Journal of Molecular Liquids 200 (2014) 448–459

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Journal of Molecular Liquids

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Stabilities of aqueous solutions of sucrose containing ascorbic and citricacids by using FTIR spectroscopy and physicochemical studies

Laura Cecilia Bichara a, Hernán Enrique Lanús b, Silvia Antonia Brandán a,⁎a Cátedra de Química General, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, Ayacucho 471, 4000 San Miguel de Tucumán, Tucumán, Argentinab Cátedra de Fisicoquímica I, Instituto de Química Física, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, San Lorenzo 456, T4000CAN San Miguel deTucumán, Argentina

⁎ Corresponding author.E-mail address: sbrandan@fbqf.unt.edu.ar (S.A. Brandá

http://dx.doi.org/10.1016/j.molliq.2014.10.0380167-7322/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 May 2014Received in revised form 19 September 2014Accepted 28 October 2014Available online 30 October 2014

Keywords:Model juicesDensityRefractive indexSoluble solidsInfrared spectrum

Stabilities of aqueous solutions of sucrose containing ascorbic (AA) and citric acids (CA) in different concentra-tions were studied at room temperature for 92 days by using FTIR spectroscopy and physicochemical properties,such as the pH, refractive index, density, conductivity, °Brix and molar refractivity. The physicochemical proper-ties and the IR spectra of these mixtures were compared with those reported for aqueous solution of AA, CA andsucrose. The results suggest that the decreasing in the pH values of themixtures could be principally be attributedto CA. The lineal increase of the conductivities with the time for the most diluted mixture could be associatedwith AA and sucrose. The variations of n and soluble solids with the time demonstrate that the products of de-composition of each component in the mixture are stable in the studied time. The studies by IR spectroscopyshow that the stabilities of these solutions are related to CA and sucrose supported by the hydrates of sucroseformation. Four characteristic bands in the IR spectra of these mixtures in solution at 1428, 1233, 1039, 799and 581 cm−1were detected. In addition, the assignments of the bands observed in the IR spectra of themixturesin solid and aqueous solution phases in the 4000–400 cm−1 regionwere proposed. These results are very impor-tant taking into account that the mixtures simulate diluted model orange juices.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The identification of the main components of an orange juice byusing FTIR spectroscopy is a very difficult task due to overlapping ofbands and, so far, there are no studies related to their quick identifica-tions neither with their completed assignments by using FTIR spectros-copy [1–9]. It is known that the organic acids of higher proportion in afruit orange are the ascorbic and citric acids and that the acid contentin a natural juice is one of the important factors to control quality attri-butes in the orange fruit by means of the FTIR spectroscopy [5–9]. Webelieve that with the aid of a model orange juice which is preparedwith the ascorbic and citric acids and sucrose in the same proportionsthat as orange fruit [10,11] it is possible to know the profile of the infra-red spectrum of these mixtures and, this way, it is possible to performby simulation the completed assignments of the bands observed in thenatural orange juices. The assignments reported in the literature forascorbic and citric acids and sucrose in solution [12–16] would allowus the assignments of the bands observed in the IR spectra of the mix-tures in order to avoid falsifications or adulterations in the final productsby means of a quick technique, as is the FTIR spectroscopy. Nowadays, it

n).

is possible to identify the bands of each component inmixtures of ascor-bic and citric acids and sucrose inwater by combining the infrared spec-trum together with studies simultaneous of their physicochemicalproperties with the storage time. Hence, in this work as part of our in-vestigations on characterizations of spectroscopic compounds of indus-trial importance [17–20] we have considered the study, on one hand, ofaqueous solutions of different concentrations of ascorbic acid, citric acidand sucrose and, on the other hand, of mixtures of those three compo-nents prepared taking into account the proportions reported in the lit-erature for an orange fruit [10,11]. These mixtures of differentconcentrations of ascorbic acid, citric acid and sucrose constitutemodel orange juices. Thus, the purposes of this work are three, first, torecord and to analyze the infrared spectra for each one of the aqueoussolutions of ascorbic and citric acids and sucrose and, also of the mix-tures with the storage time during 92 days, second, to determine andto evaluate some physicochemical properties of these systems withthe storage time of 92 days, and, finally, to perform the complete as-signments of each spectrum for a storage time of 92 days. We thinkthat this study will allow the quick identification of the constituents ofmajor proportion in a natural orange juice which are the ascorbic andcitric acids and sucrose by using IR spectroscopy. The comparisons ofnatural orange juices with model juices by using IR spectroscopy willbe the object of the next paper.

Table 1Concentrations of the solutions studied expressed in molar fractions (X).

Solutions Ascorbic acid (AA) Citric acid (CA) Sucrose Mixturea

1 X1 = 0.9772 X1 = 0.9791 X1 = 0.9775 X1 = 0.9652X2 = 0.0228 X2 = 0.0209 X2 = 0.0225 X2 = 0.0347

2 X1 = 0.9853 X1 = 0.9854 X1 = 0.9845 X1 = 0.9756X2 = 0.0147 X2 = 0.0146 X2 = 0.0145 X2 = 0.0244

3 X1 = 0.9905 X1 = 0.9905 X1 = 0.9900 X1 = 0.9871X2 = 0.0095 X2 = 0.0095 X2 = 0.0100 X2 = 0.0129

4 X1 = 0.9963 X1 = 0.9963 X1 = 0.9962 X1 = 0.9960X2 = 0.0037 X2 = 0.0037 X2 = 0.0038 X2 = 0.0040

X1 and X2 indicate the molar fractions of the solvent and solute, respectively while thenumbers from 1 to 4 represent themore concentrated and diluted solutions, respectively.Mixture: AA + CA + sucrose.

a From Refs. [10,11].

449L.C. Bichara et al. / Journal of Molecular Liquids 200 (2014) 448–459

2. Material and methods

2.1. Aqueous solution ascorbic acid (AA, H2A), citric acid (CA) and sucrose

Pure anhydrous Mallinckrodt commercial samples of ascorbic(C6H8O6) and citric (C6H8O7) acids and sucrose (C12H22O11)were used di-rectly without posterior purification. The concentrations of the solutionsprepared for each component and for the mixtures of the three compo-nents, in accordance with a natural orange juice [10,11], expressed asmolar fractions are presented in Table 1. X1 andX2 indicate themolar frac-tions of the solvent and solute, respectivelywhile the numbers from1 to 4represent the more concentrated and diluted solutions, respectively.Solutions were prepared by weighing the required quantity of eachsolid and dissolving it in degassed and nitrogen-flushed conductivitywater (K b 1 μS cm−1); these solutions were used after their preparation.

