Contribution of Phenolic Compounds to Smoke Flavor - ACS

Chapter 13

Contribution of Phenolic Compounds to Smoke Flavor Joseph A. Maga

Downloaded by UNIV OF CINCINNATI on May 30, 2016 | http://pubs.acs.org Publication Date: October 1, 1992 | doi: 10.1021/bk-1992-0506.ch013

Department of Food Science and Human Nutrition, Colorado State University, Fort Collins, CO 80523

The roles of hemicellulose, cellulose and lignin thermal degradation on the formation of phenolic-based wood smoke volatiles are discussed. Compositional differences between hardwoods/softwoods and heartwood/sapwood as related to smoke volatile formation are also described. Phenols are the major contributors to wood smoke aroma, but other compound classes are also important. Various phenols (guaiacol, 4-methylphenol, 2,6-dimethoxyphenol) have been described as possessing smoky aromas. The smoking of certain foods probably represents one of the earliest known forms of food processing and preservation. However, in modern society, with many forms of preservation available, foods are now primarily smoked to provide unique flavors in the finished product. As a result, various smoke source media, such as hickory, maple and mesquite, are now commonly available to provide diverse smoked flavors. Through the years, various researchers have attempted to characterize the compounds that are responsible for the unique sensory properties associated with wood smoke. One would predict that this would be a somewhat simple task based on the relatively few number of components found in wood. However, when one considers factors such as wood moisture content, temperature of smoke generation, instability of intermediates, and the low odor thresholds for some of the compounds produced, a very complex and confusing picture can result. One must also consider the fact that when wood smoke is exposed to food, food components can also serve as intermediates in the formation of smoke flavor compounds detected at the time of food consumption. As a result, to date, over 400 compounds have been identified as volatile components of wood smoke. As can be seen in Table I, most of the identified compounds are carbonyls and phenols. However, as with all flavor chemistry, the presence of a compound does not necessarily mean that it makes any significant contribution to flavor. Therefore, i f one were to evaluate the actual flavor contribution of each of the

0097-6156/92/0506-0170$06.00/0 © 1992 American Chemical Society Ho et al.; Phenolic Compounds in Food and Their Effects on Health I ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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Contribution of Phenolic Compounds to Smoke Flavor

410 compounds identified, it becomes quickly apparent that phenolic compounds are major contributors to wood smoke flavor. Table I.

Compound Classes Identified in Wood Smoke


Class

Number Identified 131 75 48 46 22 22 16 50

Carbonyls Phenols Acids Furans Alcohols Esters Lactones Miscellaneous Total

410

Therefore, the major objectives of this presentation are to discuss the role of smoke generation variables, including wood composition, on the formation of phenolic compounds in the resulting smoke and to present data on their sensory properties. Wood Composition The major component of most woods is cellulose, which is a long chain glucose polymer. Hemicellulose is another major component and is usually composed of a combination of five-carbon sugars such as arabinose and xylose, and six-carbon sugars including glucose, mannose and galactose. The third major component is lignin, which can be classified as a phenolic-based compound containing various possible combinations of differently bound hydroxy- and methoxy-substituted phenylpropane units. Other minor components in most woods include volatile oils, terpenes, fatty acids, simple carbohydrates, polyhydric alcohols, nitrogen and phenolic compounds, and inorganic constituents (i).

With some wood sources, bark composition should also be considered. Bark is primarily composed of suberin, which is an ester of various aliphatic hydroxy acids and numerous phenolic acids bound in various configurations. Hardwood Versus Softwood From a structural standpoint, hardwoods are more complex than softwoods. Also, most hardwoods have a higher proportion of pentosans than hexosans in their hemicellulose fraction than softwoods. The lignin fraction also differs between hardwoods and softwoods with approximately three more syringlypropane units than guaiacylpropane units in hardwood as in softwood, and in general, softwoods have more lignin than hardwoods. The mannan content of most softwoods is normally 10-15%, while in hardwoods, it rarely exceeds 3%. Also, the

Ho et al.; Phenolic Compounds in Food and Their Effects on Health I ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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PHENOLIC COMPOUNDS IN FOOD AND THEIR EFFECTS ON HEALTH I

xylan content of hardwoods (10-20%) is usually twice that of softwoods. Softwoods also contain more resin extractives than hardwoods. Because of these compositional differences, wood smoke generated from hardwood and softwood differ in their relative amounts of phenolics. As seen in Table II, hardwood smoke is higher in syringol derivatives while softwood smoke has a greater proportion of guaiacol derivatives.


