Dehydrating Food | How It Works

Dehydration is one of the world’s oldest methods of food preservation, with the principles of this process influencing some of the most modern methods of food science. Here, we will lift the lid on the long and varied history of dried food and discover how it has influenced the remarkable technologies behind food we eat every day.

The ancient technique of dehydrating food

Drying food in the sun has been going on since the beginning of recorded human history. All you need is direct sunlight for a number of days and a low humidity atmosphere of below 20%.¹ So, your geography matters, which is why sun-dried foods are such a central part of certain cultures. Evidence shows that Middle Eastern and Oriental cultures dried foods as early as 12,000 BCE. Around 2,800BCE the ancient Egyptians used to dry fish, poultry, dates and raisins for long-term storage. In the Middle Ages (between 630CE to 1630CE) cocoa leaves and fruits were sun-dried in Mexico and Peru.²

And some of the ancient dehydration methods are still the best ones, take for example sun-dried tomatoes. After being cleaned, sorted and pretreated with sulphites, halved tomatoes are laid out on wooden drying trays in the sun for 9 to 14 days.⁶ When compared with mechanical alternatives, sun-drying tomatoes is still the best method to prevent loss of nutrients and allow flavours to concentrate into its distinctive taste.⁷

The first food dehydration machine

The first automated drying process was created in 1795 by French inventors Masson and Chollet. The machine fed sliced fruit and vegetables into a hot-air dehydration chamber, with a controlled temperature of 40°C dehydrating produce ready to be pressed and sealed in tin foil.³ Since then, a variety of techniques for dehydrating food have been developed. Here we will look at how two of the most common drying methods work.

Spray drying food

The dairy industry is one of the largest processors of dehydrated food. Many dairy products are spray dried, meaning they are atomised into a fine mist then brought into contact with hot air. The moisture is instantly removed, leaving only the milk powder as a product. This method is used to make powders such as milk, whey and yeast, as well as some ice creams, cheese and fruit juices. Although spray drying food gives you the product quickly, it is a very expensive technique that requires a lot of energy.²

Freeze drying food

As the food is frozen, the moisture in it is turned into ice. It is then put into a vacuum at -45°C and the ice is removed by sublimation, meaning it goes directly from a solid to a gas, without ever entering liquid form. The advantage is that the food will maintain its structure and flavour, and almost any food can be freeze dried.¹ Like spray drying however, this method can also be expensive. To make up for high production costs, it is mainly used on high-value products for consumers, such as coffee.

Pros and cons of dehydrated food

Food dehydration reduces the size and weight of the food, while keeping some of its nutritional content. For example, if you dry 9.1kg of plums you will get 1.1kg of prunes. This makes it much easier to transport and store, not to mention an amazing longevity of 6 months to a year, compared to a typical 3-5 days for a fresh plum.

So, what are the downsides? Firstly, dehydrated food gets a bad reputation for changing the flavour of food, often not for the better, as well as creating a tough and leathery texture. On top of that, many foods lose vitamins and other useful nutrients in the process. For example, apples, apricots, peaches and plums loose 6% of their vitamin A, 55% of thiamin, 10% of niacin, and a huge 56% of their vitamin C when they are dried.³

Retaining nutritional value

On the other hand, if you were to measure the amount of nutritional content by weight, then dried foods have the advantage over its fresh counterpart.³ It is much easier to eat 10 dried apricots than 10 fresh apricots in one sitting. So, the sheer quantity you can easily consume may make up for lower levels of nutrients per fruit. Added calories and fibre also making it a firm favourite among campers and those going on long expeditions.

In some cases, the nutritional value and vitamins can be retained through blanching food before drying them, a process that often used when drying vegetables.⁵ When preserving fruit, a sulphur gas or sulphite dip is preferred before starting the dehydration process as a way to slow down the oxidation (darkening of the fruit) as well as slow the breakdown of vitamins A and C.

Fun Fact: Ever since humans have been exploring space, dehydrated food has been a staple part of the diet. During the 1965 Gemini mission, astronauts rehydrated freeze-dried foods like chicken soup and butterscotch pudding with a water gun.⁴

Despite its bad reputation, dehydrated food is has always been a staple part of our diets, due to its simplicity and efficiency in preserving food, and it looks set to continue to influence modern food technology. 

Were you surprised to learn that dehydrated food is one of the oldest methods of food preparation? Let us know in the comments below!