Thermally Generated Flavors - ACS Publications - American Chemical

Chapter 26

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 1, 2018 | https://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch026

Ammonium Bicarbonate and Pyruvaldehyde as Flavor Precursors in Extruded Food Systems 1

2

Henry V. Izzo , Thomas G. Hartman , and Chi-Tang Ho 1

1

2

Department of Food Science and The Center for Advanced Food Technology, Rutgers, The State University of New Jersey, New Brunswick, NJ 08903

Extrusion technology is fast becoming one of the most important mechanisms to add value to a variety of food products. One very important class of extruded products is "ready-to-eat" breakfast cereals. An attempt to enhance the flavor quality of breakfast cereals has resulted in investigating the application of flavor precursors to extruded food systems. The effects of the addition of ammonium bicarbonate and pyruvaldehyde on the aroma profile produced in extruded wheat flour were examined. The results indicate that the addition of these compounds yielded enhanced levels of heterocyclic pyrazines which imparted more of a toasted aroma character to the product. Extrusion cooking is classified as a high temperature short time process (HTST) in which a raw material is fed into a heated barrel and conveyed through it by the action of a rotating screw. Camire (7) summarized extrusion as an operation which combines the processes of heating, transport, mixing, working, and forming. Extrusion has been used to alter the texture and appearance of raw food materials and is used to produced a wide array of "ready-to-eat" breakfast cereals and snack foods (2). Extruders offer the food processor the convenience and flexibility of a continuous process that requires little space and capital investment. However, extrusion is not without its shortcomings, especially in reference to flavor generation (3). As was mentioned previously, the process is continuous. This leaves little time for favorable roasted, toasted, or bakery-type aromas to develop in food systems. Therefore the extent of flavor formation is minimal, resulting in products of a bland or uncooked aroma character. It is for this reason that alternative forms of flavoring extruded foods have been applied. Extruded products can be flavored either before or after extrusion. If flavoring is added to the mix before extrusion, then the product exhibits a more uniform flavor character. However, high losses of flavor are incurred due to thermal degradation and flash-off at the die (4). For this reason overdosing the flavor system 10 to 50 times is usually necessary. Depending on the flavor one is trying to achieve 0097-6156/94/0543-0328$06.00/0 © 1994 American Chemical Society

Parliment et al.; Thermally Generated Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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329

in the product, this can become a costly endeavor. T o avoid these problems, flavoring can be added to the product after emerging from the die. This can be done by either applying dry flavor mixtures or spraying the product with flavoring agents dissolved i n o i l . Even when these applications are successfully achieved, the interior of the product still exhibits a bland taste devoid of toasted flavors. Given the shortcomings of these two techniques, novel research focuses on the use of reactive flavor precursors added to the extruder feed before processing. The hypothesis behind this is governed by the idea that the extruder w i l l provide the precursors with enough thermal and mechanical energy to induce the formation of the aroma chemical during the process. During extrusion starch and protein are fragmented (5), resulting in the formation of simple sugars and free amino acids. These compounds then react to form various dicarbonyl and amino carbonyl compounds that condense during the final stages of the Maillard reaction to generate heterocyclic flavor compounds. Since the residence time i n the extruder is limited, this sequence of chemical events may not progress to completion. B y adding flavor precursors to the m i x before extrusion, the production o f volatiles is enhanced because the initial events that lead to flavor formation are surpassed. The objective of this research was to exploit this idea in an extruded wheat flour system in an attempt to remove the "raw dough" flavor character. This work focuses on the effect of extrusion on flavor formation in wheat flour, as well as how the addition of an exogenous nitrogen and dicarbonyl source effects the final aroma of the product.

