Factors In?uencing Microbial Growth in Food

Factors In?uencing Microbial Growth in Food

INTRINSIC FACTORS OR FOOD ENVIRONMENT

Intrinsic factors of a food include nutrients, growth factors, and inhibitors (or anti- microbials), water activity, pH, and oxidation–reduction potential. The in?uence of each factor on growth is discussed separately. But, as indicated previously, in a food system the factors are present together and exert effects on microbial growth in combination, either favorably or adversely.

A-Nutrient And Growth

Microbial growth is accomplished through the synthesis of cellular components and energy. The necessary nutrients for this process are derived from the immediate environment of a microbial cell and, if the cell is growing in a food, it supplies the nutrients.
These nutrients include carbohydrates, proteins, lipids, minerals, and vitamins. Water is not considered a nutrient, but it is essential as a medium for the biochemical reactions necessary for the synthesis of cell mass and energy.
All foods contain these ?ve major nutrient groups, either naturally or added, and the amount of each nutrient varies greatly with the type of food. In general, 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.
Some foods such as milk and many prepared foods have all ?ve nutrient groups in suf?cient 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 also 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 speci?c extracellular enzymes (or exoenzymes) and hydrolyzing the complex molecules to simpler forms outside before transporting them inside the cell. Molds are the most capable of doing this. However, this provides an opportunity for a species to grow in a mixed population even when it is incapable of metabolizing the complex molecules.
Microbial cells, following death and lysis, release intracellular enzymes that can also catalyze break- down of complex food nutrients to simpler forms, which can then be utilized by other microorganisms.

1- Carbohydrate in foods
Major carbohydrates present in different foods, either naturally or added as ingredients, can be grouped on the basis of chemical nature as follows:

Monosaccharides
Hexoses: glucose, fructose, mannose, galactose
Pentoses: xylose, arabinose, ribose, ribulose, xylulose
Disaccharides
Lactose (galactose + glucose) Sucrose (fructose + glucose) Maltose (glucose+glucose)
Oligosaccharides
Raf?nose (glucose + fructose + galactose)
Stachyose (glucose + fructose + galactose + galactose)
Polysaccharides
Starch (glucose units) Glycogen (glucose units) Cellulose (glucose units) Inulin (fructose units)
Hemicellulose (xylose, galactose, mannose units) Dextrans (a-1, 6 glucose polymer)
Pectins
Gums and mucilages

Lactose is found only in milk and thus can be present in foods made from or with milk and milk products. Glycogen is present in animal tissues, especially in liver. Pentoses, most oligosaccharides, and polysaccharides are naturally present in foods of plant origin.
All microorganisms normally found in food metabolize glucose, but their ability to utilize other carbohydrates differs considerably. This is because of the monosaccharides and disaccharides inability of some microorganisms to transport the speci?c inside the cells and the inability to hydrolyze polysaccharides outside the cells. Molds are the most capable of using polysaccharides.
Food carbohydrates are metabolized by microorganisms principally to supply energy through several metabolic pathways. Some of the metabolic products can be used to synthesize cellular components of microorganisms (e.g., to produce amino acids by amination of some keto acids). Microorganisms also produce metabolic by- products associated with food spoilage (CO2 to cause gas defect) or food bioprocessing (lactic acid in fermented foods).
Some are also metabolized to produce organic acids, such as lactic, acetic, propionic, and butyric acids, which have an antagonistic effect on the growth and survival of many bacteria. Microorganisms can also polymerize some monosaccharides to produce complex carbohydrates such as:
a- dextrans
b- capsular materials
c- cell wall (or outer membrane and middle membrane in Gram-negative bacteria).
Some of these carbohydrates from pathogens may cause health hazards (forming complexes with proteins), some may cause food spoilage (such as slime defect), and some can be used in food production (such as dextrans as stabilizers). Carbohydrate metabolism pro?les are extensively used in the laboratory for the biochemical identi?cation of unknown microorganisms isolated from foods.
2-Proteins in foods

The major proteinaceous components in foods are simple proteins, conjugated proteins, peptides, and non-protein nitrogenous (NPN) compounds (amino acids, urea, ammonia, creatinine, trimethylamine). Proteins and peptides are polymers of different amino acids without or with other organic (e.g., a carbohydrate) or inorganic (e.g., iron) components and contain ca. 15 to 18% nitrogen. Simple food proteins are polymers of amino acids, such as albumins (in egg), globulins (in milk), glutelins (gluten in cereal), prolamins (zein in grains), and albuminoids (collagen in muscle). They differ greatly in their solubility, which determines the ability of microorganisms to utilize a speci?c protein.
Many microorganisms can hydrolyze albumin, which is soluble in water. In contrast, collagens, which are insoluble in water or weak salt and acid solutions, are hydrolyzed only by a few microorganisms. As compared with simple proteins, conjugated proteins of food on hydrolysis produce:
a- metals (metalloproteins such as hemoglobin and myoglobin)
b- carbohydrates (glycoproteins such as mucin)
c- phosphate (phosphoproteins such as casein)
d- lipids (lipoproteins such as some in liver).
Proteins are present in higher quantities in foods of animal origin than in foods of plant origin. But plant foods, such as nuts and legumes, are rich in proteins. Proteins as ingredients can also be added to foods.
Microorganisms differ greatly in their ability to metabolize food proteins. Most transport amino acids and small peptides in the cells; small peptides are then hydrolyzed to amino acids inside the cells, such as in some Lactococcus spp. Microorganisms also produce extracellular proteinases and peptidases to hydrolyze large proteins and peptides to small peptides and amino acids before they can be transported inside the cells. Soluble proteins are more susceptible to this hydrolytic action than are the insoluble proteins.
Hydrolysis of food proteins can be either:
a- undesirable (texture loss in meat)
b- desirable (?avor in cheese).
Microorganisms can also metabolize different NPN compounds found in foods.
Amino acids inside microbial cells are metabolized via different pathways to synthesize cellular components, energy, and various by-products. Many of these by- products can be undesirable (e.g., NH3 and H2S production causes spoilage of food, and toxins and biological amines cause health hazards) or desirable (e.g., some sulfur compounds give cheddar cheese ?avor). Production of speci?c metabolic products is used for the laboratory identi?cation of microbial isolates from food. An example of this is the ability of Escherichia coli to produce indole from tryptophan, which is used to differentiate this species from non-indole-producing related species (e.g., Enterobacter spp.).

