Okafor N. Modern Industrial Microbiology and Biotechnology

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!" Modern Industrial Microbiology and Biotechnology

the metabolic products of the yeast – CO

, alcohol, and water vapor to expand to the final

size of the loaf. The protein becomes denatured beginning from about 70°C; the denatured

protein soon sets, and imposes fixed sizes to the air vesicles. The enzymes alpha and B

amylases are active for a while as the temperature passes through their optimum

temperatures, which are 55-65°C and 65-70°C respectively. At temperatures of about

10°C beyond their optima, these two enzymes become denatured. The temperature of the

outside of the bread is about 195°C but the internal temperature never exceeds 100°C. At

about 65-70°C the yeasts are killed. The higher outside temperature leads to browning of

the crust, a result of reactions between the reducing sugars and the free amino acids in the

dough. The starch granules which have become hydrated are broken down only slightly

by the amylolytic enzymes before they become denatured to dextrin and maltose by alpha

amylase and B amylase respectively.

(ii) The liquid ferment system. In this system water, yeast, food, malt, sugar, salt and,

sometimes, milk are mixed during the pre-fermentation at about 30°C and left for about 6

hours. After that, flour and other ingredients are added in mixed to form a dough. The rest

is as described above.

(iii) The straight dough system: In this system, all the components are mixed at the same

time until a dough is formed. The dough is then allowed to ferment at about 28-30°C for 2-

4 hours. During this period .the risen dough is occasionally knocked down to cause it to

collapse. Thereafter, it follows the same process as those already described. The straight

dough is usually used for home bread making.

The Chorleywood Bread Process

The Chorleywood Bread Process is a unique modification of the straight dough process,

which is used in most bakeries in the United Kingdom and Australia. The process, also

know as CBP (Chorleywood Bread Process) was developed at the laboratories of the Flour

Milling & Baking Research Association (Chorleywood, Herefordshire, UK) as a means of

cutting down baking time. The essential components of the system are that:

(a) All the components are mixed together with a finite amount of energy at so high a

rate that mixing is complete in 3-5 minutes.

(b) Fast-acting oxidizing agents (potassium iodate or bromate, or more usually

ascorbic acid) are used.

fast-acting yeasts are employed.

(d) No pre-fermentation time is allowed and the time required to produce bread from

flour is shortened from 6-7 hours to 1½-2 hours.

19.2.3 Role of Yeasts in Bread-making

Methods of Leavening: Leavening is the increase in the size of the dough induced by gases

during bread-making. Leavening may be brought about in a number of ways.

(a) Air or carbon dioxide may be forced into the dough; this method has not become

popular.

(b) Water vapor or steam which develops during baking has a leavening effect. This

has not been used in baking; it is however the major leavening gas in crackers.

Production of Fermented Foods !"

dough and oxygen was then released with catalase.

(d) It has been suggested that carbon-dioxide can be released in the dough by the use

of decarboxylases, enzymes which cleave off carbon dioxide from carboxylic acids.

This has not been tried in practice.

(e) The use of baking powder has been suggested. Baking powder consists of about

30% sodium bicarbonate mixed in the dry state with one of a number of leavening

acids, including sodium acid pyrophosphate, monocalcium phosphate, sodium

aluminum phosphate, monocalcium phosphate, glucono-delta-lactone. CO

evolved on contact of the components with water: part of the CO

is evolved during

dough making, but the bulk is evolved during baking. Baking powder is suitable

for cakes and other high-sugar leavened foods, whose osmotic pressure would be

too high for yeasts. Furthermore, weight for weight yeasts are vastly superior to

baking powder for leavening.

(f) Leavening by microorganisms, may be done by any facultative organism releasing

gas under anaerobic conditions such as heterofermentative lactic acid bacteria,

including Lactobacillus plantarum or pseudolactics such as Escherichia coli. In

practice however yeasts are used; even when it is desirable to produce bread

quickly such as for the military or for sportsmen and for other emergency

conditions the use of yeasts recommends itself over the use of baking powder.

