F I C D F P
Drying is probably the oldest dried foods are easier and less method of food preservation. Early expensive to store and
mankind quickly learned that plac- transport than other types of ing food in the sun until it no long- preserved foods.
er contained water was a good way Foods can be dried without to make the food last longer. To- destruction of the cellular tis- day, while sun drying is still used sue of the food.
in some parts of the world, modern drying techniques are available Limitations of Drying which allow control of factors, like
drying rate and temperature, which Drying is a fairly slow com- affect the quality of the final prod- pared to other forms of food uct. preservation. Depending on the
product and the drying condi- Drying preserves food by remov- tions, drying times can be as ing moisture from them. This stops long as 12 to 24 hours.
microbial growth and prevents Mold can grow on partially Dr. Pamela Brady spoilage. Although removing the dried foods, so once drying is Food Science Department moisture helps extend the shelf-life started it should continue with-
of the product, care must be taken out interruption. during the drying process to pre- Not all foods dry well. For serve as much of the product’s nu- some fruits and several vegeta-
tritive value, natural flavor, and bles other preservation meth- cooking quality as possible. ods such as canning or freezing are better methods of preserva-
It is important to remember that tion. drying does not improve the quali- Appropriate pretreatment is ty of a food product. For this rea- often necessary to prevent dis-
son, only fully-ripened produce coloration and other undesira- should be used for drying. If a ble changes that may occur
product is not suitable for eating during drying. fresh, it is not suitable for drying. Drying causes physical chang- es, like shrinkage. For exam- Advantages of Drying Food ple, plums become prunes and grapes become raisins. Due to
It is the simplest of all food these changes adding water
preservation methods. back the product may not re- Removal of water results in turn it to its original state. products that weigh less and take up less space. This means
INSTITUTE OF FOOD SCIENCE AND ENGINEERING ARKANSAS FOOD INNOVATION CENTER
THE DRYING PROCESS enzymatic action prior to being the product should be cooled as placed in the dehydrator. soon as the blanching process is There is more to producing good complete to prevent overcooking. quality dried products than simply Blanching Steam blanching usually results in removing the water. Food often Blanching is the process most of- less leaching of solids and greater requires a pretreatment to halt en- ten recommended for inactivating retention of water soluble vita- zyme activity. The food is then enzymes in vegetables and some mins than occurs with water dried to remove moisture and halt fruits before drying. Blanching is blanching. the growth of microorganisms. a heat treatment that inactivates enzymes but is not severe enough Emerging techniques such as high Preparing Foods for Drying to cook the food. It may be done pressure processing and infrared with water, steam or high pres- blanching are offering food pro- Selection and Preparation of sure processing. cessors new approaches to pre- Produce treating products for drying. Water blanching involves placing Select high quality, fresh, fully the product in boiling water for a Other Pretreatment Methods ripened produce for drying. Im- specified amount of time. Once Blanching can be used to prevent mature materials will lack flavor the blanching time is over, the enzymatic activity in fruits, how- and color. Over-mature produce product is removed from the boil- ever, blanching may cause the may be fibrous or mushy. Foods ing water and immediately fruit to darken slightly. It may that are not in good condition for plunged into cold water to stop also give a cooked flavor to some eating fresh and/or cooking are the cooking process. fruits, such as apricots, peaches, not suitable for drying. and pears, and blanched fruits A variation on water blanching may be soft and a little difficult to Prepare the product immediately used for “candied” dried fruit is handle. For these reasons, other after harvest and begin drying it blanching in a sugar syrup. This methods may be chosen for pre- as quickly as possible. Most prod- type of blanching helps hold the treating fruits. ucts should be washed thoroughly color of fruits very well, but the to remove any dirt or spray. Sort fruit will take longer to dry and Some pretreatment methods used and discard any imperfect materi- will tend to attract insects if not with fruits include: al. Decay, bruises, or mold on any packaged properly. Dipping the fruit in a solution piece may affect an entire batch. of a food acid, such as ascor- Fruits such as grapes, prunes, and bic acid or citric acid, to inac- Trim, peel, cut, slice, or shred the small dark plums, cherries, figs tivate the enzymes on the sur- product. Pieces should be rela- and some firm berries like blue- face of the pieces. Any edible tively uniform. Thinner, smaller berries have relatively tough skins acid will inactivate the en- pieces