Experimental Materials. H i g h gluten wheat flour (14% protein) was obtained from the B a y State M i l l i n g Company (Clifton, NJ) and is sold under the trade name Bouncer. Pyruvaldehyde and ammonium bicarbonate were purchased from the S i g m a Chemical C o . (St. Louis, M O ) . Formulation of Precursor Solutions. Precursor solutions were m i x e d immediately prior to extrusion. The first solution consisted o f a m m o n i u m bicarbonate in distilled water at a 154 mg/mL level. The second solution consisted of pyruvaldehyde and ammonium bicarbonate at levels of 46 mg/mL and 154 mg/mL, respectively. Solutions were kept cool until used. Extrusions. A l l extrusions were carried out on a Werner Pfliederer Z S K - 3 0 c o rotating twin-screw extruder (Werner Pfliederer, Ramsey, N J ) . T h e unit was equipped with a die exhibiting two 3 mm diameter openings. T h e length and diameter of each screw were 900 mm and 30 mm, respectively. T h e barrel was induction heated and possessed five independently controlled heating zones. Product temperatures were recorded by a thermocouple inserted at the die plate. Wheat flour was fed into the unit with a K - T r o n series 7100 volumetric feeding system ( K - T r o n Corp., Pitman, NJ). A metering pump (U.S. Electric Motors, M i l f o r d , C T ) was used to add the liquid feed (precursor solutions). A total of three extrudates were produced: extruded wheat flour (no precursor added), extruded wheat flour + ammonium bicarbonate, extruded wheat flour + ammonium bicarbonate + pyruvaldehyde. A l l extrudates were produced under identical conditions of temperature, feed moisture, and screw speed ( 1 8 0 ° C melt temperature, 16% feed moisture, 450 rpm). T o facilitate the production of the precursor-added samples, the extruder was equilibrated with water, fed v i a a reservoir. Once proper conditions were met, the liquid feed was switched by way of a valve to a second reservoir containing the reactants. Samples were then collected 5


330

THERMALLY GENERATED FLAVORS


minutes after the switch. A l l extrudates were stored at 10°C in one-quart Mason jars until analyzed. Volatile Isolation. Each extrudate was ground i n the presence o f dry ice i n a benchtop grinder (Glen M i l l s , Maywood, NJ) and passed through a 24 mesh sieve to achieve uniform particle size. Ten grams (dry basis) of each sample was weighed into a glass cylinder which was connected to a thermal desorption sample-collecting system (Scientific Instrument Services, Ringoes, NJ). A t this time an internal standard (1 m L of 1 m g / m L toluene-d# in methanol) was spiked directly into the matrix of the solid sample to facilitate quantitative analysis. Purge and trap isolation was then performed on the sample and the volatiles were trapped onto a polymer cartridge consisting of Tenax (Alltech) and Carbotrap (Supelco). The conditions for trapping were as follows: 80°C heating block temperature, 40 m L / min nitrogen gas flow, and 1 hour purge time. After trapping was completed, the polymer cartridges were purged an extra 30 minutes with nitrogen (40 mL/min) at ambient temperature to remove excess water. Volatile Analysis. T h e trapped volatiles were desorbed directly into the G C column ( 2 2 0 ° C , 5 min, helium flow 1 mL/min) using a M o d e l T D - 1 short-path thermal desorption apparatus (Scientific Instrument Services, Ringoes, N J ) (6). Separation o f the volatiles was accomplished using a V a r i a n 3 4 0 0 gas chromatograph equipped with a nonpolar fused silica capillary column (60 m χ 0.32 m m i . d . , 0.25 mm film thickness, D B - 1 ; J & W Scientific). T h e G C was operated with an injector temperature of 250°C, a helium carrier gas flow rate of 1 m L / m i n , and a split ratio of 10:1. The program for volatile separation was as follows: initial column temperature of -20°C with a 5 minute hold during thermal desorption and a temperature increase of 10°C/min from -20°C to 280°C with a 20 minute isothermal hold. The separated volatiles were then detected and identified with a Finnigan M A T 8320 high resolution mass spectrometer. The ionization was set at 70 e V and the source temperature was 250°C with a filament emission current of 1 m A . Spectra obtained were identified by utilizing an on-line computer library ( N B S ) and the Eight-Peak Mass Spectra Series (7). Linear retention indices were determined through the use of a C5-C26 w-parrafin standard (Alltech Associates) according to the method of Majlat et al. (8).

