Food Spoilage General Principles – Chemical Changes Caused by Microorganisms

Causes Of Spoilage – General Principles

When food is called “spoiled”, it usually means that there is decay or decomposition of an unfavorable nature. Foods unfit for consumption are not considered spoiled. Spoiling could be caused by one or more of these:

• Activity and growth of microorganisms, or higher forms, occasionally. Sometimes, there are multiple organisms involved.
• Insects.
• Action of enzymes in plant or animal food.
• Purely chemical reactions are those that are not catalyzed or mediated by enzymes in the tissues or microorganisms.
• Physical changes such as drying, freezing, pressure, drying and burning can cause physical damage. The discussion that follows will focus on spoilage due to microorganisms.

Classification Of Foods By Ease Of Spoilage

Foods can be divided into three groups based on their ease of spoilage.

1. Stable or nonperishable foods:  These foods are spoil when they are handled carelessly.  Stable or nonperishable and include sugar, flour, dry beans, and other products
2. Semiperishable food: These foods can be preserved in unspoiled condition for up to a year if they are stored properly and properly handled.  e.g., potatoes, some varieties of apples, waxed rutabagas, and nutmeats.
3. Perishable foods: These are the most important everyday foods that can spoil quickly if not given special preservation methods. This category includes meats, fish, poultry and most fruits and veggies, as well as eggs and milk. Although most foods can be classified into one of the three categories, some foods are close enough to the edge to be difficult to identify.

Undesirable Changes in food due to sporilage

The signs of food deterioration are the loss of the aroma, flavour, texture and nutritional qualities of foods. The various types of adverse changes that occur as a result of food spoilage are described in the following manner:

• Color change: fruit like apples and bananas become black after storage for a long time. This reduces the quality of food items.
• The smell changes: Rancid scent of fats and oils that have been spoilt and the bitter taste of curd, or sour scent of starchy foods.
• Change in consistency: Separation from milk curdling milk, stickiness, and undesirable viscosity of cooked dal that has been spoiled curries and curries as well as cooked vegetables.
• Change in texture: Certain vegetables such as potatoes, brinjal and carrot are subjected to excessive softening , leading to rotting.
• Change due to mechanical damage: Damages caused by mechanical force, such as eggs with cracked shells, the mechanical loss of vegetables and fruits in transport also cause food spoilage.

Factors affecting food sludge

The different types of spoilage for an item of food will depend on the following factors:

• A food’s composition: The nature of food’s composition affects the likelihood of spoilage. For instance, the the presence of carbohydrates and proteins particularly sugars are favored by microorganisms to provide energy sources. A small percentage of microorganisms use fat to generate energy.
• The structure of the food item:
  The healthy tissues that makeup food that are inside are either sterile or have a low Microbial content. Skin, rind or a shell in food serve as its shield against spoilage microorganisms.
• Microorganisms that are involved:
  The kinds of microorganisms that are present in food are determined by the composition of the food.
• Conditions for storage of the food items:
  Conditions for food storage can affect the development of microorganisms. Even if appropriate storage for food items is completed, food loses its freshness and nutritional value if it’s kept over a long period of time.

Factors Affecting The Growth Of Microorganisms In Food

Associative Growth

• Most food spoilage or fermentations are caused by the interaction of microorganisms.
• The competition between different types of bacteria, yeasts and molds in food determines which one will outgrow others and cause their particular type of spoilage.
• When conditions are favorable, bacteria can grow faster than yeasts and yeasts more quickly than molds. Yeasts can outgrow bacteria when they are dominant in the first place, or when the conditions are slowing the growth of bacteria.
• Molds can thrive only in conditions that are more favorable for them than yeasts and bacteria.
• There are many types of bacteria, and each one is more popular than the other.
• Similar to the yeasts, if one type is preferred, another will usually outgrow the other; and, among molds, one type will find conditions that are more favorable than others.
• However, microorganisms do not always act in an antagonistic or anti-biotic manner toward one another. They may be sometimes symbiotic to one another, or may grow simultaneously, without appearing to help or hinder the other.
• Two types of microorganisms might be synergistic. This means that they can work together to produce changes such as fermentations.
• Pseudomonas synthcyanea* grown in milk alone produces a light-brownish tint, while Streptococcus Lactis causes no color changes in milk. However, when the two organisms are combined, a brighter blue color is formed due to a pH effect on P. syncyanea’s brown pigment.
• Metabiotic effects are the most significant effect of microorganisms on other organisms. They occur when one organism creates conditions that favor growth of the other.
• Although both organisms can be growing simultaneously, it is more common for one to succeed over the other.
• Metabi-osis is a hallmark of natural fermentations and decompositions.
• At room temperature, raw milk supports an acid fermentation by Streptococcus Lactis and coliform bacteria. The bacteria then become intolerant to the acid. The acid-tolerant lactobacilli then increase the acidity until they stop. Film yeasts and molds then grow on top of the acid, reducing it until proteolytic bacteria can be active.
• The order of the organisms in a normal succession is: first, Miscellaneous Bacteria, chiefly coliform; then, Leuconostoc mésenteroides; next, Lactobacillus Plantarum; last, Lactobacillus Brevis.