The Process of Leavening: The events taking place in dough during primary fermentation

i.e. fermentation before the dough is introduced into the oven may be summarized as

follows. During bread making, yeasts ferment hexose sugars mainly into alcohol (0.48

gm) carbon dioxide (0.48 gm) and smaller amounts of glycerol (0.002-0.003 gm) and trace

compounds (0.0005 gm) of various other alcohols, esters aldehydes, and organic acids.

The figure given in parenthesis indicate the amount of the respective compounds

produced from 1 gm of hexose sugars. The CO

dissolves continuously in the dough, until

the latter becomes saturated. Subsequently the excess CO

in the gaseous state begins to

form bubbles in the dough. It is this formation of bubbles which causes the dough to rise

or to leaven. The total time taken for the yeast to act upon the dough varies from 2-6 hours

or longer depending on the method of baking used.

19.2.3.1 Factors which effect the leavening action of yeasts

(i) The nature of the sugar available: When no sugar is added to the dough such as in the

traditional method of bread-making, or in sponge of sponge-doughs and some liquid

ferments, the yeast utilizes the maltose in the flour. Such maltose is produced by the

action of the amylases of the wheat. When however glucose, fructose, or sucrose are

added these are utilized in preference to maltose. The formation of ‘Malto-zymase’ or the

group of enzymes responsible for maltose utilization is repressed by the presence of these

sugars. Malto-zymase is produced only at the exhaustion of the more easily utilizable

sugars. Malto-zymase is inducible and is produced readily in yeasts grown on grain and

which contain maltose. Sucrose is inverted into glucose and fructose by the saccharase of

the cell surface of bakers yeasts. While fructose and glucose are rather similarly

fermented, glucose ís the preferred substrate. Fermentation of the fructose moeity of

!" Modern Industrial Microbiology and Biotechnology

sucrose is initiated after an induction period of about 1 hour. It is clear from the above that

the most rapid leavening is achievable by the use of glucose.

(ii) Osmotic pressure: High osmotic pressures inhibit yeast action. Baker’s yeast will

produce CO

rapidly in doughs up to a maximum of about 5% glucose, sucrose or fructose

or in solutions of about 10%. Beyond that gas production drops off rapidly. Salt at levels

beyond about 2% (based on flour weight) is inhibitory on yeasts. In dough the amount

used is 2.0-2.5% (based on flour weight) and this is inhibitory on yeasts. The level of salt

addition is maintained as a compromise on account of its role in gluten formation. Salt is

therefore added as late as possible in the dough formation process.

(iii) Effect of nitrogen and other nutrients: Short fermentations require no nutrients but for

longer fermentation, the addition of minerals and a nitrogen source increases gas

production. Ammonium normally added as yeast food is rapidly utilized. Flour also

supplies amino acids and peptides and thiamine. Thiamine is required for the growth of

yeasts. When liquid pre-ferments containing no flour are prepared therefore thiamine is

added.

(iv) Effect on fungal inhibitors (anti-mycotic agents): Anti-mycotics added to bread are all

inhibitory to yeast. In all cases therefore a compromise must be worked between the

maximum level permitted by government regulations, the minimum level inhibitory to

yeasts and the minimum level inhibitory to fungi. A compromise level for calcium

propionate which is the most widely used anti-mycotic, is 0.19% (based on flour weight).

(v) Yeast concentration: The weight of yeast for baking rarely exceeds 3% of the flour weight.

A balance exists between the sugar concentration, the length of the fermentation and the

yeast concentration. Provided that enough sugar is available the higher the yeast

concentration the more rapid is the leavening. However, although the loaf may be bigger

the taste and in particular the texture may be adversely affected. Experimentation is

necessary before the optimum concentration of a new strain of yeast is chosen.

