G06: Sciences - Physical and Chemical Changes: Physical Methods

Human beings have always taken some measures to preserve food. Ancient people learned to leave animal flesh and fruits and vegetables in the sun and wind to remove moisture. Since microorganisms need water to grow, drying the food slows the rate at which it spoils. Today food processors provide a diet richer and more varied than ever before by using six major methods. They are canning, drying or dehydration, freezing, freeze-drying, fermentation or pickling, and irradiation.

Knowing how to preserve food has been essential throughout our history as humans. Consider that before the advent of refrigeration, which was originally devised in the 18th century, but was not perfected and widespread until the 20th century, most of civilisation had to make do without refrigeration and freezing.

Dried foods are tasty, nutritious, lightweight, easy to prepare, easy to carry and easy to use. Low humidity, low heat and good air circulation are critical for successful drying.

Drying foods is one of the every earliest methods of preservation. It is one of the simplest methods too, but, how does it work?

It is all to do with the moisture or liquid that is removed. Water is required by microorganisms that could spoil food and by enzymes that can degrade foods. Removing the moisture inhibits these spoiling mechanisms allowing the food to last much longer.

Canning is a method to preserve food in jars at high temperatures for a long period of time, killing microorganisms and inactivates enzymes that could cause food to spoil. The heating process pushes air from the jar, creating a vacuum seal as food cools.

Freezing keeps food safe by slowing the movement of molecules, causing microbes to enter a dormant stage. Freezing preserves food for extended periods because it prevents the growth of microorganisms that cause both food spoilage and foodborne illness.

Smoking is the process of flavouring, cooking, or preserving food by exposing it to the smoke from burning or smouldering plant materials like wood and that too hardwood. Smoking of food is done at the commercial as well as the home level. The meats are hung or placed on racks in an enclosed area so that the smoke does not escape and instead penetrates foods. Commercial smoking is sometime carried out with steam pipes that supplement the wood fire.

As recently as the nineteenth century, humans risked serious illness or even death by drinking liquids—such as milk, juice, or even water—that were several days old. By contrast, today’s beverages have a long shelf life thanks to the pasteurization process, named for the nineteenth century French scientist Louis Pasteur.