Peach skin is a byproduct from the further processing of fresh peaches with the potential to be recovered and utilized as a natural antioxidant. Color analysis, phenolic content and antioxidant activity of peach skin from 13 varieties of peaches grown in South Carolina were determined. Color analysis indicated that Norman, Cary Mac, Ruby Prince and Flame Prince differed from other varieties of peaches. Antioxidant activity of peach skin extracts were evaluated by the total phenolics (TP), DPPH free radical scavenging (DPPH), ferric reducing antioxidant power (FRAP) and ferrous ion chelating (FIC) assays. The range of total phenolics content was 8.38 - 18.81 (gallic acid equivalent mg/g dry weight). The total phenolic content was highly correlated to DPPH and FRAP activity in peaches ranging from 8 - 23 AE/mg and 5 - 12 AE/mg, respectively. Three peach varieties with skins having the greatest antioxidant capacity were Red Globe, Scarlet Prince, and O’Henry.
Peach (Prunus persica) is one of the most popular fruits worldwide partly due to its good taste and nutrient composition [
Antioxidants act by several mechanisms; thus no one assay can capture the different modes of action and therefore several different methods are employed to measure antioxidant capacity. Since 1958, numerous in vitro antioxidant assays have been proposed including 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging assay [
2,2-diphenyl-1-picrylhydrazyl (DPPH), iron chloride hexahydrate (FeCl3∙6H2O), 3-(2-pyridyl)-5,6-diphenyl- 1,2,4-triazine-p,p’-disulfonic acid monosodium salt hydrate(ferrozine), Folin & Ciocalteu’s phenol reagent, sodium carbonate, iron chloride tetrahydrate (FeCl2∙4H2O) were purchased from Sigma-Aldrich. 2,4,6-tris(2- pyrodyl)-s-triazine (TPTZ) was purchased from Sigma-Fluka. L-ascorbic acid, gallic acid were purchased from sigma life science. Ethyl alcohol (absolute, anhydrous, ACS/USP Grade) was purchased from Pharmco-AAPER. Acetic acid glacial was purchased from BDH. Hydrochloric acid solution certified 0.1 N was purchased from Fisher Scientific.
13 cultivars of peaches from South Carolina, namely Summer Gold, Contender, July Flame, Scarlet Prince, Fire Prince, Cary Mac, Ruby Prince, Red Globe, Norman, Bounty, Early August Prince, Flame Prince, O’Henry at harvest stage between July 12 and August 15, 2013, were obtained from a SC producer. These varieties were chosen since these were the most popular during the peak growing season in this region of the Southeast US.
Color of peach skin was measured on a model CR-400 chroma meter (Minolta Co. Ltd., Japan) by placing the colorimeter orifice directly on the peach surface prior to skin removal. Four peaches of each variety were randomly picked, four loacations on each peach were measured using the chroma meter. The color was expressed as CIE 1976 L*, a*, b*, chroma (C*), hue (h*), with L* representing the lightness of the color (L* = 0 yields black and L* = 100 indicates diffuse white), a* represents the redness to greeness of color (a* negative values indicate green while positive values indicate magenta), b* represents the yellowness to blueness of color (b* negative values indicate blue and positive values indicate yellow). C* represents the intensity or purity of color, h* represents hue of color.
Based on the size of different peach varieties, 10 to 15 peaches of each variety were picked for sampling. They were subdivided into 4 groups randomly. Peach skins were knife-peeled by hand and placed in marked sample bags and sealed. Those bags were frozen at −80˚C until analyzed. For each peach variety, four antioxidant assays were performed with one bag of peach skins each replication. In total four replications of each antioxidant assays were analyzed.
