Journal of Food Engineering 96 (2010) 421–426

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Quality evaluation of grape juice concentrated by reverse osmosis Poliana D. Gurak a, Lourdes M.C. Cabral b,*, Maria Helena M. Rocha-Leão a, Virgínia M. Matta b,1, Suely P. Freitas a a b

Federal University of Rio de Janeiro, Cidade Universitária – Ilha do Fundão, Rio de Janeiro, RJ, Brazil Embrapa Food Technology, Av das Américas, 29501, 23020-470, Rio de Janeiro, RJ, Brazil

a r t i c l e

i n f o

Article history: Received 5 May 2009 Received in revised form 5 August 2009 Accepted 15 August 2009 Available online 28 August 2009 Keywords: Phenolic compounds Reverse osmosis Anthocyanins Grape juice

a b s t r a c t The objective of this work was to evaluate the concentration of grape juice by reverse osmosis (RO). Preliminarily, a factorial design was carried out in which the independent variables were transmembrane pressure (40, 50 and 60 bar) and temperature (20, 30 and 40 °C) of the process, and the dependent variables were pH, content of soluble solids, acidity, concentration of phenolic compounds and those of monomeric and total anthocyanins, colour index, colour density, and permeate flux. None of the experiments resulted in significant changes in the juice characteristics. The best process conditions, 60 bar transmembrane pressure and 40 °C, was selected based on the resulting high permeate flux value. Subsequently, a new trial was performed in order to determine whether increasing the temperature from 40 to 50 °C would result in any changes in the juice characteristics. The transmembrane pressure was kept at 60 bar, which was also the maximum value that could be applied by the equipment. Under these conditions, an increase in permeate flux was achieved with no significant difference in the physical or chemical parameters of the product compared to the best condition corresponding to the factorial design. The physical and chemical properties of the concentrated juice increased in proportion to the volumetric concentration factor, indicating the technical feasibility of reverse osmosis for pre-concentrating grape juice. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Today, the consumption of natural food products is associated with a healthier life style, reduction in the risk of some diseases and a better way of life. Most natural foods that are packaged and distributed require some processing. Commercially available foods have a variety of effects on the human body, depending on the industrial processes to which they are subjected during production, commercialisation and consumption. Grape juice consists largely of water (81–86%), with a high concentration of the sugars glucose and fructose. It presents an elevated acidity due to the presence of tartaric, malic and citric acids. These acids ensure a low pH value, guaranteeing equilibrium between acidic and sweet tastes. Regarding its mineral elements, a high potassium value and low sodium value are found. Among the bioactive compounds present in grape juice, phenolic constituents are of great importance because their characteristics are directly or indirectly related to the quality of the juice and affect its colour and astringency (Girard and Mazza, 1998). Due to its rich constitution, grape juice is considered a differentiated beverage with positive energetic, nutritional and bioactive effects (Rizzon and Miele, 1995; Mazarotto, 2005). * Corresponding author. Tel.: +55 21 2410 9623; fax: +55 21 2410 1090. E-mail address: [email protected] (L.M.C. Cabral). 1 Tel.: +55 21 3622 9600; fax: +55 21 2410 1090. 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.08.024

Many researchers have suggested that there is an association between the consumption of bioactive compounds and the prevention of some diseases. Evidence for such an association has increased interest in the behaviour of bioactive compounds during various industrial processes. Thus, in addition to increasing nutritional and pharmacological interest in phenolic compounds, attention to technological aspects of food production has also been growing (Dubick and Omaye, 2001; Gorelik et al., 2008; Hertog et al., 1997; Hollman and Katan, 1999; Leontowicz et al., 2007). Concentration of single-strength grape juice promotes its partial dehydration, resulting in an increase of soluble solids content, improving its conservation and facilitating storage and transport of the product. Concentration of grape juice is traditionally accomplished by vacuum evaporation, which can result in loss of volatile aromatic components, freshness, colour, and vitamins, as well as in a reduction of the antioxidant properties provided by its high content of anthocyanins and phenolic compounds (Mazarotto, 2005). Reverse osmosis is a membrane separation process in which a hydraulic pressure that is greater than the osmotic pressure of the solution is applied so that water permeates from a high to low solute concentration (Girard and Fukumoto, 2000). This process can be applied to concentrate fruit juices, reducing the damage caused by thermal treatment and resulting in superior maintenance of their nutritional and sensory characteristics. The use of reverse osmosis in the concentration of many fruit juices