Table 2Physicochemical properties for ascorbic acid solutions with the storage time.

Ascorbic acid

Solution N° 1

T (days) d (g/cm3) k (μS/cm)

0 1.0761 2.517 1.0872 2.5514 1.0890 2.6631 1.0715 2.0292 1.0695 3.91

Solution N° 20 1.0485 2.317 1.0519 2.3914 1.0640 2.5131 1.0506 2.0692 1.0343 4.71

Solution N° 30 1.0319 2.057 1.0383 2.1714 1.0410 2.3731 1.0369 2.2692 1.0218 3.86

Solution N° 40 1.0097 1.3927 1.0137 1.60314 1.0110 1.80231 1.0069 1.74092 1.0164 4.840

RM (cm3 mol−1)

T (days) N° 1 N° 2

0 4.4631 4.1887 4.4252 4.18114 4.4157 4.13231 4.4834 4.18692 4.4897 4.288

After the measurements the solutions were kept free of the light and theheat.

2.2. Physicochemical properties

The pH, refractive index, density, conductivity and the quantity of totalsoluble solids expressed as °Brixwere measured at 25 °C for all the solu-tions at different concentrations. The accuracy of density determinationwas about 1 × 10−4 g/cm3 while the refractive index was determinedwith a precision of 1 × 10−4. The measurements were repeated manytimes. An automatic digital Refractometer, Leica, model AR600 for the re-fractivity index measurement with an accuracy of 0.01% was used. A dig-ital pH-meter Mettler, model MA235 for the pH measurement and adigital Conductimeter, Mettler, model MC226 were employed.

2.3. Infrared spectra

The infrared (IR) spectra of all the solutions were recorded betweenAgCl windows from 4000 to 400 cm−1. FTIR GX1 Perkin Elmer spec-trometer, equippedwith a DTGDdetector cooled at liquid nitrogen tem-peraturewas used for all measurements. All spectrawere recordedwitha resolution of 1 cm−1 and 60 scans.

3. Results and discussion

3.1. Physicochemical properties

3.1.1. Densities (d)Tables 2, 3, 4 and 5 summarize the studied physicochemical proper-

ties for the ascorbic and citric acids and sucrose and, for themixtures of

pH n 25 °C °Brix

2.29 1.3629 19.42.24 1.3636 19.82.33 1.3634 19.71.93 1.3630 19.52.39 1.3622 19.1

2.40 1.352 12.72.35 1.3526 13.02.62 1.3524 13.01.94 1.3526 13.02.47 1.3514 12.3

2.45 1.3461 8.92.43 1.3462 9.02.50 1.3466 9.11.96 1.3412 8.92.58 1.3446 8.7

2.50 1.3373 3.22.54 1.3372 2.92.60 1.3382 3.52.08 1.3374 3.12.72 1.3368 2.6

N° 3 N° 4

8 4.0221 3.83057 3.9803 3.81430 3.9921 3.83489 3.9514 3.84219 4.0811 3.8122

Table 3Physicochemical properties for citric acid solutions with the storage time.

Citric acid

Solution N° 1

T (days) d (g/cm3) k (μS/cm) pH n 25 °C °Brix

0 1.0776 8.68 1.92 1.356 15.27 1.0807 8.44 1.81 1.3564 15.514 1.0842 8.32 1.94 1.3564 13.431 1.0753 5.49 2.00 1.3567 15.592 1.0794 9.27 2.14 1.3562 15.3

Solution N° 20 1.0569 8.34 2.01 1.3509 11.97 1.0628 8.11 1.93 1.3508 11.814 1.0702 7.91 2.00 1.3510 12.031 1.0686 5.32 2.03 1.3510 12.092 1.0588 7.36 2.19 1.3506 11.8

Solution N° 30 1.0372 7.19 2.18 1.3422 6.97 1.0381 6.99 2.08 1.3432 6.914 1.0351 6.84 2.14 1.3436 7.331 1.0327 4.44 2.23 1.3434 7.292 1.0313 6.29 2.40 1.3426 6.6

Solution N° 40 1.014 4.98 2.41 1.3362 2.97 1.0135 4.79 2.25 1.3365 2.514 1.0273 4.66 2.34 1.3352 2.831 1.0140 3.05 2.35 1.337 2.892 1.0068 4.27 2.50 1.3364 2.6

RM (cm3 mol−1)

T (days) N° 1 N° 2 N° 3 N° 4

0 4.3874 4.1918 3.9952 3.81507 4.3793 4.1675 4.0022 3.820014 4.3651 4.1408 4.0181 3.755531 4.4035 4.1470 4.0037 3.823392 4.3823 4.1810 4.0223 3.8444

450 L.C. Bichara et al. / Journal of Molecular Liquids 200 (2014) 448–459

aqueous solutions of the three components at 25 °C, respectively asfunctions of the storage time during the 92 days while Fig. 1 showsthat density varies with the time for the four solutions. Note that thegraphics for the AA, CA and sucrose solutions show approximately thesame variations with the storage time for the more concentrated solu-tions (N°1). In the three (a), (b) and (c) graphics, the density valuesfor all the solutions show a quick increase more or less at the 15 daysand later a decrease slightly by the time up to the 92 days, with excep-tion of the more diluted solution (N°4), which shows an increase in7 days for AA and sucrose while in CA the increase is observed in15 days. Also, a minimum of 30 days was observed for AA and CA and,later increase the density values for AA up to 92 days while decreasingthe values for CA and sucrose. For AA, the d values agreedwith those re-ported by Shamin at 25 °C [21]. For this acid, the increase in the valuesat 15 days for all the aqueous solutions, especially those diluted (N°4)could be in part related to the formation of the oxidized A species ofAA because in solution its form undergoes a decrease in volume(−0.50 Å3), as reported by Bichara and Brandán [14]. On the contrary,the decomposition of this acid in ascorbate ion (HA−) or in its dimericform (dehydroascorbic acid, A2) could justify the decreasing up to30 days due to both species in aqueous solution increasing their vol-umes (6.50 and 2.90 Å3, respectively) [14]. For the solution of 4 sucrosein water, probably the decrease in the conductivity values justifies thediminishing of the density value for 7 days, as we will see later inSection 3.1.3. For citric acid, the increase in the density values at15 days could be in part supported by the formation of themonocitrate,citrate or sub-citrate ions because the theoretical B3LYP/6-31G* calcula-tions predicted for these ions in solution, in relation to the values in gasphase, are decreasing in the volumes of −0.1, −1.4 and −1.0 Å3,respectively as observed in Table S1. For sucrose, the increase in the

