Classification of Foods
Foods can be classified in different ways based on their perishability and pH value.
Understanding how food is classified helps in determining its shelf life and the best
methods for storage and preservation.
Classification Based on Perishability
There are three main categories of foods based on how long they can be stored without
spoiling:
1. Stable Foods: These are foods that can be stored for a long period without
spoilage, typically ranging from three months to three years. Stable foods have a
low moisture content, usually below 15%, which helps prevent spoilage. Examples
of stable foods include sugar, dry cereals, tamarind, honey, legumes, breakfast
cereals, and pasta.
2. Semi-Perishable Foods: These foods can be kept fit for consumption for a
relatively long time with minimal care. Their shelf life is usually a few weeks to a
few months, and they have a moisture content ranging from 60% to 90%.
Semi-perishable foods are more likely to spoil due to the growth of mold or
bacteria, but with proper handling, they can last a while. Examples include ginger,
garlic, onions, potatoes, fried snacks, and some types of cheese.
3. Perishable Foods: These foods have a very short shelf life, usually only a few
hours to a few days, even under good storage conditions. They contain a high
amount of moisture, typically between 80% to 95%. Storing perishable foods in a
refrigerator can extend their shelf life for up to four weeks. Common examples of
perishable foods are fish, meat, milk, fruits, and vegetables.
Classification Based on pH Value
The pH value of food influences how microbes grow and how food should be processed
or preserved. Foods are classified based on their pH into four categories:
1. High Acidic Foods: These foods have a pH value of less than 3.7. They include
citrus fruits and fermented pickles. The spoilage organisms that affect these foods
are mostly yeasts and molds. However, boiling water is usually enough to kill
these spoilage agents. Some aciduric bacteria can cause problems during storage,
but they are not very heat-resistant, so heat treatments can remove them.
2. Acidic Foods: Foods in this category have a pH between 3.7 and 4.5. Examples
include guavas, oranges, mangoes, pineapples, apples, and tomatoes. These foods
are mainly spoiled by enzymes and non-spore-forming bacteria. These spoilage
agents can be easily destroyed by normal processing temperatures of around
100°C. Food poisoning microorganisms do not grow in acidic foods, so these
foods do not produce toxins.
3. Medium Acid Foods: These foods fall within a pH range of 4.5 to 5.0. Examples
include most meat and vegetable mixes with tomatoes, soups, sauces, and other
products that combine acidic ingredients. These foods are treated similarly to
acidic foods in terms of heat treatment.
4. Low Acid Foods: Foods in this group have a pH greater than 5.0. Examples
include most vegetables (like okra, carrots, peas), maize, meat, milk, eggs, and
fish. These foods are spoiled by enzymes and bacteria, especially spore-forming
bacteria, which are heat-resistant and can survive in these foods. To extend the
shelf life of low acid foods, high-temperature treatments are required to destroy
these heat-resistant spores.
Bacteria Types Based on Temperature
There are different types of bacteria that grow in different temperature ranges, which can
affect food preservation and spoilage:
● Thermophilic Bacteria: These bacteria thrive in high temperatures, ranging from
41°C to 122°C. Many thermophiles are archaea, and they can live in hot
environments like hot springs.
● Mesophilic Bacteria: These bacteria grow best at moderate temperatures, between
20°C and 45°C. Their optimum growth temperature is between 30°C and 39°C.
● Psychrophilic Bacteria: These bacteria can grow in very low temperatures,
ranging from −20°C to +10°C. They are found in cold environments such as the
Polar Regions and deep sea.
