The Science of Dried Fruit
– By Angela Liu, CrispyGreen.com
Most people have eaten dried fruit in one form or another. It’s probably safe to say that dried fruit has been part of the human diet for as long as humans have been around. Dried fruit is a good source of energy and nutrients, and typically, it lasts longer (has a better shelf-life) than fresh fruit.
Dried fruit can be prepared in different ways. The process for making dried fruit is also known as dehydration, which is a general term to describe removing water from the fruit. The most common dehydration methods are sun (solar) drying, oven drying and hot air drying. Since the middle of last century, vacuum drying, and freeze drying have been introduced, and more recently, microwave drying and heat pump drying have been used commercially in preparation of dried fruit.
Dehydration, depending on the method used, also causes various physical and chemical changes to the fruit itself, most noticeably: shape, color, flavor, texture, and, less visibly, nutritional and microbial profiles. All of these characteristic alterations are results of the drying conditions such as temperature, humidity, pressure, and velocity of the drying medium. Since the drying conditions are different for each drying method, the resulting products can be vastly different in taste, physical appearance and nutritional value. Therefore, not all dried fruit is the same!
Generally, dried fruits prepared with high temperature drying methods, such as sun, oven and hot air drying, have a harder and denser texture; in some cases, they can be chewy and tough to bite. That’s because the evaporation of moisture from the fruit under these conditions collapse the cellular structures and make the fruit more leathery. The high temperature can also cause greater degradation to the chemical property of the fruit; thus, the products retain less of the fruit flavor and nutrients, such as vitamins, flavonoids and other phytochemicals, while also reducing physical appearance of the fruit (more shrinkage and darker in color).
More advanced dehydration technologies employ a vacuum which allows for a rapid removal of moisture from the fruit. The best method of this sort is freeze drying. Freeze drying works by freezing the fruit and then reducing the surrounding pressure (vacuum) to allow the frozen water in the fruit to sublimate directly from ice to vapor (it’s the same principle as how dried ice turns directly into CO2 gas). Freeze dried fruits have very different physical properties than heat dried fruits. The most distinguishable feature is its airiness and crunchiness. Because the water escapes the fruit as gas, it leaves many small air pockets in the fruit, which create this unique texture that is light and crispy.
From the physical stand point, freeze dried fruits retain most of the shape and the color of the fresh fruits. They also have very similar flavor profile. The porosity of freeze dried fruit also makes it easy to rehydrate and reconstitute to its pre-drying state. Because of the low drying temperature, the retention of vitamins in freeze-dried products has shown to be significantly higher than in heat-dried products.1
In the recent years, microwave (MW) drying has been developed commercially with a key benefit of increasing the drying rate. MW interacts directly with the polar water molecules within the moist materials to generate heat. The MW energy is converted into thermal energy in a short period of time, thus greatly speed up the drying process. However, MW drying has some major drawbacks that include uneven heating, possible textural damage, and limited penetration.2 Most commonly, microwave drying is used in combination with other drying methods such as air drying to achieve the desired outcome.
In summary, there are many ways to dry fruit and the results differ considerably. Knowing the basic science underlying the various drying processes can help you understand the difference between different types of dried fruit and allow you make wiser choices as a consumer.
1. S. Sablani, Drying of Fruits and Vegetables: Retention of Nutritional/Functional Quality, Drying Technology, 24:2, 123-135
2. Zhang, et al, Trends in microwave-related drying of fruits and vegetables, Trends in Food Science & Technology, 17 (2006), 524-534