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has been evaluated, and the results are very promising: concentrated fruit juices of good quality with a soluble solids content ranging from 28° to 35 °Brix are obtained (Sá et al., 2003; Cassano et al., 2003; Cianci et al., 2005; Jesus et al., 2007; Kozák et al., 2008). Kiss et al. (2004) claimed that the use of membrane separation processes to concentrate liquids is less costly than classical vacuum evaporation, justifying their application in the food industry. The aim of this work was to evaluate reverse osmosis as a process for the concentration of grape juice and to determine the effects of the processing parameters on permeate flux, volumetric concentration factor and the quality of the final product. 2. Materials and methods 2.1. Raw material Single-strength grape juice, produced at the industrial processing unit of Embrapa located in the southern region of Brazil, was used as raw material.

Table 2 Process conditions in stationary state (out of factorial design). Trial number

Temperature (°C)

Transmembrane pressure (bar)

8 9a 10a 11a

40 (+1) 50 50 50

60 (+1) 60 60 60

a

Condition out of the factorial design.

the effect of increasing the temperature above 40 °C. In this process, only the temperature of the process was changed since the maximum transmembrane pressure achievable by the equipment is 60 bar, the maximum value used in the factorial design. Therefore, the higher temperature trial was conducted at 50 °C and 60 bar. The process was repeated three times, and the data obtained were compared with data from the best conditions of the preliminary factorial design (Table 2). The statistical Tukey test was performed using Excel software (Microsoft, Office Excel 2000).

2.5. Juice concentration 2.2. Juice processing Reverse osmosis of grape juice was carried out in a reverse osmosis system (Lab Unit 20) supplied by DSS (Denmark). The system was equipped with a plate and frame module composed of HR98PP thin film composite membranes (DSS, Denmark), the characteristics of which were effective membrane area of 0.68 m2 and rejection of 95% to a 0.25% NaCl solution at 25 °C. 2.3. Factorial design In order to evaluate the effects of temperature and transmembrane pressure on the physical, chemical and nutritional characteristics of the product, complete factorial design was carried out with temperature and transmembrane pressure as independent variables, and permeate flux and parameters related to juice quality, as dependent variables (Table 1). During these experiments, the juice was not concentrated, as the retentate and permeate fractions were recirculated to the feed tank under controlled temperature and transmembrane pressure over a two-hour period. All assays in levels –1 to +1 were conducted in duplicate, and the average values of these assays were used to analyse the data. The minimum and maximum levels of each parameter were chosen based on scientific articles on fruit juice concentration: temperature (20–40 °C) and transmembrane pressure (40–60 bar) (Cassano et al., 2003; Rektor et al., 2004; Cianci et al., 2005; Jesus et al., 2007; Kozák et al., 2008). The data from the factorial design experiments were evaluated with the aid of Statistica for Windows, version 6.0 (Statsoft Inc., Tulsa, UK).

2.6. Process evaluation All the processes were evaluated with respect to permeate flux, volumetric concentration factor (VCF) and the quality characteristics of the juice. The permeate flux (measured every 15 min in triplicate) and the volumetric concentration factor (VCF) were calculated using the following equations (Eqs. (1) and (2)):

V ; At VR ; VCF ¼ VF J¼

ð1Þ ð2Þ

where V is the volume permeated during a determined time t, A is the membrane surface, VR is the final retentate volume and VF is the initial feed volume.

2.7. Analytical methods

2.4. Reverse osmosis at higher temperature An additional reverse osmosis process without concentration, beyond the factorial design, was carried out in order to determine Table 1 Process conditions for the reverse osmosis trials based on factorial design. Test

Temperature (°C)

Transmembrane pressure (bar)

1 2 3 4 5 6 7

20 40 20 40 30 30 30

40 40 60 60 50 50 50

(–1) (+1) (–1) (+1) (0) (0) (0)

Concentration tests were carried out at the transmembrane pressure and temperature defined in the first step. The process was conducted in a fed batch mode, starting with 20 L of singlestrength juice in the feed tank. As increased juice concentration in the feed tank would be expected to result in low values of permeate flux, the system was fed with 1 L of single-strength juice at intervals of 10 min, with the goal of keeping the concentration of the juice in the feed tank as low as possible. This process was repeated three times.

