Growth conditions of microorganisms in food

Overview

Microorganisms require water, energy, nitrogen, minerals, possibly of oxygen and / or growth factors for their development. Thus, food is as a supportive environment for the development of microbial germs.

However, despite the presence of nutrients, the growth of microorganisms is counteracted by other factors that are linked on the one hand, physical and chemical characteristics of the food (pH, a_w, potential redox, etc.). and secondly, the environment of the product (temperature, humidity, electromagnetic radiation, etc.).. These parameters have a selective effect on the microbial flora of a food they promote or inhibit the development of such or such microorganisms.

In order to control the microbiological quality of food, it is necessary to know the effect of each parameter on the growth of microorganisms, but also the effect of interactions that may exist between different parameters.

Effect of food structure on the growth of microorganisms

The unprocessed food are often protected from the external environment by envelopes (seed coat, skin, shell, etc..) Which are a natural barrier to penetration by microorganisms.

Following the harvesting operations, storage and processing of fresh food products (mechanical harvesting, freezing-thawing, grinding, mixing, etc..), We are witnessing the deterioration of the protective envelope which promotes microbial growth.

Some foods, like milk, have no cellular structure and are therefore more vulnerable than others.

Effect of pH on the growth of microorganisms

In aqueous solution, the pH is defined as:

pH = - log [H⁺]

When acid is added to water, the hydrogen ion concentration [H⁺] increases and is higher than the same concentration of ions in the water. Thus, the solution is acidic (over the concentration of H⁺ions is high), over the pH is low, and vice versa. The pH value can vary from 0 to 14, the value 7 corresponds to a neutral pH (pH of pure water).

Microorganisms can grow on a wide range of pH ranging from 2 to 11. However, microbial resistance to pH is highly variable and differs from one microbial group to another (Table 1). Depending on the pH optimum for growth, microorganisms are classified into three groups:

1. Acidophilus: Microorganisms with a pH optimum growth between 0 and 5.5.
2. Neutrophils: Microorganisms with a pH optimum for growth between 5.5 and 8.
3. ALKALOPHILICITY: Microorganisms with a pH optimum for growth between 8 and 11.5.

Table 1: Limits of growth of some microorganisms

Bacteria, in general, thrive best in environments with a pH close to neutral (6 to 7.5). Some bacteria, like lactic acid bacteria and acetic bacteria, can grow even at pH below 4.

The decrease in pH also affects the heat resistance of spores, we consider that below pH 4.5, the heat resistance of bacterial spores is zero.

Yeasts and molds are generally acid-resistant, their optimum pH for growth is between 4 and 6 with extreme values from 2 to 9 for the yeast and 2 to 11 for mold.

Depending on the pH, we divide the food products in weak acid with a pH above 4.5 and acidic with a pH below 4.5. This classification is based on the fact that pathogens can grow in foods with a pH below 4.5.

Stabilization of acidic products requires only a low heat treatment to remove germs alteration (yeasts, molds, lactic acid bacteria and acetic). As against the low-acid foods require, for their stabilization, sterilization treatment to remove pathogens and corruption, including bacterial spores.

The pH of foods depends on the quantities of acid or alkaline substances present, but also the buffering effect of the product, which is mainly related to protein content. Plant products are often acidic (pH of 2 to 5), while animal products generally have a pH close to neutral (6 to 7). The pH plays a crucial role in the specificity of the microbial flora of each product.

Effect of water activity on the growth of microorganisms

Water activity (a_w) indicates the availability of water from a medium for chemical reactions, biochemical, a change of state or transfer through a semi permeable membrane.

Water activity (a_w) is the ratio of the water vapor pressure of the feed (water vapor pressure at the surface of the product) and the vapor pressure of pure water the same temperature θ °.

Bacteria can grow on food products with a_wbelow 0.90. Molds and yeasts are inhibited to a aw respectively 0.7 and 0.8 except certain molds and yeasts osmophiles that can grow up to a w of 0.6. In most cases, a_wlimits for growth of a microorganism is less than the a_wlimit required for the production of its toxin.

The minimum values required by some microorganisms are collected in Table 2 below.

Fresh food (fruits, vegetables, meats, etc..) Generally have a_w from 0.97 to 0.99. They are thus a favorable environment for the development of all microorganisms unless there are other factors that limit their growth. This is the case, for example, fruit and vegetables, which generally have acidic pH favoring the development a fungal flora (yeasts and molds).

On food products with low a_w, such as bread and pastries, it is generally the growth of mold.

Of the preparations very salty (salted anchovies ...), we can assist in the development of bacterial halophiles (Halobacterium, Halococcus). The latter are growing even at water activities of 0.75 and require NaCl concentrations from 12 to 15%. Although they are not pathogens, these bacteria can cause alteration of the microbial food.

In a food, a water activity of 0.7 is considered a lower limit with all the guarantees of microbial stability. But 0.91 is a limit below which the growth of microorganisms is highly constrained. This limit was adopted by the EU directive of 1977 for food at room temperature and is even raised to 0.95 provided that it comes with a pH less than or equal to 5 2.

Effect of redox potential and oxygen on the growth of microorganisms

Redox potential (E_hexpressed in volts) measures the ease with which a win or lose electrons. An oxidizing environment is when it picks up electrons (the E_h is positive) and it is simplistic when it loses electrons (the E_his negative).

