The effects of time, energy, and operational mode of microwave drying on the protein
and chlorophyll contents of blind nettle extract were studied. Comparison of the modes
showed that drying was faster in the normal than in the impulse mode. The kinetics
of water loss followed a saturation-type behaviour. A virtual but significant increase in
protein concentration was observed for the normal-mode, 300 W treatment. A constantly
high energy input for a long time might damage proteins through the Maillard reaction.
Normal-mode technique did destroy part of the chlorophyll content. It was concluded
that the faster the drying, the more chlorophyll remained

Bogenrieder, C.

Dermelj, M.

Hídvégi, Máté

Lásztity, Radomir

English

Periodica Polytechnica (Budapest University of Technology and Economics)

PERIODICA POLYTECHNICA SER. CHEAt. ENG. VO£. 39, NO. 1, PP. 77-81, (1995) EFFECT OF MICROWAVE VACUUM DRYING ON PROTEIN AND CHLOROPHYLL CONTENTS OF BLIND NETTLE (Urtica urens L.) M. DER?v1ELJ, 1 C. BOGENRIEDER. 2 Mate HIDVEGI3 and Radomir L.A.SZr'ITy3 1 School of Chemistry, University of Ljubljana Ljubljana, Slovenia 2 School of Chemical Engineering, University of Stuttgart Germanv 3Department of Biochemistry' and Food Technology Technical University of Budapest, H-1521 Budapest, Hungary Received: June 15, 1995 Abstract The effects of time, energy, and operational mode of microwave drying on the protein and chlorophyll contents of blind nettle extract were studied. Comparison of the modes showed that drying was faster in the normal than in the impulse mode. The kinetics of water loss followed a saturation-type behaviour. A virtual but significant increase in protein concentration was observed for the normal-mode, 300 \V treatment. A constantly high energy input for a long time might damage proteins through the :"laillard reaction. ~ormal-mode technique did destroy part of the chlorophyll content. It was con-cluded that the faster the drying, the more chlorophyll remained. Keywords: microwave vacuum drying, herbal extracts, protein, chlorophyll, blind nettle. Introduction Conventional drying procedures of foods need a long time or large amounts of energy. In many cases use of vacuum during the drying process has been proved to be advantageous. Recently, an interesting variant of this technique, microwave vacuum drying, has been developed and has become popular in food processing (VVIEsE.:moFER and vVESTER?lIEIER. 1989). YIi-cro\vave vacuum drying has been especially useful in drying pastes, pow-ders or porous materials. In Hungary, microwave vacuum drying has been used for producing instant herbal extracts, teas and instant coffees. In microwave vacuum dryers, as with conventional microwave equipment, the heat needed for drying is generated by the absorption of electromagnetic radiation by the molecules of the substance to be dried. The energy is transformed into kinetic energy, especially by water. The increase in ki-netic energy makes the molecules vibrate more intensively. This process is followed by a temperature increase and evaporation of water. The warm-78 .\1. DERMELJ et al. ing up is homogeneous. Therefore, no temperature gradient is generated across the sample, thus the drying process is in a steady state. The vacuum produced in the dryer lowers the vapour pressure of the liquid, thus, the combination of vacuum and microwave energy results in a gentle but quick drying. A literature search has provided no information regarding the effects of microwave vacuum drying on nutritionally or pharmaceutically relevant compounds. The scope of the present study was therefore to investigate if microwave vacuum drying has any destructive effect on the sample being dried. As a test material, a dried extract of blind nettle \vas chosen, due to its significance in homoeopathic medicine. Materials and Methods Microwave Vacuum Drying Equipment: Labotron 600 A PVI, Industries Micro-Ondes Internationales. France. Operational modes: (1) l\: or'mal: operation with continuous microwave en-ergy input. 300 VY or 600 W for given time periods. (2) Impulse: operation with microwave energy input, (300 v'.c for 10 sand 0 VY for 13 s) or (600 VV for 10 sand 0 VV for 13 s) for given time periods. Operational times: 1, 3, .), 7, 10 or 15 min. Nettle Samples Preparation: Freshly harvested leaves of blind nettle (U rtica urens, L.), solar-dried, were extracted with hot water for 15 min, then filtered off. The extract was concentrated and crystalline maltodextrin was added (40%, DM basis) to the extract. The mixture was spray-dried with an outlet air temperature of 80 QC (original sample). The original sample was then remoisturized up to 30% moisture content. 5-5 grams of this paste \vere then micro-wave vacuum dried. l'vficrowave Experiments Samples in open glass containers were placed symmetrically on the rotating disc of the equipment and covered by a glass bell. Three parallel samples JIICROIVAFE FACUUM DR1'ING OF BLIND NETTLE 79 'were used in each experiment. Nlicrowave vacuum treatments were carried out in the t\VO operational modes for different operational times. The samples were then pO\\'dered and analysed. A nalytical Methods lvIoisture content: AACC method (Amer. Assoc. Cereal Chemists, 1969). Protein content: Lovvry method in a continuously flowing ('Contiflo') sys-tem (L(SZTlTY et aL 1975). Chlorophyll content: AOAC method (Assoc. Official Analytical Chemists, 19(0). ~ 15 E 95 >-L-o 91 87 83 (' "" ;' 300W oA---- ..... --------,.