Thermally Generated Flavors - American Chemical Society

Chapter 28

Formation and Degradation of Tryptophan Amadori Products during Extrusion Processing

Downloaded by UNIV LAVAL on April 28, 2016 | http://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch028

V. A. Yaylayan, J. Fichtali, and F. R. van de Voort Department of Food Science and Agricultural Chemistry, McGill University, 21111 Lakeshore Road, Ste. Anne de Bellevue, Quebec H9X 3V9, Canada In situ generation of Maillard reaction intermediates by extrusion processing was investigated using a model system consisting of 15% (w/w) glucose, 5% (w/w) tryptophan and 80% (w/w) inert microcrystalline cellulose under pre-selected conditions of temperature, moisture content, screw speed and feed rate. Samples were analyzed by HPLC for the presence of Amadori rearrangement products (ARP), hydroxymethyl furfural (HMF) and maltol. Two temperature ranges were investigated, one 70-90°C and the other 80100°C. When components were optimized individually using the Generalized Distance Approach (GDA) algorithm, in both temperature ranges ARP formation was highest at lower temperatures, HMF production was favored at about 16°C above the ARP optimum in both temperature ranges, while maltol production was favored at a temperature about 3-4°C lower than the HMF optimum temperature. Slightly higher moisture contents favour the production of ARP, while lower values favour HMF and maltol. Maltol shows the largest increase in yield in the second temperature range, indicating that ARPs are decomposed preferentially to produce maltol. Similar trends were observed when the formation of all components were optimized simultaneously: the yield of ARP dropped somewhat in the higher temperature range whereas maltol doubled, and HMF increased slightly. The results indicate that a controlled continuous production of Maillard reaction flavor precursors is possible by extrusion.

Extrusion combines various unit operations such as mixing, cooking and texturizing into a single continuous process (I), which makes it an attractive option for the food processor and as a result, it has been exploited extensively by the food industry. In terms o f flavor, extrusion tends to be limiting because o f chemical degradation due to oxidation, hydrolysis and other reactions occurring under high temperature short time ( H T S T ) extruder operating conditions (2) and also due to the volatiles being flashed off at the die. O n the other hand, systems containing reducing sugars and proteins or amino acids can rapidly undergo the Maillard reaction (3), one o f the key flavorproducing reactions occurring in food systems during extrusion. A s such, the extruder could serve as a continuous reactor for the in situ production o f flavor

0097-6156/94/0543-0348$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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precursors from selected sugar/amino acid mixtures. There is no information in the literature regarding the application of extrusion processing to produce Maillard reaction flavors per se, most studies deal with the stability of added aroma compounds (4, 5). Some work has been done by Ho et al. (6) on the formation of volatile compounds in corn-based model systems and determining Maillard reaction products by gas chromatography/mass spectrometry (GC/MS). They noted that formation of flavour related compounds, including pyrazines were favored at higher temperatures and lower moisture conditions. Cheftel (7) indicated that the Maillard reaction is favored at temperatures greater than 180°C and high shear conditions (>100 rpm) in combination with moisture contents of 15% or less. A recent review by Maga (2) summarizes flavor formation and retention during extrusion. Sugar/amino acid model systems have served as a common means of studying the development of Maillard flavor and color precursors, specifically the Amadori rearrangement products (ARP). The objectives of this study were: i) To assess production and yield of tryptophan A R P and its key degradation products in a model system using the extruder as a continuous reactor; ii) To assess the effect of key process variables (i.e., temperature, moisture content, screw speed) on extrusion dependent variables (product temperature, pressure at the die, specific energy consumption) and Maillard reaction product yield; iii) To predict the optimum reaction conditions for product formation using the Generalized Distance Approach algorithm (8) and iv) To compare the results with the aqueous model decomposition system. Materials and Methods Tryptophan, D-glucose, hydroxymethyl furfural (HMF), maltol, and microcrystalline cellulose were obtained from Aldrich Chemical Company (Milwaukee, WI) and used without further purification. A l l the solvents were of HPLC grade (BDH, U.S.A), water was obtained from a Milli-Q reagent grade water system (Millipore Corp., Bedford, M A ) and all HPLC mobile phases were degassed under vacuum. Tryptophan Amadori product was synthesized as reported by Yaylayan and Forage (9) and the extrudates analyzed by HPLC. Extrusion Processing and Optimization. Tryptophan (5 kg), glucose (15 kg) and cellulose (80 kg) were mixed in a vacuum dispersion mixer (Day Mixing, Cincinnati, Ohio) to obtain a homogeneous mass. The moisture content of the mixture was determined to be 4.2% by oven drying (2 hr at 60°C). The extruder used was a Baker Perkins MPF-50D (APV Baker, Inc., Grand Rapids, MI) co-rotating intermeshing twin screw extruder with the barrel configured in a 20:1 L/D ratio (barrel length to screw diameter), and screw profile was designed to obtain good mixing and conveying performance. The screw configuration in sequence from the feeder to the die consisted of: 300 mm, feed screw; 50 mm, 30° forwarding paddles; 50 mm, short pitch screw; 50 mm, single lead screw; 37.5 mm, 60° forwarding paddles; 37.5 mm, 30° reversing paddles; 100 mm, single lead; 50 mm, 60° forwarding paddles; 37.5 mm, 30° reversing paddles; 100 mm, single lead; 50 mm, 90° paddles; 75 mm, single lead; 37.5 mm, 60° forwarding paddles and 75 mm, single lead. A die having two 9 mm circular orifices was used, and the temperature was controlled over nine zones along the barrel. Pressure, product temperature at the die and torque were measured, and the specific energy consumption (SEC) was calculated (10). Preliminary experiments were carried out to determine viable operating ranges (i.e., screw speed, temperature, moisture) from which the basic experimental design was developed, which lays out the operating conditions for the extrusion experiments. A simple coded factorial design varying only moisture and temperature

