INTRODUCTION
Microorganisms use our food supply as a source of nutrients and energy. They
increase their numbers by utilizing nutrients. This can result in a deterioration
of the food. They produce enzymatic changes and off-flavours in food by
breaking down a nutrient or synthesizing new compounds. Thus, they “spoil”
our food and make it unfit for consumption.
To prevent this we reduce the
contact between microorganisms and our foods (prevent contamination) and
also eliminate microorganisms from our foods, or adjust conditions of storage
in such a way that their growth is prevented (preservation) and thus, there is
no spoilage of food.
If the microorganisms involved are pathogenic, then their presence in our food
will lead to outbreak of food borne diseases also.
Many of our foods support
the growth of pathogenic microorganisms or serve as a source of them. Here
again, we attempt to prevent their entrance and growth in our foods or
eliminate them by processing.
Interactions between microorganisms and our foods are also beneficial.
Many
of the cultured products consumed and enjoyed for example cultured
buttermilk, yoghurt, sauerkraut, pickles and tofu are produced as a result of
beneficial activities of microorganisms.
Food is the substrate for growth of microorganisms, so the characteristics of a
food are important. Food or substrate will determine which microorganisms
can or cannot grow on it so there is a need to understand the characteristics of
the food or substrate.
Then only one can make predictions about the microbial
flora that may develop and flourish in it. This microflora will bring about the
biochemical changes in food due to their activities. The types of biochemical
changes will determine whether those changes are beneficial or harmful.
Knowledge of the factors that favour or inhibit the growth of microorganisms
is very important.
It will help us in understanding the principles of food
spoilage and preservation. The chief compositional factors of food that
influence microbial activity are hydrogen-ion concentration, moisture,
oxidation-reduction (O-R) potential, nutrients, biological structure and
presence of inhibitory substances.
HYDROGEN-ION CONCENTRATION
The acidity and alkalinity (pH) of an environment has a strong influence on
the activity and stability of macromolecules such as enzymes. These enzymes
play an important role during growth of microorganisms and in their
metabolism. Thus, growth and metabolism of microorganisms are influenced
by pH.
the activity and stability of macromolecules such as enzymes. These enzymes
play an important role during growth of microorganisms and in their
metabolism. Thus, growth and metabolism of microorganisms are influenced
by pH.
Effect on Microbial Growth
Every microorganism has a minimal, a maximal, and an optimal pH for
growth. In general, bacteria grow in the pH range of 6.0–8.0, yeasts 4.5-6.0
and filamentous fungi 3.5-4.0. Molds can grow over a wider range of pH than
most yeasts and bacteria, and many molds grow at acidities too high for yeasts
and bacteria.
growth. In general, bacteria grow in the pH range of 6.0–8.0, yeasts 4.5-6.0
and filamentous fungi 3.5-4.0. Molds can grow over a wider range of pH than
most yeasts and bacteria, and many molds grow at acidities too high for yeasts
and bacteria.
Most fermentative yeasts grow well in pH range of about 4.0 to
4.5, eg. fruit juices, and film yeasts grow well on acid foods, such as
sauerkraut and pickles. On the other hand, most yeast do not grow well in
alkaline foods and thus do not have a significant role to play in the spoilage of
food products with high pH.
4.5, eg. fruit juices, and film yeasts grow well on acid foods, such as
sauerkraut and pickles. On the other hand, most yeast do not grow well in
alkaline foods and thus do not have a significant role to play in the spoilage of
food products with high pH.
However, large number of yeasts grow well in
near neutral pH. There are some exceptions for example some bacteria can
grow in moderate acidity particularly those bacteria that produce large acids as a result of their activities like lactobacilli and acetic acid bacteria. These have
pH optima between 5.0 and 6.0 and others like the proteolytic bacteria can
grow in foods with a high (alkaline) pH, as found in the stored egg white.
near neutral pH. There are some exceptions for example some bacteria can
grow in moderate acidity particularly those bacteria that produce large acids as a result of their activities like lactobacilli and acetic acid bacteria. These have
pH optima between 5.0 and 6.0 and others like the proteolytic bacteria can
grow in foods with a high (alkaline) pH, as found in the stored egg white.
Bacteria are more sensitive to pH than molds and yeasts, with the pathogenic
bacteria being the most sensitive amongst them. The pH values of some of the
common foods along with the pH range for growth of some groups of
microorganisms and a few of food associated pathogenic bacteria are given in
Table 2.1.
bacteria being the most sensitive amongst them. The pH values of some of the
common foods along with the pH range for growth of some groups of
microorganisms and a few of food associated pathogenic bacteria are given in
Table 2.1.
