Chapter 48
Thermal Generation of Aromas Downloaded from pubs.acs.org by UNIV OF TEXAS SW MEDICAL CTR on 10/09/18. For personal use only.
Design of Flavors for the Microwave Oven The Delta T Theory Nadim A. Shaath and Nehla R. Azzo Research and Development Laboratory, Felton Worldwide Inc., 599 Johnson Avenue, Brooklyn, NY 11237 Over 500 raw materials used to create flavors were analyzed through a series of experiments designed to characterize their heat absorption in the microwave oven. From the data gathered, we have proposed the Delta T (ΔT') theory to describe the behavior of flavors in the microwave environment. TheΔT'values calculated for these raw materials, which comprise a range of functional groups, allow for the extrapolation of our data to the thousands of raw materials currently used in creating food flavors. This ultimately will enable the design of flavors which are customized for microwave food applications. By the year 1990, at least one microwave oven (MWO) will be found in 85-90* of U.S. homes (1). This increasing number of microwave units has created the demand for the introduction of many MWO-related food products. Early consumer trials of cooking in the MWO were from scratch, resulting in food products that were non-palatable and unappetizing. The next trials were of packaged foods originally meant for the conventional oven which were labeled as "microwaveable". The results of these trials were also unfavorable. Conse quently, the challenge recognized by food, packaging and flavor companies was to design products exclusively for the MWO that delivered traditionally accepted tastes. In contrast to conventional ovens that generate and transmit heat through conduction and convection, the MWO produces electro magnetic waves that penetrate food and cause friction among i t s components, generating heat (2). Microwaves radiate at a frequency of 3x10 to 1x10 MHz with a corresponding wavelength of 3x10 to 1x10 cm (3). In the U.S., the most common frequency used in household units is 2,450 MHz (2,450 million cycles/second) (2). This radiation, with its alternating electromagnetic fields, causes increased movement of polar and ionic molecules. Microwaves are absorbed by foods and oils yet are reflected by metal; glass, paper and ceramics are transparent (2). It is this inconsistent behavior of objects with microwaves that must be considered in the packaging 0097-6156/89/0409-0512$06.00/0 © 1989 American Chemical Society
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513
and oven d e s i g n as w e l l as i n t h e food and f l a v o r development. The uniqueness and advantageous q u a l i t i e s o f t h e MWO as opposed t o t h e c o n v e n t i o n a l oven may become more prominent w i t h t h e p r o p e r h a r n e s s i n g o f d i f f e r e n t e n e r g i e s and i n t e r a c t i o n s . The p o s s i b i l i t i e s f o r new and r e v o l u t i o n a r y methods o f p r e p a r i n g meals a r e t h e r e f o r e many and s i g n i f i c a n t . Problems A s s o c i a t e d w i t h M i c r o v a v e
Heating
The problems a s s o c i a t e d w i t h t h e b e h a v i o r o f food p r o d u c t s i n the microwave oven a r e e i t h e r i n h e r e n t t o t h e p r o p e r t i e s o f microwaves o r a r e a f u n c t i o n o f t h e f o o d , p a c k a g i n g and oven d e s i g n . The microwave oven i s d e s i g n e d w i t h m e t a l w a l l s as a r e f l e c t o r t o c o n t a i n t h e microwaves i n s i d e t h e c a v i t y . However, t h i s c r e a t e s problems such as h o t and c o l d s p o t s g e n e r a t e d from t h e phenomenon o f s t a n d i n g waves ( 4 ) . Hot s p o t s a r e a r e a s w i t h i n t h e oven c a v i t y t h a t e x p e r i e n c e e x c e s s i v e amounts o f energy and c o l d s