19.2.3.2 Flavor development

The aroma of fermented materials such as beer, wine, fruit wines, and dough exhibit some

resemblance. However, the aroma of bread is distinct from those of the substances

mentioned earlier because of the baking process. During baking the lower boiling point

materials escape with the oven gases; furthermore, new compounds result from the

chemical reactions taking place at the high temperature. The flavor compound found in

bread are organic acids, esters, alcohols, aldehydes, ketones and other carbonyl

compounds. The organic acids include formic, acetic, propionic, n-butyric, isobutyric,

isocapric, heptanoic, caprylic, pelargonic, capric, lactic, and pyruvic acids. The esters

include the ethyl esters of most of these acids as would be expected in their reaction with

ethanol. Beside ethanol, amyl alcohols, and isobutanol are the most abundant alcohols.

In oven vapor condensates ethanol constitutes 11-12 % while other alcohols collectively

make up only about 0.04%. Besides the three earlier-mentioned alcohols, others are n-

propanol, 2-3 butanediol, b-phenyl ethyl alcohol. At least one worker has found a

correlation between the concentration of amyl alcohols with the aroma of bread. Of the

aldehydes and ketones acetaldehyde appears to be the major component of pre-

fermentation. Formaldehyde, acetone, propinaldehyde, isobutyraldehyde and methyl

Production of Fermented Foods !"!

ethyl ketone, 2-methyl butanol and isovaleradehyde are others. A good proportion of

many of these is lost during baking.

19.2.3.3 Baking

Bread is baked at a temperature of about 235°C for 45–60 minutes. As the baking

progresses and temperature rises gas production rises and various events occur as

below:

• At about 45°C the undamaged starch granules begin to gelatinize and are attacked

by alpha-amylase, yielding fermentable sugars;

• Between 50 and 60°C the yeast is killed;

• At about 65°C the beta-amylase is thermally inactivated;

• At about 75°C the fungal amylase is inactivated;

• At about 87°C the cereal alpha-amylase is inactivated;

• Finally, the gluten is denatured and coagulates, stabilizing the shape and size of

the loaf.

19.3 FERMENTED FOODS MADE FROM MILK

19.3.1 Composition of Milk

Milk is the fluid from the mammary glands of animals which is meant for feeding the

young of mammals. It is a complex liquid consisting of several hundred components of

which the most important are proteins, lactose, fat, minerals, enzymes, and vitamins in

which emusified fat globules and casein micelles are present. Its composition varies from

breed to breed, as shown in Table 19.1.

Proteins: Milk proteins are divided into two: caseins and whey proteins. Caseins consist

of carbohydrate, phosphorus, and protein ( glyco-phspho-protein) and make up 85% of

the total milk proteins. Casein exists in milk as the calcium salt, ie, as calcium caseinate in

globules (micelles) ranging from 40-300 mµ in diameter. Casein exists in four types

designated, a b, k and g and depending on their electric charges. The proportion of the

various types in milk depends on the breed of the cow producing the milk. The letter ‘s’

after a

- caseins indicates its sensitivity to precipitation by calcium.

Table 19.1 Chemical composition (%) of milk from various mammals

Animal Water Fat Protein Lactose Ash

Ass 89.0 2.5 2.0 6.0 0.5

Buffalo 82.1 8.0 4.2 4.9 0.8

Camel 87.1 4.2 3.7 4.1 0.9

Cow 87.6 3.8 3.3 4.7 0.6

Goat 87.0 4.5 3.3 4.6 0.6

Mare 89.0 1.5 2.6 6.2 0.7

Reindeer 63.3 22.5 10.3 2.5 1.4

Sheep 81.6 7.5 5.6 4.4 0.9

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Whey proteins consist of different components which are normally stable to acid, but

very sensitive to heat. b-Lactoglobulin forms about 66% of the total whey proteins,

followed by a-Lactabumin (22%). The immune globulins from about 10% of the the total,

and contribute towards the immunity derived by the young from the consumption of

colostrum.

Lactose: The main carbohydrate in milk is lactose, which is found only in milk. It is a

dissacharide of glucose and galactose and has a low sweetening ability, as well as low

solubility in water.

