Factors Influencing Microbial Growth

In FoodDr. Mohammad shubair

The ability of microorganisms (except viruses) to grow or multiply in a food is determined by the food environment as well as the environment in which the food is stored ( intrinsic and extrinsic environment of food respectively). Both factors are interrelated.

Intrinsic Factors or Food Environment • Nutrients and Growth There are five major nutrient groups necessary for microbial growth; carbohydrates, proteins, lipids, minerals, vitamins

The amount of each nutrient varies greatly with the type of food e.g. meat is rich in protein, lipids, minerals and vitamins but poor in carbohydrates.

Foods from plant sources are rich in carbohydrates but can be poor sources of proteins, minerals and some vitamins.

Milk and many prepared from it have all five nutrient groups in sufficient amounts for microbial growth.

Microorganisms normally present in food vary greatly in nutrient requirements, with bacteria requiring the most, followed by yeasts and molds.

Microorganisms differ greatly in their ability to utilize large and complex carbohydrates(e.g. starch and cellulose), large proteins(e.g. casein in milk), and lipids.

Microorganisms capable of using these molecules do so by producing specific extracellular cellular enzymes(exo-enzymes) and hydrolyzing the complex molecules to simpler forms outside before transporting them inside the cell, molds are the most capable of doing this.

This process provides an opportunity for a species to grow in a mixed population even when it is incapable of metabolizing the complex molecules.

Intracellular enzymes are released after death and lysis of microorganisms , these enzymes can also catalyze breakdown of complex food nutrients to simpler forms which can then be utilized by other microorganisms.

Carbohydrates In Foods • They are grouped on the basis of chemical

nature as follows:Monosaccharides(hexoses and pentoses)Disaccharides(lactose, sucrose, maltose)Oligosaccharides(raffinose, stachyose)

Polysaccharides(starch, glycogen, cellulose, inulin, hemicellulose, dextran, pectins, gums).• Lactose is found only in milk and foods made

from or with milk and milk products.• Glycogen is present in animal tissues,

especially in liver.• Pentoses, most of oligosaccharides and

polysaccharides are naturally present in foods of plant origin.

All microorganisms normally found in food metabolize glucose and their ability to utilize other carbohydrates differ considerably. This is because of the inability of some microorganisms to transport them inside the cells and inability to hydrolyze polysaccharides outside the cells.

Molds are the most capable of using polysaccharides.

Food carbohydrates are metabolized by microorganisms to supply energy and synthesize essential components through several metabolic pathways.

Microorganisms produce metabolic by-products which may cause food spoilage. Others produce organic acids (acetic, lactic, propionic, butyric) which have an antagonistic effect on growth and survival of many microorganisms. Others can be used in food production(dexrtans as stabilizers).

Crabohydrate metabolism profiles are used in the laboratory for biochemical identification of microorganisms.

Proteins In Foods Food protienaceous compounds include simple proteins,

conjugated proteins, peptides and non protein nitrogenous compounds(NPN; amino acids, urea, ammonia, creatinine).

Simple food proteins are polymers of amino acids e.g. albumins in egg, globulins in milk, glutelins (glutenin in cereal), prolamins (zein in grains), albuminoids(collagen in muscles).

Proteins differ in their solubility which determins the ability of microorganisms to utilize a specific protein. Many microorganisms can hydrolyze albumin which is soluble in water, in contrast, collagens(insoluble) are hydrolyzed by a few microorganisms.

Conjugated proteins of food on hydrolysis produce metals(hemoglobin, myoglobin), carbohydrates(glycoproteins such as mucin), phosphates(phosphoprotein such as casein) and lipids(lipoprotein in liver).

Foods of animal origin are rich in proteins. Plant foods such as nuts and legumes are also rich in

proteins. Microorganisms differ greatly in their ability to

metabolize food proteins. Most microorganisms transport amino acids and small peptides in the cells(they are hydrolyzed to amino acids inside the cells e.g. Lactococcus spp.).

Microorganisms produce extracellular proteinases and peptidases to hydrolyze large proteins and peptides to small peptides and amino acids before transport.

Hydrolysis of food proteins can be undesirable e.g. texture loss in meat or desirable e.g. flavor in cheese.

Amino acids inside microbial cells are metabolized via different pathways to synthesize cellular components, produce energy, and various by- products.

Many of these by- products can be undesirable; NH3, H2S which cause spoilage of food and toxins and biological amines which cause health Hazards, other by-products are desirable (some sulfur compounds give cheddar cheese flavor). Production of specific metabolic products is

used for laboratory identification of microorganisms in food e.g. E. coli produces indole from tryptophan which is used to differentiate this species from non-indole producing related species e. g. Enterobacter Spp.

