8
Isoprenoids

8.1 Introduction

The term isoprenoid encompasses a diverse and complex range of hydrocarbons and their oxygenated derivatives (cf. terpenoids, which applies to oxygenated terpenes, although the terms are often used synonymously) based on the repeated presence of the C₅ isoprene unit (2‐methylbuta‐1,3‐diene). The carbon skeleton may consist of simple inclusion of isoprene units, giving rise to compounds that are multiples of C₅ (Figure 8.1), or there may be deviation from the basic addition of C₅ units through loss or shifting of a fragment, such as a methyl group. Isoprenoids can exist as saturated/unsaturated and cyclic/acyclic hydrocarbons, and can contain alcohol, aldehyde, ketone, ester, ether, and acetal functionalities. Volatile isoprenoids are widespread aroma and flavor compounds produced enzymatically by different plants as secondary metabolites via the terpenoid pathway [1, 2].

Figure 8.1 Representation of isoprenoid biosynthetic pathway from activated isoprene units (DMAPP and IPP), giving rise to a diverse range of natural products based on this C₅ structural unit

8.2 General chemical and sensory properties of isoprenoids

Of importance to wine aroma are the isoprenoids classified as monoterpenoids (C₁₀ compounds), sesquiterpenoids (C₁₅ compounds), and C₁₃‐norisoprenoids (C₁₃ compounds derived from tetraterpenoids¹) (Table 8.1). Many of these impart desirable aromas and show dependence on grape variety. Isoprenoids tend to be very non‐polar compounds, although oxygenation increases water solubility compared to the hydrocarbon variants (e.g., the monoterpene myrcene has log P = 4.3 and water solubility = 0.004 g/L, whereas its alcohol derivative geraniol has log P = 3.6 and water solubility = 0.4 g/L). While aliphatic hydrocarbon forms are usually not highly reactive, the presence of different functional groups (i.e., alkenes, alcohols, ketones) can increase reactivity and allow for the formation of new aroma compounds (e.g., see Figure 8.2).

Table 8.1 Indicative odor descriptors, detection thresholds, and concentration ranges for classes of isoprenoids found in wine [3–18]

Examplesa	Structureb	Odor descriptorc	Threshold (μg/L)c	Ranged,e (μg/L)
Monoterpenoid
Wine lactone		Coconut, woody, sweet	0.01	0.1f
(–)‐cis‐Rose oxide		Floral, green, rose	0.2	Trace–22
1,8‐Cineole (eucalyptol)		Eucalyptus, fresh	1.1	ND–33
Linalool		Floral, citrus	25	ND–370
Geraniol		Floral, citrus	30	ND–290
Citronellol		Rose, citrus	100	1–50
Hotrienol		Floral, citrus	110	3–240
α‐Terpineol		Lilac	250	ND–400
Nerol		Floral, green	400	ND–360
Sesquiterpenoid
(–)‐Rotundone		Black pepper	0.016	Trace–0.56
Nerolidol		Floral, apple, green	10	ND–29
Farnesol		Floral, rose	20	ND–180
C13‐Norisoprenoid
β‐Damascenone		Cooked apple, quince, floral	0.050	0.3–45
β‐Ionone		Violet, wood, raspberry	0.090	ND–18
1,1,6‐Trimethyl‐1,2‐dihydronapthalene (TDN)		Kerosene, petrol	2	ND–54

^a Common names are widely used in the literature since the systematic names are more complex; for example, geraniol is (E)‐3,7‐dimethyl‐2,6‐octadien‐1‐ol and β‐damascenone is (E)‐1‐(2,6,6‐trimethyl‐1‐cyclohexa‐1,3‐dienyl)but‐2‐en‐1‐one.

^b Stereochemistry is shown for compounds that have data reported for individual stereoisomers. Note that other compounds with chiral centers also exist as stereoisomers but are not always studied individually and are therefore reported together as a single component.

^c Descriptor and threshold data may refer to matrices such as air, water, hydroalcoholic solution, and white or red wine.

^d Trace, present but not quantified.

^e ND, not detected.

^f Determined in a study of two wines.

Figure 8.2 Acid‐catalyzed reactions of monterpenoids through carbocation intermediates, showing (a) hydrolysis of geraniol and subsequent rearrangement to form other monoterpenoids, including linalool, α‐terpineol, and nerol, and (b) formation of 1,8‐cineole from grape‐derived linalool through hydrolysis and intramolecular cyclization

Isoprenoids are often present in water‐soluble bound forms (i.e., non‐volatile glycosides) in the grape berry, many of which can be hydrolyzed under mildly acidic wine conditions to release the aglycone (Chapter 23.1). Some glycosides are not hydrolyzed effectively at wine pH (e.g. unactivated alcohols such as citronellol), and harsher conditions, including lower pH and higher temperature, are required. In contrast, enzymatic hydrolysis of glycosides (i.e., glycosidases from yeast or commercial preparations) under normal winemaking conditions yields the corresponding volatile compounds (Chapter 23.1), which may then undergo acid‐catalyzed rearrangement during aging at wine pH (Chapter 25) to afford different aroma compounds, potentially altering the aroma profile of a wine as a result.

8.3 Monoterpenoids

Characteristics of key monoterpenoids in wines are shown in Table 8.1. This class of isoprenoids is typically associated with white wines produced from aromatic Muscat grape varieties, where concentrations may exceed threshold by 100‐fold [8]. Monoterpenoids are also found at suprathreshold concentrations in aromatic non‐Muscat cultivars such as Traminer and Riesling, and at subthreshold concentrations in neutral varieties including Cabernet Sauvignon, Merlot, Chardonnay, and Sauvignon Blanc [8]. The grape‐derived monoterpenoids of greatest significance to wine aroma include linalool and geraniol (“floral” character of most Muscat varieties) and (–)‐cis‐rose oxide (which contributes to a “lychee” aroma in Gewurztraminer [19]); the impact of selected grape varieties on wine concentrations of these three aroma compounds is highlighted in Table 8.2. Wine lactone, found in wine as a single stereoisomer out of eight possibilities [20], may also be an important contributor to the aroma of varietal wines [21, 22], although surveys of typical wine concentrations are absent in the literature. With descriptors such as “coconut” and “woody”, wine lactone was reported at suprathreshold concentrations in Scheurebe and Gewurztraminer wines [6], and potentially arises from acid‐catalyzed cyclization of grape‐derived acid or glucose ester forms [23] (Chapter 25). Citronellol, nerol, α‐terpineol, and dozens of other monoterpenoids (and some monoterpenes such as limonene, p‐cymene, γ‐terpinene, and myrcene²) have also been identified in grape musts and wines [5], although often at concentrations well below their sensory thresholds. Note that monoterpenoids can be found as free volatile compounds in grapes, but largely exist as glycosylated (up to 95% of the total) or polyhydroxylated precursors (Chapter 23.1), both of which are odorless.

Table 8.2 Effect of grape variety on typical concentrations of linalool, geraniol, and (–)‐cis‐rose oxide in wines [6, 14, 18, 19, 24–26]

Grape varietal	Concentration range in wine (μg/L)
	Linaool	Geraniol	cis‐Rose oxide
Muscat	78–462	44–256	ND
Gewurztraminer	60–225	45–221	8–21
Riesling	NDa–230	ND–109	ND
Sauvignon Blanc	10–58	ND–6	NRb

^a ND, not detected.

^b NR, not reported.

Apart from the major impact of cultivar, growing conditions may have a slight effect on monoterpenoids in grapes. Beyond that, absolute or relative concentrations in wine will be influenced by extraction from grapes and transformations of precursors or free volatiles during fermentation and storage [5, 8]. The extent of precursor hydrolysis and rearrangement is affected by temperature, ethanol content, and pH, as described further in Chapters 23.1 and 25. Briefly, factors that can influence wine aroma through changes to monoterpenoid profile include:

• Extraction. Extended skin‐contact, harder pressing of grape musts, increased temperature and use of pectolytic enzymes typically increase extraction of free and bound monoterpenoids.
• Enzymatic transformations. Microorganisms are capable of hydrolyzing monoterpenoid glycosides, as can exogenous glycosidases added during winemaking. Additionally, the fermentation process may result in enzymatic reduction of alkenes and acetylation of alcohols.
• Chemical transformations. Acid‐catalyzed hydrolysis or rearrangement of precursors yields a range of monoterpenoids,³ which may undergo further reactions at ordinary winemaking pH and temperature (Figure 8.2a). Lower pH and elevated temperature (e.g., from flash pasteurization or during storage) will accelerate these reactions.⁴

Most monoterpenoids in wine are derived from grape secondary metabolites, but 1,8‐cineole (eucalyptol, Table 8.1) can have an alternate origin. Predominantly limited to red wines, 1,8‐cineole also happens to be one of the principal essential oils in Eucalyptus leaves. It could be envisaged to form through acid‐catalyzed cyclization of α‐terpineol, which arises from linalool (or the monoterpene limonene) (Figure 8.2b); transformation of such compounds can occur under acidic model wine conditions to yield small amounts of 1,8‐cineole [13]. A slightly more important pathway appears to be airborne transfer of volatiles from Eucalyptus trees growing in the vicinity of grapevines [16], as previously suggested [27]. However, the major source of 1,8‐cineole in red wines appears to be from Eucalyptus leaves and other matter (i.e., material other than grapes (MOG), such as grape vine leaves and stems) being present during red wine fermentations [16] (Figure 8.3). Studies such as these highlight the need to consider alternative (i.e., environmental) sources of wine aromas that are not definitively linked to grape and yeast metabolites or maturation and storage phenomena (beyond the known exogenous contaminants, Chapter 18).

Figure 8.3 1,8‐Cineole concentrations in (a) Shiraz wines made either without skin contact (i.e., rosé style), with maceration on skins (control), and maceration with the addition of small proportions of grapevine leaf and stem (grape leaf/stem) or Eucalyptus leaves and bark (Euc leaf), and (b) extracts from Shiraz grapes, grapevine leaves, and grapevine stems sampled from a vineyard at increasing distances from a stand of Eucalyptus trees, with Row 60 being furthest away.

Data from Reference [16]

8.4 Sesquiterpenoids

A number of sesquiterpenoids have been identified in wine as free volatile compounds, including farnesol, nerolidol, and rotundone (Table 8.1). Presumably hydroxylated variants exist in glycosidic forms, based on their presence in other plants [28], an assumption supported by the increase in concentration of farnesol during malolactic fermentation or hydrolysis of grape glycosidic extracts [29]. However, in contrast to other isoprenoid secondary metabolites, no sesquiterpenoid glycosides have been identified to date. Sesquiterpenes such as α‐cedrene, α‐farnesene, α‐ylangene, and α‐guaiene have also been found in wine but are not studied in depth due to their low concentrations with respect to sensory thresholds. Nonetheless, as is common in the plant kingdom, a wide range of functional sesquiterpenes and related compounds are biosynthesized in grapes (and by yeast to some extent) in cultivars such as Cabernet Sauvignon, Shiraz, Riesling, Gewurztraminer, and some Portuguese varieties (e.g., References [30] to [32]).

From a flavor perspective, the primary aroma compound, rotundone, is the most notable sesquiterpenoid found in wine. Rotundone possesses a characteristic “black pepper” aroma and a low odor detection threshold (16 ng/L) and was first identified in the essential oil of nut grass (Cyperus rotundus, hence the name given to this bicyclic ketone). It is also found in high concentrations in black and white pepper and at lower levels in various herbs [33]. Rotundone was first identified in Shiraz wines (up to 145 ng/L) [33] and has since been detected in many other red wines, such as Cabernet Sauvignon, Durif, Mourvedre, Vespolina, and Schioppettino (up to ca. 560 ng/L in the latter two varieties). It may also be found in white wines, particularly Gruner Veltliner (up to 266 ng/L) [34].

Beyond grape variety, rotundone concentrations increase with grape maturation, and higher concentrations are correlated with cooler viticultural regions and cooler vintages; analogously, shading and cooler berry temperatures are reported to lead to higher rotundone, even within the same cluster [35, 36]. Similar to flavonoids (Chapter 11), rotundone is located primarily in the grape skins, which explains the higher concentrations in red wines. Rotundone is highly hydrophobic, and ~90% is bound to lees and marc, with further losses occurring during filtration operations [37]. Inclusion of grape leaves and stems can increase rotundone by up to 6‐fold as compared to fermentation of grapes alone [16]. This reinforces the notion presented above (for 1,8‐cineole) that MOG (and non‐grapevine material) included in a fermentation can have important influences on wine aroma.

8.5 C₁₃‐Norisoprenoids

An array of C₁₃‐norisoprenoids have been found in all of the internationally important varietal wines, for example, Chardonnay, Riesling, Semillon, Sauvignon Blanc, Cabernet Sauvignon, Pinot Noir, and Shiraz [11]. From a sensory standpoint, the more important contributions to wine aroma appear to be β‐damascenone, β‐ionone, and TDN (Table 8.1), although several others (vitispirane, Riesling acetal, actinidols, and (E)‐1‐(2,3,6‐trimethylphenyl)buta‐1,3‐diene (TPB)) have also been detected. More generally, norisoprenoids (especially C₁₃ derivatives; C₉–C₁₁ also exist) are ubiquitous flavor and fragrance compounds, and arise from the enzymatic or chemical breakdown of carotenoid pigments (C₄₀) such as neoxanthin and β‐carotene [38], either directly or through intermediate glycosides.

As is a common theme for the isoprenoid class of compounds, the acidic nature of grape and wine matrices (and the presence of enzymes/act of fermentation) means that significant transformations of non‐volatile carotenoid‐derived precursors take place to yield compounds that then impact on wine aroma. Formation of β‐damascenone, for example, involves oxidative cleavage of neoxanthin in grapes to yield grasshopper ketone, its subsequent enzymatic reduction to an allenic triol intermediate, and finally acid‐catalyzed hydrolysis steps during winemaking (potentially of glycosylated intermediates as well, Chapter 23.1) to produce β‐damascenone (Figure 8.4a). In contrast, β‐ionone can arise directly in grapes after oxidative cleavage of β‐carotene (Figure 8.4b). Carotenoid cleavage dioxygenases (CCDs) are the enzymes responsible for the regiospecific oxidative degradation of a range of carotenoids, leading to different norisoprenoid and polyene primary cleavage products [1, 11, 39].

Figure 8.4 Formation of C₁₃‐norisoprenoids through oxidative degradation of carotenoid (a) neoxanthin to form β‐damascenone after reduction (designated as [H]) and acid‐catalyzed hydrolysis (Chapter 23.1) and (b) β‐carotene to give two molecules of β‐ionone directly (whereas α‐carotene gives one molecule of α‐ionone and one of β‐ionone)

Of the C₁₃‐norisoprenoids, the best studied are TDN and β‐damascenone. TDN has an aroma reminiscent of “kerosene” and a sensory threshold of 2 μg/L (Table 8.1). It has been detected in several varietal wines, such as Chardonnay, Sauvignon Blanc, Pinot Noir, and Cabernet Sauvignon at near‐threshold concentrations (e.g., mean = 1.3 μg/L in non‐Riesling wines, maximum = 6.4 μg/L in a Cabernet Franc) [14]. However, TDN is best associated with Riesling wines and particularly aged Rieslings, where it can be present at concentrations over 50 μg/L and dominate the aroma of the wine. The sensory effects of lower concentrations of suprathreshold TDN, as are found in younger Riesling wines (e.g., mean = 6.4 μg/L, maximum = 17.1 μg/L) are not clear [14], but these concentrations do not appear to lead to consumer rejection [40]. Apart from the grape variety, the factors that affect TDN are:

• Wine age, pH, and storage temperature. TDN is at negligible concentrations in grapes, but can instead be formed by acid‐catalyzed hydrolysis of compounds such as Riesling acetal and glycosidic precursors during storage [11, 14]. Older wines, wines with lower pH, and those stored at elevated temperatures exhibit higher concentrations of TDN (Chapter 25).
• Growing conditions. Cluster light exposure and warmer growing conditions can result in 2–4‐fold increases in TDN precursors. The biological mechanism for this effect is still unresolved, but may be related to altered carotenoid profiles or carotenoid degradation.
• Absorption. Once formed, TDN is stable and may accumulate to concentrations above 50 μg/L over time. However, being a highly non‐polar compound, it may be lost due to scalping by non‐polar packaging materials⁵ (i.e., absorption of aroma volatiles by cork or synthetic closures [41]).

β‐Damascenone (“cooked apple”) is widely detected in wines, and unlike TDN its concentration does not appear to be dependent on variety [42]. Although β‐damascenone has a very low sensory threshold in water and is often reported to have the highest odor activity of any compound in wine, it is not an impact odorant and its aroma is rarely if ever the dominant sensation perceived in a wine [42]. Instead, it appears to modify the perception of other odorants – low concentrations of β‐damascenone can enhance fruitiness associated with esters while suppressing green characters from methoxypyrazines [43]. β‐Damascenone also possesses a highly matrix‐dependent sensory threshold – its detection threshold in a red wine is reported to be over 3 orders of magnitude higher than its threshold in water [44]. While true of all compounds, the statement that OAVs merely act as a guide to the impact of an odorant (see the Introduction chapter) is of particular relevance to β‐damascenone.

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Notes

1 More specifically, C₄₀ isoprenoid pigments known as carotenoids are cleaved to yield a range of structurally diverse compounds collectively termed apocarotenoids (i.e., norisoprenoids) differing in carbon chain length and functionality. Such compounds include the phytohormone abscisic acid (C₁₅), vitamin A (C₂₀ retinoids), and volatile compounds with C₉, C₁₀, C₁₁, and C₁₃ carbon skeletons. Volatile norisoprenoids constitute an important group of natural aroma and flavor molecules, of which the C₁₃-norisoprenoids are the most widely abundant compounds derived from carotenoids.

2 The non-polar nature of these monoterpene hydrocarbons would limit extraction during winemaking and there may be losses due to volatilization or binding to solids.

3 Products of acid hydrolysis can be different than those derived from hydrolysis by glycosidases, with the latter tending to better preserve the natural monoterpenoid profile of the grapes.

4 In a similar way, isolation conditions employed during analysis can contribute to reactions that alter the apparent distribution of monoterpenoids determined in wine.

5 This is also the case for other non-polar compounds, such as naphthalene and long-chain ethyl esters, which are absorbed by cork and especially synthetic closures, but not screw caps. More polar compounds, such as oak volatiles, short-chain ethyl esters, and methoxypyrazines, tend not to be affected by closure type. The absorption of non-polar aroma compounds on hydrophobic surfaces would appear to be a generally applicable phenomenon in winemaking.