Chemical Composition, Physicochemical and Technological Properties of Selected Fruit Peels as a Potential Food Source

ABSTRACT

Fruits after processing leave large proportion of peels, which is a nuisance to the environment as a solid waste. The aim of this study was to ascertain the chemical composition, physicochemical properties, and technological properties of selected such fruit peels to determine their suitability for use as natural food ingredients. Peels from four fruit varieties namely pineapple (PP), orange (OP), yellow passion fruit (PFP) and avocado (AP) were collected from fruit waste of Sri Lankan food industries. Proximate compositions of those were analyzed by AOAC approved methods in dry matter basis. Bulk density (BD), color values (L*, a*, b*), water-holding capacity (WHC), oil- holding capacity (OHC), emulsifying Capacity (EC), emulsifying stability (ES), least gelation concentration (LGC), foaming capacity (FC) and foaming stability (FS) of the peel powders were evaluated by using appropriate physical and chemical tests. All the readings were taken in triplicates. The results showed that AP and OP are rich sources of lipids while all the peels are rich in crude fibers. PFP showed significantly high WHC (99.00 ± 0.00%) as well as prominent OHC (30 ± 0.00%) and SC (16.94 ± 0.47). OP and PP can contribute as good bulking agents. Whereas, AP observed weak gelling and foaming abilities. Color values of four peels were significantly different. These results confirmed the utilization of peels of desired physicochemical and technological properties as sources of food fibers or bulk ingredients in food applications requiring oil and moisture retention. Among studied fruit peels PFP having good potential for use as excellent food hydrocolloid.

KEYWORDS:

• Fibers
• food hydrocolloids
• fruit peels
• technological properties

Introduction

Fruits and vegetables account for 45% or the highest food waste generates globally (FAO, 2011). Among them, 14.4% generated at the agricultural stage followed by, consumption (8.8%), processing (8.5%), distribution (7.1%), and postharvest (6.9%) stages (Galanakis, 2015; Royte, 2016). In Sri Lanka, 30% of annual food production wastes without utilization (Samarawicrama, 2016). The issue of food waste can be great prominence in the efforts to combat hunger, raise income, improve food security, and reduce environment pollution (Tielens and Candel, 2014).

Numerous researchers have shown that fruit wastes are rich in nutrients (dietary fiber, vitamins, and minerals), phytochemicals, antioxidants, food ingredients like pectin, natural colors, antifungal compounds, and antimicrobial compounds, etc.(Hassan et al., 2011; Maniyan et al., 2015; Opara et al., 2009). Since fruit wastes are cheap raw materials, those with these properties can favorably utilize in food product development at a very low cost. For instance, pectin, a heteropolysaccharide extracted from apple pomace or citrus peels is used as a commercially acceptable thickening agent, particularly in jellies and jams (May, 1990). Further, mango (Banerjee et al., 2016), banana (Castillo-Israel et al., 2015), pineapple (Ukiwe and Alinnor, 2011), etc. as well as a combination of fruit wastes also successfully tested for pectin isolation (Christy et al., 2014) . Moreover, fruit peels can act as a substrate for the production of another thickening agent called bacterial cellulose (Kumbhar et al., 2015). Furthermore, fruit peels are rich in dietary fibers, which are possible binding and bulking agents and fat mimics. Yet, such kind of technological applications are limited for locally available fruits and vegetables.

Currently, in line with the green concepts, certain Universities and Research Institutes in Sri Lanka having growing interest in this research area. However, before acquiring fruit wastes for mass scale food productions, we should have a clear idea on their composition, physicochemical, and technological properties. Hence, this work is directed toward gathering relevant data for selected fruit peels from industrial effluents.

Materials and Methods

Fruit Peel Powders (FPPs) Preparation

Peels were prepared according to the method described by Do Espírito Santo et al. (2012). Peels of pineapple (Ananas comosus), orange (Citrus reticulata), yellow passion fruit (Passiflora edulis) and avocado (Persea americana) were obtained from a leading fruit processing industry in Sri Lanka. The byproducts were collected just after the industrial processing and stored in a bench scale freezer, in order to avoid the microbial contamination on organic materials. In the next day, the peels were thawed, clean under running tap water and decontaminated in 5 ppm SMS (Sodium Metabisulfate) solution for 30 min. The peels were then dried in an oven (Memmert UN110, USA) with circulating air at 50°C until constant weight. The dry peels were reduced to fine powder in a blender (SL-PB-426, Sisil, Singer, Sri Lanka). Particle size was standardized using sieves (Jainson, India) with mesh diameter 300 µm. The resulting fiber powders were stored in air-tied polythene bags and kept in a refrigerator at 4°C. Approximately, 1 kg of each peels were collected from single batch of fruit, and a single homogeneous powder was used for all experiments.