density values at 15 days could be attributed to the hydrolysis of su-crose, while the decrease in the density values could be attributed tothe dihydrate and the pentahydrate sucrose formation. On the contrary,for the four solutions of the mixture (Fig. 1d) the behaviors with thetime are similar among them but different from the other ones, it isfirst a little increased up to 15 days as observed and, then the values de-crease up to 92 days. Here, the three components show separately dif-ferent variations of the densities with the concentration of thesolutions and with the storage time, for these reasons, the slight in-crease in the density values of these mixtures during the first 15 dayscan be attributed indistinctly to the ascorbic acid, citric acid or sucrosebecause those graphics showed a nondefined tendency. These resultsshow clearly that the densities of the mixture are more stable thaneach solution in particular.

3.1.2. pH valuesThe pH values for the four solutions determined at 25 °C with the

storage time of 92 days are presented in Tables 2 to 5while the graphicsof the densities with the time are observed in Fig. 2. Here, the pH valuesfor the AA, CA and sucrose solutions show different variations with thetime, being observed a notable increment in the pH values after the30 days for all the solutions of AA and CA (Fig. 2a and b). For AA(Fig. 2a), from the initial pH values up to 15 days the values increaseand then decrease to a minimum up to 30 days after which it increasedin the values with the time up to 92 days. The AA decompositionincreases strongly in solution, for this reason, the formation of thediffer-ent species formed (HA− and A2

− anions, oxidized A and dimeric A2

forms), as reported by Bichara and Brandán [14], could justify the vari-ations observed. This property shows clearly that the decomposition ofthis acid is favoredby thedilution. For CA, the variations in the pH values

Table 4Physicochemical properties for sucrose solutions with the storage time.

Sucrose

Solution N° 1

T (days) d (g/cm3) k (μS/cm) pH n 25 °C °Brix

0 1.1335 15.93 4.79 1.3810 29.97 1.1419 18.72 4.60 1.3814 30.114 1.1410 22.80 4.25 1.3814 30.131 1.1246 6.00 3.66 1.3822 30.592 1.1268 15.30 2.88 1.3822 30.5

Solution N° 20 1.1157 14.10 4.88 1.3706 247 1.1195 15.08 4.96 1.3710 24.214 1.1130 18.80 4.70 1.3710 24.331 1.1022 12.00 4.41 1.3708 24.192 1.0862 19.00 3.15 1.3700 23.6

Solution N° 30 1.0701 15.87 4.61 1.3552 14.77 1.0760 29.20 3.96 1.3554 14.814 1.0645 97.10 3.56 1.3552 14.731 1.0652 32.00 3.24 1.3554 14.892 1.0468 63.00 2.69 1.3550 14.6

Solution N° 40 1.0215 31.10 3.91 1.3422 6.37 1.0416 25.50 4.25 1.3422 6.314 1.0245 44.20 5.90 1.3418 6.031 1.0257 19.00 3.58 1.3420 6.192 1.0190 66.00 2.52 1.3426 6.5

RM (cm3 mol−1)

T (days) N° 1 N° 2 N° 3 N° 4

0 5.2191 4.6560 4.3456 3.96227 5.1816 4.6749 4.3286 3.969614 5.1524 4.6908 4.3528 3.953631 5.2373 4.7345 4.3507 3.951292 5.2270 4.7950 4.4227 3.9835

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suggest the formation of mono-citrate (C6H7O7−1), citrate (C6H5O7

−3)and sub-citrate (C6H4O7

−4) ions in solution. For sucrose, from the firstday up to the 92nd day the pH values decrease for the three more con-centrated solutions while for the most diluted a notable increase in the15th day is observed. Probably, with the dilution the dihydrate andpentahydrate-sucrose is quickly formed and, for this reason, the in-crease in the basicity could be attributed to the presence ofmost neutralspecies in solution and less ions in the aqueousmedia. Here, the proper-ties observed for sucrose in water are strongly dependent of the hydra-tion effect, as reported by Bobrovnik [22]. Hence, sucrose has hydroxylicH+ that interchangeswithwater bymeans of H bonds of different types,such as between water molecules, between sucrose molecules and be-tweenwater and sucrose. Then, for concentrated solution the hydrationgrade produces different stability regions and in low concentrations ofsucrose each sucrose molecule is surrounded by a certain water mole-cule. Graphically, we observed that the results obtained for AA, CA andsucrose didn't influence the pH values of the mixtures, as observed inFig. 2d. In particular, the decreasing in the pH values of the mixture onthe 7th day is principally related to species derived from of CA becausethe solutions of this acid have a similar behavior.

3.1.3. Conductivities (k)The conductivity values for the four studied solutions at 25 °C versus

the storage time of 92 days can be seen from Tables 2 to 5while in Fig. 3are presented the different behaviors of the conductivities for all the so-lutions as a function of the time. The graphics show clearly different be-haviors for the four solutions, thus, for AA (Fig. 3a) the conductivitiesincrease from the first day up to the 15 days, later a decreasing is ob-served up to the 30 days and, then, experiment an increasing up to

