Chapter 22
Interaction between Sulfur-Containing Flavor Compounds and Proteins in Foods Donald S. Mottram and Ian C. C. Nobrega Department of Food Science and Technology, The University of Reading, Whiteknights, Reading RG6 6AP, United Kingdom
When volatile disulfides were added to aqueous solutions containing protein, full recovery of the disulfides could not be achieved. In some protein systems the levels of disulfides recovered were almost 100 times lower than from simple aqueous solution. In addition significant quantities of the disulfides were converted to the corresponding thiols. Similar effects were shown in meat systems. Other heterocyclic sulfur compounds, such as thiophenes, were not affected by the presence of proteins. The interactions between proteins and disulfides may be explained by interchange redox reactions between the disulfides and sulfhydryl and disulfide groups of proteins. This demonstrates that chemical, as well as physical, interactions between aroma compounds and food components can be important determinants of food flavor.
Sulfur-containing volatile compounds generally have potent aromas with low odor threshold values, and many are important in determining the aroma characteristics of foods. Thiol-substituted furans, such as 2-methyl-3-furanthiol (1) and 2furanmethanethiol (3), and the corresponding disulfides, 2 and 4, have been shown to have meat-like or roasted, coffee-like aromas at low concentrations. During thermal processing, such compounds may be formed in the Maillard reaction (1-3) or from the degradation of thiamin (4,5). These thiols and disulfides are widely used as components of flavorings for soups, savory products and meat substitutes, where they are either added to the flavorings as nature-identical chemicals or as components of reaction-product flavorings. Recently it has been shown that disulfides, such as bis(2-methy 1-3-fury 1) disulfide (2) and bis(2-furylmethyl) disulfide (4) could not be recovered completely when added to aqueous solutions containing protein. Furthermore, some of the
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© 2000 American Chemical Society
275 disulfides were converted to the corresponding thiols. This paper discusses such protein - disulfide interactions and how they may influence the aroma characteristics of heated foods containing sulfur aroma compounds. The data presented demonstrate that chemical interactions between aroma compounds and food components are important factors in the binding of flavor in food.
1
2
3
4
Recovery of Disulfides from Aqueous Solutions containing Proteins When a commercial savory flavoring, containing thiol and disulfide flavor components, was heated at 100°C in aqueous solution with egg albumin during simultaneous steam distillation and solvent extraction (SDE), considerable changes in the relative concentration of the sulfur compounds were observed (6). The flavoring comprised a complex mixture of aroma compounds in which the major constituents were terpenes and fiiran disulfides. Heating egg albumin with the flavoring during SDE showed a similar recovery of the terpenes to that achieved in from an aqueous control without protein. However, there was a marked effect on some of the sulfur compounds. In particular, there was a decrease of almost 100-fold in the recovery of the major disulfide component, bis(2-furylmethyl) disulfide (4), from the egg albumin system compared with the aqueous control. Furthermore, none of disulfide 2 was recovered from the protein-containing system, although, compared with 4, it was present at much lower levels in the flavoring. In contrast to the loss of the disulfides, a considerable increase (almost 10-fold) in the quantities of 2-furanmethanethiol was found in the egg albumin system. It was suggested that this could be due to interchange reactions between disulfide and thiol groups on the flavor compounds with sulfhydryl groups of the protein. This was investigated further by adding disulfides 2 and 4 to a series of aqueous systems containing different proteins (casein or egg albumin) or a carbohydrate (maltodextrin). A similar effect to that shown with the commercial flavoring was observed with a marked loss of the disulfides in the presence of protein and a large increase in the level of the corresponding thiol (Table I). The recovery of the disulfide 4 from water was almost 100% and a similar recovery was obtained for a maltodextrin system, although the recovery of 2 was not complete. It appeared that all the lost disulfides were not recovered as thiols indicating that interaction had occurred with the protein. When the protein in the system was casein the disulfide losses and conversion to thiol were considerably less than in the systems containing egg albumin.