Food – pH range – Organism – pH range
Citrus fruits – 2-5 – Molds – 0-11
Soft drinks – 2.5-4 – Yeasts- 1.5-8.5
Beer – 3.5-4.5 – Lactic acid bacteria – 3.2-10.5
Meat – 5.5-6.2 – Staphylococcus aureus – 4-9.8
Fish – 6.5-7.3 – Salmonella spp. – 4.1-9
Egg white – 8.6-9.6 – Escherichia coli – 4.3-9
Milk – 6.5-7 – Yersinia enterocolitica – 4.5-9
Flour – 6.2-7.2 – Clostridium botulinum – 4.8-8.2
Vegetables – 4.8-7 – Clostridium perfringens – 5.4-8.7
Fermented shark – 10-12 Bacillus cereus – 4.7-9.3
pH minima and maxima of microorganisms also varies due to other important
factors like temperature, moisture content, salt concentration, redox potential
etc. For example, in the presence of 0.2 M NaCl, Alcaligenes faecalis can
grow over a wider pH range than in the absence of NaCI or in the presence of
0.2 M sodium citrate.
factors like temperature, moisture content, salt concentration, redox potential
etc. For example, in the presence of 0.2 M NaCl, Alcaligenes faecalis can
grow over a wider pH range than in the absence of NaCI or in the presence of
0.2 M sodium citrate.
The pH minima of certain lactobacilli also depends upon
the type of acid used, for example with citric, hydrochloric, phosphoric and
tartaric acids growth can occur at lower pH than in presence of acetic or lactic
acids.
the type of acid used, for example with citric, hydrochloric, phosphoric and
tartaric acids growth can occur at lower pH than in presence of acetic or lactic
acids.
In general, yeast and molds are more acid-tolerant than bacteria.
When microorganisms are grown at pH either higher or lower than their
optimum pH there is an increase in lag phase of the microbe. The increased
lag would be of longer duration if the food has a good buffering capacity in
contrast to one that has poor buffering capacity. Good buffering capacity of
food would result in slower change in pH of food due to microbial activity.
When microorganisms are grown at pH either higher or lower than their
optimum pH there is an increase in lag phase of the microbe. The increased
lag would be of longer duration if the food has a good buffering capacity in
contrast to one that has poor buffering capacity. Good buffering capacity of
food would result in slower change in pH of food due to microbial activity.
A
respiring microbial cell is adversely affected by pH since it affects the
functioning of enzymes and the transport of nutrients into the cell. In addition
to the effect of pH on rate of growth of microorganisms, pH also affects rate
of survival of microorganisms during storage, heating, drying and other forms
of processing.
respiring microbial cell is adversely affected by pH since it affects the
functioning of enzymes and the transport of nutrients into the cell. In addition
to the effect of pH on rate of growth of microorganisms, pH also affects rate
of survival of microorganisms during storage, heating, drying and other forms
of processing.
Many times the initial pH may be suitable, but growth of the
organism itself may alter the pH, thereby making it unfavourable. Conversely,
the initial pH may be restrictive, but the growth of a limited number of
microorganisms may alter the pH to a more favourable range for the growth of
many other microorganisms.
organism itself may alter the pH, thereby making it unfavourable. Conversely,
the initial pH may be restrictive, but the growth of a limited number of
microorganisms may alter the pH to a more favourable range for the growth of
many other microorganisms.
The inherent pH of foods varies, although most are neutral or acidic. Materials
with an alkaline pH generally have a rather unpleasant taste with some
exceptions like egg white where the pH increases to around 9.2, as CO2 is lost
from the egg after laying. The pH of a product can be easily determined with a pH meter.
with an alkaline pH generally have a rather unpleasant taste with some
exceptions like egg white where the pH increases to around 9.2, as CO2 is lost
from the egg after laying. The pH of a product can be easily determined with a pH meter.
However, this value alone is not sufficient for predicting microbial
spoilages. It is also desirable, for example, to know the acid responsible for a
given pH, because some acids, particularly the organic acids, are more
inhibitory than others.
spoilages. It is also desirable, for example, to know the acid responsible for a
given pH, because some acids, particularly the organic acids, are more
inhibitory than others.
Effect on Microbial Ecology and Food Spoilage
The acidity of a product plays an important role in deciding the type
microflora present in food and the rate and type of its spoilage. For example,
most of the meats and seafoods have a final ultimate pH of about 5.6 and
above. Thus, these products are susceptible to bacterial as well as to mold and
yeast spoilage.
microflora present in food and the rate and type of its spoilage. For example,
most of the meats and seafoods have a final ultimate pH of about 5.6 and
above. Thus, these products are susceptible to bacterial as well as to mold and
yeast spoilage.