Extraction procedures were based on Lim [
Total phenolic compounds in each peach skin extract were determined with Folin-Ciocalteu reagent according to the method of Singleton [
Radical scavenging activity of peach skin extract was measured according to the method of Molyneux [
Reducing power of peach skin extract was determined by FRAP assay described by Benzie and Strains [
Ferrous ion-chelating potential of peach skin extract was determined according to the method of Gulcin [
where A0 is the absorbance of the control, A1 is the absorbance of peach skin sample or EDTA, Conc1 is the concentration of peach skin extracts.
An analysis of variance (ANOVA) was conducted to compare the effect of peach varieties on peach skin color (L, a*, b*, hue, chroma), total phenolic content and antioxidant capacity (DPPH, FRAP, FIC), and when peach variety significantly (P < 0.05) affected an individual peach property, the Tukey post hoc test was used to determine if there was a significant difference (P < 0.05) among 13 varieties of peaches (SAS, 2011). The relationship of four antioxidant assays and peach skin color with phenolic content of peach skin was determined using the Pearson coefficient correlation.
Lightness and hue values varied among the 13 varieties evaluated with Cary Mac, Ruby Prince, Flame Prince being significantly lighter (P < 0.05) than all other varieties except for Contender (
Phenolic content differed across the varieties of peaches tested. Among all the varieties, Red Globe had the highest phenolic content while Ruby Prince had the lowest mean content of phenolics (
had a large variation while the Scarlet Prince variety had both a high phenolic content and a relatively small range of variation in phenolic content (
Color | L* | C* | h* | a* | b* | |||||
---|---|---|---|---|---|---|---|---|---|---|
Cultivar | Estimate means | Standard deviation | Estimate means | Standard deviation | Estimate means | Standard deviation | Estimate means | Standard deviation | Estimate means | Standard deviation |
Bounty | 43.32C | 7.06 | 39.69B | 7.72 | 37.76BCD | 12.03 | 30.24ABC | 6.40 | 24.48DEF | 9.12 |
Cary Mac | 59.07A | 4.97 | 45.49AB | 2.16 | 59.82A | 11.14 | 22.48D | 7.74 | 38.61AB | 4.77 |
Contender | 55.30AB | 6.40 | 47.52A | 3.40 | 49.72AB | 9.66 | 30.08ABC | 5.19 | 36.00ABC | 6.82 |
Early August Prince | 47.03BC | 4.22 | 46.27AB | 3.75 | 40.37BCD | 4.66 | 35.05A | 2.80 | 30.00ABCDE | 4.41 |
Fire Prince | 45.98BC | 9.19 | 42.92AB | 5.76 | 41.93BC | 13.69 | 30.43 ABC | 5.58 | 28.62BCDE | 10.28 |
Flame Prince | 58.83A | 8.34 | 46.92AB | 3.98 | 59.58A | 13.14 | 22.59D | 6.75 | 39.92A | 8.62 |
July Flame | 39.73CD | 4.23 | 41.47AB | 5.39 | 32.98CD | 5.74 | 34.45AB | 3.66 | 22.73EF | 5.76 |
Norman | 33.56D | 5.77 | 30.26C | 8.82 | 27.11D | 6.44 | 26.41CD | 6.61 | 14.43F | 6.69 |
O’Henry | 45.89BC | 8.72 | 43.33AB | 4.83 | 40.18BCD | 11.50 | 32.04ABC | 3.99 | 27.95CDE | 9.03 |
Red Globe | 47.73BC | 13.72 | 40.33AB | 9.69 | 42.36BC | 19.58 | 26.39CD | 6.68 | 27.62CDE | 15.07 |
Ruby Prince | 58.30A | 7.38 | 44.31AB | 2.83 | 50.27AB | 11.05 | 27.63BCD | 5.94 | 33.67ABCD | 6.56 |
Scarlet Prince | 47.48BC | 8.13 | 45.10AB | 6.61 | 42.55BC | 11.52 | 32.11ABC | 5.78 | 30.59ABCDE | 9.05 |
Summer Gold | 41.81CD | 7.09 | 42.10AB | 7.40 | 34.05CD | 8.55 | 34.03AB | 4.35 | 24.10DEF | 8.45 |
A-E mean within the same column with the same subscript are not significantly different (P > 0.05). n = 16.
