polyphenols from plant materials: extraction...

ISSN: 1579-4377

POLYPHENOLS FROM PLANT MATERIALS: EXTRACTION AND ANTIOXIDANT POWER.

Daniel Franco1, Jorge Sineiro2*, Mónica Rubilar3, Marivel Sánchez2, María Jerez2, Manuel Pinelo4, Noelia

Costoya2, María José Núñez2

1Centro de Investigaciones Agrícolas de Mabegondo (CIAM). Santirso de Mabegondo. 15318. Abegondo. A Coruña

2Department of Chemical Engineering, School of Engineering, USC. Rúa Lope Gómez de Marzoa, s/n. 15782. Santiago de Compostela. Spain

3Centro de Genómica Nutricional Agro-acuícola (CGNA).Universidad de La Frontera. Av, Francisco Salazar, 01145. Temuco, Chile

4Department of Chemical and Biochemical Engineering, Center for Bioprocess Engineering, Building 229, Technical University of Denmark

*[email protected]

ABSTRACT Experimental results on the extraction of several agricultural byproducts using conventional solvents are presented and discussed. The effects of the main process variables (including solvent polarity, liquid to solid ratio, temperature, time) on the yields and antioxidant activity have been studied for each one. Enzyme-aided aqueous extraction of waste vegetable materials with the aim of improving the polyphenols extraction performance was optimized. Additionally, studies regarding the effect of mechanical pulsation are presented. KEYWORDS Polyphenols, plant materials, extraction, antioxidant power

Sineiro et al. EJEAFChe, 7 (8), 2008. [3210-3216]

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INTRODUCTION

A scope of our research in recent years

The search for new antioxidants from vegetable materials has taken a very high attention in the last decade. Many research groups put their focus on the extraction of antioxidants from plants and agro-industrial byproducts, putting this topic at vanguard of search. The use of tea extracts in food products as yoghourts and in cosmetics has been popularized. Our research group started to investigate in this field as a way to valorize the polyphenols (mainly hydroxycinnamic acids) bound to sunflower proteins, which need to be removed in order to obtain a high-quality protein [1]. Valorization of byproducts by the extraction of polyphenols and their use as antioxidants led us to study their extraction from the residues derived from non-conventional oilseeds, to achieve a process with the maximum overall benefit. In this way, we have investigated the antioxidant activity of extracts from Gevuina avellana and Rosa rubiginosa defatted seeds and hulls [2- 4]. The antioxidant activity of the ethanolic extracts from R. rubiginosa was due to the presence of trans-retinoic acid and that one of G. avellana hulls extracts was likely due to the combined presence of benzoic and cinnamic acids and flavan-3-ols.

Thereafter, we studied the extraction of polyphenols and the antioxidant activity of ethanolic, methanolic and aqueous extracts from several agricultural byproducts, as apple pomace [5], grape pomace [6-9], almond hulls [5-6, 10] or murta leaves [11]. Most of these residues contained benzoic and cinnamic acids, together with and flavonoids; these were mainly flavan-3-ols and flavonol glycosides (major flavonoids in murta extracts). In the last years, we put our focus on the extraction of procyanidins, mainly from grape pomace and pine bark [12-13], achieving for this latter material extracts with a content of procyanidins as high as 87% in weight.

The existence of several hydroxyl groups bonded to an aromatic ring provides the molecule with the ability of donating a proton to a radical, so acting as a possible chain-breaking molecule or antioxidant upon secondary oxidation. The stable radical, α,α diphenyl-β-picrylhydrazyl (DPPH) has been widely used for the screening of substances with potential antioxidant activity [14]. The evaluation of antioxidants must be followed also by assays with other systems, as lipids in micellar systems [15-16], bulk oils [2], foods, etc., because the more potent proton donor is not necessarily the best antioxidant in lipidic systems.

In this paper we will summarize some of the aspects widely reported as affecting the polyphenolics extraction yield, through some results obtained by our laboratory in last years, discussing some results of polyphenols extraction yields and DPPH scavenging activity, as affected by several operational variables.