the 92 days, being notable it increase for the most diluted solution.The increase in the conductivity values could be related in part tothe presence of H+, OH− and ascorbate ions in the medium and of theA2− species derived from the ascorbic acid decompositionwhile the de-crease in the conductivity could be justified by the presence of neutralspecies, such as the A and A2 forms, as reported in the literature [14].For CA (Fig. 3b), the conductivity values decrease with the time up toa minimum value for 30 days and then the values increase up to the92nd day. In this case the increases in the values are justified by thepresence of citrate, mono-citrate and sub-citrate ions while low quanti-ties of these ions justify the observed decreasing. In the case of sucrose,the behaviors of the conductivities of the solutions are different fromthose observed for AA and CA. Here, from the most diluted to themost concentrated solution a maximum and two minima are observedat the same time, whose conductivity values increase with the dilution,thus, for solutions 3 and 4 the values are higher than the other ones. Inall the cases the maximum is observed at the 15th day while the posi-tion of the minimum changes with the solution. For solutions 1, 2 and3, the minima are located at the first and 30th days while in solution 4the position of the minima are located at the 7th and 30th days. Obvi-ously, the maximum value at the 15th day is in agreement with the de-crease in the pH value or an increase in the basicity for solution 4, thus,the increasing of the conductivity with the dilution is justified by theformation of few quantities of dihydrate and penta hydrate-sucrose, asobserved experimentally by Max and Chapados [23] while the decreas-ing in the values is attributed to a high quantities of these two hydrates.The results for AA, CA and sucrose suggest that the behavior of the solu-tionmost diluted of themixtures (Fig. 3d) is different from solutions 1, 2and 3 because the conductivity has a behavior lineal with the storage

Table 5Physicochemical properties for the mixtures of ascorbic and citric acids and sucrose in aqueous solutions of different concentrations with the storage time.

AA + CA + sucrose

Solution N° 1

T (days) d (g/cm3) k (μS/cm) pH n 25 °C °Brix

0 1.1494 2.89 2.05 1.3880 33.77 1.1562 2.76 1.99 1.3886 34.014 1.1566 2.56 2.03 1.3888 34.231 1.15806 2.80 2.09 1.3890 34.292 1.1454 2.72 2.32 1.3890 34.2

Solution N° 20 1.0734 3.43 2.25 1.3612 18.57 1.0846 3.34 2.19 1.3619 18.814 1.0862 3.13 2.22 1.3624 19.131 1.0797 3.28 2.26 1.3624 19.092 1.0733 3.29 2.44 1.362 18.8

Solution N° 30 1.0460 3.23 2.37 1.3499 11.27 1.0543 3.18 2.28 1.3514 12.314 1.0514 3.02 2.34 1.3518 12.531 1.0582 3.23 2.39 1.3520 12.692 1.0541 3.37 2.52 1.3514 12.3

Solution N° 40 1.0180 2.27 2.55 1.339 4.27 1.0197 2.33 2.51 1.3359 4.214 1.0167 2.40 2.53 1.3394 4.531 1.0223 2.85 2.55 1.3392 4.392 1.0146 3.95 2.60 1.3392 4.3

RM (cm3 mol−1)

T (days) N° 1 N° 2 N° 3 N° 4

0 6.9370 5.7967 5.0403 4.22267 6.9056 5.7468 5.0199 4.180614 6.9064 5.7455 5.0390 4.232531 6.9008 5.7514 5.0091 4.207192 6.9771 5.8088 5.0208 4.2390

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time. Note that solution 4 of AA presents a similar variation than the onecorresponding with solution 4 of the mixture. Hence, the results fromthe first day up to the 30th day for this diluted solution could supportthe influence of this acid while from the 30th day up to the 92nd dayas well AA and sucrose could justify the variation of this property inthe mixture.

3.1.4. Refractive index (n)The values obtained for all the studied solutions are given from

Tables 2 to 5 while the graphics of the refractive index for those solu-tions against the storage time of 92 days at 25 °C can be seen in Fig. 4.The analyses of the graphics show first, that there are no significantvariations in the n values with the storage time and are only observedslightly decreasing in solution 3 of AA up to the 30th day (Fig. 4a) andin solution number 4 of CA (Fig. 4b) and the mixture (Fig. 4d) up tothe 15th day. The latter observation shows clearly that the variation ofn at the 15th day can be attributed to the CA because it is the only com-ponent that changed at the same time. In general, these results for thefour solutions demonstrate that the n values are stable with the storagetime. In otherwords the decomposition of the products does not changedue to the law of conservation of the mass.

3.1.5. Quantity of soluble solid (°Brix)Tables 2 to 5 summarize the values obtained in the determination of

thequantity of total soluble solids, expressed as °Brix for all the solutionsat 25 °C versus the storage time during the 92 days while in Fig. 5 thecorresponding variations of °Brix in function of the time are presented. The analysis of the graphics for the AA solutions practically does notvary with the storage time which suggests, despite of their decomposi-tion in water, that the quantity of soluble solids are stable in the studied

time because it is onlywhen there is an increase in the temperature thatthe AA undergoes other decomposition processes [24]. The solutions forthe CA (Fig. 5b) show practically a similar behavior and certain stabilitywith the time but, with exception of the most concentrated solutionwhose °Brix value shows a decreasing in 15 days. This fact for this solu-tion is related to the increase in the density value at that time, asobserved in the Fig. 1b perhaps due to the formation of citrate,monocitrate and subcitrate ions. Note that in Fig. 2b the most concen-trated solution shows the lower pH values than the other ones dueprobably to the presence in higher quantity of those ions. A very impor-tant result is that for the four solutions of the mixtures (Fig. 5d) thequantities of soluble solid values remain practically constant with thestorage time showing that they are stable by the time and that thedecomposition of the products does not change due to the law of con-servation of the mass, as explained above.

3.1.6. Molar refractivityThemolar refractivity [RM] for all the aqueous solutions at 25 °C was

calculated in accordance with the expression reported in the literature[25] and the results are summarized from Tables 2 to 5 for the AA, CA,sucrose and the mixture solutions, respectively while, the correspond-ing values for each solution were represented in function of the storagetime in Figure S1 (Supporting information). Taking into account that themolar refractivity is an additive property it is possible to calculate thetheoretical molar refractivities for the water and for AA, CA, and sucroseand the mixture by means of the atomic refractivities. Thus, for waterthe calculated molar refractivity is 3.7034 cm3/mol with a relativeerror of 0.58% in relation to the theoretical value while, for the mixturethe error is 2.83%. Figure S1 shows that for AA the behavior of [RM]withthe time is dependent on the concentration of the solutions, thus the

Fig. 1. Graphical presentation of density against time, for (a) ascorbic acid, AA (b) citric acid, CA (c) sucrose and (d) mixture of the three components at 25 °C. The numbers from 1 to 4represent the more concentrated and diluted solutions, respectively.