Results and Discussion A summary o f the volatiles produced from the extrusion of wheat flour with and without added precursors is depicted in Table I. Each compound is presented with its corresponding retention index and resulting semiquantitative data. A total o f 23 volatile compounds were identified i n the study. These included 8 aldehydes, 4 alcohols, 3 ketones, 2 furans, 5 pyrazines, and 1 sulfur-containing compound. It is interesting to note that the majority of the aroma compounds present i n the unextruded wheat are aldehydes and alcohols, products derived mainly from lipid degradation. The flour itself however is very low in lipid content, only about 4%. It is possible that the concentration of such volatiles was enhanced during the milling process leaving these components as the major volatiles. When the flour is extruded however, with or without added precursors, the concentration of volatile aldehydes and alcohols diminishes dramatically, especially for hexanal and hexanol. When the flour is extruded, heterocyclic pyrazines are generated which are derived from ami no-car bony 1 interactions. Pyrazines possess a roasted, toasted, or


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Flavor Precursors in Extruded Food Systems

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Table I. Volatiles Generated in Extruded Wheat Flour Systems

COMPOUND

R.I.

d

c

b

a

EX+AB EX+AB+PA EX UNEX Volatile Concentration (ppm)


Aldehydes 641 680 785 890 944 1048 1098 1153

3-Methylbutanal Pentanal Hexanal Heptanal Benzaldehyde Ethylpentanal Nonanal 2-Nonenal

-

.1139 .3233

.0208 .0423 .8382 .0169 .1382 .0472 .0178

.0059 .0221 .5559 .0171 .0735 .0242 .0609

-

-

-

.3114 4.6571 .1334 -

.0198 .0212 .3591 .0094 .0789 .0103 .1218 .0286

Alcohols Pentanol 2-Furanmethanol Hexanol l-Octen-3-ol

781 855 867 979

.5949 7.1221 .2846

870 880 1025

-

-

.0174 .0248 .0144

-

-

.0345 .0312

.0430 .0191

.1549 .1948

.0102 .0172 .1140

.0107 .0116 .0568

.0077 .0191

676 995

.0603 .4249

.0148 .0666

.0060 .0870

.0202 .0640

720 819 905 911 1005

-

.0119 .0261

.1000 .0210

-

-

-

-

-

.3231 .8595 .2582 .0205 .0107

959

-

-

-

.0096

Ketones 5-Methyl-2-hexanone 2-Heptanone 3-Octen-2-one

-

Furans 2-Propylfuran 2-Pentylfuran Pyrazines Pyrazine Methylpyrazine 2,6 - Dimethylpyrazine 2,5 - Dimethylpyrazine 2-Vinyl-5-methylpyrazine

-

Sulfur-Containing Dimethyl trisulfide

b

a

e

U N E X = Unextruded wheat flour; E X = Extruded wheat flour; A B = Ammonium d

bicarbonate added; PA = Pyruvaldehyde added


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T H E R M A L L Y GENERATED FLAVORS


nutty aroma character and are important flavor components in a variety of heated foods (9). When wheat flour is extruded with ammonium bicarbonate, a more reactive ammonia source, the amount of total pyrazines increases. Presumably, this is because the reactive ammonia introduced into the sample is participating i n the formation of Amadori products, catalyzing sugar degradation and contributing nitrogen moieties to the pyrazine structure. In the sample which contains wheat flour, ammonium bicarbonate, and pyruvaldehyde, the concentration of pyrazines increased dramatically. This illustrates the importance of the presence of both a reactive carbonyl source and a reactive amino source to the "time limited" formation of such compounds in an extruded system. Pyrazine F o r m a t i o n . If one looks more closely at the trends o f pyrazine formation in each of the extruded samples, it can be seen that in the case of the wheat flour sample alone and that with ammonium bicarbonate, only unsubstituted pyrazine and methylpyrazine are formed. The sample with added ammonium bicarbonate produced higher levels of unsubstituted pyrazine. This means that the degradation of any sugars hydrolyzed from the starch backbone during extrusion produced two-carbon and three-carbon fragmentations, which in turn resulted in the production two- and three-carbon amino carbonyl fragments. It is these fragments which are known to condense to form pyrazine and methylpyrazine (10). The question then arises as to why no dimethyl pyrazine was detected in these samples, since this compound can be formed by the condensation of two three-carbon amino carbonyl fragments. A possible explanation for this can be hypothesized by examining studies on the kinetics of alkylpyrazine formation (77). It has been shown that the energy of activation for the formation of pyrazine and methylpyrazine is lower than that of dimethylpyrazine and other highly-substituted pyrazines. Therefore, it is possible that in this system the condensation of similar two-carbon amino carbonyl fragments or the condensation of a two- carbon fragment with a three-carbon fragment is more chemically favored than is the condensation between two three-carbon amino carbonyl fragments. In the sample containing ammonium bicarbonate and pyruvaldehyde, these two pyrazines are the major components, probably due to the fact that their formation is kinetically favored. However, 2,6dimethylpyrazine, 2,5-dimethylpyrazine, and 2-vinyl-5-methylpyrazine are also produced. The fact more pyrazine and methylpyrazine are generated in this sample, coupled with the appearance of other substituted pyrazines, shows that the addition of reactive ammonia and dicarbonyl sources is needed to produce greater quantities and diversities of these compounds.

