Microbiology of Foods

UNIT 1 MICROBIOLOGY OF FOODS

Structure
1.1. Introduction
1.2 Food Microbiology – Basic Concept
1.3 History of Food Microbiology
1.4 Role of Microbiology in Biotechnology
1.5 Role of Microorganisms in Fermented Foods
1.5.1 Fermented Baked Preparations
1.5.2 Fermented Vegetable Foods
1.5.3 Fermented Soyabean Products
1.5.4 Fermented Dairy Products
1.5.5 Other Fermented Food Preparations
1.5.6 Economically Important Fermentation Products
1.5.7 Other Uses of Microbes in Industry
1.6 Let Us Sum Up
1.7 Glossary
1.8 Answers to Check Your Progress Exercises

1.1 INTRODUCTION

The first unit in this course introduces you to the discipline called food microbiology.
In this unit, we will try to understand the historical aspects and development of food
microbiology over the years. Food microbiology, as a discipline, has evolved to
accommodate various modern developments. We will look at the role of microbiology
in biotechnology and see how efficiently a microorganism can be improved by bio-
engineering. Further, we will understand the role of microorganisms in preparing
different fermented products and discuss other uses of microorganisms in the food
industry.
Objectives
After studying this unit, you will be able to:
understand the history and historical development of food microbiology,
describe what is biotechnology and its role in the food industry,
enumerate the various fermented food preparations produced by the use of
microorganisms, and
discuss other uses of microorganisms in the food industry.

1.2 FOOD MICROBIOLOGY – BASIC CONCEPT

We start our learning of food microbiology by first understanding what is microbiology.

Microbiology is the branch of biology that deals with microorganisms and their
effects on other living organisms. Microorganism, as you may already know, is a
microscopic organism, such as a bacteria, virus, algae, fungus, protozoan etc. So why
do we need to study and learn about these microbes? Microbes can spoil the foods,
can cause food borne diseases and interestingly, some of them are useful. The
relationship between microorganisms and food has been a subject of study for long
and has been recognized as a separate area of study referred to as ‘food microbiology’.
It is, in fact, a branch of microbiology concerned with the relationships between
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Food Microbiology and microorganisms and food. Food microbiology concerns with the interactions between
Safety microorganisms, food and us the community. It covers food borne disease, food
hygiene, food spoilage, fermented foods and beverages, use of microorganisms to
produce food ingredients and processing aids, microbiological aspects of quality control,
conventional and novel methods for the microbiological analysis of foods and aspects
of food legislation. Food microbiology therefore, is a vast field of study in itself. Let
us look at the origin of this field of study, next.

1.3 HISTORY OF FOOD MICROBIOLOGY

The microorganisms were first observed using primitive microscopes as early as the
late 1600s. The science of microbiology is barely 150 years old. A dramatic
development and broadening of the subject of microbiology has taken place since
World War II.

It is extremely difficult to pinpoint the precise beginnings of man’s awareness of the

presence and role of microorganisms in foods, evidence available at this time indicates
that this knowledge preceded the establishment of microbiology as a science. The era
prior to this may be further divided into what has been called as man’s ‘food gathering
period’ and the ‘food producing period’.

The food-gathering period may be from origin of man to 10,000 years ago. During
this period, man was presumably carnivorous in his eating habits, with plant foods
coming into his diet later in this period. The food producing period dates from 10,000
years ago to present time. Between 3000 BC and 1200 BC, the Jews employed salt
in the preservation of various foods. The use of curds involving fermentation of milk
was known in India since the Vedic period. The epic Mahabharath dating 5000 BC
contains references to milk products like curd and butter.

It is presumed that man first encountered problems of spoilage and food poisoning
early in the period with the advent of prepared foods. The problem of disease
transmission by food and faster spoilage due to improper storage both made their
appearance.

The first man to suggest the role of microorganism, in spoiling food was A. Kiremer.
In 1658, he examined decaying bodies, meat, milk or other solutions and saw what
he referred to as “worms” invisible to the naked eye. Subsequently, L. Pasteur (1837)
was the first man to appreciate and understand the presence and role of microorganisms
in food. In 1860, he employed heat to destroy undesirable organisms present in wine
and beer.

