Microwave heating of water-ethanol mixtures

Microwave heating of water-ethanol mixtures

G. Charalambous (Ed.), Food Flavors: Generation, Analysis and Process Influence © 1995 Elsevier Science B.V. All rights reserved 1065 MICROWAVE HEAT...

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

1065

MICROWAVE HEATING OF WATER-ETHANOL MIXTURES A.Paoli and A.Schiraldi DISTAM, Universita di Milano, Via Celoria 2, 20133 Milano, Italy Abstract The 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 irradiated surface, the extintion coefficient and heat capacity of the chemical compound considered. When a homogeneous mixture is subjected to microwave irradiation, the heating rate depends 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 the pure water data go through a maximum at the azeotropic composition.

1. INTRODUCTION Extended use of domestic microwave ovens to thaw and even cook a number of foods has raised the problem of reducing the aroma flash off when the concerned chemical compounds can 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 to produce 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 operational criteria. One of these (5) states that the susceptibility of a given compound. A, to the microwave action can be appreciated from the resulting heating rate, (dT/dt). A simple tool to evaluate the effects produced by a given MW exposure is represented by the ratio AT'=-

(dT/dt), (dT/dt) ,

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

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

^

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

1066

Ar-

\^ 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 with the values reported in literature: as a rule, a poor agreement was found either v^ equation 1 or vs other authors' experimental data. This stimulated the present investigation that was mainly aimed to reexamine the experimental and formal approach to AJ'. The simple case of water-ethanol mixtures was considered to emphasize the effect of the competition between different compounds that strongly interact with 2450 MHz microwaves. 2 . MATERIALS AND METHODS Deionized distilled water and pure ethanol (Merck) were used. 15 g samples of water-ethanol mixtures of various composition were exposed for 90 seconds to the microwaves in a properly designed oven (ALM 1600, SFAMO, Plombieres, France), the source power being adjusted at 100 Watt. The sample were contained in 25 mL glass beckers settled over the base disk of the oven in positions of ascertained (7) irradiation density. An optical fiber thermocouple dipped in the sample at 0.5 cm from the upper surface was used to measure the temperature at 10 s intervals (see Figure 1). The maximum temperature reached did not exceed 50°C.

OPTICAL FIBER SEIVSORS

IZ3

u Figure J. Experimental assembly to determine AJ'

1067 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 20 calorimeter. Figure 2 shows some typical examples of the Cp data obtained.

4.5

4H EtOH 50%

^

3.5

^

3

EtOH 96%

EtOH 100% 2.5 H

—I

0

20

40

1—

60 T/'*C

—I—

80 80

100

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 given composition, X(EtOH), of the water-ethanol mixture.

1068

EtOH 96% EtOH 100% EtOH 80% EtOH 60% EtOH 40% EtOH 20% H2O

100

Figure 3. Temperature records at various A4W exposure time. Since these experimental data concern open samples, such a behaviour might be due to the fact that a major part of the trapped MW energy produces a temperature increase during the early exposure, whereas this effect is later on reduced by the endothermic process of evaporation. These data were fit with simple exponential Sanctions, T(t) ^ 20 + a t, that allowed the relevant (dT/dt)-vs-t trend to be drawn. The corresponding AT'-vs-TtmcQS were thence easily obtained (see Figure 4): these clearly show that AT strongly depends on the reached temperature and goes through a broad maximum at about 30°C.

EtOH 96%

EtOH 20%

20

25

30

35

40

45

H2O

50

55

T rc Figure 4. AT -vs- Tfor various Water - Ethanol mixtures

60

1069 This picture is more realiable than the l^T'-vs-t traces (see Figure 5) obtained according to equation 2: these indeed give the false impression that AF' would be poorly affected by the exposure time {i.e., the temperature increase).