will dry faster than larger, with a waxy coating. These tough zymes but if the acid used is thicker ones resulting in some surfaces serve to seal in moisture. one already present in the product being over-dried before To allow interior moisture to be fruit, no new flavor will be other pieces are dried enough. drawn out during drying, the added. skins are often cracked or Soaking the fruit for about 10 Pretreating “checked” to remove this waxy minutes in a salt solution will substance. To crack the skins, the work as a pretreatment. This Enzymes in plant material are re- fruit is dipped into briskly boiling treatment has the disad- sponsible for color and flavor water, then in a very cold water vantage of introducing a salty changes during ripening. Unless before drying. flavor to the fruit. these enzymes are stopped, A fruit juice high in ascorbic changes will continue during dry- For steam blanching, the product acid (vitamin C) can also be ing and storage. Fruits and vege- is heated by exposing it to steam, used as a pretreatment dip. tables that will be dried are often rather than placing it in boiling This is generally not as effec- given a treatment to slow or stop water. As with water blanching, tive as a pure ascorbic acid
2 dip and tends to add some of loaded into the drier. The drier is ing air flow from the bottom. the color and flavor of the fruit turned on. When the product is juice to the product being dry, the trays are removed from Because the trays become warm treated. Juices high in ascorbic the drier and the product removed during the drying process, some acid include lemon, orange, from the trays. These driers are foods, especially those high in pineapple, grape, and cranber- fairly simple in design but are also sugar, will tend to stick to the tray. ry. somewhat inefficient for large This makes it difficult to remove Suggested preparation methods scale operations since they are not the food when drying is finished. and pretreatments for selected operational while the trays are be- fruits and vegetables are provided ing filled, loaded into the dryer in Appendices 1 and 2. and unloaded when drying is com- plete. Batch dryers are most often Drying Equipment used for small-scale operations and in pilot plants, like AFIC. There are a number of types of dryers/dehydrators available and Continuous driers are designed so the choice of which is best for a that the food enters the dryer on a particular operation varies with conveyer belt, passes through the factors such as: drying treatment, and then exits Figure 2. A variety of kinds of tray liners are available to keep fluids and a) the nature of the raw material, without stopping the dryer. Since small foods from falling through per- b) the quantity of material to be product can be placed on the con- forations in trays and to prevent dried and veyor continuously, there is no foods from sticking to the trays.. c) post-drying handling and pack- need to stop the process between (Photo courtesy of Commercial De- hydrator Systems, Inc.) aging requirements. batches. These dryers are best when there is a need to run large All food dehydrators have three amounts of product over long pe- This sticking can be prevented by things in common. They all use riods of time. lining the tray with a tray liner heat high enough to draw out the such as parchment paper, or a moisture but not so high it cooks Food Trays mesh liner made of food-grade the food; they use dry air to ab- plastic or Teflon. Spray-on or sorb the moisture released by the Regardless of type of dryer, the wipe-on releasing agents can also food; and air movement to carry food should be spread in a single be used to prevent sticking. the moisture away from the food. layer on the drying surface. This will assure all pieces of the prod- Temperature and Air Speed are Batch (tray) dryers handle a single uct have equal exposure to heat Critical batch of product at a time. The and air movement in the dehydra- product is placed on trays that are tor. The key to successful drying is to remove moisture as quickly as Most drying surfaces, are made so possible at a temperature that does that air can circulate around the not seriously affect the flavor, tex- food both from the top and the ture, and color of the food. In bottom. This makes drying faster. most dryers this is accomplished by a flow of hot air pulling mois- Stainless steel trays with perfora- ture from the surface of the prod- tions in them are often used in uct. The challenge is to determine batch dryers. For products like the appropriate levels of heat and liquids, fruit leathers, and seeds, air flow to dry the product with a that would fall or flow through the minimum loss of quality. Figure 1. In a batch (tray) dryer perforations, a solid tray may be Since many food components are product is dried on trays inserted into used or a tray liner may be used to the dryer. (Photo courtesy of Com- sensitive to heat, determination of mercial Dehydrator Systems, Inc.) cover the holes while still allow- the best drying temperature for
3 Table 1. Water activity values of selected foods. (Adapted from Water Activity of Foods Table, Penn State Extension - http://extension.psu.edu/food/preservation/issues/water-activity-of-foods/water-activity-of- foods-table)
