SAQ 8. Infectious agents are organisms that can be passed to, and between, people in the process of infection transmission. Many infectious agents bacteria, viruses, fungi and protozoa are microorganisms that are too small to be seen except with a microscope; the adult stages of disease-causing parasites e.
Microbiology is the science that deals with the study of microorganisms. Although infections often result in disease, it is possible to be infected with a pathogen and still appear healthy. However, the infectious agent can still be passed on to others, for example by spreading into food handled by the infected person.
The majority of foodborne diseases those caused by infectious agents transmitted to people in the food we eat are due to bacteria, but as you will see in Study Session 9, viruses , parasites and toxins can also cause foodborne diseases. Bacteria are the most abundant of all organisms. Bacteria are unicellular organisms made of one cell and are very small in size, ranging from 0. Bacteria reproduce asexually. There are pathogenic bacteria capable of causing human illness and food spoilage, but there are also beneficial species of bacteria that are essential to good health and a healthy environment.
For example, beneficial bacteria live in our gut and help us digest our food; some bacteria are used to produce foods such as yoghurt and cheese; and others break down wastes in the environment. Some bacteria are capable of forming highly resistant and endurable structures called spores. Bacterial spores are resistant to heat, freezing, drying, chemicals and other adverse environments.
This means the spores can survive the normal processes of food storage and preparation. Two examples of spore-forming bacteria important in food contamination are Bacillus and Clostridium. Temperature, humidity, oxygen and water are important for bacteria to grow and multiply.
Under favourable conditions a growing bacterial population can double at regular intervals ranging from about 15 minutes to several hours. This means that the numbers of bacteria in food can increase rapidly and soon become hazardous to health, particularly if the food has a favourable temperature and water content. In the next section, we look in detail at factors that can promote or delay bacterial growth in our food.
The growth of microorganisms in food products can be affected by extrinsic factors and intrinsic factors , as you will see below. By understanding the factors affecting the growth of microorganisms in food we can know how to keep food safe to eat. This knowledge can also help us to work out how to preserve food for longer. Extrinsic factors are factors in the environment external to the food, which affect both the microorganisms and the food itself during processing and storage.
Extrinsic factors include temperature, humidity and oxygen. Different microorganisms grow over a wide range of temperatures. Some microorganisms like to grow in the cold, some like to grow at room temperature and others like to grow at high temperatures.
This is of paramount importance in food safety, because if you know the temperature growth ranges for dangerous microorganisms it helps you to select the proper temperature for food storage to make them less able to grow and reproduce.
The humidity of the storage environment is an important factor for the growth of microorganisms at the food surfaces. If you store food in a dry atmosphere , microorganisms are less able to grow than if the food is stored in a humid moist environment. Therefore, dry conditions are better for food storage than moist conditions.
Many microorganisms need oxygen in order to develop and reproduce: these are called aerobic microorganisms. A good example is Escherichia coli , a faecal bacterium which grows readily on many foods.
If you keep food in a low oxygen environment, aerobic bact eria cannot grow and multiply. Conversely, there are some microorganisms that grow without oxygen, called anaerobic microorganisms. An example of this is Clostridium botulinum , the bacterium causing botulism, which can survive in very low oxygen environments such as tinned foods. Intrinsic factors exist as part of the food product itself.
For example, meat has certain characteristics that may promote the growth of certain microorganisms. The following common intrinsic factors affect the growth and multiplication of microorganisms in foods. Environments that are acidic have pH values below 7; those that are alkaline have pH values above 7. Most microorganisms grow best at close to the neutral pH value pH 6. Only a few microorganisms grow in very acid conditions below a pH of 4.
Bacteria grow at a fairly specific pH for each species, but fungi grow over a wider range of pH values. For example, most meats naturally have a pH of about 5. At this pH meat is susceptible to spoilage by bacteria, moulds and yeasts; however the pH of meat can be lowered by pickling, which makes it less favourable as an environment for microorganisms to grow in.
Microorganisms need a moist environment to grow in. The water requirements of microorganisms are described in terms of water activity represented by the symbol a W , a measure of how much water is present. Most foodborne pathogenic bacteria require a W to be greater than 0. Think of some foods that store well when they are dry but become contaminated quickly when they are wet. You may have thought of different examples: the one that we thought of is rice.
