Thermally Generated Flavors - American Chemical Society

Chapter 38

Microwave and Thermally Induced Maillard Reactions Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 26, 2016 | http://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch038

V. A. Yaylayan, N . G. Forage, and S. Mandeville Department of Food Science and Agricultural Chemistry, McGill University, 21111 Lakeshore Road, Ste. Anne de Bellevue, Quebec H9X 3V9, Canada

The effect of amino acid type on the generation of Maillard aromas under microwave irradiation in an open system was evaluated by mixing different combinations of amino acids with the same reducing sugars and characterizing the aromas produced as caramel, meaty, nutty, fragrant, vegetable and baked. The amino acids were divided into five categories; aliphatic, aromatic, basic, acidic and sulfur-containing. Certain trends emerged after analysis of the results that relate the presence of specific amino acid category in the reaction mixture to a corresponding aroma note produced after microwave heating. The presence of amino acids with alkyl side chains were found to be essential for the generation of caramel notes, sulfur-containing amino acids for meaty type notes and basic amino acids for nutty and baked notes. Selected formulations were also subjected to conventional heating and their sensory properties and chemical composition (by GC/MS analysis) were compared to those from microwave treated samples. No significant differences were observed between the two samples. Developing natural flavors and colors for the microwave poses one of the next challenges to the food industry. The lack of Maillard flavor development during microwaving and the loss of added flavors are primary factors which contribute to the low acceptability of many microwaved food products (1). However, in spite of these shortcomings, the sales of microwave food products have experienced a larger growth compared to overall food sales in the last few years. The most common approach used today in the industry to promote flavor development in the microwave is the use of susceptor packaging. While susceptors are effective in promoting surface browning, they do not allow the full development of flavors (2). On the other hand, the "Delta T" theory provides some guidelines as to the type of added flavors that can be used in microwave formulations to minimize their loss (3). To improve the quality of microwave foods, and to better understand the differences between microwave and conventional heating, the chemical composition of model systems subjected to both modes of heating have been compared. Parliment (4) studied the products of the Maillard reaction between glucose and proline formed under microwave and conventionally heated systems and found that certain products were predominant in

0097-6156/94/0543-0449$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 CALIFORNIA SAN DIEGO on December 26, 2016 | http://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch038

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THERMALLY GENERATED FLAVORS

the microwave treated samples and others in the conventionally heated mixtures. Yeo and Shibamoto (5) reviewed the chemical composition of microwaved and conventionally heated foods and model systems. Some of the conclusions drawn from different studies were in contradiction, such as the relative amounts of pyrazines and sugar fragmentation products in microwave and conventionally heated samples. On a more fundamental level, microwave irradiation has been used to perform variety of organic single-step reactions such as esterification, hydrolysis, cyclization, Diels-Alder, Sjq2-type reactions etc., and the results have been compared to classical reflux method by different researchers (6, 7), according to these studies there are no fundamental differences between microwave and reflux methods of heating, the reactions proceeds through the same mechanisms, to produce the same products, in comparable yields, however, i f the microwave heating is done under closed system, then the rate of the microwave reaction becomes faster up to several hundred times, due to the superheating of the solvent. For example, the percent yield of η-propyl benzoate after heating equal amounts of reactants for 4 min in an oil bath at 160°C and in the microwave oven in an open vessel was 29 and 25 % respectively. The same reaction produces a yield of 79% for 6 min of microwave heating in closed system, compared to 4 h refluxing which produces the same yield. In a closed system microwave increased the rate of the esterification reaction by 40 times (7). Materials and Methods Sample Preparation. Vials containing aqueous solutions of Maillard precursors were subjected to microwave irradiation or to conventional heating or both, in an open system in a domestic dual microwave/conventional oven, operated at full power (640 W). The samples were irradiated by microwave until all the water was evaporated (2-4 minutes) and the residue was dark brown, to mimic actual cooking conditions that require surface drying of foods. Thus, the compounds identified by GC/MS represent the volatiles that were trapped in the residue or undecomposed non-volatile components. In order to insure that both treatments produced the same extent of Maillard reaction for comparison purposes, the conventional heating time was adjusted such that after similar dilutions, both samples absorbed to the same extent at 460 nm in the spectrophotometer. The samples were diluted with 1.0 mL of methanol, and the absorbance was measured (460 nm) prior to injection into the GC/MS. Similarly, the same samples were also subjected to conventional heating until most of the water has evaporated (20-50 minutes), diluted to 1.0 mL with methanol and the absorbance at 460 nm was measured. The process was repeated at different time intervals, until the same absorbance was achieved as for the microwave treated sample. On the average, one minute of microwave heating time was equivalent to 12 minutes of conventional heating time to produce the same extent of browning. GC/MS Analysis. A Hewlett Packard GC/Mass selective detector (5890 GC/5971B MSD) was used for the separation and acquisition of the electron impact (70 eV) mass spectra of the compounds. Two μL of the methanol solution was injected in the split mode (15:1) on to a fused silica capillary column (HP Ultra-2, 50-m χ 0.2 mm i.d. χ 0.33 μπι film thickness). The column was held at 70°C for 2 min., then increased at a rate of 5°/min to 250°C and held for 5 minutes. Carrier gas (helium) flow rate was 0.85 cm^/min; injection port temperature was 250°C. Tentative identification of the compounds were achieved by comparison of the fragmentation patterns of the unknowns with those in the Wiley/NBS mass spectral library through a P B M library search routine. Model systems. Model systems were prepared by mixing the same reducing sugar(s) with different combination of amino acids (see Table I) from each group with all