Yield and Chemical Composition

Percentage yield was calculated as follows:

$\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\mathrm{\%}=\left(\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}/\mathrm{w}\mathrm{e}\mathrm{t}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\right)\ast 100.$

Moisture content was determined after oven drying to a constant weight at 105°C. The crude protein, crude fat, ash contents, and total fiber content were determined in dry basis by using AOAC Method 920.152, 940.26, 963.15, and 991.43, respectively (Cunniff, 1996; Howitz, 2000). Carbohydrates content was determined in dry basis according to Romelle et al. (2016) by the following equation.

$\mathrm{C}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{\%}=100-\left(\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{t}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{\%}+\mathrm{a}\mathrm{s}\mathrm{h}\mathrm{\%}+\mathrm{l}\mathrm{i}\mathrm{p}\mathrm{i}\mathrm{d}\mathrm{\%}+\mathrm{c}\mathrm{r}\mathrm{u}\mathrm{d}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{\%}\right).$

Physiochemical Properties

pH

pH was measured according to the method of Suntharalingam and Ravindran, 1993 using a pH meter (HI-2211, Hanna, USA). Peel powder (10 g) was shaken with 100 ml distilled water, allowed to stand for 30 min, filtered and the pH of filtrate was measured.

Instrumental Color Analysis

The color of the samples was measured using a Hunter Lab color meter (CR 400, Conika Minolta, Japan). Measurements were taken directly at three different locations, after standardization with a white calibration plate (L* = 94.12, a* = 0.29, and b* = 2.73). Color was expressed in Hunter Lab units L*, a* and b*, where L* indicates lightness, a* indicates hue on a green (–) to red (+) axis, and b* indicates hue on a blue (–) to yellow (+) axis (Ozcan and Kurtuldu, 2014).

Technological Properties

Bulk Density (BD)

Sample (5 g) was weighed into 50 ml measuring cylinder. The measuring cylinder was then tapped continuously until a constant volume was obtained. Based on the weight and volume, the apparent (bulk) density was calculated (Tagodoe and NIP, 1994).

Swelling Capacity (SC)

Powder (0.5 g) was weighed in a measuring cylinder (10 ml), added distilled water up to 10 ml and the total volume acquired by the sample was measured. After that, it was gently stirred to eliminate trapped air bubbles and left on a level surface, at room temperature, 24 h to settle the sample. The latter volume occupied by sample was measured and SC was expressed as ml of water/ g of dry base (Larrauri, 1999, as cited in Mora et al., 2013).

Water and Oil Absorption Capacities

Powder (1 g) was mixed with distilled water (10 ml) for 30 s, allowed to stand at room temperature for 30 min and centrifuged (Hettich, Germany) at 3000 rpm for 30 min. The volume of the supernatant was noted. Water absorption (mg/ml) was calculated as the difference between the initial volume of water added to the sample and the volume of the supernatant. The same procedure was carried out to determine the oil absorption capacity by replacing water with vegetable oil (Beuchat´s method, 1977, as cited in Acuña et al., 2012).

Emulsifying Properties

Powder (5 ml) dispersion in distilled water (10 ml) was homogenized with 5 ml of oil for 1 min and the emulsions were centrifuged at 1100 rpm for 5 min. The height of emulsified layer and height of the total content of the tube (TC) were measured. Emulsifying Capacity (EC) was calculated as EC (%) = (ELH/TC) × 100. Emulsion stability (ES) was tested by heating the emulsion at 80°C for 30 min, then centrifuging at 1100 rpm for 5 min. ES was calculated as ES (%) = (ELHA/TCA) × 100; where ELHA = height of emulsified layer after heating and TCA = height of total content before heating (Neto et  al., method (2001, as cited by Acuña et al., 2012).

