Factors affecting the growth of microorganisms in food

Foods that are both animal and plant source play a significant role in the development of microbial communities. The ability of microorganisms develop or multiply in such food is contingent on the environment. The intrinsic, extrinsic and implicit food factors and techniques for food processing are all involved in the growth of microorganisms. These factors influence the growth of microbial colonies in food, as well as the particular pathways that they utilize to create the energy needed and other metabolic products.

A. Intrinsic factors

The self-parameters within animals and plants are known as intrinsic factors. These comprise:

1. pH

In general, yeasts and molds are able to grow with lower pH levels than Gram-negative bacteria. Gram-negative bacteria have a greater sensitivity to pH levels below in comparison to Gram-positive bacteria. The pH range for growth for molds ranges from 1.5 to 9.0 and the range for yeasts is 2.0 to 8.5 and for Gram-positive bacteria 4.0 to 8.5 as well as Gram-negative bacteria, 4.0 to 8.5; and for Gram-negative bacteria, 4.5 and 9.0. In accordance with the pH levels, microorganisms may be classified as:

• Neutrophils thrive in an alkaline pH between 5 and 8.
• Acidophiles thrive best when they have the pH of 5.5.
• Alkaliphiles thrive best when they have the pH of 8.5.

Microorganisms	pH required for microbial growth
Minimum	Optimum	Maximum
Bacteria
Bacillus subtilis	4.0	5.4 – 6.3	9.4 – 10
Clostridium Botulinum	4.8 – 5.0	6.0 – 8.0	8.5 – 8.8
Clostridium perfringens	5.0 – 5.5	6.0 – 7.6	8.5
Escherichia coli	4.3 – 4.4	6.0 – 8.0	9.0 – 10
Lactobacillus (most)	3.0 – 4.4	5.5 – 6.0	7.2 – 8.0
Pseudomonas aeruginosa	5.6	6.6 – 7.0	8.0 – 9.0
Salmonella Typhi	4.0 – 4.5	6.5 – 7.2	8.0 – 9.0
Staphylococcus aureus	4.0 – 4.7	6.0 – 7.0	9.5 – 9.8
Yeasts
Hansenula	–	4.5 – 5.5	–
Saccharomyces cerevisiae	2.0 – 2.4	4.0 – 5.0	–
Saccharomyces rouxii	1.5	3.5 – 5.5	8.5 – 10.5
Molds
Aspergillus niger	1.2	3.0 – 6.0	–
Aspergillus oryzae	1.6 – 1.8	–	9.0 – 9.3
Mucor	–	3.0 – 6.1	9.2
Penicillium	1.9	4.5 – 6.7	9.3
Rhizopus nigricans	–	4.5 – 6.0	–

Based on pH, foods can be grouped as

Foods	pH	Examples
Highly acidic	<3.7	Berries, Sauer-Kraut
Acidic	3.7-4.6	Tomato, Pineapple
Medium acidic	4.6-5.3	Asparagus, Pumpkins, Beets, Spinach
Low acidic	>5.3	Pear, Corn, Bean, Meat, Fish

2. Water activity (aw)

The activity of water (aw) refers to the quantity of water used to biological processes that can be decreased through an impact of osmotic. The food industry makes water available in a variety of ways, like

• Solutes and ions bind solutions of water
• Hydrophilic colloids
• Crystallization of water, or the water of hydration

Microorganisms require water in its most accessible form to thrive in food. The food’s water activity can be measured in the ratio of volume of water that the foods vaporizes compared to pur water of the exact temperature. It can range from 0 to 1, since every food has any water activity that is either 1, or 0.

Different foods and water-related activities include:

Foods	Water activity (aw)
Fresh fruits and vegetables Fresh poultry or fish Fresh meat Eggs Cheese(most type) Nuts Jam Flour Dried fruits Sugar Cereals	0.97-1.00 0.98-1.00 0.95-1.00 0.97 0.91-1.00 0.66-0.84 0.75-0.80 0.67-0.87 0.51-0.89 0.10 0.10-0.20

Bacteria require greater amount of water activity to growth than fungi. The bacteria cannot develop below 0.91 and molds may be as small as 0.80. Gram-negative bacteria are more vulnerable to low water activity than gram positives.

Based on the range of water activity microorganisms may be classified in the category of

• Halotolerant, which can develop when there are large amounts of salt.
• Osmotolerant can develop when there are the highest concentrations of organic compounds that are unionized, such as sugars
• Aerotolerant can develop on dry food.

Some microorganisms as well as their needed range of the aw include:

Microorganisms	Water activity requirement
1.Bacteria Bacillus cereus Clostridium botulinum Clostridium perfringes Escherichia coli Lactobacillus Pseudomonas aeruginosa Salmonella	0.92-0.95 0.90-0.98 0.93-0.97 0.94-0.97 0.90-0.96 0.96-0.98 0.93-0.96
2. Most yeastsHansenula Saccharomyces cerevisiae Saccharomyces rouxii	0.89-0.93 0.90-0.94 0.62-0.81
3. MoldsAspergillus niger Aspergillus flavus Mucor Penicillium Fusarium	0.80-0.84 0.7-0.90 0.80-0.93 0.78-0.90 0.80-0.92

3. Oxidation-reduction potential (Eh)

The redox potential or oxidation-reduction of a substance can be defined as the measurement of a exchange of electrons among atoms and molecules. The oxidation-reduction potency is typically described as Eh and is measured in millivolts (mV). The Redox potential of food items is contingent on the

• The pH of food
• The availability of oxygen (physical state packaging)
• Capacity for buffering or poising
• The composition of food (such as the composition of protein, ascorbic acid and reduction of sugars)

A variety of food and their Eh line:

Food	E(mV)
Raw meat Raw minced meat Wheat Barley Spinach Grape Lemon	-200 +225 -320 to -360 +225 +74 +409 +383

The Eh range that different groups of microorganisms are able to develop can be found as below:

• Aerobes that grow best between +500 and 300 mV, such as Molds yeasts, Bacillus, Pseudomonas, Moraxella and Micrococcus.
• Facultative anaerobes can develop most effectively at +300 to +100 mV like the bacteria that produce lactic acids as well as those from the family Enterobacteriaceae.
• Anaerobes can thrive in the range of +100-250 mg/mV or less, like Clostridium species.