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

38. YAYLAYAN ET AL.

Microwave and Thermally Induced Maillard Reactions 451

possible permutations in specific ratios. The mixtures were subjected to microwave heating in an open vial until the development of brown color and evaporation of most of the water. The sensory evaluation by an untrained panel of the resulting aromas were than recorded.


Results and Discussion There are no satisfactory theoretical explanations or practical solutions to the problem of Maillard flavor generation during microwave heating. Attempts have been made to compare the chemical composition of microwaved and conventionally heated Maillard model systems, however, this type of comparisons can be extremely misleading due to the variations in the time-temperature exposure of the two systems, specially in closed containers where due to extreme high pressures, the solvent can superheat and accelerate the rate of the chemical reactions (7). In order to draw meaningful conclusions about the real differences between microwave and conventional heating, the two systems should undergo Maillard reaction to the same extent. In addition, due to variations in the time-scale between the two modes of heating, complex multistep reactions such as Maillard reaction, can remain incomplete in the microwave i f enough time is not allowed for the reaction to proceed However, during a chemical reaction, the yield of a particular product can be increased by increasing the reactivity and /or the concentrations of the reactants, specially if time is a limiting factor which is the case in the microwave. If one of the reasons for the failure of microwave to develop flavors is the absence of reactive Maillard precursors in sufficient concentrations to effect browning and flavor formation in the limited time-scale of microwave, than adding reactive precursors in sufficient concentrations to the food products, the Maillard reaction can be initiated at microwave conditions. Fresh baked, roasted, and chicken flavor formulations have been already developed in our laboratories using this strategy that can produce the desired effect in the microwave oven (8). However to facilitate the choice of amino acids, the effect of different amino acids was evaluated in terms of their potential to produce specific aroma notes in the microwave. The Effect of Amino Acid Side Chain on the Type of Aroma Produced in the Microwave. In order to find trends within specific groups of amino acids, and their effect on the type of aromas produced in the microwave, the amino acids were divided into five categories based on the chemical nature of their side chains (Table I). The aroma produced by the model Maillard systems were categorized into eight aroma types listed in Table I which shows a partial list of different mixtures of amino acids examined. Inspection of the results reveals that meaty, nutty and baked aroma notes are the most common in the Maillard reaction mixtures subjected to microwave heating, this is the same trend observed in conventional heating. A n interesting type of aroma that can be produced in the microwave is the fragrance reminiscent of different fruits. Generally it was found that amino acids with alkyl side chains (Aa: glycine, alanine, leucine, isoluecine and valine) were essential for the production of caramel notes. This is mainly due to the fact that Amadori products formed from such amino acids produce relatively large amounts of maltol by dehydration reactions. Maltol is known to produce a caramel type of aroma. Sulfur containing amino acids seem to be essential components in model systems producing meaty type aromas. Under conventional heating, sulfur containing compounds such as mcrcaptans, thiazoles and thiophenes are predominant in meaty aromas. The characteristic feature of the amino acid requirement for the nutty notes was the presence of either basic amino acids (Be category: glutamine and asparagine) or the presence of aliphatic amino acids (Ab category: serine and threonine). One of the main components of roasted nutty aromas are the derivatives of pyrazines, and this fact might explain the requirement of basic amino acids for the generation of these aromas.


452



Table I. Different Aromas Produced by the Model Amino Acid-Sugar Mixtures Caramel

Meaty

Nutty

Meaty+Veg.

