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Journal of Food Engineering 81 (2007) 236–242

Study of solid–liquid extraction kinetics of total polyphenols fromgrape seeds

Ana Bucic-Kojic *, Mirela Planinic, Srecko Tomas, Mate Bilic, Darko Velic

Department of Process Engineering, Faculty of Food Technology, Josip Juraj Strossmayer, University of Osijek, F. Kuhaca 18,

P.O. Box 709, HR-31001 Osijek, Croatia

Received 12 April 2006; received in revised form 5 September 2006; accepted 29 October 2006Available online 13 December 2006

Abstract

Grape seeds are a waste by-product obtained after wine or juice making and present a good source of functional compounds, such aspolyphenols. Extraction is a very important step for later use of phenolic components. The effect of different temperatures and solid–liquid ratios on the solid–liquid extraction yield of total polyphenols and the effect of four classes of particle size on the extraction kinet-ics of total polyphenols with 50% of aqueous ethanol were studied. Temperature, solid–liquid ratio and milling degree had positiveinfluence on extraction rate and extent of extraction. The 1.47–6.68% of total polyphenols per dry matter of grape seeds was extractedduring 200 min. Extraction kinetics was estimated using non-experimental two-parameter equation – modified Peleg’s model. Resultsshowed a good prediction of Peleg’s model for extraction kinetics in all experiments (q > 0.994), which gave the possibility for estimationof initial rate and extent of solid–liquid extraction.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Grape seeds; Total polyphenols; Solid–liquid extraction; 50% Aqueous ethanol; Temperatures; Solid–liquid ratio; Particle size; Peleg’s model

1. Introduction

The interest in the investigation of active components,especially polyphenols, from natural sources (fruits, vegeta-bles, cereals, herbs) has greatly increased in recent years.The reason for that is restricted use of synthetic antioxi-dants such as butylated hydroxyanisole (BHA) and butyl-ated hydroxytoluene (BHT) in foods because of theirpossible undesirable effects on human health (Jayapraka-sha, Selvi, & Sakariah, 2003).

During winemaking or juice making from grape highquantities of waste by-products (grape pulp, seeds, skin)remain, which are used only as a feed for animals due totheir high fibre content (Palma, Pineiro, & Barroso,2001). However, these by-products are still a good andcheap source of high-quality polyphenolic compounds

0260-8774/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2006.10.027

* Corresponding author. Tel.: +385 31 224 334; fax: +385 31 207 115.E-mail address: ana.bucic@ptfos.hr (A. Bucic-Kojic).

which can be used in different therapeutic procedures withthe purpose of free radical neutralisation in biological sys-tems (Heim, Tagliaferro, & Bobilya, 2002; Yilmaz &Toledo, 2004). Grape seeds contain 5–8% polyphenolsaccording to Shi, Yu, Pohorly, and Kakuda (2003a).Extraction is a very important stage in isolation, identifica-tion and use of phenolic compounds and there is no singleand standard extraction method. Solid–liquid extraction ofphenolic compounds with different solvents (Baydar, Oz-kan, & Sagdic�, 2004; Lapornik, Prosek, & Golc Wondra,2005; Pekic, Kovac, Alonso, & Revilla, 1998) and in somecases extraction with supercritical fluid (Palma & Taylor,1999) are the most common by used techniques for isola-tion of these compounds. To describe extraction mecha-nism in the literature, Fick’s second law of diffusion isusually used (Cacace & Mazza, 2003; Stangler Herodez,Hadolin, Skerget, & Knez, 2003). Many authors investi-gated solid–liquid extraction of natural antioxidants andtheir properties from grape seed (Jayaprakasha et al.,

