Biological Pathways for the Formation of Oxygen-Containing Aroma

Chapter 14 Biological Pathways for the Formation of Oxygen-Containing Aroma Compounds 1,2

Devin Peterson and Gary A. Reineccius

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Downloaded by UNIV LAVAL on October 6, 2015 | http://pubs.acs.org Publication Date: August 7, 2002 | doi: 10.1021/bk-2002-0826.ch014

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Department of Food Science and Nutrition, University of Minnesota, 1334 Eckles Avenue, St. Paul, MN 55108 Current address: Department of Food Science, Pennsylvania State University, 215 Borland Laboratory, University Park, PA 16802

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While sulfur- and nitrogen-containing aroma compounds are often considered to be more potent than oxygen-containing aroma compounds, oxygen-containing aroma compounds are widely distributed in foods and often make a very significant contribution to aroma. This is true across food categories including raw plant materials, fermented, and thermally processed foods. Wide arrays of oxygenated aroma compounds (e.g. acids, alcohols, aldehydes, furans, pyrans, lactones, etc.) have sufficiently low sensory thresholds and defined characters to be important aroma contributors. This paper will discuss some of the biological pathways leading to both the most abundant and some of the more potent oxygenated aroma compounds.

Oxygen-containing aroma compounds are ubiquitous in food systems. The prevalence of these compounds in food aromas can be associated with two factors; oxygen is the most abundant element on the surface of earth, and oxygen is a reactive atom that readily participates in chemical reactions to form innumerable compounds (/). While oxygen-containing aroma compounds are typically not considered as potent as sulfur- or nitrogen-containing compounds,

© 2002 American Chemical Society

In Heteroatomic Aroma Compounds; Reineccius, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

227


228 these compounds have been found to make a major contribution to numerous foods (e.g. fruits, beer, cheeses, etc.). Much of the early flavor research focused on oxygen-containing aroma compounds. This is primarily because of the abundance of oxygenated compounds in the volatile isolates. If one considers the Maillard reaction, the Strecker aldehydes typically quantitatively dominate the aroma isolate. While in this case, they may not be the most sensorially important volatiles in the profile; they generally are the most abundant. The same might be said for oxygencontaining volatiles in the profile of fermented foods (e.g. acids) and many plant aroma profiles (e.g. esters). These compounds again are significant in making a contribution, albeit negative, to storage off- flavors due to lipid oxidation. There is little question that oxygen-containing volatiles are widely found in foods and often play a key role in defining aroma. Since other sections of this text consider the formation of oxygencontaining aroma compounds by non-biological pathways, this manuscript will focus on biological pathways. We will illustrate the typical pathways for the formation of some of the more abundant oxygenated volatiles (e.g. carbonyls and esters) in plants. We will also present how five of the more potent oxygencontaining aroma compounds (3-hydroxy-4,5-dimethyl-2(5H)-furanone, βionone, β-damascenone, l-octen-3-one, and (E,Z)-2,6-nonadienal; thresholds < 0.0 ^ g / L ) are formed in nature. We will not go back to the origin of the precursors of these example compounds (e.g. fatty acids, carotenoids or amino acids) since pathways for their biosynthesis can be found in most college texts.

Abundant Oxygenated Aroma Compounds Fatty Acid Precursors Several different pathways for the formation of volatiles may involve fatty acids as precursors. One pathway involves the enzyme-catalyzed oxidation of unsaturated fatty acids (2). This mechanism yields large quantities of carbonyls, some of which become reduced to alcohols and potentially esterified to yield esters. A second pathway is normal fatty acid metabolism. Fatty acid metabolism provides a pool of volatiles serving both as native aroma compounds or as precursors in further reactions. While later in this chapter we present specific pathways for fatty acid oxidation leading to the formation of 1octen-3-ol and 2,6-nonadienol, we will introduce these two pathways in a more general sense here. In terms of lipid oxidation mechanisms, lipoxygenase is the first enzyme involved in this process forming a peroxide of an unsaturated fatty acid by adding molecular oxygen. Isomerases may then rearrange this peroxide which


229 ultimately affords more choices in degradation products. The peroxide is subsequently broken down by additional enzymes to form a wide variety of carbonyls. Many of the alcohols and acids are enzymatically converted to esters, which provide the basis of many fruit flavors. The cleavage points and some resultant degradation products of linoleic and linolenic acid oxidation are shown in Figure 1.

Linoleic Acid CH CH CH CH CH CH=CH-CH -CH=CH-(CH ) COOH


3

Hexenal Hexenol

2

2

2

2

2

2

7

(E,E)-2,4-Decadienal (E)-2-Heptenal l-Oeten-3-ol (Z)-3-Nonenal l-Octen-3-one (E)-2-Nonenal (E)-Oct-2-enal l-Nonen-4-one

Linolenic Acid CH CH -CH=CH-CH -CH=CH-CH -CH=CH-(CH ) COOH 3

2

2

(Z)-3-Hexenol (Z)-2-Hepten-2-ol (Z)-3-Hexenal (E)-2-Hexenol (E)-2-Hexeanl

2

(Z)-5-Octen-2-ol

2

7

(Z,Z)-3,6-Nonadienal (E,Z)-3,6-Nonadienal (E,E)-3,6-Nonadienal

Figure 1. Aroma compoundsfromthe oxidative degradation of linoleic and linolenic acids (2).

Considering that each plant has its unique distribution of unsaturated fatty acids, enzymes and environment (e.g. pH), it is easy to understand that each plant will have similarities but yet differences in aroma. Normal fatty acid metabolism also provides a host of oxygenated aroma


230 compounds. This pathway provides a wide range of fatty acids, alcohols, aldehydes, methyl ketones and lactones and is illustrated in Figure 2 (5). The products of this pathway are generally even numbered in carbon atoms (except for the methyl ketones).

(a) Malonyl-ACP

CO + ACP-SH s

Vi

/

El (I) R - C H - C O S - A C P (a)

>(II) R - C H . C O - C H - C O S - A C P

2


E2

2

2

E3 (III) R - C H . C H O H - C H - C O S - A C P

>

2

2

>

(IV) R - C H . C H = C H 2

COS-ACP

\ R-CH=CHCH -COS-ACP 2

E4 > R - C H - C H - C H - C O S - A C P (this molecule re-enters the cycle at (a) above to increase the carbon chain by 2 carbons per cycle) 2

2

2

Figure 2. Fatty acid biosynthesis in plants (3). Compound I gives saturated fatty acids; II and III give methyl ketones and secondary alcohols; and IV gives unsaturatedfatty acids, unsaturated aldehydes unsaturated alcohols and γlactones. El acylACP, malonyl-ACP ligase; E2 β-oxoacetyl-ACP.NADH oxidoreductase; E3 D-p~hydoxyacyl-ACP:NAD(P) oxidoreductase; and E4 α,βdehydroacyl-ACP:NADH(NADPH) oxidoreductase.

Amino A c i d Precursors It is obvious that the above mentioned pathways yield straight chain compounds. Straight, branched chain and aromatic volatiles are produced from amino acids. The pathways proposed for the formation of branched chain oxygen-containing volatiles in banana are represented by the mechanism shown in Figure 3. This involves the decarboxylation and deamination of the base amino acid to yield an aldehyde. The aldehyde can be oxidized to an acid or reduced to an alcohol. These acids and alcohols are potentially esterified to form esters.


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Biological Pathways for the Formation of Oxygen-Containing Aroma

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