Influence of Microwave Heating on Flavor - American Chemical Society

Chapter 49 Influence of Microwave Heating on Flavor James A. Steinke, Christine M . Frick, Jo A. Gallagher, and Kenneth J. Strassburger

Downloaded by CORNELL UNIV on August 28, 2016 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0409.ch049

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Flavor systems which perform well in conventionally prepared food are frequently unacceptable when incorporated in microwave heated food products. A frequent problem associated with flavor systems during microwave heating is the loss of flavor attributed to disproportionate distillation or degradation of selected flavor components during microwave heating. High pressure liquid chromatography and gas chromatographic techniques were used to quantitate these flavor components in microwave heated systems. Formation of Strecker aldehydes and loss of flavor components were much greater during microwave heating than conventional heating to comparable temperatures. The loss of a homologous series of volatile acids varied widely depending on the composition of the microwave medium. Factors which affect the dielectric property of the microwave medium such as water and salt concentration had a significant impact on the loss of volatile components. Consumer interest i n microwave products i s at an a l l time high. How ever, quality of microwave products i s frequently marginal. Numerous microwaveable products on the market are the result of a l a b e l change rather than a change i n product formulation or packaging materials. A d d i t i o n a l l y , products are introduced as dual microwave and conven t i o n a l heated products with flavor systems which were not optimized for either process. Quality i s marginal. Consumers were w i l l i n g to s a c r i f i c e product quality for convenience. However, t h i s attitude i s changing as a result of the large number of new microwave products being introduced. Microwave-only products require a basic under standing of the problems associated with microwave heating. Conventional heating and microwave heating of food products result i n s i g n i f i c a n t l y d i f f e r e n t end products. Foods heated conven t i o n a l l y are subjected to r e l a t i v e l y high surface temperatures, 350-450 degrees F., which r e s u l t s i n product surface dehydration. 0097-6156/89/0409-0519$06.00/0 © 1989 American Chemical Society Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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THERMAL GENERATION O F AROMAS

The low water a c t i v i t y at the product surface favors hundreds of flavor reactions through c l a s s i c a l Maillard Browning mechanisms. Product surface also develops a brown color and crisp texture. Microwave heating, however, produces a very d i f f e r e n t product because the mechanism of microwave heating i s so d i f f e r e n t . Generation of heat within food products i n the microwave process i s caused by molecular f r i c t i o n attributed to the breaking of hydrogen bonds associated with free water molecules and ionic migration of free s a l t s i n an e l e c t r i c a l f i e l d of rapidly changing p o l a r i t y . Highest temperatures are t y p i c a l l y 190-212 degrees F. Water vapor migrates to the product surface causing evaporative cooling and moisture con densation at the surface. Low product temperatures and the high water a c t i v i t y minimize flavor, color, and texture development during microwave heating of food systems. Flavors added to microwave food systems have a greatly expanded role compared to flavors added to products prepared by conventional heating. The flavors must provide not only the characterizing flavor ( i . e . , lemon, butter, v a n i l l a , e t c . ) , but also the t y p i c a l roasted, toasted, and baked flavors which do not develop i n microwave heated products. New flavors designed for use i n microwave products must mask the raw uncooked flavor c h a r a c t e r i s t i c s and other undesirable flavor notes frequently found i n many microwave bases. Microwave flavors must also deliver pleasant aromas into the room during the microwave process. Development of these flavors for microwave a p p l i cation i s dependent upon a fundamental understanding of microwave heating on flavor performance i n food systems. There i s l i t t l e available l i t e r a t u r e on the interaction of flavor components with food systems during microwave heating. How ever, numerous authors have reported on the d i e l e c t r i c properties of nonflavor food ingredients during microwave processing (1,2,3,4). Individual flavor components are subjected to losses through d i s t i l l a t i o n , flavor binding by starches and proteins, and chemical degradation during the microwave process. Specific data on flavor loss by d i s t i l l a t i o n as affected by the various media and chemical modification of flavor precursors i s presented i n t h i s paper. Data on flavor binding during microwave processing i s addressed i n a sub sequent paper. MATERIALS AND METHODS Preparation of Strecker Aldehyde Samples. Twenty grams of an aqueous solution containing 0.5% amino acid and 1.0% d i a c e t y l were sealed i n a 30 ml v i a l p r i o r to heating. Samples were heated either 4 minutes i n a G.E. Space Maker 600 Watt Microwave Oven or held 60 minutes i n a 190 degree F. water bath. Microwave samples were heated i n 20 second i n t e r v a l s and subsequently cooled u n t i l a t o t a l microwave heating time of 4 minutes was achieved. Maximum tempera ture of the microwave sample never exceeded 190 degrees F. The amino acids evaluated were glycine, alanine, and v a l i n e . Samples were sealed to prevent the loss of formaldehyde, acetaldehyde, and isobutraldehyde which were formed by Strecker degradation of glycine, alanine, and v a l i n e , respectively, when heated i n combination with the d i a c e t y l . Samples were analyzed by gas chromatographic headspace analysis.