Aa Aa +Ab Aa + Ba

S S+Aa S+Bb S+Ar+Ab S+Ar+Bb

Bc+Ba Bc+Ba+Bb Be Ab+Bb Ab+Ba

S+Ar S+Ar+Ac S+Ar+Ba+Bb+Bc

Fragrant

Roasted vegetable

Baked potato

Baked

Ar AH-Aa ArfAb ArfBa

Ar+S+Ac+Aa+Bb Ar+S+Ac+Aa+Bc

S+Ab S+Ac+Aa+Ab S+Ac+Ar+B

Ba Ba+Bb Ba+Bb+Bc+Ac Ba+Ac

Aa Ab Ar Ba

(glycine, alanine, leucine, isoluecine, valine) Bb (lysine, arginine, histidine) (serine and threonine) Be (glutamine and asparagine) (tryptophan, phenylalanine and tyrosine) Ac (glutamic acid and aspartic acid) (proline, hydroxyproline) S (cysteine, cystine and methionine)

A l l the model systems producing mixed meaty vegetable aroma notes contained both sulfur and aromatic amino acids. Such aroma notes are usually characterized by the presence o f sulfur containing and aromatic compounds. Aromatic amino acids are essential for the production o f fruity aromas i n combination with aliphatic or basic amino acids. The importance o f aromatic amino acids lies in the fact that they can be converted into aromatic aldehydes and ketones essential for fruity-type flavors, such as benzaldehyde. O n the other hand, the two model systems that produced roasted vegetable notes contained aromatic, sulfur, acidic and aliphatic (Aa) amino acids in addition to either B b category amino acids (lysine+arginine+histidine) or Be (glutamine+asparagine). A c i d i c , aliphatic and sulfur-containing amino acids seem to promote baked potato notes i n Maillard model systems. The essential feature o f the Maillard model systems producing baked aroma notes under microwave conditions is the presence o f basic amino acids especially B a category amino acids (proline + hydroxy proline). This is i n accordance w i t h observations under conventional heating conditions. Analysis o f baked flavors have shown that many important aroma compounds contain a proline derived moiety, such as 2-acetyl-pyrroline.

Comparison of the Microwave and Thermally Induced Flavors. Based on the information gathered on the effect o f amino acid type on the generation o f M a i l l a r d aromas i n the microwave, six formulations were optimized (samples 1-6 i n Table II) for microwave generation o f aromas that require surface drying for their formation (baked and roasted). The same formulation was subjected both to microwave irradiation and to conventional heating in an open system as described under "Materials and Methods". The compounds identified in all the samples by G C / M S are shown i n Table II. A s expected, only a small number o f components (between 9 and 17) were identified i n each sample, for a total o f 31 compounds i n a l l samples. Under the experimental conditions at w h i c h the flavors were produced, there was no significant difference i n the type o f compounds generated between microwave


38. YAYLAYAN ET AL.


Table II. Compounds Identified in Microwave (M) and Conventionally ( Q Heated Flavor Mixtures Name

M

C

Sample


Furan Derivatives Furan 2-Furan carboxaldehyde 5-Methyl-2-furan carboxaldehyde 2-Furan methanol HMF 2,3-Dimethyl-4-hydroxy-3(2H)-fliranone 2-(5H)-Furanone 2-Methylfuran Furanylmethylpyrrole 2,3-Dihydro-4-hydroxy-2,5-dimethyl-3-furanone l-(2-Furanyl)ethanone 4-Hydroxy-but-2-enoic acid lactone 2,5-Furandione

+ + + + +

1,2, 3,4, 5, 2, 4 , 5 1,2, 3 , 4 , 5 1,2, 3 , 4 , 5 1 3.4 6 3.5 5 5 5 5

+ + + + + +

Pyran Derivatives 2,3-Dihydro-3,5-dihydroxy-6-methyl 4(H)pyran-4-one + 3,5-Dihydroxy-2-methyl-4(H)pyran-4-one +

+ +

1,2, 3,4

3,4,5

N-Heterocycles 2- (lH)-Pyridinone 3- Methylpyrrole Pyrazine

+

S-Containing Compounds 3-Methylthio propanal Dimethyl disulfide Methanthiol Dimethyl trisulfide

+ + + +

Aliphatic Acetic acid Formic acid Acetaldehyde Butanal 2- Methylpropanal 3- Methylbutanal 2-Methylbutanal 1 -Hy droxy-2-propanone

+ +

+ + + + +

1,2, 3 , 4 , 5 2 6 3 2,6 2,6 2, 3,6 2,4

Aromatic Benzene acetaldehyde

Sample 1 Meat aroma, Sample 2 Fragrant, Sample 3 Potato aroma, Sample 4 Baked aroma, Sample 5 Bread aroma, Sample 6 Baked potato aroma



