Ind. Eng. Chem. Res. 1993,32, 2357-2364
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Influence of the Development of Particle Morphology upon Rates of Loss of Volatile Solutes during Drying of Drops Jorge M. Sunkelt and C. Judson King' Department of Chemical Engineering, University of California, Berkeley, California 94720
An electron-capture detector was used to monitor instantaneous rates of loss of SF6 during drying of suspended drops of several different aqueous solutions. Simultaneously the appearance of the drop was recorded with a videocamera, thereby enabling the rate of evolution of SF6 to be related to changes in particle morphology. Early in drying, rates of loss of SF6 decrease toward zero as selective diffusion develops. For coffee extract frothy arms of liquid protrude from the drop surface later in drying and produce surges of SF6 evolution. For maltodextrin solutions there was substantial loss of SF6 due to repeated expansion, bursting, and collapse, as bubbles formed repeatedly within a drop. Drops of nonfat milk remained smoother and more spherical in appearance, with much less tendency for surface disruptions. Nonetheless, a t higher temperatures milk drops showed periods of substantial SF6 loss later in drying.
Introduction The flavor and aroma of a food beverage typically reflect a complex bouquet of numerous highly volatile organic compounds. Balanced and substantial retention of these substances is desirable for consumer appeal. Although widely used as a method of dehydration, spray drying has suffered from poor retention of volatile flavor and aroma. As understanding of the mechanisms of loss of volatile solutes has grown (Thijssenand Rulkens, 1968;King, 19881, it has become possible to produce spray-dried products with considerably improved flavor and aroma. An understanding of factors governing loss and retention of highly volatile organic solutes in spray drying is also important for control of emissions of volatile organics (VOC's) and deodorizing during drying. For some time it has been established that substantial losses of volatile compounds occur during atomization in spray drying (Kieckbusch and King, 1980), and that selectivediffusion sets in and greatly reduces or eliminates loss once the surfaces of the drops have reached a high enough dissolved-solid content (Thijssen and Rulkens, 1968;Etzel and King, 1984). It has also been known that complex changes in the morphologies (size, shape, and appearance) of drops occur during spray drying. These changes can expose interior liquid (Charlesworth and Marshall, 1960;El-Sayed et al., 1990),with consequent potential for further volatiles loss. Whereas the extents and mechanisms of volatiles loss during atomization and by diffusion from spherical droplets are reasonably well understood (King,1988,Kerkhof andThijssen, 1977),there is very little information in the literature from which one could gauge the extent and nature of volatiles loss, if any, due to morphological developmentof drops during drying. Any experimental effort to determine the effect of morphological development upon volatiles retention by means of experiments in a spray dryer is thwarted by the facts that gas-phase mixing is intense and droplets or particles sampled at any location have different sizes and have arrived through different temperature fields. We have carried out experiments previously samplinga single stream of uniform drops falling through a controlled and reproducibletemperature field (Greenwaldand King, 1982; Alexander and King, 1985;Wallacket al., 1990). However,
* To whom correspondence should be addressed.
+ Present address: Department of Chemical Engineering, University of Washington, Seattle WA 98195.
