.68	1.00
TDF	−0.43	0.02	0.52	0.28	1.00
IDF	−0.03	−0.24	0.94	0.66	0.68	1.00
SDF	−0.42	0.32	−0.63	−0.53	0.27	−0.52	1.00
WHC	0.21	0.85	0.09	−0.63	0.23	0.13	0.07	1.00
OHC	−0.10	0.21	0.70	0.12	0.52	0.73	−0.39	0.59	1.00

Bold numbers indicate significant difference (P < 0.05).

Antioxidant Activity Measurement

The DPPH scavenging activities of different DFs from the six selections of fruit peels are given in Table 3. The rambutan peel DF showed the highest DPPH radical scavenging activity followed by Kaeo mango peel DF, Chok Anan peel DF, and santol peel DF, respectively. The rest of the samples showed the comparatively lower DPPH antioxidant activity. It was clearly seen that scavenging activity of Trolox used as a positive control was relatively less pronounced than those of DFs from the peels of rambutan, santol, and mangoes, indicating that these DF samples were a good source of antioxidant. The best fit was found with a correlation of R² = 0.87 for a power model expression (y = 258,310x^−1.6885). These results suggested that total phenolic contents and antioxidant activity in DFs from fruit peels did not behave linearly, which was in accordance with the results of Scalzo et al.;^[43] Deepa et al.;^[44] Al-Saikhan et al.;^[45] and Tyug et al.^[46] The low linear regression coefficient or non-linear relationship between total phenolic content and antioxidant activity could not be concluded that phenolic compounds were not responsible of the antioxidant activity. However, this could be explained that the large variation found in the antioxidant activity values was attributed to the differences in the phenolic profiles both qualitative and quantitative among genotypes.^[47] There was no correlation between total flavonoid content and antioxidant activity of the DFs from the fruit peels.

Acute Oral Toxicity Test

In the acute oral toxicological test for all DF samples, all tested rats appeared normal and no mortality occurred within 14 days of observation periods. There was no significant difference in mean of body weight gain of the treated and control rats. Necropsy of all treated rats at the termination did not show any significant pathological changes. These results confirmed that the acute oral minimum fatal dose of all DFs from the seven fruit varieties was greater than 2000 mg/kg body weight indicating their safety for food application.

CONCLUSION

The tropical fruit peels analyzed in this study were very rich in dietary fiber (52–84 g/100 g dry matter). Dietary fibers from different peels displayed different benefits. Kaeo mango peel DF, Chok Anan mango peel DF, and santol peel DF exhibited noticeable properties, as revealed by the high TDF, high SDF, well-balanced DF fraction, low fat, protein and ash content, high total polyphenol and flavonoid contents, high antioxidant capacity, and moderate ability to hold water and oil. Durian peel DF showed the greatest ability to hold both water and oil and high in TDF and IDF contents. However, it had significantly low polyphenol and flavonoid contents and antioxidant capacity. Rambutan peel DF had significantly high polyphenol and flavonoid contents, and high antioxidant ability. Longan peel DF showed the highest TDF and IDF contents containing cellulose and lignin. Longong peel DF exhibited relatively poor DF quality: high fat and protein contents and low antioxidant activity. The chemical compositions, dietary fiber compositions, functional properties, antioxidant capacity, and no acute toxicity made the peels of tropical fruits a great potential to be used as low-caloric functional ingredients for dietary fiber enrichment.