mo heating ethanol-water

8/11/2019 MO Heating Ethanol-water

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G. Charalambous (Ed.), Food Flavors: Generation, Analysis and Process Influence 1995 Elsevier Science B.V. All rights reserved 1 0 6 5

MICROWAVE HEATING OF WATER-ETHANOL MIXTURES

A.Paoli and A.Schiraldi

DISTA M, Universita di Milano, Via Celoria 2, 20133 M ilano, Italy

AbstractThe susceptibility of a given chemical compound shows to the microwave action can be

described with the resulting heating rate, that depends on the distance from the irradiatedsurface, the extintion coefficient and heat capacity of the chemical compound considered.

When a homogeneous mixture is subjected to microwave irradiation, the heating ratedepends on the average values of the above parameters. In the case of water-ethanol mixtures,the experimental heating rate scaled with respect to those of th pure water data go through amaximum at the azeotropic composition.

1. INTRODUCTION

Extended use of domestic microwave ovens to thaw and even cook a number of foods hasraised the problem of reducing the aroma flash off when the concerned chemical compoundscan directly interact with 2450 MHz microwaves (1,2). On the other hand, microwave heating

was proven (3, 4) to promote aroma release from vegetal tissues and therefore proposed toproduce concentrated essential oils or MW aromatized products.Significant help in either kind of investigations would come from the possibility to predict the

effects of the MW treatment; efforts have therefore made to envisage simple operationalcriteria. One of these (5) states that the susceptibility of a given compound. A, to themicrowave action can be appreciated from the resulting heating rate, (dT/dt). A simple tool toevaluate the effects produced by a given MW exposure is represented by the ra tio

(dT/dt),AT =-

(dT/dt) ,^ ^ water

that directly implies water as a reference compound (5, 6). AT can be tentatively predicted as

p ( ^ ) C > ( ^ ) x e x p [ - 2 a ( w ) z ] ^

where p, Q?, a and z stand for density, heat capacity at constant pressure, MW attenuationparameter and depth from the exposed surface, respectively. It was also suggested (6) that ATcould be reasonably approximated as


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A r -\^ end ^ start ^

W end~ ^ start)

2)

water

where the temperature change across a given exposure time is considered.For a number of aroma compounds the relevant AJ' was experimentally determined (3, 4) in

our laboratory and, when possible, compared w ith the values reported in literature: as a rule, apoor agreement was found either v^ equation 1 or vs other authors' experimental data. Thisstimulated the present investigation that was mainly aimed to reexamine the experimental andformal approach to AJ'. The simple case of water-ethanol mixtures was considered toemphasize the effect of the competition between different compounds that strongly interactwith 2450 MHz microwaves.

2 . MATERIALS AND METHODS

Deionized distilled water and pure ethanol (Merck) were used. 15 g samples of w ater-ethanolmixtures of various composition were exposed for 90 seconds to the microwaves in a properlydesigned oven (ALM 1600, SFAMO, Plombieres, France), the source power being adjusted at100 Watt. The sample were contained in 25 mL glass beckers settled over the base disk of theoven in positions of ascertained (7) irradiation density. An optical fiber thermocouple dipped inthe sample at 0.5 cm from the upper surface was used to measure the temperature at 10 sintervals (see Figure 1). The maximum temperature reached did not exceed 50C.

uIZ3

OPTICAL FIBER SEIVSORS

Figure J. Experimental assembly to determine AJ'


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Density and MW attenuation factor of the water-ethanol mixtures were drawn from literature(8 , 9), whereas their heat capacity was experimentally determined with a Mettler DSC 20calorimeter. Figure 2 shows some typical examples of the Cp data obtained.

4.5

4 H

^ 3.5

^ 3

2.5 H

EtOH 50%

EtOH 96%

EtOH 100%

I 1

40 60T/ '*C

I

8020 40 60 80 100T/ *C

Figure 2. Specific heat capacity at constant pressure of some Water - Ethanol mixtures.

3. RESULTS AND DISCUSSION

For every investigated mixture the temperature increase v^ the MW irradiation time, /,showed a bending trend. Figure 3 shows the bunch of curves, each referred to a givencomposition, X(EtOH), of the water-ethanol mixture.


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This picture is more realiable than the l^T -vs-t traces (see F igure 5) obtained according toequation 2: these indeed give the false impression that AF' would be poorly affected by theexposure time {i.e., the temperature increase).

2.2

2

1.8

i_1.6

1.4

1.2

1

0.8

EtOH 96%

EtOH100%

EtOH 80%

_ E t O H 6 0 %

EtOH 40%

- EtOH 20%

1

60Time (sec)

20 40 80 100

Figure 5. AT -vs - exposure time for various Water - Ethanol mixtures.

Equation 1 allowed determination of a predicted A7''-v5-X(EtOH) trend according to theliterature p and a data, and our experimental Cp results. The relevant trends vs X(EtOH) arereported in Figure 6. Since the sample considered are in the liquid state, where convectionrapidly modify the temperature profile, the predicted AT was referred to a very smallpenetration depth: in practice the trend reported in Figure 6 can be directly referred to theexposed surface.

It must be emphasized that the predicted AT does not imply significant changes within the20 - 40C range; the trend reported in Figure 6 can be therefore considered reliable at anyintermediate temperature.

Figure 7 reports the comparison between predicted and measured AT (at three differenttemperatures, 20, 30 and 40 C).


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mo heating ethanol-water

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