Correlation Between Browning Index and Browning Parameters During the Senesence of Longan Peel

Abstract

The correlations of browning index of longan peel with different browning parameters were studied. The index was determined by rating the visible browning appearance of longan peels stored for eight days at 25°C by eight individuals. The result was then tested for correlation with CIE color parameters (L, a, and b), hue angle (h°), polyphenoloxidase activity, and percentage of ion leakage of the peels. The correlation was also confirmed using oxalic acid solution as an effective anti-browning agent. The results suggested that with or without the organic acid treatment, good correlations were observed only with a value and hue angle (h°), indicating alteration of the darkness. Polyphenoloxidase activity as well as the leakage, however, did not correlate with browning index.

Keywords:

• postharvest browning
• correlation
• polyphenoloxidase activity
• ion leakage
• color changes

INTRODUCTION

Enzymatic browning is a sign of deterioration that is used to indicate the quality of fruits and vegetables (Wills et al., 2007). After the ripening of fruits, cell wall and cellular membranes lose their integrity more quickly, which then stimulates the enzymatic oxidation. The browning process starts from o-dihydroxylphenol, such as catechol, mainly found in the pericarp of fruits being oxidized by polyphenoloxidase (PPO) to browning intermediate, o-benzoquinones. Then, it followed by the condensation or polymerization of the o-quinones to brown products called malanins (Wills et al., 2007; Martinez and Whitaker, 1995; Yoruk and Marshall, 2003).

Browning index (BI) has been used to determine browning assessment in the pericarp of litchi and longan and other fruits (Caro and Joas, 2005; Jiang, 2000; Jiang et al., 2004; Joas et al., 2005; Lichter et al., 2000; Qu et al., 2006; Ray et al., 2005; Sommano et al., 2009; Su et al., 2005; Tian et al., 2005; Zhang et al., 2001; Zheng and Tian, 2006). This index is assessed by rating the visible brown area on each fruit pericarp based on the given scales. However, this method seems to utilize only a personal skill that can vary from person to person. Thus, there is a need to standardize the browning index with quantifiable measurements. Several research studies have been conducted in an attempt to identify the relationship between BI and other browning parameters in order to predict browning behavior of longan and litchi. Previous work on the relationship between BI and anthocyanin, for instance, revealed the inverse relationship (Jiang, 2000; Zhang et al., 2001; Underhill and Critchley, 1994). Zhang et al. (2001) predicted that anthocayanase might play an important role to the browning of litchi pericarp involved in the anthocyanase-anthocyanin-PPO reaction.

Oxalic acid solution has been shown to control postharvest browning of longan and litchi fruits (Zheng and Tian, 2006; Wangchai et al., 2006). This experiment studied the relationship between BI and various kinds of browning assessments. The correlation was confirmed by using oxalic acid as a potential anti-browning agent.

MATERIALS AND METHODS

Fruit Sample

Longan cv. Daw was harvested at a study plot of Mae Jo University, Chiang Mai, Thailand in late December, 2006. The fruits with uniform size were free from disease and were hand-harvested and packed in a basket covered with leaves, which was immediately transferred to the laboratory, Faculty of Agriculture, Chiang Mai University.

Anti-Browning Agent

Oxalic acid solution (Fisher Chemical, Leicestershire, UK) was prepared at the concentration of 1.0% (w/v). Stalks were removed and the fruits were soaked in the solution for 3 min. The fruits were then air dried and stored at room temperature for browning assessments.

Browning Assessments

The browning behavior assessments were analyzed in terms of BI, color, PPO activity, and relative leakage rate.

Browning Index

Similar to sensory evaluation, the browning index was assessed by measuring the visual brown area on each fruit pericarp by eight individuals with the scale of 1 (no browning, excellent quality), 2 (slight browning), 3 (<25% brown), 4 (25–50% brown), 5 (>50% brown) (Jiang, 1999 and Jiang, 2000). The index (%) was calculated using the following formula:

Approximately 50 fruits were investigated every 2 days for 8 days. At day = 0 the fruits showed excellent quality.