Foaming Properties

Powder (10 g) was dispersed in distilled water (100 ml), and the suspension was mixed vigorously for 2 min using a blender. The initial solution volume (V1) and final volume after mixing (V2) were noted down. FC was calculated as FC = ((V2-V1)/V1) × 100. FS was determined as the foam volume that remained after 8 h and was expressed as a percentage of the initial foam volume (Coffman and Garcia, 1977 as cited by Acuña et al., 2012).

Gelation Properties

Powder suspensions of 2–10% w/v were prepared in distilled water. Each prepared dispersion (10 ml) were transferred into a test tube, heated in a boiling water bath for 1 h, cooled rapidly under running tap water, and further cooled for 2 h in a refrigerator at 4°C. The least gelation concentration was taken as the concentration at which the sample did not fall from the inverted test tube Coffman and Garcia (1977) (as cited in Acuña et al., 2012).

Results and Discussion

Yield and Chemical Composition of Fruit Peels

The yield of fruit peels are largely vary with the cultivar, maturity level and the processing conditions (e.g., drying method, drying temperature, particle size) (Garcia-Amezquita et al., 2018). Therefore, uniform processing conditions were maintained throughout the test period. Yield and the proximate composition of selected fruit peels are presented in Table 1.

Table 1. Proximate composition of selected fruit peel powders

Property	Sample
	AP	PP	PFP	OP
Yield*	62.21 ± 0.45^a	17.07 ± 0.17^d	14.70 ± 0.07^c	50.39 ± 1.71^b
Moisture (%)	37.79 ± 0.45^c	82.93 ± 0.17^a	85.21 ± 0.01^a	49.61 ± 1.71^b
Ash (%)	2.94 ± 0.05^c	4.56 ± 0.03^b	6.32 ± 0.08^a	4.92 ± 0.04^b
Protein (%)	0.25 ± 0.01^a	0.17 ± 0.03^b	0.17 ± 0.01^b	0.28 ± 0.00^a
Lipid (%)	35.22 ± 0.58^a	0.99 ± 0.16^c	0.47 ± 0.03^c	16.20 ± 0.18^b
Fiber (%)	53.14 ± 0.17^a	11.66 ± 0.23^d	32.85 ± 0.02^b	20.14 ± 0.88^c
Carbohydrates (%)	7.98 ± 0.66^c	82.61 ± 0.10^a	59.01 ± 1.28^b	58.62 ± 0.42^b

Mean ± SD. Values followed by different letters in the same row are significantly different (p < 0.05) according to Tukey’s test (calculations were done in dry basis except for yield and moisture%)

Among four tested peels, AP showed the highest yield (62.21 ± 0.45%) and the PFP (14.70 ± 0.07%) showed the lowest. Yield of PP (17.07 ± 0.17%) and OP (50.39 ± 1.71%) was higher with compare to the findings of Romelle et al. (2016) as 9.17 ± 0.67% and 14.27 ± 0.05%, respectively. .

The moisture content of the peel powders ranged from 37.79 ± 0.45 to 85.21 ± 0.01%. If not processed further, these agro wastes contains high moisture percentages are susceptible for microbial fermentation, produce putrid odors, soil pollution, harborage for insects and can give rise to serious environmental pollution (Shalini and Gupta, 2010).

In the four analyzed fruit peel powders, the protein content ranged from 0.17 ± 0.03 to 0.28 ± 0.00%, which are low with compare to the results of Morais et al. (2017) for dried peels. However, findings of Nicolini et al. (1987) and López-Vargas et al. (2013) were observed the less protein content of OP (1.27%), PFP (0.35%) in dry basis. Proteins may be denatured during the drying process at 50°C because temperatures above 41°C will break the interactions in many proteins and denature them (Clement et al., 2017).

The lipids content of fruit peel powders ranged from 0.47 ± 0.03 to 35.22 ± 0.58% with PFP having the lowest content and AP with the highest level. Low crude fat content of PFP was confirmed by the findings of López-Vargas et al., 2013. However, the fat content of AP and OP were somewhat higher than similar findings (Egbuonu and Osuji, 2016; Mariane et al., 2016). Nevertheless, OP is using for commercial oil extraction (ReserchItali, 2017).

