The objectives of this unit are to enable you to understand the important genera
of microorganisms associated with food. This unit gives a brief account of the
morphological, physiological and cultural characteristics of various
microorganisms.
After going through this unit, you should be able to:
• know the various types of microorganisms;
• explain the requirements for their growth;
• learn the classification of these organisms based on their characteristics;
and
• distinguish between the useful and harmful microorganisms.
INTRODUCTION
We already know that the microorganisms use our food as a source of nutrients
for their own growth. This, of course can result in deterioration of the food. By
increasing their numbers, utilizing nutrients, producing enzymatic changes and
contributing off flavors by means of breakdown of a product or synthesis of
new compounds they can “spoil” a food.
When the microorganisms involved
are pathogenic, their association with our food supply is critical from a public
health point of view. Therefore a classification of different organisms and their
growth requirements is required to prevent spoilage of foods.
VARIOUS TYPES OF MICROORGANISMS
Microbes are single-cell organisms so tiny that millions can fit into the eye of a
needle. They are the oldest form of life on earth. Microbe fossils date back
more than 3.5 billion years to a time when the Earth was covered with oceans
that regularly reached the boiling point, hundreds of millions of years before dinosaurs roamed the earth. Without microbes, we couldn’t eat or breathe.
Without us, they’d probably be just fine.
Bacteria
Many of us know bacteria only as “germs,” invisible to naked eyes that can
invade our bodies and make us sick. Few know that many bacteria not only
coexist with us all the time, but help us do an amazing array of useful things
like make vitamins, break down garbage, and even maintain our atmosphere.
These are unicellular microorganisms that are classed as plants. A bacterial cell
is about 1µm in length and somewhat smaller in diameter. Bacteria are
classified according to their shape. Cocci are spherical, bacilli are cylindrical
and spirilla and vibrios are spiral. Bacterial spores are more heat resistant than
yeast or mold spores to most processing conditions. Bacteria, with a few
exceptions cannot grow in acid media in which yeasts and molds thrive. They
multiply by ‘binary fission’.
When a bacterium becomes mature it divides into
two, these two become four and so on. Bacteria can be found virtually
everywhere. They are in the air, the soil, and water, and in and on plants and
animals, including us. A single teaspoon of topsoil contains about a billion
bacterial cells (and about 120,000 fungal cells and some 25,000 algal cells).
The human mouth is home to more than 500 species of bacteria. Some bacteria
(along with archaea) thrive in the most forbidding, uninviting places on Earth,
from nearly-boiling hot springs to super-chilled Antarctic lakes buried under
sheets of ice. Microbes that dwell in these extreme habitats are aptly called
extremophiles.
The growth of bacteria is very rapid and depends upon the
nature of the food material, moisture, temperature and air. Some bacteria do
not grow in air but temperature plays a major role in their growth, the optimum
being generally 37°C for bacteria pathogenic to humans.
Bacteria are very sensitive to acids and are destroyed in their presence even at
temperature of boiling water.
Hence, most fruits being acidic can be easily
sterilized at 100°C whereas vegetables being non-acidic require a higher
temperature of 116°C.
A bacterium’s genetic information is contained in a single DNA molecule
suspended in a jelly-like substance called cytoplasm. In most cases, this and
other cell parts are surrounded by a flexible cytoplasmic membrane that is
itself surrounded by a tough, rigid cell wall. A few species, such as the
mycoplasmas, don’t have cell walls.
Even though bacteria have only one cell each, they come in a wide range of
shapes, sizes, and colours.
The important groups of bacteria are:
a) Bacillus: rod-shaped.
b) Coccus: spherical.
c) Coccobacillus: oval-shaped.
d) Aerobes: require atmospheric oxygen for growth, e.g., Acetobacter aceti.
e) Facultative anaerobes: can grow with or without atmospheric oxygen.
f) Obligate anaerobes: do not grow in atmospheric oxygen.
g) Mesophiles: require a temperature below 38°C for growth.
h) Obligate thermophiles: grow between 38°C and 82°C.
i) Facultative thermophiles:
grow over a wide range of temperatures covered
by mesophiles and obligate thermophiles and below.
j) Psychrotrophs: grow fairly well at refrigeration temperatures and some can
even grow slowly at temperatures below freezing.