Lipids In Foods They include free fatty acids, glycerides, phospholipids, waxes and sterols. Lipids are relatively higher in foods of animal

origin than in foods of plant origin. Nuts, oil seeds, coconuts, olives have high

amounts of lipids. Cholesterols are present in foods of animal origin

or foods containing ingredients from animal sources.

Many microorganisms can produce extracellular lipases which can hydrolyze glycerides to fatty acids and glycerol.

Fatty acids can be transported in cells and used for energy, glycerol can be metabolized separately.

Some microorganisms produce extracellular lipid oxidases, which can oxidize unsaturated fatty acids to produce different aldehydes and ketones(molds are more capable of producing such enzymes).

Pseudomonas, Achromobacter and Alcaligenes can produce these enzymes.

In many foods, the action of these enzymes is associated with spoilage(such as rancidity), whereas in other foods the enzymes give desirable flavor(mold-ripened cheese).

Some intestinal microorganisms can metabolize cholesterol e.g.Lactobacillus acidophilus.

Identification of many microorganisms is based on the different short chain fatty acids they produce.

Minerals And Vitamins In Foods

Microorganisms need several elements in small amounts such as P, Ca, Mg, Fe, S, Mn, K.

Many organisms can synthesize B vitamins. Different foods contain most B vitamins. Many vitamins are used by microorganisms in

different biochemical reactions as coenzymes.

Water Activity Water activity(Aw) is a measure of the

availability of water for biological functions and relates to water present in food in free form.

In a food system, total water or moisture is present in free and bound forms.

Bound water is the fraction used to hydrate hydrophilic molecules and to dissolve solutes and is not available for biological functions, thus it does not contribute to Aw.

• The free water in food is necessary for microbial growth.

• It is necessary to transport nutrients and remove waste materials.

• It carries out enzymatic reactions. • It participates in the synthesis of cellular materials.• It plays in other biochemical reactions such as

hydrolysis of a polymer to a monomer(e.g. proteins to amino acids).

• Each microbial species or a group has an optimum , maximum and minimum Aw level for growth.

pH And Growth On the basis of pH foods can be grouped into:• High acid foods(pH<4.6).• Low acid foods(pH4.6 or above). Most fruits, fruit juices, fermented foods,

salad dressings are high acid foods. Most vegetables, meat, fish, milk are low acid

(high pH) foods. Tomato is a high acid vegetable(pH 4.1-4.4).

The acid in foods can be present naturally(as in fruits), produced during fermentation(fermented food), or added during processing(salad dressings).

Foods can also have compounds that have a buffering capacity e.g. milk, meat, they do not show pH reduction.

The pH of a food affects microbial growth and viability. Each species has an optimum and a range of pH for growth.

Molds and yeasts are able to grow at lower pH than do bacteria.

Gram negative bacteria are more sensitive to low pH than Gram positive bacteria.

Temperature Temperature of growth is used in the laboratory to enumerate and

isolate microorganisms from foods. On the basis of their temperature of growth, microorganisms

important in food are divided into three groups, each group has an optimum temperature(OT) and temperature range(R) of growth:

• Thermophiles: OT=55C, R=45-70C.• Mesophiles: OT=35C, R=10-45C.• Psychrophiles: OT=15C, R=-5-20C. Food can be stored at refrigerator temperature or below(freezing).

Some relatively stable foods are kept between 10-35C. Different temperatures are also used to stimulate desirable

microbial growth in food fermentation.

Redox Potential And Microbial Growth

According to the presence or absence of free O2 microorganisms are grouped into: aerobes, anaerobes, facultative anaerobes and microphiles. For aerobes O2 is needed for energy generation as it

acts as the final electron acceptor through aerobic respiration.

Facultative anaerobes can generate if free O2 is available, or they can use bound O2 in compounds such as NO3, SO4 as final electron acceptors through anaerobic respiration.

If O2 is not available, other compounds are used to accept electrons or H through anaerobic fermentation e.g. acceptance of H from NADH2 by pyruvate to produce lactate.

Anaerobic and facultative anaerobic microorganisms can only transfer electrons through fermentation.

Obligate anaerobes cannot grow in presence of small amounts of O2 as they lack superoxide dismutase which is necessary to scavenge the toxic O2 free radicals.

Microaerophiles grow better in the presence of less O2.

Bacillus, Pseudomonas, Moraxella, Micrococcus, Molds, and yeasts have aerobic species.

Lactic acid bacteria and enterobacter are facultative anaerobes.

Clostridium is an anaerobic bacteria. Campylobacter is a microaerophile. Putrifaction of meat by clostridium Spp. Underanaerobic conditions causes its spoilage.Growth of penicillium Spp. In blue cheese

produces desirable characteristics.