COMPARATIVE STUDY OF SPECIFIC HEAT CAPACITIES OF SOME VEGETABLE OILS OBTAINED BY DSC AND MICROWAVE OVEN

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1 Jounal of Temal Analysis and Caloimety, Vol. 79 (2005) COMPARATIVE TUDY OF PECIFIC HEAT CAPACITIE OF OME VEGETABLE OIL OBTAINED BY DC AND MICROWAVE OVEN J. C. O. antos 1*, M. G. O. antos 2, J. P. Dantas 1, Mata M. Conceio 2, P. F. Ataide-Filo 3 and A. G. ouza 3 1 Depatamento de Química, CCT, Univesidade Estadual da Paaíba, Campus I, Bodocongó, Campina Gande, PB, Bazil 2 Depatamento de Química, Univesidade Fedeal do Rio Gande do Note, Campus Univesitáio, Natal, RN, Bazil 3 Depatamento de Química, CCEN, Univesidade Fedeal da Paaíba, Campus I, Joo Pessoa, PB, Bazil Conventional metods ave been poposed to detemine temal popeties of edible vegetable oils. Te evaluation of te applicability of DC and micowave oven (MO) metods to detemine te specific eat capacities of te edible vegetable oils was pefomed. It was obseved tat te specific eat capacities of eac edible oil inceased as a function of te satuation of te fatty acids. Keywods: DC, edible oils, micowave, specific eat, temal analysis Intoduction It is essential to know te temal beavio of te edible vegetable oils, tei cemical composition and pysical popeties fo a igoous contol of pocesses and setting up standads fo eac specific use [1 3]. Te specific eat capacity (C p ) can be consideed as one of te possible pysical popeties. Te knowledge of te specific eat capacities of te oils and fats ae quite useful to detemine tei beavio duing diffeent tecnological pocesses. Despite te specific eat capacities ae simila fo te tiglyceides in tei oiginal pysical state, tey can incease as a function of te unsatuation of te fatty acids in te liquid and also in te solid state. It is impotant to empasize tat te numeical values of te specific eat capacities of liquid fats ae twice lage compaed to tat of solid fats and also tat te fom as a ige eat capacity tan te fom exibits. All tese ae diectly elated to te mobility of te oil and fat molecules in te diffeent pysical states [4]. Te mola eat as a geat impotance in temocemical pocesses and it is elated to te eat of tansfomation, Q p, at constant pessue, p, asitissown below: dh=q p (1) Q p depends on te eat cange and does not depend on te evesibility of tese pocesses. Te specific eat capacity (C p ) is calculated by Eq. (2): C p dh Qp (2) dt dt In te last yeas, new analytical metods ave been developed by eseaces to caacteize te pocessing and stoage conditions. Te application of temoanalytical metods fo tis study and te caacteization of oils and fats as inceasing inteest. Teefoe, using temoanalytical metods, as TG, DTA and DC fo te caacteization of oils and fats as distinguised ole in food industy [5 9]. In te pesent wok, te specific eat capacities of some edible vegetable oils wee detemined by DC and compaed to te esults obtained by micowave oven (MO) pocedue. Expeimental Mateials ix edible vegetable oil samples, soybean, olive, apeseed, ice, con, sunflowe and two mixtues, olive (3%)/soybean (%) and olive (30%)/sunflowe (70%), wee used in tis study. Te composition of edible vegetable oil samples was listed in Table 1. DC measuements A imadzu model DC-50 Diffeential canning Caloimete was used. Te edible vegetable oil samples wee placed in aluminum cucibles, eated wit =5 C min 1 up to 30 C and wee kept fo 5 min ten eated wit =10 C min 1 up to 200 C, unde a dynamic atmospee of nitogen (50 ml min 1 ). Te initial sample masses wee mg. Tee mea- * Auto fo coesopndence: oleos@ig.com.b /$20.00 Akadémiai Kiadó, Budapest, Hungay 2005 Akadémiai Kiadó, Budapest pinge, Dodect, Te Netelands

2 ANTO et al. Table 1 Composition of some edible vegetable oil samples Fatty acids/% Edible oils Monounsatuated Polyunsatuated atuated Atificial antioxidants Olive Rapeseed unflowe Con citic acid/tbhq oybean citic acid/tbhq Rice ban oybean/olive unflowe/olive citic acid suements wee caied out fo eac sample: 1) measuements wit empty cucible; 2) measuements wit a efeence mateial (alumina) wit a known specific eat capacity; 3) measuements of te specific eats of te edible oil samples. Micowave oven measuements A BRATEMP model 38MWO micowave oven witout any adaptation was used to obtain te specific eat by te micowave oven metod. Te expeiments consisted in monitoing te measuements of te eating tempeatue of wate (used as standad) and of te edible oil samples unde contolled conditions (P=900 W, t=30, 60 and 90 s). Te sample mass was 150 g and a temomete was used to measue te tempeatue peiodically. Results and discussion DC measuements Te elationsip between te eat capacity of te sample C s (sample pan suppot+sample pan+sample), of te efeence mateial C (efeence mateial suppot+efeence mateial pan+efeence mateial) and te eating ate () is expessed as: C C T T (3) R wee T s is te tempeatue of te oil sample; T is te tempeatue of te efeence mateial; R is te instument constant (esistance between sample, efeence mateial and funace) and T s T is te DC displacement, wic is popotional to te diffeence between C s and C. If te DC displacement is and te popotionality constant is k, te expession will be: C C =k (4) Consideing tat te eat capacities of te edible vegetable oil sample suppot and te efeence mateial suppot can be epesented by C and C, te specific eat capacities of te efeence mateial and of te oil sample can be epesented by c 0 and c, futemoe te mass of te efeence mateial and of te oil sample ae m 0 and m, espectively, te following elationsips ae obtained: C C k1 (5) ( C m c ) C k (6) ( C mc) C k3 (7) wee 1, 2, 3 ae te temal displacements of te DC elated to te blank, te efeence and te sample. Figue 1 sows te ecoded DC cuves fo an edible vegetable oil sample and fo te blank and efeence mateial. Fig. 1 DC cuves: a edible oil sample, b blank and c efeence mateial Fom Eqs (5) (7) te following equation is obtained: mc 3 1 (8) mc J. Tem. Anal. Cal., 79, 2005