So then, starting from 150 years ago till date, food microbiology, as a discipline, has
evolved to accommodate various modern developments. We learnt that microorganisms
can cause food spoilage and disease, though not all microorganisms are harmful.
Some organisms play a beneficial role in nutrition and well being of humans. This
aspect has been studied and great advancements have been made in this area. Some
of the recent developments in food microbiology are discussed next:

a) Probiotics: The word ‘Probiotic’ is a Greek word and it means “for life”. It refers
to microorganisms and their culture products, which contribute to the intestinal
microbial balance, thus benefiting the host by protecting against disease or
improvising its nutrition. It is well known that probiotics, like lactobacillus,
assist in the digestion of lactose, inactivate toxins, bind cancer causing chemicals,
modulate the gut flora and reduce cholesterol absorption in the gut. We will read
about probiotics in greater details in the Advance Nutrition Course in Unit 11.

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b) Biotechnology: You must have heard the word ‘biotechnology’. In today’s world, Microbiology of Foods
it is one of the most extensively used branches of science to develop/ generate
better quality foods in the market. Biotechnology is a series of enabling
technologies that involve the manipulation of living organisms or their sub-
cellular compounds to make or modify products to improve plants or animals or
to develop microorganisms for specific uses. If these specific uses are meant to
enhance the production, processing and distribution of safe, nutrition foods, then
it is “food biotechnology”. Microorganisms including bacteria and moulds have
been used for the production of fermented meat, vegetable products as well as
wine, beer etc. and for producing food additives like flavour enhancers, stabilizers,
colours and preservatives. However, with the advert of newer genetic engineering
techniques developed in the last 30 years, tremendous developments have taken
place in food biotechnology. Genetically engineered crops, processing and
ingredients are gaining a regular approval and are entering the markets. Let us
learn more about this new, interesting branch of science geared towards developing
better quality of food in the next section.

1.4 ROLE OF MICROBIOLOGY IN BIOTECHNOLOGY

The word biotechnology is derived from the word “bio” meaning ‘life or living
systems’, while the word “technology” is defined as ‘scientific methods for achieving
a practical purpose’. Biotechnology, hence, is the use of biological processes to make
or change a product. Biotechnology is not new to the food sector, since human beings
have been exploiting microorganisms for the production, processing and preservation
of foods for centuries. Biotechnology is also used to genetically modify plants or
animals and control particular attributes. How is this done? Let us look at some of the
uses of biotechnology through genetically modifying foods and improving the
nutritional status of the population.

Genetically Modified Foods

Modern biotechnology techniques are now used by the scientists to be able to identify
individual genes that control particular characteristics. The selected gene can be
transferred to another plant or animal to bring about a desired change. We have talked
about ‘genes’ here. What are genes? Genes are composed of specific lengths of
deoxyribonucleic acid (DNA) strands intertwined in a spiral called as the double helix
as shown in Figure 1.1. You may recall reading about DNA in the Nutritional
Biochemistry Course, Unit 2, as well. It is this DNA which gives the individual
characteristics to all plants and animals. For e.g. the colour of a flower’s petals, fair
or dark skin complexion or the brown or blue coloured eyes.

Figure 1.1: Double helical structure of DNA

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Food Microbiology and The use of biotechnology and understanding the nature of DNA has led to what we
Safety call as ‘genetic modification’. It is a technique of changing by inactivating, deleting
or inserting genes to produce a desired characteristic. In this technique, selected
individual genes are transferred from one organism (microbes, plant or animal) to
another organism. When genes are transferred from one species to another, transgenic
organisms are produced. Foods produced by genetically modified technique are called
Genetically Modified or GM Foods. The term ‘Recombinant technology’ or
‘Biotechnology’ is often used to describe genetic modification process.

What is the genetic modification process? Let us next briefly review the process.
Initially, the gene that carries the desirable characteristic is identified. Then a gene
from a second strain carrying the desired trait is inserted, which produces a genetically
modified variety which is identical to the original variety with the improved desirable
characteristics.

The first GM plants were created in 1983. Since then a variety of crop plants such
as maize, soybean, rice, rapeseed (mustard), tomato, cotton, potato etc have been
modified by this technique. What are the benefits of genetic modification? Among the
many benefits of genetic modification, reduction in the use of pesticides or herbicides,
higher yields, better quality food, foods with greater shelf life, nutritional improvement
and enhancement in processing qualities are some of the important benefits of genetic
modification. Let us understand this concept better by looking at the benefits of
genetically modifying a few of the food items:

a) Tomatoes: It was discovered in the Nottingham University in the UK that it is

possible to slow down the softening process of tomatoes by genetically modifying
the tomato plant. This helped to increase the shelf-life of tomato, keeping it fit
to eat for longer and reducing waste during processing.