Conclusions The data suggest that the addition of precursors to extruded food systems can enhance the formation of pyrazine aroma compounds and thus boost toasted aroma character. The addition of ammonium bicarbonate to the wheat flour systems did not enhance the formation of pyrazines as much as did the addition of both ammonium bicarbonate and pyruvaldehyde. T h i s suggests that even though starch fragmentation and protein hydrolysis occur during extrusion, the levels of macromolecular breakdown are still too low to facilitate adequate flavor generation. After breakdown, the resulting sugars and amino acids released from the macromolecules must be further degraded to form amino carbonyl fragments, and finally aroma compounds. Time in the extruder is too short to complete these sets of reactions. Since little can be done during extrusion to lengthen the duration of


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Flavor Precursors in Extruded Food Systems 333

reaction, we must increase the reactivity as well as the amount of available reactants so as to achieve more concentrated and diversified levels of pyrazines. Consequently, the addition of reactants in the form of specific Maillard precursors is a promising way to enhance and control the aromas generated in extruded foods.


Acknowledgments This is New Jersey Agricultural Experiment Station Publication No. D-10544-10-92 which was supported by state funds and the Center for Advanced Food Technology. The Center for Advanced Food Technology is a member of the New Jersey Commision for Science and Technology. We thank Mr. Karl Karmas for his assistance in the mass spectrometric analysis and Mrs. Joan Shumsky for her secretarial aid. Literature Cited

1. Camire, M. E.; Camire, Α.; Krumhar, K. CRC Crit. Rev. Food Sci. Nutr. 1990, 29, 35-57. 2. Cheftel, J. C. Food Chem. 1986, 20, 263-283. 3. Parliment, T. H. In Thermal Generation of Aromas ; Parliment, T. H.; McGorrin, R. J.; Ho, C. -T., Eds.; ACS Symposium Series No. 409; American Chemical Society: Washington, D. C., 1989; pp. 2-11. 4. Delache, R. Getriede, Mehl and Brot, 1982, 36, 246-248. 5. Wen, L.F; Rodis, P.; Wasserman, B. P. Cereal Chem. 1990, 67, 268-275. 6. Hartman, T. G.; Karmas, Κ.; Chen, J.; Shevade, Α.; Deagro, M.; Hwang, H. I. In Phenolic Compounds in Food and Their Effects on Health I: Anal Occurance, and Chemistry. ; Ho, C.-T.; Lee, C. Y.; Huang, M. -T., Eds.; ACS Symposium Series No.506; American Chemical Society: Washington, D.C., 1992; pp. 60-76. 7. Mass Spectrometry Data Center, Eight Peak Mass Spectra, The Royal Society of Chemistry: Nottingham, U. K., 1984. 8. Majlat, P.; Erdos, Z.; Takacs, J. J. Chromatography. 1974, 91, 89-103. 9. Maga, J. A. CRC. Crit. Rev. Food Sci. Nutr. 1983, 16, 1-48. 10. Shibamoto, T.; Bernhard, R. A. J. Agric. Food Chem. 1976, 24, 847851. 11. Leahy, M. M.; Reineccius, G. A. In Thermal Generation of Aromas. ; Parliment, T. H.; McGorrin, R. J.; Ho, C. -T., Eds.; ACS Symposium Series No. 409; American Chemical Society: Washington, D. C., 1989, pp. 197-208. RECEIVED

June 30, 1993


Thermally Generated Flavors - ACS Publications - American Chemical

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