Antioxidant Potential and Polyphenol Content of Selected Food

Abstract

Phenolic compounds are natural antioxidants present in plant foods, which are important to human health. This study aimed to measure the total antioxidant potential with the FRAP (Ferric Reducing Antioxidant Power) method and the total polyphenol content of 44 common consumed foods; represented by vegetables, pulses, fruits, cereals and breads. The antioxidant potential ranged: in vegetables 0.033–3.209, in pulses 0.342–0.387, in fruits 0.312–2.833, and in cereals and breads 0.062–1.709 mmol/100 g in fresh mass, respectively. The antioxidant potential of the foods tested was related to the total polyphenol contents.

Keywords:

• Antioxidant potential
• Polyphenols
• Food

INTRODUCTION

Oxidative stress plays a major role in the pathogenesis of most chronic diseases, such as cancer, cardiovascular disease, obesity, diabetes type 2, and hypertension. It is induced by an excessive production of reactive oxygen species (ROS), which affect the integrity of the antioxidant defence system of the body. An over-production of free radicals is responsible for the cellular injury and cell death.[1]

Natural antioxidants present in a diet demonstrate protective qualities against free radicals. The total antioxidant activity of food is due to the integrated action of different compounds.[2] Foods, such as fruits and vegetables are reported to contain a wide variety of antioxidant components, including vitamin C, β-carotene and polyphenols.[3,4] Polyphenols are the most common and widely distributed class of plant secondary metabolites and several thousand different compounds have been identified. They are polyhydroxylated phytochemicals, which contain the two main classes, i.e., flavonoids and phenolic acids. Flavonoids comprise a large group of polyphenols divided into several subclasses, such as flavones (apigenin, luteolin), flavanones (naringenina, hesperetin), flavonols (quercetin, myricetin, kaempferol), flavanols (catechin, epicatechin), and anthocyanidins (cyanidin, malvidin).[5] Polyphenols are major coloring components of flowering plants. They cannot be synthesized by humans, but are an integral part of human diet and play many different roles in human life. Frequent intake of foods rich in polyphenols is associated with the reduced risk of cancer, cardiovascular diseases and stroke.[6]

Precise estimation of the average flavonoid intake is influenced by several factors which include: varied flavonoid availability from food sources, differences in their distribution in plants, and the magnitude of food intake by humans.[7] Recent studies show that flavonoid intake may vary among different nations. The mean total intake of flavonoids in Belgian adults was estimated on 203 mg/person/day (range 19.0–1027.0 mg/person/day),[8] whereas in Spanish diets it provided up to 2590–3016 mg/person/day.[9] These variations in the flavonoid intake may in part depend on the personal dietary habits and food preferences, but they are also influenced by the quantity of phenolic components, which may differ in plants depending on a geographical region and methods of plant cultivation.[10]

The consumption of antioxidant-rich foods is the simplest and most effectively possible way to prevent diseases related to oxidative stress. Therefore, the purpose of this study was to evaluate the polyphenol content and the antioxidant activity of selected food products most commonly consumed in Poland. The FRAP antioxidant activity and total polyphenol contents were determined in vegetables, pulses, fruits and cereal products to create databases of foods according to the antioxidant quality.

MATERIALS AND METHODS

Food Samples

Forty-four food products (19 vegetables; 2 pulses; 12 fruits; 11 cereal products), one kilogram of each product, were randomly purchased in triplicate at different local food markets. Edible parts of selected vegetables and fruits (approximately 200 g) were washed separately, sliced into 5 mm strips and dried using an air-drier (MPM Product, Poland) at 60–70°C to dry matter for about 20 h.[11] Citrus fruits, kiwi fruits and bananas were peeled before drying. Fruits were destoned. Pulses and cereal products (approximately 100 g) were dried at 120°C for 60 min in a convection air-oven (BMT, Czech Republic). Dried products were pulverized in a grinder and stored at room temperature in a desiccator in plastic containers until analysis.[12]

Food Extraction

Pulverized samples (0.25 g) were placed in test tubes with 10 ml of methanol/water (50:50, v/v). The pH was adjusted to 2 using 2M HCl. The tubes were thoroughly shaken at room temperature for 1 h, and then centrifuged at 4000 g for 10 min. Supernatants were collected in clean dry test tubes. Then the residues were extracted again with 10 ml of an acetone/water mixture (70/30, v/v). The methanol and acetone extracts were combined and used to determine the total polyphenol content and antioxidant activity.