b) Maize: The European corn was made pest-resistant by inserting a gene from the
naturally occurring soil bacterium Bacillus thuringiensis. The gene produces a
protein that acts as an insecticide, but is harmless to other creatures.

c) Golden rice: The natural varieties of rice do not provide vitamin A. Vitamin
A deficiency, as you may already know, could lead to blindness, decreased
immunity to diseases and deaths of more than a million children. So, the scientists
in Switzerland genetically enhanced rice to be rich in -carotene (a precursor of
vitamin A, to be converted to vitamin A in the body) by re-engineering the genes
that imparts yellow colour to daffodils. Interesting isn’t it!

d) Vaccines: In US, foods such as potatoes, tomatoes and bananas that can carry
vaccine for the infectious liver disease – Hepatitis B, have been successfully
produced on a small scale. Feasibility studies are being conducted to investigate
whether these modified crops would help to deliver the vaccines to people living
in developing countries. If the positive outcomes are indicated in these studies,
it would help to save lives of many millions.

In India, so far 3 hybrids of cotton containing Bt gene produced by Mahyco/Monsanto

has been approved for commercialization by the Genetic Engineering Approval
Committee (GEAC) in 2002. One variety of cotton, again containing Bt gene produced
by rasi seed was allowed seed production for one hybrid. Other GM crops undergoing
field trials include:
Mustard containing barnase-bar star gene, produced by the company Proagro and
another variety produced by Jawahar Lal Nehru University, Delhi.
Rice and brinjal with Bt gene and tomato by the Indian Agricultural Research
Institute, New Delhi.
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 Potato containing lysine protein gene from amaranth plant by Jawahar Lal Nehru Microbiology of Foods
University, New Delhi.
Besides the benefits, apprehensions about the use of genetically modified foods have
been expressed and you will study about them in the next Unit. In fact, because of
these apprehensions, the Government of various countries have introduced legislations
to cover the development, release, cultivation, sale, import etc. of these foods. We will
get to know more on this topic subsequently.

So far we have looked at the benefits of genetically modifying a few of the food
items. We saw how corn was made pest-resistant by inserting a gene from the naturally
occurring soil bacterium. Likewise, microorganisms have other beneficial roles as
well. Microorganisms i.e. bacteria, yeasts and moulds have been used since the
beginning of the recorded history for the production of fermented dairy, cereal, meat
and vegetable products, as well as, for fermenting the beverages such as wine and
beer. Many ingredients used in foods as vitamins, stabilizers, flavour and flavour
enhancers, colours and preservatives are produced by microbes.

Next, we shall briefly focus on a variety of fermentation food preparations, method

of preparation and microorganisms involved in this process. This information will
help you understand the role of microorganisms in food fermentation and in the food
industry. But first, let us take a break and recapitulate what we have learnt so far.

Check Your Progress Exercise 1

1) State whether the following statements are true or false. Correct the false
statement.
a) All microbes are harmful, since they spoil foods and cause food-borne
diseases.
b) Man was primarily carnivorous during the food-gathering period.
c) A. Kiremer was the first one to discover microorganisms in food.
2) What is food microbiology? What areas does it cover?
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
3) Define the following terms:
a) Probiotics
.................................................................................................................
.................................................................................................................
b) Biotechnology
.................................................................................................................
.................................................................................................................
c) Genetic modification
.................................................................................................................
.................................................................................................................
.................................................................................................................
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Food Microbiology and
Safety 1.5 ROLE OF MICROORGANISMS IN FERMENTED
FOODS

Fermentation is one of the oldest forms of food preservation technologies in the

world. Indigenous fermented foods such as bread, cheese and wine have been prepared
and consumed for thousands of years and are strongly linked to culture and tradition,
especially in the rural households and village communities.

The development of fermentation technologies is lost in the mists of history.

