CHAPTER 10: PROCESSING OF HORTICULTURAL CROPS

When conditions are not suitable for storage or immediate marketing of fresh produce, many horticultural crops can be processed using simple technologies. There are many processing methods that can be used by small-scale handlers, including drying, fermenting, canning, freezing, preserving and juicing. Fruits, vegetables and flowers can all be dried and stored for use or sale in the future. Fermentation is popular throughout the world as a food preservation method, and over 3,500 individual fermented foods have been described by Campbell-Platt (1987). Fruits and vegetables can be canned or frozen, and fruits are often preserved in sugar or juiced.

Processed products must be packaged and stored properly in order to achieve their potential shelf life of up to one year. Dried products must be packaged in air-tight containers (glass or plastic bottles or sealed plastic bags). Canned and bottled products must be adequately heat processed using high quality containers that provide good seals. Dried and canned or bottled products are best stored in a cool, dark place.

Postharvest handling, transport and marketing of processed products can be much simpler and less costly than for fresh products, since refrigeration is unnecessary. Dried products take up much less space than their fresh equivalents, further reducing transport and storage costs.

Source: Bills, J. and Bills, S. 1974. Home Food Dehydrating. Bountiful , Utah : Horizon Publishers

Intermediate Technology Publications* in association with CTA published a guide to appropriate equipment entitled Small-Scale Food Processing (1992) edited by Fellows and Hampton. We encourage you to use this directory to find more information on the processes introduced in this manual, or to locate specific equipment and local manufacturers.

* Intermediate Technology Publications, 9 King Street , London WC2E 8HW , UK

Processing equipment

A catalog of postharvest processing equipment is available from Intermediate Technology Publications. Included are driers, storage containers, cleaners, hand mills, power mills, shellers, decorticators (seed removers), oil processing equipment, fruit presses, and root crop cutters/graters. Some examples are shown below.

Two-man cassava grater:

Four-bladed root chopper:

Hand-operated fruit press:

Cherry pitter:

Source: Intermediate Technology Publications, 1987. Post-harvest Crop Processing: Some tools for agriculture. Intermediate Technology Publications. London , England . 29 pp.

Preparation for processing

Some produce requires blanching before freezing or drying. Blanching by boiling water bath or in steam ends certain enzymatic reactions in the product and helps retain color and flavor after processing. Always rinse blanched produce under very cold water or dip blanched produce into an ice water bath to stop the cooking process and quickly bring the temperature down.

Blanching times for selected commodities {use one gallon of water per pound (8 liters per kg) of produce}. Add one minute for each 2000 ft altitude if you live at elevations over 4000 ft.

Sources: Chioffi, N. and Mead, S. 1991. Keeping the Harvest. Pownal , Vermont : Storey Publishing.
McClure, S. 1992. The Harvest Gardener. Pownal , Vermont : Storey Publishing.

Fruits such as apples, pears, peaches and apricots are sometimes treated with sulfur being dried. Sulfuring {burn one tablespoon of sulfur powder per pound (12g per kg) of fruit} or sulphiting (dip fruit in a 1% potassium metabisulfite solution for one minute) helps prevent darkening, loss of flavor and loss of vitamin C.

Sulfur has been a source of allergic reactions in some people, so packages of sulfured product should always be clearly labeled. Vitamin C can be used as an alternative pre-treatment to prevent browning during the drying process. Use 30 ml ascorbic acid powder in one liter (or 2 tablespoons in one quart) of lukewarm water. Slice the fruits directly into the solution, remove with slotted spoon, drain well and pat dry.

For best results when drying fresh produce, fruits should be sliced or quartered, and vegetables should be thinly sliced, chopped or diced. Solar drying of fruits will take 2 to 3 days or longer, while most chopped or diced vegetables will dry in 1 to 2 days.

Sulfuring times for selected fruits:

Sources: Miller, M. et al. 1981. Drying Foods at Home. University of California , Division of Agricultural Science, Leaflet 2785.
Hobson, P. 1994. Making and using dried foods. Pownal , Vermont : Storey Publishing.