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

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THERMALLY GENERATED FLAVORS


was used for two separate extrusion runs having distinct temperature ranges (70-90°C and 80-100°C) but common moisture variation (55-63%), with the feed rate and screw speed held constant. A separate experiment was carried out to study the effect of screw speed. The extruder was run at each set of design conditions until the operating conditions stabilized. Samples were then taken, placed in plastic bags after cooling and frozen at -25°C until analyzed. The response data (i.e., the concentration of ARP, maltol and H M F produced) was then used to determine the optimal conditions for their formation by the Generalized Distance Approach using a design radius of 1.0 (8, 11). Compromise optima in turn were obtained from the G D A data using the Multiple Response (MR) program developed by Conlon and Khuri (12). H P L C Analysis. Extrudate samples were taken under selected conditions of temperature, moisture content, screw speed and feed rate for analysis of HMF, maltol and ARP. Two grams of each sample was diluted to 100 mL with distilled water and filtered through a 0.45 mm, type H A Millipore filter (Waters Scientific) before injection onto a HPLC column equipped with a Rheodyne injector having a 20loop. A Beckman System Gold modular HPLC was used for analyses, consisting of a Model 166 variable wavelength U V detector set at 280 nm and a model HOB solvent delivery system controlled by a NEC lap-top computer and connected to a Shimadzu CR-18 integrator (10-mV full scale). Analyte H P L C separation was carried out using either a 5μ, 2.0x150 mm Beckman C-18 Ultrasphere column or a Merck 5μ, 2.0x150mm C-18 Lichrosphere, 100-RP-18 column, equipped with a Licro CART 4-4 guard column. Both columns were operated at ambient temperature, 20 \ih was injected for analysis and the values obtained represent the average of three injections. The analyte peaks were identified and confirmed by comparison of their retention times with commercial or synthesized standards using two mobile phases, wavelength ratioing, and spiking (9). Quantitation was performed by the use of appropriate calibration curves. Results and Discussion Processing Conditions. The model system chosen was tryptophan/glucose, which has been studied previously by the authors (9, 13) in an aqueous matrix. Because of the extensive raw material requirements and inherent expense imposed by full scale extrusion experimentation, the sugar/amino acid mixture was diluted by microcrystalline cellulose which does not interfere with the Maillard reaction (14) and is insoluble in water, making it simple to extract the reaction products from the extrudate. Tryptophan Amadori compound (a reactive intermediate) and its breakdown products, H M F and maltol were used as indicator compounds of the extent of Maillard reaction. Initial experimental designs were predicated on using relatively high temperatures and low moisture conditions as suggested by Cheftel (7); however, preliminary trials indicated that viable extrusion conditions could only be obtained using >55% moisture and temperatures lower than 110°C. These limiting conditions were likely due to the effect of the microcrystalline cellulose matrix (-80%) rather than the amino acid/sugar mixture. Based on the experimental design conditions, the barrel temperature did not affect torque, pressure at the die or specific energy consumption (SEC); however, moisture was a limiting variable. Temperature, which normally affects viscosity, has no effect in this system as cellulose is insoluble and does not melt. Moisture acts as a lubricant, reducing torque, pressure and energy consumption. The effect of screw speed on torque, SEC, and pressure was studied independently. Although the torque decreases exponentially as screw speed increases, SEC reaches a minimum around 200 rpm while the pressure tends to maximize at the screw speed. Screw speed was also studied and has a complex effect on torque, specific energy and pressure, which were related to Maillard product formation (10).