Measurement of Total Polyphenol Contents

The total polyphenol content of food was determined using the Folin-Ciocalteau method.[13] In brief, 0.2 ml of an extracted sample was mixed with a 1 ml Folin-Ciocalteau reagent previously diluted in distilled water (1:10) and 0.8 ml of 7.5 % (w/v) sodium carbonate. The absorbance was measured after 30 min at 765 nm on a Spekol 10 spectrophotometer, Carl Zeiss Jena, Germany. The results were expressed as Gallic acid equivalents (GAE).

FRAP Assay

The FRAP (Ferric Reducing Antioxidant Power) was determined according to Benzie and Strain.[14] This method is based on the reduction of Fe³⁺- 2,4,6-tripyridyl-s-triazine (TPTZ) complex to the TPTZ-Fe²⁺ form. A freshly prepared FRAP reagent (1.5 ml) was warmed to 37°C; and a reagent blank reading was taken at 593 nm (A₁) on a Spekol 10 spectrophotometer, Carl Zeiss Jena, Germany. Then, 50 μl of the sample and 150 μl of distilled water was added to the FRAP reagent. The absorbance (A₂) was measured following incubation at 37°C for 4 min. In the FRAP assay, the antioxidant potential of a sample was determined from the plotted standard curve using FeSO₄•7H₂O at a concentration range between 100 and 1000 μmol/l. The presented data are the means of two measurements.

Statistical Analysis

The Statistica 6.1 software (StatSoft, Inc.) was applied for the data computation. Results were expressed as mean values ± standard deviation. Correlations between variables were calculated with the Pearson^'s test. Results were considered statistically significant at p < 0.05.

RESULTS AND DISCUSSION

Vegetables

Antioxidant activity and total polyphenol contents in the present study showed variation within the vegetables tested (Table 1). Fennel and red cabbage were characterized by the highest antioxidant activities (3.209 ± 0.78 and 2.864 ± 0.59 mmol/100 g, respectively), as well as the highest mean content of polyphenols (283 ± 69 and 316 ± 11 mg/100 g, respectively) in fresh mass. Whereas, the lowest FRAP (0.033 ± 0.01 mmol/100 g) and the lowest mean polyphenol content (17 ± 3 mg/100 g) were found in cucumbers. Other studies showed similar tendencies, however the differences between the results were observed. The FRAP antioxidant activity of red cabbage, for instance, was about 35% higher in the present study than in the study of Halvorsen et al.[15] Another example are the results obtained for fennel. Fennel is widely used in Poland for spicing food. Several studies found that fennel demonstrated high antioxidant potential and polyphenol content.[16,17] Parejo et al.[18] identified 42 phenolic compounds in fennel, which are very important plant constituents because of their scavenging ability. The high antioxidant potential of red cabbage was attributed to the presence of antioxidant phytochemicals, especially antioxidant vitamins, including ascorbic acid and ɑ-tocopherol, and phenolic compounds—mainly anthocyanins.[19]

Table 1 Total polyphenol content and antioxidant activity in vegetables and mushrooms in order of the FRAP antioxidant activity