A low cost sulfuring box can be constructed from a large cardboard box that is slashed in several places to allow adequate ventilation. Trays for drying be stacked using bricks and wooden spools as spacers. The trays must be made completely of wood, since sulfur fumes will corrode metal The entire assembly must be located out of doors, preferably on bare soil. Use one tablespoon of sulfur powder per pound (35 mls per kg) of fruit. Place the sulfur in a container well away from the side of the box since it will become quite hot. Seal the bottom edges of the box with soil.

Source: Miller, M. et al. 1981. Drying Foods at Home. University of California , Division of Agricultural Science, Leaflet 2785.

Wooden box:

Source: Intermediate Technology Publications, 1987. Post-harvest Crop Processing: Some tools for agriculture. Intermediate Technology Publications. London , England .

Solar drying

Horticultural produce can be dried using direct or indirect solar radiation. The simplest method for solar drying is to lay produce directly upon a flat black surface and allow the sun and wind to dry the crop. Nuts can be dried effectively in this way.

Simple direct driers can be made from trays of screening material propped upon wooden or concrete blocks to allow air to circulate under the produce. A layer of cheesecloth can be draped loosely over the produce, protecting it from insects and birds while drying. Check the produce each day, and move it under cover if rain threatens.

A simple method for solar drying is to construct a raised platform from wood and cover the frame with loosely woven mats. In the illustration below, sliced fresh tomatoes are being dried in direct sunlight on straw mats. Air can pass over and below the produce, speeding drying and reducing losses due to overheating.

Source: Kitinoja, L. 1992. Consultancy for Africare/ USAID on food processing in the Ouadhai, Chad , Central Africa . Extension Systems International, 73 Antelope Street , Woodland , California 95695 .

Aluminum foil can be utilized to reflect the sun onto the drying trays. The example shown below uses a plastic sheet to trap some of the heat and speed drying time.

Source: Intermediate Technology Publications, 1987. Post-harvest Crop Processing: Some tools for agriculture. Intermediate Technology Publications. London , England . 29 pp.

All the trays, screens or mats used for drying produce must be kept clean. Trays made from stainless steel, plastic or nylon are much easier to keep clean than are wooden trays. Some fruits juices will adhere to the surfaces with each handling. This leads to an accumulation of dirt and buildup of mold that can contaminate and affect the appearance of the dried product. Use a strong detergent and a still brush to scrub the trays, screens or mats clean. Allow them to dry in the sun before using them to dry product.

A good material for settling dust on roads, paths or earthen floors is calcium chloride. When spread on the earth it absorbs moisture from the air and keeps the soil moist. Rake the calcium chloride into the surface at a rate of ½ pound per square yard.

To reduce mold growth on trays, screens and mats during the off season, wash and dry them thoroughly, then store them in a well ventilated area.

Source: Mrak, E.M. and Phaff, H.J. 1949. Sun-drying fruits. California Agricultural Experiment Station Circular 392.

In order to improve the efficiency of drying, some sort of structure must be used to capture solar radiation. Various types of solar driers have been developed and are illustrated below.

* indicates glazed surface

Source: Fuller, R.J. 1993. Solar Drying of Horticultural Produce: Present Practice and Future Prospects. Postharvest News and Information 4(5): 131N-136N.

More complex models of solar driers have glass or clear plastic windows that cover the produce, providing some protection from insects while capturing more of the heat of the sun.

Direct solar drier:

Indirect driers are constructed so the sun shines upon a solar collector (a shallow box, the insides painted black, topped with a pane of glass) heating air which then moves upward through a stack of four to six trays loaded with produce.

Source: Yaciuk, G. 1982. Food Drying: Proceedings of a Workshop held at Edmonton , Alberta , 6-9 July 1981. Ottawa , Ontario : IDRC 104 pp.

Forced-air dehydrators

Nut crops can be dried in bulk using a dehydrator that combines a steady stream of air with an external source of heat. The plenum chamber below the produce is covered with a floor of perforated sheet metal or wooden slats. A fan located between the furnace and the plenum chamber moves the hot air through the drying produce.

Source: FAO. 1985. Prevention of Post-Harvest Food Losses: A Training Manual. Rome : UNFAO. 120 pp.

Oil-burning dehydrators

The batch-dryer illustrated below is constructed of wood, has an axial type fan and burns kerosene or diesel oil. A wide variety of dryers are available from manufacturers around the world.