Effect of Environmental Conditions

The environment is what determines which microorganisms in food will outgrow others, and how they cause the food’s characteristic change or spoilage. These factors are interrelated and together determine which organisms will grow and what effects they have. These factors include the chemical and physical properties of food, oxygen availability, and temperature.

1. Physical State and Structure of the Food

• It is possible for food to spoil based on its physical condition, its colloidal nature and whether it has been heated, moistened or frozen.

• One of the most important factors that influence microbial growth is water availability, location and quality.
• Water can be considered both a chemical compound that is necessary for growth as well as a part of the physical structure.
• It is well-known that microorganisms need moisture to grow and that they thrive in a large supply.
• The organisms must have access to this moisture, that is, they should not be tied up with solutes or hydrophilic colloids such as agar.
• Salt and sugar dissolve in water, causing an osmotic tension that draws water out of cells.
• Dry food like bread is more susceptible to molds. Honey and sirups have a high sugar content, which makes them more attractive for the growth of osmophilic yeasts. Moist, neutral foods like milk, meat, and eggs are most commonly spoiled by bacteria.
• Grape juice may be favorable to yeasts due to its high sugar content and low pH. However, it will support the growth bacteria if the incubation temperatures for fermentative yeasts are too high or low.
• Refrigerated foods can mold when there is air, but may suffer bacterial spoilage if there isn’t.
• Although honey’s sugar content is too high to be used by most yeasts, but not for all molds, molds are rarely affected by honey because it contains fungistatic substances.
• Aw levels as low as 0.70 make it unlikely that food will spoil if kept at room temperature. This is the moisture level in dry milk at 8 to 11 percent total moisture. It also includes flour at 13-15 percent, nonfat milk at 15 per cent, dried whole eggs at 10 to 11, flour at 13-15 percent, dry milk at 15 per cent, dehydrated fat-free milk at 15 percentage, seeds of leguminous plants at 15 Percent, dehydrated vegetables and fruits at 14-20 percent, 18-25 percent, and 18 percent starch.
• Microorganisms can alter the amount of moisture available by changing the substrate or releasing metabolic water.
• Bacillus subtilis is believed to be responsible for the production of ropiness, or bread-like texture, by releasing moisture from starch decomposition. This makes it more suitable for its own growth.
• The destruction of moisture-holding tissue, such as fruits, by molds may make water accessible to yeasts and bacteria.

Freezing

• If the temperature is low enough, freezing will prevent microbial growth. However, it can also damage tissues. So juices that are released upon thawing favor microbial proliferation.
• As the temperature drops, freezing increases the concentrations of solutes in the frozen portion. This slows down and ultimately stops the growth of organisms that can grow below 0 C.
• Hydrophilic colloids are also affected by freezing, which results in water being removed from them that cannot be absorbed completely on thawing.

Heated processing

• The chemical composition and structure of food may be altered by heat processing. This includes releasing or tying up water, destroying colloidal suspensions or gels or emulsions, and altering the food’s ability to absorb moisture and oxygen.
• It is possible for protein to become more readily available to organisms than in its native state.
• Gelation of starch or protein can result in the release of moisture, which makes them more difficult to decompose.
• The reasons given are why cooked food is often more easily broken down than fresh food.

Changes in colloidal constituents

• Although the causes of changes in colloidal components of food may differ from those caused by freezing or heating, such as sound waves, the results are the same.
• Water and fat emulsions are more susceptible to spoilage. The spoilage rate will be greater if water is the continuous phase, and fat the discontinuous, as in French dressing. Butter, on the other hand, is more likely to go rancid.

2. Chemical Properties of the Food

How suitable a food is as a medium for microorganism growth will depend on its chemical composition. Every organism is unique in its ability to use certain substances as energy sources, carbon sources, or nitrogen sources.

The properties of food determine the number and type of organisms that can grow in or possibly spoil it. These include (1) pH, or hydrogen ion concentration, (2) nutrition availability, (3) moisture availability and (4) O-R Potential.

Chemical Changes Caused By Microorganisms in Food

Many chemical reactions are possible due to the variety of organic compounds found in food and the many types of microorganisms capable of decomposing them. This can lead to many products. This discussion will focus on the most important types of food constituents and their chief products.

