Influence of DNA on Volatile Generation from Maillard Reaction of

Chapter 30

Downloaded by STANFORD UNIV GREEN LIBR on July 26, 2012 | http://pubs.acs.org Publication Date: December 1, 2003 | doi: 10.1021/bk-2004-0871.ch030

Influence of DNA on Volatile Generation from Maillard Reaction of Cysteine and Ribose 1

2

1

Yong Chen , Chee-Kok Chin , and Chi-Tang Ho 1

2

Departments of Food Science and Plant Sciences, Cook College, Rutgers, The State University of New Jersey, New Brunswick, NJ 08901-8520

The effects of DNA on thermal flavor generation were investigated using the Maillard reaction model system containing cysteine and ribose, which were heated to the roasting temperature of 180 °C for 2 hours at pH 5 andpH8.5. The volatiles and semi-volatile compounds identified from the liquid phase of the reaction systems of ribose and cysteine with or without DNA indicated that DNA affected volatile formation in a complex manner. A typical compound derived from DNA-involved thermal reaction was 2-furfuryl alcohol, which elicits a sugary aroma. DNA mitigated the sulfurous, meaty aroma by reducing the concentration of some well-known meaty flavor compounds such as 2-methyl-3furanthiol, 2-methyl-3-thiophenethiol, 2-furfurylthiol and their associated dimers as well as some thiophenes. On the other hand, DNA promoted the formation of several important nitrogen-containing volatiles such as methylpyrazine, 2acetylthiazole, cyclopentapyrazine and 2,6-dimethylpyrazine, which are known to elicit roasty, nutty flavor notes. Although DNA can act as a nitrogenous source, it should not be regarded as a major donor in the formation of nitrogen-containing compounds.

© 2004 American Chemical Society In Nutraceutical Beverages; Shahidi, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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A number of studies utilizing various analytical techniques in recent years have revealed some very important aroma compounds imparting meat flavor perception (1-3). Out of more than 1000 volatiles identifiedfromcooked meats, a few have been characterized as meaty flavor impact compounds. In general, thiol-substituted furans and thiophenes and related disulfides possess strong meat-like and/or roast aromas with low odor threshold values. For instances, 2methyl-3-furanthiol and its disulfide bis-(2-methyl-3-furyl)-disulfide were identified as major contributors to the meaty aroma of cooked beef, chicken, and pork (4,5) while 2-furfurylthiol was identified as a contributor to roasty and coffee-like notes (6). It is well known that the Maillard reaction between reducing sugars and amino acids is one of the crucial routes for the formation of these compounds during cooking. Studies have shown that cysteine and pentoses are important precursors participating in the thermal reaction to generate meaty aroma compounds. In meats ribose is one of the major sugars, and it originates principally from ribonucleotides, in particular, adenosine triphosphate (7). Postmortem effects result in a large amount of nucleotides from degradation of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Besides serving as the source for pentose sugar, nucleotides have been found to act synergistically with glutamic acid or monosodium glutamate (MSG) to enhance meaty, brothy, and MSG-like taste while suppressing sulfurous notes (8). Two nucleotides, inosine-S'-monophosphate (IMP) and guanosine-5monophosphate (GMP), which accumulate in post-slaughter muscle as a result of the hydrolysis of inosine triphosphate and guanosine triphosphate, respectively (7), are taste active. They both have been used as flavor enhancers in savory foods and are believed to contribute to the "umami" taste (9). Thermally, these nucleotides are not stable. Their contents can be decreased by half within approximately 30 min at 121 °C in highly acidic conditions (10). The thermal degradability of these nucleotides makes them potential modifiers of thermally generated flavors. However, previous studies related to the influence of nucleic acids on meaty flavor generation have primarily focused on IMP. In a meat model system, IMP enhanced the formation of thiols and novel disulfides containing furan moieties (1). Volatiles generated from the reaction of both alliin-IMP and deoxyalliin-IMP elicited a pungent garlic flavor with roasted notes, caused by sulfur-containing compounds plus a number of pyrazines and thiazoles, including methylpyrazine, ethylpyrazine, 2,5dimethylpyrazine, 2-propylthiazole, and 2-ethyl-4-propylthiazole (77). The level of IMP was 0.106-0.443% in beef and 0.075-0.122% in chicken; chicken appeared to contain a somewhat higher amount of AMP, CMP, UMP than beef (10). It was reported that the normal concentration of DNA in longisimus muscle was about 1091.9 μg/g muscle (12). The high level of DNA in meats potentially has some effects on meat flavor production during cooking. However, information regarding the overall effect of DNA on meaty flavor is virtually unavailable. The influence of DNA on the formation of volatile compounds at both pH 5 and 8.5 using a Maillard reaction model system of ribose and cysteine was examined in this study.