2.2 EtOH 96%

2

EtOH100%

1.8 EtOH 80%

i_1.6 1.4

_ E t O H 60%

1.2

EtOH 40% -

1 0.8

EtOH 20%

1—

20

40

60 Time (sec)

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 the literature p and a data, and our experimental Cp results. The relevant trends vs X(EtOH) are reported in Figure 6. Since the sample considered are in the liquid state, where convection rapidly modify the temperature profile, the "predicted" AT" was referred to a very small penetration depth: in practice the trend reported in Figure 6 can be directly referred to the exposed surface. It must be emphasized that the "predicted" AT does not imply significant changes within the 20 - 40°C range; the trend reported in Figure 6 can be therefore considered reliable at any intermediate temperature. Figure 7 reports the comparison between "predicted" and measured AT (at three different temperatures, 20, 30 and 40 °C).

1070 1.2

-

H.U

* " - - - . ^ C p / (J/g K) 4 1

?

3.5 -

p i

[.^i^^^^)^^''^^^^^^

O)

3 — Q.

o

^ y

3 -

\ ^*S,^ \

* / ' „

AT-

1

1

20

40

^

1- 0.2

,..---•

1^ 0.5 -

^

E

-^ ' t0.4

^*"'

f^

^

1

\

y

1.5 ^

0.8

0.6

l{cm')

^/

2 H-

<

-«-Hjr

y

2.5 -

^

E

^

j _ ^

60

80

100

X (%EtOH) Figure 6. Density, p, attenuation factor, a, specific heat, Cp, and predicted ST' vs the % weigh composition. These data refer to 25°C. 3.5

2.5-1

• Calc. trend T = 20°C T = SOX T = 40X

2 1.5 1 0.5-j 0

—I—

20

—I

1—

40

60

80

100

X (EtOH)

Figure 7. Comparison hePA^een predicted and experimental tsJ*

1071 Since equation 1 does not account for any evaporation, such a comparison should be more reliable when limited to the early phase of the MW irradiation, namely at temperatures largely below the boiling point. According to the above considerations, the larger the observed AJ', the smaller the MW energy spent to sustain sample evaporation; the maximum AT" values should therefore correspond to the pure heating effect of the microwaves. When these AJ' data are compared with the "predicted" AJ' significant deviations appear with a maximum about the azeotropic composition (Figure 8).

1.4

Figure 8. Excess AT' with respect to equation I at 30°C. Such a behaviour would therefore reveal that the azeotropic mixture, even at temperature far below the respective boling point, can be single out from the composition range, probably because of a peculiar structural arrangement at the molecular level, that allows a more effective MW energy partition than any other Water-Ethanol mixture to the overall temperature increase.

1072 4. CONCLUSIONS The results of this work clearly show that the quantity AJ' is indeed useful to predict the heating rate produced by MW radiation, provided that it can be determined in the early exposure. Once properly evaluated, AF can be also of some help to understand the direct interaction between microwaves and molecules.. In the present case it was possible to single out the azeotropic mixture even at temperatures far below the respective boiling point, in spite of the fact that all the other physical properties do not reveal any singularity at this composition. References 1

J.A. Steinke, C. Frick, K. Strassburger, J. Gallagher, Cereal Foods World, 34 (1989) 330-332.

2

C. Whorton and G. Reineccius, ibidem, 35 (1990) 553 - 559

3

M.Riva, L.F. Di Cesare and A.Schiraldi. G.Charalambours (ed). Food Flavor Ingredients and Composition. Amsterdam, (1993) 327.

4

L.F. Di Cesare, M.Riva, GSansovini and A.Schiraldi. H.Maarse and D.G Van Der Reij (eds). Trends in Flavoue Research. Amsterdam - London - New York - Tokio, (1994) 121.

5

R.E. Mudgett, Food Technol, 40 (1986) 84 - 98

6

N.A. Shaath and N. R. Azzo, (1989) in "Thermal Generation of Aroma", T. H. Parliment, R J. McGorin and C-T. Ho, Eds), Am. Chem. Soc. Publ, chap. 48, 512 525

7

M. Riva, L. Franzetti, A. MattioU and A. Galli, Ann. Microbiol. Enzymol., 43 (1993) 115

8

Handbook of Physics and Chemistry, 1982

9

RE. Mudgett, D. I. C. Wang and S. A. Goldblith, J. Food Sci., 39 (1974) 632 - 635

Research supported by National Research Council of Italy, Special Project RAISA, Subproject N. 4, Paper N. 1487