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Maillard Reaction. The Maillard reaction, known to be an important flavor producing reaction in food systems, is usually associated with baked and roasted products, and is used commercially by the flavor industry to produce reaction flavors (15). The flavor charactersistics produced depends primarily on the precise nature and ratio of the amino acids and sugars, reaction conditions (primarily time, temperature and moisture) and the nature of any thermal degradation reactions which may contribute to the overall flavor profile. Some of the steps of the reaction mechanism are well known based on model system studies; however, the accurate prediction of flavors produced is still largely an art. The degradation of tryptophan A R P produces a complex mixture of products. Maltol and H M F can be used as indicator compounds, although they may also be formed at much slower rates from the decomposition of glucose alone (9). A R P decomposition basically takes place via two well established pathways: a) 1,2enolization producing mainly H M F leading to browning, and b) 2,3-enolization producing maltol and leading mainly to flavor formation (15). However, other mechanisms have also been proposed (9). The information available on this reaction in the model aqueous matrix is presented below. Considering the reaction of tryptophan with glucose, it is important to point out that the measured values of the Amadori products represent the accumulated amounts (the difference between the amount of A R P formed and the amount decomposed). Hence the concentrations of ARP, H M F and maltol produced are interrelated, A R P accumulation and decomposition being a dynamic process and the optimization of any one compound automatically implies reduced levels of the others. Decomposition of Tryptophan ARP in a Model Aqueous System. The products formed from the thermal decomposition of tryptophan Amadori product alone have been studied previously in an aqueous matrix, at two temperatures (110°C, 140°C) and concentrations (1.53 mg/mL, 1.33 mg/mL) (9, 13). Table I. Relative Rates of Decomposition of Tryptophan ARP Relative Rates

Temperature

Concent, of ARP

(°Q

(mg/mL)

110->140

1.53

3.5 χ

110->140

1.33

4.7 χ

110

1.33->1.53

1.4 χ

140

1.33-^1.53

1.08 χ

—> change to

The results from these studies have shown that the first order rate constant for the disappearance of A R P in the absence of sugar was dependent on the temperature and the water content of the reaction mixture. The rate of decomposition of the more concentrated solution was 3.5 times faster at 140°C than at 110°C and 4.7 times faster in the less concentrated solution at 140°C than at 110°C. Comparing the rates at the same temperature but at different concentrations indicated that at 110°C, the rate is 1.4 times faster in the more concentrated solution and at 140°C the rate is faster by only 1.08 times (see Table I). These results indicate that in dilute solutions,


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decomposition of A R P proceeds by hydrolytic rate-determining reactions, while thermally induced non-hydrolytic reactions (such as C-C and C-N bond cleavages) become important as the temperature is increased. The value of the activation energy (Ea) for the decomposition of ARPs was found to depend on the water content of the reaction mixture; higher the water content, the higher is the value of Ea. Consequently, dilute solutions of Amadori products are more sensitive to changes in temperature than concentrated solutions. Formation of HMF and Maltol from Tryptophan ARP in a Model Aqueous System. The formation of H M F and maltol from A R P was also studied (9). The generation of hydroxymethyl furfural (HMF) in food or in model systems is a useful indicator of the extent of Maillard reaction. Maltol on the other hand, is associated with caramel flavor and is used as a flavor potentiator in non-alcoholic beverages. According to Table II, increasing the temperature by 30°C increases the rate of formation of H M F by 1.5 times at both A R P concentrations. However, increasing the water content increases the rate at both temperatures by 2.5 times, indicating that a hydrolytic reaction is involved in the rate determining step. However, activation energies at both water concentrations are the same; these observations indicate that the reaction is more sensitive to the water content than to variations in temperature. On the other hand, increasing the temperature in the more concentrated solution of the A R P by 30°C increases the rate of formation of maltol by 1.5 times, and by 2.8 times in the less concentrated solution, indicating that thermally induced nonhydrolytic reactions (such as C-C and C-N bond cleavages) might be involved in the rate-determining step. However, increasing the water content, decreases the rate at 110°C by 2.5 times and by 1.4 times at 140°C, indicating that dehydration steps are also involved in the formation of maltol from the ARP. The same conclusion can be reached by comparing the activation energies at two concentrations. Both H M F and maltol can also be formed directly from the degradation of glucose alone, however at much slower rates.