0888-588519312632-2357$04.00/ 0
even then individual drops develop substantially different morphologies and degrees of expansion,probably because of differences in the number and effectiveness of bubblenucleation sites present within different drops. We have therefore chosen to carry out studies of the drying of individual drops. However, for individual drops quantitative monitoring of highly volatile solutes poses problems. Measurement of the amount of volatile solute in the drop requires detecting very small quantities and is probably restricted to a single analysis at some point during drying. Continuous measurment of the amount of solute in the gas requires quantitative detection at very low concentrations. We have chosen the latter route. Verderber and King (1992) initially developed and presented an approach whereby volatile flavor and aroma compounds are modeled experimentally by a highly halogenated volatile substance, SF6, which is detected continually in the gas stream downstream of a singledrying drop by means of an electron-capture detector. The six very electronegativefluorine atoms on SF6and the extreme sensitivity of the electron-capture detector to such atoms give this method of analysis more than sufficient sensitivity. This technique is coupled with in-situ videorecording of the appearance of the drop, thereby enabling the instantaneous rate of loss SF6 loss to be correlated with the appearance of the drop. SF6 is an acceptable model volatile solute because losses of all highly volatile compounds are governed by condensed-phase mass transfer and are therefore mechanistically equivalent. Verderber and King (1992)demonstrated that this technique is workable and that large losses of this volatile tracer occurred late in drying for drops of various aqueous solutions, coincident in time with expansions and surface eruptions. In the present work we have improved the apparatus of Verderber and King to enable operation a t higher temperatures, more frequent sampling of the gas, and less loss of information due to axial mixing of the gas. The improved apparatus has been used to discern the effects of morphological development upon loss of SF6 during drying of drops of aqueoussolutions of maltodextrin,coffee extract, nonfat milk, and mixtures of coffee and nonfat milk.
Apparatus and Procedure A schematicdrawing of the apparatus is shown in Figure 1. Flowing nitrogen (99.998% purity, Matheson Corp.) 0 1993 American Chemical Society
2358 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993
T
Teflon Needle V.IW
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40150 Male-Female Ground Glass Joint
Figare 1. Experimental apparatus.
was used as the heating medium. Traces of oxygen and water were eliminated by contacting the gas stream with a heated zirconium-based catalyst in a high-capacity gas purifier (Supelco Corp.). Ultrahigh gas purity avoids interferences due to oxygen atoms. Purified nitrogen was delivered to the drying column by steel and rubber tubing. Thenitrogen flowrate, measuredby arotameter, was 1.00 Llmin, measured at room temperature and pressure. The gas heating section consisted of a quartz glass tube, 60 cm long and 3.6 cm outside diameter (0.d.). The gas inlet was located 6 cm above a female ground-glass joint, fused to the bottom of this section. Another section of quartz tubing, 55 cm long, 1.5 cm o.d., and closed at the bottom, served as the heater and was fused to a male ground-glass joint placed in the aforementioned female joint. Nickel-chromium alloy resistance heating wire (Omega Corp.), 3.5 m long, 0.81 mm in diameter, and insulated with approximately lo00 2.8-mm-o.d., singlehole fish-spine ceramicbeads, was coiled around and along the heating tube. The ends of this wire were silver soldered to two tungsten wires which passed through the groundglass joint to a Variac voltage regulator. Heat losses to the surroundings were minimized by wrapping moldable insulation, 2.5 cm thick, around the heating section. The drop-viewing section of the column was a 6-cmlong, 4-cm-0.d. length of quartz glass tube, fused to the top of the heating section. A type K thermocouple was placed at the level of the drop, midway radially from the pendant drop to the inner surface of the tube. This thermocouple was attached to an Omega Series 920 temperature controller which governed power to the Variac. Drops were held in place along the axis of this tube by a flexible silica capillary hollow fiber (Polymicro Technologies). This fiber entered through an extended 12/30 male glass joint, 5 cm long and sealed at the bottom with a high-temperature,Teflon-coated,siliconerubber septum. Fibers of 75,100, or 150pm inside diameter were used, the