Some bacteria have natural colours.
Certain species contain pigments, such as
various chlorophylls, that make them naturally green, yellow, orange, or
brown. Colonies of millions of bacteria may appear pink, yellowish, or white.
Important Food Spoilage Bacteria
Group Genus
Acetics Acetobacter and Gluconobacter
Lactics Lactobacillus, Leuconostoc,
Pediococuus, Sreptococcus
Butyrics Clostridium
Propionics Propionibacterium
Proteolytics Bacillus, Pseudomonas, Closridium,
Proteus etc.
Some useful bacteria
The following bacteria are of great importance in the food processing industry.
Acetobacter sp.
These bacteria, also known as “vinegar bacteria”, cause significant spoilage in
the wine industry but are necessary for vinegar production. The important
species are Acetobacter aceti, A. orleansis and A.schutzenbachi. They are very
small, usually non-motile and generally do not form spores.
These bacteria are
aerobes and in the presence of oxygen convert ethyl alcohol to acetic acid.
These bacteria can be easily destroyed by heating to 65°C.
Lactobacillus sp.
Different organisms of this group, also known as “lactic acid bacteria”, have
different properties but all of them produce lactic acid from carbohydrates.
The
important species include Lactobacillus plantarum, Pediococcus cerevisiae,
Leuconostoc mesenteroides, Streptococcus faecalis and Lactobacillus brevis.
These bacteria cause “lactic souring” and spoil wines, which can be easily
prevented by maintaining a sulphur dioxide concentration of 0.007 per cent in
wine.
Yeasts
Yeasts are unicellular fungi which are widely distributed in nature. They are
somewhat larger than bacteria. The cell length is about 10µm and the diameter
is about a third of this. Most yeasts are spherical or ellipsoidal. Yeasts that
multiply by means of ‘budding’ are known as ‘true yeasts’. Yeasts grow
luxuriously at a moderate temperature in a solution of sugar in plenty of water.
Under suitable conditions the sugar is converted into alcohol and carbon
dioxide is evolved. This is the reason that carbon dioxide is evolved from food
materials spoiled by yeasts and pushes out corks from bottles with great force.
Most of them do not develop in media containing more than 66% sugar or
0.5% acetic acid. Boiling destroys the yeast cells and spores completely.
Some
of the yeasts which grow on fruits are Saccharomyces, Candida and
Brettanomyces.
Pseudo-yeasts
These are like true yeasts but do not form spores. All the members of this
group are particularly unsuitable for fermentation purposes as they produce
off-flavours and cloudiness.
Yeasts causing food spoilage
Yeast Product Spoilage
Saccharomyces Low sugar products
Candida High-acid foods, salty foods, butter
Brettanomyces Beers, wines
Zygosaccharomyces (osmophillic) Honey, syrups, molasses, wines, soy
sauce
Pichia Wines
Hansenula Beers
Torulopsis Milk products, fruit juices, acid
foods
Rhodotorula Meat, sauerkraut
Fungi (Molds)
Fungi are eukaryotic organisms.
This means that their DNA-containing
chromosomes are enclosed within a nucleus inside their cells. (The
chromosomes of bacteria and archaea are not walled off inside nuclei, making
them prokaryotic organisms). Molds are multicellular, filamentous fungi which
are devoid of chlorophyll.
They are larger than yeasts. They are strict aerobes
and require oxygen for growth and multiplication and tend to grow more
slowly than bacteria.
Fungi are lower thallophytic plants but do not make their own food via
photosynthesis like green plants. They feed on organic matter like rotting
leaves, wood, and other debris, or upon the tissues of living plants and animals.
