Chapter 37
Nonequilibrium Partition Model for Prediction of Microwave Flavor Release 1
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Ernst Graf and Kris B. de Roos
2
1
Tastemaker, 1199 Edison Drive, Cincinnati, OH 45216 Tastemaker, Nijverheidsweg 60, 3771 M E Barneveld, Netherlands
2
During microwave cooking of food many flavor components are rapidly flashed off. Such selective volatilization may result in off-odors, flavor profile distortion, or even a complete lack of flavor. A rigorous physicochemical model was developed to predict flavor retention. This mathematical description accounts for volatility, hydrophobicity, and compartmentalization of the flavor compound within the food matrix. The model lends itself to (1) predict the behavior of a flavor mixture in food during microwave cooking; (2) design a flavor composition and delivery system for unique microwave applications; and (3) optimize the food formulation to enhance the intended flavor functionality. Creative exploitation of the synergistic interactions between flavor and food is paramount to the systematic engineering of microwave foods with superior flavor performance.
Demographic changes and increasing affluence in Western societies have resulted in a growing consumer demand for convenience foods, particularly microwaveable single serve items. Current market penetration of the microwave oven in the US is estimated at 90% of all households and the market for microwaveable foods is predicted to reach about 28 billion dollars per year by 1993 (1). Despite their request for convenience, consumers are unwilling to compromise taste and texture quality. This provides a serious technological challenge but also a competitive business opportunity to the sophisticated food processor and ingredient supplier. A number of flavor problems often limit the use of microwave ovens (2,3). Generation of Maillard browning is mostly absent during microwave cooking due to vast differences in the mechanism of heat transfer. High ambient temperatures in convection ovens dehydrate the food product surface and create an environment conducive to flavor generation, i.e. high temperature at a low water activity for an extended time period. During microwave cooking, however, the product is heated internally and moisture migrates to the food surface, causing evaporative cooling and
0097-6156/94/0543-0437$06.00/0 © 1994 American Chemical Society
Parliment et al.; Thermally Generated Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 28, 2018 | https://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch037
438
THERMALLY GENERATED FLAVORS
high water activity. Development of baking flavors will not occur under these unfavorable conditions and at the drastically reduced cook times. This lack of flavor and aroma generation necessitates addition of flavors to microwave food to increase palatability and consumer acceptance. However, microwave cooking of food results in rapid loss of many flavor components at variable rates, leading to flavor profile distortion or even a complete lack of flavor. In addition to these flavor problems, microwave heating often generates off-odors due to volatilization of minor constituents that are essential to the overall flavor quality yet are malodorous as a single chemical, such as butyric acid in a butter flavor. Finally, with some products consumers desire sustained aroma release throughout the entire cook cycle - instead of an intense initial burst - that mimicks baking in the traditional convection oven. There exists only a limited number of publications on the preparation of microwave food flavors (4-7). Most of this research has been of empirical nature and often entails some form of flavor immobilization by enrobement. Currently, the only systematic approach to describing the behavior of a flavor in a microwave food is the Delta-T theory (8,9). Shaath and Azzo proposed a model in which the release of a flavor can be predicted from its heat capacity and dielectric properties. Unfortunately, all of the microwave absorptivity measurements were performed on pure compounds and the results are completely irrelevant to a flavor diluted to 10 ppm or less. Flavor compounds with high dielectric constants absorb more microwave radiation than those with low dielectric constants. The absorbed radiation is then dissipated to the surrounding medium as heat energy, but flavor volatility is not affected. Due to the high dilution of most flavors, their dielectric properties fail to measurably influence the temperature of the solution. A similar conclusion concerning the inadequacy of the Delta-T theory was reached previously (10). The Delta-T theory assigns absolute microwave volatility constants to flavors and does not recognize any effects of the medium on the flavor volatility. Steinke et al. demonstrated the large dependence of aroma release on the type of solvent (11,12). In fact, we will show below that the numeric value of the partition coefficient of a flavor compound between two solvents is directly proportional to the ratio of its volatility from the same two solvents. Therefore, aroma release of a very hydrophobic compound from water may exceed its release from oil by a factor of 10,000. The current paper presents a rigorous physicochemical mathematical model that takes into account flavor volatility, hydrophobicity and its compartmentalization in the food matrix. This novel nonequilibrium partition model lends itself to accurately predict the behavior of a flavor mixture in any food during microwave cooking, to prevent some of the aforementioned problems specific to microwave flavors, and to customize food systems with unique flavor performances. Materials and Methods Determination of Partition Coefficients. Oil-to-air partition coefficients were measured at 25°C, 55°C, 80°C, and 100°C using capillary tubes packed with X A D - 4 beads according to the method of Etzweiler et al. (13). Water-to-air partition coefficients were determined at 25°C, 90°C, and 95°C by monitoring the decrease in aqueous concentration of the flavor compounds during stripping with nitrogen gas at a flow rate of 12.5 ml/min. To avoid evaporation of water from the solution the nitrogen was presaturated with water vapor. Samples of
Parliment et al.; Thermally Generated Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
37.