Fig. 2.Graphical presentation of pH against time, for (a) ascorbic acid, AA (b) citric acid, CA (c) sucrose and (d)mixture of the three components at 25 °C. Thenumbers from1 to 4 representthe more concentrated and diluted solutions, respectively.

453L.C. Bichara et al. / Journal of Molecular Liquids 200 (2014) 448–459

Fig. 3.Graphical presentation of conductivity against time, for (a) ascorbic acid, AA (b) citric acid, CA (c) sucrose and (d)mixture of the three components at 25 °C. The numbers from1 to 4represent the more concentrated and diluted solutions, respectively.

454 L.C. Bichara et al. / Journal of Molecular Liquids 200 (2014) 448–459

four solutions show different decreasing values with time. The moreconcentrated solutions, these are solutions 1 and 2 evidence a higherdecrease in the 15th day while solution 3 presents two minima in the7th and 30th days and solution 4 has only a minimum at the 7th day.From the 30th day and up to the 92nd day all the solutions have

Fig. 4.Graphical presentation of refractive index against time, for (a) ascorbic acid, AA (b) citricto 4 represent the more concentrated and diluted solutions, respectively.

different behaviors with the time, as observed in Figure S1. Note thatthe decreasing of [RM] for solutions 1 and 2 is justified by the increasein the corresponding density values, as can be seen in Fig. 1a. In similarform, we observed that for the solutions of CA the slight variations of[RM], especially in the 15th day, are also attributed to the density values

acid, CA (c) sucrose and (d)mixture of the three components at 25 °C. The numbers from1

Fig. 5. Graphical presentation of soluble solids (°Brix) against time, for (a) ascorbic acid, AA (b) citric acid, CA (c) sucrose and (d) mixture of the three components at 25 °C. The numbersfrom 1 to 4 represent the more concentrated and diluted solutions, respectively.

Fig. 6. Comparison between the experimental infrared spectra of the four aqueous solu-tions of ascorbic acid (AA) at different concentrations during the first day and at the92ndday. Thenumbers from1 to 4 represent themore concentrated and diluted solutions,respectively.

Fig. 7. Comparison between the experimental infrared spectra of the four aqueous solu-tions of citric acid (CA) at different concentrations during the first day and at the 92ndday. The numbers from 1 to 4 represent the more concentrated and diluted solutions,respectively.

455L.C. Bichara et al. / Journal of Molecular Liquids 200 (2014) 448–459

Fig. 8. Infrared spectra of the four aqueous solutions of sucrose at different concentrationsat the first day and at the 92nd day. The numbers from 1 to 4 represent the more concen-trated and diluted solutions, respectively.

Fig. 9. Infrared spectra of the four aqueous solutions of themixtures at different concentra-tions at thefirst day and at the 92ndday. The numbers from1 to 4 represent themore con-centrated and diluted solutions, respectively.

456 L.C. Bichara et al. / Journal of Molecular Liquids 200 (2014) 448–459

and, in particular, the significant decreasing for the most diluted solu-tion is strongly related to their density (Fig. 1b). For the solutions of su-crose, only themost concentrated solution in the 15 days showed a littledecreasing on the [RM] value. Themost important result obtained in thisstudy is observed for the mixtures of aqueous solutions because theircomponents in the proportions which were prepared produce [RM]values practically constantwith the storage time. These results probablyindicate thatwhen themolar refractivity is constant there are no chang-es expected in the properties of these solutions due to polarizationunder the influence of the electric fields, as reported by Pacak [26].These consequences are very important if these mixtures simulatemodel orange juices.

3.1.7. Infrared spectraIn this study, we present only the analyses of the IR spectra in the

aqueous solutions at different concentrations of AA, CA, sucrose and ofthe mixtures of the three components during the first day and at the92nd day in which they were studied due to the more significant varia-tions that they show. These spectra of the four solutions of AA, CA, su-crose and the mixtures of the three components are given from Figs. 6to 9, respectivelywhile Figure S2 presented the IR spectra of themixtureof ascorbic and citric acids and sucrose in solid state and in aqueoussolutions for the most concentrated solution 1 at the first day and atthe 92nd day. The observed wavenumbers for the mixture in solidphase and for the solution 1 during the first day and at the 92nd daycompared with the bands observed for AA, CA and sucrose togetherwith the corresponding assignments can be seen in Table 6. A generalobservation is the notable diminishing in the number of bands whenthe mixture changes of solid phase to solution are due to the wideningof the bands as a consequence of the dilution.

3.1.7.1. IR spectra of ascorbic acid. Fig. 6 shows for the four AA solutionsthe variations of the IR spectra recorded on the first day and at the92nd day. During the first day of preparation of the solutions, in the re-gion of higher wavenumber it is observed that when the dilution in-creases the bands at 3409 cm−1 undergo widening due to theformation of H bonds and to the decomposition of this acid in ascorbateions and in the oxidized A species, as indicated in Table 6 [16–18]. Themost important changes in the region between 1800 and 400 cm−1

are, for example, the intensities of the two bands at 1753 (C_Ostretching) and 1672 cm−1 (C_C stretching) change notably from solu-tion 1 to solution 4, thus, the first band decreases with hydration whilethe other one increases their intensity. These observations were justi-fied by Hvoslef and Klaeboe [27] because both modes in aqueous medi-um increase the double bond character. Note that in the 92nd day thedecrease in the intensities of both bands up to that the band at1753 cm−1 for solution 4 completely disappears. Also, the bands of me-dium intensities at 1353, 1139 and 1065 cm−1 change slightly the posi-tion and decrease their intensities with hydration. On the other hand, inthe lower wavenumber region the band at 686 cm−1 has a significantwidening with dilution. Thus, for solution 4 most diluted at the 92ndday we observed a notable reduction in the number of bands, as canbe seen in Fig. 6. The results reveal clearly the decomposition of thisacid with the hydration and the formation of new species, as reportedby Bichara et al. [12,14].