(–1) (–1) (+1) (+1) (0) (0) (0)

Samples of single-strength juice, juice processed after 2 h without concentration (factorial design), and concentrated juice were analysed with respect to the following physical and chemical parameters: pH and total soluble solids (AOAC, 1995); total titrable acidity (Riberéau-Gayon et al., 1976); colour index (Riberéau-Gayon et al., 2006); monomeric anthocyanins and colour density (Wrolstad et al., 2005); total anthocyanins (Francis, 1982); degradation index (Wrolstad et al., 2005); total phenolic compounds (Singleton and Rossi, 1965, adapted and optimised by Georgé et al., 2005); antioxidant activity (determined at 30 min by reaction with DPPH (2,2-diphenyl-1-picrylhydrazyl) radical according to Brand-Williams et al. (1995) with the modifications proposed by Kim et al. (2002)).

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3. Results and discussion The results of the factorial design showed that there was no significant difference in any of the quality parameters of juice concentrated under any of the conditions tested. As expected, permeate flux varied with the different conditions (Table 3 and Fig. 1). The process carried out at 40 °C and 60 bar resulted in the highest permeate flux, whereas the process performed at 40 bar gave the lowest permeate flux. Processes conducted at 30 bar and 50 °C had a permeate flux similar to those conducted at 60 bar and 20 °C, emphasising the effect of temperature on permeate flux. This effect is directly related to the feed viscosity, which is exponentially dependent on temperature (Arrhenius Law). The colour of the grape juice is a very important characteristic, as it is the first indicator of quality observed by the consumer. Tonality and intensity of colour can provide information with respect to possible defects or the quality of the raw material. The colour can vary according to the origin and region where the grapes were harvested, as well as due to the particular juice production technology applied (Downey et al., 2006). Colour is also strongly related to the physical and chemical characteristics of pigments, such as anthocyanins, that are present (Bautista-Ortín

et al., 2007). In processes conducted under various conditions of temperature and transmembrane pressure with recirculation of the juice, no significant changes in colour or in total or monomeric anthocyanin values were noted. Several authors have suggested that phenolic compounds are the main source of antioxidant activity in fruits. This was evident in the present work, since grape juice contains large amounts of phytochemical compounds such as anthocyanin pigments and only traces of vitamin C (Kuskoski et al., 2005). Rektor et al. (2004) investigated a variety of membrane separation processes for the preservation and concentration of grape must. These authors reported that, during reverse osmosis, the retention of anthocyanins in purple grape must was 99.5% at 35 °C and 50 bar, evidencing the efficiency of this process. The best process condition (40 °C and 60 bar) was selected based on the high value of permeate flux achieved. As the limit pressure of the reverse osmosis equipment (60 bar) had already been reached, these results induced the planning of a new reverse osmosis experiment at a higher temperature. These conditions were 50 °C and 60 bar. The result of this experiment showed that a further increase in permeate flux occurred when the temperature of the process was increased from 40 °C to 50 °C.

Table 3 Physical and chemical characteristics, and permeate flux of the grape juice submitted to the different operating conditions corresponding to the experimental design.

a b c d e f

Temperature (°C) Pressure (bar)

20a 40a

20a 60a

40a 40a

40a 60a

30 50

30 50

30 50

Total acidityb pH Soluble solids (°Brix) Colour indexc Colour density Degradations index (anthocyanin) Total anthocyanins (mg/100 g)d Monomeric anthocyanins (mg/L)d Phenolics compoundse Permeate flux (L/hm2)f

0.69 ± 0.02 2.98 ± 0.06 13.5 ± 0.6 2.086 ± 0.222 6.56 ± 0.54 2.38 111.3 ± 3.1 85.1 ± 2.8 1.87 ± 0.19 9.70 ± 0.09

0.71 ± 0.00 2.93 ± 0.01 14.3 ± 0.2 2.124 ± 0.117 7.05 ± 0.35 2.43 112.3 ± 2.2 88.2 ± 3.0 2.08 ± 0.02 20.38 ± 0.48

0.70 ± 0.00 2.93 ± 0.01 13.6 ± 0.2 2.029 ± 0.109 6.67 ± 0.33 2.49 108.9 ± 2.8 84.1 ± 4.1 2.04 ± 0.08 11.77 ± 0.00

0.68 ± 0.02 2.92 ± 0.02 13.7 ± 0.0 2.050 ± 0.023 6.73 ± 0.04 2.47 107.0 ± 0.1 84.1 ± 1.0 1.94 ± 0.10 25.4 ± 1.20

0.68 2.96 13.6 2.030 6.64 2.44 109.8 82.8 1.98 19.79

0.69 2.95 13.9 2.177 7.07 2.64 113.7 78.2 2.05 19.69

0.72 2.95 14.3 2.120 6.51 2.85 113.0 72.6 1.93 19.20

Mean of two repetitions. Expressed in g/100 g acid tartaric. Expressed in unit of absorbance. Expressed in malvidin 3,5 diglucosides. Expressed in g/L of acid Gallic. Stabilized flux.