larger diameters being deployed when the liquid solution forming the drops was more viscous. The outside diametersof these fiberswere 100,16O,and390pm,respectively. The extended glass jacket provided an air gap between the jacket and the fiber so as to hinder vaporization of the liquid inside the fiber. Drops ranging from 0.5 to 1.8mm in diameter were formed at the tip of the capillary fiber. A VHS videocamera (HitachiVM-3150A)equipped with a 12-72-mm macro lens was located at drop level, 1.5 cm radially away from the outer wall of the viewing tube. In order to reduce heat losses, half of the circumference of the viewing tube opposite the videocamera was insulated with moldable insulation. The entire sequence of appearances of a drop during drying was discernible from thevideorecording. Drop sizeswere measuredon thevideo screen, by comparison with the known diameter of the thermocouple tip. Initial drops were assumed to be spherical. Later in drying, the reported diameters are the maximum diameter of the drop visible on the video screen. Another tube ran at a 45O angle to the axis of the column. Within this section was a Teflon-coated, stainless steel staticmixer, 1.3cmindiameterand24.1 cmlong,consisting of 12 single helical stages in series, each stage rotated by 90° with respect to the next. The static mixer was chosen to even out the composition of the gas stream without much axial dispersion. A rubber stopper sealed the end of this tube. Two perpendicular side tubes, opposite one other, were fused to the upper end of the mixing tube. One of these side tubes served as an outlet to the gas chromatograph (GC), while the second served as a vent. The gas flow to the GC, as measured with a soap-bubble meter, was regulated at 0.5 L/min by means of a 2-mm Teflon needle valve located in the vent tube. The entire gas sampling section was wrapped with heating tape connected to a second temperature controller. The temperature indicated by a type K thermocouple under the heatingtape was kept at the same temperature as that oftheviewingsection, soastominimiienatwal convection. A Varian Model 3700GC, equipped with a63Ni electroncapture detector (ECD) and a digital integrator, was used to measure the SF6 concentration. Gas samples were injected via a manually operated six-port, high-temperature gas sampling valve with a 0.5-mL sample loop. Calibrations were obtained by use of a standardized mixture of 28 ppb SF6 in nitrogen (Matheson Corp., Newark, CA) and four sample loops of 0.5-, 1.0-, 3.0-, and 5.0-mL volume (Sunkel, 1992). A column of Molecular Sieve 5A (Linde Corp.), 3 mm i.d. and 60 cm long, was used in the GC and took up water vapor from evaporation of the pendant drop, so that peaks for only SF6emerged. Gas samples were taken every 6-10 s, depending on the time required for the ECD output to return to the base line following a peak. The gas residence time between the sampling section in the drying apparatus and the ECD was approximately 11 s. Maltodextrin (Maltrin M150) was obtained from Grain Processing Corp., Muscatine, IA. Solutions of coffee extract were prepared from unagglomeratedspray-dried instant coffee powder (Procter & Gamble Co., So. San Francisco, CA). Solutions of milk were prepared from commercial nonfat dry milk solids (Lucerne, Safeway Stores, Inc.). Solutions were made up by weight in degassed, deionized water. Coffee solutions were centrifuged for 30 min at 2000 rpm. The silica fiber was glued to a syringe needle which was connected to a glass barrel filled with the feed liquid solution. This solution had been brought toward saturation by bubbling sF6 gas through it in a 100-mL. three-neck flask a t room temperature and
Ind. Eng. Chem. Res., Vol. 32, No. 10,1993 2359 2 168 secl
3 174 acrl
Figure 2. Sequence of frames from the ndeorewrding of a drop of 30% (wiw)coffee extract drying in nitrogen at 180 O C .
was then drawn into the glass syringe barrel by driving the syringe needle through a rubber septum on the threeneck flask and allowing the pressure to fill the syringe. In order to retard bacteriological growth and promote saturation, the saturator was placed in direct contact with ice. The solutions contained in the syringe were allowed to equilibrate toward room temperature for 5 min before a drop was formed. In some experiments drops were formed and then allowed to fall from the fiber into the heating section, under the presumption that pyrolysis would then cause the drop to release all, or nearly all, of the SF6 dissolved in it. The fiber was withdrawn from the chamber immediately after the drop had fallen. Experimental Results a n d Discussion Characterization of Apparatus. Following injection of 2 mL of air through the fiber tip into nitrogen flowing at 220 "C, essentially all of the air was detected by the ECD between 12 and 27 8 later (Sunkel, 1992). This gives a measure of the extent of axial dispersion. SF6 released from an evaporating drop of otherwise pure water was detectd during a period of 12 to about 60 s following drop formation. SFe loss was spread over still longer times for drying of drops of solutions. Hence axial dispersion does not dominate the response. Coffee Extract. Figure 2 is photographic sequence showingtypical morphological development during drying of drops of coffee extract. Bubbles become apparent on the drop surface after about 70 s and become longer protrusions after about 100 s. The drop is in an expanded shape during these events, but inflates and collapses frequently. Once one of these protrusions, or fingers, developed, additional protrusions followed at the same weak point, either ramifying from the original finger or replacing the original finger after it had collapsed. Formation of such protrusions or fingers during drying of coffee extract has been observed previously (El-Sayed et al., 1990). This and related phenomena of expansion, bursting, and cratering have been related to the drop temperature (El-Sayed et al., 1990; Greenwald and King, 1982). The temperature of a drop is initially a t the wet-
1.4
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sno
innn
ISOO
lono
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Figure 3. Release of SFs (top) and change in diameter (bottom)for apendantdropof30% (w/w) wffeeextractdryinginnitmgenat180 "C.