3.1.7.2. IR spectra citric acid. Analyzing the IR spectra for the solutions ofCA of Fig. 7, we observed that in the region of higher wavenumbersthere are no significant changes with the dilution while the moreimportant modifications are observed in the 1800–600 cm−1 region.Thus, the three IR bands in the spectrum in solid phase in 1756, 1708and 1698 cm−1 disappear in aqueous solution given two bands at

Table 6Observed wavenumbers (cm-1) and assignments for ascorbic and citric acids and sucrose and the more concentrate mixture (model orange juice).

Ascorbic acid (H2A)a Citric acidb Sucrosec Mixtured

IR,Raman

Assignment IR,Raman

Assignment IR,Raman

Assignmentc IRSolid

IR Solut

3523 s νO-H (H2A, HA-, A2-, A) 3535 νO-H 3564 m ν(O-H)w 3565 3538 vs,br3498 νO-H 3469 sh ν O-H 3525

3409 vs νO-H (H2A, HA-, A) 3394 s ν O-H 34123316 s νO-H (H2A, HA-) 3350 νO-H 3337 s ν O-H 3389 3302 vs.br

3247 νO-H 3257 sh ν O-H 33223217 m νO-H (H2A) 3132 m νaCH2 3226 3087 vs,br3030 s νO-H (H2A) 3035 νaCH2 op 3052 sh νC-H 30343002 m νaCH2 (H2A), νC-H (A) 2994 νaCH2 op 2993 w νaCH2 29972978 m νa CH2 (H2A) 2975 νaCH2 ip 2970 w νaCH2 29712960 w νa CH2 (A2-, A) 2961 w νsCH2 op 2958 sh νC-H2944 m νC-H (H2A), νa CH2 (HA-) 2933 w νO-H 2943 m νsCH2 2944 2945 sh2916 m νC-H (H2A, HA-), νs CH2 (H2A) 2916 m νC-H 29152903 vs νC-H (H2A, HA-), νs CH2 (H2A, A2-, A) 2902 m νsCH2 2984 sh2854 w νO-H(H2A),νsCH2(HA-)νC-H(H2A,HA-,A2-,A) 2849 sh2737 w νC-H (A2-) 2763 sh2641 sh νO-H (A2-) 2650 sh1753 m ν C = O (H2A, HA-, A2-, A) 1756 vs νsC = O1 1733 w δH2O 1757

1708 vs νaC = O3 1714 w δH2O1672 s ν C = C (H2A), δCOH (A2-), 1662 vw δH2O 1674 1677 vs1667 vs ν C = C (HA-),ν C = O (A), δ CH2 dim(H2A) 1698 sh νsC = O3 1648 s δH2O

1538 vw ν(C-C)1525 vw δCH2

1517 vw δCH2

1495b m νC-Cdim (H2A), wagCH2 (HA-), δ CH2 (H2A, A) 1493 vs δsCOH 1496 vw wagCH2 15021459 m wagCH2 (H2A), δ CH2 (H2A, HA-, A2-) 1469 w νaC-C2 1463 w ρC-H 1465 1455 sh1438 sh δCOH(HA-),wagCH2(A),δHOC(H2A),νC-C(H2A) 1430 m νC-C, δCH2 ip 1432 w wagCH2 1432

1426 w τ(O—H),τwH2O(2) 1428 s1413 sh τ(O—H),τwH2O(2)1399 sh ρC-H 1392

1387 w wag CH2 (H2A) 1389 m δaCOH, wagCH2op 1388 w δ(O-H),ρ’(C-H)1363 sh δCOH (HA-), wagCH2 (A2-), δOCH (A) 1365 w wag CH2 ip 1365 m τ(O—H),τwH2O(2) 13661353 m δCOH (H2A, HA-, A2-, A), δ CCO (H2A) 1358 w ρCH2 ip 1354 sh ρC-H 1347 w1344 vw δCCH(H2A,HA-,A2-),δΟCH (A) δCOH(H2A,HA-) 1340 sh δsCOH 1348 m ρC-H 13431321 m δCOH (H2A), δCCH (HA-) 1325 vw δsCOH 1325 m ρCH2 δ(O- H) 1323 1323 sh1302 sh ρCH2 (H2A, HA-), δ ΟCH (A2-)ν C-O (H2A), τw CC2 (H2A) 1308 w wagCH2 ip,δaCOH 1302 w ρ’(C-H) 1306

1292 w ρCH2 ip, wagCH2 ip 1293 w ρC-H1274 s δOCH (HA-,A2-,A),ρCH2 (A), wagCH2 (HA-),δCOH(H2A),δCCH(H2A) 1271 w δO-H 1276 1265 sh1246 m δCOH (H2A), ρCH2 dim (H2A), δ OCH (HA-), ρ CH2 (A2-, HA-) 1242 m νsC-O 1241 m δO-H 1246

1233 m1221 w τwCC2 (H2A), δ OCH (A2-),δCOH(A) 1214 m ρCH2 ip 1212 w δO-H 12271197 w δCOH (H2A, HA-), ν C-C (HA-, A2-),τw CC2 (H2A, A) 1191 vw δO-H 1200

1174 s ν C-O 1172 m δO-H 11771163 m νC-O 1152 m

1139 m τwCC2 (HA-, A2-), ν C-O (H2A, A), ν C-C (H2A) 1140 s νC-O 1141 m νC-O 11401130 s νC-O

1120 νC-O (H2A), δ COH (H2A) 1126 m νC-O 11201112 s νC-C (HA-, A),ν C-C (A2-) 1115 m νC-O

1105 s νC-O 1107 w1074b m νC-O (H2A, HA-, A2-, A) 1081 w δaCOH 1074 m νC-O 1071 1079 sh1065b m νC-O (H2A), ν C-C (H2A), τw CH2 (A2-) 1069 vs νC-O

1053 m νC-C 1055 s νC-O 10541046 sh νC-O (H2A, HA-, A2-, A) 1042 sh τwH2O(2),τ(O—H)

1036 sh wagCH2 op,τwCH2op 1039 s1026 vs νC-C (HA-), ν C-O (H2A, A) 1022 w νC-C 1028

1015 m τwH2O(2),τ(O—H) 10151004 m νC-O

990 m νC-O (HA-, A2-), ν C-C (H2A, HA-, A) βC = O (A) 994 s νC-O 991 998 sh966 vw νsC-C2 966 sh νC-O 945945 w νsC-C1 946 w τR1(A6) 922 925 w

