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

640 Part 5: Fruits, Vegetables, and Cereals

Hydrogen

Sulphide

Sulphate Sulphate

Adenylyl sulphate

Phospoadenylyl sulphate

Sulphite Sulphite

Hydrogen sulphide

Homocysteine

Methionine

S-adenosylmethionine

Cystathionine

Cysteine

o-Acetyl

homoserine

1

Gluthatione

S-adenosylhomocysteine

CH 3 -THF

CH 2 -THF

THF

Serine

Glycine

CH 3 -X

X

2

3

4

5

6

7

8

9

10

11

12

13

14

Figure 33.4.The remethylation, transulfuration, and sulfur assimilation pathways. Genes and enzymes catalyzing individual reactions are as

follows: 1, sulfate permease; 2, ATP sulfurylase; 3,MET14: adenylylsulfate kinase (EC 2.7.1.25); 5,MET10: sulfite reductase (EC 1.8.1.2); 6,

sulfite permease; 7,MET17:O-acetylhomoserine (thiol)-lyase (EC 2.5.1.49); 8,CYS4: cystathionineβ-synthase (CBS; EC 4.2.1.22); 9,

CYS3: cystathionineγ-lyase (EC 4.4.1.1), 10,MET6: methionine synthase (EC 2.1.1.14); 11,SAM1andSAM2:S-adenosylmethionine

synthetase (EC 2.5.1.6); 12,SAH1:S-adenosylhomocysteine hydrolase (EC 3.3.1.1); 13,SHM1andSHM2: serine hydroxymethyltransferase

(SHMT; EC 2.1.2.1); 14,MET12andMET13: methylenetetrahydrofolate reductase (MTHFR; EC 1.5.1.20). “X” represents any methyl group

acceptor; THF, tetrahydrofolate; CH 2 -THF, 5,10-methylenetetrahydrofolate; CH 3 -THF, 5-methyltetrahydrofolate. (Partly adapted from Chan

and Appling 2003.)

At the end of the primary fermentation and during the matu-

ration, the excess H 2 S is reutilized by the yeast. A warm con-

ditioning period at 10a12` ◦C may be used to remove excessive

levels of H 2 S.

Brewing yeasts produce H 2 S when they are deficient in the

vitamin pantothenate (Walker 1998). This vitamin is a precursor

of coenzyme A, which is required for metabolism of sulfate into

methionine. Therefore, panthothenate deficiency may result in

an imbalance in sulfur amino acid biosynthesis, leading to excess

sulfate uptake and excretion of H 2 S (Slaughter and Jordan 1986).

Sulfite is a versatile food additive used to preserve a large

range of beverages and foodstuffs. In beer, sulfite has a dual

purpose, acting both as an antioxidant and an agent for masking

of certain off-flavors. Some of the flavor stabilizing properties

of sulfite is suggested to be due to complex formation of bisul-

fate with varying carbonyl compounds, of which some would

give rise to off-flavors in bottled beer (Dufour 1991). Especially,

the unwanted carbonyltrans-2-nonenal has received particular

attention, since it is responsible for the “cardboard” flavor of

some types of stale beer. It has been suggested that it would be

better to use a yeast strain with reduced sulfite excretion during

fermentation and to add sulfite at the point of bottling to ensure

good flavor stability (Francke Johannesen et al. 1999). There-

fore, a brewer’s yeast disabled in the production of sulfite has

been constructed by inactivating both copies of the two alleles

of theMET14gene (which encodes for adenylylsulfate kinase).

Fermentation experiments showed that there was no qualitative

difference between yeast-derived and artificially added sulfite,

with respect totrans-2-nonenal content and flavor stability of

the final beer.

The elimination of the gene encoding sulfite reductase

(MET10) in brewing strains ofSaccharomycesresults in in-

creased accumulation of SO 2 in beer (Hansen and Kielbrandt

1996a). The inactivation ofMET2resulted into elevated sulfite