A. Bucic-Kojic et al. / Journal of Food Engineering 81 (2007) 236–242 237

2003; Shi et al., 2003a; Yilmaz & Toledo, 2004) and fromother plant materials (Bandoniene, Pukalskas, Venskuto-nis, & Gruzdiene, 2000; Moure et al., 2001; Skerget et al.,2005) as well as methods for their identification (Guendez,Kallithraka, Makris, & Kefalas, 2005; Tsao & Deng, 2004).However, literature data about optimisation (Pinelo, DelFabbro, Manzocco, Nunez, & Nicoli, 2005; Shi et al.,2003b), modelling and simulation of solid–liquid extractionprocess are scarce. Therefore, there is a need for mathemat-ical modelling, as a useful engineering tool, which consider-ably facilitates optimisation, simulation, design and controlof processes and contributes to utilization of energy, timeand solvent. On the other hand, different mathematicalmodels are often used for the description of sorption pro-cesses (dehydration/rehydration) of food materials. Oneof them is a two-parameter, non-exponential Peleg’s model(Abu-Ghannam & McKenna, 1997; Hung, Liu, Black, &Trewhella, 1993; Maskan, 2002; Moreira Azoubel &Xidieh Murr, 2004; Palou, Lopez-Malo, Argaiz, & Welti,1994; Park, Bin, Reis Brod, & Brandini Park, 2002; Peleg,1988; Planinic, Velic, Tomas, Bilic, & Bucic, 2005; RuızDıaz, Martınez-Monzo, Fito, & Chiralt, 2003; Sacchetti,Gianotti, & Dalla Rosa, 2001; Sopade & Obekpa, 1990;Turhan, Sayar, & Gunasekaran, 2002).

Because of a similarity between extraction and sorptionkinetics, our goal was to examine the applicability ofPeleg’s equation for modelling the extraction of total poly-phenols from grape seeds. The aim of this work was to alsodetermine the best temperature, particle size and solid–liquid ratio for solid–liquid extraction of total polyphenolsunder experimental conditions. Quantification of individ-ual polyphenols in extract was not in the issue of thispaper.

Fig. 1. The influence of extraction temperature and solid–liquid ratio(L/S) on extraction yield of total polyphenol concentrations from non-classed milled grape seeds.

2. Materials and methods

2.1. Sample preparation

White grapes were produced in a private plantationlocated in Eastern Slavonia. Grape seeds were residualafter wine making which were manually separated fromthe pomace and sun dried (dry matter content �91%).Dried seeds were stored at room temperature and milledin coffee grinders before extraction.

Determination of dry matter content: Dry matter contentof grape seeds was determined by drying of milled grapeseeds (5 g) at 105 �C to constant mass. Analyses were donein duplicate and the average dry matter content (wdb) wasexpressed in percents (%) and calculated by the followingformula:

wdb ¼m2

m1

� 100 ð1Þ

where m1 is a mass of grape seed before drying (g) and m2 isthe mass of grape seed after drying (g).

Sample preparation for the investigation of extraction

yield: Grape seeds were milled in a coffee grinder (SMK

101, Gorenje) for 2 min with 15 s intervals to avoid heatingof the samples.

Sample preparation for the study of extraction kinetics:

Grape seeds were milled in a coffee grinder with the possi-bility of particle size regulations (MKM 7000, Bosch) andseparated by sieving into four particle classes:(>0.63 mm), (0.63–0.4 mm), (0.4–0.16 mm) and (0.16–0.125 mm).

After grinding, the samples were immediately vacuumpacked and stored at +4 �C before extraction.

2.2. Extraction process

Extracts from milled grape seed were prepared by using50% aqueous ethanol as a solvent. Each extraction was per-formed in triplicate.

Determination of extraction yield: In the test tubes, 0.5 gof non-classed sample was mixed with 5 ml, 10 ml, 15 ml or20 ml of solvent to obtain different solid–liquid ratios (L/S)of 10 ml/g, 20 ml/g, 30 ml/g and 40 ml/g, respectively. Testtubes were incubated in the water bath at different temper-atures (25 �C, 40 �C, 50 �C, 60 �C, 70 �C, 80 �C) for120 min with 20-s shaking of each test tube at 15 min inter-vals by Vortex (Vibromix 10, Tehtnica).

Monitoring of extraction kinetics: The highest extractionyield was obtained at a solid–liquid ratio of 40 ml/g asshown in Fig. 1. So, the solid–liquid ratio of 40 ml/g wasused for the purpose of kinetic study at different extractiontemperatures (25 �C, 50 �C, 80 �C). Extraction kinetic wasobserved on three classes of particles: (>0.63 mm), (0.63–0.4 mm), (0.4–0.16 mm) at 25 �C, 50 �C and 80 �C and onone more class (0.16–0.125 mm) at 80 �C. The extractionkinetics was observed during 200 min with periodical shak-ing as previously described.