aw Food Examples
Fresh and canned fruits, vegetables, meat and fish; milk; cooked sausages; breads; foods 0.95 with up to 4% sucrose or 7% NaCl
Some cheese (Cheddar, Swiss, Provolone); cured meat; fruit juice concentrates; foods with 0.91 55% sucrose or 12% NaCl Fermented sausage; sponge cakes; dry cheese; margarin; foods with 65% sucrose or 15% 0.87 NaCl Most fruit juice concentrates; sweetened condensed milk; chocolate, maple, and syrups; 0.80 flour; rice; fruit cake and other high-sugar cakes
0.75 Jam, marmalade, marzipan, marshmallows
Rolled oats containing approx. 10% moisture; jelly; molasses; some dried fruits; nuts 0.65
0.60 Dried fruits with 15-20% moisture, caramel, honey
Noodles, spaghetti, etc with 12% moisture; spices with 10% moisture 0.50
0.30 Cookies, crackers, etc. containing 3-5% moisture
Whole milk powder containing 2-3% moisture; dried vegetables containing about 5% mois- 0.03 ture; dehydrated soups, some cookies and crackers your product is critical to assuring While there are elaborate mathe- mendations should be considered the quality and safety of the dried matical models to calculate the only a starting point for experimn- product. If the temperature is too best temperatures and air move- tation with your product to define low in the beginning, microorgan- ment rates, there are a number of the best drying conditions to give isms may survive and even grow factors individual to your product a good product. before the food is adequately that must be considered. These dried. If it is too high, the surface include such things as the original Testing for Dryness will dry out too quickly, basically condition of the product, product sealing internal moisture in the components such as sugar and salt A water activity (aw) test should be product (case hardening) and food levels, the size and shape of the performed to determine the safety components sensitive to heat may pieces, the humidity of the envi- of the finished product. be damaged. ronment, and the method of opera- tion of your dehydrator. Water activity is a measure of the The rate of air movement is also amount of water available to sup- critical. Heated air absorbs mois- In general, a temperature of 140oF port the growth of bacteria, mold, ture from the product. Movement provides a good drying rate with and yeast. The goal of drying is to of the air then takes this moisture minimum loss of food quality. lower the water activity of the away. The goal is to establish a Seeds, grains, herbs and spices are food to a point where microorgan- rate of air movement that allows often dried at slightly lower tem- isms that cause foodborne disease good moisture removal. peratures. However, these recom- can no longer grow.
4 Water activity is measured on a pieces must be consistently dry fruits, lemon juice or ascorbic scale that ranges from 0 which is throughout the package. acid should be added to the puree no water available to 1, pure wa- to prevent browning. Fruits should have no signs of ter. Most fresh produce has an aw above 0.95 and that is enough to mold, have a leathery and pliable Spices such as cinnamon, cloves, support the growth of bacteria, texture (for example, if the piece ginger, or nutmeg, or flavorings mold, and yeasts (Table 1). It is of fruit is folded over itself, it like vanilla extract or lemon or important to note that aw is the springs back), not be sticky or lime peel may be added to amount of water available, not the brittle, and be free from cores and achieve unique flavors. total water content of the food. seeds or peel. Vegetables should Some water in the food may be be brittle and crisp, and be dry Purees are spread into a thin layer tied up by food components, like enough to rattle. Herbs should be on drying trays. Trays should be sugar or salt, so not available for like dust when rubbed between lined with an appropriate liner or use by bacteria. fingers. with plastic wrap to prevent the puree from flowing through the Each type of microorganism has a Recommended storage times for holes in the tray and to make it unique aw range for survival. Alt- dried fruits and vegetables range easier to remove the dried leather hough most bacteria require an aw from six months to one year. from the tray. If desired, puree of at least 0.91, some can survive may be portioned into uniform at levels as low as 0.75. Foods LEATHERS portions rather than being poured that depend on water activity as a out as a single sheet. means of preventing bacterial Leathers are made by drying pu- growth should have an aw of 0.85 reed fruit or vegetables on a flat Leathers should be dried at or lower. Some yeasts and molds surface. They get their name from 140oF. Leathers dry from the out- can grow at aw levels as low as the fact that, when dry, the prod- side to the center so the edges 0.60. Preventing spoilage by uct is shiny and has the texture of may appear dry before the centers these organisms requires lowering leather. They are often packaged are dried. Dried leathers should the aw to below this level. rolled into tube-like shapes. peel easily from the tray liner/ plastic wrap. Packaging and Storage Any type of fruit or vegetable may be made into a leather, how- JERKY Dried foods can easily reabsorb ever, fruit leathers are more com- moisture from the air so must be mon than vegetable ones. Purees Jerky, or dried meat, is a favorite packaged as soon as they are of more than one type of fruit of hikers and campers because it cooled. Packaging material and/or vegetable may be made for is as much as 75% lighter than should protect the product from unique flavor combinations. fresh meat. Because most of the exposure to moisture. Because moisture is removed, the meat light can cause many dried foods To make leathers, select ripe or can be stored without refrigera- to deteriorate, packaging that pre- slightly overripe produce. Wash tion. Jerky can be made from al- vents exposing the product to and prepare as if for eating fresh. most any lean meat. light may be desirable. Purees may be made from canned or frozen product so produce can The USDA recommends that, to Product color should be uniform be canned or frozen right after make jerky safely, red meats throughout the package. Discol- harvest then made into leathers should be steamed or roasted to o o ored pieces that appear toasted or later. 160 F and poultry to 165 F (as burned should be avoided. Pieces measured with a meat thermome- should be uniform sized and Produce should be pureed until ter) before drying. This step as- shaped without ragged edges. All smooth. If using light colored sures that any bacteria present
5 will be destroyed. After heating Brady, P.L 1995. Drying Vegeta- the meat to the indicated tempera- When jerky is dried, a piece of bles. AR Coop. Exten. Serv. Fact- ture, a constant dehydrator temper- cooled meat will crack but not sheet FSHED-56. ature of 130 to 140oF should be break when bent. After removing maintained during drying. At these the jerky from the dehydrator, pat Brady, P.L 1995. Drying Leath- drying temperatures, drying is fast off any surface oil and cool before ers,Jerky, Herbs, Seeds, Popcorn, enough to get the meat dry before packaging. and Nuts. AR Coop. Exten. Serv. it spoils and enough moisture is Factsheet FSHED-57. removed to prevent microorgan- FREEZE DRYING isms from growing. Driscoll, R. 2004. Food Dehydra- Freeze drying is a method of dry- tion. Chapt 2 in Food Processing To make jerky, slice the heated ing food by freezing it, then put- Principles and Applications, J.S. meat with the grain into strips 1/4– ting the frozen food in a vacuum. Smith and Y.H. Hui , eds. Black- to 1/2-inch thick and 1 to 1 1/2 The vacuum causes the frozen wa- well Publishing. inches wide. Remove any fat be- ter in the food to evaporate. This cause fat tends to become rancid ice-to-vapor process, called subli- Harrison, J.A. and Andress, E.L. during drying. mation, results in a dried product 2010. Preserving Food: Drying that is essentially the same size Fruits and Vegetables. GA Coop. Salted jerky is made by sprinkling and shape as the original product Exten. Serv. FDNS-E-43-10. the strips liberally with salt and, if but much lighter due to the remov- desired, other seasonings. Salted al of the water. Because freeze- Potter, N.N. and Hotchkiss, J.H. strips are stacked in a bowel or dried products have spaces where 1998. Food Dehydration and Con- crock and weighted down so they the ice had been, they readily ab- centration. Chapt. 10 in Food Sci- become immersed in the liquid sorb water when rehydrated. The ence.Springer Science and Busi- that the salt draws from the meat. downside of freeze drying is that it ness Media. The strips are allowed to stand in requires specialized equipment to this brine for several hours before produce and is relatively energy FSIS. 2013. Jerky and Food Safe- being drained and placed in a de- intensive. ty. USDA Food Safety Factsheet. hydrator. Available online at: http:// REFERENCES www.fsis.usda.gov/wps/portal/ Unsalted jerky is made skipping fsis/topics/food-safety-education/ the brining step. The seasoned Brady, P.L 1995. Drying Fruits. get-answers/food-safety-fact- meat is placed directly in the de- AR Coop. Exten.Serv. Factsheet sheets/meat-preparation/jerky-and- hydrator. The meat may be mari- FSHED-55. food-safety/ct_index nated before drying if additional flavoring is desired.
The author gratefully acknowledges the assistance of Dr. Griffiths Atungulu, Department of Food Science, University of Arkansas and Coleen Hildebrandt, Regional Sales Manager, Commercial Dehydrator Systems, Inc. in the preparation of this material.
This publication was prepared by the Arkansas Food Innovation Center (AFIC) as an outreach to entrepreneurs. Funding for the preparation of this material was provided in part by grants from the U.S. Department of Agriculture, Arkansas Agriculture Depart- ment, and the Southern Extension Risk Management program.
The Arkansas Food Innovation Center is part of the University of Arkansas System Division of Agriculture’s Institute of Food Sci- ence and Engineering. For more information about AFIC, please visit afic.uark.edu
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6 Appendix 1. Preparation and pretreatment recommendations for drying selected fruit.