EFFECT OF SOLVENT POLARITY ON ANTIRADICAL POWER.

Influence of process variables

The extraction yield and the antioxidant activity of the extracts from plants highly depend on the solvent polarity, which determines both qualitatively and quantitatively the extracted antioxidant compounds. The highest yields are usually achieved with ethanol and methanol and their mixtures with water, although other solvents have been widely used in the


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extraction of polyphenols from plants, as ethyl acetate or acetone. Water and ethanol are those most widely used because of their low toxicity and high extraction yield, with the advantage of modulating the polarity of the solvent by using ethanol/water mixtures at different ratios. The main drawback of the aqueous extraction is the low yield in antioxidants with low polarity or liposoluble antioxidants as, for example, the carotenoids. Solubility of polyphenols depends mainly on the hydroxyl groups and the molecular size and the length of hydrocarbon.

Some results obtained from Rosa rubiginosa and Gevuina avellana Mol. seeds can be seen in Table 1. There is a balance between polarity and polyphenolics extraction yield: ethyl acetate was the less polar solvent, rendering low extraction yield and a moderate activity as DPPPH inhibition percentage, whereas ethanol, which was slightly more polar, was the best extracting solvent. In this case those more polar solvents, as methanol or water, did not render the best extracts, because the major radical scavenger was trans-retinoic acid.

Table 1.- Proton donor capacity, as DPPH inhibition, of the extracts from Rosa rubiginosa and Gevuina avellana.

Solvent Concentration (g/L)

Seed DPPH inhibition (%)

Polarity

Ethanol 1 Rosa rubiginosa 80.5 5.2 Ethyl acetate 1 “ 53.9 4.3

Water 1 41.04 9 Methanol 1 52.2 6.6 Ethanol 1 Gevuina avellana 13.2 5.2

Ethyl acetate 1 9.24 4.3 Water 1 13.9 9

Methanol 2 14.2 5.4 Methanol 8 14.33 5.4

From these observations, we often used ethanol and water, controlling variables as

temperature, time of extraction and liquid/solid ratio for the extraction. The results for aqueous and ethanolic extraction of phenolics from grape pomace,

according to an experimental design for analyzing the effects of time of extraction (t, minutes), liquid-to-solid ratio (L/S, mL/g) and temperature (T, ºC) are shown in Table 2. Higher polyphenols concentration was found with ethanolic extracts at 50 °C, also being the extracts, which showed the highest DPPH radical scavenging activity. In these conditions polyphenols concentration values with ethanol were about twice those found in water. Table 2.- Experimental design to study the effects of temperature, liquid-to-solid ratio and temperature on the

polyphenols and DPPH scavenging activity of Garnatxa grape pomace extracts [7]. Conditions: Phenols concentration (ppm) DPPH inhibition (%) Experiment t L/S T Ethanol Water Ethanol Water

1 30 5 25 19.7±0.1 4.8±0.47 7.65±0.41 1.20±0.05 2 30 5 50 38.8±0.3 18.3±0.14 13.40±1.02 4.70±0.42 3 30 1 25 62.4±0.3 22.1±2.20 21.70±1.45 5.80±0.48 4 30 1 50 140.5±0.7 42.2±4.11 48.73±1.87 10.20±1.00 5 90 5 25 28.9±0.2 20.0±1.10 10.81±0.56 4.70±0.21 6 90 5 50 51.8±0.3 36.3±2.80 18.60±1.36 7.40±0.46 7 90 1 25 88.9±0.5 30.0±2.45 32.00±2.21 6.80±0.51 8 90 1 50 170.9±10.7 82.8±8.10 68.00±2.84 20.70±1.93


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As can be seen in Table 2, phenols concentration and antiradical power were directly related. Besides, when extracts are concentrated, they can saturate DPPH, thus not being the inhibition percentage a good indicator. In these cases, EC50 value, the polyphenols concentration which achieves a 50% of DPPH inhibition, is preferred.