In both ways, the extracts were separated from roughparticles after extraction by decantation and were centri-fuged for 5 min on 15 000g (Multifuga 3 L-R Heraeus,Kendro Laboratory Products). The supernatants weremeasured and pooled to 25 ml by using 50% aqueous eth-anol. The extract prepared in this way was used for furtherspectrophotometric determination of total polyphenols.

238 A. Bucic-Kojic et al. / Journal of Food Engineering 81 (2007) 236–242

2.3. Determination of total polyphenols

The concentration of total polyphenols in the extractswas determined by spectrophotometric Folin–Ciocalteaumethod according to Lapornik et al. (2005) with somemodifications. 0.2 ml of the extract was mixed with1.8 ml of distilled water and 10 ml of 10-fold dilutedFolin–Ciocalteau reagent. After 30 s to 8 min, 8 ml of7.5% of sodium carbonate solution was added. All testtubes with the mixture were shaken for 10 s on the Vortexand put on to incubation in a water bath at 45 �C. Absor-bance was measured after 15 min at 765 nm (UV/VIS 554,Perkin–Elmer Spectrophotometer) against blank sample.Blank sample was prepared with water instead of theextract. Determination of total phenolic compounds wascarried out in a duplicate and calculated from the calibra-tion curve obtained with gallic acid, which was used as astandard and results were expressed as mg GAE/ml. Thefinal results were calculated by the following formulataken from Waterman and Mole (1994), which wasadapted to experimental conditions and expressed in gallicacid equivalent per a dry basis of grape seeds (mg GAE/gdb)

C ¼ c � ðV 2 þ V 3Þ �V 1

V 2

� �� 1

ms

� 100

wdb

ð2Þ

where C is concentration of total polyphenols (mg GAE/gdb), c the concentration of total polyphenols (mg GAE/ml), V1 the volume of gained extract after solid–liquidextraction (ml), V2 the volume of extract taken from V1

for the preparation of diluted extract (ml), V3 the volumeof extraction solvent used for the dilution of extract to atotal volume of 25 ml (ml), ms the mass of grape seedused in extraction (g) and wdb the content of dry matter(%).

2.4. Kinetic of solid–liquid-extraction

As mentioned in the introduction and confirmed by thisexperimental work, the extraction curves (concentration oftotal polyphenols vs. time) have similar shape with thesorption curves (moisture content vs. time), so, all thesecurves could be described by the same model proposedby Peleg (1988) which in case of extraction would assumethe form:

CðtÞ ¼ C0 þt

K1 þ K2 � tð3Þ

where C(t) is the concentration of total polyphenols at timet (mg GAE/gdb), t the extraction time (min), C0 the initialconcentration of total polyphenols at time t = 0 (mg GAE/gdb), K1 is Peleg’s rate constant (min gdb/mg GAE) and K2

is Peleg’s capacity constant (gdb/mg GAE). Since C0 in allexperimental runs was zero, Eq. (3) was used in the finalform

CðtÞ ¼ tK1 þ K2 � t

ð4Þ

The Peleg rate constant K1 relates to extraction rate (B0) atthe very beginning (t = t0)

B0 ¼1

K1

ðmg GAE=gdb minÞ ð5Þ

The Peleg capacity constant K2 relates to maximum ofextraction yield, i.e. equilibrium concentration of ex-tracted total polyphenols (Ce). When t ?1, Eq. (6) givesthe relations between equilibrium concentration and K2

constant

Cjt!1 ¼ Ce ¼1

K2

ðmg GAE=gdbÞ ð6Þ

2.5. Statistical methods

Statistica 6.0 (Stat Soft Inc., USA) was used for dataanalysing. The randomized complete block designed(RCBD) was served as experimental scheme for two facto-rial analyses. The analysis of variance (ANOVA) and F-testwere used to assess the factor influence on extraction andtheir possible interaction under conditions in this study.The parameters of modified Peleg’s model (constants K1

and K2) were determined from experimental data usingnon-linear regression (Quasi-Newton method). The con-cordance between experimental data and calculated valuewas established by the correlation coefficient (q) and theroot mean squared deviation (RMSD) as follows:

RMSD ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

n

Xn

i¼1

ðexperimental� calculatedÞ2s

ð7Þ

3. Results and discussion

Dry matter content in all experimental runs was deter-mined and results were expressed on dry basis, which gen-erally provides more accurate and reliable datacomparison. Solid–liquid extraction was carried out by50% aqueous ethanol for 200 min, which gave concentra-tion of total polyphenols in the range from 14.72 mgGAE/gdb to 66.81 mg GAE/gdb and represented 1.47–6.68% per dry matter of seeds that was compatible withthe results reported by Shi et al. (2003b). They wereextracted 15.153–39.246 mg GAE/gseed of total polyphe-nols by using 50% aqueous ethanol. Other authors alsoobtained similar results using different solvents. Jayap-rakasha et al. (2003) gained 38% (w/w) and 46% (w/w)of total polyphenols (catehin equivalents) per crudeextract by using acetone–water–acetic acid and metha-nol–water–acetic acid, which when recalculated on grapeseed were 2.13% and 2.57%, respectively; Baydar et al.(2004) extracted 9.65% and 10.86% of total polyphenolson seed base by using acetone–water–acetic and ethyl–methanol–water, respectively, Guendez et al. (2005)obtained the average of 3.8 mg GAE/gseed extracted byethyl acetate.

0

20

40

60

80

0 50 100 150 200t (min)

C (

mg

GA

E/g

db)

(> 0.63) mm (0.63 - 0.4) mm

(0.4 - 0.16) mm (0.16 - 0.125) mm

Fig. 2. The influence of particle size on total polyphenols extraction at80 �C and L/S = 40 ml/g for different particle classes (symbols –experimental data; lines – approximation curves).

A. Bucic-Kojic et al. / Journal of Food Engineering 81 (2007) 236–242 239

3.0.1. Influence of solid–liquid ratios and temperatures on the

extraction yield of total polyphenols

The estimation of extraction yield was based on theconcentrations of extracted total polyphenols from non-classed milled grape seed. The concentrations were calcu-lated according to Eq. (2) and the results are shown inFig. 1. Statistical data processing was carried out by6 � 4 RCBD and the results are shown in Table 1. Onthe basis of F-value magnitudes, no statistical by significantdifference (P = 0.05) between six repetitions of each extrac-tion was found, but high by significant F-value of combina-tions occurred (six temperatures and four solid–liquidratios). These results implied significant difference of thepolyphenols concentration depending on temperature andsolid–liquid ratio. Further ANOVA analysis showed thattemperature and solid–liquid ratio had statistical by signif-icant influence on extraction yield of total polyphenols.Besides, interaction between extraction temperature andsolid–liquid ratio was observed, which indicated thatextraction temperature had a different influence on extrac-tion yield at different solid–liquid ratios. The L/S = 40ml/g gave mostly the best extraction yield at all tested tem-peratures, so this solid–liquid ratio was used for the exam-ination of extraction kinetics. The highest polyphenolsyield (30.243 mg GAE/gdb) was obtained at 80 �C andL/S = 40 ml/g (Fig. 1).

3.0.2. Solid–liquid extraction kinetics of total polyphenols

The influence of particles size on solid–liquid extractionkinetics at 80 �C and solid–liquid ratio of 40 ml/g can beseen in Fig. 2. The results and extraction curves indicatedthe exponential growth of extraction rate in the time forall examined particle classes. The total concentration ofextracted polyphenols was the highest during extractionfrom the smallest particle class (0.16–0.125 mm) in theamount of 66.810 mg GAE/gdb, and the lowest in the caseof the biggest particle size (>0.63 mm) in the amount of23.907 mg GAE/gdb. However, coefficient of variation ofrepetition was increased from the highest to the lowest par-

Table 1Results of statistical analysis (6 � 4 RCBD)

Source of variation df SS

Total 143 1427.76Repetitions 5 1.80Combinations 23 1359.08Error 115 66.885

Effects

Temperature of extraction (#) 5 759.59Solid–liquid ratio (L/S) 3 542.16Interaction of effects (# � L/S) 15 57.33

df – degrees of freedom; SS – sum of squares; s2– variance, Fexp, Fcrit – expera Statistically insignificant.b Statistically significant.

ticles (2.33%, 2.49%, 6.57% and 12.61%, respectively),which implied the highest data dispersions for (0.16–0.125 mm) particle class as a consequence of the possibleagglomeration of smaller particles, so this fraction wasomitted in the further experimental work (extraction at25 �C and 50 �C). A similar pattern of extraction curves(curves are not shown) was obtained at temperatures25 �C and 50 �C for three particle classes but with differentextraction extent (Table 2). From these results, great tem-perature influence on extraction kinetics of total polyphe-nols from grape seeds can be observed. In addition tothat, high initial rate of polyphenols extraction (first40 min) can be seen from extraction curves, followed byslower extraction rate, and asymptotically approachingthe equilibrium concentration.