Parliment et al.; Thermal Generation of Aromas ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

49. STEINKE ET AL.

Influence of Microwave Heating on Flavor


Preparation of V o l a t i l e Acids Solutions i n Various Media. Samples of 150 g 90/10 oil/water blend containing 500 ppm concentrations of acetic, propionic, butyric, v a l e r i c , and caproic acids were microwaved 0, 1, 2, and 3 minutes i n a 600 Watt G.E. Microwave Oven. Temperatures were recorded and a duplicate sample was heated to the same temperatures i n a conventional oven. Samples were heated i n open containers to permit the loss of acids during heating. Samples containing 150 g of 500 ppm acids were also prepared using 3% added sodium chloride i n the 90/10 oil/water blend, water, and vegetable o i l as the microwave medium. These samples were heated 0, 1, 2, and 3 minutes i n the microwave. Changes i n the acid concen t r a t i o n were determined by high pressure l i q u i d chromatography with an organic acid column and an aqueous mobile phase. Preparation of Microwave and Conventional Cakes. Diacetyl and acetoin were added at 200 ppm to a commercially available cake mix. The conventional cake was baked 35 minutes at 250 degrees F. i n a standard General E l e c t r i c e l e c t r i c oven. The microwave cake was baked 6.5 minutes i n a 600 Watt G.E. Space Maker Microwave Oven. Diacetyl and acetoin concentrations were determined by gas chromato graphic headspace analysis as previously described f o r quantitation of the Strecker aldehydes. Microwave and Conventional Heating Systems. A General E l e c t r i c Space Maker 500 Watt Microwave Oven at 2450 MHZ was used i n the preparation of a l l microwave samples. A standard General E l e c t r i c Hotpoint elec t r i c range or a hot water bath was used to prepare conventional heated samples. Gas Chromatography. A Varian 3700 gas chromatograph equipped with a flame i o n i z a t i o n detector, a Hewlett-Packard Model 19395A Headspace Sampler with direct i n j e c t i o n , and a Hewlett-Packard 3357 Laboratory Automation System were used. A 60 mx 0.32 mm DB-5 fused s i l i c a cap i l l a r y column was i n s t a l l e d i n the gas chromatograph. Helium at 25 cm/sec was employed as the c a r r i e r gas. Column was equipped with a 50:1 s p l i t t e r system. Temperature of the i n j e c t i o n pact was 250 degrees C, temperature of detector was 250 degrees C. Column was maintained at 140 degrees C throughout the analysis. A l l samples were equilibrated 30 minutes at 50 degrees C i n the Hewlett-Packard headspace sampler p r i o r to i n j e c t i o n on the column. High Pressure Liquid Chromatography. The high pressure l i q u i d chro matography system used consisted of a Varian L.C. Model 5000 with a column heater, 50 u l i n j e c t o r loop, Varian Autosampler and a Varian U.V.-50 variable wavelength detector. A l l solvents used were HPLC grade from Mallinckrodt Chemicals. Analyses were performed on a Bio Rad Organic Acid Column HPX-87H (250 x 7.6 mm), without the use of guard columns. Flow rate was 0.7 ml/min. An i n j e c t i o n volume of 50 u l was used. Detection parameters were 210 mm at 0.1 AUFS. Column was maintained at 55 degrees C. Mobile phase was 0.016 M H2SO4. Concentration of each analyte (acetic, propionic, butyric, v a l e r i c , or caproic) i n each sample was calculated as follows: analyte concentration = analyte sample peak area/analyte standard peak area x analyte standard concentration x d i l u t i o n factor. The


521

522

THERMAL GENERATION OF

percentage of acid l o s t was acid concentration.

AROMAS

determined by comparison with o r i g i n a l

RESULTS & DISCUSSION Strecker Aldehyde Formation. Formaldehyde, acetaldehyde and i s o butyraldehyde were formed by Strecker degradation of glycine, alanine and valine, respectively. Relative concentrations of aldehydes pro duced by microwave and conventional heating to comparable temperature i s shown i n Table I. S i g n i f i c a n t l y higher concentrations were observed for microwave heated samples.