GRAF & DE ROOS
Nonequilibrium Partition Model
439
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 28, 2018 | https://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch037
200 μΐ were analyzed by HPLC after 15, 60, 120, and 240 min. The partition coefficients were calculated from the fraction of the flavor retained in the aqueous solution. Partition coefficients at 100°C were calculated by extrapolation using the Clausius-Clapeyron equation. Microwave Cake Preparation. Microwave cake batters (2000 g) - 24.9% flour, 24.6% sugar, 2.9% vegetable shortening, 1.1% Tween 60 (emulsifier), 0.1% methanolic flavor solution (10 to 30 ppm of 5 different flavor compounds in batter), variable amounts of water (24.0% to 46.4%), and variable amounts of soy oil (0% to 22.4%) - were prepared by creaming the sugar with the shortening and then blending in the remaining ingredients to uniformity. The combined amount of water and oil was kept constant at 46.4%. Triplicate batter aliquots of 100 g were placed in glass dishes and baked in the microwave oven for 1, 2, and 3 minutes at 640 watts (calibrated using 2000 g of water). Batter temperature was monitored continuously using a fluoroptic fiber probe (Luxtron instrument). Moisture loss was determined gravimetrically at the end of each time interval by measuring total weight loss. Some samples were also baked in a convection oven at 230°C for 20 and 40 minutes. Interior cake temperature was determined with a thermometer after both time intervals. The following 5 flavor compounds were selected for the microwave release study: 2,3-dimethylpyrazine, indole, naphthalene, oc-ionone, and δ-2-decenolactone. Criteria for choosing these flavor compounds included their range of volatility and hydrophobicity, as well as their ease of analytical extraction, separation, and detection. Similarly, the above microwave cake model was developed to (1) allow for a wide range of water-to-oil ratios without phase separation; (2) use a pourable system to minimize variability in sample weight, volume, and geometry; and (3) to evaluate aroma release in a real food product in order to eliminate any potential colligative and other interactive solute effects of the flavor constituents that might cause boiling point elevations or depressions and steam distillation in a pure liquid. Determination of Flavor Retention in Cake Batter. Cake batter (50 g) was weighed accurately into a separatory funnel. A n 18% aqueous NaCl solution (120 ml) was added, followed by methanol (160 ml), dichloromethane (320 ml), and 0.4% (w/v) methanolic m-dimethoxybenzene (50 μΐ). The mixture was stirred vigorously for 30 min and the resulting slurry was allowed to separate into two phases. The organic phase was centrifuged - i f necessary - and concentrated by distillation through a Vigreux column until the temperature started to rise. Dichloromethane (250 ml) was added and distillation was continued to complete azeotropic removal of methanol. The concentrate (10 ml) was filtered and analyzed by HPLC. The method for the flavor determination in the cake was the same as that in batter, except water was added prior to extraction to compensate for moisture loss during baking. Quantitative flavor analyses were performed by reverse phase H P L C on a 250 χ 4 mm Ultrasphere ODS column with a guard column. The compounds were eluted at 30°C at a flow rate of 1 ml/min using the following linear gradient systems: C H 3 C N : H 2 0 (1:4) for 2.5 min, C H 3 C N : H 2 0 (3:1) for the subsequent 22.5 min, and
100% C H 3 C N for the last 10 min. The eluate was monitored at 235 and 270 nm with a multiple wavelength detector. The column was washed with methylene chloride.
