Chapter 3: Apply knowledge of food micro-organisms

Food micro-organisms play a critical role in the field of Nutrition and Dietetics, influencing food safety, quality, and nutritional value. Understanding these micro-organisms is essential for dietitians and nutritionists to advise on proper food handling, processing, and preservation techniques that prevent foodborne illnesses and nutrient degradation. In Kenya, where diverse food sources and preservation methods exist, knowledge of food micro-organisms enables professionals to safeguard public health in wards, feeding kitchens, and community nutrition programs. This chapter explores the fundamental concepts of food micro-organisms, their types, and the factors affecting their growth in foods.

3.1 Food micro-organisms

Microorganisms are present in soil, water, air, plants, animals and people, so raw foods carry them from the field or the abattoir to the kitchen. Some of these organisms are useful because they turn milk into cheese and cereal into sour porridge. Others spoil food, and a few cause illness. A dietitian who plans meals, advises households on storage or investigates a case of food poisoning needs to recognise the groups of microorganisms found in food, know what they need in order to grow and understand why some foods spoil faster than others. This topic defines the key terms, describes the main groups of food microorganisms and explains the conditions that control their growth. Their importance, the methods of testing them and the ways of controlling them are taught in the topics Significance of food micro-organisms, Analysis of food micro-organism and Ways of controlling food micro-organism control.

3.1.1 Definition of terms

Clear terms help health workers and food handlers to describe a problem in the same way. The two terms below are the basis of the topic, and a short list of related words follows them.

3.1.1.1 Microorganisms

Microorganisms (microbes) are living organisms that are too small to be seen with the naked eye and that can be seen only with a microscope. The main groups are bacteria, fungi (which include moulds and yeasts), viruses and protozoa. Microorganisms are found almost everywhere and many are harmless or useful. For example, harmless bacteria live in large numbers in the human large intestine, where they ferment some of the dietary fibre that is not digested earlier in the gut.

3.1.1.2 Food microorganisms

Food microorganisms are the microbes that are found in, on or carried by food. They reach food from soil, water, air, dust, the intestines of animals, insects and the hands, clothes and equipment of food handlers. They can be placed in three practical groups:

• Spoilage microorganisms: these grow in food and cause changes in smell, taste, colour or texture, such as sour milk, slimy meat or mouldy bread. They make the food unpleasant or unfit to eat, though they do not always cause illness.
• Pathogenic microorganisms: these cause illness when they or their toxins are eaten with food. Food that carries them often looks, smells and tastes normal.
• Useful microorganisms: these are used on purpose to make or improve food, as in the making of cheese, bread and vinegar.

Related terms

• Contamination: the entry of harmful microorganisms, chemicals or physical objects into food.
• Spoilage: the change of food into a state that is not acceptable for eating because of the growth of microorganisms or the action of enzymes and chemical changes.
• Foodborne pathogen: a microorganism that can cause disease when it is eaten in food, such as Salmonella and some strains of Escherichia coli.
• Fermentation: the controlled use of microorganisms to change the food's flavour, texture and keeping quality.
• Preservation: treatment of food to slow or stop the growth of microorganisms and so extend its shelf life.

3.1.2 Types of food microorganisms

The three groups of microorganisms usually met in everyday food handling are bacteria, moulds and yeasts. Bacteria are single-celled organisms. Moulds and yeasts are fungi: moulds are multicellular and grow as threads, while yeasts are single cells. Viruses and protozoa can also be carried in food or water and can make people ill. They do not multiply in food, because they need a living host, so the control of those organisms depends on clean water, safe handling and hygiene and not on the storage conditions described in this topic.

3.1.2.1 Bacteria

Bacteria are single-celled organisms that are measured in thousandths of a millimetre. They have three basic shapes: round (cocci), rod-shaped (bacilli) and spiral (spirilla). They multiply by binary fission, in which one cell divides into two. Under good conditions of warmth, moisture and nutrients, some bacteria can divide as often as every 20 minutes.

Some bacteria form spores, which are tough resting structures that survive boiling and drying and germinate when conditions improve. Species of Bacillus and Clostridium are spore-formers. Other bacteria produce toxins in food, and some of these toxins are not destroyed by cooking.

Bacteria in food fall into three groups:

• Spoilage bacteria: for example, Pseudomonas species spoil chilled meat, fish and milk and cause slime and off-odours.
• Pathogenic bacteria: Salmonella (raw poultry, eggs and meat), pathogenic Escherichia coli (undercooked minced meat and contaminated water), Staphylococcus aureus (carried on skin, in the nose and in infected cuts, and producing a heat-stable toxin in food), Bacillus cereus (cooked rice and porridge left warm for hours), Clostridium botulinum (grows without oxygen and makes a powerful toxin in poorly processed low-acid canned foods), Listeria monocytogenes (grows slowly even in the refrigerator) and Campylobacter (raw poultry and raw milk).
• Useful bacteria: lactic acid bacteria, such as Lactobacillus and Lactococcus, are used to make cheese and to ferment vegetables and cereals.

3.1.2.2 Mould

Moulds are fungi that grow as a network of fine threads (hyphae) called a mycelium, which can be seen as fuzzy, cotton-like or powdery patches in white, green, black, blue or grey. They spread by microscopic spores in the air, which settle on food and grow when moisture and warmth are enough. Moulds need oxygen, and they grow on a wide range of foods including bread, cheese, fruit, grain, nuts and cooked foods, over a wide pH range. They can also grow on foods that are dry, salty or sugary, where most bacteria cannot.

