2
Carbohydrates

2.1 Introduction

Carbohydrates are the most abundant class of biological molecules on Earth. Colloquially, “sugar” is used as either a synonym for carbohydrate or to refer to sucrose (table sugar, a disaccharide composed of glucose and fructose). In food chemistry and in this text, the term sugar will refer to low molecular weight carbohydrates, especially those with a sweet taste (e.g. glucose, sucrose).

Sugars are biologically important because they represent a water‐soluble energy source. In most organisms glucose is used as a major energy substrate as part of glycolysis, and in humans and most other animals, glucose is used to circulate energy throughout the body. In grapes and many other plants, sugars are transported in the form of sucrose, and will accumulate in the grape berry during ripening in the form of glucose and fructose (Chapter 20). These sugars are the primary substrate for yeast to produce ethanol during alcoholic fermentation (Chapter 22.1) and sugars in wine can contribute to perceived sweetness. Higher molecular weight polymers of sugars (polysaccharides) are major components of the cell walls in grapes and other plants. Finally, covalent bonding between non‐sugar compounds (aglycones) and sugars to form glycosides will result in increased solubility and other chemical changes to the original compound (Chapter 23.1)

2.2 Nomenclature, representation, and occurrence of sugars

The simplest carbohydrates are the monosaccharides, which include the aldoses (polyhydroxylated aldehydes) and ketoses (polyhydroxylated methyl ketones). The name, carbohydrate, refers to the empirical formula C_x(H₂O)_x of aldoses and ketoses, whose general structures are shown in Figure 2.1.

Figure 2.1 General structures for aldose and ketose sugars

These sugars can be further distinguished by the following properties:

• Carbon number. Pentoses (5 carbons) and hexoses (6 carbons) are the most common in wine.
• Fermentability. Sugars may be distinguished by whether they can be fermented by wine yeast under ordinary circumstances.
• Derivatives. The basic monosaccharide structure may be modified by reduction, oxidation, etc.

In grapes, the most quantitatively important sugars are hexoses: fructose (a ketose) and glucose (an aldose). These compounds are reducing sugars, that is, they are capable of reducing Cu(II), Fe(III), and other transition metals under alkaline conditions. The reaction involves oxidation of the aldehyde group of aldoses, but ketoses are also measured because they can isomerize to form aldoses under basic conditions. Importantly, many oligosaccharides (e.g., sucrose) lack a reactive carbonyl group and will not be included in measurements of reducing sugars.

In aqueous solution, hexoses and pentoses predominantly exist as hemiacetal rings, formed by the reversible intramolecular reaction of the carbonyl group with an –OH group. The resulting forms are referred to as either furanoses (5‐member rings) or pyranoses (6‐membered rings). For each ring form, the hemiacetal group can exist as one of two stereoisomers, referred to as anomers (α‐ or β‐; Figure 2.2). The different forms exist in equilibrium, and at room temperature the most abundant isomers of glucose and fructose are the β‐pyranose forms. The hemiacetal form of sugars can react with alcohols to form acetals (see Chapter 9). The resulting compounds are called glycosides. Glycosidic bonds between the hemiacetal of one sugar ring and the –OH group of another sugar result in the formation of disaccharides, trisaccharides, and larger polysaccharides. Sucrose, for example, is a disaccharide of glucose and fructose. A more complete description of sugar nomenclature can be found in an introductory biochemisty or food chemistry textbook [1].

Figure 2.2 Glucose, a hexose sugar, represented as an open‐chain Fischer projection (left) and as a chair conformation of its pyranose form (right). The C1–C6 numbering system for hexoses is also shown on the left

Typical concentrations of sugars in wine are reported in Table 2.1, and their structures are shown in Figure 2.3. In grapes, fructose and glucose increase during ripening from near‐undetectable before veraison to a total concentration in the range of 180–250 g/kg at harvest (Chapter 20). In the wine industry, it is more common to report total soluble solids (TSS) rather than individual sugars in juices. Typically, TSS measurements are performed by density or refractometry, and are calibrated against sucrose standards. Because sugars represent only 90–95% of soluble solids, these measurements are more accurately described as “apparent sugars”. TSS can be reported in units of Brix, where 1 Brix = 1% w/w soluble solids as sucrose, but may also be reported as Baumé, calculated as Brix ÷ 1.8. Baumé is a rough measure of the “potential alcohol” of a wine: that is, the % v/v ethanol concentration that would be achieved if the juice was fermented to dryness.

Table 2.1 Concentrations and taste properties of major sugars in wine

Sugar or sugar derivatives	Typical concentration in dry wine (g/L)a	Taste threshold in H2O (g/L) [4–6]	Sweet intensity of a 10% w/w solution [5]b	Notes
Sugars
Fructose	0.2–4	1.8–2.4	114	Major grape sugar, hexose
Glucose	0.5–1	3.6–12	69	Major grape sugar, hexose
Sucrose	0–0.2	3.6	100	Disaccharide of glucose and fructose
Arabinose	0.5–1	2.5		Non‐fermentable pentose, component of glycosides and pectin
Galactose	0.1	9.0		Isomer of glucose, component of pectin
Rhamnose	0.2–0.4			Deoxy sugar Forms glycosides
Sugar alcohols
Glycerol	7–10	5.2–7.7		Fermentation metabolite
Mannitol	0.01–0.05	7.3	69	Indicative of grape rot or wine spoilage
Arabitol		6.5
Sorbitol (glucitol)	0–0.05	6.2	51
Inositols, total	0.2–0.7	3.2
Sugar acids
Gluconic acid	Up to 2c		Sour	Indicative of grape rot
Galacturonic acid	0.1–1		Sour/astringent	Major component of pectin
2‐Oxogluconic acid	Up to 0.1c			Indicative of grape rot

^a Sugars and sugar alcohol concentrations compiled from multiple sources from Reference [7]. Data on sugar acids compiled from Reference [8].

^b Reference is sucrose = 100.

^c High values in grapes afflicted by rot.

Figure 2.3 Structures of major sugars and sugar derivatives found in wine

During alcoholic fermentation, hexose sugars are largely converted to ethanol and CO₂, but some residual sugar (RS) will still be detectable in wines. Potential sources of residual sugars in finished wines include:

• Incomplete fermentation, either because fermentation is stopped or because sugars are non‐fermentable (e.g. arabinose, xylose)
• Back‐sweetening of wine with sucrose, grape must, or other sources after fermentation is complete
• Hydrolysis of glycosides during storage (Chapter 23.1)
• Extraction from oak [2]

The species included in RS measurements will vary with methodology. For example, enzymatic methods will generally only measure fructose, glucose, and possibly sucrose. Methods based on copper reduction assays will include other reducing sugars (e.g., arabinose), but not sucrose. Typical dry table wines will have residual sugar concentrations between 1 and 4 g/L, but sweet wines may have greater than 100 g/L (Chapter 19). Because yeast are glucophilic, fructose is usually at higher concentrations than glucose [3], although this difference will be obscured by back‐sweetening. Several other monosaccharides are present in wine at concentrations ranging from 0.1 g/L to 1 g/L, including arabinose, galactose, and rhamnose (Table 2.1). Most of these sugars cannot be fermented by wine yeast strains.

2.3 Physical, chemical, and sensory properties of sugars

Sugars are hydrophilic due to their large number of –OH groups (Figure 2.2). Monosaccharides are soluble in both ethanol and water, but solubility (particularly in ethanol) decreases with increasing chain length [1]. Sugars can participate in a range of reactions in aqueous solutions, including wine.

Enzymatic reactions of sugars in grape must or non‐stabilized wine include:

• Glycolysis – the metabolism of sugars (Chapter 22.1)
• Enzymatic oxidation to form sugar acids or enzymatic reduction to form sugar alcohols
• Enzymatic hydrolysis of glycoside bonds, for example, hydrolysis of sucrose to yield glucose and fructose by invertase.

Important non‐enzymatic reactions in finished wine include:

• Acid hydrolysis of glycoside bonds – for example, sucrose added to wine will be ~50% hydrolyzed to fructose and glucose when stored at room temperature for two months at pH 3.0 [9].
• Sugar degradation – under acidic conditions, sugars can be degraded to furanic carbonyl compounds (furfural, 5‐(hydroxymethyl)furfural (HMF), sotolon) and related products, particularly at higher temperatures (Chapter 25).
• Carbonyl chemistry – the carbonyl group of reducing sugars exists primarily as a hemiacetal and thus is less available to react than analogous carbonyls discussed in Chapter 9. Nonetheless, the carbonyl group can react with bisulfite to yield an adduct (Chapter 17). Reactions with amine groups to yield browning products, as occurs in the thermal processing of many foods (Maillard reactions) is expected to be negligible due to the low pH and low amino acid concentrations of wine (Chapter 5).

The notable flavor property of most sugars is sweetness (Table 2.1). Most monosaccharides have sweet detection thresholds in the range of 10–50 mM (0.2–1.0% w/w). Of the sugars found in wine, the most potent is fructose, which is almost twice as sweet as glucose and 15% sweeter than sucrose at a concentration of 10% w/w [5]. The sweet taste of sugars will increase perception of body and mask other taste and tactile sensations like sourness, bitterness, astringency, and pungency, and vice versa [10]. For example, increasing the sugar concentration of a citric acid solution results in a decrease in perceived acidity [11]. Similar observations have been made in wine reconstitution and omission studies near the sensory thresholds of sugars, in which increasing the concentration of sugars decreases astringency and sourness [6]. The sweetness threshold of sugars also provides a rationale for labeling or classifying wines as “dry” versus “sweet” based on quantitative analysis of residual sugars. For example, in EU regulations, “dry” (or its translation) indicates <4 g/L of residual sugar, corresponding roughly to the detection threshold for sugars in wines.

Sugars are non‐volatile and have no aroma – the sweet caramel aroma of cooked sugar is due to degradation products like sotolon, discussed later (Chapter 9). However, the presence of sugar has well‐known cognitive effects on non‐taste attributes of food flavor. For example, in coffee, addition of sweeteners increases the retronasal intensity of “caramel,” while decreasing “roasty” and “coffee” flavors [12]. The reverse can also happen, where sweet aromas can increase perception of sweetness by taste [13]. This phenomenon of combining information from two different sensory modalities is referred to as synesthesia [14]. High sugar concentrations can also have quantifiable effects on the volatility of odorant compounds, by decreasing the availability of water for solvation (“salting‐out”). The magnitude of these effects are usually small, with <20% increase in volatility observed in a 15% w/v sucrose solution versus water for three representative odorants (isoamyl acetate, ethyl hexanoate, eugenol) [15].

2.3.1 Sugar alcohols

Polyalcohols (polyols) describe aliphatic compounds with multiple –OH groups and no other functional groups (Figure 2.2). The term is used synonymously with sugar alcohols in some texts, since they are often derived from sugars and usually have a sweet taste. The major class of sugar alcohols in wine are the alditols, which are formed by reduction of the sugar carbonyl group to an alcohol during fermentation. They are named by replacing the –ose suffix of the corresponding aldose sugar with –itol.

The major sugar alcohol in found in wine is glycerol, which is usually the most abundant compound in dry wines after water and ethanol. Glycerol is common to all alcoholic fermentations, where it has several important physiological functions as an osmoregulant and in redox balancing (Chapter 22.1). Glycerol concentrations are usually in the range of 7–10 g/L [16], but may be over 15 g/L in high sugar fermentations like ice wines [17]. Other alditols, such as mannitol, arabitol, and sorbitol, are present in the range of 20–300 mg/L (total < 1 g/L, Table 2.1). Higher concentrations of sugar alcohols may be a sign of microbial spoilage; for example, high concentrations of mannitol formed by enzymatic reduction of fructose are indicative of lactic spoilage [18]. Other quantitatively important polyalcohols are 1,2‐propanediol (propylene glycol) and 2,3‐butanediol, formed by reduction of lactic acid and acetoin, respectively. Another class of polyalcohols found in wines is the inositols (cyclitols), which have perhydroxylated cyclohexane rings. The inositols are components of phosphoinositols, used by yeast and other eukaryotes in cell membranes and signal transduction. One major (myo‐) and two minor (scyllo‐ and chiro‐) inositols are observed, and typical total concentrations are ~300 mg/L [19].

Sugar alcohols are chemically and microbiologically stable under reductive wine conditions. Sugar alcohols lack a reactive carbonyl group, and thus will not participate in electrophilic reactions as is observed with sugars. Sugar alcohols do not appear to be metabolized by commercial yeast or LAB strains under normal wine conditions, although some rare spoilage LAB can degrade glycerol [18].

Sugar alcohols have about 50% of the sweet intensity of sucrose on a w/w basis (Table 2.1). In wine, only glycerol is usually present at concentrations above its taste threshold (5.2 g/L) [4]. Historically, glycerol was implicated as an important contributor to the mouthfeel of a wine, but the actual impact on mouthfeel appears to be minor. For example, addition of >25 g/L of glycerol to a model wine was necessary to cause a detectable change in mouthfeel [4], and in a non‐targeted study on white wines, no correlation was observed between glycerol and wine body [20]. Although these results cast doubt on the impact of glycerol in isolation, a reconstitution study on dry red wines observed that elimination of all alditols (glycerol, sorbitol, mannitol, and others) results in a significant decrease in wine mouthfulness/body [6].

2.3.2 Sugar acids

Aldonic acids have a carboxylic acid group in place of the aldehyde group and are named by replacing the –ose suffix of the corresponding aldose sugar with –onic acid, for example, glucose and gluconic acid. Uronic acids have a carboxyl acid group in place of the terminal hydroxyl group. Uronic acid derivatives of aldoses are named by replacing the –ose suffix of the corresponding sugar with –uronic acid, e.g. glucose and glucuronic acid. Confusingly, uronic acid derivatives of ketoses are named as oxo derivatives of aldonic acids, for example, 2‐oxogluconic acid. Uronic acids possess an electrophilic aldehyde group and at high concentrations can contribute to SO₂ binding (Chapter 17). These sugar acids (Figure 2.2, Table 2.1) are generally formed by enzymatic oxidation and high concentrations, particularly of gluconic acid, are associated with grape spoilage organisms such as Gluconobacter [21]. One exception is galacturonic acid, which is typically present at several hundred mg/L in dry wines, and is likely formed by pectin hydrolysis during or after fermentation.

The impact of uronic and aldonic acids in wine is poorly studied. They will contribute to titratable acidity (pK_a = ~3.5), but at typical concentrations found in wine (<0.5 g/L as tartaric acid equivalents) they are likely to be of negligible sensory importance. Although higher concentrations are found in wines produced from grapes compromised by Acetobacter and Gluconobacter, it is likely that other off‐flavors (e.g. volatile acidity) will have a more noticeable role.

2.4 Polysaccharides

Polysaccharides are polymers of carbohydrates. The major polysaccharides in grapes are structural components:

• Cellulose, a water‐insoluble β‐(1 → 4) polymer of d‐glucopyranose
• Pectin, a water‐soluble heteropolymer that consists primarily of α‐(1 → 4) d‐galacturonic acid linkages
• Hemicellulose, a diverse class of heteropolymers related to pectin that consists of xylose, arabinose, galactose, glucose, and several other sugars.

In wine, polysaccharides may be derived from both grape and yeast cell walls (Table 2.2). Because of their polymeric nature, polysaccharides are not ordinarily characterized as individual molecules. Instead, it is common to isolate and quantify fractions based on size (e.g., by gel permeation chromatography) or charge (ion exchange chromatography). The fractions may then be hydrolyzed to determine the constituent sugars [22].

Table 2.2 Typical concentrations and sources of major polysaccharides in wine. Adapted from Reference [23]

		Concentration (mg/L)
Class	Source	Red wine	White wine	Typical MW (kDa)	Charge
Rhamnogalacturonans (RG‐I and ‐II)	Grape	50–250	10–50	10	Acidic
Arabinogalactan proteins (AGPs)	Grape	100–150	50–150	100–250	Neutral
Mannoproteins	Yeast	100–150	100–150	50–500	Neutral

Grape polysaccharides can decrease yield and extraction during winemaking (Chapters 19 and 21) and decrease the efficiency of filtration (Chapter 26.3), and winemakers will often use carbohydrase enzymes to decrease these impacts. Grape polysaccharides will be further degraded by microbial enzymes during fermentation and are not efficiently transferred to wine during fermentation either due to degradation or poor solubility.

Two exceptions are rhamnogalacturonans I and II (RG‐I and RG‐II), pectin fragments that contain a high degree of branching from several classes of sugar residues. Arabinogalactan proteins (AGPs) consist primarily of arabinose, galactose, and glucuronic acid, and also contain about 10% of hydroxyproline‐rich protein. Both appear to be less susceptible to hydrolysis than other pectic or hemicellulosic substances. The major yeast‐derived polysaccharides are mannoproteins, the primary constituents of yeast cell walls. They are released during yeast autolysis and thus are expected to be at higher concentrations in wines aged on lees. They consist primarily of mannose, with smaller amounts of protein. More detailed information on polysaccharide composition can be found elsewhere [22, 23].

Polysaccharides are widely used in the food industry to increase perceived viscosity and mouthfeel in beverages, although typically at higher concentrations than those in wine [1]. The effects of wine polysaccharides on mouthfeel and other flavor properties are an active area of investigation. Addition of isolated polysaccharide fractions to model wine are reported to slightly increase body, and the acidic RG‐II fraction also decreased perceived astringency [24]. Polysaccharides may also have a modest indirect effect on wine aroma by decreasing volatility of some odorants [25], although the effect appears to be small. Mannoproteins can inhibit potassium bitartrate crystallization (Chapters 26.1 and 27) and stabilize the foam of sparkling wines [26]. Finally, some polysaccharides are associated with wine faults. For example, β‐glucans produced by Pediococcus (and other bacteria) can cause a visible fault called “ropiness” (viscous and oily texture) [18] which can also affect mouthfeel and filterability.

References

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Notes

1 Allowances may be made for the effects of acidity on perceived sweetness, such that a high acid wine may have as much as 9 g/L of residual sugar in some EU regions. Also, in most New World countries, use of the term “dry” is not regulated.

2 The salting-out effect of sugars is of minimal significance to wine aroma, but will impact ethanol measurement by the ebulliometry technique, which relies on differences in boiling point in ethanol–water mixtures versus water. The presence of sugars will decrease the boiling point of wine and result in incorrectly high apparent ethanol contents, such that a correction factor must be applied for RS >20 g/L.

3 In other foods, sugar alcohols like sorbitol and mannitol are often used as sweeteners in chewing gums, since they are not fermentable by oral microflora and thus do not promote dental caries. They can also be used as part of low-calorie products because they are poorly absorbed during digestion, although this has the unfortunate side-effect of serving as a laxative.