Effect of Cysteine Addition on the Volatile. Compounds Released at the Die during Twin-screw. Extrusion of Wheat Flour. Chin-Fa Hwang, William E. Riha III, ...
Lebensm.-Wiss. u.-Technol., 30, 411–416 (1997)
Effect of Cysteine Addition on the Volatile Compounds Released at the Die during Twin-screw Extrusion of Wheat Flour Chin-Fa Hwang, William E. Riha III, Baoping Jin, Mukund V. Karwe, Thomas G. Hartman, Henryk Daun and Chi-Tang Ho* C. F. Hwang, W. E. Riha III, B. Jin, M. V. Karwe, H. Daun, C.-T. Ho: Department of Food Science, Cook College, Rutgers, The State University of New Jersey, New Brunswick NJ 08903 (U.S.A.) M. V. Karwe, T. G. Hartman, C.-T. Ho: The Center for Advanced Food Technology, Cook College, New Jersey Agricultural Experiment Station, New Brunswick NJ 08903 (U.S.A.) (Received June 25, 1996; accepted September 20, 1996)
Numerous sulfur-containing volatile compounds were released from an extruder when cysteine was added to wheat flour before extrusion. The volatiles released were collected in a specially designed cryogenic apparatus at the die of the twin-screw extruder. Five concentrations of cysteine were used (0 to 10 g/kg). Among the identified sulfur-containing compounds, meat-like flavor compounds, such as 2,4-dimethyl-3-thiazoline, 2-methylthiazolidine, 3,5-dimethyl-1,2,4-trithiolane, and 2,4,6-trimethyl-perhydro1,3,5-dithiazine, were collected in the highest amounts. Several nonsulfur containing compounds such as methylpyrazine, 2,6-dimethylpyrazine, and 2-acetyl furan were also found in the condensate. The total amount of volatiles collected increased when more cysteine was added to the flour, but only a small fraction of added cysteine formed flavor volatiles in both the condensate and extrudate.
©1997 Academic Press Limited Keywords: volatile compounds; extrusion; cysteine; Maillard reaction; sulfur-containing compounds
Introduction Extrusion cooking has a great effect on the flavor profiles of food products manufactured by this method. Temperature, moisture content, shear, pressure, and residence time are main conditions influencing the generation of flavor volatiles. Composition of raw materials used for extrusion is also an important factor for developing characteristic flavor and obtaining high quality extrudates. In order to enhance the flavor and create a pleasing aroma, flavor compounds are usually added to the surface of the product after extrusion. There has been increasing interest in adding flavor precursors to the feed before extrusion to obtain the desired flavors. Izzo and Ho (1) used autolysed yeast extracts (AYE) as the precursors to study thermally generated flavors during extrusion. Tanaka et al. (2) evaluated phenol formation using a phenyl-α-glucoside as a precursor. Izzo et al. (3) examined the aroma profile produced in extruded wheat flour by adding ammonium bicarbonate and pyruvaldehyde as flavor precursors. They found that addition of these compounds may enhance the levels of heterocyclic pyrazines which contributed to the roasted or toasted *To whom correspondence should be addressed.
aroma character of the product. Bailey et al. (4) investigated the effect of whey protein concentration on the volatile compounds in an extruded corn meal product. Cysteine is known to produce meaty flavor by pyrolysis or through Strecker degradation with dicarbonyl compounds. Cysteine contains not only an α-amino group which can react in the Maillard reaction to produce pyrazines, but also contains a sulfur-containing thiol group. Application of amino acids, such as cysteine, as the precursor to extrusion cooking to study the functional properties and generated flavors of wheat flour extrudates has been reported recently (5, 6). Riha III et al. (6) studied the volatile compositions of wheat flour extrudates with different amounts of cysteine addition. A total of 18 sulfur-containing compounds, four pyrazines, and one furan were found from the extrudate. Although the concentration of most volatiles increased with cysteine addition, volatile production from the extrudate did not seem to increase significantly when addition of cysteine was over 0.5%. We assumed that a large number of compounds would be released from the die due to steam distillation as the extrudate emerges from the extruder die. In order to fully evaluate the volatiles produced from this extrusion, a collection system was used to trap volatiles
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which were released at the extruder die. Therefore, the objectives of this research focused on the investigation of released flavor compounds from the die due to the addition of cysteine to wheat flour prior to extrusion, as well as comparison of addition of this amino acid affecting the compounds recovered from the extrudate and released at the die.