In Nutraceutical Beverages; Shahidi, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Experimental Materials


D-ribose, tridecane and anhydrous sodium sulfate were purchased from Aldrich Chemical Company (Milwaukee, WI). DNA and L-cysteine were bought from Sigma Chemical Co. (St. Louis, MO). Methylene chloride used was of HPLC grade and purchased from Fisher Scientific (Fair Lawn, NJ). Thermal Reactions The powdered DNA was made from Herring sperm and had a fishy odor. For each reaction involving DNA, 3 g of DNA powder was weighted and thermally dissolved in distilled water. The solution was extracted with 120 ml (40 χ 3) of hexane to remove possible lipid residues. L-cysteine and D-ribose (0.01 mol) with or without 3 g of DNA were dissolved in 100 ml of distilled water. The addition of DNA into the solution ofriboseand cysteine lowered the pH by about 1 unit. The pH of solutions was adjusted to 5.0 or 8.5 with sodium hydroxide prior to thermal reaction in 150 ml Hoke stainless steel cylinders (Hoke Inc., Clifton, NJ), which were heated at 180 °C in an oven for 2 h. The reactions were immediately stopped by cooling under a stream of cold water. Other thermal reactions of DNA, between DNA and ribose or cysteine were all conducted in a similar manner and under the same conditions. Liquid/liquid Extraction of Volatile/Semivolatile Compounds After cooling, the brown reaction mixture was mixed with 0.5 ml of a solution of internal standard (tridecane, 1 mg/ml) and extracted with methylene chloride (50 ml χ 3 times). The extract was dehydrated by anhydrous sodium sulfate and concentrated under a nitrogen flow to 10 ml in a flask and then transferred to a Kuderna-Danish concentrator and further concentrated to 1-1.5 ml before subjecting to further analysis. GC/Mass Spectrometry Analysis The concentrated isolates from different reaction mixtures were analyzed by GC/mass spectrometry (GC/MS), using a Hewlett-Packard 6890 GC equipped with a fused silica capillary column (60 m χ 0.25 mm i.d.; 1 μιη thickness, DB1) coupled to a Hewlett-Packard 5973 series mass selective detector. Mass spectra were obtained by electron ionization at 70 eV and a source temperature of 250 °C.


430 Identification of the Volatiles


The identification of volatile compounds was based on GC/MS analysis. The compounds from the isolate were identified by comparing the mass spectral data with those of authentic compounds available in the Wiley 275 library or previous publications (2,75-75). Quantification of the volatile/semivolatile compounds from the liquid phases was based on using tridecane as an internal standard.

Results and Discussion The effect of DNA on volatile generation was investigated using Maillard reaction solutions of ribose and cysteine with or without DNA at both starting pH of 5 and 8.5, which were heated at roasting temperature of 180 °C for 2 h. As indicated by Table I, pH changed 2-3 units after the reactions. Changes of pH are common in the reaction systems in the absence of buffering agents (16). Table I. Final pH, final appearance andflavordescription of the thermal reaction mixtures of ribose and cysteine with/without DNA Solutions of Model Systems DNA

DNA + Ribose

Ribose + Cysteine

Cysteine + DNA

Ribose + Cysteine + DNA

Initial pH

Final pH

Aroma Characteristic

5

4.27

Fruity, sugary

8.5

5.55

Slightly burnt

8

3.61

Sugary

8.5

4.25

Sugary, fruity

5

3.56

Sulfurous, meaty

8.5

5.25

Sulfurous, burnt, nutty

5

5.31

Sulfurous

8.5

7.08

Sulfurous, burnt

5

3.67

Sulfurous, meaty

8.5

5.42

Nutty, burnt

Organoleptically, the odor elicited from the reaction of ribose and cysteine under acidic conditions was sulfurous and meaty, but under basic conditions, the odor had additional roasty and burnt notes, and the sulfurous, meaty aroma