--;' "" ,1''' 3 6 I 9 12 I 11> 15 Time,min Fig, 1. Change of dry matter content. ",lode: normal Results and Discussion The results of analysis are summarized in Table 1 and demonstrated in (Figs. 3-6). Changes in dry matter content as a function of time are shown in Figs. 1-2. 80 ! ~ 99 o >. L-o 91 87 '" .... , I I , 83 I , , I M. DERMELJ et al. ....,.~ .. 300W _---- ... ~---- .... --)111"'~" '" Time,min Fig. 2. Change of dry matter content. Mode: impulse The higher the energy input, the faster the drying process regardless of the mode used (Figs. 1-£). Comparison of the modes showed that drying was faster in the normal than in the impulse mode. The kinetics of \vater loss followed a saturation type behaviour \vhich is a general characteristic. In Figs. 3-4 the time-dependent changes in protein contents determined are shown. A virtual but significant increase in protein concentration could be observed in the normal mode. 300 \V treatment, which can be explained by assuming that microwave vacuum drying makes proteins more available, i.e. more reactable with e. g. Folin reagent. It seems, however, hard to explain why the lower the energy input the better the availability of proteins in the normal mode. A constantly high energy input for a long time might damage proteins through the occurrence of Maillard-type reactions, etc. This virtual increase in protein content could not be observed so evidently in the impulse mode. It may reflect that the impulse technique is more gentle than the normal mode. The normal-mode technique did, however, destroy part of the chlorophyll content. As the destruction was larger at lmver energy input (Fig. 5), it was concluded that the faster the drying, the more chlorophyll remains protected. A small energy input for a long time may destroy more chlorophyll than higher energy for a short time. 0 -0 -C 'Qj 15 '-0.. 0 0 c iii 15 cl: ! 21 20 19 MICROWAI'E VACUUM DRYING OF BLIND NETTLE " " " "...------- ... ,,/ 600 W 170~------~------~------~------~------~j~ Trme,min Fig. 3. Change of protein content. Mode: normal ! 20 19 17~-------L--------~------~--------~-------7.~~ o Time,min Fig. 4. Change of protein content. ?vlode: impulse 81 82 M. DERMELJ et al. A '0> 18 z:: c \ N '$2 15 \ X >, .£ a. 12 0 '-g .£ U 9 L-~ ____ ~ ______ J-______ ~ ______ ~ ______ ~ __ ~ 60 ! '0> 24 z:: c N '$2 21 x >, .£ g- 18 o ::c u 3 6 9 12 15 Time,min Fig. 5. Change of chlorophyll content. Mode: normal I I I I I I I \ I \ \ \ \ \ " I , I , I , I , I , , .... 300W I 1'/ I 3 .... .... I I ........ 600W , .... ' .... I r 11!> o 6 9 12 15 Time,min Fig. 6. Change of chlorophyll content :'lode: impulse In the impulse mode (Fig. 6) the above observation ,vas but partly valid: over the 9 min treatment the higher energy input resulted in greater loss of chlorophyll. .\fICROW·AVE VACUUM DR1·ING OF BLLVD ,'fETTLE 83 Table 1 Average values * of protein and chlorophyll contents (D?vl basis), microwave vacuum dried blind nettle samples. Sample Operational Operational Power Protein Chlorophyll mode time (\V) (%) (nmol/g) (min) 1+ 18.1 1711 2 n 300 17.7 629 :3 n .300 IS.7 972 -1 n .J 300 17.9 1316 ·5 n 7 300 lS.3 915 6 n 10 300 20.3 1087 7 n 15 .300 20.8 S58 S n 1 600 18.1 1316 9 n :3 600 lS.5 1602 10 n .5 600 lS.4 1201 11 n 7 600 IS. 1 1602 12 n 10 600 lS.9 1487 13 n 1·5 600 19.0 1204 14 1 300 17.4 1373 15 3 300 lS.4 1316 16 .5 300 17.6 1716 17 7 300 IS.4 14S7 18 10 :300 19.4 1888 19 15 300 18.3 1888 20 1 600 18.2 1373 21 :3 600 19.0 2288 22 ;) 600 18.6 1716 2:3 7 600 17.8 2116 24 10 600 17.4 1659 2.5 IS 600 17.9 1258 Based on three determinations + Original sample n i\ormal Impulse Microwaye vacuum drying was found to be suitable for drying herbal extracts. It was important to adjust the modes, energy inputs and time for achieving quick but gentle drying. In the normal mode smaller energy input proved to be efficient and more useful. vVith the impulse technique a higher energy input made the drying process faster but contributed more significantly to the loss of the active compound than a smaller energy input. 84 Af. DERMELJ et al. References 1. Amer. Assoc. Cereal Chemists (1969): Approved Methods of the AACC, Method 44-15A. AACC, Inc., St. Paul, lvIN. 2. Assoc. Official Analytical Chemists (1970): Official Methods of Analysis. Method 3.10.5-3.108. AOAC Washington, DC. 3. LAsZTITY, R. - ORSI, F. - VARGA, J. (1975): Neue Wege und :\foglichkeiten der Verwendung von automatischen Analysengeraten zur Untersuchung von Getrei-deerzeugnissen. Bericht iiber die 6. Tagung internationale Probleme der modernen Getreideverarbeitung und Getreidechemie. Inst. f. Getreideverarbeitung, Bergholz-Rehbriicke, pp. 229-241. 4. WIESENHOFER, R. - WESTERMEIER, P. (1989): Vakuumtrockner mit Mikrowelle. Chem. Tech. Vol. 18 (5), pp. 66-69. 

EFFECT OF MICROWAVE VACUUM DRYING ON PROTEIN AND CHLOROPHYLL CONTENTS OF BLIND NETTLE (Urtica urens L.)

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EFFECT OF MICROWAVE VACUUM DRYING ON PROTEIN AND CHLOROPHYLL CONTENTS OF BLIND NETTLE (Urtica urens L.)

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