Table I. Strecker-Aldehydes Produced by Microwave and Conventional Heating of Amino Acids and Diacetyl Aldehydes Formaldehyde Acetaldehyde Isobutyraldehyde

Conventional (ppm)

Microwave (ppm)

10 10 30

20 30 130

Other reactions, such as the formation and degradation of 1-amino-ldeoxyketoses, were previously reported to proceed to a much greater extent i n microwave products than i n those prepared by conventional heating systems (5). Chemical reactions occur during microwave processing of food systems, however, their contribution to flavor appears to be minimal. The v o l a t i l e aldehydes quickly f l a s h o f f during subsequent heating. The desirable baked, toasted, and roasted flavors t y p i c a l of M a i l l a r d Browning do not develop i n microwave heated food products. Loss of V o l a t i l e Acids During Microwave and Conventional Heating. The e f f e c t of microwave and conventional heating on the loss of v o l a t i l e acids i n an oil/water (90/10) mixture i s shown i n Table I I . Acid losses, regardless of the carbon chain length, were much greater i n the microwave heated systems. Significant losses of acetic (33%), propionic (20%), and butyric (9%) were observed during microwave heating to 120 degrees F. Losses observed i n conventional heating to t h i s temperature were n e g l i g i b l e . Since the temperature of the bulk l i q u i d was the same i n both the microwave and conventional heated samples, the observed losses of v o l a t i l e acids was not a t t r i buted to temperature. Mechanisms of heating, however, were s i g n i f i c a n t l y d i f f e r e n t . The d i e l e c t r i c properties of water and o i l d i f f e r r a d i c a l l y . A high water concentration i n food systems greatly increases i t s d i e l e c t r i c properties. O i l , however, contributes r e l a t i v e l y l i t t l e to the d i e l e c t r i c behavior of a food system (1). Consequently, i n the 90/10 oil/water mixture, the microwave energy was directed primarily at the 10% aqueous phase. Acids added to this 90/10 mixture w i l l p a r t i t i o n into this aqueous phase to the extent of their r e l a t i v e s o l u b i l i t y i n the two phases. Greatest losses were observed for acetic acid which exhibits the greatest s o l u b i l i t y i n water and was concentrated i n the aqueous phase. Losses of the more nonpolar acids, i . e . caproic, were also much greater i n microwave samples. Losses of the r e l a t i v e l y


49.

STEINKE ET AL.

523

Influence ofMicrowave Heating on Flavor

nonpolar acids were attributed to their orientation at the oil/water interface and proximity to the p r e f e r e n t i a l l y heated aqueous phase during microwave processing. Loss of the acids during microwave heating was favored because of the large water losses associated with this process and the corresponding loss of water soluble v o l a t i l e acids through steam d i s t i l l a t i o n . Conventional heating of these systems to the same temperature demonstrated much lower losses of v o l a t i l e acids. Conventional heating was t y p i c a l convective heat transfer and, therefore, dependent upon the s p e c i f i c heat of the oil/water mixture. There was no p r e f e r e n t i a l heating of the aqueous phase.


Table I I .

The E f f e c t of Microwave vs. Conventional Heating on the Loss of Acids i n an Oil/Water (90/10) Mixture

Type of Heating

Acids (% Loss) Acetic

Propionic

Butyric

Valeric

Caproic

120 Deg. F. Microwave Conventional

33 1

20 0

9 0

0 0

0 0


66 12

44 10

22 3

8 1

1 0


80 20

62 12

44 5

26 2

17 0

Salt Addition to the Microwave Medium. Addition of 3% sodium chloride to the 90/10 oil/water mixture had a s i g n i f i c a n t impact on the loss of v o l a t i l e acids. Data i s summarized i n Table I I I . Loss of acids was much greater i n systems with added s a l t . The increased loss i s attributed to the change i n d i e l e c t r i c properties because of dissolved s a l t i n the water. Table I I I . E f f e c t of Salt on Acid Losses During Microwave Heating Acid (% Loss) Added Salt

Acetic

Propionic

Butyric

Valeric

Caproic

No Salt

66

44

22

8

1

3% Salt

85

72

54

32

5

The Loss of V o l a t i l e Acids i n Water. The loss of v o l a t i l e acids i n water during microwave heating i s summarized i n Table IV. Greatest losses were observed f o r the more nonpolar although less v o l a t i l e acids. This was expected and i s consistent with vapor pressure data generated for v o l a t i l e flavor compounds i n d i l u t e water systems i n a non-microwave environment (6,7). The more polar acids have a greater a f f i n i t y f o r the aqueous medium and, therefore, exhibit less loss