431

became less obtrusive. The presence of DNA in the reaction systems imparted some sugary, fruity, and sweet notes in addition to the aroma characteristics perceived in the absence of DNA. Compounds identified from the liquid phases of the reactions between ribose and cysteine with or without DNA are listed on Table II. The data provided clear evidence that DNA can act as a major source for pentose. The addition of DNA increased the levels of furans, especially 2-furfuryl alcohol, furfural and difiirylmethane. In the reaction systems without DNA, 2furfurylalcohol was formed in small quantities but addition of DNA increased its concentration by many folds. Believed to have a fruity and sugary aroma, 2furfuryl alcohol has been identified as a major component from baked sweet potato (77) and rice cake (75). It can also be generated in large amounts from the reaction between cysteine and glucose (79) and found to have strong inhibitory ability toward hexanal oxidation in commercial beer (20). Furfural serves as an important precursor for the formation of other furanoids and heterocyclic compounds such as thiophenes and pyrroles. When H S or N H are present, replacement of the oxygen on the furan ring with sulfur or nitrogen results in the formation of the corresponding thiophenes or pyrrole derivatives. Consistent with previous research reports (1,21,22), the results indicated that more sulfur compounds were produced at lower pH. The meaty flavor impact compound 2-methyl-3-furanthiol and its oxidized dimer bis (2-methyl-3furyl) disulfide were formed in a much larger amount under acidic conditions and DNA appeared to inhibit their formation. These two compounds have the potency to dictate the characteristics flavor of meat. It has been shown that compounds containing a 2-methyl-3-furanyl group could originate from the reaction of hydrogen sulfide with 4-hychoxy-5-methyl-3(2H)-fiiranone (HMF) (23), which may derive from pentoses in the Maillard reaction or from the dephosphorylation of ribose phosphate (16,24). It is well known that formation of HMF from pentose sugar is favored under more acidic conditions. A significant amount of 2-furfurylthiol was generated from reaction of ribose with cysteine at pH 5, much less formed at pH 8.5. This is in accordance with previous studies showing that 2-furfurylthiol formation was greatly affected by pH and lowering the pH would significantly increase its formation (25). The formation of 2-furfurylthiol probably resulted from the reaction of H S with the ribose breakdown product furfural (23). Although DNA increased the amount of furfural, it did not show the same effect on 2-furfurylthiol formation. Instead, it greatly reduced the quantity of 2-furfurylthiol and its corresponding dimer, bis (2-furfuryl) disulfide. Possessing a characteristic coffee-like aroma, 2furfurylthiol has been identified from the reaction between cysteine and inosine 5-monophosphate (26). The thiophenoids were the most numerous compounds identified. Among them, 2-methylthiophene was the most abundant, followed by thieno[3,2-b] thiophene and 5-methylthieno[2,3-d] thiophene. In general, addition of DNA 2

3

2


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Table II. Volatile Compounds Generated from the Liquid Phases of the Thermal Reactions of Ribose and Cysteine, with/without DNA at pH 5 or pH8.5


Compounds Furanoids Furfural 2/3-Furfuryl alcohol 2-Acetyl-5-methylfuran Difurylmethane 2-(2-Furanyhnethyl)-5-methylfuran 1 -(2-Furyl)-2-propanone 2,2*-( 1,2-Ethenediyl)tofiiran 5-Methyl-2(5H)-fwanLone

RCS

fl

0.172 0.023 0.006 0.016 0.020

Amount (mg / g ribose) RCD5* RC8.5 RCPS-S* C

0.487 1.471 0.013 0.247 0.007 0.003

0.011 0.027

0.049

0.003

0.022

Thiophenoids 2-Methylthiophene 3-Methylthiophene 2, 3-Dimethylthiophene 2-Acetylthiophene 2-Ethyltkiophene 2-Piopylthiophene 2-Acetyl-3/5-methylthiophene Dimethylformylthiophene 2/5-Methyl-thieno[2,3-b]thiophene 3-Thiophenecarboxaldehyde 2-Hydroxymethylthiophene 3/5-Methyl-2-thiophenecarboxaldehyde 2-Formyl-3-methylthiophene 2-Fonnyl-2,3-dihydrothiophene 2/3-Thiophenemethanol Thieno[3,2-b]thiophene Dihydrothienothiophene Methyldihydrothienothiophene 5-Methylthieno[2, 3~d](hiophene 2-Thiophenethiol 2-Mefhyl-3-thiophenethiol 2-Thienylmethanol Dihydro-3-(2H)-thiophenone Dihydro-2-methyl-3(2H)-thiophenone 5-Methyl-2(5H)-thiophenone 1 -(2-Thienyl)-1 -propanone

0.468 0.078 0.122

0.132 0.022 0.054

0.035 0.009 0.011 0.058 0.016

0.002 0.005 0.017 0.006 0.045 0.135 0.009 0.007

0.051 0.148

0.133 0.047

0.071 0.125 0.157 0.096 0.164 0.007 0.064 0.014

0.015 0.036 0.026 0.065 0.026 0.017

0.023 5.021

0.423 0.074 0.106 0.011 0.081 0.010

0.237 0.039 0.021 0.009 0.007

0.028

0.016

0.048 0.028 0.044 0.090 0.044 0.127 0.036 0.095 0.170 0.070 0.007 0.161 0.015


0.023 0.005 0.082 0.081 0.195 0.113 0.051 0.033 0.022 0.007 0.045 0.117 0.008 0.149

433 Table II. Continued


Amount (mg / g ribose) Compounds R C 5 a RCD5 6 R C 8.5C R C D Furanoids Other sulfur-containing compounds

2-Methyl-3-furanthiol bis(2-Methyl-3-furyl)disulfide 2-Furfurylthiol bis(2-Furfuryl)disul£ide 3-Mercapto-2-pentanone Furfuryl sulfide 2-[(Methylthio)methyl]furan 2-Methyl-3-(methylthio)-butane 2, 3-Dihy

Influence of DNA on Volatile Generation from Maillard Reaction of

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