Fat: Fat consists of one molecule of glycerol and three of fatty acids. Over 60 different acids

are known in butter, many of them, being of low molecular weight of about 10 carbon

atoms or less, and include saturated and unsaturated fatty acids.

Enzymes: Enzymes found in milk include proteases, carbohydrases, esterases, oxidases/

reductases.

Minerals: Milk is a major source of calcium; other minerals in milk are phosphorous,

magnesium, sodium, potassium, as well as sulphate and chloride ions.

When fat is removed from milk such as during butter making, the remnant is skim milk.

On the other hand, when casein is removed such as during cheese manufacture, the

remnant is known as whey. Whey is high in lactose and its disposal sometimes poses

some problem as not all microorganisms can break down whey. It is however used in the

production of yeasts to be used as food or fodder.

19.3.2 Cheese

Cheese is a highly proteinaceous food made from the milk of some herbivores. Cheese is

believed to have originated in the warm climates of the Middle East some thousands of

years ago, and is said to have evolved when milk placed in goat stomach was found to

have curdled. The scientific study and manipulation of milk for cheese manufacture is

however just over a hundred years old. Most cheese in the temperate countries of the

world such as Western Europe and the USA is made from cow’s milk, the composition of

which varies according to the breed of the cattle, the stage of lactation, the adequacy of its

nutrition, the age of the cow, and the presence or absence of disease in the breasts

(udders), known as mastitis. In some subtropical countries milk from sheep, goats, the

lama, yak, or ass is also used. Sheep milk is used specifically for the production of certain

special cheese types in some parts of Europe (e.g. Roquefort in France, and Brinsen in

Hungary). Milk from the water buffalo may be used in India and other countries, while

milk from the reindeer and the mare may be used in northern parts of Scandinavia and in

Russia, respectively. Cheese made from the milk of goat and sheep has a much stronger

flavor than that made from cow’s milk. This is because the fat in goat and sheep milk

contain much lower amounts of the lower fatty acids, caproic, capryllic, and capric acids.

These acids confer a sharp taste (similar to that of Roquefort cheese) to cheese made from

these mammals. Future discussion of cheese in this chapter will however refer to that

made from cow’s milk.

About a thousand types of cheese have been described depending on the properties

and treatment of the milk, the method of production, conditions such as temperature, and

the properties of the coagulum, and the local preferences.

Production of Fermented Foods !"#

19.3.2.1 Stages in the manufacture of cheese

The manufacture of all the types of cheeses include all or some of the following processes:

(a) Standardization of milk

The quality of the milk has a decided effect on the nature of cheese. Cheese made from

skim milk is hard and leathery; the more fat a cheese contains the smoother its feel to the

palate. The fat/protein ratio is often adjusted through fat addition in order to yield a

cheese of consistent quality. In the US, pasteurization (High Temperature Short Time) or

(Long Temperature Short Time) must be given to milk to be in certain types of cheeses,

such as cottage or cream cheese. For others the milk need not be pasteurized. but must be

stored at for at least 60 days at 2°C. If however the ‘starter’ is slow acting or souring is

delayed, food-poisoning staphylococci could develop and produce toxins in the cheese.

Sub-pasteurization temperatures are often the legal compromise. Pasteurization gives a

better control over the processes of cheese production. However, the organisms present in

raw milk are important during the ripening processes.

The milk may also be homogenized by forcing it at high speed through small orifices to

reduce the milk fat globules for use in producing soft cheeses.

(b) Inoculation of pure cultures of lactic acid bacteria as starter cultures

In the past, lactic acid was produced by naturally occurring bacteria. Nowadays they are

inoculated artificially, by specially selected bacteria termed starters. Indeed lactic acid

formation is necessary in all kinds of cheese. The propagation and distribution of lactic

acid bacteria for use in cheese manufacture is an industry in its own right in the United

States. For cheese prepared at temperatures less than 40°C strains of Lactococcus lactis are

used. For those prepared at higher temperatures the more thermophilic Streptococcus

thermophilus, Lactobacillus bulgaricus, and Lact. helveticus are used.