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Table I. Quantities of disulfides and thiols recovered from aqueous systems containing protein or carbohydrate substrates. Compound
Quantity recovered fag) Water alone
bis(2-furylmethyl) disulfide
476 (1)
Egg Albumin
Casein
Malto dextrin
3(1)
377 (63)
492 (10)
2-furanmethanethiol
l(l)
127 (14)
12(4)
10)
bis(2-methyl-3-furyl) disulfide
na
6(2)
272(21)
378 (24)
2-methy 1-3 -furanthiol
na
226 (54)
67 (8)
1(1)
a
NOTE: Initial addition was 500 μg of each disulfide. Each value is the mean of triplicate determinations and standard deviations are shown in parentheses, na, not determined. The disulfides contained very small amounts of the corresponding thiols as impurities. SOURCE: Adapted from Reference 7. a
Effect of Protein on Sulfur Compounds obtained from a Ribose- Cysteine Reaction System These investigations of interactions between proteins and disulfides have now been extended to determine the effect of protein on other volatile sulfur compounds. When cysteine and pentoses, such as ribose, are heated, complex mixtures of volatiles are formed, which have meaty, savory aromas. Such mixtures are used as reaction product flavorings. Cysteine - ribose reaction mixtures have been the subject of a number of investigations in this laboratory (7-9) and over 100 compounds have been identified. These are dominated by sulfur compounds, including compounds where the sulfur is contained in a heterocyclic structure and others where the sulfur is in the form of a thiol or disulfide group. Recently we reported over 20 disulfides in the volatiles from a cysteine - ribose reaction system. These were symmetrical and unsymmetrical disulfides formed from furanthiols, thiophenethiols, and mercaptoketones (8). Heterocyclic compounds produced in the system included thiophenes, thiophenones, dithianones, and thienothiophenes. Such a system has now been used to determine the effect of protein on a range of volatile sulfur compounds. The reaction system was prepared by heating cysteine and ribose (0.3 mmol each) in 6 m L aqueous phosphate buffer (pH 5.6) at 140 °C for 30 min (8). The mixture was then mixed with 150 mL 5% egg albumin before extracting by simultaneous distillation extraction using procedures described previously (6). A control determination was carried out using a similar system without albumin . Quantities of some of the major volatiles from the systems are shown in Table II. The heterocyclic sulfur compounds were generally unaffected by the egg albumin. The mercaptoketones appeared to be present at a higher concentration in the system containing albumin although the differences were not significant (p > 0.5 in Student t-
277 Table IL Approximate quantities (ng/10 mg ribose) of selected volatile sulfur compounds recovered by SDE from cysteine and ribose model systems in the absence (blank) or presence or egg albumin. Blank
Egg albumin
1390
1970
3-Mercapto-2-pentanone 6
628
725
2-Mercapto-3-pentanone
497
550
1297
1014
Compound 3-Mercapto-2-butanone
5 7
2-Methyl-3-furanthiol 1 2-Methyl-3-thiophenethiol 8
426
262
2-Furanmethanethiol 3
1643
1285
3-Thiophenethiol 9
743
471
2-Methy 1-3-fury 1 l-methyl-2-oxopropyl disulfide 10
63
37
2-Methyl-3-fiiryl l-ethyl-2-oxopropyl disulfide 11
48
tr
2-Methy 1-3-fiiryl l-methyl-2-oxobutyl disulfide 12
21
tr
2-Furylmethyl l-methyl-2-oxopropyl disulfide 13
21
14
Bis(2-methyl-3-furyl) disulfide 2
259
27
2-Methyl-3-furyl 2-methyl-3-thienyl disulfide 14
57
tr
Bis(3-thienyl) disulfide 15
43
nd
2-Methy 1-3-fury 13-thienyl disulfide 16
88
13
2-Methyl-3-thienyl 3-thienyl disulfide 17
38
nd
2,3-Dimethylthiophene 18
24
25
1712
1675
2-Formylthiophene 20
404
302
3-Methyl-2-formylthiophene 21
463
519
3-Methyl-l,2-dithian-4-one 22
224
235
4,5-Dihydro-2-methyl-3(2//)-thiophenone
19
715
641
a dihydrothienothiophene
1712
1604
a methyldihydrothienothiophene
492
468
2,3-Dihydro-6-methylthieno[2,3c]furan
23
N O T E : values are means of triplicate collections (average C V = 20%); tr, trace (