bulbvalueandthenincreasesasresistaneetomasstransfer within the drop develops during drying. Bubbles of dissolvedgasnucleatewhenthedrop temperature becomes high enough to give sufficient supersaturation. The bubbles grow by addition of water vapor as the drop temperature increases further, developing overpressure and consequentbursting and/or protrusion tendencies as the drop temperature surpasses 100 'C (Greenwald and King, 1982). At higher temperatures of the drying gas, bubbles form more readily and the drop temperature
2360 Ind. Eng. Chem. Res., Vol. 32, No. 10,1993 pg SF61ml of sarurated H20(Gerrard. 1980) 0 Measwed pg SF6 / ml Of saturated H 2 0 Morphological SF6 loss / r n l coffee drop
Pre-selective diffusion SF6 loss / rnl coffeedrop Burned 10. 20. and 30% coffee. SF6 loss / rnl drop
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Water 'Temperaturc ("C) Temperature of Drying Gas ("C) Figure 4. Losses of SFs from drops of coffee extract solution and water. F3
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pg SF6/ml of saturated H 2 0 (Gerrard.1980). Measured pg SF6 / ml Of saturated H 2 0 Q Morphological SFa loss I "430% coffee drop
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21 2s .?o 301) 115 13s IXO 220 2211 Water Temperature ( " C ) Temperature of Drying Gas ("C) Figure 6. Losses of SFSfrom dmps of 30% (w/w) coffee extract dried at different gas temperatures.
exceeds 100 "C earlier, with consequent greater and more frequent activity. The top portion of Figure 3 shows the release of sF6 from such a drop, as a function of the cumulative flow of drying gas (again expressed a t rmm temperature and pressure) since the s t a r t of drying. The cumulative flow can be converted to time through the lo00 mL/min flow rate. T w o periods of SF6 release are apparent. The first corresponds to diffusionalloss from the liquid drop before any significant morphological development sets in. Loss during this first period falls to essentially zero after 900 mL of drying gas has passed over the drop in the case of Figure 3. The lower half of Figure 3 depicta the change in the maximum diameterofthedropduringdrying,asmeasured directly from the videotape by reference to the known thermocouple size. The diameter is normalized by the initial diameter (1.81mm). After 1400 mL of drying gas,
the diameter begins to increase sporadically. Actually the diameter fluctuates many times more than are shown in the figure. What is shown is intended to be representative of what occurs. As the diameter increases because of internal hubhle expansion and fluctuates because of repeated bubble growth, bursting, and collapse, renewed release of SFe occurs. SFe peaks later in drying correspondedwith individualmorphologicalevents seen on the videotapes, when allowance was made for the 11-s lag. The additional loss is associated with exposure of protrusions and/or is due to bursting of the surface which releases SF6-saturated gas from internal bubbles. Data of the sort given in Figure 3 are reported by Sunkel (1992) for all runs made in this research. The number of runs under any one set of conditions was not sufficient to permit statistical analysis. However, Figure 3 and other data of the same sort (Sunkel, 1992) display three important results:
Ind. Eng. Chem. Res., Vol. 32,No. 10,1993 2361 pure water at threetemperatures, averagingdatareported byGerrard (1980) from threedifferentsource. The fourth bar correspondsto the measured loss from a drop of water that was allowed to fall from the fiber tip into the heating section. The fifth, sixth, and seventh bars show the total losses measured from suspended drops of 109% ,20% ,and 30% coffee extract solutions. These losses are divided into two portions, those occurring before the onset of selective diffusion and those occurring during morphological development. The final three bars denote total losses measured from drops of solutions of coffee extract of the three concentrations that were allowed to fall from the fiber tip into the heating section. Losses from the suspendeddrops appear to he somewhat greater than those from the drops that fell from the fiber tip. This could reflect some loss from solution in the tip of the fiber as well as from the drop, or else it could stem from experimental error. The fiber was withdrawn immediately after the drop was formed and fell in the experiments with solutions and water where drops were allowed to fall into the heating section, whereas the fiber was of necessity left in place during drying of suspended drops. Comparison of bars five through seven with bars eight through ten suggests that the losses of SFa from the o'750 500 I000 IS00 2000 2500 3000 3500 suspended drops are essentially total, if the assumption is made that losses from drops released into the heating ml drying gas . .. section =e total or nearly so. Interestingly, the drops of Figure 6. %lease of SF, (top)and change in diameter (bottom)for less concentrated coffee solutions release (and therefore a pendant drop of 30% (w/w) aqueous d t o d e x t r i n drying contained) substantially more sF6 than is released and in nitrogen at 180 'C. contained in drops of either more concentrated solutions 1. The onset of selective diffusion is directly confirmed, or water. This unexpected resultmay reflectone or more since a large fraction of the SF6 remains in the drop after of the following phenomena: the rate of release drops to zero after 900 mL of gas. 1. There were changes in solubility and/or differences 2. Morphologicaldevelopment does cause a substantial in the approach to saturation of the initial solution. loss of the volatile solute, in this case more than was lost 2. There wassubstantialretentionof SFsindrieddrops before the onset of selective diffusion. 3, L~~~o c c a s ~ o n e ~ ~ y m o r p ~ o ~ o g ~ c ~of~ solutions e v ~ ~ ~with p ~ ~higher ~ ~ ~ initial e e ~ concentrations, both suspended and allowed to fall into the heating section; to be directly associated with formation of protrusions Le., pyrolysis did not cause total loss of SFa after all. and/or bursting through the surface. 3. Loss occurred from solution in the tip of the fiber Figure 4 shows the total losses of SF6 under a variety before it was withdrawn in the experiments where the of conditions. All data are normalized by drop volume. drops fell, as well as for suspended drops. The first three bars correspond to the solubility of SF6 in
Water Temperature ( " C ) Temperature of Drying Gas ("C) Figure 1. Losses of SFs from drops of maltodextrin solution and water.
.
2362 Ind. Eng Chem. Res., Vol. 32, No. 10,1993 I pg SF6 / m l of saruraied H20(Gerard,
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Water Temperature ("C) Temperature of Drying Cas ("C) Figure 8. Losses of SFe from drops of 30% (wlw) maltcdextrin solution extract dried at different gas temperatwea
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Figure 10. Release of SFe (top) and change in diameter (bottom) for a pendant drop of 30% (wlw) nonfat milk solids solution drying in nitrogen at 115 "C.
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Fmure 9. Sequence of frames from the videorecording of a drop of 30% (w/w) aquwua nonfat milk solids drying in nitrogen at (a) 150 and (b) 180 "C.