Ascorbic acid was used as the standard for the DPPH assay thus results were expressed as ascorbic acid equivalent (AE). The Red Globe variety had the greatest radical scavenging activity (
Ascorbic acid was also used as the standard reference for FRAP analysis therefore results were expressed as ascorbic acid equivalents (AE). The order from highest to least reducing ability of peach varieties was Red Globe, Scarlet Prince, Bounty, Norman, O’Henry, July Flame, Fire Prince, Summer Gold, Early August Prince, Cary Mac, Flame Prince, Contender, Ruby Prince (
Results of ferrous ion chelating (FIC) assay differed from the other antioxidant assays which may be due to the fact that FIC assay measures chelating ability, a different mode of antioxidant action compared to other assays used. The order of highest to lowest chelating ability was Early August Prince, Ruby Prince, Norman, Flame Prince, Summer Gold, Contender, Cary Mac, Bounty, July Flame, Scarlet Prince, Fire Prince, Red Globe,
O’Henry (
There is no one assay that measures all aspects of antioxidant capacity. Therefore, antioxidant assays based on different modes of action were performed to evaluate overall antioxidant capacity. Correlation of different antioxidant assays was performed to verify the relationship between assays in evaluating different peach varieties. P value of all the Pearson correlation was less than 0.05. TP and DPPH assays were the most closely correlated with an R value of 0.92.
Pearson correlation coefficients (R), N = 13 | ||||
---|---|---|---|---|
Prob > |r| under H0: Rho = 0 | ||||
DPPH assay | FRAP assay | FIC assay | ||
TP assay | R | 0.92 | 0.88 | −0.76 |
P | <0.0001 | <0.0001 | 0.0024 | |
DPPH assay | R | 0.87 | −0.69 | |
P | <0.0001 | 0.0084 | ||
FRAP assay | R | −0.66 | ||
P | 0.014 |
Correlation of color and total phenolic content was performed to determine the relationship of color and phenolic content. P values of all Pearson correlations were greater than 0.05 and R values were all close to 0 (
The total phenolic content and antioxidant activity of peach skin were affected by peach cultivar. The range of total phenolics content was 8.38 - 18.81 (Gallic Acid Equivalent mg/g dry weight). Babbar [
Previous research has showed that total phenolic content of O’Henry peach skin evaluated by HPLC-DAD
Pearson correlation coefficients (R), N = 13 | ||||||
---|---|---|---|---|---|---|
Prob > |r| under H0: Rho = 0 | ||||||
Total phenolic | L* | a* | b* | C* | h* | |
R | −0.2365 | −0.0212 | −0.2045 | −0.2234 | −0.19 | |
P | 0.4365 | 0.9451 | 0.5027 | 0.4632 | 0.5341 |
method was 120.2 mg/100g fresh weight [
Correlations among TP, DPPH and FRAP assays were all high while the correlation coefficient of FIC assay to the other assays was not as high. This is case since the FIC assay measures chelating ability while the other assays measure phenolics and reducing or radical scavenging which are closely related to phenolic structure. While both DPPH assay and FRAP assay measure reducing reactions, the antioxidant capacity evaluated of DPPH assay was generally higher than FRAP assay. DPPH assay is based on the transfer of hydrogen atom, while FRAP assay depends on the electron transfer, particularly reducing of Fe(III) [
Since peach variety affected antioxidant capacity and phenolic content of skin, use of skin to produce a natural antioxidant additive would require quality control to ensure consistent antioxidant product. Further studies may include HPLC analysis of peach skin components and antioxidant activity test in vivo. Also, since peach skin extracts have strong antioxidant activity, production of a natural additive for use as a food additive could be an economic benefit to the peach industry.