Anthropologists have suggested that it was the production of alcohol that motivated
primitive people to settle down and become agriculturists. Some even think that the
consumption of fermented foods is pre-human. The first fermented foods consumed
probably were fermented fruits. Hunter-gatherers would have consumed fresh fruits
but at times of scarcity would have eaten rotten and fermented fruits. Repeated
consumption would have led to the development of the taste for fermented fruits.
There is a reliable information that fermented drinks were being produced over 7,000
years ago in Babylon (now Iraq), 5,000 years ago in Egypt, 4,000 years ago in
Mexico and 3,500 years ago in Sudan.
Bread-making probably originated in Egypt over 3,500 years ago. Several triangular
loaves of bread have been found in ancient tombs. Fermentation of milk started in
many places with an evidence of fermented products in use in Babylon over 5,000
years ago. There is also evidence of fermented meat products being produced for King
Nebuchadnezer of Babylon. China is thought to be the birth-place of fermented
vegetables and the use of Aspergillus and Rhizopus moulds to make food. The book
called “Shu-Ching” written in the Chou dynasty in China (1121-256 BC) refers to the
use of “chu”- a fermented grain product.
Having looked at the history of fermentation, let us review and see what is fermentation?
Fermentation is the slow decomposition process of organic substances induced by
microorganisms, or by complex nitrogenous substances (enzymes) of plant or animal
origin. It can be described as a biochemical change, which is brought about by the
anaerobic or partially anaerobic oxidation of carbohydrates by either microorganisms
or enzymes.
You know that microorganisms are naturally found in foods, since there is no
environment where some type of microbes cannot live. These microbes either living
or dead and their cellular byproducts all have specific uses in some foods. These
include such products as fermented food products. Fermented foods use microbes to
convert the original food into a fermented product by the use of specific microbes.
These microorganisms use the original product for growth and reproduction, and in
the process, they excrete byproducts into the environment surrounding themselves and
the food. These byproducts plus the part of the original product that is not consumed
is the fermented food. Fermented foods include fermented dairy, meat, fish, cereals,
fruits, vegetable products etc. They may be fermented separately or in conjunction
with each other to produce the desirable end product.

From our discussion, so far, it is evident that, fermentation involves the introduction
of the desirable microbes into the original product. Some of the common microbes
used in food fermentation are highlighted herewith.
Lactococcus lactis — used in dairy fermentation.
Steptococcus thermophilus — used in dairy fermentation.
Leuconostoc sp. — used in wine making, dairy fermentation.
Pediococcus sp. — meat fermentation, vegetable fermentation, ripening of some
16 cheeses.
 Lactobacillus sp. — meat fermentation, vegetable fermentation, dairy fermentation, Microbiology of Foods
sourdough bread.
Bifidobacterium sp. — added to dairy products to promote intestinal health.
Propionibacterium sp. — Swiss cheese.
Yeasts — bread, beer, wine, liquors.
Moulds — ripening cheeses, soy sauce.
Lactobacillus delbruekii, subspecies bulgaricus and streptococcus thermophilus
— making of yogurt.
Although fermentation of foods has been in use for thousands of years, it is likely that
the microbial and enzymatic processes responsible for the transformations were largely
unknown. It is only recently that there has been a development in the understanding
of these processes and their adaptation for commercialization. Let us now get to know
about a few of the fermented food preparation used commercially. We shall begin our
study on fermentation products with the baked preparations.

1.5.1 Fermented Baked Preparations

In baked products such as bread and bun, the yeast Saccharomyces cerevisiae which
is popularly known as “baker’s yeast”, helps by raising the dough giving it the texture
and also adding flavours. The different ingredients added gives distinctly different
tastes to each of the products. The naan, which is popular in India, is made from
maida (refined wheat flour) to which salt, yeast or curd is added. It is kneaded
vigorously for 15 minutes adding vegetable oil for softening. It is allowed to ferment
for 30 minutes – 1 hour. It is then baked rapidly for 5 to 10 minutes. Intense heat
causes centre of the dough to expand rapidly and create a central pouch. Saccharomyces
cerevisiae is mainly responsible for leavening by carbon dioxide production.

1.5.2 Fermented Vegetable Foods

We all consume vegetables either in raw or cooked form as salad preparations, soup,
dry vegetables and curry or as stuffing in a variety of snacks. But, do you know, we
can even consume these by making fermented products. Can you think of any such
food item? Yes, of course, it is pickles which all of us relish sometimes or the other.
Let us learn about these fermented vegetable products and see which microorganisms
are involved in their preparation.
i) Sauerkraut
Sauerkraut is a fermented fresh cabbage product. It is popular in USA and Europe.
The main organism which is involved in the fermentation of this pickle is lactic
acid bacteria, Leuconostoc mesenteroides followed by Lactobacillus plantarum.
ii) Cucumber pickle
Cucumber pickle is a fermentation product of fresh cucumbers. Several lactic
acid bacteria are involved in the preparation of this pickle. Lactobacillus plantarum
is the most important organism required for the fermentation of cucumber pickle.
Next, we shall look at the commonly consumed fermented soya products.

