Food Biochemistry and Food Processing

27 Bakery and Cereal Products 629

sulting in products like succinate, glycerol and ace-
tate (Gobbetti and Corsetti 1996, Condon 1987,
Stolz et al. 1993).

Proteolysis and Amino Compounds

In a sourdough, the flour contributes considerable
amounts of amino acids and peptides; however, in
order to satisfy nutritional requirements of growing
LAB and provide sufficient amino compounds, pre-
cursors, for flavor development, some proteolytic
action is necessary. The LAB have been suspected
as the main contributors of proteinase and peptidase
activities for release of amino acids in sourdoughs
(Spicher and Nierle 1984, Spicher and Nierle 1988,
Gobbetti et al. 1996), although the flour enzymes
may also have considerable input (Hammes and
Gänzle 1998). In addition, lysis of microbial cells,
particularly yeast cells, add to the pool of amino
acids; a stimulant peptide containing aspartic acid,
cysteine, glutamic acid, glycine, and lysine that ap-
pears in the autolytic process ofC. millerihas also
been identified (Berg et al. 1981).Lactobacillus san-
franciscensishas been found to have a regime of
intracellular peptidases, endopeptidase, and pro-
teinase, as well as a dipeptidase and proteinase in the
cell envelope (Gobbetti et al. 1996). Limited autoly-
sis of lactobacillus populations in sourdoughs may
add to the repertoire of enzymes that will release
amino acids from flour proteins, including those
from proline-rich gluten in wheat. Some of the
enzymes have been purified for further characteriza-
tion (Gobbetti et al. 1996), and they express interest-
ing activity levels at sourdough pH and temperatures.
The addition of exogenous microbial glucose oxi-
dase, lipase, endoxylanase,-amylase, or protease in
the production of sourdough with 11 different LAB
cultures showed positive effects on acidification rate
and level for only three cultures, oneLeuconostoc
citreum, oneLactococcus lactissubsp.lactisand one
Lb. hilgardii.Lactobacillus hilgardiiwith lipase,
endoxylanase or-amylase showed increased pro-
duction of acetic acid.Lactobacillus hilgardiiinter-
acted with the different enzymes for higher stability
and softening of doughs (Di Cagno et al. 2003).
Recent work withLb. sanfranciscensis,Lb. brevis,
andLb. alimentariusin model sourdough fermenta-
tions showed, by using two-dimensional electrophor-
esis, that 37–42 polypeptides had been hydrolyzed.
The polypeptides varied over wide ranges of pIs and

molecular masses, and they originated from albu-

min, globulin, and gliadin, but not from glutenin. Free

amino acid concentrations increased, in particular

those of proline and glutamic and aspartic acid. Pro-

teolysis by the lactobacilli had a positive effect on

the softening of the dough. A toxic peptide for celiac

patients, A-gliadin fragment 31–43, was degraded by

enzymes from lactobacilli. The agglutination of hu-

man myelogenous leukemia–derived cells (K562) by

toxic peptic-tryptic digest of gliadins was abolished

by enzymes from lactobacilli (Di Cagno et al. 2002).

Volatile Compounds and Carbon Dioxide

Both yeasts and LAB contribute to CO 2 production

in sourdough products, but the importance of the

two varies. In bread production with only the (natu-

ral) sourdough microflora, the input from LAB may

even be decisive for leavening because the counts

and kinds of yeast may not be optimal for gas pro-

duction. Relatively low temperature (e.g., 25°C) and

low dough yield (e.g., 135) would select for LAB

activities and less yeast metabolism. More complete

volatile profiles were obtained at higher tempera-

tures (e.g., 30°C) and with a more fluid dough. Of

course increasing the leavening time may give sub-

stantially richer volatile profiles (Gobbetti et al.

1995). If baker’s yeast, S. cerevisiae, is added to op-

timize and speed up the production process, the con-

tribution from yeasts will dominate (Gobbetti 1998,

Hammes and Gänzle 1998).

Bread made with chemical acidification without

fermentation starter failed in sensory analysis. This

indicates that fermentation with yeasts and LAB is

important for good flavor, although high quality raw

materials and proofing and baking are also decisive

factors. Flavor compounds distinguishing the differ-

ent metabolic contributions in sourdough are as fol-

lows (Gobbetti 1998):

• Yeast fermentation (alcoholic):2-methyl-1-
  propanol, 2,3-methyl-1-butanol


• LAB homofermentative:diacetyl, other carbonyls


• LAB heterofermentative: ethyl acetate, other
  alcohols and carbonyls.

Antimicrobial Compounds from Sourdough

LAB

The primary antimicrobial compounds produced by

sourdough LAB are lactic and acetic acid, diacetyl,