Food Biochemistry and Food Processing

27 Bakery and Cereal Products 633

“sweet bushera.” After 2–4 days, the product be-
came sour and alcoholic and was consumed by adults.
However, the sweet bushera showed very high
counts of coliforms and had a reputation for causing
diarrhea (Muyanja 2001). Clearly, the development
of defined starter cultures would improve the safety
of this and similar products.
Some recent examples of studies on the micro-
bial flora of spontaneously fermented cereals are
shown in Table 27.3. For each product, several dif-
ferent types of organisms have been isolated. In
other words, a specific product is not produced from
fermentation by a specific organism or organisms.
Lb. plantarumseems to be the most commonly iso-
latedLactobacillusspecies in fermented cereals. In
addition, heterofermentative LAB such as leuconos-
tocs, Lb. brevis, and Lb. fermentum frequently
occur. Yeasts are always present in spontaneously
fermented products, but few studies have character-
ized the predominating species. However, Jespersen
(2003) reported thatS. cerevisciaeis the predomi-
nant yeast in many African fermented foods and
beverages.
All spontaneously fermented products contain, or
have contained, many different types of microorgan-
isms. These have grown in the product and will have
metabolized some of the cereal components, thereby
making a contribution (positive or negative) with
their metabolites to the overall sensory characteris-
tics of that product. However, studies on sponta-
neously fermented products have focused on LAB
and yeasts since these organisms are often associat-
ed with other, better known, fermented products and
have a history of safe use in food. Stanton (1998)
proposed that the nature of the substrate (raw mate-
rial) and the technology used to produce fermented
foods are the predominating factors that determine
the development of microorganisms and, thereby, the
properties of a product.

DESIRABLEPROPERTIES OF THEFERMENTING
MICROFLORA

The most important property of a starter culture for
a fermented cereal is the ability to quickly produce
copious amounts of lactic acid in order to achieve a
rapid decline in pH and retard the growth of path-
ogens and other undesirable organisms. Some work-
ers (Sanni et al. 2002) have sought amylolytic LAB
strains, as this could remove the need for using the

highly contaminated malted flour in a product. The

starter should also be able to hydrolyze the cereal

protein in order to obtain the amino acids sufficient

for rapid growth, and it should produce desirable

and product-typical aroma and flavor compounds,

but not off-flavors. Some products are characterized

by a foaming consistency, and heterofermentative

organisms (LAB or yeasts) are required for this pro-

perty. Bacteriocin-producing strains have also been

sought (Holzapfel 2002) in an attempt to increase

the microbiological safety of the products. Starter

cultures must also be commercially propagable and

be able to survive preservation methods without loss

of viability, activity, or metabolic traits.

MICROBIOLOGICAL ANDBIOCHEMICAL

CHANGES INTRADITIONALFERMENTED

CEREALS

Few studies have been made on the biochemical

changes that take place in traditional fermented

cereals. Mugula et al. (2003) analyzed samples of

naturally fermented togwa made from sorghum and

maize, to which togwa was backslopped and malt

was added. The development of groups of microor-

ganisms, organic acids, soluble carbohydrates, and

volatile components was studied during the 24-hour

fermentation. Maltose and glucose increased during

the first part of the fermentation due to the action

of cereal amylases, but later were reduced as the

growth of LAB and yeasts increased. The pH drop-

ped from around 5.0 to 3.2 in 24 hours, and this was

mirrored by a rise in lactic acid to about 0.5%.

Ethanol and secondary alcohols and aldehydes in-

creased during the secondary part of the fermenta-

tion. Malty flavors are typical for fermented cereal

products and may be produced during grain malting.

Secondary aldehydes and alcohols are responsible

for these flavors and may also originate from micro-

bial metabolism of the branched-chain amino acids

leucine, isoleucine, and valine. These compounds

are produced by yeasts, some LAB, and probably

also by other microorganisms in the product.

Many spontaneously fermented cereals also have

a very short shelf life, since fermentation continues

in the absence of refrigeration. Off-flavors, in partic-

ular vinegary notes, are a common problem. The very

low pH in fermented cereal products may be senso-

rially compensated for by saccharification by -

amylase.