The Fig. 3a–c shows the effect of extraction tempera-tures (25 �C, 50 �C, 80 �C) on the extraction kinetics oftotal polyphenols for each particle class (>0.63, 0.63–0.4and 0.4–0.16 mm). In the case of particle size (>0.63 mm)the highest concentration of total polyphenols wasobtained at 80 �C, and the lowest concentration at 25 �C(Fig. 3a, Table 2). The same relation was obtained for par-ticle class (0.63–0.4 mm) (Fig. 3b, Table 2). The extractionrate and extraction extent obtained from particle class (0.4–0.16 mm) were a little higher at 50 �C than 80 �C (Fig. 3c,Table 2). This can be explained by possible agglomeration

s2 Fexp Fcrit P = 0.05

0.36 0.62a 2.2959.09 101.60b 1.620.58

151.92 261.20b 2.29180.72 310.72b 2.69

3.82 6.57b 1.85

imental and critical Fisher coefficients.

Table 2Values of extraction extent, Peleg’s constants (K1 and K2) for solid–liquid extraction with correlation coefficient (q) and the root mean squared deviation(RMSD) at different particle size and extraction temperature

Particle size (mm) Temperature (�C) Extraction extent (mg GAE/gdb) K1 (min � gdb/mg GAE) K2 (gdb/mg GAE) q RMSD

(>0.63) 25 14.7193 1.1541 0.0651 0.9944 0.417950 20.2015 0.6571 0.0481 0.9969 0.425880 23.9074 0.7101 0.0393 0.9983 0.3847

(0.63–0.4) 25 22.0303 0.4256 0.0448 0.9957 0.529750 23.7196 0.3037 0.0416 0.9981 0.378780 26.5784 0.3056 0.0375 0.9967 0.5564

(0.4–0.16) 25 52.6011 0.0814 0.0193 0.9972 0.9945

50 56.4722 0.0645 0.0175 0.9985 0.799080 54.3745 0.0869 0.0184 0.9985 0.7664

(0.16–0.125) 80 66.8097 0.0400 0.0156 0.9945 1.7490

0

20

40

60

80

0 50 100 150 200

t (min)

C (

mg

GA

E/g

db)

25˚C 50˚C 80˚C

0

20

40

60

80

0 50 100 150 200

t (min)

C (

mg

GA

E/g

db)

25˚C 50˚C 80˚C

0

20

40

60

80

0 50 100 150 200t (min)

C (

mg

GA

E/g

db)

25˚C 50˚C 80˚C

a

b

cFig. 3. The temperature influence on solid–liquid extraction of totalpolyphenols for different particle classes of grape seeds: (a) (>0.63 mm),(b) (0.63–0.4 mm) and (c) (0.4–0.16 mm) (symbols – experimental data;lines – approximation curves).

240 A. Bucic-Kojic et al. / Journal of Food Engineering 81 (2007) 236–242

of smaller particles, as mentioned earlier, which can causeexperimental errors. Moreover, extraction of proteins isbetter at lower temperatures by alcohol solution with highwater ratio (Shi et al., 2003b) and it is known that proteinsgive positive reaction with Folin–Ciocalteau reagent, whichcan cause interference in spectrophotometric determinationof total polyphenols.

This course of extractions (Figs. 2 and 3a–c) shows sim-ilarity with course of sorption processes, which can be welldescribed by Peleg’s model (Peleg, 1988; Planinic et al.,2005). The modified Peleg’s model (Eq. (4)) adapted toexperimental condition was used for experimental dataapproximation. The experimental extraction extent, calcu-lated parameters of modified Peleg’s model (constants K1

and K2), correlation coefficient (q) and RMSD are shownin Table 2. The correlation coefficients were high in allexperiments (q > 0.994) which implied good concordancebetween experimental and calculated data. The RMSDhad relatively low values for (>0.63 mm) and (0.63–0.4 mm) particle classes, while with further decrease of par-ticle size this RMSD value was a little higher. The values ofextraction rate constant (K1) and constant of extractionextent (K2) showed a tendency of decrease with the increaseof extraction temperature and the decrease of particles size.The difference in extraction extent was analysed by usingthe scheme of 32 RCBD. The results indicated no signifi-cant difference (P = 0.01) in total polyphenol concentra-tions after 200 min (extraction extent) between sixrepetitions of each extraction, but high significant F-valueof combinations (three temperatures and three particleclasses). Interaction of factors (temperature and particlesize) was also observed. So, this observation implied a sig-nificant difference in extraction extent in view of extractiontemperatures and particles size.