Materials and Methods Extrusion Crystalline L-cysteine (Sigma, St. Louis, MO) was ground in a mill and passed through a 0.12 mm (i.d.) mesh seive (Type ZM1, Glenmills INC, Maywood, NJ). Cysteine was ground at medium speed for 5 s in order to prevent heat generation and cysteine oxidation. The cysteine powder was mixed thoroughly for 15 min with wheat flour in a mixer (Bounder, Bay State Milling Company, MA) at concentrations of 0, 2.5, 5.0, 7.5, and 10 g/kg. The high gluten wheat flour containing predetermined amounts of cysteine was used as the feed material for extrusion. Extrusion was carried out with a ZSK-30 co-rotating twin-screw extruder (Werner Pfleiderer Corp., Ramsey, NJ). Its detailed configuration and extrusion conditions were the same as reported earlier (6). A total of five condensates were collected representing the four concentrations of cysteine added and the control. The following conditions were used: feed moisture content 16%, total mass flow rate at the die 225 g/min, screw speed 500 rpm, die temperature 185 °C, specific mechanical energy 1709 kJ/kg, torque 63%, and average residence time 46 s. The extrusion conditions were equilibrated by running the extruder with plain wheat flour. Thereafter, the flour with cysteine was added to the feeder. Samples were run in order of increasing cysteine concentration. The moisture content of the raw flour (95 g/kg) was increased to 160 g/kg by pumping additional water to the wheat flour during extrusion. The extrusion was run in duplicate.
Collection and analysis of volatiles released at the die Volatiles released at the extruder die were collected via a special condensation apparatus (7). This apparatus consisted of a large drum which was tightly connected to the extruder below the cutter to collect the extruded product and a vapor port which was connected to the volatile collection unit. The collection unit consisted of three traps. The first trap was chilled to 0 °C, and the second and third traps were maintained at –100 °C. A relatively low vacuum of approximately 200–400 mmHg was used to collect the volatiles flashed off with the steam at the extruder die during the 15-min collection time. Condensates collected in the three traps were combined and spiked with tridecane as the internal standard. The volatiles were then extracted twice with 200 mL of methylene chloride and concentrated using a Kuderna-Danish concentrating apparatus. Finally, a
stream of nitrogen gas was used to concentrate samples to 1 mL.
Gas chromatography-mass spectrometry Identification of peaks from the condensates was achieved using a Finnigan MAT 8230 mass spectrometer in the electron ionization mode (70 ev) coupled to a Varian 3400 gas chromatograph. A DB-1 column, 60 m long and 0.32 mm i.d. (J & W Scientific, Folsom, CA) was used for the analysis. Conditions for analysis of volatiles were: temperature program from 40 to 280 °C at 5 °C/min without initial hold time and with 20 min final hold time at a split of 50:1. The injector temperature was held at 270 °C. Identifications were made by on-line comparison of the mass spectra of the unknowns with the National Bureau of Standards computerized data base. A Finnigan Matt SS300 data system was used for data acquisition.