Table II. Phenolic Differences in Hardwood Versus Softwood Wood Smoke

Compound Guaiacol 4-Methylguaiacol 4-Ethylguaiacol 4-Vinylguaiacol 4-Allylguaiacol Syringol 4-Methylsyringol 4-Vinylsyringol 4-Allylsyringol 4-Propenylsyringol

Relative % in Softwood Hardwood Smoke Smoke 8 7 3 3 2 12 9 3 3 10

10 10 5 5 4 3 3 1 1 2

SOURCE: Adapted from ref. 2.

Heartwood Versus Sapwood The amount of heartwood (center of tree) to sapwood (outer portion of tree) can significantly vary with different woods and also within the same species. The faster a tree grows, the less heartwood it contains. Also, the proportion of heartwood decreases with tree height. From a compositional standpoint, heartwood is lower in moisture than sapwood; and with softwoods, heartwood has less lignin and cellulose but more extractives than sapwood, while no consistent trend is apparent in hardwoods. Thermal Reactions Among the major wood components, hemicellulose is the first to undergo thermal destruction, followed by cellulose and finally lignin. The temperatures required for these reactions are summarized in Table III. From a theoretical standpoint, smoke can be generated at a relatively low temperature representing primarily hemicellulose degradation; but under these conditions, lignin may not be completely degraded and thus the resulting smoke will have a different composition than if a higher temperature were utilized. The presence or absence of air can also significantly influence the end products formed during the thermal degradation of wood. In


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173 Contribution of Phenolic Compounds to Smoke Flavor

the absence of air, a combination of water, organic liquids and gases are released, leaving a mixture of tar, pitch and charcoal. In the presence of air, combustion results, degrading both volatile and nonvolatile components as well as producing additional volatiles. The influence of wood smoking temperature on resulting smoke phenol content can perhaps best be appreciated by viewing the data shown in Table IV. As temperature is increased, phenol content also increases. However, at higher temperatures the phenol content begins to gradually decline, probably due to thermal instability.


Table III. Thermal Degradation of Wood °C 90-170 200-260 260-310 310-500 >500

Reaction Free and bound water losses Hemicellulose degradation Cellulose degradation Lignin degradation Secondary reactions


Table IV. Wood Smoking Temperature Versus Phenol Content Temperature (°C) 400 450-500 500-600 700-800 900-1050

Content (mg/100g) 350 800 2960 2810 1945


Hemicellulose Degradation Hemicellulose, upon thermal degradation, primarily yields furan derivatives and aliphatic carboxylic acids. However, both furans and acids are not very thermally stable, especially at higher temperatures, and thus each undergo extensive oxidation. As seen in Figure 1, both the acetyl and 0-methyl groups of hemicellulose can react to produce volatile degradation products. Cellulose Degradation The thermal degradation of cellulose occurs by two distinct temperature dependent pathways. At lower temperatures, bond scission, elimination of water, formation of free radical, carbonyl, carboxyl


174


and hydroperoxide groups, and the evolution of carbon monoxide and dioxide gasses occur. At higher temperatures, a series of reactions including transglycosylation and fission occur producing a series of anhydro sugars and lower molecular weight compounds. As seen in Figure 2, numerous intermediates can form, thereby producing a variety of catalysts that can be involved in further classical carbohydrate thermal degradations.


Lignin Degradation

Phenols and derivatives are most directly related to lignin degradation. Initial degradation by fission occurs with the heterocyclic furan, pyran rings and ether linkages of lignin. This eventually results in the production of guaiacol, which in turn can further degrade to form phenol and cresols. Since hardwoods have higher amounts of methoxy substitutions than softwoods, the dimethoxy rings can result in the formation of syringol and an entire series of parasubstituted compounds in addition to guaiacol. The formation of ferulic acid is also a key step in lignin degradation since it can undergo further decarboxylation to produce the series of compounds shown in Figure 3. In the presence of air, oxygenated compounds such as vanillin are formed. As can be seen from above, wood composition and smoke generation temperature can significantly influence the resulting volatile composition of wood smoke. Phenols and Wood Smoke