1.5.3 Fermented Soyabean Products

Do you enjoy Chinese food? If you have tried making it at home, you would have
realized that soya sauce is a basic ingredient in Chinese cooking. In fact two of the
most commonly consumed soya products are tempeh and soya sauce. We will briefly
focus on the preparation of these products.
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Food Microbiology and i) Tempeh
Safety
Tempeh is a highly popular soyabean preparation in Indonesia. The chief organism
in this preparation is the species belonging to mould Rhizopus oligosporus. The
soyabean mash is wrapped in a banana leaf or a plastic box and the mash is
inoculated with tempeh fungus by the addition of a portion of previous batch and
allowed to ferment for about 24-48 hours at a temperature 30-40°C, until there
is a good mycelium growth. This is then sliced and prepared as per the taste, such
as roasting or frying. The taste of the tempeh is considered to be bland, but it is
highly nutritious.

ii) Soya sauce

Soya sauce is a very popular preparation of Japan which has received wide
acceptance in many countries. This is prepared by inoculating an Aspergillus
species, mostly Aspergillus oryzae in a mixture of soaked and steamed soya bean
with roasted wheat in the ratio of 2:1. The mixture is allowed to be incubated for
3 to 5 days. Subsequently, it is subjected to various processing with Lactobacillus
bacteria and the yeast Saccharomyces rouxii. After 3 months, the final product is
filtered, pasteurized and bottled for use.

We will learn more about the fermented soya products in the Principles of Food
Science Course, Unit 11.

1.5.4 Fermented Dairy Products

A large number of fermented dairy products are available in our markets. These are
prepared at our homes as well. How are they made and what are the microorganisms
involved in their preparation?
The fermented dairy products assume greater importance in the human diets in India,
as invariably, the diets mostly include milk byproducts especially the cheese, butter,
yogurt, dahi etc. A variety of bacterial yeasts and moulds are involved with the
fermentation of dairy products. We will begin our discussion with cheese.
i) Cheese
There are several varieties of cheese manufactured all over the world. All types
of cheese are the byproducts of lactic fermentation of milk. There are several
varieties of cheese which are classified as hard, semi-hard and soft cheese. These
are prepared with culturing of the milk either with bacterial or mould species.
Among the several varieties, the popular ones are cheddar cheese and swiss
cheese which are known as ‘hard cheese’ whereas roquefort cheese (blue cheese)
is a semi-soft cheese and the soft variety is the camembert cheese. The cheddar
cheese originated from England and was adopted in USA, the colour of which
ranges from white to orange-yellow, depending upon the colour added. The curing
is done with the help of Streptococcus and Lactobacillus. The cheese is without
the gas holes (the eyes), which characterizes the Swiss cheese. The Swiss cheese
is cultured with the help of a mixed culture, L. bulgaricus, Streptococcus
thermophilus and Propionibacterium shermanii which imparts the characteristic
eye formation. The roquefort cheese is prepared by the inoculation of curd with
Penicillium roqueforti and the camembert cheese is produced by the fermentation
with Penicillium camemberti.

ii) Dahi and Yogurt

Dahi or curd is the Indian variety, a version of yogurt and is widely used in the
daily menu of an average Indian. Several organisms are involved in the preparation
of dahi which contains lactic acid. Yogurt is the preparation with the action of
two organisms, Streptococcus thermophilus and Lactobacilius bulgaricus. The
18 ideal ratio of these two organisms is 1:1 for best results. In common usage, dahi
 refers to ‘the domestically prepared fermented milk using a starter of the previous Microbiology of Foods
day’ while yogurt refers to the ‘industrially produced fermented milk using a
particular bacteria as the starter.’

iii) Butter
The microorganisms which are involved in the preparation of butter are
Streptococcus lactis and Streptococcus cremoris which convert the lactose in the
milk to lactic acid. Then the organisms like Streptococcus diacetilactis, Leuconostoc
dextranicum and Leuconostoc citrovorum are involved in imparting the aromatic
flavours to the butter. The preparation of commercial butter involves aging of
cream overnight at 5-10°C and culturing for 15-16 hours with bacteria.