Results and Discussion The volatile compounds identified and quantified in the condensates are presented in Table 1. The compounds generated during extrusion and collected at the extruder die included pyrazines, furans, thiophenes, thiazoles, thiazolines, dithiazines, cyclic sulfur-containing compounds, and others. The concentrations of most compounds increased when more cysteine was added. However, the apparent decrease in the concentration of pyrazine may be due to high amounts of hydrogen sulfide produced from cysteine during extrusion. Hydrogen sulfide favored the formation of sulfurcontaining compounds, and might inhibit pyrazine formation. The concentrations of furfural alcohol, 1H-pyrrole, and isopropyl 2-butenoate did not significantly change or decreased slightly. It is possible that these compounds had been saturated when lower amounts of cysteine were added. Alkylpyrazines are generally recognized as important contributors to the flavors of all roasted, toasted, or similarly processed foods (8). As shown in Table 1, methylpyrazine and 2,6-dimethylpyrazine are the two most abundant pyrazines found in the condensates. The amounts of pyrazines generated from the Strecker degradation of cysteine were much lower than those of sulfur-containing compounds. The reason for this might be the low concentration of dicarbonyl compounds degraded from sugar during extrusion. Only small amounts of furans were found, which could have been formed by pyrolysis of sugar present in the wheat flour or from the breakdown of starch. 2-Acetylfuran was the most abundant of furans in the condensates. Sulfurcontaining compounds identified in this study, especially thiazoles and thiophenes, are known to significantly contribute to the thermally produced meaty flavors (9). Thiazoles are similar to pyrazines in flavor character and produce roasted, toasted or nutty aromas. Thiophenes tend to have sulfury or onion-like characters (10). Hydrogen sulfide, which is formed
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Cysteine added (g/kg) Compounds
0
2.5
5.0
7.5
10.0
Pyrazines pyrazine methylpyrazine 2,6-dimethylpyrazine 2-ethyl-5-methylpyrazine 3-ethyl-2,5-dimethylpyrazine
– – – – –
38 176 179 51 –
83 465 398 43 60
55 426 349 44 67
31 428 409 64 86
Furans furfuryl alcohol 2-acetylfuran 2-furancarboxylic acid
– – –
40 – –
50 50 –
45 67 –
44 91 20
Sulfur-containing compounds thiophene 2-methyl-5-formyl-thiophene 2-(vinylthio) thiophene Thiazole 4-methyl-5-ethylthiazole 2-propyl-5-methylthiazole 2-methyl-4-isopropylthiazole 4-methyl-3-thiazoline 2,4-dimethyl-3-thiazoline 2,4,5-trimethyl-3-thiazoline 2,4,5-trimethyl-3thiazoline (isomer) 2-methyl-4-ethyl-3-thiazoline 2-ethyl-4,5-dimethyl-3-thiazoline 2,5-dimethyl-4-ethyl-3-thiazoline 2-methylthiazolidine 2-butylthiazolidine 4,5-dimethyl-3-oxazoline 2,4-dimethyl-5-ethyl-3-oxazoline 2,4,5-trimethyl-3-oxazoline 3,5-dimethyl-1,2,4-trithiolane 3,5-dimethyl-1,2,4trithiolane (isomer) 2,4,6-trimethyl-2H-1,3,5dithiazine 2,4,6-trimethyl-2H-1,3,5dithiazine (isomer) trithiepane 4,7-dimethyl-1,2,3,5,6pentathiepane 4,6-dimethyl-1,2,3,5-tetrathiane 3,6-dimethyl-1,2,4,5-tetrathiane Others 2-butanone 1H-pyrrole 4-methyl-1-pentanol isopropyl 2-butenoate
– – – – – – – – – –
27 97 124 142 – 61 76 40 – – 35 155 119 292 278 349 35 65 118 202 90 229 253 289 – 34 90 259 38 264 381 501 259 1246 1254 1473 39 190 227 274
– – – – – – – – – –
84 35 65 28 34 – 29 – 42 92
357 175 158 141 350 – 55 – 186 34
–
92
32
396 510 246 395 172 252 159 211 963 1600 – 60 59 72 – 20 210 245 110 579 141
Table 2 Amounts of sulfur-containing compounds and yield of sulfur from the condensate collected at the extruder die Cysteine Total volatiles Sulfur-containing Yield of sulfur added (g/kg) (µg/g)a compounds (µg/g)a (%)b 0 2.5 5.0 7.5 10.0
75 –
184 –
517 102
– – –
– 36 –
– 41 –
– 64 35
64 324 125
– – – –
– 46 – 42
– 55 – 41
55 54 – 34
254 50 20 24
15
10
5
0
0
2.5
5.0
7.5
10.0
Cysteine added (g/kg)
Fig. 1 Comparison of the total concentration of sulfurcontaining compounds in the extrudate (C) and condensate (j)
3000
– None or trace amount (less than 10 ng/g of reaction mixture).