Thermal destruction of SFe is unlikely, since it is a relatively stable compound. Finally,the amount of SFe loas occurring before selective diffusion sets in is smaller for drops of solutionswith higher initial concentrations. This reflects an earlier onset of selective diffusion, as the critical surface concentration
for selective water removalis established earlier indrying. This effect is offset by greater losses due to morphological development for solutions with higher initial concentrations. These latter losses reflect a greater amount of fingering activity and/or the fact that the conditions lead to near total loss, and more SFe was left after selective diffusion has set in. Figure 5 shows similar results for drops of 30% (w/w) coffee solution drying a t gas temperatures ranging from 115to 220 O C . Losses duringtheselectivediffusion period are lower at higher temperatures, reflecting an earlier onset of selective diffusion due to build-up of greater concentration gradients due to faster drying at higher temperatures. Fingering actions and losses associated with
Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 2363
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Figure 11. Losses of SFBfrom drops of 30% (wlw) nonfat milk solids solution extract dried at different gas temperature%.
morphologicaldevelopment,on theother hand, aregreater at higher temperatures, and again the combination of the two types of loss may be nearly total. Maltodextrin. Drops of aqueous maltdextrin solution gave a boiling appearance late in drying, whereby bubbles rapidly and repeatedly formed and broke through the surface. This same behavior was observed by El-Sayedet al. (1990). There was no formation of the protruding fingers characteristic of coffee extract. Figure6reportsdataforlossof SFeanddiameterchange for drops of 30% w/w maltodextrin solution dried at 180 O C . Again, the periods of loss before selective diffusion and morphological development set in and after morphological development begins are separate and distinct. The second large SF6peak occurred as drops reached the period of agitated boiling and repeated, frequent surface eruptions. For maltodextrin, axial mixing averages out SF6 release from individual bursts through the surface, as opposed to the situation for coffee extract, where fingers are sustained for longer periods and separate peaks are thereby found. The effect of initial maltodextrin concentration is shown in Figure 7. The results and interpretation are similar to those for coffee extract, except for the inversion of results for 10% and 20% initial drop concentration. Figure 8 displays the effect of the temperature of the drying gas for drops of 30% (w/w) initial concentration. At higher temperatures boiling is more intense and drying is more rapid; consequently losses due to the boiling effect occur earlier and are larger. There may be an intermediate optimum temperature for minimizing loss. Nonfat Milk. Figure 9 is a photographic sequence for the drying of a drop of 30% (w/w) dissolved nonfat milk solids during drying at (a) 150 and (h) 180 "C. Milk drops appear to set a firm and smooth spherical surface early in drying, possibly due to denaturation and/or demulsification of milk proteins (El-Sayed et al., 1990). Only at higher temperatures of drying do surface disturbances and protrusions occur, and to much lesser extents than for coffeeextractor maltcdextrin at agiven temperature. Milk also undergoes browning (probably through the Maillard reaction) late in drying at higher temperatures, as can he seen in frames 5 and 6 of Figure 9h. Figure 10 shows the loss of sF6 from a drop containing 30% (w/w) nonfat milk solids, dried a t a nitrogen
temperature of 115 'C. SF6 losses were detected only during the initial period before the onset of selective diffusion. The lack of subsequent losses reflects the lack ofsurfacedeformation later in thedryingprocess.although some interior voidage did develop, as evidenced by the diameter reduction being less than would correspond to thedegreeofdrying.aswellas byasmallsurfacedepression late in drying. As the gas temperature was increased to 135 "C and higher, SFs peaks of increasing size appeared late in the drying process (Sunkel, 1992). For nitrogen temperatures ranging from 135 to 180 "C the milk drops ballooned to a greater size, forming a glossy sphere before eventually browning. At temperatures above 180 "C a few small ramiform arms were observed late in drying, one