Parliment et al.; Thermally Generated Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
440
THERMALLY GENERATED FLAVORS
Results and Conclusions When a flavor compound is allowed to equilibrate between an emulsion and air in a closed system, the fraction of the compound present in the liquid phase is given by its mass balance (equation 1). From this mass balance we can easily derive the mathematical extraction equilibrium based on partition coefficients (equation 2) by substituting partition coefficients for flavor concentrations ( c = P c , o ï aw w^ ao) * by subsequently multiplying the numerator and denominator by P / c . In the case of a single-phase liquid system, such as pure water, the term V in equation 2 becomes zero and the extraction equilibrium for a water-air system can be reduced to equation 3. In equation 2 either P or P can also be expressed in terms of P according to equation 4, since a complete mathematical description of all three partition equilibria in a closed ternary system requires the measurement of only two partition coefficients. The partition coefficient P is directly proportional to the hydrophobicity of the flavor compound, while P and P are a direct measure of its volatility in water and in oil, respectively. c
a
>
c
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 28, 2018 | https://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch037
a o
>
a w
=
w
a n (
w
0
a o
a w
o w
o
w
a w
j _ c aVa
c 0 V0
+
+
c oV o
VwP ao
^ _
+
VaP aw Ρao +
a o
c WW V +
^ c ww ,V %A
A#
Vο P aw VwP ao +
^ Vο P aw
(3)
f = Va Ρ aw ,0
9
P
o ow w =
'°9»*waw P
,0
9 °a o ao P
4
( )
where: f
V v v
= = =
w 0
a
^ow ^ao ^aw Mw [c] Ma 0
~ =
=
=
= =
fraction o f flavor retained i n liquid or food volume o f water (ml) volume o f oil (ml) volume o f air (ml) oil-to-water partition coefficient ([c]o/[c] ) air-to-oil partition coefficient ([c] /[c] ) air-to-water partition coefficient ( [ c ] / [ c ] ) flavor concentration in water (g/1) flavor concentration in o i l (g/1) flavor concentration in air (g/1) w
a
0
a
w
The pertinence of equation 2 to a real food system was tested in a whole milk model. Five flavor compounds with a wide range of volatility and hydrophobicity were dissolved in both water and olive oil and their liquid-to-air partition coefficients were determined from headspace analyses. These partition coefficients were employed to calculate flavor retention in milk using equation 2. Milk was assumed to consist of a Parliment et al.; Thermally Generated Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 28, 2018 | https://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch037
37. GRAF & DE ROOS
441
Nonequilibrium Partition Model
4% oil-in-water emulsion. Table I demonstrates excellent agreement between observed and predicted flavor retention in milk. This example clearly illustrates the general validity of the above flavor equilibrium partition model for estimation of aroma release from food products. In order to apply the static equilibrium equation to dynamic nonequilibrium microwave systems, we used the multiple extraction model which is premised on the following assumptions: During baking in the microwave oven the batter or cake is extracted consecutively with infinitesimal volumes of air or water vapor. During each successive extraction, food-to-air equilibrium is achieved only at the interphase. However, continuous flavor uniformity throughout the food and oil-to-water equilibrium of flavor compounds are maintained by the mass transfer of moisture due to rapid steam generation. The fraction of a flavor compound remaining in the cake after η extractions is given by equation 5.
f =
VwΡao 1
f
" e+
f
e
V~ Ρaw Ρao r
Vο Ρ aw +
VwP ao +
(5)
Vο Ρ aw
where: f η
e
= =
fraction of food being extracted number of successive extractions
For each food the variables f , η and V must be determined by measuring the retention of a test sample of flavor compounds with known partition coefficients. In principle, η is very large and f infinitely small. Therefore, in practice, we substitute for f a very low value (
