Food Spoilage: Intrinsic and Extrinsic Factors

Food Spoilage: Intrinsic and Extrinsic Factors

What is Food Spoilage?

Food spoilage is an unpleasant appearance, smell, and bad taste of a food which not favorable for human consumption. Food spoilage is caused by different types of microorganism such as: Bacteria, fungus, yeasts etc.

Food spoilage has been a continuing problem ever since humans first discovered they could produce more food than they could eat in a single meal. We all know of Marco Polo’s travels to China in the 13th century to obtain spices and explore new trade routes.

What often is not mentioned is that the spices brought back from China were more than just a luxury; the spices were essential for improving the smell and taste of spoiled food. Refrigeration was virtually unknown, and canning was yet to be invented.

Food Spoilage Comes from Several Microbial Sources

Spoiled foods usually have an unpleasant appearance, aroma, and taste. Sometimes, however, spoilage can be difficult to detect, such as when staphylococci deposit exotoxins in food or when few bacteria are present to cause a perceptible change.

Contaminating microorganisms can be transmitted to foods in several ways. Airborne pathogens can fall onto fruits and vegetables and then penetrate the product through an abrasion of the skin or rind, whereas crops carry soilborne bacterial pathogens to the processing plant. Shellfish concentrate pathogens by straining contaminated water and catching the microbes in their filtering apparatus, and rodents and arthropods transport pathogens on their feet and
body parts as they move about among foods.

Human handling of foods also provides a source for transmission. For example, bacterial pathogens from an animal’s intestines can be transmitted to and contaminate meat handled carelessly by a butcher.

Several factors that can determine food spoilage

Because food is basically a culture medium for microorganisms, the chemical and physical properties in and surrounding the food have a significant impact on the type of microorganisms growing on or in the food.

Food Spoilage
Several intrinsic and extrinsic factors determine whether foods are likely to spoil quickly or resist spoilage.

Intrinsic Factors

Conditions naturally present in foods that influence microbial growth are called intrinsic factors. These include the following:

  • Water: One of the prerequisites for all life is water. Therefore, food must be moist, with a minimum water content of 18% to 20% before contamination by microorganisms and spoilage can occur. Microorganisms do not grow in foods such as dried beans, rice, and flour because of their low water content.
  • pH: Another important factor is a food’s acidity. Most foods fall into the slightly acidic range on the pH scale, and numerous bacterial species multiply under these conditions. In foods with a pH of 5.0 or below, acid-loving molds often are the spoilage organisms. Citrus fruits, for example, generally escape bacterial spoilage but are susceptible to mold contamination.
  • Physical Structure: Another property of a food is its physical structure. A raw steak, for example, is not likely to spoil quickly because microorganisms cannot penetrate the solid meat. However, raw, ground hamburger meat can deteriorate rapidly because microorganisms exist both within the loosely packed ground meat as well as on the surface.
  • Chemical Composition: A food’s chemical composition (nutrients) can encourage microbial growth. Fruits support organisms metabolizing sugars and carbohydrates, whereas meats support protein decomposers. Starch-hydrolyzing bacterial cells and molds often are found on potatoes, corn, and rice products.

The food industry recognizes three groups of foods based on their intrinsic properties. Highly perishable foods are those that spoil rapidly. They include poultry, eggs, meats, most vegetables and fruits, and dairy products. Foods such as nutmeats, potatoes, and some apples are considered semiperishable, meaning they spoil less quickly. Nonperishable foods are often stored in the kitchen pantry. Included in this group are cereals, dried rice and beans, macaroni and pasta products, flour, and sugar.

Perishability of Foods
Examples of highly perishable, semiperishable, and nonperishable foods. The physical and chemical properties of these foods are reliable indicators as to their rate of perishability

Extrinsic Factors

Environmental conditions surrounding the food (food storage and packaging) are extrinsic factors influencing microbial growth and food spoilage:

  • Oxygen: Properly vacuum-sealed cans of food are without oxygen gas (anaerobic) and thus do not support the growth of aerobic microbes. An improperly prepared food contaminated with the anaerobe C. botulinum provides a suitable environment for growth, and one can contract botulism by consuming botulism toxins that have been released in the food.
  • Temperature: Refrigerator temperature is usually too cold for the growth of most spoilage organisms and freezer temperature halts the growth of microbes. However, the warm hold of a cargo ship and a humid, hot storeroom of a warehouse are environments conducive to the growth of many spoilage microbes.

A good example for the relationship of intrinsic and extrinsic factors with microorganisms is dairy products such as milk. Milk is an extremely nutritious food for both humans and microbes. It is an aqueous solution (87% water) with a pH of about 7.0 and contains proteins, carbohydrates, and fats.

About 2.5% is a protein, consisting of casein and lactalbumin. Carbohydrates make up about 5% of the milk. The major carbohydrate is lactose, sometimes referred to as milk sugar. The last major component of milk is butterfat, which comprising about 4% of the milk, is removed in the preparation of nonfat (skim) milk or low-fat milk.

However, when bacteria contaminate milk, the cells secrete enzymes that digest fats into fatty acids. We say the milk becomes spoiled; it is rancid and unfit to drink.

A common type of milk spoilage often takes place in the kitchen refrigerator or dairy case at the supermarket. Here, species of Lactobacillus, Streptococcus, or Paenibacillus (a spore-forming genus), which are ubiquitous in nature, are introduced during milking and processing. Not all these cells are killed by pasteurization, so the survivors can multiple slowly, fermenting the lactose into lactic and acetic acids. Enough acid can develop to change the structure of the protein and cause it to curdle (coagulate).

Milk also can be contaminated by gram-negative rods, such as Escherichia coli and Enterobacter aerogenes. These bacterial species produce acid and gas from lactose. The acid curdles the protein, whereas the gas forces the coagulated casein (curds) apart, sometimes so violently that they explode out of the container. The result is called “stormy fermentation.” Clostridium species also cause this reaction.

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