Table Π. Relative Rates of Formation of HMF and Maltol from TRP-ARP Temperature

(PQ

Concentration of ARP (mg/mL)

Rate of formation of HMF

Rate of formation of maltol

110->140

1.53

1.5 χ Î

1.6 χ Τ

110->140

1.33

1.5 χ Î

2.8 χ î

110

1.53-^1.33

2.5 χ î

2.5 x i

140

1.53->1.33

2.5 χ î

1.4 x i

-> change to , Τ increase, i decrease

Separate Optimization of Maltol, HMF and ARP Formation in the Extruder. Using the Generalized Distance Approach (GDA), one can predict the optimum concentrations and processing conditions for each component from the concentrations of the selected Maillard intermediates and the experimental design process conditions. The mathematical basis for this procedure was developed well over a


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decade ago (8), however it has only recently been applied to assist in the development and optimization of multivariate extrusion processes (16). G D A permits one to find compromise conditions for the input variables that are somewhat favorable to all responses. This implies that the multiresponse function deviates as little as possible from the individual optima, and the deviation is formulated as a distance function which is minimized over the experimental region. By evaluating the Maillard reaction system by this means, one can determine whether the selected processing conditions favor the 1,2 or 2,3 enolization degradation pathway, based on whether H M F or maltol is dominant. The method also permits the prediction of compromise conditions where all components are optimized simultaneously, by formulating responses as a distance function minimized over the experimental region.

Table ΙΠ. Optimum Concentrations Calculated by Generalized Distance Approach for Each Maillard Reaction Component and the Respective Processing Conditions Required to Produce the Optima Listed

Response

Optimum (g/L)

Yield (%)

Moisture (%)

Temperature (°Q

(Temperature Range 70-90°C) 0.04760

2.70

58.6

70.0

HMF

b

0.00116

0.16

55.9

86.3

MAL

C

0.03880

5.50

55.3

83.4

ARP

a

(Temperature Range 80-100°C) ARP

a

0.03450

1.95

58.6

80.0

HMF

b

0.00138

0.19

56.1

97.2

MAL

C

0.06970

9.87

55.2

93.3

a

C

ARP = Amadori rearrangement product, °HMF = Hydroxymethylfurfural, MAL = Maltol

Reproduced with permission from ref. 10. Copyright 1992.

Table III provides a summary of the optimum concentrations and yield for each Maillard component and the respective processing conditions for each of the two temperature sets. It can be seen that in both temperature ranges A R P development is optimized at lower temperatures, although the optimum shifts from 70 to 80°C in the higher temperature range and the yield drops slightly. H M F production is favored at about 16°C above the A R P optimum in both temperature


354


ranges, while maltol production is favored at a temperature about 3-4°C lower than H M F production. Slightly higher moisture contents favour the production of ARP, while lower values favour H M F and maltol. Maltol shows the largest increase in the second temperature set, with the yield almost doubling indicating that A R P is decomposing preferably to maltol, thereby reducing the total A R P present. This observation is consistant with the results obtained in the aqueous model studies where a similar trend was observed (see Table II). Table Iv. Compromise Optima Calculated for the Two Temperature Sets and the Associated Extrusion Conditions Required


Response

Compromise Optimum (g/L) 70-90°C

%Yield

80-100°C

VoYxeld

Temperature (°C)

72.9

97.2

Moisture (%)

56.2

56.2

ARP

(g/L)

0.04200

2.40

0.03100

1.75

H M F (g/L)

0.00098

0.14

0.00138

0.19

M A L (g/L)

0.03170

4.49

0.0631

8.94

a

C

ARP = Amadori rearrangement product, °HMF = Hydroxymethylfurfural, MAL = Maltol Reproduced with permission from ref. 10. Copyright 1992.