Vegetables and mushrooms	N	Mean dry mass content (%)	Polyphenol content in dry mass (mg/100 g)	Polyphenol content in fresh mass (mg/100 g)	FRAP in dry mass (mmol/100 g)	FRAP in fresh mass (mmol/100 g)
Fennel	3	17.1 ± 2.6	1717 ± 601	283 ± 69	19.44 ± 6.8	3.209 ± 0.78
Red cabbage	3	9.7 ± 1.5	3307 ± 530	316 ± 11	30.16 ± 9.0	2.864 ± 0.59
Sorrel	3	10.2 ± 1.7	1877 ± 298	205 ± 67	17.68 ± 2.3	1.919 ± 0.53
Beetroots	3	11.7 ± 2.4	1527 ± 110	179 ± 47	11.60± 2.7	1.309 ± 0.04
Radish	3	5.2 ± 0.3	1411 ± 127	74 ± 10	8.35 ± 0.9	0.436 ± 0.07
Onions	3	10.7 ± 0.2	1020 ± 30	109 ± 2	2.67 ± 0.5	0.286 ± 0.05
Cabbage	3	8.7 ± 0.7	733 ± 50	64 ± 5	3.23 ± 0.5	0.284 ± 0.06
Red pepper	3	10.6 ± 1.0	1287 ± 150	137 ± 26	2.52 ± 0.5	0.267 ± 0.07
Champignons	3	8.1 ± 0.5	687 ± 100	56 ± 12	3.05 ± 0.7	0.248 ± 0.06
Chinese cabbage	3	6.6 ± 1.7	700 ± 200	49 ± 27	3.45± 0.7	0.236 ± 0.11
Parsley roots	3	24.0 ± 3.6	307 ± 50	73 ± 6	0.87 ± 0.2	0.204 ±.026
Chives	3	6.3 ± 0.1	1180 ± 110	74 ± 6	3.13 ± 0.2	0.196 ± 0.01
Tomatoes	3	6.0 ± 1.3	980 ± 90	60 ± 13	3.22 ± 0.4	0.194 ± 0.05
Carrots	3	11.3 ± 0.3	307 ± 60	35 ± 7	1.70 ± 0.5	0.192 ± 0.06
Leeks	3	12.8 ± 1.5	567 ± 160	70 ± 12	1.22 ± 0.4	0.152 ± 0.03
Celery roots	3	11.6 ± 2.8	333 ± 50	39 ± 14	1.15 ± 0.1	0.132 ± 0.03
Lettuce	3	4.2 ± 0.1	507 ± 40	21 ± 2	2.75 ± 0.5	0.115 ± 0.02
Potatoes	3	20.1 ± 2.1	155 ± 9	31 ± 2	0.38 ± 0.1	0.076 ± 0.02
Cucumbers	3	3.8 ± 0.6	453 ± 10	17 ± 3	0.87 ± 0.2	0.033 ± 0.01

Pulses

Pulses contain a wide range of polyphenolic compounds, including flavonols, flavone glycosides, flavanols, and oligomeric and polymeric proanthocyanidins, located essentially in their seed coat, which can contribute to their antioxidant capacity.[20] The antioxidant capacity and the mean polyphenol content in the present study found: in peas (respectively) 0.387 ± 0.07 mmol/100 g and 191 ± 26 mg/100 g, in beans – 0.342 ± 0.11 mmol/100 g, and 142 ± 20 mg/100 g in fresh mass (Table 2). Halvorsen et al.[15] reported that different varieties of pulses contain variable amounts of antioxidants, measured by the FRAP assay. Our findings show a 3-fold lower FRAP value for beans and peas compared to an Italian study.[21]

Table 2 Total polyphenol content and antioxidant activity in pulses in order of the FRAP antioxidant activity

Pulses	N	Mean dry mass content (%)	Polyphenol content in dry mass (mg/100 g)	Polyphenol content in fresh mass (mg/100 g)	FRAP in dry mass (mmol/100 g)	FRAP in fresh mass (mmol/100 g)
Peas	3	87.9 ± 1.5	217 ± 30	191 ± 26	0.44 ± 0.1	0.387 ± 0.07
Beans	3	90.5 ± 2.0	157 ± 30	142 ± 20	0.38 ± 0.1	0.342 ± 0.11