Various researchers (4-15) have reported that phenols, as a compound class, are the major contributors to wood smoke aroma. However, other researchers tend to dispute the above claim since they reported on other compound classes that possess typical smoke aroma properties. As examples, the data summarized in Tables V and VI show that phenols are not totally responsible for smoke aroma. There is no question that phenols can be generated at relatively high concentrations in wood smoke, and when their relatively low odor thresholds are considered, there is no question that they do make a significant contribution to wood smoke aroma. As seen in Table VII, guaiacol, 4-methylguaiacol, syringol and phenol are the major phenols found in wood smoke. In the case of commercial liquid smokes, up to 1.06 mg/ml of phenols have been reported (17). Table V. Contribution of Smoke Subfractions To Wood Smoke Aroma Smoke Subfraction Phenolic only Phenolic plus carbonyl Phenolic plus neutral plus basic

Aroma Quality Good Better Best

SOURCE: Adapted from ref. 13



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A c e t y l Groups

O-Methvl Groups

F u r f u r a l and Hydroxymethylfurfural

Mucic A c i d

^

Levulinic Acid

Pyromucic A c i d

Furan

γ-Hydroxyvaleric Acid

γ-Valerolactone

Figure 1. Thermal degradation of hemicellulose.

Cellulose •l 4-Anhydride r

> l 6-Anhydride r

l 6-Anhydride

> l 2-Anhydride r

1,4:3,6-Dianhydride

r

/

O l i g o - and P o l y s a c c h a r i d e s ^ !

Dehydration/Decomposition Products

Volatiles

Figure 2.

Thermal degradation of cellulose.




Lignin

F e r u l i c Acid

4-Methylguaiacol

Acetovani1 lone

j Vanillin !

V V a n i l l i c Acid

Guaiacol

Figure 3. Ferulic acid thermal degradation.


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Table VI. Aroma Intensities of Various Wood Smoke Subfractions Wood Smoke Source

Phenolic

Apple Chestnut Cherry Red Oak White Oak

3.40 3.21 3.59 3.17 3.44

Basic 5.32 3.44 4.61 4.07 3.52

Carbonyl

Neutral

4.02 3.86 5.40 6.03 5.46

2.28 1.82 4.13 2.92 2.63



Table VII. Major Phenols Identified in Wood Smoke Compound

Whole Smoke (mg/1) 417 333 392 59

32 14 7 _6

1201

59

Guaiacol 4-Methylguaiacol Syringol Phenol Total

Vapor Phase (mg/1)

SOURCE: Adapted from ref. 5. It should be noted that wood smoke is not the only source of phenols in certain smoked foods. Spices such as cinnamon, thyme, pepper, nutmeg, marjoram and cloves have also been shown to produce phenols when heated (15). Sensory Properties Various researchers have attempted to evaluate the sensory properties of individual phenols relative to wood smoke. For example, in Table VIII it can be seen that among the three compounds evaluated, by far the most potent was 4-methylguaiacol in both odor and taste sensitivity. A panel was also asked to characterize the taste and odor of the same three compounds and 58% of the panel reported that guaiacol had a smoky taste while 72% thought that 2,6-dimethoxyphenol had a smoky odor (18). From these data, it would appear that phenols have different odor and taste properties thus complicating their overall degree of influence relative to smoke flavor. Flavor is commonly defined as a combination of aroma and taste properties. The odor properties of various phenols have been characterized; and, as seen in Table IX, various descriptors related to wood smoke have been reported. Phenol compound volatility also is a factor relative to smoke flavor contribution (20). The low boiling phenol portion (60-90°C) obtained from the distillation of smoke condensate was found to


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primarily contain phenol, cresols and quaiacols and was described as having a hot and bitter taste; whereas the fraction distilled at 92132°C and containing isoeugenols and syringols had a pure and characteristic smoke flavor. In turn, the high boiling phenol fraction (132-200 C) had an acid, chemical sensory property. e

Table VIII. Reported Flavor Thresholds (PPM) and Corresponding Flavor Indices* of Phenols


Compound

Taste Threshold

Taste Index

Odor Threshold

Odor Index

0.065 0.013 1.65

6,400 90,000 1,400

0.09 0.021 1.85

4,600 58,800 1,200

4-Methylguaiacol Guaiacol 2,6-Dimethoxyphenol

* Concentration in smoke condensate + mean threshold concentration SOURCE:

Adapted from ref. 18.