Ash content of the peel powders ranges from 2.94 ± 0.05 to 6.32 ± 0.08%. The ash level observed in OP (4.92 ± 0.04%) and PPP (4.56 ± 0.03%) were comparable to the content obtained with peels of sweet orange (4.89 ± 0.06%) and pineapple (4.39 ± 0.14%) by Romelle et al., 2016; Egbuonu and Osuji, 2016, respectively. The values of AP, PP, and PFP were similar to the findings of; Morais et al. (2017).

The crude fibers and carbohydrates content of fruit peel powders ranged from 11.66 ± 0.23 to 53.14 ± 0.17% and from 7.98 ± 0.66 to 59.01 ± 1.28%, respectively. Carbohydrate content of OP (58.62 ± 0.42) was close to the value (53.27 ± 0.10) of Romelle et al., 2016 for orange peels. Findings of Morais et al. (2017) was adjacent with the fiber contents of AP, PP and PFP.

Physicochemical and Technological Properties

Physicochemical properties of tested peel powders are stated in Table 2, and the technological properties are in Table 3. The properties were tested for PP, PFP, and OP by several researches. However, only a few researches have concentrated on the physiochemical and technological properties of AP. The findings were scanty for local fruit waste generate from manufacturing industries. This functional properties are associated strongly with the chemical structure of polysaccharides and proteins of the peels, which largely affected by grinding, drying, heating, extrusion cooking etc. (Abou-Arab et al., 2016; Garcia-Amezquita et al., 2018).

Table 2. Physicochemical properties of selected fruit peels powders

Sample	pH	Color values
		L*	a*	b*
AP	6.23 ± 0.02^a	55.31 ± 0.24^d	8.25 ± 0.03^a	14.80 ± 0.09^c
PP	4.24 ± 0.01^b	60.79 ± 0.60^c	6.10 ± 0.04^b	22.76 ± 0.06^b
PFP	4.46 ± 0.03^b	82.17 ± 0.08^a	2.5 ± 0.12^c	15.68 ± 0.07^c
OP	4.30 ± 0.01^b	77.26 ± 0.82^b	−2.76 ± 0.01^d	25.74 ± 0.71^a

Mean ± SD. Values followed by different letters in the same column are significantly different (p < 0.05) according to Tukey’s test

Table 3. Technological properties of selected peels powders

Property	Sample
	AP	PP	PFP	OP
Bulk density (g/ml)	0.29 ± 0.00^b	0.39 ± 0.01^a	0.29 ± 0.00^b	0.37 ± 0.01^a
Swelling Capacity (ml/g)	4.36 ± 0.29^c	7.57 ± 0.29^b	16.94 ± 0.47^a	8.98 ± 0.14^b
Water Holding Capacity (ml/g)	6.33 ± 0.14	4.30 ± 0.29	9.63 ± 0.21	6.33 ± 0.14
Oil Holding Capacity (ml/g)	3.10 ± 0.16	1.90 ± 0.08	2.85 ± 0.05	2.40 ± 0.03
Emulsifying Capacity %	36.26 ± 0.20^a	28.00 ± 0.00^b	28.14 ± 1.03^b	27.47 ± 0.53^b
Emulsifying Stability %	41.76 ± 0.97^a	28.00 ± 0.00^b	29.17 ± 0.00^b	25.33 ± 1.33^c
Least gelation concentration %	>10	9.00 ± 0.00	8.00 ± 0.00	3.00 ± 0.00
Foaming capacity %	8.47 ± 0.29^d	22.67 ± 0.67^a	10.00 ± 0.00^c	16.67 ± 1.76^b
Foam stability%	11.84 ± 0.40^b	8.84 ± 0.25^c	18.28 ± 0.96^a	12.26 ± 1.24^b

Mean ± SD. Values followed by different letters in the same row are significantly different (p < 0.05) according to Tukey’s test

No significant differences (P < 0.05) were found in pH of PP, PFP, and OP samples. Due to the acidic pH values they can be used in foods as jams, juices or yogurt. In contrast, highest pH value observed in AP (6.23 ± 0.02), near to neutral pH. pH of PFP was in line with the findings of López-Vargas et al. (2013).

Color is one of the important quality parameters in food products. In here, PFP have the highest lightness (82.17 ± 0.08) while AP have the least (55.31 ± 0.24). Observations of López-Vargas et al. (2013) was showed similar trends for the color values of PFP. Greenness and yellowness of the peels were high may be due to the pigments like chlorophyll and carotenoids (Figure 1). However, diminish of such pigments and maillard browning happened after drying (Abou-Arab et al., 2016). Color changes impart by incorporation of fruit peel powders limited their potential applications in food products. Pleasant aroma was observed in dried OP and PP. Enhancement of the aroma during drying process was recognized may be due to the releasing of volatile aroma compounds in fruit peels at 50°C. Findings of Chairote and Intachum (2016) showed new aroma compounds produced in chili pepper after the heating process. Incensement of some aroma compounds and reduction of some other are possible due to heating (Cai et al., 2017). GCMS chromatography study should run to find the effect exactly by quantifying the aroma compounds.

Figure 1. Appearance of Peel Powders

BD range from 0.29 ± 0.00 to 0.39 ± 0.01 g/ml. There was no significant difference observed in the BD of AP and PFP. Similar trends in PP and OP. BD depends on particle size and initial moisture content. Both are similar in tested powders. High BDs suggests their suitability for use in food preparations. On contrast, low bulk density would be an advantage in the formulation of complementary foods (Akpata and Akubor, 1999; Suresh, 2013).

Highest SC was observed in PFP (16.94 ± 0.47 ml/g), which is adjacent with the SC of wheat flour (17.60 ± 1.85 ml/g) in the findings of Suresh (2013). Similarly, WHC of PFP as high as 9.63 ± 0.21 ml/g could be attributed to the presence of higher amount of carbohydrates and fiber in this powder. WHC is the ability of a sample to bind water when exposed to an external stress (Kunzek et al., 1999; Ngoc et al., 2012). High SC and WHC indicates that the materials tested may increase in medium viscosity, form gels, effect of satiety and increased fecal bolus therefore decreasing glucose, fat and cholesterol absorption. Lowest values for SC and WHC was observed in AP (4.36 ± 0.29 ml/g) and PP (4.30 ± 0.29 ml/g), respectively (Acuña et al., 2012).

A maximum lipophilic tendency (OHC) of 3.10 ± 0.16 ml/g was observed in AP. This result suggested that AP has more hydrophobic interaction sites than other peels. However, all the peel powders are potentially useful in flavor retention, to improve palatability and extension of shelf life in bakery or meat products, in which fat absorption is desirable.

The EC is a molecule’s ability to act as an agent that facilities solubilization or the dispersion of two immiscible liquids. ES is the ability to maintain the integrity of an emulsion (Cho and Almeida, 2012). None of studied peel powders is excellent emulsifiers with the range of 28.00 ± 0.00 to 36.26 ± 0.20% and 28.00 ± 0.00 to 41.76 ± 0.97% for EC and ES, respectively. Because the values are very less than the EC value of commercially available good emulsifier, guar gum which having EC of 90% and ES of pomegranate bagasse was 90.7–93.8% (Mora et al., 2013). The water soluble and salt soluble proteins are emulsifying the fat globules by forming a protein matrix on their surface (Pomeranz, 2012). The powders are poor source of protein may be attribute to the less EC and ES.

Fiber, polysaccharide, and pectin contents are largely effect in gelation properties. In the study, gels were formed even at 2%. But they are not fully stable (Figure 2). AP observed the least gelation properties even though had the highest fiber content, may be due to less pectin contents. OP observed the lowest LGC among four peels.

Figure 2. Test for find least gelation concentration (peel concentrations were increased from right to left as 2 to 10%)

FC is depending on flexible protein molecules that could decrease surface tension (Abou-Arab et al., 2016). In this study, poor protein content of the samples might be attribute to less values for FC (8.47 ± 0.29 to 22.67 ± 0.67%) and FS (8.84 ± 0.25 to 18.28 ± 0.96).

Conclusion

In order to extend the shelf life of fruit peels, removal of additional moisture content is must. However, denaturation of protein due to drying is a drawback. Yet, All the peel powders are good and cheap sources of fiber and ash. Moreover, AP and OP are rich in crude fat as well. Since plant oils are rich in high-density lipids (HDL), they can use to replace animal fat sources. OP and PP have significantly pleasant aroma, which increase with the drying time. PFPP have excellent water absorption capacity and swelling capacity, which have potential to use as food hydrocolloid. This study revealed the possibility of using fruit peel wastes in numerous food applications.

Additional information

Funding

This work was fully funded by Sabaragamuwa University of Sri Lanka through the attribution of a research grant [SUSL/RG/2017/05].