Preserving food is critically important due to the need to create food products that are resistant to spoilage for long periods of time. Drying is one of the oldest preservation methods. Dried food encompasses a wide range of types of food, such as fruits, spices, snack food, and dried meats.
Moisture is one key component in food spoilage; drying out the food eliminates this path of food spoilage and therefore helps preserve it. Dried foods contain the same caloric and vitamin content as the food had before the drying, since only water is being lost. Drying also increases the flavor of food, due to the food becoming more concentrated. Drying results in an extremely long shelf life if stored properly. A wide range of packaging can be used. Since drying reduces the food’s overall weight and volume, it can also lower shipping costs.
To prepare the food for the drying process, it must be properly washed to remove dirt and decrease the number of bacteria present. Then the food must all be properly sized. The drying process requires a high surface area to volume ratio, as this allows more moisture to evaporate out of the food. The following sections describe these preparation processes for each type of food.
Fruits are harvested and sent to food processing plants, where they are individually prepared through washing, grading, cutting, and peeling.
Washing and Grading
After being harvested, fruit is washed and sorted, either mechanically or by hand. In washing, the fruits move along a conveyor belt or agitating/revolving screens while being washed with high-pressure streams of water to remove external contaminants such as dirt as seen below. The fruit is next passed onto screeners or rollers that sort for size, with significantly undersized specimens and particulate contaminants falling through as waste. The desired produce is then further sorted, or “graded” according to maturity, size, and shape. Grading may be performed manually, although newer optical sorting methods such as using pulsed LED light to quickly sort food according to configurable quality and size thresholds are faster and more accurate.
Cutting, Trimming and Coring
After grading, fruits are further processed through cutting, trimming, and coring. These steps ensure that a ready-to-eat, high-quality product is delivered, without the need for further preparation by the consumer. Fruit may be cut, sliced, and cored through the use of mechanical appliances similar to the mechanical peeler shown in the Peeling section below. The individual pieces of fruit are held or skewered in place while a blade performs the required operation, and the produce moves along a conveyor to the next step in processing.
Peeling removes the fruit’s skin. Depending on the type of fruit, as well as company preference, this may be accomplished through mechanical peeling, steam peeling, or lye peeling.
Mechanical peeling, such as using a blade to peel a rotating orange, is typically used for fruits. Other types of equipment, such as abrasion devices and rotating sieve drums are also common.
Steam peeling takes advantage of high-temperature steam under high pressure to loosen the skin. Fruits are placed in an insulated retort, which may rotate or otherwise agitate its contents to facilitate peel removal. When the pressure inside the retort is suddenly released, steam under the skin rapidly expands and pops the skin. After steaming, the retort contents are fed through a hopper below the retort onto a roller coated conveyor or pinch roller, where the skins are pulled away under a spray of water.
Lye peeling is accomplished by conveying or submerging produce through a hot solution of lye (sodium hydroxide). The alkali solution is extremely caustic and requires a post-wash in a revolving drum fitted with water sprays to remove residual lye, shown below. To sufficiently render the product safe for consumption and wash away the loosened peels, the water sprays reach pressures of 80 – 100 psi. Once thoroughly rinsed, the final taste of the produce should be unaffected.
Similar to the ingredient preparation steps for fruits, meat products are cleaned, and portioned. The methods of washing and grading are extremely similar to the above process for fruits.
Cutting and Trimming
After being collected from slaughter conditions, which comprise the butchering, skinning, and cleaning of the animal, meats undergo size reduction by cutting for ease in handling and increased surface area for drying. These processes are usually performed manually by factory workers on a conveyor system as shown above.
Spices are harvested much like fruits and vegetables and go through a washing and grading process very similar as well. Since spices encompass a wide range of foods, the processes used to prepare them vary widely from almost no needed preparation to intensive processes to prepare the spice for drying. The process that spices undergo is described in more detail in the Spices and Extracts module.
There are many methods of drying foods, including the use of convection drying, bed dryers, drum drying, freeze-drying, microwave-vacuum drying, tray dryers, combined thermal hybrid drying, sunlight, commercial food dehydrators, and household ovens. The method used depends on the food being produced and the preferences of the manufacturer.
After being sized and properly washed, fruits are treated with preservatives to prevent browning during the drying process. A mixture of citric acid, calcium chloride, and erythorbic acid or ascorbic acid is mixed in water and then applied to the fruits. The fruit can be treated with a 0.1 to 0.3% sulfur dioxide treatment to achieve this as well.
The most common method of dehydrating fruits is convection drying. In convection drying, warm air is passed over the product, evaporating moisture from the food. The food can be either placed on a conveyor or it can be stationary, and the air can flow either in parallel or perpendicular to the food. There can be slots or holes in the floor/tray and air can be blown up through the holes or down through them. Longer periods of time are needed for bigger, higher-moisture fruits. Once the fruit has reached the specified moisture content as determined by lab tests, it is sent to the packaging step.