4. Nutrient content

The microorganisms require nutrients, like carbohydrates, proteins sulfur, phosphorus, vitamins, lipids, nitrogen, energy, water and minerals to support their development and for carrying out metabolic functions. Food is the most effective source of nutrients to support the growth of microbial life. The microorganisms present in food are diverse in the requirements for nutrients and those that can use nutrients in large quantities are typically the most prevalent in food items.

The basic carbohydrates and amino acids are first utilized before being followed by more complex versions that these nutrients are found in. The nutritional requirements for Gram-positive bacteria are more than yeasts, and they are were followed by Gram-negative. Molds have the least nutrient needs.

5. Presence of antimicrobial constituents

Different foods have antimicrobial elements that help to prevent attacks of microbial agents on foodborne pathogens. Foods made from animal and plant products has antimicrobial ingredients.

Antimicrobial components	Food	Organisms inhibited
Anthocyanin pigment	Fruits	Bacteria
Dimethoxyisoflavone	Peanut	Aspergillus flavus
Tannin	PeanutsMint	Aspergillus parasiticus
p-coumaric acidferulic	Grapes	Aspergillus parasiticus, Aspergillus parasiticus, Saccharomyces cerevisiae, E. coli, Staphylococcus aureus, Bacillus cereus
Cinnamaldehyde	Oils(essentials)	Gram-positive bacteria
Capsaicinoids	Pepper	Salmonella typhi, Pseudomonas
Phenols and flavonoids	Pomegranate peel	Gram-positive bacteria
Allicin	Garlic	Gram-positive and Gram-negative bacteria
Curcumin	Tumeric	S. Typhi, Listeria monocytogens, E. coli, Staphylococcus aureus
Chitosan	Shells of marine crustaceans	Gram-negative bacteria
Defensin	Mammals cells and tissues	Bacteria and fungi
Lactoperoxidase	Milk	S. Typhi, Shigella, Pseudomonas, and coliforms
LysozymeAvidin	Egg	E. coli, Klebsiella pneumoniae, Serratia marcescens, and P. aeruginosa
Lactoferrin	Milk	Gram-negative and Gram-positive bacteria, fungi, and parasites

6. Biological structures

Certain food items have biochemical structure that blocks the entry of microbes. The natural cover protects against destruction and decreases the risk of spoilage due to microbial. Structures like the outer skins of fruits as well as the shells of nuts and egg shells meat have fascia and skin, that block the infiltration of foodborne pathogens as well as spoilage microorganisms.

B. Extrinsic factors

The variables that can be controlled by external conditions are known as extrinsic elements. These comprise:

1. Temperature of storage

The reactions of enzymatic enzymes and the growth of microbial species are influenced by the environment temperature. The temperature of growth for molds and yeasts exhibit the wide range of temperatures from 10 to 35 deg C.

Based upon temperature, the bacteria may be classified into

• The psychrotrophs are cold-tolerant, ubiquitous microorganisms that thrive at temperatures ranging from between 0 to 20degC. This includes Pseudomonas spp as well as Enterococcus spp.
• Mesophiles are microorganisms that develop between 25 and 40 degrees Celsius and have an optimal temperature of growth close to 37degC. This includes species such as Salmonella, Staphylococcus, Clostridium, Shigella, and Bacillus.
• Thermophiles are microorganisms which grow at temperatures above 45 degrees Celsius, and have an optimal growth temperature of 50degC to 70degC. They include species like Bacillus, Clostridium, and Geobacillus.

2. Relative Humidity

The relative humidity refers to the amount of water in the food or atmosphere. The relative humidity may affect the level of activity in water (aw) amount in the food, and consequently affect the development of microorganisms. For example, dry grain kept in a climate that has high humidity can soak in water and then undergo decay of mold.

3. Presence and Concentration of gases in the environment

Gases like Carbon dioxide (CO2) as well as Ozone (O3) as well as oxygen (O2) are able to exert an immediate toxic effect that may hinder growth and growth. Ozone (O3) along with oxygen (O2) are extremely toxic to anaerobic microbes and carbon dioxide has a powerful effect against aerobes that are obligate.

C. Implicit factors

The third element that plays a major role in determining the type of the microbial life cycle in food is known as implicit elements. This element defines the characteristics of the organisms and how they react to conditions. Microorganisms may inhibit or promote the growth of each other.

The various reactions which could be beneficial or harmful to microorganisms include predation, parasitism as well as amensalism, commensalism asymbiosis, allotropy, and neutrality. Organisms can produce substances that are toxic or inhibitory to other organisms like antibiotics, bacteriocins and antibiotics, hydrogen peroxide and organic acids.

D. Food Processing factors

In the process of processing food microorganisms may be exposed to a variety of chemical or physical pressures. The most common processing stresses are heating as well as freezing, drying as well as osmotic and radiation effects and other chemicals. Heating can reduce the level of microbial activity in food products by destroying the cytoplasmic membrane and altering the metabolic and enzymatic processes.

Freezing inhibits your microbe’s ability to grow through having the inhibitory effect of lower pH and an increase in Aw. Drying inhibits the growth of microbial colonies since it triggers metabolic damage that hinder the proliferation of cells.