Lactic acid has the following effects:

(i) It causes the coagulation of casein at pH 4.6, the isoelectric point of that protein,

which is used in the manufacture of some cheeses, e.g. cottage cheese.

(ii) It provides a favorably low pH for the action of rennin the enzyme which forms the

curd from casein in other types of cheeses.

(iii) The low pH eliminates proteolytic and other undesirable bacteria.

(iv) It causes the curd to shrink and thus promotes the drainage of whey.

(v) Metabolic products from the lactic acid bacteria such as ketones, esters and

aldehydes contribute to the flavor of the cheese.

Problems of lactic acid bacteria in cheese-making

(i) Attack by bacteriophages: Bacteriophages sometimes attack the lactic acid starters

and besides choosing strains that are resistant to phages, rotations (i.e., using

different lactic mixtures every three or four days) helps eliminate them.

(ii) Inhibition by penicillin and other antibiotics: Lactic acid bacteria, being Gram-positive

are particularly susceptible to penicillin used to treat diseased udder in mastitis if

the antibiotic finds its way into the milk; other antiobiotics also have an inhibitory

effect on them.

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(iii) Undesirable strains: Some strains of lactic acid bacteria are undesirable in cheese

making because they produce too much gas, undesirable flavors, or produce

antibiotics against other lactic acid bacteria. They arise by mutation.

(iv) Sterilant and detergent residues: Sterilant and detergent residues may inhibit the

growth of starter bacteria. The minimum concentration required for inhibition

varies with the different anti-microbial agents and between different strains of

starter bacteria. Residues gain entry to milk at the (a) farm, (b) during

transportation to the factory, and (c) the factory due to careless use of sterilants or

detergents, incomplete draining or inadequate rinsing of equipment. The

inhibitory effects of sterilant and detergent residues are prevented by the correct

and ethical use of these materials. Proper use includes the use of the chemical at

the correct concentration and adequate rinsing and draining. Their presence is

mitigated by dilution with uncontaminated milk.

The classical material used in the formation of the coagulum is ‘rennet’ which is derived

from the fourth stomach, abomasum or vell of freshly slaughtered milk-fed calves.

Besides those of calves, the abomasum of kids (young goats), lamb or other young

mammals have been used. Rennet is produced by soaking and/or shredding air-dried

vells under acid conditions with 12-20% salt. Extracts from young calves contain 94%

rennin and 6% pepsin and from older cows, 40% rennin and 60% pepsin. Rennin

(chymosin) is the enzyme responsible for the coagulation of the milk. Pepsin is proteolytic

and too high an amount of pepsin can result in the hydrolysis of the coagulum and a

resulting low yield of cheese, and a bitter taste may result from the amino acids. Due to the

high cost of animal rennet, other sources, mostly of microbial origins, have been found

(Table 19.2).

Table 19.2 Some commercial microbial rennets and their microbial sources

Commercial Rennet Microbial source

Harmilase Mucor miehei

Rermilase Mucor miehei

Fromase Mucor miehei

Emposase Mucor pusillus

Meito Mucor pusillus

Suparen Endothia parasitica

Surd curd Endothia parasitica

Mikrozyme Bacillus subtilis

The major effect of the milk-clotting enzymes is the conversion of casein from a

colloidal to a fibrous form. First the pH of the milk is brought down from pH 6.8 -7 to pH

5.5 by the action of lactic acid bacteria which produce lactic acid from lactose in the milk.

On addition of rennet, the active component, rennin, catalyses the hydrolyses of k-casein

to release para-k-casein and k-casein macropeptide. The latter goes into whey, while the

para-k-casein remains part of casein micelles, which now bind together to form the curd

following the removal of carbohydrates with the k-casein macropeptide and the exposure

Production of Fermented Foods !"%

of binding surfaces. The events up this coagulation are aided by lowered pH and by

increasing temperatures up to 45°C. Most of the bacteria, fat, and other particulate matter

are entrapped in the curd. When casein is removed the remaining liquid containing

proteins, lactalbumin, globulin, and yellow-green riboflavin (vitamin B

) is whey. The

whey proteins may be precipitated by heat, but not acid or rennin and they are used in

making whey cheese. The enzymes used in cheese making are now obtained from

microorganisms, mainly fungi.

