Quality Characteristics of Citrus Peel Jelly

Abstract

Citrus peels are high in flavonoids and can help with nausea, indigestion, and phlegm. Furthermore, the peel is higher in dietary fiber and phenolic compounds than the fruit. However, every year, around 40,000∼120,000 tons of citrus peels are discarded as waste. As a result, citrus peel jelly was created, which can be reused as a functional food. In this study, salinity, color, texture, and antioxidant properties were measured by adding citrus peel powder at 0%, 1%, 3%, 5%, and 7%, respectively. The salinity decreased as the amount of addition increased (P<0.001). The L-value of chromaticity decreased significantly (P<0.001). The a-, b-value increased significantly (P<0.001). As the addition amount increased, the hardness decreased significantly (P=0.002). Total polyphenols, flavonoids, 2,2-diphenyl-1-picrylhydrazyl scavenging capacity, and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) scavenging capacity all increased statistically significantly (P<0.001). Through this study, we confirmed the quality characteristics of citrus peel jelly. Citrus peel jelly, which is high in antioxidant activity, is expected to increase the use of peel and functional foods.

INTRODUCTION

Citrus peels are the dried rinds of Citrus unshiu of the Rutaceae family when it bears mature fruits. In Korea, it has been used as an herbal medicine since ancient times. Citrus peel relieves nausea, indigestion, and phlegm and contains many flavonoids including naringin, hesperidin, diosmin, and tangeretin (Cho, 2021). Furthermore, studies show that citrus peel can help prevent osteoporosis, depression, and anxiety (Lim et al., 2014; Mannucci et al., 2018). The peel of citrus fruits contains more dietary fiber and phenolic compounds than the fruit itself (Brito et al., 2014). However, raw fruits are mainly consumed, and the rinds are discarded as waste during the processing. From 2016 to 2020, the production of open-field citrus in Jeju Island, Korea, averaged 476,000 tons per year (Jeju Citrus Federation, 2022). Citrus peels are estimated to generate about 40,000∼120,000 tons of waste each year (Cho, 2021).

Jelly is a gel-like food and is a sugar favorite food with less than 20% water content. Depending on the type of gelling agent used such as agar, pectin, gelatin, or starch, various textures can be provided. According to a report on the consumption trend of jelly, the jelly market is growing rapidly because women can relieve stress by chewing jelly and eating it without burden when their mouth is dry (Korea Agro-Fisheries & Food Trade Coporation, 2017). Simultaneously, as public interest in health and snacks grows, so does research into the addition of various functional substances to jelly. The quality characteristics of jelly made with citrus peels powder, which is high in antioxidant activity, were analyzed in this study.

MATERIALS AND METHODS

Materials

Citrus peel powder (Green Natural), agar (Hwain Trading), sugar (CJ CheilJedang), and oligosaccharide (CJ CheilJedang) for the manufacture of citrus peel jelly were used to proceed.

Manufacture

The manufacture of jelly was referred to in the previous study, the sansuyu puree agar jelly manufacturing method of Jeong et al. (2017). Table 1 shows the mixing ratio for each material. Fig. 1 and 2 depict the manufacturing method and the finished jelly. With reference to Kim et al. (2010) for the addition ratio of citrus peel powder, jelly was prepared at 0, 1, 3, 5, and 7% ratios.

Table 1.

Formulation of jelly added with citrus peel powder

Ingredient (g)	Sample
	0%	1%	3%	5%	7%
Citrus peels powder	0	2	6	10	14
Water	170	170	170	170	170
Agar	5.4	5.4	5.4	5.4	5.4
Sugar	12	12	12	12	12
Frutooligosaccharide	12	12	12	12	12

Fig. 1.

Procedure for the preparation of jelly added with citrus peel powder.

Fig. 2.

Visual comparison of jelly added with citrus peel powder.

Salinity and chromaticity

A salinity meter was used to measure salinity (HI-96821, Hanna Instruments SRL). After homogenizing the sample with 3 g of citrus peel jelly in 27 mL of distilled water, the supernatant was measured three times. Chromaticity was measured with a color meter (CR-400, Minolta Co., Ltd.), and L- (brightness), a- (redness), and b- (yellowness) values were repeatedly measured three times.