Temperature

30.529

Pressure

Temperature by Pressure

9.505829

4.529412

p=.05 Effect Estimate (Absolute Value) Fig. 1. Pareto chart for permeate flux.

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Table 4 Permeate flux, and physical and chemical evaluation of the original grape juice and that processed for 2 h by reverse osmosis at 60 bar, and 40 °C and 50 °C (without concentration). Condition

Initial juiceA

Temperature (°C) Pressure (bar) Total acidityB pH Soluble solids (°Brix) Colour indexC Colour density Degradations index (anthocyanin) Total anthocyanins (mg/100 g)D Monomeric anthocyanins (mg/L) D Phenolics compoundsE Antioxidant activity (30 min)F Permeate flux (L/hm2) G

40 60 0.710 ± 0.013 2.94 ± 0.02 13.7 ± 0.0 2.078 ± 0.929 6.821 ± 0.100 2.37 ± 0.10 110.99 ± 2.66 92.11 ± 5.76 2.096 ± 0.021 8.703 ± 0.150 –

After 2 h of processA 50 60 0.678 ± 0.004 2.91 ± 0.03 14.0 ± 0.0 2.123 ± 0.126 6.908 ± 0.047 2.64 ± 0.01 109.56 ± 2.34 78.47 ± 2.70 2.019 ± 0.028 8.195 ± 0.990 –

40 60 0.693 ± 0.020a 2.92 ± 0.02a 13.7 ± 0.0a 2.040 ± 0.022a 6.655 ± 0.049a 2.43 ± 0.14a 107.40 ± 0.12a 86.74 ± 10.16 a 2.02 ± 1.108a 9.544 ± 0.299a 25.37 ± 1.20a

50 60 0.667 ± 0.010a 2.91 ± 0.00a 13.9 ± 0.0a 2.061 ± 0.063a 6.624 ± 0.354a 2.57 ± 0.12a 105.38 ± 3.74a 77.43 ± 3.60a 1.95 ± 0.06a 8.167 ± 0.187a 36.49 ± 1.58b

a and b = average with same letters in one line denoting no statistical difference at a 95% significance level. A Mean of three repetitions. B Expressed in g/100 g tartaric acid. C Expressed in unit of absorbance. D Expressed in malvidin 3,5 diglucosides. E Expressed in g/L of acid Gallic. F Expressed in lmol/g de Trolox. G Stabilized flux.

It is important to observe that there was no significant variation (p < 0.05) in any of the quality parameters of grape juice processed at 50 °C when compared with juice processed using the optimum conditions of the factorial design, 40 °C and 60 bar (Table 4). Statistical analysis was not performed for the single-strength grape juice because all experiments were carried out with the same batch of grape juice. The negligible influence of the temperature of the reverse osmosis process on juice quality can be attributed to the extraction step that is used during the production of the juice; this is accomplished by cooking the fruits at high temperature to obtain optimal extraction of the phenolic compounds (Frankel et al., 1998). However, it is also true that exposure of grape juice to high temperature for long periods during extraction, pasteurisation, concentration and/or storage can cause losses of phenolic compounds, as well as degradation of anthocyanins. This was confirmed by Larrauri et al. (1997), who evaluated the effects of various temperatures (60 °C, 100 °C and 140 °C) during concentration on grape juice quality. These investigators showed that, up to 60 °C, no significant

changes in colour characteristics, antioxidant activity (ferric thiocyanate method) or content of phenolic compounds of the grape juice were observed. When the drying temperature was 100 °C or 140 °C, a significant reduction in total extractable polyphenols (18.6% and 32.6%), as well as decreases of 28% and 50%, respectively, in the antioxidant activity of the samples was observed (Larrauri et al., 1997). In this study, it was observed that permeate flux decreased during the concentration process. This is a classical behaviour that can be attributed to polarisation concentration on the membrane surface at the first instants of the process (not measured), as well as to fouling phenomena and to increases in the osmotic pressure and viscosity of the juice due to soluble solids concentration (Fig. 2). All three processes conducted under the same conditions were finished after 1.5 h. This value was previously established, as it was important to determine membrane behaviour under selected conditions of transmembrane pressure and temperature. The concentration of the grape juice in the feed tank reached 28.5 °Brix,

Fig. 2. Permeate flux and soluble solids concentration during the concentration of grape juice at 50 °C and 60 bar transmembrane pressure (a, b and c represent triplicates).

