FACTORS THAT INFLUENCE MICROBIAL ACTIVITIES IN FOOD

Internal

1. Nutrient contents

2. Water activity

3. Redox potential

4. pH

5. Osmotic pressure

External

1. Temperature

LECTURE 4

FACTORS THAT INFLUNCE MICROBIAL ACTIVITIES IN FOOD

Numbers of microorganisms in food and their growth kinetic

The significance of food contamination depends on the numbers and types of microorganisms and the opportunity for growth

Levels of contamination will range from calculated upper limits to extremely lower limit. Two guide microorganisms were used:

Escherichia coli representing bacteria Sacchromyces cerevisiae representing yeasts

Upper limit: The maximum level of contamination when the organism occupies the available space may be calculated:

1 or 2 x 1012 bacterial cells /ml or /g1 x 1012 yeasts cells / ml or /g

Lower limits: The minimum level of contamination found in food may be less than 1 cell / ml or /g

Sterile: means the absence of all living organisms

Growth kinetics

The growth curve is characterized by 4 basic phases:

1) lag phase2) exponential phase3) stationary phase4) death phase

Lag phase

During this phase the cell numbers do not increase. The microorganisms adapt to the environment and intra cellular metabolic pathways are established.

Exponential phase

Microbial numbers increase at the maximum rate allowed by environmental conditions

The length of this phase varies usually it last for 2 hours, but may extend over several hours.

The overall process of multiplication involved both the absorption of nutrients from the substrate into the cell and the removed of waste or catabolic product from the cell into the substrate.

Thus microbial growth results in physio-chemical changes of the environment. These become great enough to make the environment unsuitable for further growth.

In this condition, the microbial population has reached to the stationary phase

Stationary phase

At this stage, due to nutrient shortage and accumulation of waste products, a few cells die and a few cells multiply, keeping the living population stable.

Death phase

The population enters into the death phase where the rate of cell death is higher than the rate cell multiplication

The shape of the growth curve is affected by factors

a) temperature higher or lower than optimumb) limitation of nutrients for the microorganismc) presence of natural antimicrobial compound or

disinfectants

Internal

1. Effect of nutrients on bacterial growth rates

Microorganisms require foods for energy, growth and proper cell functioning.

Nutrients include carbohydrate, protein, lipid, minerals and vitamins

If nutrients are abundant, growth rate is faster.

Microbes normally found in food vary greatly in nutrient requirement.

Bacteria ↓ Nutrient requirement

YeastMold

They are also vary in their ability to utilize complex carbohydrate and production of extracellular enzymes in order to hydrolyze complex molecules.

Food is important to generate energy - mainly derived from carbohydrate sources. Other carbon compounds that can be used as energy source are alcohol, organic acids, amino acids, peptides etc.

It is also important for growth - mainly derived from protein sources, nitrogen sources, peptides, amino acids, urea, ammonia etc.

Vitamins and minerals are important for cell functioning.

2. Effect of water activity on bacterial growth rates

Water activity is a measure of the availability of water for biological functions.

Relates to water present in "free" and "bound" forms.

Bound water: Dissolve solutes: Not available for biological functions : Does not contribute to Aw

Aw in food differ between 0.1 - 0.9

Cereals, cracker, sugar, salt 0.1-0.2Honey, chocolate <0.6Jam, jelly 0.6-0.85Condensed milk 0.85-0.93Tomato paste, fruit juice 0.93-0.98

Aw can be reduced by removing water and can be increased by absorption of water. This is important in the control of microorganism.

Aw ↓ Remove of water

Aw ↓ Absorption of water

Microorganisms need water to grow. Free water is necessary for microbial growth especially in process like transport of nutrients, removal of waste materials and carry out enzymatic reactions.

Each microbial species has an optimum, maximum and minimum Aw level for growth.

Bacteria 0.9Molds 0.6Yeasts 0.85

Perishable foods such as milk, meat etc are easily spoilt because of the high Aw.

As Aw is lowered, the ability to grow will reduce. Most spoilage bacteria do not able to grow Aw0.9

To prevent spoilage, we need to reduce the water activity by desiccation, freezing etc.

Shelf life is extended as more water is reduced.

Molds and yeast are more tolerant to low water activity than the bacteria. They play an important role in spoilage of bread and dried foods.

The most xerophillic bacteria can only withstand up to 0.65 whereas fungi 0.6

If Aw is reduced to 0.65 / 0.60 the food cannot spoil for at least one year (if other parameters are also controlled).

3. Effect of pH on bacterial growth rates

Food can be grouped as:

a) Low acids foods (pH > 4.6) Meat, fish, milk and soups

b) High acids foods (pH<4.6)Fruits and juices, vegetables and salad dressings.

Microorganisms grow in different pH range:

Molds pH1.5 - pH9.0Yeast pH2.0 - pH8.5Bacteria pH6.5 - pH7.5

Most bacteria have limited pH range. Optimal pH value for growth is near neutral.

The ability of microorganism to grow well will be inhibited if the pH of the surrounding is higher or lower than the optimal pH value.

Most pathogenic bacteria are very sensitive to low pH value. Soft drinks and fruits are very acidic and not suitable for bacterial growth.

Proteolytic bacteria can grow in media with high pH.

Vegetables have higher pH than fruits. Therefore they are subjected to spoilage by bacteria.

Molds have a wider pH range. Therefore they are able to tolerate low pH better than bacteria and yeast. Molds can cause spoilage of soft drinks, fruits and honey.