p o t s a r e a r e a s within the c a v i t y t h a t r e c e i v e n e g l i g i b l e energy ( 5 ) . A l s o , the frequency o f 2,450 MHz has a wavelength o f 12.2 cm which i s o f a s i m i l a r magnitude t o t h a t o f most food p r o d u c t s . T h i s c r e a t e s an i n c r e a s e d p e r c e n t a g e o f microwave r a y s r e f l e c t e d o f f t h e s u r f a c e o f foods ( 6 ) . When c o o k i n g a product i n t h e MWO, t h e a i r i n s i d e t h e oven c a v i t y remains c o o l ( a i r i s t r a n s p a r e n t t o microwaves). This produces a food t h a t i s heated from t h e i n s i d e b u t whose s u r f a c e temperature i s l o w e r due t o c o n t a c t w i t h t h e c o o l e r oven a i r ( 2 ) . Because o f t h i s , i t s h o u l d a l s o be noted t h a t microbes and b a c t e r i a c o u l d s u r v i v e a t food s u r f a c e s due t o l o w e r temperatures ( 7 ) . Lack o f c r i s p i n g and browning a r e two drawbacks o f t h e MWO ( 8 ) . Crispy t e x t u r e and brown c o l o r n o r m a l l y a c h i e v e d i n c o n v e n t i o n a l c o o k i n g a r e l a c k i n g i n microwave c o o k i n g . I n c o n v e n t i o n a l ovens, h i g h temperatures d e h y d r a t e a p r o d u c t ' s s u r f a c e , p r o d u c i n g a c r i s p y c r u s t on t h e e x t e r i o r which a l s o h e l p s t o p r o t e c t t h e i n t e r i o r from m o i s t u r e and f l a v o r l o s s . I n t h e MWO c r i s p i n g does n o t o c c u r , and s o g g i n e s s may r e s u l t as w e l l . Sogginess d e v e l o p s i n microwaved food p r o d u c t s s i n c e m o i s t u r e (and v o l a t i l e c o n s t i t u e n t s ) a r e d r i v e n t o the food s u r f a c e ( 9 ) . The s u r f a c e m o i s t u r e p e r s i s t s because o f t h e c o o l oven a i r s u r r o u n d i n g t h e f o o d . The u n c r u s t e d p r o d u c t i n the MWO remains prone t o t h e s t e a m i n g - o f f o f m o i s t u r e , f l a v o r and o t h e r volatile food components. The f l a s h i n g - o f f " and " m o d i f i c a t i o n " o f f l a v o r s i n microwaveable foods a r e o f t e n common and p r e d i c t a b l e occurrences. Because o f t h e dynamic e f f e c t s o f e l e c t r o m a g n e t i c energy on t h e f l a v o r s themselves, t h e p a r t i a l o r t o t a l v o l a t i l i z a t i o n o f f l a v o r i s o f t e n t h e cause f o r b l a n d t a s t e and t h e d e v e l o p ment o f o f f - f l a v o r i n f o o d s . Microwave energy can cause p h y s i c o c h e m i c a l changes i n s p e c i f i c f l a v o r components, r e s u l t i n g i n s e r i o u s d i s t o r t i o n of the f i n a l taste p r o f i l e . The s h o r t e n e d c o o k i n g times common t o t h e MWO do n o t a l l o w f o r p y r o l y s i s , c a r a m e l i z a t i o n o r f l a v o r - g e n e r a t i n g r e a c t i o n s such as Maillard, S t r e c k e r d e g r a d a t i o n and Amadori rearrangements t o o c c u r ( 1 0 ) . T h i s commonly l e a v e s microwave foods ( e s p e c i a l l y baked goods) w i t h a raw, uncooked and underdeveloped t a s t e . These problems a r e c u r r e n t l y b e i n g approached through v a r i o u s oven m o d i f i c a t i o n s , packaging d e v i c e s such a s s u s c e p t o r s and h e a t i n g e l e m e n t s , as w e l l as t h e p r o p e r d e s i g n o f f l a v o r s and food p r o d u c t s f o r t h e MWO. f ,