Moulds cause spoilage by softening, discolouring and giving a musty smell to food. Some also produce toxins (mycotoxins). Aspergillus flavus and related moulds grow on maize, groundnuts and other grains and nuts that are harvested or stored damp in warm weather and produce aflatoxins. Aflatoxins damage the liver and are not destroyed by cooking, so mouldy grain and nuts must be discarded. Other moulds are useful: Penicillium species ripen certain cheeses, and Rhizopus is used to make the fermented soya product tempeh.

3.1.2.3 Yeast

Yeasts are single-celled fungi, larger than bacteria, that usually reproduce by budding, in which a small bud grows from the parent cell and separates. They grow best in foods that are sugary, moist and slightly acid, and they tolerate acid and high sugar better than most bacteria do. They grow with or without oxygen. When they ferment sugar they produce alcohol and carbon dioxide gas.

Yeast is useful in the making of bread, where the carbon dioxide raises the dough, and in beer and traditional brews. It can also be a cause of spoilage: yeasts ferment fruit juice, jam, honey and syrups, producing gas, froth and an alcoholic or yeasty smell, and they may form a film on the surface of pickles and brines. A swollen, fizzing bottle of juice is a sign of yeast fermentation and the product is discarded.

3.1.3 Factors affecting the growth of microorganisms in foods

Microorganisms grow when the food and its surroundings meet their needs. The factors fall into two groups: those of the food itself (nutrients, acidity and water availability) and those of the environment (temperature, oxygen and time). Preservation methods work by changing one or more of them. For this reason, a dietitian who knows these factors can predict which foods spoil quickly and advise how to store them.

Temperature

Every microorganism has a range of temperature within which it can grow. Mesophiles, which include most food poisoning bacteria, grow best at about 30 to 40 °C, close to body temperature. Psychrotrophs can grow slowly at refrigerator temperatures, and thermophiles grow best above about 45 °C. The range between about 5 °C and 60 °C, in which bacteria multiply quickly, is called the danger zone. Hot-held food is kept at 63 °C or above. Cold slows or stops growth but does not kill most microorganisms, while heat above about 70 °C destroys the active forms of most of them. Spores can survive these temperatures. Cooked food should therefore not stand in the danger zone for long.

Acidity (pH level)

The pH of a food measures its acidity. Most bacteria grow best at a pH near neutral, about 6.5 to 7.5, and growth slows as the food becomes more acid. Yeasts grow best in slightly acid conditions, and moulds can grow over a wide pH range. Clostridium botulinum does not grow in food with a pH below 4.6, which is why acid foods such as fruit and pickles can be heat processed more gently than low-acid foods such as meat, beans and vegetables. Fermented foods keep better because the acid made during fermentation lowers pH.

Moisture content

All microorganisms need available water. The measure used is water activity (aw), which runs from 0 for no free water to 1 for pure water. Fresh foods such as milk, meat and fruit have a water activity of about 0.95 or higher. Drying, salting and adding sugar lower water activity. The approximate lowest water activity at which each group can grow is shown below.

This is why dried grain and fish are safe when properly dry, but go mouldy first when they become damp.

Oxygen availability

Microorganisms differ in their need for oxygen. Aerobes need oxygen, anaerobes grow only without oxygen, and facultative organisms can grow with or without it. Microaerophiles need a little oxygen. Moulds are aerobic, so sealing food airtight limits mould growth. Anaerobic bacteria such as Clostridium botulinum can grow in sealed cans and vacuum packs if the food is not processed correctly.

Nutrient availability

Microorganisms need sources of energy and building materials: carbohydrate, protein, fat, minerals and vitamins. Foods that are rich in protein and moisture, such as meat, fish, milk and eggs, support rapid growth of bacteria, so they spoil quickly if they are not kept cold. Spices and onions contain natural antimicrobial substances, and honey resists spoilage mainly because its water activity is very low, so these foods are less prone to spoil.

Time

The longer food stays under favourable conditions, the larger the number of microorganisms. A growing population goes through four phases: a lag phase, when cells adjust and number changes little; a log (exponential) phase, when the number doubles at each generation; a stationary phase, when growth equals death as nutrients run out; and a death phase. The effect of the log phase is large. If one bacterium divides every 20 minutes, there are three divisions in one hour:

Number after 1 hour = 2 × 2 × 2 = 8

Real growth is slower than this when conditions are not ideal, but the table shows why cooked food must not be left standing in the danger zone for hours.

Chapter Summary

This chapter explored the fundamental concepts surrounding food micro-organisms, beginning with clear definitions of microorganisms and food microorganisms to establish a solid understanding of their nature. It then detailed the primary types of food microorganisms, including bacteria, moulds, and yeasts, emphasizing their distinct characteristics and roles in food. The discussion highlighted various factors influencing the growth of these microorganisms in food, such as temperature, moisture, and pH, which are critical for managing food safety and quality. The significance of food microorganisms was examined, illustrating their impact on food spoilage, fermentation, and potential health risks. Methods for analyzing food microorganisms were considered, focusing on techniques used to detect and quantify microbial presence in food products. Finally, the chapter reviewed effective strategies for controlling food microorganisms, essential for preserving food and preventing contamination. Together, these topics provide a comprehensive foundation for applying knowledge of food micro-organisms in food processing and preservation.