924 vw τwCH2 (H2A, A) τ (OH) (A2-),ν C-C (HA-) 914 sh τwCH2 op 912904 w τwCH2 op, νC-O 899 sh τR1(A5) 872

870 νC-O (A) 881 w τwCH2 ip 885 sh ν(C-O) 852820b m νC-C (H2A), τwCH2 (HA-, A2-, A), ν C-O (HA-) 842 vw νaC-C1 836 s δ(OCC),τwCH2,νC1-

C5)825

795 sh γCOO2 799 vs783 vw γC-C (H2A, HA-) 785 vw wagH2O(1)756b s τ(OH)ip dim (H2A), βC-O (A2-) 755 sh βR1(A6) 759722 m γC = O dim (H2A), γC-C (A) 729 sh δCOO5 722 w βR2(A6) 736711b vw γC = O (H2A), βC-O (HA-) 714 sh βR3 (A6) 713 sh697 s τw CH2 dim(H2A), βC = O (H2A), βR1 (A2-), ν C-C (A), γC = O

(HÁ-)700 w δCOO4 701 vw ρH2O

(continued on next page)

457L.C. Bichara et al. / Journal of Molecular Liquids 200 (2014) 448–459

Table 6 (continued)

Ascorbic acid (H2A)a Citric acidb Sucrosec Mixtured

IR,Raman

Assignment IR,Raman

Assignment IR,Raman

Assignmentc IRSolid

IR Solut

686 w βC = O (H2A), ν C-C (H2A) 686 w γCOO5 686 w δC-C-C 686675b m ν C-O (H2A, HA-), τ (OH) (HA-) 666 vw γCOO3 666 sh δO-C-O649 τ(OH)op dim(H2A), γC = O (A2-) 640 w τ(O-H) 644 w ρH2O 643628 m βR1 dim(H2A), βC = O (A), βR1 (HA-) 627 vw τ(O-H),δCOO3 636 m ρH2O591 w γC-O (HA-), ν C-C (A2-)βR1 (H2A, A), βR2(HA-) 599 s τ(O-H),δCOO5,

δCCC,ρCOO5

594 m βR1(A5) 600

581 vs565b m δOCH(H2A), γC-O (H2A, A2-),βR2 (H2A, A2-, A), wagCC2 (A) 571 w γCOO4 570 w δO-C-O 573

541 w δCOO1 548 s ρH2O(1) 553520 sh γCOO6 537 w τO-Hw

496 w δCCO(H2A), wagCC2 (A2-) 504 w γCOO1 504 m τwH2O482 w τO-H

473 w γC = O (A) 477 sh γCOO6 474 m δO-C-C 474 477 w467 w wagH2O

449 w ρCC2 (H2A), Wag CC2(H2A, HA-)τ(OH) (A) 438 vvw ρCOO4 451 w τwH2O412 s τO-H 424 404 sh

396 w τ(OH) (H2A), βC-O (HA-, A2-) 397 w δCCC 393 s τO-H378 s τO-H

363 m τ(OH) (H2A), γC-O (A2-)ρ CC2 (A2-), βC = O (A) 367 w ρCOO3, ρCOO1 368 vs δC-C-O357 sh δC-C-O

340b w γC-O (H2A, HA-), γC = O (A) τ (OH) (H2A), βC-O (A2-), δ CCO (A) 348 w ρCOO6 342 sh τO-H323 vw δCCC 329 w τO-H

320 sh δO-C-C292b w βC-O(H2A), δCCO (A2-), βC = O (A), γC-O (HA-) 306 w δCCC 309 vw τw (O-C)

254 sh δCCC 252 sh δ(OCC),ρ’(C-C)238b m τ(OH) dim(H2A),δ CCC (A2-),τ (OH) (H2A, HA-, A), β C-O(HA-) 247 vw δCCC 241 sh δO-C-C223 m τ(OH) (H2A, HA-), β C-O(H2A), τCC (A2-), δ CCC (A) 237 vw δCCC 235 m δO-C-C

212 w δCCC op 212 vs δO-C-C208 m δCCC (H2A), τ (O-H) (A2-) 204 sh νOH—Ow180 w γC-C dim(H2A) 180 vvw τw2O(2),τ(O—H)

172 vw νOH—Ow166 vvw τR1(A5)

163 m τCCCO dim(H2A) 160 vw τO-Hw

148 m γC-O(H2A,) δ OCH (H2A), 140b νs (O-H–O)# 153 vvw τO-Hw

138 m τR1 (H2A, HA-,A) 138 vvw τR3(A6)122 w τR1 (H2A), τCCCC (HA-) 129 vvw νOH—Ow113 w τ CCCO (H2A), τR2 (A2-), δ CCC (A) 105b νa (O-H–O)#, δCCC ip 114 vvw OH—Ow91 m τ CCCO (H2A), τCCCC (HA-) 88b δCCC ip 90 τwO-C81 s τR2 (H2A) ,τCCCO (A2-), τCCCC (A) 82 τwO-C73 s τR2 (H2A, τR1 (A2-), τCCCO (A), τw Ring(HA-) 68b τwCC ip 70 τR2 (A5)43 vw τw Ring (H2A), τCCCC (A2-), τR2 (A), τR1 (HA-) 44b τwCC op 41 τ(O—H)

ν, stretching; δ, scissoring; wag, wagging; γ, out- of plane deformation; β, in- plane deformation; ρ, rocking; τ, torsion, τ,w, twisting;a, antisymmetric; s, symmetric; R, ring, s, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad; dim, dimer.

a From Ref. [12,14].b From Ref. [13].c From Ref. [19].d This work.

458 L.C. Bichara et al. / Journal of Molecular Liquids 200 (2014) 448–459

1727 and 1636 cm−1 when the solutions are more concentrated whilein solution 4 it is observed only one of these bands [19]. For this acid,the most important result is the high stability of the four solutionswith the dilution observing that the spectra at the 92nd day are practi-cally the same than those recorded on the first day, as observed in Fig. 7.These results are in agreement with those observed by the refractiveindex, °Brix and molar refractivity studies.