Texture

The texture of the jelly with citrus peel powder was analyzed using Texture Profile Analysis with a CTX Texture Analyzer (AMETEK Brookfield). Moreover, the following properties were measured: adhesiveness, hardness, cohesiveness, and springiness. Table 2 shows the measurement conditions. Measurements were repeated three times for each sample.

Table 2.

Measurement condition for the texture analyzer

Measuring	Condition
Test speed	30 mm/s
Trigger force	10 g
Distance	5 mm
Sample diameter	30 mm
Sample height	30 mm
Probe	10 mm

Antioxidant properties

Total polyphenol, flavonoids, 2,2-diphenyl-1-picrylhydra-zyl (DPPH) radical scavenging capacity and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging capacity were calculated for antioxidants of citrus peel powder jelly. The total polyphenol content was applied with reference to Zoecklein et al. (1990). The Folin-Ciocalteu reagent is reduced by the polyphenolic compound in the sample to produce a molybdenum blue color. After mixing 0.4 mL of 10% sodium carbonate, 0.4 mL of Folin-Ciocalteu reagent, and 0.4 mL of the supernatant, it was left for 30 min. It was measured with a spectrophotometer at 760 mm (UV-1800, Shimadzu Corp.). The content of flavonoids was measured with reference to Davis (1947). After mixing 1 mL of 90% diethylene glycol, 0.1 mL of 1 N NaOH, and 0.1 mL of the supernatant, the mixture was left at room temperature for 1 h and measured at 420 nm. Kang et al. (1996) were referred for the method of measuring DPPH radical scavenging capacity. Furthermore, 0.1 mL of the supernatant was added to 0.9 mL of 0.2 mM DPPH solution and shaken for 10 sec with a vortex mixer. After blocking the light and leaving it at room temperature for 30 min, the absorbance was measured at 517 nm. The result value was expressed as radical scavenging activity and was compared to the control group without the addition of the sample. Verzelloni et al. (2007) were referred to for ABTS radical scavenging capacity. Moreover, 0.9 mL of an ABTS radical solution prepared by reacting a 2.7 mM potassium persulfate solution and a 7.4 mM ABTS solution in a 1:1 ratio was reacted with 0.1 mL of the sample supernatant for 10 min to measure at 734 nm. The result value was expressed as radical scavenging activity and was compared to the control group without the addition of the sample. The antioxidant activity was measured three times.

$$\text{DPPH radical scavenging capacity (\%)=}\frac{1-\text{Absorbance of sample addition group}}{\text{Absorbance of control}}\times 100$$

$$\text{ABTS radical scavenging capacity (\%}=\frac{1-\text{Absorbance of sample addition group}}{\text{Absorbance of control}}\times 100$$

Statistical analysis

The mean±standard error was obtained using the analysis of variance with SPSS (ver. 22.0 for windows, IBM Corp.) package. The significance was determined at the P<0.05 level. The least significant deviation method was used for the post hoc test.

RESULTS AND DISCUSSION

Salinity and chromaticity

Table 3 shows the salinity and color of citrus peel jelly. Further, the salinity was found to decrease significantly as the amount of citrus peel powder added increased (P<0.001). Son and Lee (2014)’s Salicornia herbacea L. powder-added salinity tended to increase as the addition amount increased. Since the salinity of the citrus peel is thought to be low, the partial salinity of the jelly is relatively reduced as the amount of addition increases. As for the color of the jelly added with citrus peel powder, it was confirmed with the naked eye that the yellow-green color became darker as the amount added increased. As a result, as the amount of citrus peel powder added increased, the L-value decreased significantly (P<0.001). As the amount of addition increased, so did the a- and b-value (P<0.001). Jeong et al. (2017) also discovered that the a- and b-values increased as the addition amount of sansuyu puree agar jelly increased. Jeong and Kim (2008) also demonstrated that as more citrus concentrate was added to the jelly, the b-value increased. The inherent yellowness of citrus peel is thought to have influenced the brightness and yellowness. Furthermore, it is believed that browning occurred as a result of thermal decomposition of polyphenols during the boiling process of citrus peel powder, affecting the color.

Table 3.