Similarly, most vegetables have higher pH values than fruits,
and thus vegetables would be more prone to bacterial than fungal spoilage
since such pH values favour bacterial growth. Soft-rot producing bacteria such
as Erwinia carotovora and pseudomonads play a significant role in their
spoilage.
and thus vegetables would be more prone to bacterial than fungal spoilage
since such pH values favour bacterial growth. Soft-rot producing bacteria such
as Erwinia carotovora and pseudomonads play a significant role in their
spoilage.
In fruits, however, a lower pH (below 4.5) prevents bacterial growth
and yeasts and molds dominate spoilage.
Fish is spoiled more rapidly than meat under chilled conditions. This is due to
the fact that the pH of post-rigor mammalian muscle is around 5.6 and this
contributes to the longer storage life of meat. On the other hand, fish have a
pH between 6.2-6.5.
and yeasts and molds dominate spoilage.
Fish is spoiled more rapidly than meat under chilled conditions. This is due to
the fact that the pH of post-rigor mammalian muscle is around 5.6 and this
contributes to the longer storage life of meat. On the other hand, fish have a
pH between 6.2-6.5.
Shewanella (formerly Alteromonas) mainly causes
spoilage under chilled conditions. It is a pH-sensitive microbe and hence,
plays a significant role in fish spoilage but not in normal meat (pH<6.0).
Those fishes that have a naturally low pH such as halibut (pH~5.6) as a result
have better keeping qualities than other fish.
spoilage under chilled conditions. It is a pH-sensitive microbe and hence,
plays a significant role in fish spoilage but not in normal meat (pH<6.0).
Those fishes that have a naturally low pH such as halibut (pH~5.6) as a result
have better keeping qualities than other fish.
Thus, a food with inherently low
pH would tend to be more stable microbiologically than a neutral food.
Upon the death of a well-rested meat animal, the usual 1% glycogen is
converted into lactic acid, which directly causes a depression in pH values
from about 7.4 to about 5.6. Most of the bacteria cannot tolerate lower pH,
hence meat has a longer storage life.
pH would tend to be more stable microbiologically than a neutral food.
Upon the death of a well-rested meat animal, the usual 1% glycogen is
converted into lactic acid, which directly causes a depression in pH values
from about 7.4 to about 5.6. Most of the bacteria cannot tolerate lower pH,
hence meat has a longer storage life.
Meat from fatigued animals spoils faster
than that from rested animals. This is because most of the glycogen present
had already been used during its lifetime and hence, final pH attained upon
completion of rigor mortis is not as low as that of a well-rested animal. Thus,
bacteria are able to grow and spoil it.
than that from rested animals. This is because most of the glycogen present
had already been used during its lifetime and hence, final pH attained upon
completion of rigor mortis is not as low as that of a well-rested animal. Thus,
bacteria are able to grow and spoil it.
The excellent keeping quality of certain foods is related to their restrictive pH,
for example fruits, soft drinks, fermented milks, sauerkraut and pickles which
have an acidic pH. Fruits, soft drinks, vinegar, and wines have an excellent
keeping quality mainly due to pH, which falls far below the point at which
bacteria normally grow.
for example fruits, soft drinks, fermented milks, sauerkraut and pickles which
have an acidic pH. Fruits, soft drinks, vinegar, and wines have an excellent
keeping quality mainly due to pH, which falls far below the point at which
bacteria normally grow.
Fruits generally undergo mold and yeast spoilage, and
this is due to the capacity of these organisms to grow at pH values < 3.5,
which is considerably below the minima for most food spoilage and all food
poisoning bacteria.
this is due to the capacity of these organisms to grow at pH values < 3.5,
which is considerably below the minima for most food spoilage and all food
poisoning bacteria.
Some foods have a low pH because of inherent acidity; others, for example,
the fermented products like sauerkraut, pickles and fermented milks have a
low pH because of acidity produced due to the activity of microorganisms.
the fermented products like sauerkraut, pickles and fermented milks have a
low pH because of acidity produced due to the activity of microorganisms.
This acidity is also known as biological acidity and is generally due to the
accumulation of lactic acid during fermentation. Regardless of the source of
acidity, the effect upon keeping quality appears to be the same. This ability of
low pH to restrict microbial growth has been employed since the earliest times
for preservation of foods using acetic acid and lactic acids.
accumulation of lactic acid during fermentation. Regardless of the source of
acidity, the effect upon keeping quality appears to be the same. This ability of
low pH to restrict microbial growth has been employed since the earliest times
for preservation of foods using acetic acid and lactic acids.
