Important Sulfur-Containing Aroma Volatiles in Meat - American

Chapter 15

Important Sulfur-Containing Aroma Volatiles in Meat

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 16, 2016 | http://pubs.acs.org Publication Date: July 29, 1994 | doi: 10.1021/bk-1994-0564.ch015

Donald S. Mottram and Marta S. Madruga University of Reading, Department of Food Science and Technology, Whiteknights, Reading RG6 2AP, United Kingdom

Sulfur-containing furans and thiophenes and related disulfides are known to possess strong meat-like aromas and exceptionally low odor threshold values. Such compounds have been found in model Maillard reaction systems and in cooked meat where they are considered to contribute to the characteristic aroma. Possible precursors of these compounds in meat are pentose sugars and cysteine. One of the main sources of pentoses in meat is theribonucleotide,inosine-5'-monophosphate (IMP), which accumulates in meat during post-mortem glycolysis. In this study the role of IMP as a precursor of meat flavor has been examined by heating muscle with and without added IMP. A number of thiols and novel disulfides containing furan groups were isolated from the meat systems, with much larger amounts formed in the meat containing IMP. The amounts of these sulfur compounds were also higher in meat systems in which the pH had been reduced by the addition of acid.

The investigation of characteristic flavors associated with cooked meats has been the subject of much research over the past four decades but, although compounds with "meaty" aromas had been synthesized, compounds with such characteristics were not found in cooked meats until recently (1). In the search for compounds with characteristic aromas it was found that furans and thiophenes with a thiol group in the 3-position possessed meat-like aromas (2). The corresponding disulfides formed by oxidation of furan and thiophene thiols were also found to have meat-like characteristics, and exceptionally low odor threshold values (3). A number of such compounds are formed in heated model systems containing hydrogen sulfide or cysteine and pentoses or other sources of carbonyl compounds (4,5). The thermal degradation of thiamine also produces 2-methyl-3-fiiranthiol and a number of sulfides and disulfides (6,7).

0097-6156/94/0564-0180$08.00/0 © 1994 American Chemical Society

Mussinan and Keelan; Sulfur Compounds in Foods ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


15. MOTTRAM & MADRUGA

Sulfur-Containing Aroma Volatiles in Meat

181

Despite the acknowledged importance of these furan- and thiophenethiols and their disulfides in meat-like flavors, it is only recently that compounds of this type have been reported in meat itself (8-10). Grosch and co-workers (11) have suggested that the 2-methyl-3-furanthiol and its disulfide are more important in the aroma of boiled meat than in roast beef where alkylpyrazines, 2-acetyl-2-thiazoline and 2,5dimethyl-4-hydroxy-3(2fl)-furanone make greater contributions. The Maillard reaction between reducing sugars and amino acids is primarily responsible for the formation of these meaty aroma compounds during the cooking of meat, although the thermal degradation of thiamine may also be important. In the Maillard reaction the precursors for 2-methyl-3-furanthiol are likely to be pentose sugars and cysteine. Indeed early work on meat flavor showed that meaty aromas were generated by heating cysteine with pentoses. In meat the principal sources of pentose sugars are theribonucleotides,in particular inosine-5'-monophosphate (inosinic acid) which accumulates in post-slaughter muscle from the hydrolysis of adenosine triphosphate, theribonucleotideassociated with muscle function. IMP has been used as a flavor enhancer in savory foods and is believed to contribute to the "umami" taste which is considered to be an important part of the taste characteristics in meat. However, its thermal degradation and reaction with cysteine and hydrogen sulfide during cooking may be one of the main routes to the characteristic aromas of cooked meat. The formation of 2-methyl-3-furanthiol and related compounds in model systems is influenced by factors such as pH, temperature and buffer, as well as the nature and concentration of reactants (5,11,12). The Maillard reaction is known to be pH dependent, alkaline conditions favoring the formation of colored compounds and nitrogen-containing volatiles, while other volatiles are only formed at lower pHs (13-15). Recently the effect of small changes in pH between 4.5 and 6.5 have been examined in meat-like model systems containing cysteine andriboseor 4-hydroxy-5methyl-3(2#)-furanone (5,12,16). Small changes in pH had a major effect on certain classes of volatiles; nitrogen heterocyclics such as pyrazines were only produced at pH above 5.5, while the formation of 2-methyl-3 furanthiol and its disulfide were favored by lower pH. In unbuffered model systems, large changes in pH occur during heating; therefore, buffers such as phosphate or pyrophosphate are generally used. It is known that phosphate can catalyze the Maillard reaction (17) and alternative means of maintaining a constant pH during the reaction would be preferable. Meat normally has a pH in the range 5.6-6.0 and has a high buffering capacity which results in little pH change during cooking. It would therefore provide the most appropriate medium in which to study the effect of pH on the Maillard derived volatiles associated with meat flavor. In the work reported in this paper, meat has been used as the substrate in which to study the effect of pH on the formation of thiols and disulfides, using acid or alkali to adjust the pH. The importance of IMP concentration in meat on the formation of sulfur-containing furans during cooking was also examined in meat containing added IMP.