Salinity and color value of jelly added with citrus peel powder

Sample	0%	1%	3%	5%	7%
Salinity (g/100 g)	1.03±0.03c	1.07±0.03c	0.93±0.03b	0.80±0.00a	0.80±0.00a
L-value	54.71±0.05e	51.66±0.05d	47.46±0.06c	42.45±0.09b	39.68±0.04a
a-value	—1.11±0.04c	—4.04±0.06a	—2.70±0.08b	—0.39±0.04d	1.49±0.03e
b-value	—4.80±0.05a	10.84±0.10b	20.34±0.14c	27.17±0.07d	34.50±0.08e

Means in a row by different letters (a-e) are significantly different by least significant deviation at P<0.001.

Texture

Table 4 shows the results of the texture of the jelly. The amount of citrus peel powder added reduced adhesion but this was not statistically significant (P=0.151). Hardness decreased as the amount added increased (P=0.002), and cohesiveness increased and then decreased with a maximum value of 0.61 in the 5% addition group (P=0.568). The springiness decreased and increased repeatedly (P=0.097). Jeong et al. (2017) also showed that the adhesiveness decreased as the amount of sansuyu puree agar jelly increased. Even in Lee et al. (2010)’s jelly containing black garlic, the hardness decreased as the amount added increased. In the above study, it is assumed that as the powder added increased, the adhesiveness decreased and the hardness increased due to the intervention of the powder particles in the jelly-forming structure. As a result of the above experiments, cohesiveness and springiness are thought to be unaffected by powder intervention.

Table 4.

Texture of jelly added with citrus peel powder

Sample	0%	1%	3%	5%	7%	P-value
Adhesiveness (mJ)	2.16±1.20	4.65±1.04	3.66±0.54	3.60±0.13	2.06±0.13	0.151
Hardness (g)	333.97±77.55bc	406.80±15.61c	252.50±7.71b	246.57±4.45b	109.90±4.76a	0.002
Cohesiveness	0.62±0.26	0.36±0.06	0.48±0.06	0.61±0.03	0.51±0.04	0.568
Springiness (mm)	9.47±0.15	9.62±0.09	8.83±0.47	9.23±0.69	7.81±0.52	0.097

Means in a row with different letters (a-c) are significantly different by least significant deviation at P<0.05.

Antioxidant properties

Table 5 displays the antioxidant activity of jelly. The total polyphenol content in the control group was 1,416.23 mg/mL, while the 1% supplement group had 1,791.70 mg/mL, the 3% supplement group had 1,762.13 mg/mL, the 5% supplement group had 2,042.37 mg/mL, and the 7% supplement group had 2,343.07 mg/mL. It decreased slightly at 3% but increased significantly overall (P<0.001). Flavonoids increased significantly in the control group to 52.27 mg/mL, 1% in the addition group at 137.40 mg/mL, 193.00 mg/mL in the 3% addition group, 279.80 mg/mL in the 5% addition group, and 359.23 mg/mL in the 7% addition group (P<0.001). DPPH increased significantly with the increasing addition amount reaching 9.06% in the control group, 22.64% in the 1% addition group, 26.37% in the 3% addition group, 33.67% in the 5% addition group, and 37.41% in the 7% addition group (P<0.001). ABTS significantly increased as the amount added increased to 5.46% in the control group, 34.69% in the 1% supplement group, 46.73% in the 3% supplement group, 69.26% in the 5% supplement group, and 80.94% in the 7% supplement group (P<0.001). In the citrus peel jelly of the previous study according to Cho (2021), total polyphenols, flavonoids, DPPH, and ABTS all gradually increased significantly. Through this experiment, it was confirmed that citrus peel powder jelly also had an antioxidant effect.

Table 5.

Total polyphenol, flavonoid, and antioxidant activities of jelly added with citrus peel powder

Sample	0%	1%	3%	5%	7%
Polyphenol (mg/mL)	1,416.23±66.19a	1,791.70±3.72c	1,762.13±12.82b	2,042.37±11.23c	2,343.07±17.95d
Flavonoid (mg/mL)	52.27±0.27a	137.40±1.25b	193.00±1.48c	279.80±16.47d	359.23±2.05e
DPPH (%)	9.06±2.45a	22.64±1.5b	26.37±1.5b	33.67±1.33c	37.41±1.47c
ABTS (%)	5.46±2.17a	34.69±1.37b	46.73±1.25c	69.26±0.82d	80.94±0.71e

Means in a row with different letters (a-e) are significantly different by least significant deviation at P<0.001.

DPPH, 2,2-diphenyl-1-picrylhydrazyl; ABTS, 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid).