Food Biochemistry and Food Processing (2 edition)

BLBS102-c33 BLBS102-Simpson March 21, 2012 14:5 Trim: 276mm X 219mm Printer Name: Yet to Come

634 Part 5: Fruits, Vegetables, and Cereals

Sulphite

Sulphate

Sulphite

Glucose

Glucose

Maltose Maltotriose

Sucrose homocysteine

H 2 S H 2 S

Amino acids

Fructose Fructose-6-P

Pyruvate Keto acids

Acetaldehyde

Vicinal

diketones

Amino acids

Fatty acyl CoA

Organic acids Acetyl CoA

Fatty acids

Esters

Ethanol

Higher

alcohols

Lipids

Figure 33.2.Interrelation between yeast metabolism and the

production of bioflavoring by-products.

(aldehydes and vicinal diketones), and sulfur-containing com-

pounds (Hammond 1993). In addition, some speciality beers can

contain important concentrations of volatile phenol compounds

(Vanbeneden et al. 2007).

Biosynthesis of Higher Alcohols

During beer fermentation, higher alcohols (also called “fusel

alcohols”) are produced by yeast cells as by-products and repre-

sent the major fraction of the volatile compounds. More than

35 higher alcohols in beer have been described. Table 33.3

gives the most important compounds, which can be classified

into aliphatic (n-propanol, isobutanol, 2-methylbutanol (or ac-

tive amyl alcohol), and 3-methylbutanol (or isoamyl alcohol)),

and aromatic (2-phenylethanol, tyrosol, tryptophol) higher alco-

hols. Aliphatic higher alcohols contribute to the “alcoholic” or

“solvent” aroma of beer, and produce a warm mouthfeel. The

aromatic alcohol 2-phenylethanol has a sweet rose-like aroma

and has a positive contribution to the beer aroma. It is believed

that this compound masks the dimethyl sulfide (DMS) percep-

tion (Hegarty et al. 1995). The aroma of tyrosol and tryptophol

are undesirable, but they are only present above their thresholds

in some top-fermented beers.

Higher alcohols are synthesized by yeast during fermenta-

tion via the catabolic (Ehrlich) and anabolic pathway (amino

acid metabolism) (Ehrlich 1904, Chen 1978, Oshita et al. 1995,

Hazelwood et al. 2008). In the catabolic pathway, the yeast

uses the amino acids of the wort to produce the correspond-

ingα-keto acid via a transamination reaction. Isoamyl alcohol,

isobutanol, and phenylethanol are produced via this route from

leucine, valine, and phenylalanine, respectively. An outsider in

this pathway is propanol, which is derived from threonine via

an oxidative deamination. The excess oxoacids are subsequently

decarboxylated into aldehydes and further reduced (alcohol de-

hydrogenase) to higher alcohols. This last reduction step also

regenerates NAD+.

Dickinson and coworkers looked at the genes and enzymes,

which are used byS. cerevisiaein the catabolism of leucine

to isoamyl alcohol (Dickinson et al. 1997), valine to isobu-

tanol (Dickinson et al. 1998), and isoleucine to active amyl

alcohol (Dickinson et al. 2000). In all cases, the general se-

quence of biochemical reactions is similar, but the details for

the formation of the individual alcohols are surprisingly differ-

ent. The branched-chain amino acids are first deaminated to the

correspondingα-ketoacids (α-ketoisocapric acid from leucine,

α-ketoisovaleric acid from valine, andα-keto-β-methylvaleric

Table 33.3.Major Higher Alcohols in Beer (Partly Adapted From NN 2000.)

Compound

Flavor Threshold

(mg/L) Aroma or Taste(b)

Concentration Range

(mg/L) Bottom

Fermentation

Concentration Range

(mg/L) Top Fermentation

n-Propanol 600 (c), 800(b) Alcohol 7–19 (12)(a),(f), 6–30(j) 20–45(i)

Isobutanol 100 (c), 80–100(g),

200 (b)

Alcohol 4–20 (12)(f), 6–32(j) 10–24(i)

2-Methylbutanol 50 (c), 50–60(g),70(b) Alcohol 9–25 (15)(a), 12–16(j) 80–140(i)

3-Methylbutanol 50 (c), 50–60(g),65(b) Fusely, pungent 25–75 (46)(a), 30–60(j) 80–140(i)

2-Phenylethanol 5 (a),40(c), 45–50(g),

75 (d), 125(b)

Roses, sweetish 11–51 (28)(f), 4–22(g),

16–42(h), 4–10(j)

35–50(g),8–25(a), 18–45(i)

Tyrosol 10 (a), 10–20(e),20(c),

100 (d), (g), 200(b)

Bitter chemical 6–9(a), 6–15(a) 8–12((g), 7–22(g)

Tryptophol 10 (a), 10–20(e), 200(d) Almonds, solvent 0.5–14(a) 2–12(g)

Sources: (a) Szlavko (1973), (b) Meilgaard (1975a), (c) Engan (1972), (d) Rosculet (1971), (e) Charalambous et al. (1972), (f) Values in

48 European lagers (Dufour unpublished date), (g) Reed and Nogodawithana (1991), (h) Iverson (1994), (i) Derdelinckx (unpublished data),

(j) immobilised cells (Willaert and Nedovic 2006).

aMean value.