3 VEGETABLE OIL ince te specific eat capacity of te efeence mateial is known, te specific eat capacity of te edible oil samples can be calculated using Eq. (9): mc c (9) m 2 1 1, 2 and 3 ae continuous functions elated to te tempeatue, consequently te specific eat capacities can be detemined continuously [5]. Fo all te investigated edible oils, te specific eats do not vay substantially and tese values can be used fo industial engineeing puposes. Te specific eat values obtained fom te DC cuves of te investigated vegetable edible oils in te ange of C ae listed in Table 2. In te investigated tempeatue ange, te specific eat capacity values fo te olive oil samples and fo te mixtues containing olive oil wee ige tan tose of te ote oils. Tese diffeences can be attibuted to a ige concentation of monounsatuated fatty acids ae pesent in te olive oil, as it as been indicated in Table 1 [6]. In geneal, te esults demonstate tat te specific eat capacity pobably depends on te unsatuation degee of te analyzed oils. It can be deduced fom te expeimental data tat fo te edible vegetable oils te specific eats did not vay substantially in te studied tempeatue ange. A typical C p cuve obtained fo ice and sunflowe oils is sowninfig.2. Compaing te esults fo te edible oil samples wit tose obtained by ote autos [4, 7, 10], a small Fig. 2 pecific eat capacity vaiation of te ice and sunflowe oils as a function of tempeatue diffeence in te specific eat values wee obseved, wic pobably can be attibuted to te used metod. Micowave oven metod Te evaluation of te specific eat capacity values was done by calculations wic took into account te effectively absobed powe by te samples duing eating, as it is sown by Eq. (10): P kc m T p (10) t Table 2 pecific eat capacities (C p ) of te edible oils between C obtained by DC Tempeatue/ C C p /J g 1 K 1 olive soybean con ice sunflowe apeseed olive/sunflowe olive/soybean J. Tem. Anal. Cal., 79,

4 ANTO et al. Table 3 pecific eat capacities (C p ) of te edible vegetable oils Edible oils Tempeatue/ C Micowave oven C p /J g 1 K 1 DC Olive oybean Con Rice unflowe Rapeseed Olive+sunflowe Olive+soybean wee P is te absobed powe (W), k is te convesion facto fom cal/s to watt, C p is te specific eat of te sample, m is te sample mass and T is te vaiation of tempeatue duing te exposition time (t) [11]. Befoe pefoming te expeiments wit te vegetable edible oil samples, wate was used as [11]. Te tempeatue vaiation was measued upon funising a ig powe fo a defined time inteval to 1501 g of wate. Using Eq. (10) and consideing tat te specific eat of wate is 4.18 J g 1 K 1,itwas possible to calculate te powe tat te wate effectively adsobed duing te micowave teatment. Using te values of te powe effectively absobed by te wate and te tempeatue vaiation of te oils wit te same mass and exposition time, te specific eat capacities of te edible vegetable oils could be detemined using Eq. (10). Te specific eat capacity values obtained by te MO metod (Table 3) sow tat tee ae no significant diffeences in te values of te edible vegetable oil samples analyzed, indicating te low sensitivity of te metod. Compaing tese esults wit tose obtained by DC, it was obseved tat specific eat values ae ige fo all samples, except fo tese containing olive oil. Tese diffeences can be attibuted to te amount of te oil samples used in te micowave, wic ae muc ige tan tat ave been used in te DC expeiments. Conclusions It was obseved tat te specific eat capacities of eac edible oil inceased as a function of te satuation of te fatty acids, despite of being simila fo te tiglyceides in tei oiginal state. Teefoe it is concluded tat te specific eats of te edible vegetable oils depend on te composition of fatty acids. Tus, it is impotant to know te specific eats of edible vegetable oils at constant pessue, as tese data ae elpful duing long time of stoing, feeze-dying, etc. Te development of an altenative metod using DC tecnique fo detemination of specific eat capacities of edible vegetable oils was studied in tis wok, is diffeent of te metods employed by ote eseaces. Te MO was used as a compaative tecnique and te esults obtained indicated tat te micowave oven metod is unsatisfactoy. It was concluded, DC is elpful tool to measue tese values, being moe apid tan te micowave oven measuements, equiing smalle sample amounts and often yielding geate accuacy. 286 J. Tem. Anal. Cal., 79, 2005

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