524

THERMAL GENERATION OF AROMAS

on heating. The extent of losses for a l l acids during microwave heating, however was much greater than expected. Table tV, Time (minutes)


1.0 2.0 3.0

The Loss of Acids i n Water Acids (% Loss)

Acetic

Propionic

Butyric

Valeric

Caproic

9 16 20

10 19 29

11 22 37

12 26 45

14 30 73

Loss of V o l a t i l e Acids i n Vegetable O i l . P o l a r i t y of the microwave medium had a s i g n i f i c a n t e f f e c t not only on the type of acid l o s t but also the extent of that l o s s . Acetic acid, the most polar acid, exhibited the smallest loss during microwave heating i n an aqueous medium. Losses of acetic acid i n a nonpolar medium, however, were the greatest as shown i n Table V. Losses of v o l a t i l e acids i n vegetable o i l were minimal compared to losses i n aqueous systems. Loss of v o l a t i l e acids can be minimized by changing or modifying the microwave medium. Table V.

Loss of Acids Microwaved i n Vegetable O i l Acids (% Loss)

Time (minutes)

Acetic

Propionic

Butyric

Valeric

Caproic

1.0 2.0 3.0

1 15 23

0 2 11

0 0 0

0 0 0

0 0 0

Table VI summarizes the e f f e c t of heating medium on the loss of acids after 3 minutes of microwave heating. Loss of v o l a t i l e acids varied widely dependent on the microwave medium. Acetic and caproic acids had losses ranging from 20-80% and 0-73%, respectively, depending on medium composition. The d i e l e c t r i c property, s p e c i f i c heat, or other physical/chemical properties of i n d i v i d u a l flavor compounds can provide valuable insight into the p o t e n t i a l behavior of these compounds during the microwave process. The d i e l e c t r i c property of the t o t a l food system and the a f f i n i t y of the flavor compound f o r the microwave medium, however, were p r i m a r i l y responsi ble f o r the behavior of these flavor compounds during microwave heating. Table VI.

Medium Composition

The E f f e c t of Medium Composition on the Loss of Acids During Microwave Heating Acids (% Loss) Acetic

Oil 23 90/10 Oil/Wat er 80 Water 20

Propionic

Butyric

Valeric

Caproic

11 62 29

0 44 37

0 26 45

0 17 73


49.

Influence of Microwave Heating on Flavor

STEINKE ET AL.

525

Microwave Cake and Conventional Cakes. Losses of d i a c e t y l and acetoin (Table VII) were much greater i n the microwave cake than i n the cake prepared by conventional heating. Losses i n the microwave were attributed to v o l a t i l i z a t i o n , although flavor binding by starches and proteins i s also a factor. Table VII.

The Effect of Microwave vs. Conventional Heating on the Loss of Diacetyl and Acetoin

Type of Cook


Conventional Cook Microwave Cook

Concentration (ppm) Acetoin Diacetyl 80 50

80 65

Microwave food products are rarely as simple as the water and o i l systems discussed above and caution must be exercised i n predicting the reaction of i n d i v i d u a l flavor components i n complex food systems containing s a l t , proteins, sugars, starches, and other food ingredients. Liquid products quickly dissipate the microwave energy and result i n a more uniform product. Solid food products, multiphase systems, or frozen products develop hot spots during heating which further complicate flavor delivery i n these systems. Performance of the flavor i n the microwave i s dependent not only on the physical/chemical properties of individual flavor components, but more importantly, on the interaction of these components with complex food systems.

Literature Cited 1. Bengtsson, N.E.; Risman, P.O., J. Microwave Power, 1971 6(2), 107-123. 2. Nelson, S.O., Trans. Amer. Soc. Agric. Eng., 1980, 23, 1314-1317. 3. Ohlsson, T.; Bengtsson, N.E.; Risman, P.O., J. Microwave Power, 1974, 9, 129-145. 4. To, E.C.H.; Mudgett, R.E.; Wang, D.I.C.; Goldblith, S.A.; Decareau, R.V., J. Microwave Power, 1974, 9(4), 303-316 5. Barbiroli, G.; Garutti, A.M.; Mazzaracchio, P., 1978, Cereal Chem., 1978, 55, 1056-1059. 6. Buttery, F.G.; Ling, L.C.; Guadazni, D.G., J. Agric Food Chem., 1969, 17, 385-389. 7. Nawar, W.W., J. Agric. Food Chem., 1971, 19, 1057-1059. RECEIVED

January 31, 1989


Influence of Microwave Heating on Flavor - American Chemical Society

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