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The Delta T (AT) Theory (11,12) Our approach was t o understand the n a t u r e o f the i n t e r a c t i o n o f f l a v o r s w i t h microwave energy. The f a c t o r s t h a t a f f e c t microwave h e a t i n g o f foods i n c l u d e the s p e c i f i c heat ( s ) , t h e d i e l e c t r i c c o n s t a n t (e')» the l o s s tangent and the d i e l e c t r i c l o s s f a c t o r ( 1 3 ) . However, i n the MWO, i n h e r e n t p r o p e r t i e s o f the oven and i n d i v i d u a l foods become more prominent, such as t h e d e n s i t y , shape, size, m o i s t u r e c o n t e n t , q u a n t i t y and c o n s i s t e n c y o f t h e f o o d , as w e l l a s the s t a r t i n g temperature and power s e t t i n g o f the oven ( 1 4 ) . I t i s thus c l e a r l y e v i d e n t t h a t many complex f a c t o r s a r e r e s p o n s i b l e f o r the b e h a v i o r o f foods i n the MWO. T h e r e f o r e we d e c i d e d t o d e v e l o p a s i m p l i f i e d approach t h a t can a s s i s t o u r f l a v o r c h e m i s t s i n d e s i g n i n g new f l a v o r s f o r t h e MWO, one which combines the m a j o r i t y o f t h e f a c t o r s t h a t a f f e c t microwave h e a t i n g . With t h i s g o a l i n mind we conducted a s e r i e s o f experiments t h a t t e s t e d the e f f e c t s o f microwaves on i n d i v i d u a l f l a v o r i n g r e d i e n t s . S i n c e s p e c i f i c h e a t s , d i e l e c t r i c c o n s t a n t s and d i e l e c t r i c l o s s f a c t o r s a r e not a v a i l a b l e i n the l i t e r a t u r e f o r most o f the commonly used f l a v o r i n g r e d i e n t s , we searched f o r a l t e r n a t e c r i t e r i a t o e v a l u a t e f l a v o r raw m a t e r i a l s . Thus, e a r l y i n 1985 we embarked on a r e s e a r c h p r o j e c t t o a n a l y z e over 500 c h e m i c a l s , s o l v e n t s , e s s e n t i a l o i l s and n a t u r a l f l a v o r i n g components t h a t were c a r e f u l l y s e l e c t e d to r e p r e s e n t t h e thousands o f raw m a t e r i a l s used i n the f l a v o r industry. A l l were a n a l y z e d i n d i v i d u a l l y i n the MWO t o d e t e r m i n e their characteristic behavior within the e l e c t r o m a g n e t i c environment. The b a s i c d e s i g n o f t h e experiment c o n s i s t e d o f p l a c i n g a preweighed sample o f m a t e r i a l a d j a c e n t t o an e q u i v a l e n t preweighed sample o f water as a s t a n d a r d i n the c e n t e r o f a 700 Watt MWO. Each sample was t e s t e d under v a r i o u s c o m b i n a t i o n s o f power s e t t i n g s (10, 30, 50, 80 and 100*) and h e a t i n g times (20, 60, 150, 300 s e c ) . Temperatures were measured a t each power and time s e t t i n g . E l e v e n c o m b i n a t i o n s o f power v s . time s e t t i n g s were conducted f o r each c h e m i c a l and t h e i r average temperature v a l u e was computed. Data was collected and a p a t t e r n emerged which reflected the tested m a t e r i a l ' s unique heat a b s o r p t i o n i n the microwave oven. Each component was g i v e n a r e p r e s e n t a t i v e v a l u e , D e l t a T' ( A T ' ) , w h i c h c o r r e s p o n d s t o the r a t i o o f t h e temperature i n c r e a s e o f the sample to t h e temperature i n c r e a s e o f the s t a n d a r d . Thus, AT (sample) AT ( s t a n d a r d )
A T /
where *
T
- final " T
T
initial
The e x p e r i m e n t a l d e s i g n i n c l u d e d AT' c a l c u l a t i o n s f o r a number o f commonly-used f l a v o r c h e m i c a l s i n c l u d i n g a homologous s e r i e s o f thioalcohols, a l d e h y d e s , ke tones, e s t e r s , l a c t o n e s , amines, compounds and hydrocarbons. T a b l e I c o n t a i n s t h e AT' d a t a on two homologous s e r i e s ,
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Design of Flavors for the Microwave Oven
a l i p h a t i c a l c o h o l s and a l d e h y d e s , compared a g a i n s t 5 (15) and 8' (16) d a t a . The predominant e f f e c t o f i n c r e a s i n g c h a i n l e n g t h i s a
Table I .