Color Measurement

Color of the whole longan fruit was measured by a Minota Chroma meter (Minota®, CR300, illuminant D65 Minolta Holdings, Ltd., Tokyo, Japan). The parameters (L, a, and b) were measured in three positions of the same fruit. Fifty fruits will be determined for each treatment and storage time intervals. Hue angle (h°) was also calculated with the following formula:

Polyphenol Oxidase Activity

Samples were cut and chilled at 4°C prior to enzyme extraction. For the extraction, 3 g of the aril was ground with extraction solution, at a ratio of 8:1 extracting solution and the aril, respectively. The extraction solution was prepared by mixing phosphate buffer solution (0.1 M and pH 6.4) containing 1% polyvinyl pyrrolidone (PVPP) (Nacalai Tesque, Japan). The mixture was incubated overnight at 4°C. After transferring to a capped glass vial, the homogenized mixture was then centrifuged for 20 min at 15,000 rpm at 4°C. PPO activity was assayed by measuring the oxidation of 0.1 M 4-methylcatechol as substrate according to the method described by Jiang (1999). A protein assay was conducted using the Bradford protein assay (Bradford, 1976). One unit of enzyme activity was defined as the amount that caused a change in absorbance of 0.001 absorbance units per minute. The measurement was carried out at day = 2, 4, 6, and 8. Five replications were performed.

Measurement of Relative Leakage Rate

The method used has been described by Zheng and Tian (2006) with a slight modification. Fifteen pieces of longan pericarp discs, 15-cm diameters, were rinsed and incubated for 3 hr in 60.0 mL deionized water. Then the solution was measured for initial leakage solution with a conductivity meter (HI8819N, Hanna Instrument, Kehl Germany). After being boiled for 5 min, each sample was then left to cool at room temperature and the final leakage (total electrolyte) was measured. Percentage leakage rate was calculated as percent of initial electrolytes.

Statistical Analysis

The correlation between browning index and PPO activity was statically analyzed by Pearson's correlation analysis using Satistix® ver8.0 program (Tallahassee, FL, USA).

RESULTS AND DISCUSSION

Browning Index and Use of Oxalic Acid as Anti-Browning Agent

The results showed that the browning index judged by eight participants increased with time (Table 1). After 8 days, the longan had a moderate brown color (BI = 2.38). Su et al. (2005) defined that longan with a browning index higher than 2.0 was considered to deteriorate for marketing quality. Thus, based on our results, longan had a shelf life as short as 1 week at room temperature. The lightness (L value) of the longan peel significantly changed with time [Fig. 1 (2)]. Change in hue angle was also found, but no significant difference was identified. The a and b values, indicating redness and yellowness, respectively, slowly increased from the beginning of storage [Fig. 1 (3) and 1 (4)]. PPO activity was the greatest at the beginning of storage and dramatically decreased over time. PPO activity decreased dramatically after day 2 of storage and remained relatively low from days 4 to 8. Jiang et al. (2002) reported one factor associated with the loss of PPO activity, that is the loss of moisture content from the fruit, most noticeably happened at the beginning of the storage.

FIGURE 1 Browning parameters assessed over 8 days of analysis: (1) browning index (BI); (2) lightness (L value); (3) a value; (4) b value; (5) Hue angle (h°); (6) PPO activity; (7) % leakage. Error bars = ±SD.

TABLE 1 Average Number (%) of Longan in Each Class of Browning Index and Browning Index in Each Day of Analysis

	Percentage of longan in each class of browning index (%)
Day(s)	1	2	3	4	5	Average BI
2	54.58	32.86	9.42	3.42	0	1.62
4	46.58	36.86	14.58	2	0	1.89
6	40	36.86	17.42	4.58	1.14	1.72
8	17.42	40.28	31.14	9.42	1.72	2.38

The increase in ion leakage through cell membranes facilitates enzymatic browning through failure of cell compartmentalization (Lichter et al., 2000). Therefore, this parameter was also used to evaluate fruit quality. However, in the experiment, the relative leakage of longan fluctuated over time.

Oxalic acid was used together with fungicide to inhibit browning in litchi fruit (Zheng and Tian, 2006). It appeared that at concentrations of 2 and 4 mM oxalic acid can effectively control postharvest browning by inhibiting the degradation of anthocyanin. Moreover, pH also plays an important role in pigment stability. At a low pH (pH = 1), litchi seemed to retain its redness better than it did at higher pHs (Zhang et al., 2001). Thus, when oxalic acid was applied as an anti-browning agent, it could increase peel acidity and maintain the redness of the peel. Oxalic acid was also used in controlling pericarp browning of the longan (Wangchai et al., 2006). In combination with ozone, it was found that longan maintained a good quality by minimizing pericarp browning and slightly controlling postharvest diseases. In this experiment, the solution (1%) was used to inhibit browning of longan peel by soaking the fruits for 3 min. However, results on browning assessments illustrated that the solution could not control browning in longan peel. The oxalic solution was effective at 5.0 and 10.0% (Wangchai et al., 2006). Thus, the concentration of oxalic acid at 1.0% in our experiment may not be adequate for this purpose.

Correlation of Browning Index and Browning Parameters

The results in Table 2 illustrated that there was no correlation. The experiment illustrated that there was no correlation between BI and L as well as b values. L ranges from black = 0 to white = 100 illustrating lightness of the peel and positive b value indicates yellow color. Since the correlation of these two values to BI was not found, the postharvest browning might not relate to the brightness and yellowness. Nonetheless, a value and hue angle were well correlated with BI. This could be explained by the increase in a value with browning of the longan peel may be as a result of the change in greenness of the peel. A similar relationship was also observed when an organic acid solution and chitosan were used for controlling pericarp browning of litchi (Joas et al., 2005). Hue angle was around 80° at the beginning of the storage indicating the yellowish-brown color. After 8 days in storage, a decrease in hue angle was found. Therefore, the visual brown color is probably caused by the increasing in red-purple color where hue angle, 0° is red-purple, 90° is yellow.

TABLE 2 Correlation Browning Index and Browning Parameters

	Browning parameters
	BI		L		a		b		h°		PPO		Leakage
	Con	Oxalic	Con	Oxalic	Con	Oxalic	Con	Oxalic	Con	Oxalic	Con	Oxalic	Con	Oxalic
Correlation against BI	—	—	−0.219	−0.184	0.772	0.963	0.486	0.242	−0.834	−0.974	−0.389	−0.494	−0.195	−0.157
Significance	—	—	NS	NS	NS	*	NS	NS	NS	*	NS	NS	NS	NS
Con = control (without antibrowning agent). * = significantly correlated at 95% interval. NS = not significantly correlated.

PPO activity is a direct indicator for deterioration of longan fruit (Jiang et al., 2002; Jiang and Li, 2001). In the current experiment, the correlation between BI and PPO activity was not observed. Although postharvest browning depends largely on PPO activity, browning appearance may not be correlated with this activity (Martinez and Whitaker, 1995; Yoruk and Marshall, 2003). PPO activity measures the rate of o-quinone formation (colorless), which can be detected at its maximum wavelength (Martinez and Whitaker, 1995). The visible brown pigment, however, is the polymerized form of o-quinone. Su et al. (2005) observed that the high activity of PPO in longan was found at the beginning of the storage when no skin browning occurred. Jiang et al. (2004) also added that PPO activity during the storage of litchi was inconsistent.

The leakage of the peel disc was also determined. The correlation between BI to this factor was not identified, which can be implied that the ion leakage is independent of the visual browning appearance of the longan peel. Lichter et al. (2000) also suggested that the increase in conductivity does not relate to obvious change for the improvement in fruit color.

CONCLUSIONS

The relationship between browning index was found only with parameter a and hue angle from the color measurement. This relation was associated with the change from light brown color to darker color: green (as indicated by a value) or red (as indicated by h°). There was no correlation between BI and lightness (L) (and b value), which designates black and yellow color. As PPO activity measured the rate of o-quinone formation in longan peel, the higher rating of visual browning color (the polymerized form, melanin) did not relate to an increase in PPO activity. The conductivity, also, was not correlated to BI.