Check Your Progress Exercise 2

1) Fill in the blanks:
a) Saccharomyces cerevisiae, popularly known as ............................. i s
used in the fermentation of dough by raising the dough and giving it
............. and ................................ .
b) A few fermented dairy products are .............................,
.......................,..........................................and ............................ .
c) Dahi is ......................................... prepared while yogurt is
................................ prepared.
d) Roquefort cheese is prepared by inoculation with ................... bacteria
and camembert cheese with .................... bacteria.
e) Organisms .........................., ....................................... and
........................................are involved in imparting aromatic flavours to
the butter.
2) Match the following:
A B
a) Sauerkraut i) Rhizopus oligosporus
b) Soya sauce ii) Soyabean and roasted wheat product
c ) Tempeh iii) Fermented fresh cabbage
d) Yoghurt iv) S.thermophilus and L.bulgaricus
e) Butter v) S.lactis and S.cremoris

1.5.5 Other Fermented Food Preparations

In this section, we shall discuss food preparations that involve a combination of raw
food products, which on fermentation, give appetizing and nutritious preparations.
Can you list a few of such fermented food products which are formed by combination
of raw food materials. Yes, these include idli, dosa, vada, dhokla etc. Let us get to
know about these fermented products, starting with idli – a salty sponge cake, prepared
from cereal and pulse.
i) Idli
Idli is prepared from rice and black gram dal. They are cleaned, washed and
ground in equal proportions and left overnight for fermentation. The
microorganisms involved in this preparation are Leuconostoc mesenteroides, which
grows first in the batter followed by Streptococcus faecalis and Pediococcus
cerevisiae. Once the batter rises sufficiently, it is steam-cooked and served.
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Food Microbiology and ii) Vada
Safety
Vada is prepared by soaking black gram dal in water for sometime, and then
ground to paste. It is then left to ferment at 23-32°C for 3-12 hours, usually
overnight. It is then made into balls and deep fried in vegetable oil. The Black
gram paste is fermented by lactic acid bacteria, with heterofermentative
Leuconostoc mesenteroides being the major organism. The organism produces
carbon dioxide which aerates the product.

iii) Dosa
Dosa is a light, shallow-fried, thin pan-cake. It is prepared from fermenting rice
and black gram overnight. The aeration of batter is caused by lactic acid bacterial
fermentation by Streptococcus faecalis with carbon dioxide production by
Leuconostoc mesenteroides.

iv) Bhatura
Bhatura is prepared from refined wheat flour, salt and sometimes with pepper,
cumin or turmeric and made into dough with water. Curd is used as a starter and
the dough is fermented at 20-30°C overnight. It is rolled and flattened into discs
and deep fried in vegetable oil. The major organisms involved in fermentation
are Streptococcus and Lactobacillus species, introduced by curd.

v) Dhokla
Dhokla is similar to idli, where rice and Bengal gram dal are used in making it
yellow-coloured. Leuconostoc mesenteroids and Streptococcus faecalis are the
bacteria involved in the fermentation. The batter is poured into large sheets, then
steamed and cut into pieces and seasoned.

Apart from helping man in preparing his foods, the microorganisms have contributed
to a great extent in perpetuating man’s desire for alcoholic beverages, which are the
products of plant fermentation. Now let us see how these alcoholic products are
prepared.

1.5.6 Economically Important Fermentation Products

Alcoholic drinks fall into two broad categories: wines and beers. Wines are made
from the juice of fruits and beers from cereal grains. Primitive wines and beers have
been produced, with the aid of yeasts, for thousands of years, although it was not until
about four hundred years ago that microorganisms associated with the fermentation
were observed and identified. It was not until the 1850’s that Louis Pasteur demonstrated
unequivocally the involvement of yeasts in the production of wines and beers. Since
then, the knowledge of yeasts and the conditions necessary for fermentation of wine
and beer has increased to the point where pure culture fermentations are now used to
ensure consistent product quality. Originally, alcoholic fermentations would have been
the spontaneous events that resulted from the activity of microorganisms naturally
present. These non-scientific methods are still used today for the home preparation of
many of the world’s traditional beers and wines. Let us learn about these products.

i) Beer
Beer is an alcoholic product, produced by brewing. It is a principal malt beverage
where the fermentation of carbohydrates to alcohol takes place. Barley is used in
the preparation of beer. Yeasts play a major role in the preparation of beer. Lager
beer is produced by Saccharomyces uvarum, which settles at the bottom of the
fermenting vat and is known as the ‘bottom yeast’.