1. Changes in Nitrogenous Organic Compounds

• Most nitrogen found in food is found in proteins. These proteins must be hydrolyzed using enzymes of microorganisms, enzymes of food, or enzymes of food to polypeptides and simpler peptides or amino acids before they can be used as nitrogenous foods for most organisms.
• Proteinases are enzymes that hydrolyze proteins into peptides. This can give foods a bitter taste.
• The hydrolysis of polypeptides into simpler peptides, and then to amino acids is catalyzed by peptididases. These amino acids give flavor to certain foods, whether they are desirable or not. For example, amino acids make ripened cheeses more flavorful.
• These hydrolyses don’t usually produce any particularly undesirable products.
• However, anaerobic decomposition may produce noxious odors. This is called putrefaction. This results in sulfur-containing, foul-smelling products such as hydrogen,methyl, and/or ethylsulfides, mercaptans, along with ammonia and amines (e.g. histamines, tyramines, piperidines, putrescine and cadaverine), indole and skatole and fatty acids.
• Microorganisms can deaminate, decarboxylate, or both amino acids to produce products.
• Escherichiacoli produces, among other things, glyoxylic and acetic acids, as well as ammonia from Glycine. Pseudomonas produces methylamine, carbon dioxide, and Clostridium gives acetic, ammonia and methane.
• These three organisms are made from alanine and produce (1) an aketo acid and ammonia and (2) acetic acids, ammonia and carbon dioxide and (3) propionic, acetic, ammonia and carbon dioxide.
• E. coli makes pyruvic acid from serine and ammonia from Clostridium species.
• As mentioned previously, sulfur in sulfur-bearing amino acid may be reduced to foul-smelling sulfur sulfides and mercaptants.
• Desulfotomaculum Nigrimificans (formerly C.nigrificans), is an obligate anaerobe that can reduce sulfate and sulfide, and also produces hydrogen sulfide using cystine.
• Other nitrogenous compounds that can be decomposed include (1) amides and imides and urea. Ammonia is the main product. (2) Guanidine and creatine yield urea. (3) Amines, purines and pyrimidines may also yield ammonia or carbon dioxide and organic acids (mostly lactic and acetic).

2. Changes in Nonnitrogenous Organic Compounds

Non-nitrogenous food for microorganisms that are used mainly to get energy, but also possible as carbon sources, include carbohydrates, organic acid and ketones, alcohols and glycosides, cyclic compound, and lipids.

Carbohydrates

• Microorganisms prefer carbohydrate, if they are available, to other energy-yielding food. Complex di-, tria-, and polysaccharides are usually hydrolyzed into simple sugars before they can be used.
• An aerobic monosaccharide such as glucose would be oxidized into carbon dioxide and water, and anaerobically would undergo decomposition using any one of six types of fermentation.
  • (1) An alcoholic fermentation (as by yeasts) with ethanol or carbon dioxide as the main products.
  • (2) A simple lactic fermentation (e.g., homofermentative lac acid bacteria) with lactic acids as the main product.
  • (3) A mixed lactic fermentation is one that uses heterofermentative lac acid bacteria. It includes lactic, acetic, ethanol, glucoserol and carbon dioxide as its chief products.
  • (4) The coliform type fermentation, which is by coliform bacteria with lactic and acetic acids, formic acids and ethanol, carbon dioxide and hydrogen as probable products.
  • (5) The propionic fermentation by propionibacteria produces succinic, acetic and propionic acids as well as carbon dioxide.
  • (6) the butyric-butyl-isopropyl fermentations, by anaerobic bacteria, yielding butyric and acetic acids, carbon dioxide, hydrogen, and in some instances acetone, butylenes glycol, butanol, and 2-propanol.
• There are many other products that can be made from sugars, depending on the microorganisms active. These include higher fatty acids and other organic acids as well as ketones.

Organic Acids 

• Many organic acids found in food as salts are oxidized to carbonates by organisms, which causes the medium to become alkaline.
• Aerobically, the organic acids can be oxidized to complete carbon dioxide and water. This is similar to what film yeasts do.
• Acids can be oxidized to simpler acids or other products that are similar to sugars.
• Coenzyme A aids in the degrading of saturated fatty acids and lower ketonic derivatives to acetic, two carbons at once.
• Although unsaturated and hydroxy fatty acids can be partially degraded in a similar way, they must be converted into a saturated acid (or ketonic derived) for complete beta-oxidation.

Other Compounds 

• Alcohols are usually oxidized to the appropriate organic acid, e.g. ethanol to acetic.
• Glycerol can be dissimilated into products similar to glucose.
• After hydrolysis to release sugar, glycosides will dissipate characteristically.
• Acetaldehyde can be reduced to ethanol or oxidized to acetic Acid.
• It is difficult to attack cyclic compounds.

Lipids 

• Microbial lipase hydrolyzes fats to glycerol, fatty acids, and then they are dissimilated in the same way as described previously.
• Although microorganisms can be involved in the oxidation fats, autooxidation is more common.
• The constituents of phosphate, Glycerol, Fatty Acids, and Nitrous Base, e.g. Choline, may be degraded from phospholipids.
• Lipoproteins consist of cholesterol esters and proteins.

Pectic Substances 

• Protopectin is the water-insoluble parent substance of plants and is transformed to pectin. This water-soluble polymer galacturonic acid contains methyl ester links and various degrees of neutralization by different cations.
• It forms gels when it is combined with sugar and acid. Pectinesterase hydrolyzes the methyl ester linkage in pectin to produce pectic acid or methanol.
• To produce smaller chains and eventually free D-galacturonic acids, polygalacturonases break down the linkage between galacturonic units of pectin and pectic acid. This may lead to simple sugars.