182

SULFUR COMPOUNDS IN FOODS


Experimental Samples (100g) from a beef fillet (M. psoas major), obtained from a local meat supplier, were chopped in a laboratory blender and mixed with 12 ml water containing sufficient hydrochloric acid (1 M) to adjust the pH to 5.0, 4.5 or 4.0. The pH of the untreated meat was 5.6 and blanks were prepared without pH adjustment. Other samples were prepared with water containing 2.7 g inosine-5'-monophosphate (free acid obtained from yeast, Sigma Chemical Co). The addition of IMP resulted in a drop in pH of approximately 1.1 pH units, therefore the pH of some of these samples was adjusted to 5.6 by the addition of sodium hydroxide (1 M). The meat samples were then heated in glass bottles in an autoclave at 140 °C for 30 min. All the reactions were carried out in triplicate. Headspace volatiles from the cooked meat were analyzed by headspace concentration followed by GC or GC-MS. The meat was placed in a 250 ml conical flask, fitted with a Drechsel head, and oxygen-free nitrogen was passed over the sample, for 2 h at a rate of 40 ml/min, to sweep the volatiles onto a trap packed with Tenax GC (SGE Ltd) (18). The sample was maintained at 60 °C throughout collection. After collection, the volatiles were thermally desorbed, using a modified injector port, directly onto the front of a DB-5 fused silica column (30 m χ 0.32 mm i.d.) in the oven of a Hewlett Packard HP5890 GC. The oven was held initially at 0 °C for 5 min while the volatiles were desorbed from the Tenax trap (held at 250 °C in the modified injector). The temperature was increased to 60 °C over 1 min, and then held for 5 min at 60 °C before programming to 200 °C at a rate of 4 °C/min. The column effluent was split equally between an FID detector and an odor port. GC-MS analyses were performed under similar GC conditions using a Hewlett Packard HP5988A mass spectrometer. Quantitation was based on peak area integration of the GC-MS chromatograms using 2-dichlorobenzene as internal standard. This compound, in ethanolic solution (Ιμΐ containing 65 ng/μΐ), was added to the Tenax trap just before collection. Results and Discussion The heated meat systems contained several thiols, namely 2-methyl-3-furanthiol, 2furylmethanethiol, 2-mercapto-3-pentanone and 3-mercapto-2-butanone, and di- and trisulfides containing 2-methyl-3-furyl and/or 2-furylmethyl groups (Figure 1). Dimethyldisulfide, dimethyltrisulfide and dimethyltetrasulfide were also produced presumably from methanethiol, which was not isolated by the headspace entrainment method. Methanethiol also gaveriseto methyl disulfides containing 2-methyl-3-furyl and 2-furylmethyl moieties. The concentrations of many of the compounds were low, and some were only detected in systems containing added IMP. No thiol-substituted thiophenes were found, although significant quantities of formyl- and acetylthiophenes, thiophenones and dithiolanones were present in all the cooked meat prepara tions. Many of these thiols and disulfides have not been previously reported in meat systems (Table I). Their identifications were based on mass spectra and comparison of linear retention indices (LRI) with those of authentic compounds. Authentic samples of the mercaptoketones and most of the di- and trisulfides were not available


Sidfur-Containing Aroma Volatiles in Meat

MOTTRAM & MADRUGA


SH

ΙΓΛ SH

SH

1

2

S CH n

'

3

SH

S

// V =

_

s

8 R j CH3 ,

6 n =2 7 n =3

9

R

l

v / >V\> R2

=

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11 n = 2 12 n = 3

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Sn-CH 13 n = 2 14 n = 3

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16 R-j

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17 R = C H , R = H 1

2

=

5

18 R-j C^Hg ,

2

R2

=

C H 3

Figure 1. Thiols and sulfides identified in the headspace volatiles of heated meat systems.


Mussinan and Keelan; Sulfur Compounds in Foods ACS Symposium Series; American Chemical Society: Washington, DC, 1994. 1

813 908 873 913 1175 1392 1220 1451 1535 1635 1466 1584 1687 1932 1552 1584 1649 1659

3-mercapto-2-butanone * 2-mercapto-3-pentanone * 2-methyl-3-furanthiol 2-fiirylmethanethiol 2-methyl-3-furyl methyl disulfide 2-methyl-3-furyl methyl trisulfide * 2-furylmethyl methyl disulfide 2-furylmethyl methyl trisulfide * bis(2-methyl-3-furyl) disulfide 2-methyl-3-furyl 2-furylmethyl disulfide 2-methyl-3-furyl 2-oxopropyl disulfide * 2-methyl-3-furyl l-methyl-2-oxobutyl disulfide * bis(2-furylmethyl) disulfide bis(2-furylmethyl) trisulfide * 2-furylmethyl 2-oxopropyl disulfide * 2-furylmethyl l-methyl-2-oxopropyl disulfide * 2-furylmethyl 2-oxobutyl disulfide * 2-furylmethyl l-methyl-2-oxobutyl disulfide

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Important Sulfur-Containing Aroma Volatiles in Meat - American

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