AT' V a l u e s o f Two Homologous S e r i e s , w i t h S p e c i f i c Heat and D i e l e c t r i c C o n s t a n t s
Chemical
AT'
Value
s
(15)
e'
(16)
Aliphatic Alcohols A l c o h o l C-2 A l c o h o l C-6 A l c o h o l C-8 A l c o h o l C-10
1.73 1.13 0.97 0.75
0.59 0.59 0.59 0.54
24 13 10 8
A l i p h a t i c Aldehydes Aldehyde C-2 Aldehyde C-3 Aldehyde C-5 Aldehyde C-6 Aldehyde C-10
1.28 1.18 0.98 0.93 0.71
0.57 0.56 0.52 0.52 0.50
27 18 12 11 7
d e c r e a s e i n r e l a t i v e p o l a r i t y as e v i d e n c e d by the d e c r e a s i n g AT' v a l u e s and e' c o n s t a n t s . Note t h a t a c o r r e l a t i o n between the AT' v a l u e and the e' c o n s t a n t can be made o n l y i f s remains r e l a t i v e l y u n i f o r m throughout he homologous s e r i e s . I n t e r f u n c t i o n a l group comparisons a l s o r e v e a l , as e x p e c t e d , t h a t the more p o l a r a l c o h o l s g e n e r a l l y have a h i g h e r AT' v a l u e than the c o r r e s p o n d i n g a l d e h y d e s . T a b l e I I l i s t s 13 o f the most commonly-used c h e m i c a l s i n the f l a v o r i n d u s t r y . For example, c i n n a m i c aldehyde i s used f o r the g e n e r a t i o n o f c a s s i a o r cinnamon f l a v o r s , carvone f o r spearmint flavors, c i t r a l f o r c i t r u s f l a v o r s , and benzaldehyde for cherry f l a v o r s . As can be seen, the known s v a l u e s f o r t h e s e compounds a r e
T a b l e I I . AT' V a l u e s o f Common F l a v o r i n g Raw M a t e r i a l s w i t h S p e c i f i c Heat and D i e l e c t r i c C o n s t a n t s Chemical D i h y d r o coumarin gamma-Undecalactone Cinnamic a l d e h y d e Carvone Glycerin Citral Benzaldehyde Benzyl alcohol Ethanol E t h y l benzoate Water A l c o h o l C-8 Aldehyde C-10 Myrcene
AT'
Value 2.65 2.64 2.58 2.38 2.30 2.29 2.19 1.81 1.73 1.13 1.00 0.97 0.71 0.09
s
(15) 0.37 0.38 0.37 0.40 0.57 0.39 0.40 0.44 0.59 0.39 1.00 0.59 0.50 0.50
8'
(16) 25 23 17 15 42 19 19 13 24 6 78 10 7 3
515
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THERMAL GENERATION OF AROMAS
not f a i r l y c o n s t a n t , hence t h e c o r r e l a t i o n o f e' c o n s t a n t s w i t h t h e degree o f c h e m i c a l p o l a r i t y ceases t o e x i s t . The AT' v a l u e , on t h e o t h e r hand, a f f o r d s an e x c e l l e n t i n d i c a t o r o f a c h e m i c a l ' s unique heat a b s o r p t i o n i n the MVO. A n o n - p o l a r hydrocarbon such as myrcene (JL) has an e x t r e m e l y low AT' v a l u e o f 0.09, whereas an aldehyde such as c i n n a m i c aldehyde ( 2 ) e x h i b i t s a v e r y h i g h AT' v a l u e o f 2.58. These v a l u e s i n d i c a t e t h a t , under the same h e a t i n g c o n d i t i o n s i n t h e MVO ( f o r example, 30% power s e t t i n g and 150 s e c o n d s ) , myrcene reached a maximum temperature o f 35°C, v e r s u s 175°C f o r c i n n a m i c aldehyde. Thus, t h e d i f f e r e n c e s i n the AT' v a l u e s i n d i c a t e a d r a m a t i c d i f f e r e n c e i n t h e heat a b s o r p t i o n c a p a c i t y o f f l a v o r c h e m i c a l s i n the MVO.
C H = C H—C — H
s
6 2
Numerous a d d i t i o n a l t e s t s t o determine the e f f e c t o f the carbon c h a i n on the f u n c t i o n a l group were conducted u s i n g a homologous s e r i e s . The e f f e c t o f u n s a t u r a t i o n i s pronounced o n l y i n cases where t h e double bond i s c o n j u g a t e d w i t h the a l d e h y d i c f u n c t i o n a l group. F o r example, t r a n s - 2 - h e x e n a l (£) had a AT' v a l u e o f 1.50 a s compared t o n-hexanal w i t h a v a l u e o f 0.93. The ease o f e l e c t r o n derealization i n t r a n s - 2 - h e x e n a l , as shown below, i n c r e a s e s t h e p o l a r i t y o f the m o l e c u l e , hence i t s h i g h e r AT' v a l u e .
o" +
CH (CHg ) CH=CH—C 3
2
*
\
/
CH (CH2 ) CH—CH=C 3
2
H
B r a n c h i n g has a moderate e f f e c t on i n c r e a s i n g t h e AT' v a l u e s o f molecules. F o r example, e t h y l n - b u t y r a t e (4) has a AT' v a l u e o f 0.20, whereas e t h y l i s o b u t y r a t e ( 5 ) has a AT'~value o f 0.37.