4. Effect of redox potential on bacterial growth rates

The redox potential is a measure of potential difference in a system or food.

It can be generated by a couple reaction:

a. Oxidation: Loss of electron by a substance (i.e. the substance is an electron donor) or known as reducing agent.

b. Reduction: Gain of an electron by another substance (i.e. an oxidizing agent). The substance is an electron acceptor

The redox potential is measured in electrical units of milivolts designated as Eh.

a) In the oxidized range, the redox potential is + mV

b) In the reduced range, the redox potential is -mV.

The redox potential of a food is influenced by its chemical composition, specific processing treatment given and storage condition in relation to air (vacuum packed, liquid N2, C02 etc).

Fresh foods of plant and animal origin are in a reduced state due to presence of reducing substances such as sugars, ascorbic acids and sulphydryl group of protein.

Diffusion of oxygen into these fresh food substances can change the redox potential of the food.

Processing food such as heating can also alter the Eh (reduced or oxidized). Food stored in air can have a wide Eh range (+Mv) than when stored under vacuum.

The presence or absence of oxygen in the environment is important in the growth of microorganisms. Microorganisms can be grouped into categories based on their requirement on intolerance to oxygen

1) Aerobes• Grow in the presence of air that contains molecular

oxygen.

• Obligate aerobes require oxygen for growth and carry out aerobic respiration.

2) Microaerophiles

• Grow only at reduced concentrations ofmolecular oxygen - 5% of atmospheric oxygen concentration (20%)

3) Facultative anaerobes

• Can grow in the presence or absence of air. If oxygen is not available, they will carry out anaerobic respiration

4) Anaerobes

• Do not require oxygen for growth, therefore only in the absence of air.

• Strict anaerobes are sensitive to oxygen and even to a brief exposure to oxygen will kill such organisms e.g. Clostridium spp.

Grow of all these microorganisms and their metabolic reactions are extremely dependent on positive redox potential of food.

The range of Eh at which different groups of microorganisms grow are:

Aerobes Between +500 and +300 mVAnaerobes Between +100 and -250 mVFacultative anaerobes Between +300 and -100 mV

The presence or absence of oxygen and the Eh of food determine the growth of a particular microbial group in foods. This is important in microbial spoilage of foods and in desirable characteristics of fermented foods.

Microbial spoilage: putrefaction of meat by Clostridium spp.

Fermentation: Penicillium spp. in blue cheese

5. Effect of osmotic pressure on bacterial growth rates

Osmotic pressure is an internal experience by microorganisms.

When microorganisms is in sugar or salt solutions, water from microorganisms will migrate, which cause microorganisms to experience higher osmotic pressure.

As the solute concentration is higher, osmotic pressure is higher, thus more inhibition to growth.

Most microorganisms are not able to tolerate high osmotic pressure except the halophiles such as Pseudomonas spp. And

Vibrio spp. (0.5% - 0.3% salt concentration) and also osmpohiles such as Leuconostoc spp. Spoilage of food with higer sugar and salt content is usually caused by osmophiles and halophiles respectively.

External

1. Effects of temperature on bacterial growth rates

Temperature can greatly influence the rate of reaction or enzymatic activities.

The temperature at which food is held will influence the growth rate of microorganisms, hence the rate of spoilage.

Microorganisms grow over a wide range of temperature. With respect of temperature, there are 3 groups of microorganisms:

a) Psychrophiles: Grow best at low temperatures (<20°C), cause spoilage of refrigerated foods.

b) Mesophiles: Grow best at moderatetemperatures (20 - 40°C).

c) Thermophiles:Grow best at high temperatures (> 45/50°C).

Two other important terminologies:

a) Psychrotrophs: Microorganisms that can grow at refrigerated temperature (0 - 5°C).

b) Thermoduric: Microorganisms that can survive pasteurization process.

FACTORS AFFECTING HEAT RESISTANCE (HR) OF MICROORGANISMS

1. Time and temperature As time and temperature increase, the death rate increases

Temperature ↑Death ↑Time required for killing ↓

2. Type of microorganisms

HR related to the optimal growth temperature of the microorganisms

Thermophiles > Mesophiles > Psycrophiles

Psychrophiles grow well in refrigeration temperature as temperature is increased, death occur rapidly.

Thermophiles - high growth rate at high temperature. If the temperature is higher than the optimum temperature, growth rate declines.

Spores are more resistant than the vegetative cells.

Gram positive bacteria are more resistant than gram negative.

Molds and yeast are not heat resistant. Killed at 70°C -80°C.

3. Number of microorganisms The higher the number of microorganisms, the higher is the

degree of heat resistance due to:

a) Higher production of protective substances / protein extrcellular component.

b) More varieties of microorganisms with different heat resistant.

4. pH Microorganisms are most resistant at their optimal pH. If pH is

increased or decreased from the optimum, the HR is reduced.

High acid food required less heating

Examples: Effect of pH on B.subtilis spores pH 4.4 - 2 minutes survival time pH 7.6 - 11 minutes survival time

5. Water

High humidity (more water) - less heat resistant.Less water - more heat resistant.

More water, better heat penetration. Denaturation of protein also faster. Therefore kill microorganisms in shorter time.

6. Food ingredient / medium / substrate

A microorganism in food with high fat is more resistant due to fat protection. Long chain fatty acids provide better protection than the short chain.

Microorganisms with high proteins are more resistant. Proteins are colloids, which also provide better protection.