454


irradiated and of conventionally heated samples. l-(2-Furanyl)ethanone, 4-hydroxybut-2-enoic acid lactone, 2,5-furandione and dimethyl trisulfide were present only in the microwave treated samples, whereas acetaldehyde, 2-(lH)-pyridinone and pyrazine were only present in the conventionally heated samples. However dimethyl trisulfide was also detected in a different sample that was heated conventionally in our laboratories. MacLeod and Coppock (9), studied volatile extracts of boiled beef cooked conventionally and by microwave irradiation and found that six of the eight pyrazines identified were formed in higher amounts in the microwave. Most likely these differences are due to time-temperature exposure variations rather than differences in mechanistic pathways as suggested by others. Half of the identified compounds were either furan or pyran derivatives, there were only four N-containing heterocyclic compounds and four S-containing compounds. A l l of the aldehydes identified could arise by Strecker degradation from corresponding amino acids (Table ΙΠ), it seems Strecker aldehydes could form in the microwave as easily as under conventional heating.

Table m . Strecker Aldehydes Identified in Maillard Flavor Formulations Strecker Aldehyde

Amino acid source

Acetaldehyde Benzene acetaldehyde 3-Methylthiopropanal 2- Methylpropanal 3- Methylbutanal 2-Methylbutanal

Alanine Phenylalanine Methionine Valine Leucine Isoleucine

Figure 1 shows compounds that were identified in all of the microwave and conventionally treated samples. 2,3-Dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4one, has been shown (10) to form directly from Amadori products during mass spectrometric fragmentation in the electron impact mode by a process termed "orthoelimination". It has also been shown that it is the direct precursor of maltol, by mass spectrometric linked-scan experiments. Figure 2 shows the proposed mechanism of formation of maltol and the simultaneous release of an amino acid moiety based on mass spectrometric studies. In the initial step, the Amadori product undergoes dehydration to form a double bond between C-2 and C-3 of the sugar residue, this is followed by a cleavage of the C - N bond by thermally induced ortho-clïrmnation reaction to release the amino acid and a sugar residue. The latter isomerizes to form the pyranone, a more stable product and can than undergo further dehydration to form maltol.

Conclusion Compensation for the time-temperature variations between microwave and conventionally heated samples can be achieved by increasing the concentrations of specific reactive Maillard precursors in model systems or in formulations that can be applied to microwavable foods. In additon, surface drying of foods appears to be necessary to achieve browning.



38. YAYLAYAN ET A L


2-Furancarboxaldehyde (R=H, CHO) 5-Methyl-2-furan carboxaldehyde (R = C H , CHO) 2-Furan methanol (R=H, R^= CH OH) HMF (R = CHO, Rj = CH OH)

2,3-Dihydro-3,5-dihydroxy 6-methyl-4(H )-pyran-4-one

3

2

2

Figure 1. C o m m o n O-heterocyclic compounds identified i n all o f the samples studies

Ο Maltol Figure 2. Ortho-cYimmaûon

o f Amadori products


456



Literature Cited 1. Whorton, C.; Reineccius, G. In Thermal Generation of Aromas; Parliment, T.; McGorrin, R.; Ho, C. T., Eds.; ACS Symposium Series No 409; American Chemical Society: Washington, DC, 1989;p526. 2. Reineccius, G.; Whorton, C. In The Maillard Reaction in Food Processing, Human Nutrition and Phydsiology; Finot, P., Aeschbacher, H., Hurrell, R. F. and Liardon, R. (eds.), Birkhäuser, Basel, 1990,p197. 3. Shaath, N. A. and Azzo, N. R. In Thermal Generation of Aromas; Parliment, T.; McGorrin,R.;Ho, C. T., Eds.; ACS Symposium Series No 409; American Chemical Society: Washington, DC, 1989;p512. 4. Parliment, T. H. This Book. 5. Yeo, H.; Shibamoto, T. Trends Food Sci. Technol. 1991, 2, 329. 6. Giguere, R. J., Bray, T. L.; Duncan, S. M. Tetrahedron Lett. 1986, 27, 4945. 7. Gedye,R.N., Smith, F. E.; Westway, K. C. Can. J. Chem. 1988, 66, 17. 8. Yaylayan, V. Pending Patent Application, McGill University, 1993. 9. Macleod, G.; Coppock, B. M. J. Agric. Food Chem. 1976, 24, 835. 10. Yaylayan, V., Mandeville, S.; Pare, J. Spectroscopy, 1991, 9, 73. RECEIVED

June 30, 1993


Thermally Generated Flavors - American Chemical Society

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