(d) Shrinkage of the curd

The removal of whey and further shrinkage of the curd is greatly facilitated by heating it,

cutting it into smaller pieces, applying some pressure on it and lowering the pH. In many

types of cheeses, such as Parmesan, Emmenthal and Gruyere, there is a stage known as

‘scalding’ in which the temperature can be as high as 56°C in the preparation. Acid

produced by the lactic starters introduce elasticity in the curd, a property desirable in the

final qualities of cheese.

(e) Sa1ting of the curd and pressing into shape

Salt is added to most cheese varieties at some stage in their manufacture. Salt is important

not only for the taste, but it also contributes to moisture and acidity control. Most

importantly however it he1ps limit the growth of proteiolytic bacteria which are

undesirable. The curd is pressed into shape before being allowed to mature.

(f) Cheese ripening

The ripening or maturing of cheese is a slow joint microbiological and biochemical

process which converts the brittle white curd or raw cheese to the final full-flavored

cheese. The agents responsible for the final change are enzymes in the milk, in the rennet

and those from the added starter microorganisms as well as other micro-organisms

which confer the special character of the cheese to it. Among the cheese whose peculiar

characteristics are dependent on particular microorganisms are the blue-veined cheese

Roqueforti, Gorgonzola, Stilton, conferred by Penicillium roquefort, Swiss cheese, with its

characteristic flavor and holes produced by the fermentation products and gases from

Propionibacterium spp. Yeasts, micrococci, and Brevibacterium linens impart the

characteristic flavor of Limburger cheese. In soft cheese, such as Camembert, the protein

is completely broken down to almost amino acids, whereas in the hard cheese, the protein

remains intact.

19.3.3 Yoghurt and Fermented Milk Foods

Many types of fermented milks are produced and drunk around the world (Table 19.3)

Yoghurt is a fermented milk traditionally believed to be an invention of the Turks of

Central Asia, in whose language the word yoghurt means to blend, a reference to how the

milk product is made. Although accidentally invented thousands of years ago, yoghurt

has only recently gained popularity in the United States. While yoghurt has been present

for many years, it is only recently (within the last 30-40 years) that it has become popular.

This is due to many factors including the introduction of fruit and other flavorings into

yoghurt, the convenience of it as a ready-made breakfast food and the image of yoghurt as

a low fat healthy food.

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Table 19.3 Fermented milks and their presumed countries of origin

Name Presumed Description Cultures

country of

origin

Yoghurt Asia, Acidic, set or stirred, S. thermophilus, Lb. bulgaricus,

Balkans characteristic aroma (and Lb. acidophilus, Bifidobacterium spp.) *

Acido-

philus USA Set, stirred or liquid, Lb. acidophilus

milk mild flavor

Kafir Caucasus Stirred beverage, Lc. lactis, Lc. cremoris, Lb. kefir, Lb. casei,Lb.

creamy consistency,

characteristic taste

and aroma (CO

)

Kumiss Mongolia Frothy beverage, acid, Lb. bulgaticus, Lb. acidophilus, yeasts

refreshing taste

Lassi India Sour milk drink diluted Lactococcus spp., Lactobacillus

with water, consumed spp., Leuconostoc

salted, spicy or sweet

Dahi India Set, stirred, or liquid S. thermophilus, Lb. bulgaricus,

beverage pleasant Lc. diacetylactis,

Leben Middle East Set or stirred product, S. thermophilus, Lb. bulgaricus,

pleasant taste and aroma Lb. acidophilus,

Filmjilk Sweden Viscous stirred beverage, Lc. lactis, Lc. cremoris, Lc. diacetylactis, Ln.

clean acid taste

Villi Finland Viscous stirred product, Lc. lactis, Lc. cremoris, Lc. diacetylactis, Lc.

mildly sour

* Depending on the country

In the manufacture of yoghurt, two kinds of lactic acid bacteria, Lactococcus spp. and

Lactobacillus spp., are generally used with usually unpasteurized milk. Most commonly

used are Lactococcus salivarius and thermophilus, and Lactobacillus spp., such as Lacto.

acidophilus, bulgaricus and bifidus.