Recommended Pre-Treatment (Choose from options listed) Fruit Preparation Steam Syrup Sulphur* Blanch Blanch Other (minutes) (minutes) Apples Peel and core, cut into slices or X 3-5 10 - ascorbic acid solution or rings about 1/8-inch thick - fruit juice dip or - sulfite dip Apricots Pit and halve. May slice as de- X 3-4 10 - ascorbic acid solution or sired. - fruit juice dip or - sulfite dip Bananas Use solid yellow or slightly brown 3-4 10 - ascorbic acid solution or -flecked bananas. Avoid bruised or - fruit juice dip or overripe bananas. Peel and slice - sulfite dip 1/4-inch to 3/8-inch thick cross- wise or lengthwise. Berries Wash and drain berries For firm berries like blueberries, cranber- ries, currants, gooseberries, and huckle- berries – Plunge into boiling water to “check” (crack) skins. Stop cooking by placing hot fruit in ice water. Drain be- fore drying. For soft berries like strawberries and boy- senberries, no pretreatment is needed. Cherries Stem, wash, drain, and pit fully 10 If cut or chopped, no pretreatment needed ripe fruit. Cut in half, chop or (for sour) If whole, dip in boiling water for about 30 leave whole. sec. to check skins. Figs Select fully ripened fruit. Imma- X Whole: Plunge into boiling water to ture fruit may sour before being “check” (crack) skins. Stop cooking by drying completely. Wash or clean placing hot fruit in ice water. Drain whole fruit with a damp cloth. before drying. Leave small fruit whole. Cut large fruit in half. Grapes Whole: Plunge into boiling water to Seedless Leave whole “check” (crack) skins. Stop cooking by placing hot fruit in ice water. Drain before drying. Halves: no treatment necessary
With Cut in half and remove seeds seeds Peaches When sulfuring, cut in half and X 8 10 - ascorbic acid solution or remove pit. If desired remove skin. - fruit juice dip or For steam or syrup blanching, - sulfite dip leave whole while blanching. Half and pit after cooling. May also be quartered or sliced. Pears Cut in half and core. Peeling is X 6 10 - ascorbic acid solution or preferred. Slice or quarter, if de- - fruit juice dip or sired. - sulfite dip Plums Leave whole or if sulfuring, halve X Clean by rinsing in hot tap water. (Prunes) the fruit. (optional but gives good flavor)
* Consult with processing expert about options for use of sulfur when drying fruits.
7 Appendix 2. Preparation and pretreatment recommendations for drying selected vegetables
Recommended Pre-Drying Treatment (Choose from options listed) Vegetable Preparation Water Blanch Steam Blanch (minutes) (minutes) Asparagus Wash thoroughly. Halve large tips. 3 ½-4 ½ 4-5 Beans, green Wash thoroughly. Cut in short pieces or length- 2 2-2 ½ wise. Beets Cook as usual. Cool; peel. Cut into shoestring None strips about 1/8-inch thick. Broccoli Trim, cut as for serving. Wash thoroughly. Quarter stalks lengthwise. 2 3-3 ½
Cabbage Remove outer leaves; core and quarter. Cut into 2 ½-3 1 ½-2 strips 1/8-inch thick. (until wilted) Carrots Use only crisp, tender carrots. Wash thoroughly. Cut off roots and tops; preferably peel, cut in slices 3 ½ 3-3 ½ or strips 1/8-inch thick. Celery Trim stalks. Wash stalks and leaves thoroughly. Slice stalks into pieces about 1/8-inch thick. 2 2
Corn Husk, trim, cut kernels from the cob after blanch- 3-4 5-6 ing. Garlic Peel and finely chop bulbs. None Horseradish Wash; remove small rootlets and stubs. Peel or None scrape roots. Grate or cut into 1/8-inch slices. Mushrooms Scrub thoroughly. Discard any tough, woody stalks. Cut tender stalks into short sections. Do not None peel small mushrooms or “buttons.” Peel and slice large mushrooms. Okra Wash, trim, slice crosswise into 1/8- to 1/4-inch None disks. Onions Wash, remove outer “paper shells.” Remove tops None and root ends, slice 1/8- to 1/4-inch thick. Peas, green Shell. 2 3 Peppers and pimen- Wash, stem, core. Remove “partitions.” Cut into None tos disks about 3/8-inch thick. Potatoes Wash; peel. Cut into shoestring strips 1/4-inch 5-6 6-8 thick or slices 1/8-inch thick. Squash Winter/Pumpkin Cut or break into pieces. Remove seeds and cavity 1 2 ½-3 pulp. Cut into 1-inch wide strips. Peel rind. Cut strips crosswise into pieces about 1/8-inch thick.
Wash, trim, cut into 1/4-inch slices Summer 1 ½ 2 ½-3 Tomatoes Steam or dip in boiling water to loosen skins. Chill in cold water. Cut into sections about ¾-inch wide 1 3 or slice. Cut small cherry or plum tomatoes in half. 8