The biggest problem with food is that it is unstable. Even relatively stable food products change over time. There are two ways in which these changes occur.
- Internal changes to the chemical make up of food. A good example would be loss of vitamins
- Spoilage caused by microorganisms. Moldy bread is a good example.
Preserving food has been going on for centuries. Without it humans would have been unlikely to stop being nomadic. By preserving food it allowed them to have a food supply throughout the seasons and not move to where there was fresh food growing. Later on, preserving food allowed for travel long distances where there was no certainty that fresh food existed, including off the planet.
Preserving food is essential a way to extend the shelf life of that food. Shelf-life to food scientists has a particular meaning representing when the food quality has deteriorated either from a sensory perspective or from a chemical perspective. Obviously, sometimes these are the same thing.
There are two ways in which to consider food preservation, how does food go bad and how can we stop it. How food deteriorates depends a lot on the particular food item. For example high moisture foods are more likely to have bacterial damage than low moisture foods. Living foods under changes after harvest; this includes the fact that fruit and vegetables continue to respire even after being picked and the fact that muscle protein changes after the slaughter process. I could write about all of these changes but that would be several posts long, if not unending.
More interesting is what do we do to preserve food and what changes does that cause to the food item. The commonest way to preserve food is to reduce the available moisture content, which is also known as the water activity. Drying can be done by drying, salting, and making jams or jellies. In the latter process, the food is preserved with large amounts of sugar. This reduces the water availability because sugar is hygroscopic and holds on to that water for itself. The fruit, and it typically is fruit that is preserved in this way, is also cooked which destroys enzyme activity, unfortunately degrade thermal unstable vitamins and softens the cell walls. For fruits high in pectin, softening the cell walls releases the gum which, when the jam is cool, sets. This gives jams and jellies their firm structure. Pectin can be added to fruit low in pectin so that the preserve sets. Even though this is a way of preserving, fruit that would normally last a week or two can now be kept up for a year, changes to the fruit preserves will take place during storage. Light colored jams will darken. Unsurprisingly, reduced sugar preserves are not a long lasting as regular preserves.
Dried fruits are common and drying is also used a lot for meat preservation. Drying or dehydration is the removal of moisture, the dryer the better as far as shelf life is concerned. There are many ways to dry foods, from sun drying to freeze-drying and spray drying. The latter two being more likely commercial methods, whereas heat drying and sun drying could be carried out at home. The method of drying alters the food in different ways. For example, freeze drying results in the formation of food products that are very little changed from the original – just without any moisture. As the new Strawberry pieces in cereals show – adding moisture back results in almost original fruit piece. Well, sort of. Spray drying, which is commonly used for liquids such as milk or juices, results in a fine powder.
Salting, smoking, and curing are mostly used for meat and fish. The addition of salt or smoke causes dehydration of cells through osmosis – the water moves out of cells into the salty surroundings. This causes the cells to die or become temporarily inactivated. This includes bacterial cells as well as food cells. Thus, bacteria cannot grow. However, the food itself is very different from fresh. We have got used to this as bacon, lox, kippers and ham are all treated in this way.
As you can see removal of available water does not have to occur by dehydration, other techniques such as adding salt or sugar have the same effect.
References:
- Shephard, Sue Pickles, Potted and Canned
- Bennion, Marion and Scheule, Barabar, Introductory Foods
Lab Cat 