Other non-conventional plant material we have studied was murta (Ugni molinae Turcz.), a wild shrub grown in Chile. The antioxidant activity of murta leaves, according to phenols concentration in several extracts is shown in Figure 1. In these assays, ethanol and methanol were the best solvents and DPPH inhibition percentages were very high for polyphenols concentration values higher than 0.5 g/L, reason by which this material shows great possibilities for several fields (food, cosmetic, etc.)

Figure 1.- Effect of polyphenols concentration (as gallic acid equivalents) on DPPH inhibition of Ugni

molinae (murta) extracts in ethanol and methanol.

ENZYME-AIDED EXTRACTION OF POLYPHENOLS

Reduction of particle size increases the polyphenols extraction rate and the extraction yield. This effect is achieved usually by grinding and by enzymatic treatment, which performs at microscopic level. Several studies reported the increase of extraction yield by the action of pectinases, cellulases and hemicellulases [17-18]. It is shown in Figure 2 the effect of enzymatic treatment upon the antioxidant activity of grape pomace from a wine-making facility (the pomace did not undergo distillation) treated with Cellubrix® (cellulase and β-glucosidase activities) and Olivex® (a mixture of pectinases, cellulases and hemicellulases). The enzymatic treatment with multi-activity enzymes usually increased the polyphenolics extraction yield, but this effect was not always followed by an increase of antioxidant activity, as wase observed for Cellubrix. Differences were measurable, although small, because the extracts showed values near 90 %, thus the substrate was almost saturated. The multi-activity features of Olivex® gave as result a statistically significant increase of antioxidant activity respect to controls.

EFFECTS OF PULSING FLOW ON CONTINUOUS EXTRACTION

When extraction is performed as a continuous operation, mass transfer becomes decisive in the efficiency of the operation. The application of pulsing flow has demonstrated to be beneficial for obtaining a maximum extraction yield; its effect was studied in several


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research works as, for example, on the extraction of sunflower and grape pomace polyphenols in an immersion extractor. Pulsing flow improved the concentration gradients into the extractor, also originating micro-turbulence zones. It was not observed influence of pulsation on the antioxidant activity of extracts (data not shown), but it was observed an increase of the area under the curve, so being obtained an increase of yields. An example of grape pomace extraction with a particle size lower than 0.5 mm, as affected by ethanol flow-rate is shown In Figures 3a to 3c. The extraction yield was flow-rate-dependent: the higher the flow-rate the lower the effect of pulsing flow.

Figure 2.- DPPH scavenging activity (%) of grape pomace extracts from wineries, treated with Cellubrix® and

Olivex®. (b1, b2: “blanks”, 1h, 2h: 1 or 2 hours treatment)

a)

b)

c)

d)

Figure 3.- Effect of pulsing flow on the extraction of polyphenols from grape pomace from Garnatxa variety at 1 mL/min (a, b) and 2 mL/min (c, d) flowrates.

“a,c”: Particle size < 0.5 mm; “b,d”: Particle size between 0.5 and 1 mm.


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PERSPECTIVES

Agricultural residues have a great potential as source of antioxidants, many of which are polyphenols. Solvent and process variables must be carefully chosen to optimize their extraction. Operation method in continuous extraction (by percolation or immersion) is also important, because it improves the mass transfer, for example, by applying pulsing flow. Other strategies as enzymatic treatment can be suitable to maximize the antioxidant yields and activities. The antioxidant power usually is related to phenolics content, but not always. Antioxidant activity of every extract must be measured by several methods: radical scavenging activities (DPPH), oxidation of lipids (TBARS), micellar systems, etc., because none of them is representative of all real system; for example it is possible to obtain extracts with high radical scavenging activity but unable to protect an oil for oxidation because of its low miscibility. ACKNOWLEDGEMENTS

To Spanish Education Ministry and Xunta de Galicia by Projects AGL2006-12210-C03, PGIDIT04PXIC26503PN, and 07TAL003265PR.

REFERENCES

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polyphenols from plant materials: extraction...

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