CH, CH - C H , - CH,- C - O - C H 3
2
5
CH - CH - C - O - C H 3
2
O
4
o 5
5
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SHAATH AND AZZO
517
Geometric isomerism has an obvious e f f e c t on the p o l a r i t y o f o r g a n i c molecules a s evidenced by t h e h i g h e r AT' v a l u e f o r c i s - 3 - h e x e n o l (6) when compared to that o f trans-2-hexenol (T) a s shown below. Note that the p o s i t i o n o f the double bond i s n o t r e l e v a n t f o r the e n o l 6, s i n c e the a l c o h o l group i s unconjugated. HO-(CH,)
C H 2
S
HO-CH.
o=c(
/C= ( C
N
H 6
(AT'«1.14)
H
H
N
H
C H 3
7
7
(AT'«0.86)
AT' v a l u e s a r e thus d i v i d e d i n t o two major groups: f l a v o r components which were found to possess a h i g h AT' ( g r e a t e r than 1.0, the v a l u e f o r water), and those with low AT' ( l e s s than 1.0). High AT' components tend t o g e t h o t t e r i n t h e microwave oven and t h e r e f o r e can be used most e f f e c t i v e l y i n " r e a c t i o n " - t y p e f l a v o r s where browning and c a r a m e l i z a t i o n i s d e s i r a b l e . Conversely, low AT' v a l u e s r e f l e c t t h e reduced heat absorbance o f f l a v o r components w i t h i n the microwave oven. They a r e l e s s prone to microwave-related " m o d i f i c a t i o n s " o r " f l a s h i n g - o f f " and a r e t h e r e f o r e l i k e l y to have superior flavor retention. Experimental data f o r chemical combinations, e s s e n t i a l o i l s , and f l a v o r systems w i l l appear i n a future publication. Conclusion
The knowledge gained from the t e s t i n g and e v a l u a t i o n o f the dozen f u n c t i o n a l groups, t h e i r homologous s e r i e s and the e f f e c t o f c h a i n l e n g t h , geometry, u n s a t u r a t i o n , c o n j u g a t i o n and branching on t h e i r AT' v a l u e s has enabled us i n our c u r r e n t r e s e a r c h to assess the heat a b s o r p t i o n c h a r a c t e r i s t i c s o f hundreds o f aroma chemicals and t h e i r combinations. T h i s , i n t u r n , allows us to d e s i g n and develop numerous f l a v o r systems tailor-made f o r many microwave food applications. Literature Cited
1. Thorns, S.J.MV Foods '88, First International Conference on Formulating Food for the Microwave Oven, Chicago, March 8-9, 1988. 2. Schiffmann, R. F. Proc. Intl. Microwave Power Inst. Mtg., June 9-11, 1987. 3. Colthup, N.B.; Daly, L.H.; Wiberley, S.E. Introduction to Infrared and Raman Spectroscopy; Academic Press: New York, NY, 1964; p 2-3. 4. Ohlsson, T. Microwave World March-April 1983, p 4-9. 5. Anon. Food Technology January 1989, p 117-125. 6. Keefer, R. Microwave World Nov.-Dec. 1986, p 11-15. 7. Lin, W.; Sawyer, C. The Journal of Microwave Power and Electromagnetic Energy 1988, 23, p 183-194.
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8. Cramwinckel, A.B.; Raats, M.M.; Logman, H.W.; Butijn, C. Microwave World 1988, 9, p 9-13. 9. Craft, P.J. Food Engineering September 1981, p 66. 10. Katz, E. M W Foods '88, First International Conference on Formulating Food for the Microwave Oven, Chicago March 8-9, 1988. 11. LaBell, F. Food Processing June 1988, p 142-145. 12. Anon. Food Engineering May 1988, p 57-58. 13. Schiffmann, R.F. M W Foods '88, First International Conference on Formulating Food for the Microwave Oven, Chicago, March 8-9, 1988. 14. Schiffmann, R.F. M W Foods '88 Sponsored by PIRA Packaging Division, England, June 30-July 1, 1988. 15. Noller, C.R. Chemistry of Organic Compounds; W.B. Saunders Company: Philadelphia, PA, 1965; 3rd. Edition p 991-992. 16. Weast, R.C. CRC Handbook of Chemistry and Physics, 66th edition; CRC Press, Inc.: Boca Raton, FL, 1985-1986; p E-52, E-55. RECEIVED May 31, 1989