The initial extraction rate (Eq. (5)) and maximum ofextraction extent in experimental conditions (Eq. (6)) werecalculated from constants K1 and K2.. Fig. 4 shows theinfluence of particle size on initial extraction rate (1/K1)at 80 �C and L/S = 40 ml/g (the best experimental condi-tions, as mentioned above). The exponential decrease ofinitial extraction rate with an increase of particle size

R2 = 0.9967

0

10

20

30

0.0 0.2 0.4 0.6 0.8particle size (mm)

1/K

1 (m

g G

AE

/min

·gd

b)

Fig. 4. Effect of average particle size of each class on the initial extractionrate, 1/K1 at 80 �C and L/S = 40 ml/g (symbols – experimental data; lines– approximation curves).

A. Bucic-Kojic et al. / Journal of Food Engineering 81 (2007) 236–242 241

implied strong dependence of initial extraction rate on par-ticle size as can be seen in Figs. 2 and 3, too.

The temperature influence on initial extraction rate(1/K1) for different particles size was assessed by linearizedArrhenius equation, which gave relationship between 1/K1

and extraction temperatures as follows

ln1

K1

¼ ln K0 �Ea

RTð8Þ

where K0 is constant (mg GAE/gdb min), Ea the activationenergy (kJ/mol), R the universal gas constant (8.314 kJ/mol K), and T the absolute temperature (K).

The calculated activation energies (Eq. (8)) for averageparticles size of each class (>0.63, 0.63–0.4 and 0.4–0.16 mm) were 7.70 kJ/mol, 5.27 kJ/mol and 1.10 kJ/mol,respectively (results are not shown). These results impliedthat initial extraction rate (1/K1) tends to be more temper-ature sensitive for bigger rather than for smaller particles.In other words, raise of extraction temperature will affectthe initial extraction rate of bigger particles more than thatof smaller particles.

In the same manner, extraction extent (1/K2) in exper-imental condition was decreased with increase of particlesize and showed exponential trend, as presented inFig. 5. The influence of extraction temperature on extrac-tion extent was stronger on the bigger particles than the

R2 = 0.9573

0

20

40

60

80

0.0 0.2 0.4 0.6 0.8particle size (mm)

1/K

2 (m

g G

AE

/gd

b)

Fig. 5. Effect of average particle size of each class on the extraction extent1/K2 at 80 �C and L/S = 40 ml/g (symbols – experimental data; lines –approximation curves).

smaller particles and that was confirmed by value ofenergy activation which was in the amounts of 8.01 kJ/mol, 2.82 kJ/mol and 0.71 kJ/mol for particle size:(>0.63 mm), (0.63–0.4 mm) and (0.4–0.16 mm),respectively.

4. Conclusion

At experimental condition in this study, the yield andkinetic of solid–liquid extraction were influenced by thetemperature, solid–liquid ratio and particle size. Theextracted total polyphenols were in the range from14.72 mg GAE/gdb to 66.81 mg GAE/gdb under experimen-tal conditions.

The Peleg equation showed to be suitable to model thesolid–liquid extraction kinetics of total polyphenols fromgrape seeds at tested conditions, which was proved by highcorrelation coefficient in all experiments (q > 0.994) andrelatively low RMSD (0.3787–1.7490). The 80–90% of totalpolyphenols reached within 200 min was extracted in thefirst 40 min of extraction. The temperature and particlesize, as well as their interaction had high influence onsolid–liquid extraction kinetics. The values of initial extrac-tion rate, 1/K1, and extraction extent, 1/K2, increased withthe decrease of the particles size and showed exponentialtrend. The extraction temperature had bigger influence oninitial extraction rate and extraction extent for bigger thanfor smaller particles, which were proved by the highestvalue of energy activation. The results of this work couldcontribute in the simulation and optimization of extractionkinetics for grape seed.

Acknowledgement

This work was financially supported by the Ministry ofScience, Education and Sports of the Republic of Croatia,Project No. 0113005.

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