Food Biochemistry and Food Processing (2 edition)

BLBS102-c31 BLBS102-Simpson March 21, 2012 14:0 Trim: 276mm X 219mm Printer Name: Yet to Come

31 Bakery and Cereal Products 607

Fermented cereals are therefore usually regarded as safer than

their unfermented counterparts (Nout and Motarjemi 1997). A

weaning food made from unmalted cereals may be a cause of

malnutrition because its thick viscosity limits the nutritional in-

take of a small child. Addition of malted flour decreases the vis-

cosity so that more food can be ingested. If the fermentation flora

includes yeasts in addition to LAB, a measurable reduction of

carbohydrate will occur due to the production of CO 2 and other

volatile compounds (Muyanja 2001). Analysis of fermented ce-

real products therefore shows that the protein:carbohydrate ratio

is improved during the fermentation, and this obviously has nu-

tritional benefits.

Milling of cereals into flour is usually done prior to fermen-

tation, but in some products, for example, borde (Ethiopia), wet

milling is used. This technique can be used when mechanical

grain mills are not available and if the product is required to

be smooth and without bits of suspended bran. The starch is

also liberated from the grain more thoroughly when slurried

with water and sieved than if it has been previously dry milled

(Abegaz 2002).

A heat treatment step is found at some point in the production

technology of most fermented cereal products and may involve

boiling, steaming, or roasting. The type of heating employed is

likely to have an effect on the flavor of the product, certainly if the

temperature attained is sufficient to promote Maillard reactions.

The heat also gelatinizes the starch, making it more suscepti-

ble to amylolytic enzymes, thus providing greater amounts of

fermentable carbohydrates. However, at the same time, most of

the natural contaminating (and potentially fermenting) flora and

cereal enzymes are destroyed. Such products are also prone to

contamination after the heat treatment step, and are thereby po-

tentially unsafe should pathogenic organisms grow during the

subsequent fermentation. The traditional solution to this is to

use “backslopping,” the addition of some of a previous batch of

the product, and/or the addition of malted flour. Regular back-

slopping results in a selection of acid-tolerant organisms and

functions as an empirical starter culture.

Fermentation usually takes place at ambient temperatures, and

this may cause seasonal variations in products due to selection of

different microorganisms at different temperatures. The duration

of fermentation is largely a matter of personal choice, based on

expected sensory attributes. Heat treatment after fermentation

makes for a safer product, but it has the disadvantage of change

of taste or loss of volatile flavor and aroma compounds.

The Microflora of Spontaneously

Fermented Cereals

Spontaneously acid-fermented cereal products may contain a

variety of microorganisms, but the flora in the final product

is generally dominated by acid-tolerant LAB. Yeasts are also

invariably present in large numbers when the fermentation is

prolonged. A typical fermented cereal product contains approx-

imately 10^9 and 10^7 cfu/g of product, of LAB and yeasts, re-

spectively. However, since yeast cells are considerably larger

than bacteria cells, their metabolic contribution to product char-

acteristics is likely to be just as important as that of the LAB.

The buffer capacity of cereal slurries is low, and the pH there-

fore drops quickly as acid is produced. Pathogenic organisms

are inhibited by a fast acid production, so the addition of starter

cultures, either as a pure culture or by “backslopping” promotes

acid production and contributes to the safety of the fermented

product (Nout et al. 1989).

The potential and the need for upgrading traditional fermenta-

tion technologies have initiated considerable research (Holzapfel

2002). In some recent studies of spontaneously fermented ce-

reals, the LAB and yeasts have been isolated and identified

as a first stage towards developing starter cultures for small-

scale production of traditional fermented cereals. Muyanja et al.

(2003) recorded that bushera, a traditional Ugandan fermented

sorghum beverage that contains high numbers of LAB, was

usually consumed by children after one day of fermentation

as “sweet bushera.” After 2–4 days, the product became sour

and alcoholic and was consumed by adults. However, the sweet

bushera showed very high counts of coliforms and had a repu-

tation for causing diarrhea (Muyanja 2001). Clearly, the devel-

opment of defined starter cultures would improve the safety of

this and similar products.

Some recent examples of studies on the microbial flora of

spontaneously fermented cereals are shown in Table 31.3. For

each product, several different types of organisms have been

isolated. In other words, a specific product is not produced from

fermentation by a specific organism or organisms.Lb. plantarum

seems to be the most commonly isolatedLactobacillusspecies in

fermented cereals. In addition, heterofermentative LAB such as

leuconostocs,Lb. brevis, andLb. fermentumfrequently occur.

Yeasts are always present in spontaneously fermented prod-

ucts, but few studies have characterized the predominating

species. However, Jespersen (2003) reported thatS. cerevis-

ciaeis the predominant yeast in many African fermented foods

and beverages.

All spontaneously fermented products contain, or have con-

tained, many different types of microorganisms. 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 charac-

teristics of that product. However, studies on spontaneously fer-

mented products have focused on LAB and yeasts since these

organisms are often associated with other, better known, fer-

mented products and have a history of safe use in food. Stanton

(1998) proposed that the nature of the substrate (raw material)

and the technology used to produce fermented foods are the

predominating factors that determine the development of mi-

croorganisms and, thereby, the properties of a product.

Desirable Properties of the

Fermenting Microflora

The most important property of a starter culture for a fermented

cereal is the ability to quickly produce copious amounts of lac-

tic acid in order to achieve a rapid decline in pH and retard

the growth of pathogens and other undesirable organisms. Some

workers (Sanni et al. 2002) have sought amylolytic LAB strains,

as this could remove the need for using the highly contaminated