2500 Concentration (ng/g)
during the degradation of cysteine with a diketone (11), has been considered a precursor of various sulfurcontaining compounds associated with meat aroma. Another way to generate thiazoles and thiophenes is to exchange the oxygen atom of the furan ring or oxazoles with a sulfur atom of hydrogen sulfide. This formation pathway has been reported by Shibamoto (12). Table 2 illustrates how added cysteine affects both the amounts of volatiles and sulfur compounds recovered from the condensates. The amount of cysteine added had a significant effect on the production of sulfur-
– 0.19 0.16 0.20 0.18
20
– 1543 1510 4064 10920 63 –
0 2.87 5.83 9.81 18.18
containing compounds. The greater the amount of cysteine added, the greater the amount of sulfurcontaining compounds produced. However, the yield of these compounds was very low when compared with total amount of cysteine added. Comparing amounts of sulfur-containing compounds in the condensates with those recovered from the extrudate (6) (Fig. 1), it is apparent that the total amount of sulfur-containing compounds in the condensates increases as cysteine addition increased. The increase was not significant
829
– –
0.05 3.50 7.79 11.86 23.97
aBased on the total reaction mixture. bEstimate based on total sulfur in sulfur-containing compounds.
Concentration (µg/g)
Table 1 Concentration of volatile compounds identified from condensates collected at the extruder die (ng/g reach or mixture)
2000 1500 1000 500 0
Pyrazine
Thiophene
Thiazole
Thiazoline Dithiazine
Types of volatiles
Fig. 2 Comparison of concentration of several types of volatiles from extrudate (C) and condensate (j) of 5.0 g/kg cysteine sample
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when cysteine concentration reached 5 g/kg. These data show that more sulfur-containing volatiles were released at the extruder die than were recovered from the extruded product when cysteine concentration was high. This indicates that large amounts of sulfur are lost at the die, especially at the higher concentration (0.75% and 1.0%) of cysteine. This sulfur may be lost in the form of hydrogen sulfide or retained in covalent linkages formed between cysteine and proteins or lipids. Hydrogen sulfide was easily released and lost from the heating of cysteine (11). Li and Lee (5) reported that two routes are possible for added cysteine to form cross-linkages with wheat protein. The yield of cysteine recovered in the extrudate was about 40% (5). These linkages may affect the texture of the final product. As evaluated by an expert flavorist, the aroma of the extruded product was unacceptably strong when high cysteine amounts were added. Cysteine addition at 0.5% was considered optimum for sufficient flavor contribution (6). Several volatiles were compared between the condensate and extrudate, as shown in Fig. 2. It was interesting to find that thiazoles and thiophenes dominated in the extrudate, whereas, thiazolines and dithiazines were the major sulfur-containing volatiles in the condensate. Only trace amount of thiazolines were found in the solid extrudate. There are probably two reasons for this. The first is that thiazolines, the reduced form of thiazoles, are more polar and are therefore more volatile due to the steam at the die. The second reason is that cyclic polysulfides are not easily retained in the extrudate. In fact, these compounds are generally formed from the reaction of hydrogen sulfide with ammonia or aldehydes. It is possible that the reaction most likely takes place in the hot steam released at the die. As shown in Table 1, large amounts of sulfur-containing compounds were identified in the condensate when cysteine was added. However, no significant amounts of these compounds were found without the addition of cysteine. This suggests that the natural cysteine present in wheat flour did not become involved significantly in the formation of volatile compounds released from the die. The role of naturally present sulfur in wheat flour may be related to the cross-link structure of wheat gluten or extrudated products (5). The sulfur-containing compounds consisted of: thiophenes, thiazoles, thiazolines, thiazolidines, oxazolines, and cyclic sulfides including dithiazines. Among these sulfur-containing compounds, 2,4-dimethyl-3-thiazoline, 2-methylthiazolidine, 2,5-dimethyl-1,2,4-trithiolane, and 2,4,6-trimethyl-perhydro-1,3,5-dithiazine were the most abundant in the condensate. Although relatively small amounts of thiophenes were found, thiophenes can significantly contribute to the sensory properties of foods because of their low threshold values. Thiazoles possess extraordinarily potent sensory properties which can be described as green, roasted or nutty (13). As shown in Table 1, 2-4-dimethyl-3-thiazoline and 2,4,5-trimethyl-3-thiazo-