20
ii) Ale Microbiology of Foods

Ale is produced by the strains of S. cerevisiae, which is collected at the top and
is called as the ‘top yeast’.

iii) Rice beer

Rice beer is a low-alcohol beverage made from rice, which is more popular in
North Eastern India. The rice is milled, water is added and cooked. It is cooled
and then the starter is added and fermented for 18 hours at 20-28°C. Rice beer
is then decanted from the solid residue, which often is used as a breakfast cereal.
The starter introduces a mixture of moulds and yeasts. The rice starch is broken
down to sugars by amylase enzymes of moulds of Rhizopus, Mucor and Aspergillus
species. The yeasts of Endomycopsis and Hansenula then convert the sugars to
ethanol and carbon dioxide. The ragi starter contains moulds, yeasts and lactic
acid bacteria. It is produced by fermenting rice or other starchy powders. This is
used as a starter for several fermented foods.
iv) Wine
Wine is one of the oldest and well-known fermented alcoholic beverages produced
by the fermentation of good and sound grapes which is further processed, known
as aging, before consumption. Although there are other fruit wines, they are not
as popular as grape wine. The grape fruits are crushed to give a “must”. The
fermentation of “must” is initiated by yeasts Kloeckera apiculatus and
Metschnikowia pulcherrima together with the yeasts of Torulopsis, Candida etc.
The main fermentation is by the yeasts, Saccharomyces cerevisiae and S. uvarum
or S. bayanus which converts sugars to ethanol and carbon dioxide.
v) Champagne
Champagne is a product of secondary fermentation. Fresh must, yeast and sugar
are added to the wine selected for champagne preparation.

vi) Distilled liquor products

The distilled alcoholic products of interest are rum, brandy and whisky. These
products are manufactured from the distillation of yeast fermentations of sugar
cane juice, molasses, grains and grain products.
a) Rum
Rum is an alcoholic distillate of fermented sugarcane juice or molasses.

b) Brandy
An alcoholic beverage produced after distillation from fruit wines/grape
wine. The brandy is produced by distilling grapes or other fruit wines.

c) Whisky
Whisky is produced by distilling the fermented mash of wheat, barley, malt
and other grains with Saccharomyces cerevisiae.
After learning about the economically important fermented products, we move on to
studying the other uses of microbes in the industry.

1.5.7 Other Uses of Microbes in Industry

Here in this section, we shall study about the different microbes used in the food
industry to manufacture the products of commercial value namely vinegar citric acid,
antibiotics etc.

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Food Microbiology and i) Vinegar
Safety
If the alcohol produced by the fermentation process is further oxidized to acetic
acid by the acetic acid-producing bacteria, the product is vinegar. Vinegar is made
by different processes. It can be made from fruit juices, starchy vegetables, malted
cereals, sugars and alcohol. Vinegar is widely used as a preservative in food
preparation.

ii) Enzymes
Enzymes are the protein substances produced by the living cells which catalyzes
a biochemical reaction. They accelerate a specific chemical reaction.
Enzymes, which are known as biocatalysts, are very useful in the manufacturing
of several products of commercial value. The enzymes are widely used for the
manufacture of alcoholic beverages etc. The enzyme -amylase used in bread-
making, is commercially prepared from Aspergillus oryzae. The amyloglucosidase,
used as a substitute for malt in the production of beer and spirits, is commercially
prepared from Aspergillus niger. Pectolytic enzymes are produced from a number
of fungi for use in fruit processing. Cellulase enzyme used for removing cellulose
cloud and clarifying juices is produced from the mould Trichoderma viride and
proteases used in cheese-making from Aspergillus niger.
iii) Amino acids and Vitamins
The importance of amino acids and vitamins in human health is well-recognized.
Several microbes have been used for their productions which are biologically
suitable. Yeast is one of the best sources of the vitamin B complex. A number of
preparations of high potency vitamin B-complex made from dried yeast and yeast
extracts are available in the market. Riboflavin, one of the B-group vitamins, is
produced from the yeast Eremothecium ashbyii. Ergosterol, the precursor of vitamin
D is synthesized by a number of moulds and yeasts. -carotene is produced
commercially by fermentation using the fungus Rhodotorula.
iv) Citric acid
Citric acid is one of the widely used chemical which finds applications in several
divergent industries such as pharmaceuticals, flavouring extracts in food
preparations, dyeing etc. Citric acid is produced by the mould Aspergillus niger,
which converts sugars to citric acid where molasses is generally the raw material.
v) Antibiotics
Apart from giving man several food products, certain microorganisms, especially
moulds have given products which are life-saving. The antibiotics are the products
of living organisms, which in small proportions could be acting as inhibitory
agent for the growth of other microbes. The discovery of Penicillin produced by
Penicillium notatum by Sir Alexander Fleming in 1929 has triggered off the
manufacturing of modern antibiotics. There are over 600 antibiotics derived from
bacteria and over 150 from fungi. The genus Streptomyces has yielded a wide
range of useful antibiotics such as streptomycin, aureomycin, chloromycetin and
terramycin.
From our discussion above, it is evident that there is a tremendous scope and
potential for the use of microorganisms towards meeting the growing world demand
for food, through an efficient utilization of available natural food and feed stocks
and the transformation of waste materials.
Check Your Progress Exercise 3
1) Fill in the blanks:
a) Bottom yeast produces ......................., while top yeast produces ..........