The bacteria produce lactic acid from lactose in the milk causing the pH to drop to

about 4-5 from about 7.0. This drop in pH causes the milk to coagulate. The lactic acid

gives yoghurt its sour taste and limits the growth of spoilage bacteria. Yoghurt is flavored

usually with fruits.

19.4 FERMENTED FOODS FROM CORN

Corn is a tropical crop, but grows in the summer in temperate climates, which have

at least 90 days which are frost free. It is known as maize in some parts of the

world. Scientifically known as Zea mays L, it is used to make important fermented foods in

west Africa and it is sometimes mixed with sorghum, Sorghum bicolor Linn for this

purpose.

Production of Fermented Foods !"'

19.4.1 Ogi, Koko, Mahewu

Ogi, also known as akamu, is a Nigerian sour gruel made from maize. In Ghana the

equivalent foods are known as koko. For ogi preparation, corn is soaked in water for

about two days. Thereafter the cereal is wet-milled and sieved to remove the fibrous portions

of the maize. The starchy sediment is allowed to settle and to ferment for another two

days. The water is decanted off and the starchy sediment is ogi. It is prepared by boiling

it to form a thick gruel which can be consumed sweetened with sugar, or eaten with foods

made from beans. It may also be heated to form a stiff gel, known as agidi or eko when cool.

Ogi or the stiff gel made from it are popular weaning and convalescent foods in Nigeria.

Studies of the microbiology of ogi production show that in the early stages of

fermentation, fungi such as Fusarium and Cephalosporium which are acquired from the

field, and which form the bulk of the organisms in the first 24 hours, soon disappear to be

replaced with lactic acid bacteria especially (Lactobacillus plantarum and Lact

mesenteroides) and yeasts (Saccharomyces cerevisiae, Rhodotorula spp., and Candida

mycoderma which become the dominant organisms at the time of the milling.

The flavor of ogi has been shown to be due to the activities of lactic acid bacteria and

occasionally to yeasts and acetic acid bacteria. The following acids were identified in that

quantitative order by gas chromatography: acetic, butyric, pentanoic, isohexanoic, and

isobutyric.

The nutritional quality of ogi suffers from its method of preparation: the soaking of the

grains and the discarding of the overtails before milling leads to loss of minerals as well

as a portion of the already low quantity of protein and amino acids present in the cereals.

The flow charts for ogi production and that of a similar food, koko, from Ghana are in Fig.

19.1

Koko

Soak grains

12 hours

Soak grains

1/3 days

Grind

Mix

Sieve

Ferment

1-3 days

Ferment

1-3 days

Disperse

Decant

Cook

Sievates

discarded

Supernatant

discarded

Ogi

Fig. 19.1 Flow Charts for

Koko

and

Ogi

Production

Mahewu, also known as mogou, is a South African sour food. Although the main

organism found in locally-made mahewu is Streptococcus lactis, mahewu is made on an

industrial scale by inoculating Leuconostoc delbruckii into autoclaved 8-10% maize slurry,

fermenting the mixture for about 12 hours and spray-drying the slurry. It is an acid food

of about pH 3.5, and in order to ensure that the proper level of lactic acid is produced to

attain this pH level, buffering salts such as CaHPO

are somtimes added. It is a

convenient food consumed by miners and the dry powder needs only to be reconstituted

in cold water to get the food ready for consumption. The food is sometimes enriched with

vitamin and protein rich additives such as yeast extract.