More delicate fruits, such as berries, are better suited for the freeze-dried method, in which the fruit is frozen below the triple point of water and then placed in a low-pressure chamber and heated so that the water sublimates from the fruit. This method disturbs the structure of the food the least but is also the most expensive form of drying food, so it is reserved for the more delicate fruits that may be damaged by other forms of drying.
Meats are usually soaked in a flavor brine and then either dried using shelf dryers or commercial food dehydrators, or smoke-dried. Smoking is the most common method because it adds additional flavor. Meats do not necessarily need to be cooked if they are undergoing a drying process, but most drying processes work in unison with some form of cooking process.
The brine used in brining is a mixture of salt and other additives. Salt increases the osmotic pressure inside the cells of the meat, forcing water out, thus dehydrating the cell.
The meat smoking process cooks the meat, thereby killing bacteria, and imparts desirable flavors. The smoking process involves slow cooking at low temperatures and low humidity, which induces drying along with cooking. To smoke meat, wood is used as the fuel source and is burned very slowly in a controlled manner to provide relatively low cooking temperatures and high volumes of smoke. It can take anywhere from 4 to over 12 hours of smoking to dehydrate the meat, depending on the thickness of the pieces. Smoking also imparts a lot of flavor and may be used in general cooking to impart flavor rather than just dehydrate the meat as seen in the picture below.
Food dehydrators and shelf dryers are also used to dry meat. Food dehydrators are layers of racks that the meat is hung on and that are placed in a dry clean warm room that has hot air passed over the meat products, which dries them out over a period of twelve or more hours, and usually also cooks them.
The term spices encompass a large variety of different plant material: from dried crushed leaves to seeds, parts of flowers, and even bark, so the drying process can vary widely. The most popular method of drying spices is to sun-dry them. The spices having been previously prepared are laid out thinly on a platform usually made of wood. These platforms are left out in the sun for a day to a few weeks. The water evaporates out of the plant material and then the spices can be either crushed or ground up into a powder and packaged or left as a whole. Examples of spices dried in this way are cinnamon sticks and dried bay leaves.
Spices also require a post-drying treatment to ensure all harmful bacteria are eradicated, since there is a larger risk of bacteria being present due to the open conditions of the drying process. This process usually consists of either a steam treatment or ethylene oxide fumigation, which kills any harmful bacteria. The spices can then be ground up and packaged. The process for drying spices is described in more detail in the Spices and Extracts module.
Dried foods can be packaged in a wide variety of packaging, as long as the packaging is impermeable to moisture and oxygen. Appropriate packing materials include plastic, metal, and certain paper, with plastic generally being the cheapest and most convenient. An oxygen-absorbing pack is usually placed inside the packaging as well to improve product shelf life.
The packaging process can be manual or automatic. Automatic packaging uses packaging machines that weigh out a specified amount of product and then dispense it into the package.
Monitoring the moisture level closely is the best form of quality control. If the moisture level is kept below a certain range specific to the type of food, the food will not spoil.
The process is continually supervised to make sure that the product is up to par and any product that does not meet standards is removed from the process by hand or by machines similar to the automatic machines used in grading and sorting. Samples are weighed and tested for their moisture level to ensure that it is in the correct range to inhibit bacterial growth. Samples are also tested for harmful bacteria such as salmonella, and E. coli to ensure safety.
During packaging workers also remove any sub-par products from the line.
- Butko, Andrew. “Dried Apricots.” Wikimedia Commons, 2007, from
- Poultry Workers Cutting and Trimming Chickens [Photograph]. (2016).
- Fruit Peeler [Photograph]. (2011)
- Hogue, Theresa, and Oregon State University. “Jake Tilden-Browning Examines a Rack of Jerky in the OSU Meat Center’s Smokehouse.” Wikimedia Commons, 2009,
- U.S. Government Accountability Office from Washington, DC, United States / Public domain
- Dauthy, M. E. (1995). Fruit and Vegetable Processing (United Nations). Rome: Food and Agriculture Organization of the United Nations.
- Downing, Donald L. Processed Apple Products Workshop. New York State Experiment Station, 1992. Springer, link.springer.com/book/10.1007%2F978-1-4684-8225-6#about
- Dunn, T. J. (2015). Food Packaging. In Kirk‐Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc (Ed.).
- Gurtler, Joshua B. Microbiological Safety of Low Water Activity Foods and Spices. Springer-Verlag New York, 2016. Springer, link.springer.com/book/10.1007%2F978-1-4939-2062-4.
- Pearson, A, and T Gillett. Processed Meats. Springer Verlag, 2014. Springer, link.springer.com/book/10.1007%2F978-1-4615-7685-3#about.
- Lopez, A. (1975). A Complete Course in Canning (10th ed.). Baltimore, MD: The Canning Trade.
- United States, Environmental Protection Agency., Office of Air Quality Planning and Standards. (1995). Compilation of Air Pollutant Emission Factors.
- Austin Potter
- Daniel Watza
- Tamia Middleton