of which is shown in Figure Sb, frame 3. Morphological events observed in the milk drops coincided with SF6 peaks. However, there were also periods of loss later in drying for which there was no discernible surface disruption. Either there were disturbances on the side of the drop away from the videocamera, or surface stretching and/or high drop temperature facilitated diffusion of SF6 through the surface shell. Figure 11shows the effect of drying-gas temperature on losses from drops of 30% (w/w) nonfat milk concentrate, as well as the distribution of those losses between the periods before the onset of selective diffusion and during morphologicaldevelopment. Here thelossesduringdrying appear to be much less than total,except possibly for the 220 "C result. Mixturesof Nonfat MilkandCoffeeExtract. Some experiments were conducted with drops containing 20% (w/w)nonfatmilksolidsand 10% (w/w) coffeesolids,dried at gas temperatures ranging from 115to 220 "C. The aim was to see whether lesser morphologicaldevelopmentand volatiles loss engendered by the milk component might exert a protective effect for coffee volatiles. The results did show less morphological development than for coffee alone, especially at lower drying temperatures, but losses were substantially greater than for drops of milk solids alone (Sunkel, 1992). Conclusion
A highly sensitive electron-capture detector allows continuous monitoring of SF6 released during drying of a
2364 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993
pendant drop. The SF6 solute models volatile flavor and aroma components. Simultaneous videorecording of the appearance of the drop makes it possible to relate the loss of SF6 to morphological changesthat occur during drying. A period of initial loss tapers off toward zero, giving a direct confirmation of the development of selective diffusion during drying of drops. For drops of aqueous solutions of coffee extract and maltodextrin very substantial renewed losses of SF6 occur later in drying, concomitant with periods of morphological development-formation of protruding frothy armsfor coffee extract and repeated expansion, bursting, and collapse of bubbles for maltodextrin. Drops of nonfat milk remain smoother during drying and give much less loss of SF6 late in drying.
Acknowledgment This research was supported by the National Science Foundation under Grant No. CBT-8822989. Literature Cited Alexander, K.; King, C. J. Factors Governing Surface Morphology of Spray-Dried Amorphous Substances. Drying Technol. 1985,3,
Etzel, M. R.; King, C. J. Loss of Volatile Trace Organica during Spray Drying. Znd. Eng. Chem. Rocess Des. Dev. 1984,23, 705. Gerrard, W. Gas Solubility, Widespread Application; Pergamon: New York, 1980; pp 206-216. Greenwald, C. G.; King, C. J. The Mechanism of Particle Expansion in Spray Drying of Foods. AIChE Symp. Ser. 1982,78 (No. 218), 101.
Kerkhof, P. J. A. M.; Thijeeen, H. A. C. Quantitative Study of the Effecta of Procesa Variablea on Aroma Retention during the Drying of Liquid Foods. AIChE Symp. Ser. 1977, 73 (No. 163),33. Kieckbusch, T. G.; King, C. J. Volatile8 Loss during Atomization in Spray Drying. AIChE J. 1980,26,718. Erratum. AIChE J. 1981, 27, 528.
King, C. J. Spray Drying of Food Liquids, and Volatiles Retention. In Reconcentration and Drying of Food Materials; Bruin, S., Ed.; Elsevier: Amsterdam, 1988; pp 147-162. Sunkel,J. M. Loss of Volatile Components and MorphologicalEvents during Drying of Single Pendant Drops. M. S. Thesis, University of California, Berkeley CA,1992. Thijeaen, H. A. C.; Rulkens, W. H.Retention of Aromas in Drying Food Liquids. Zngenieur (The Hague) 1968,80, Ch45. Verderber, P. A.; King, C. J. Measurement of Instantaneous Rates of Loss of Volatile Compounds during Drying of Drops. Drying Technol. 1992, 10, 875. Wallack, D. A.; El-Sayed, T. M.; King, C. J. Changes in Particle Morphology during Drying of Drops. 2. Effects on Drying Rate. Ind. Eng. Chem. Res. 1990,29, 2354.
321.
Charlesworth, D. H.; Marshall, W. R. Evaporation from Droplets Containing Dissolved Solids. AZChE J. 1960, 6, 9. El-Sayed, T. M.; Wallack, D. A.; King, C. J. Changes in Particle Morphology during Drying of Drops of Carbohydrate Solutions and Food Liquids. 1. Effects of Composition and Drying Conditions. Znd. Eng. Chem. Res. 1990, 29, 2346.
Received for review September 28, 1992 Revised manuscript received February 1, 1993 Accepted February 12,19930
* Abstract published in Advance ACS Abstracts, August 15, 1993.