3-Lipids in foods

Lipids in foods include compounds that can be extracted by organic solvents, some of which are free fatty acids, glycerides, phospholipids, waxes, and sterols. Lipids are
relatively higher in foods of animal origin than in foods of plant origin, although nuts, oil seeds, coconuts, and olives have high amounts of lipids. Fabricated or prepared foods can also vary greatly in lipid content. Cholesterols are present in foods of animal origin or foods containing ingredients from animal sources. Lipids are, in general, less preferred substrates for the microbial synthesis of energy and cellular materials. Fatty acids can be transported in cells and used for energy synthesis, whereas glycerol can be metabolized separately.
Some microorganisms also produce extracellular lipid oxidases, which can oxidize unsaturated fatty acids to produce different aldehydes and ketones. In general, molds are more capable of producing these enzymes. However, certain bacterial groups such as Pseudomonas, Achromobacter, and Alcaligenes can produce these enzymes. Lysis of dead microbial cells in foods causes release of intracellular lipases and oxidases, which then can carry out these reactions. In many foods the action of these enzymes is associated with spoilage (such as rancidity), whereas in other foods the enzymes are credited for desirable ?avors (such as in mold-ripened cheeses). Some bene?cial intestinal micro- organisms, such as Lactobacillus acidophilus strains, can metabolize cholesterol and are believed to be capable of reducing serum cholesterol levels in humans.

4-Minerals and vitamins in foods

Microorganisms need several elements in small amounts, such as phosphorous, calcium, magnesium, iron, sulfur, manganese, and potassium. Most foods have these elements in suf?cient amounts. Many microorganisms can synthesize B vitamins, and foods also contain most B vitamins.
Some foods may have limited amounts of one or a few nutrients for rapid growth of some Gram-positive bacteria, especially some fastidious Lactobacillus species. When their growth is desired, some carbohydrates, essential amino acids, and B vitamins may be added to a food. It is not possible or practical to control microbial growth in a food by restricting nutrients.
5-Water
Water in food that is not bound to food molecules can support the growth of bacteria, yeast, and mold. The term water activity (aw) refers to this unbound water.The water activity of a food is not the same thing as its moisture content. Although moist foods are likely to have greater water activity than are dry foods, this is not always so. In fact, a variety of foods may have exactly the same moisture content and yet have quite different water activities.
The water activity (aw) of a food is the ratio between the vapor pressure of the food itself, when in a completely undisturbed balance with the surrounding air media, and the vapor pressure of distilled water under identical conditions. A water activity of 0.80 means the vapor pressure is 80 percent of that of pure water. Most foods have a water activity above 0.95 and that will provide sufficient moisture to support the growth of bacteria, yeasts, and mold. The amount of available moisture can be reduced to a point that will inhibit the growth of microorganisms.
Water activity values of selected foods
Food Water activity
Fresh meat and fish .99
Liverwurst .96
Cheese spread .95
Bread .95
Red bean paste .93
Caviar .92
Aged cheddar .85
Fudge sauce .83
Salami .82
Soy sauce .8
Jams and jellies .8
Peanut butter .7
Dried fruit .6
Cookies .3
Instant coffee .2
Predicting Food Spoilage
Water activity (aw) has its most useful application in predicting the growth of bacteria, yeast, and mold. For a food to have a useful shelf-life without relying on refrigerated storage, it is necessary to control either its acidity level (pH) or the level of water activity (aw) or a suitable combination of the two. This can effectively increase the product s stability and make it possible to predict its shelf life under known ambient storage conditions.
Food can be made safe to store by lowering the water activity to a point that will not allow pathogens such as Clostridium botulinum and Staphylococcus aureus to grow in it. The table below illustrates the water activity (aw) levels that can support the growth of particular groups of bacteria, yeast, and mold.
Effect of aw on Spoilage of Foods
aw Spoilage microorganism Food
0.90-1.00 Bacteria Cottage cheese, meat
0.85 - 9.0 Bacteria, molds, yeasts Margarine, condensed milk, whipped butter
0.80 - 0.85 Yeasts Fruit syrups
0.75 - 0.80 Xerophilic molds, molds and yeasts Dried figs, jams
0.70 - 0.75 Yeasts Confections
0.65 - 0.70 Osmophilic yeasts Molasses
0.60 - 0.65 Xerophilic molds, osmophilic yeasts Dried fruit
Semi-moist foods
For foods with a high water activity correct proper refrigeration is always necessary. These include most fresh foods and many processed foods, such as soft cheeses and cured meats. However, many foods can be successfully stored at room temperature by proper control of their water activity (aw). These foods can be described as semi-moist and include fruitcakes, puddings, and chocolate and caramel sauce.
When these foods spoil, it is usually the result of surface mold growth. Most types of mold do not grow at a water activity level below 0.8. Some will grow slowly at this aw, so it is usually recommended that products of this type not have a water activity greater than 0.75. While this will not completely prevent microbial spoilage, those few yeast and molds that do grow at lower water activities need only to be considered when special shelf life conditions must be met.