Food products are generally reducing environments. This is due to the presence in food, substances highly hydrogenated,-SH radicals, reducing sugars or other compounds such as ascorbic acid (vitamin C) or tocopherols (vitamin E). The oxidant effect of a food is mainly due to the presence of atmospheric oxygen or surface (meat) or mass (plants: parenchyma through the gaps and stomata).

Depending on their requirements in oxygen, microorganisms are classified into four categories:

1. The strict aerobes (Pseudomonas, Micrococcus, Bacillus) are microorganisms that can grow in the presence of oxygen.
2. The aerobic optional (Enterobacteria, Staphylococcus) are microorganisms that can grow in the presence and absence of oxygen.
3. Strict anaerobes (Clostridium, Bacteroides, ...): Are the microorganisms that can grow in the presence of oxygen.
4. Micro-Aérophile (Lactobacillus, Streptococcus, ...): Are the microorganisms that can grow in the presence of low amounts of oxygen.

Effect of temperature on the growth of microorganisms

Temperature is one of the most important factors influencing the growth of microorganisms. Indeed, every organism has an optimum temperature field of promoting its development. Temperatures outside this area hinder its growth.

According to their optimum temperature of growth, microorganisms are classified into several groups whose names reflect the various areas of thermal tolerance:

Psychrophiles and psychrotrophic

The Psychrophiles (Bacillus psychrophilus, Chlamydomonas nivalis, ...) are microorganisms that grow at temperatures ranging from 0 to 20 ° C with an optimum at 15 ° C. These are microorganisms adapted really cold, they encounter little in the food but in colder regions (eg polar regions).

Microorganisms belonging to the group of psychrotrophic are able to grow in the temperature range from 0 to 35 ° C with an optimum growth of 20 to 35 ° C. This is an intermediate group between Psychrophiles and mesic and is responsible for microbial deterioration of refrigerated food.

The group is represented by psychrotrophic many bacteria which are the main genres Pseudomonas, Alcaligenes, Erwinia, Corynebacterium, Flavobacterium, Lactobacillus and Streptomyces. Note also that yeasts and molds are mostly psychrotrophic.

Mesophilic

The mesophilic multiply at temperatures ranging from 20 to 40 ° C with an optimum at 37 ° C. They are found on foods stored at room temperature or refrigerated foods when the cold chain has been broken.

The main bacterial genera and species belong to the group of mesophiles. These are the common species and species pathogenic for humans and animals, they are mostly natural saprophytes.

Examples of mesophilic: Escherichia coli, Salmonella, Staphylococcus, Campylobacter, etc..

Thermophilic

The thermophiles are microorganisms that thrive in temperatures ranging from 40 to 65 ° C with an optimum at 55 ° C. They are found in soil, water and even hot springs. In food circles, they are represented especially in the genera Bacillus and Clostridium bacteria and some fungi (Aspergillus, Cladosporium).

The thermophilic sporeforming and often fall into two groups depending on the temperature to germination of spores:

1. The thermophilic mandatory if the spores can germinate and grow below 50 ° C.
2. The thermophilic optional: if the spores can germinate and grow at temperatures above and below 50 ° C.

Among the thermophilic mandatory, some can grow up to a temperature of 77 ° C and their spores can survive for several minutes at 121 ° C.

Effect of inhibitory substances on the growth of microorganisms

Inhibitors of the growth of microorganisms are antimicrobial molecules that occur naturally in some foods (plants, animals), produced by certain microorganisms or added intentionally by humans for food. In the latter case, it is considered as additives.

Some antimicrobial substances have a specific action on some microorganisms (bactericides, fungicides), while others have a spectrum of action much wider.

Lysozyme, which is found naturally in eggs and milk, has a specific antibacterial activity. While gossypol (an antioxidant present in cottonseed) and essential oils have a broader spectrum, they act as antibacterial and antifungal agents.

Among the products of microbial fermentation are lactic acid and acetic acid which have a relatively large bacterial activity. These substances are also produced by chemical synthesis and are used as additives.

The range of substances used as additives is very varied chemical compositions and modes of action heterogeneous. Among these additives are:

1. Nitrates and nitrites of sodium and potassium (NaNO₃, KNO₃, NaNO₂, KNO₂) which are known for their inhibitory action on Clostridium;
2. Sulfur dioxide (SO₂) and sulfites (sodium sulfite Na₂SO₃, sodium bisulphite NaHSO_3;disulphite sodium Na₂S₂O_5;disulphite potassium K₂S₂O_5;calcium sulfite CaSO₃, 2 (H₂O)) which are known to inhibit mainly on bacteria and fungi.
3. Propionic acid is particularly active against fungi. Propionic acid is a fermentation product found naturally in cheeses cooked;
4. Carbon dioxide (CO_2);peroxide oxygen or oxygenated water (H₂O₂) and so on.

Combined effect of several parameters on the growth of microorganisms

In a complex environment such as food, the growth of microorganisms is not subject to a single factor but to several endogenous parameters (pH, a_w, presence of inhibitors, growth factors, etc..) And exogenous (temperature, humidity , etc.).. It is therefore useful to consider the combined effect of all parameters on the growth of microorganisms.

The operation of the combined effect of several parameters allows better control of the microbiological quality of the food and the orientation of microbial metabolism.