Simultaneous Optimization of Maltol, HMF and ARP Formation in the Extruder. Table IV presents the calculated compromise optimum data, where all components are optimized simultaneously. In this situation, one is balancing the production of the three components against each other; however, similar trends are observed as before: A R P dropping somewhat in the higher temperature range, maltol doubling, and H M F increasing, but being produced only in relatively small amounts. These results are consistant with those from the aqueous decomposition studies. Moisture and temperature are but two of many variables which can be controlled during the extrusion process (1) and the G D A procedure has been successfully applied to develop and optimize the production of sodium caseinate for three product characteristics dictated by six simultaneous operating variables (17). As such, the G D A has been proven to be a very useful optimization technique; however, the derived data are only valid within the original experimental design parameters and cannot be extrapolated. Conclusion A twin screw extruder can be used as a continuous reactor for the Maillard reaction. The composition of reaction products can be based on the judicious adjustment of extrusion conditions to optimize the formation of A R P , H M F and maltol individually or simultaneously as a compromise optimum One limitation of this work was the need to use microcrystalline cellulose as a diluent, which strongly affected the extruder operating conditions (torque, specific energy and pressure). It is


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unlikely that the basic reaction pattern would change dramatically if run under the same conditions without the filler; however, the elimination of the filler would undoubtedly expand the range of operating conditions (i.e., temperature and moisture), which could increase yield and provide more direction to controlling the reaction. In order to elucidate whether there would be any commercial potential for the production of Maillard reaction intermediates for a particular sugar/amino acid mixture, additional work should be carried out without the diluent, including reaction kinetics study that requires the assessment of theresidencetime distribution in the extruder. Albeit somewhat limited, sufficient information has been obtained to confirm the basic premise that Maillard flavor intermediates may be produced by extrusion processing, and that some control can be exercised over thereactionby manipulating the extruder parameters.


Acknowledgments The authors gratefully acknowledge funding support from Conseil des recherches en pêche et en agro-alimentaire du Québec (CORPAC MCA # 2567) for this research. The authors also would like to thank the Food Research and Development Center of Agriculture Canada (Ste. Hyacinthe, Quebec) for access to the extruder. Literature Cited

1. Fichtali, J.; van de Voort, F.R..Cereal Foods World 1989, 34, 921. 2. Maga, J. Α., In Extrusion Cooking; Mercier, C.; Linko, P.; Harper, J., Eds.; American Association of Cereal Chemists, Inc., Minnesota, 1989;p387. 3. Maga, J. Α.; Kim, C. H. In Thermal Generation of Aromas; Parliment, T.; McGorrin, R.; Ho, C. T., Eds.; ACS Symposium Series No 409; American Chemical Society: Washington, DC, 1989;p494. 4. Maga, J.A.; Sizer, C. E. Lebensm. Wiss. Technol. 1979, 12, 15. 5. Crouzet, J.; Sadafian, Α.; Doko, B.; Chouvel, M. In Thermal Processing and Quality of Foods; Elsevier Applied Science Publishers, Ltd. England. 1984 p. 212 6. Ho, C. T.; Bruechert, L. J.; Kuo, M. C.; Izzo, M. In Thermal Generation of Aromas; Parliment, T.; McGorrin, R.; Ho, C. T., Eds.; ACS Symposium Series No 409; American Chemical Society: Washington, DC, 1989;p504. 7. Cheftel, J. C. Food Chem. 1986, 20, 263. 8. Khuri, A. I. and Conlon, M. Technometrics 1981, 23, 363. 9. Yaylayan, V.; Forage, N. J. Agric. Food Chem. 1991, 39, 364. 10. Yaylayan, V.; Fichtali, J.; van de Voort, F.R.Food Res. Int. 1992, 25, 175. 11. Khuri, A. I.; Cornell, J. A. Response Surfaces, Designs and Analysis; Marce Dekker, New York, 1987. 12. Conlon, M.; Khuri, A. I. MR Multiple response optimization; Technical Report No 322., Department of Statistics, University of Florida, 1988. 13. Yaylayan, V.; Forage, N. Food Chem. 1992, 44, 201. 14. Eichner, K.; Karel, M. J. Agric. Food Chem.1972, 20, 218. 15. Reynolds, T. M. Food Tech. Aust.1970, 22, 610. 16. Fichtali, J.; van de Voort, F. R; Khuri, A. I. Food Proc. Eng. 1990, 12, 247. 17. Fichtali, J.; van de Voort, F.R Milchwissenschaft 1990, 45, 560. RECEIVED

June 30, 1993


Thermally Generated Flavors - American Chemical Society

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