Table IX.

Odor Descriptions of Various Phenols

Description

Compounds

Pungent

Phenol, 0-, M- and P-cresol; 2,3- and 2,4-Xylenol

Cresolic

2,6-, 3,4- and 3,5-Xylenol; 2-Ethyl-, 3-ethyl- and 5-methylphenol; 2,3,5Trimethylphenol

Sweet, smoky

Guaiacol; 3-Methyl- and 4-ethylguaiacol

Weak, phenolic

3- Methylguaiacol

Woody

4- Allylguaiacol

Smoky

2,6-Dimethoxyphenol

Heavy, burnt

2,6-Dimethoxy-4-methyl and ethyl, propyl, propenylphenol

Heavy, sweet, burnt

Pyrocatechol; 3-Methyl-, 4-methyland 4-ethylpyrocatechol

SOURCE:

Adapted from ref. 19.


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Contribution of Phenolic Compounds to Smoke Flavor

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Conclusions Phenolic compounds do make major contributions to smoke flavor, and the amounts and types of these compounds produced during some generation are primarily dependent upon the wood source and smoke generation temperature. Literature Cited 1.


2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Maga, J. A. Smoke in Food Processing; CRC Press: Boca Raton, FL 1988; 1-27. Baltes, W.; Wittkowski, R.; Sochtig, I.; Block, H.; Toth, L. In The Quality of Foods and Beverages; Charalambous, G.; Inglett, G., Eds.; Academic Press: New York, NY, 1981, 1-14. Toth, L. Fleischwirtschaft. 1980, 60, 728. Knowles, M. E.; Gilbert, J.; McWeeny, D. J. J. Sci. Food Agric. 1975, 26, 189. Kornreich, M. R.; Issenberg, P. J. Agric. Food Chem. 1972, 20, 1109. Lustre, A. B.; Issenberg, P. J. Agric. Food Chem. 1970, 18, 1056. Radecki, Α.; Lamparczyk, H.; Grzybowski, J.; Halkiewicz, J.; Bednarek, P.; Boltrukiewicz, J.; Rapicki, W. Bromatol. Chem. Toksykol. 1976, 9, 327. Radecki, Α.; Bednarek, P.; Boltrukiewicz, J.; Cacha, R.; Rapicki, W.; Grzybowski, J.; Halkiewicz, J.; Lamparczyk, H. Bromatol. Chem. Toksykol. 1975, 8, 179. Radecki, Α.; Grzybowski, J. Bromatol. Chem. Toksykol. 1981, 14, 129. Radecki, Α.; Grzybowski, J.; Halkiewicz, J.; Lamparczyk, H. Acta Aliment. Pol. 1977, 3, 203. Luten, J. B.; Ritskes, J. M.; Weseman, J. M. Z. Lebensm. Unters. Forsch. 1974, 168, 289. Fiddler, W.; Doerr, R. C.; Wasserman, A. E.; Salay, J. M. J. Agric. Food Chem. 1966, 14, 659. Fujimaki, M.; Kim, K.; Kurata, T. Agric. Biol. Chem. 1974, 38, 45. Kasahara, K.; Nishibori, K. Bull. Jpn. Soc. Sci. Fish. 1979, 45, 1543. Baltes, W.; Sochtig, I. Z. Lebensm. Unters. Forsch. 1979, 169, 17. Maga, J. Α.; Japojuwo, O. O. J. Sensory Stud. 1986, 1, 9. Ishiwata, H.; Watanabe, H.; Watanabe,M.;Hayashi, T.; Hara, Y.; Kato, S.; Tanimura, A. Bull. Nat. Inst. Hygienic Sci. 1976, 94, 112. Wasserman, A. E. J. Food Sci. 1966, 31, 1005. Kim, K.; Kurata, T.; Fujiman, M. Agric. Biol. Chem. 1974, 38, 53. Toth, L.; Potthast, K. Adv. Food Res. 1984, 29, 87.

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Contribution of Phenolic Compounds to Smoke Flavor - ACS

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