Fruits

The mean antioxidant activity of fruits in the present study ranged between 0.312 ± 0.002 mmol/100 g in pears and 2.833 ± 1.04 mmol/100 g in strawberries (Table 3). The mean polyphenol content was contained between 72 ± 4 and 239 ± 89 mg/100 g respectively, which is consistent with a study by Brat et al.[22] who found that strawberries and grapes demonstrated the highest polyphenol contents (>180 mgGAE/100 g in fresh mass). In the present study high FRAP and polyphenol content was noted in grapefruit (1.199 ± 0.17 mmol/100 g and 154 ± 35 mg/100 g, respectively), and in oranges (1.179 ± 0.25 mmol/100 g and 148 ± 17 mg/100 g, respectively). The antioxidant activity in oranges was in agreement with a study by Halvorsen et al.[15] but was 30% higher in grapefruit. The polyphenol content in citrus fruits were in accordance with the results of Gorinstein et al.[23]

Table 3 Total polyphenol content and antioxidant activity in fruits in order of the FRAP antioxidant activity

Fruits	N	Mean dry mass content (%)	Polyphenol content in dry mass (mg/100 g)	Polyphenol content in fresh mass (mg/100 g)	FRAP in dry mass (mmol/100 g)	FRAP in fresh mass (mmol/100 g)
Strawberries	3	10.4 ± 2.0	2473 ± 1301	239 ± 89	29.27 ± 15.6	2.833 ± 1.04
Green grapes	3	15.9 ± 1.0	1057 ± 47	170 ± 18	8.71 ± 0.7	1.392 ± 0.14
Grapefruits	3	10.9 ± 0.4	1425 ± 380	154 ± 35	11.08 ± 1.8	1.199 ± 0.17
Oranges	3	11.9 ± 0.4	1247 ± 170	148 ± 17	9.99 ± 2.5	1.179 ± 0.25
Kiwi fruits	3	16.2 ± 1.4	530 ± 70	87 ± 12	5.38 ± 0.6	0.878 ± 0.18
Mandarins	3	13.3 ± 0.5	950 ± 50	126 ± 4	6.52 ± 0.7	0.865 ± 0.07
Apricots	3	14.1 ± 2.1	1053 ± 328	148 ± 47	5.80 ± 0.8	0.812 ± 0.11
Nectarines	3	11.5 ± 0.9	523 ± 235	60 ± 26	6.36 ± 4.8	0.724 ± 0.52
Peaches	3	12.3 ± 2.8	661 ± 120	83 ± 32	5.52 ± 0.9	0.695 ± 0.26
Apples	3	14.1 ± 0.3	543 ± 60	77 ± 9	4.00 ± 0.4	0.564 ± 0.07
Bananas	3	26.0 ± 0.6	400 ± 60	104 ± 14	1.67 ± 0.3	0.435 ± 0.07
Pears	3	14.3 ± 0.5	507 ± 10	72 ± 4	2.18 ± 0.1	0.312 ± 0.02

In Poland, apples are the most popular fruits among consumers.[24] The mean antioxidant activity of apples is rather low compared to other fruits. In the present study, it was 5-fold lower, while polyphenol contents were 3-fold lower than those in strawberries, which were the best sources of antioxidants in this study. Moreover, in contrast to a Canadian study, the polyphenol content in apples was 2-fold lower.[25]

Strawberries are good sources of natural antioxidants. In addition to the usual nutrients, such as vitamins, especially vitamin C, and minerals, strawberries are also rich in polyphenols, such as anthocyanins, flavonoids, and phenolic acids.[26] The acute intake of strawberries may significantly increase the plasma FRAP values in the human subjects.[27]

Cereal Products

Cereal grains contain many bioactive compounds, which are mainly present in bran layers of cereal grains, which provide protection against chronic diseases like coronary disease and some cancer.[28] In this study among the cereal products the highest antioxidant activity (1.709 ± 0,32 mmol/100 g) and the highest polyphenol content (327 ± 41 mg/100 g) in fresh mass was found in buckwheat groats (Table 4). Wholegrain bread presented the highest antioxidant activity and polyphenol content among breads, as compared to food products made of refined flour. Antioxidant activity in rice and wheat flour were 0.062 ± 0.02 and 0.078 ± 0.01 mmol/100 g, respectively. This is about 20–30-fold lower than in the cereals with the highest antioxidant activity. The lowest antioxidant activity in rice may be associated with the lowest polyphenol content (42 ± 10 mg/100 g). These findings correspond with results of a study by Pellegrini et al.,[21] who demonstrated that the FRAP value of flour was the highest in buckwheat flour (whole meal) and the lowest in wheat flour. Buckwheat is a food component of high nutritional value, that is rich in vitamin B₁ and B₂, tocopherols and phenolic substances, namely rutin, catechins and phenolic acids.[29] Holasova et al.[30] have indicated, that the tocopherols and carotenoids do not significantly participate in the antioxidant activity of buckwheat. The highest antioxidant activity was derived from the phenolics compounds such as 3-flavanols, flavonols, and phenolic acids.