Fungi, along with bacteria, are the planet’s major composters and recyclers.
Although fungi may seem like a nuisance when they grow in your fruit bowl or
refrigerator, their ability to degrade some of the toughest organic materials,
including tree wood and insect exoskeletons, means that our planet is not
cluttered with a mass of debris. Fungi secrete digestive enzymes in order to
break down complex food sources, such as animal corpses and tree stumps,
into smaller components they can absorb.
The principle parts of a mold are a web-like structure known as mycelium and
the spore. The mycelium is often white and cottony and penetrates into the
11
Classification of
Microorganisms
Important in the Food
Industry: Bacteria,
Yeasts and Mold
attacked foodstuff.
After fixing itself the mold produces viable spores which
resist the favourable conditions after the dispersal and germinate when they get
favourable conditions. They thrive best in closed, damp and dark situations
with an adequate supply or warm, moist air but require less free moisture than
yeasts and bacteria.
They prefer sugar containing substances and may spoil
jams, jellies and other sugar-based products. Acid medium favours their
growth and, therefore, they grow well in pickles, juices etc. this is the main
reason that fruit and fruit products are attacked by molds which not only
consume nutrients present in the food thereby lowering its food value but also
spoil the flavour, texture and appearance of the product. Molds are sensitive to
heat; boiling quickly destroys molds and their spores.
The most important
molds are:
a) Penicillium sp. (Blue moulds)
b) Aspergillus sp. (Black moulds)
c) Mucor sp. (Gray moulds)
d) Bysslchlamyces fulva
Classification of Microorganisms
A)
On basis of temperature for growth
Microorganisms can be classified into:
− Thermophillic: Microbes who require high temperature for their growth
and survival (optimum temperature=45-65ºC).
− Thermoduric:
Microbes which do not grow at high temperatures but
can survive in it.
− Mesophillic: Microorganisms which require optimum temperature of
20-50ºC for growth and multiplication.
− Psychrophillic: Microorganisms requiring less than 20ºC as optimal
temperature for growth.
− Psychroduric: Microorganisms which do not grow at low temperature
but can survive.
B)
On basis of oxygen requirement for growth:
− Obligate Aerobes: Require oxygen for growth and multiplication e.g.
molds.
− Obligate Anaerobes: Strictly grow only in absence of oxygen.
− Facultative: Microorganisms than can grow in both presence and
absence of oxygen e.g. yeasts.
− Microaerophillic: Organisms which are able to grow at very low
oxidation-reduction potential.
C) On basis of requirement of water activity.
In general, bacteria require more moisture than yeasts and yeasts more than
molds.
The classification according to requirement of aw is as follows:
Group of microorganism Minimal aw value
Bacteria 0.91
Yeast 0.88
Molds 0.80
Halophillic bacteria 0.75
Xerophillic fungi 0.65
Osmophillic Yeasts 0.60
− Halophillic bacteria: Bacteria which grow in high salt solutions
− Osmophillic Yeasts: Yeasts which can grow best in high concentrations
of sugar
− Xerophillic Fungi: Fungi which can grow in low water activity
D)
On basis of nutrient degradation capacity:
− Proteolytic: Microorganisms which are capable of protein degradation
because of extracellular proteinases produced.
− Lipolytic: Microbes which catalyxe the hydrolysis of fats to fatty acids
and glycerol.
− Sacchrolytic: These microorganisms hydrolyse disaccharides or
polysaccharides to simpler sugars.
− Pectinolytic:
These microorganisms hydrolyse pectin.
E) On basis of staining:
On basis of staining the bacteria can be classified as:
− Gram positive: Those bacteria that stain violet after Gram stain test. In
these the cell wall is mostly comprised of peptidoglycan layer.
− Gram negative: Those bacteria that do not stain violet after Gram stain
test. Cell wall mainly comprised of lipopolysaccharides.