line (and its isomer) are the two major compounds found in this group. 2,4-Dimethyl-3-thiazoline, found in cooked beef aroma, has a nutty, roasted and vegetable aroma and is formed by the thermal generation of cysteine Strecker aldehyde reacted with ammonia and hydrogen sulfide (14). Mussinan et al. (15) reported that 2,4,5-trimethyl-3-thiazoline can be formed from 2-keto3-butanethiol, ammonia and acetaldehyde. As shown in Fig. 2, most of the thiazolines were detected in the condensate because hydrogen sulfide was easily released from cysteine and reacted with and replaced a dicarbonyl function during extrusion cooking. In addition to the two major compounds mentioned above, high concentration of 2-methylthiazolidine was also found in the condensate. The data show that the reduced types of thiazolines or thiazolidines are more stable in the condensate. Oxazoles and oxazolines are oxygenated forms of thiazoles and thiazolines. They were formed at much lower concentrations than their sulfur-containing counterparts. This may be due to the high reactivity of hydrogen sulfide. 2,4,5-Trimethyl-3-oxazoline was the major oxazoline in the condensate. This compound was first isolated from boiled beef (16) and described as having a ‘characteristic boiled beef aroma’. Cyclic polysulfides were the most dominant of the sulfur-containing volatiles, especially the dithiazines and trithiolanes. In the sample with 10 g/kg cysteine, 2,4,6-trimethyl-perhydro-1,3,5-dithiazine reached 10.9 µg/g, while 3,5-dimethyl-1,2,4-trithiolane and its isomer was found at a concentration of 1.4 µg/g. Of these sulfur-containing volatile compounds, some are known as having meat-like flavors, such as 1,3,5-trithiane and 3,5-dimethyl-1,2,4-trithiolane. 2,4,6-Trimethyl-perhydro-1,3,5-dithiazine is the most abundant among the sulfur-containing compounds released at the extruder die. It has been described as having strong roast cereal and popcornlike notes (17) and has been found in beef broth (18) and in boiled meat (19). It was one of the major products obtained from thermal degradation of cysteine and glutathione in a water model system (20). However, 2,4,6-trimethylperhydro-1,3,5-thiadiazine, which was another major product reported by these authors was not found in the condensate. This maybe be explained by the fact that 2,4,6-trimethyl-perhydro-1,3,5-dithiazine was more stabile during storage (17). Shankaranarayana et al. (21) reported that this compound was present in the broth or the headspace of canned meat products and it is easily formed from hydrogen sulfide, ammonia, and acetaldehyde. Schutte (22) postulated a mechanism for the formation of this compound in heat processed food. 3,5-Dimethyl-1,2,4-trithiolane has been found in the aroma of boiled meat (16, 19), in beef broth (18), and in potatoes (23). This compound was found at high levels in our study. However, Zhang et al. (20) reported that high amounts of this compound were formed from glutathione degradation rather than from cysteine degradation.
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Conclusions There was significant flavor generation by using cysteine as a flavor precursor in extrusion cooking. Most of the volatile compounds released at the extruder die were sulfur-containing compounds. Total volatile compounds or sulfur-containing compounds increased as cysteine concentration increased. However, the yield of sulfur containing compounds recovered from the condensate was very low. Most of the cysteine participated in forming hydrogen sulfide and ammonia because numerous meaty flavor compounds were identified in the condensate, mainly dimethyl1,3,5-trithiolane, 2,4-dimethyl-3-thiazoline, 2,4,6-trimethyl-perhydro-1,3,5-dithiazine and others. A comparison of volatiles from the condensate and recovered from the extrudate shows that more thiophenes were identified from the extrudate while more thiazolines, thiazolidines, and dithiazines were identified in the condensate. Cysteine addition in extrusion cooking can add desirable flavors to wheat flour extrudates. A large amount of flavor compounds derived from cysteine accumulated in the condensate. Further research in producing quality of extruded food products should include a strategy to accumulate more flavor volatiles in the extrudate when using added precursors such as cysteine.
Acknowledgements This is publication No.D10544-4-96 of the New Jersey Agricultural Experiment Station reported by State Funds and the Center for Advanced Food Technology (CAFT). CAFT is a New Jersey Commission on Science and Technology Center. This work was also supported in part by U.S. Army Research Office. We thank Dr H.I. Hwang for preliminary work.
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