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 Microbiology of Foods
b) A low-alcohol beverage is ...................................... .
c) Ragi starter is produced by ............... of rice or other starchy powders
d) The additives in champagne preparation are ........................................ ,
..........................................and ...................................... .
e) Whisky is produced by the distillation of fermented mash of wheat
malt and other grains with .............. .
2) Name a few fermented products which involve the use of Leuconostoc
mesentroides as one of the microorganisms that aid in fermentation.
................................... , ......................................... , ...........................
3) How is vinegar prepared?
............................................................................................................................
............................................................................................................................
............................................................................................................................
4) What are antibiotics? Name a few antibiotics.
............................................................................................................................
............................................................................................................................
............................................................................................................................

1.6 LET US SUM UP

In this unit we started our study of food microbiology by first understanding

the concept and historical development of the discipline. The modern development
in the field of microbiology with respect to biotechnology and probiotics was
highlighted.

Further, we studied about various fermented preparations belonging to different food

groups such as vegetables, pulses, cereals, milk etc. The discussion involved a brief
description of the method of preparation and the organism used for fermentation.
Further, preparation of commonly consumed beverages differing in the alcoholic
contents was described. Finally, a few other uses of microbes in food industry were
discussed. These included vinegar and citric acid, preparations as antibiotics, and in
amino acids and vitamin synthesis.

1.7 GLOSSARY

Antibiotics : products of living organisms, which in small

proportions could act as inhibitory agents for growth
of other microbes.

Baker’s yeast : Saccharomyces cerevisiae, which helps in raising the

dough giving it the texture and flavours.

Deoxyribonucleic acid : a large molecule that contains genetic coding

(DNA) information within each cell.

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Food Microbiology and Enzymes : protein substances produced by living cells which
Safety catalyze a biochemical reaction.
Microorganism : a microscopic organism such as a bacteria, virus, alga,
fungus, protozoan etc.
Probiotics : microorganisms and their culture products which
contribute to the intestinal microbial balance.

1.7 ANSWERS TO CHECK YOUR PROGRESS

EXERCISES

Check Your Progress Exercise 1

1) a) False, Some microbes are useful for example microbes present in the intestinal
flora which protect against disease

b) True

c) True
2) Food microbiology is a branch of microbiology concerned with the relationship
between microorganisms and food. It covers food borne diseases, food hygiene,
food spoilage, fermented foods and beverages, use of microorganisms to produce
food ingredients, microbiological aspects of quality control, microbiological
analysis of food and food legislation.
3) a) Probiotics refers to microorganisms and their culture products which
contribute to the intestinal microbial balance, thus benefitting the host by
protection against disease or improvising the nutrition.
b) Biotechnology is a series of enabling technologies that involve the
manipulation of living organisms or their sub-cellular compounds to make
or modify products to improve plants or animals or to develop microorganisms
for specific uses.
c) Genetic modification is a technique for copying individual genes and
transferring them to another living organism in order to incorporate or delete
specific characteristics.
Check Your Progress Exercise 2

1) a) baker’s yeast; texture, flavours.

b) cheese, dahi, yogurt and butter.
c) domestically, industrially .
d) Penicillium roqueforti, Penicillium camemberti.
e) Streptococcus diacetilactis, Leuconostoc dextranicum, Leuconostoc
citrovorum.
2) a) - iii)
b) - ii)
c) - i)
d) - v)
e) - iv)
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Check Your Progress Exercise 3 Microbiology of Foods

1) a) larger beer, ale b) rice beer

c) fermentation d) must, yeast, sugar

e) S. cerevisiae

2) Idli, Vada, Dosa, Dhokla

3) Vinegar is produced by the oxidations of alcohol to acetic acid by acetic-acid

producing bacteria. It can be made from fruit juices, starchy vegetables, malted
cereals, sugars and alcohol.

4) Antibiotics are the products of living organisms which in small proportions could
act as inhibitory agent for growth of other microbes e.g. streptomycin, aureomycin,
chloromycetin and terramycin.

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