Inhibition of Microbes by Weak Acids
With the exception of those soft drinks that contain phosphoric acid, in most other acidic foods acidity is due to the presence of weak organic acids. These
do not dissociate completely into protons and conjugate base in solution but
establish equilibrium:
do not dissociate completely into protons and conjugate base in solution but
establish equilibrium:
HA H + + A−
The partial dissociation of weak acids, such as acetic acid, plays an important
role in their ability to inhibit microbial growth. Although addition of strong
acids has a more profound effect on pH but at the same pH, they are less
inhibitory than weak lipophilic acids.
role in their ability to inhibit microbial growth. Although addition of strong
acids has a more profound effect on pH but at the same pH, they are less
inhibitory than weak lipophilic acids.
This is because microbial inhibition by
weak acids is directly related to the concentration of undissociated acid
(Figure 2.1). These undissociated lipophilic acid molecules can pass freely
through the membrane, in doing so they pass from an external environment of
low pH where the equilibrium favours the undissociated molecule to the high
pH of the cytoplasm.
weak acids is directly related to the concentration of undissociated acid
(Figure 2.1). These undissociated lipophilic acid molecules can pass freely
through the membrane, in doing so they pass from an external environment of
low pH where the equilibrium favours the undissociated molecule to the high
pH of the cytoplasm.
At this higher pH, the equilibrium shifts in favour of the
dissociated molecule, so the acid ionizes producing protons. These protons
tend to acidify the cytoplasm. The cell tends to maintain its internal pH by
expelling protons leaking in.
dissociated molecule, so the acid ionizes producing protons. These protons
tend to acidify the cytoplasm. The cell tends to maintain its internal pH by
expelling protons leaking in.
This process requires energy and the microbe
diverts energy from growth related functions to removing protons from the
cell thereby slowing its growth. The burden on the cell becomes too great. The
cytoplasmic pH drops to a level where growth is no longer possible and the
cell eventually dies.
diverts energy from growth related functions to removing protons from the
cell thereby slowing its growth. The burden on the cell becomes too great. The
cytoplasmic pH drops to a level where growth is no longer possible and the
cell eventually dies.
Strong acids on the other hand dissociate completely into
protons and conjugate base in solution. These dissociated acid molecules
cannot pass freely through the cell membrane. Hence there is not much change
in the pH of the cytoplasm. As a result these are less inhibitory than weak
acids at the same pH.
protons and conjugate base in solution. These dissociated acid molecules
cannot pass freely through the cell membrane. Hence there is not much change
in the pH of the cytoplasm. As a result these are less inhibitory than weak
acids at the same pH.
Buffers in Foods
Some foods are better able to resist changes in pH than others. These tend to
resist changes in pH since these are buffered and the ability to resist changes
in pH is known as buffering capacity. The buffers are the compounds present
in food that resist changes in pH and thus are important. These are especially
effective within a certain pH range.
resist changes in pH since these are buffered and the ability to resist changes
in pH is known as buffering capacity. The buffers are the compounds present
in food that resist changes in pH and thus are important. These are especially
effective within a certain pH range.
Buffers permit an acid (or alkaline)
fermentation to go on longer with a greater yield of products and organisms than would otherwise be possible. In general, meats are more buffered than
vegetables. Contributing to the buffering capacity of meats are their various
proteins. Vegetables are generally low in proteins and consequently lack the
buffering capacity to resist changes in their pH by the growth of
microorganisms.
fermentation to go on longer with a greater yield of products and organisms than would otherwise be possible. In general, meats are more buffered than
vegetables. Contributing to the buffering capacity of meats are their various
proteins. Vegetables are generally low in proteins and consequently lack the
buffering capacity to resist changes in their pH by the growth of
microorganisms.
Hence these permit an appreciable decrease in pH with the
production of small amounts of acid by the lactic acid bacteria during the early
part of sauerkraut and pickle fermentations. This is desirable since it enables
the lactic acid bacteria to suppress the undesirable pectin-hydrolyzing and
proteolytic organisms which cause spoilage.
production of small amounts of acid by the lactic acid bacteria during the early
part of sauerkraut and pickle fermentations. This is desirable since it enables
the lactic acid bacteria to suppress the undesirable pectin-hydrolyzing and
proteolytic organisms which cause spoilage.
Low buffering power makes for a
more rapidly appearing succession of microorganisms during fermentation
than high buffering power. Milk is fairly high in protein (a good buffer) and
therefore permits considerable growth and acid production by lactic acid
bacteria during the manufacture of fermented milks before growth is
suppressed.
more rapidly appearing succession of microorganisms during fermentation
than high buffering power. Milk is fairly high in protein (a good buffer) and
therefore permits considerable growth and acid production by lactic acid
bacteria during the manufacture of fermented milks before growth is
suppressed.