When rice is dry it will store for a long time, but when it is cooked and wet it will go bad quite quickly and cause food poisoning. In order to grow, multiply and function normally, microorganisms require a range of nutrients such as nitrogen, vitamins and minerals.
Microorganisms therefore grow well on nutrient-rich foods. The natural covering of some foods provides excellent protection against the entry and subsequent damage by spoilage organisms.
Examples of such protective structures are the skin of fruits and vegetables such as tomatoes and bananas Figure 8. Bacteria are a major source of microbial contamination of food, i. Viruses, parasites and fungi are also able to contaminate food and cause foodborne illnesses in humans.
Microorganisms can enter food through different routes. Look at Figure 8. The most common routes of entry are discussed below. Microorganisms are found everywhere in our environment. Many types can be found in air and dust , and can contaminate food at any time during food preparation or when food is left uncovered Figure 8.
Imagine a kitchen where food is prepared and stored in rural communities, and think how easily microorganisms in the air and dust could contaminate the food. Many microorganisms present in soil and water may contaminate foods.
Microorganisms also grow on plants and can contaminate food if care is not taken to remove them by washing or inactivate them by cooking. Over the course of days and weeks at home, we are exposed to foods of different nature with different ways of cooking. How to determine what is the best method to preserve these foods given the foods are of different nature and exposed to different ways of cooking like steaming, boiling or using oil to cook?
Jaykishen Sookun - thank you for your comment. Divide leftovers into shallow containers. This encourages rapid, even cooling. Cover with airtight lids or enclose in plastic wraps or aluminum foil. Use refrigerated leftovers within 3 to 4 days, or freeze them for longer storage. I Believe if you follow guideline and inspect your make they not expired your family will be safe from food poisoning. Skip to main content Official websites use.
Menu U. A simple heat treatment blanching is used to inactivate these enzymes. Oxygen can also cause spoilage of foods spontaneously, by itself. Oxidative spoilage is the chief cause of quality loss in fats and fatty portions of foods. When lipids oxidize, short chain carbon compounds are formed; these compounds have very strong odors and flavors and are very undesirable and unacceptable. The off-odors resulting from this type of spoilage are sharp and acrid and have been described as linseed oil-like, tallowy, fishy or perfume-like.
Water is one of the most common substances on earth. It is an essential component of all foods. The amount of water in a food known as percent water influences the appearance, texture, and flavor of the food.
All living organisms as well as food contain water. The amount of water in some common foods is shown below. Although the water content of a food is expressed as a percent, this number does not reflect how the water exists in the food. Bound water is physically bound to large molecules components in the food.
It is not available to microorganisms for their growth and it cannot participate in chemical reactions. Water or moisture greatly affects the keeping qualities of food. Excessive moisture pickup can result in product spoilage and spoilage by:.
Food spoilage and spoilage can occur when there are slight changes in relative humidity. Moisture can condense on the surface of a product and this can result in many common food defects. The molding of grain, soggy cereals, and the caking and lumping of dry products like powders and cake mixes can result from excessive moisture. Other defects such as mottling, crystallization and stickiness have also been observed.
Moisture condensing on the surface of a food can also provide an environment for bacteria and molds to grow and multiply. Physical defects such as cracking, splitting and crumbling occur when excessive moisture is lost from foods.
The exposure of foods to light can result in the photodegradation or spoilage of these products. Oxidation, a chemical process that produces undesirable changes in color, flavor and nutrient content, results when air reacts with food components. When fats in foods become rancid, oxidation is responsible. Discoloration of light-colored fruits can be reduced by using an antioxidant, such as ascorbic acid or citric acid, before freezing.
Vapor-proof packaging that keeps air out helps reduce oxidation problems. Light exposure could result in color and vitamin loss. Light also may be responsible for the oxidation of fats. These creatures require food to survive and damage food, making it more vulnerable to further deterioration.
Bruises and cracks on raw produce leave areas where microorganisms easily may grow. Improperly packaged foods, dented cans and broken packages provide places for microorganisms, air, light and creatures to enter.
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