Table 4 Total polyphenol content and antioxidant activity in cereal products in order of the FRAP antioxidant activity

Cereals	N	Mean dry mass content (%)	Polyphenol content in dry mass (mg/100 g)	Polyphenol content in fresh mass (mg/100 g)	FRAP in dry mass (mmol/100 g)	FRAP in fresh mass (mmol/100 g)
Buckwheat groats	3	87.0 ± 3.4	377 ± 60	327 ± 41	1.97 ± 0.39	1.709 ± 0.32
Barley groats	3	88.7 ± 2.2	198 ± 40	175 ± 32	0.90 ± 0.36	0.796 ± 0.29
Extruded rye bread	3	92.8 ± 2.4	150 ± 60	140 ± 58	0.42 ± 0.29	0.392 ± 0.27
Extruded graham bread	3	91.9 ± 0.6	127 ± 30	117 ± 29	0.39 ± 0.21	0.359 ± 0.19
Wholegrain bread (rye)	3	58.0 ± 4.1	168 ± 30	98 ± 21	0.54 ± 0.09	0.314 ± 0.07
Wheat rolls	3	71.0 ± 0.4	113 ± 30	81 ± 18	0.32 ± 0.03	0.225 ± 0.02
White bread (wheat)	3	60.1 ± 0.5	113 ± 30	68 ± 19	0.37 ± 0.10	0.220 ± 0.06
Oats	3	90.2 ± 1.9	103 ± 20	93 ± 13	0.24 ± 0.09	0.219 ± 0.08
Noodles	3	90.8 ± 3.9	138 ± 10	126 ± 16	0.14 ± 0.05	0.125 ± 0.05
Wheat flour	3	89.7 ± 1.2	110 ± 20	99 ± 14	0.09 ± 0.01	0.078 ± 0.01
Rice	3	89.4 ± 1.9	047 ± 10	42 ± 10	0.07 ± 0.02	0.062 ± 0.02

Similarly to other studies,[31] the antioxidant activity of foods was positively related to the mean polyphenol content (Fig. 1). Polyphenols are strong in vitro antioxidants, mainly due to their low redox potential and their capacity to donate several electrons or hydrogen atoms.[32] They contain conjugated ring structures and hydroxyl groups that have abilities to scavenge free radicals and reactive oxygen species involved in oxidative processes.[33] Therefore, dietary polyphenols may protect against chronic diseases through inhibition of oxidative damage. The findings of the present study confirm a role of polyphenols for generation of antioxidant activity of foods.

Figure 1 Correlation of the antioxidant activity with the total polyphenol content in the foods tested. (Figure provided in color online.)

CONCLUSION

According to these results, the antioxidant activity of vegetables ranged from 0.033 to 3.209 mmol/100 g fresh mass, for pulses it was 0.342–0.387 mmol/100 g fresh mass, for fruits: 0.312–2.833 mmol/100 g fresh mass, and for cereal products: 0.062–1.709 mmol/100 g fresh mass. Polyphenols have been shown to be responsible for the antioxidant activity of plant materials. The total polyphenol content varied for vegetables: 17–283 mg/100 g fresh mass, for pulses: 142–191 mg/100 g fresh mass, for fruits: 72–239 mg/100 g fresh mass, and for cereal products: 42–327 mg/100 g fresh mass. Food products such as fennel, red cabbage, strawberries, and buckwheat groats, which are good sources of polyphenols, that have an impact on antioxidant potential, should be recommended for frequent consumption.

ACKNOWLEDGMENT

The technical assistance of Ms. Hanna Olechnowicz is gratefully acknowledged.