3.1.7.3. IR spectra sucrose. In the IR spectra of sucrose in water the moreconcentrated solutions practically have the same bands showing thatonly for the most diluted solution 4 the bands change with the hydra-tion, as shown Fig. 8. Probably, this fact is related to the formation ofthe dihydrate and pentahydrate-sucrose that stabilize the solutions, asobserved by Max and Chapados [23] and reported by Brizuela et al.[20]. In this case, the strong IR band at 1648 cm−1 is attributed to theOH deformation due to thewater molecules. The assignments for aque-ous saturated solutions of sucrose and solutions at different molarconcentrations of sucrose in water were recently published and charac-terized by infrared, HATR and Raman spectroscopies and, for these

reasons, they are not presented here. Tables 6 summarizes the IR andRaman bands observed for the solutions previously reported [12–17].

3.1.7.4. IR spectra mixtures (AA + CA + sucrose). The IR spectra of themixture of the three components in solid phase can be seen inFigure S2 compared with that corresponding to the most concentratedsolution in aqueous solution. The positions of the bands observed inboth spectra are presented in Table 6 togetherwith those correspondingto the AA, CA and sucrose components. The spectra show clear differ-ences notable in the positions and in the intensities of some bands.Thus, in the higher wavenumber region (Figure S3) we observed thatthe IR bands in solid phase in 3565, 3525 and 3412 cm−1 associatedwith the OH stretching of those components disappear in solution andthree new broad and intense bands are observed at 3538, 3302 and3087 cm−1 as a consequence of the H bonds formed due to the hydra-tion, as indicated in Table 6. Also, in this region a characteristic band at-tributed to thewater appears at 2125 cm−1. In the region between 1800and 400 cm−1 the more important changes in the hydration are evi-denced, as is shown in Figure S4. Hence, the two bands at 1757 and

459L.C. Bichara et al. / Journal of Molecular Liquids 200 (2014) 448–459

1674 cm−1 disappear with the dilution given place to a broad and in-tense band at 1677 cm−1 while in the region between 1520 and1175 cm−1 the set of bands observed in solid phase disappear in the so-lution given place to newbands at 1428, 1347 and 1233 cm−1. Note thatthe band at 1428 cm−1 is associated with the formation of dihydrate-sucrose [17] while the band at 1233 cm−1 is a band characteristic ofthe mixture in aqueous solution and can be clearly used to identify theAA, CA and sucrose components in solution. In the region between1172 and 956 cm−1 a typical band at 1039 cm−1, associated with thepresence of CA can be also used to identify the mixtures in aqueous so-lution. Finally, in the lower wavenumber region, it is between 956 and400 cm−1, that we observed that two bands at 799 and 581 cm−1 canbe easily used to identify the mixtures in solution of which the firstband is assigned to CA. Fig. 9 shows the IR spectra for the aqueous mix-tures on the first day and at the 92nd day while a comparison of the IRspectra of the mixture of ascorbic and citric acids and sucrose in solidstate with those obtained in aqueous solutions for themost concentrat-ed solution 1 at the first day and at the 92nd day is shown in Figure S2.Table 6 presented the bands of the mixture in solid phase and in aque-ous solution for themost concentrated solution 1. In the region of higherwavenumbers the spectra show a widening of the bands at 3565 and3389 cm−1 with the dilution but the contrary is observed with the stor-age time at the 92nd day. This way, the shoulder and the band at 1757and 1674 cm−1, respectively change with the dilution because thefirst disappears and the second one increases their intensity. Note thatthis band with the time decreases its intensity up to the 92nd day, ascan be seen in Fig. 9. The decompositions of these solutions with thestorage time are clearly observed in the shifting of some bands whilein the first day changes with the dilution for the four solutions are ob-served. The spectra at the 92nd day have shown that the bands are prac-tically the same despite the observed decreases in the intensities fromsolutions 1 to 4, as can be seen in Fig. 9. These observations suggestthat the stabilities of these solutions are related to the components ofthe mixture as CA and sucrose. These results support the use of CA asa preservative agent in the nutritious industries.

4. Conclusions

In the present work the stabilities during a time of 92 days of fouraqueous solutions at different concentrations of (i) ascorbic acid, (ii)citric acid, (iii) sucrose and, (vi) of sucrose containing ascorbic and citricacids were studied by using FTIR spectroscopy and physicochemicalproperties such as pH, refractive index, density, conductivity, quantityof total soluble solids expressed as °Brix and molar refractivity. Thestudy of the densities shows a same tendency for the four aqueous solu-tions of themixtures evidencing similar variationswith the storage timewhich could be attributed indistinctly to the ascorbic acid, citric acid orsucrose. The similar behaviors of the pH versus the time for the four so-lutions of CA and the mixtures suggest that the decreasing in the pHvalues of the mixtures could be attributed principally to CA. The linealbehavior for the most diluted mixture of the conductivity values withthe time from the first day up to the 30th day could be attributed tothe AA decomposition while from the 30th day up to the 92nd day thecomponents AA and sucrose in water could in part justify the variationof this property with the time. For the more concentrated solutions ofthemixtures probably sucrose supports the behavior of the conductivityvalues with the storage time due to their similar variations. The n andsoluble solid variations for the four solutionsdemonstrate that theprod-ucts of decomposition of each component separately and, in the

mixtures are stable with the storage time. These consequences arevery important taking into account that thesemixtures simulate dilutedmodel orange juices. The studies by IR spectroscopy show that the sta-bilities of these solutions are related principally to the citric acid andsucrose components supported by the hydrate-sucrose formation, as re-ported in the literature. The comparison of the IR spectra of themixturein solid phase with the corresponding experimental one in solution ev-idences the formation of new bands at 1428, 1233, 1039, 799 and581 cm−1 associated with the products of decomposition of each com-ponent in the mixture. Finally, the bands observed in the IR spectra ofthe solutions concentrated of AA, CA, sucrose and the mixture werecompletely assigned to their decomposition products in solution.

Acknowledgments

This work was subsidized with grants from CIUNT (Consejo deInvestigaciones, Universidad Nacional de Tucumán).

Appendix A. Supplementary data

Table S1 and Figures from S1 to S4 are the Supporting information.Supplementary data associated with this article can be found, in the on-line version, at http://dx.doi.org/10.1016/j.molliq.2014.10.038.

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