P.D. Gurak et al. / Journal of Food Engineering 96 (2010) 421–426 Table 5 Physical and chemical evaluation of single-strength grape juice, and grape juice concentrated by reverse osmosis.

A B C D E F

Parameters

Single-strength juiceA

Concentrated juiceA

Total acidityB pH Soluble solid Colour indexC 420 nm2 520 nm2 620 nm2 Colour density Total anthocyanins (mg/100 g)D Monomeric anthocyanins (mg/ L)D Degradation index (anthocyanin) Phenolic compoundsE Activity antioxidant (30 min)F

0.69 ± 0.02 2.93 ± 0.05 14.7 ± 0.31 2.63 ± 0.19 0.96 ± 0.08 1.18 ± 0.06 0.48 ± 0.05 7.49 ± 0.39 101.16 ± 13.59 71.13 ± 7.20

1.38 ± 0.05 2.78 ± 0.02 28.2 ± 0.22 5.05 ± 0.47 1.84 ± 0.20 2.34 ± 0.16 0.87 ± 0.11 14.65 ± 1.11 187.84 ± 27.27 148.12 ± 19.22

3.08 ± 0.27

3.05 ± 0.41

2.16 ± 0.13 13.63 ± 031

4.28 ± 0.03 22.23 ± 0.72

Mean of three determinations. Expressed g/100 g acid tartaric. Expressed unit of absorbance. Expressed malvidin 3,5 diglucosides. Expressed in g/L gallic acid. Expressed in lmol/g de Trolox.

corresponding to a VCF of 2.1. In the first experiment, there was a high decay of the permeate flux after 75 min of processing. Even so, the process was finished after 135 min. The soluble solids content reached 27 °Brix within 1 h of the beginning of the process. This should be taken into account when processing in a continuous mode is planned. Recently, orange juice was concentrated at 60 bar and 30 °C using the same type of membrane module described here. The soluble solids content of the concentrated orange juice reached 36 °Brix (Jesus et al., 2007). The maximum concentration value achieved in the reverse osmosis process is limited by osmotic pressure and by the viscosity of the product. The operation pressure of the system must always be greater than the osmotic pressure of the feed (Mulder, 1991). Quality evaluation of the concentrated juice showed that there were increases in its total acidity, its colour intensity, and its concentration of anthocyanin and phenolic compounds proportional to the volumetric concentration factor. The increase in soluble solids was associated with a browning of the concentrated juice. As expected, water removal decreased the luminosity of the juice. The stability of anthocyanins was favoured by the low pH, lack of vitamin C and high concentration of sugar, which physically interacts with anthocyanin in such a manner as to protect it. The pH of the concentrated juice compared to single-strength juice did not change – a consequence of the buffer characteristics of fruit juices (Table 5). The DPPH radical scavenging capacity assay showed increases relative to concentration factor of 1.63 and 1.41 at 30 and 60 min, respectively, demonstrating that grape juice contains considerable concentrations of antioxidant compounds important to the human diet. Kiss et al. (2004) evaluated the use of microfiltration associated with reverse osmosis and nanofiltration to produce a grape juice at 45 °Brix. The authors concluded that the investment and operational costs of vacuum evaporation were greater than those associated with reverse osmosis, due to the high cost of thermal energy. Pozderovic et al. (2006) demonstrated that the membrane HR98PP (DSS) presents a high retention value for volatile compounds. This is another advantage of the use of reverse osmosis in the concentration of grape juice, since, during the thermal evaporation process, losses of some substances that contribute to juice aroma can be higher than 90%.

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In this work, evaluation of the permeate stream showed that it contained no soluble solids or organic acids. This finding demonstrates the efficiency of retention of desirable compounds by the reverse osmosis membrane, a consequence of its high selectivity.

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