Source: Clarke, B. 1987. Post-Harvest Crop Processing: Some Tools for Agriculture. London , UK : Intermediate Technology Publications.

Two types of dehydrators are commonly used for drying small volumes of nut crops. A wagon with a perforated floor can be transported from the field and connected to a portable burner batch drying. A stationary "pot-hole" dehydrator is designed to move heated air along a plenum under a fixed platform Individual bins of nuts are placed upon the platform and are dried as heat rises up through the perforated floor.

Wagon Dehydrator:

Pot-Hole Dehydrator:

Source: Kader, A.A. and Thompson, J.F. 2002. Postharvest handling Systems: Tree nuts. Pp.399-406. In: Kader, AA. (Ed). Postharvest Technology of Horticultural Crops. University of California , Division of Agriculture and Natural Resources, Publication 3311.

Electric dehydrators

A simple electric dehydrator can be constructed using plywood, sheet metal a small fan, five household light bulbs with porcelain mounting fixtures and some screening material. The drier shown below is 32 inches long by 21 inches wide and 30 inches high, and contains racks for five trays. The fan and the sheet metal lining the bottom compartment help conduct heat upward through the box.

Source: Chioffi, N. and Mead, G. 1991. Keeping the Harvest. Pownal , Vermont : Storey Publishing

Oven drying

Fruits and vegetables can be dried in a home oven if the oven can be run at a low temperature. Place the prepared produce on baking or metal screen trays, set the oven temperature at 140 °F and leave the door ajar (2 to 4 inches). Drying time can be reduced if ventilation is increased by using a small fan placed outside the oven.

Source: Georgia Cooperative Extension Service. 1984. So Easy to Preserve. University of Georgia , Athens , Georgia .

Drying flowers

Cut flowers can be air dried by hanging upside down or while supported by chicken wire. Certain flowers will look more natural if left standing in a vase while they dry. Anthurium dries best if left to dry very slowly. Cut the stems at a sharp angle, and place the flowers into a vase containing two inches of water. In all cases, flowers should be left to air dry in a dark, well area.

Flowers that dry best if left standing: strawflower, delphinium, larkspur, okra pods

Flowers that dry best while hanging upside down: chysanthemum, amaranthus, African daisy, statice, marigold

African daisy supported on a screen of chicken wire:

Source: Rogers , B.R. 1988. The Encyclopaedia of Everlastings. New York : Michael Friedman Publishing Group, Inc. 191 pp.

Cut flowers can be dried quickly and easily in sand or silica gel. Sand used for drying flowers should be clean, smooth and the finer the better. Starting with one inch of sand in a container, place the flower to be dried on the sand and gently cover the entire flower with more sand. The container should be left uncovered and flowers should be dried in about three weeks. Flowers that dry well in sand are shasta daisy, lily-of-the-valley, cosmos, dahlia, sweet william carnation, stock, freesia and narcissus.

Drying flowers in sand:

Silica gel is relatively expensive but reusable if heated to dry out the gel between uses. To use, cover the flower as with sand, then tightly seal the container. Check for drying in two to three days. Silica gel is especially useful for drying fragile plants and flowers with delicate colors.

Flowers that dry best in silica gel are allium, anemone, cornflower, roses, tulip and zinnia.

Source: Rogers , B.R. 1988. The Encyclopaedia of Everlastings. New York : Michael Friedman Publishing Group, Inc. 191 pp.

Canning

Two types of canners are commonly used to process horticultural crops. The first is a water bath canner, which is a large pot with a loose cover and a rack to hold jars off the bottom The pot should be deep enough to cover the canning jars by one to two inches and still have another inch of space to allow brisk boiling. The diameter of the pot should be no more than four inches wider than the diameter of the stove's burner to ensure even heating.

Acidic foods such as fruits, tomatoes, pickles and relishes, and high sugar foods such as jams, jellies, syrups and marmalades can be safely processed using a boiling water bath.

Source: Georgia Cooperative Extension Service. 1984. So Easy to Preserve. University of Georgia , Athens , Georgia .

A pressure canner is recommended for processing low acid foods such as vegetables. A pressure canner is a specially made heavy pot with a locking lid, an inner rack and a steam vent in the kid. The vent can be adjusted using a weight, value or screw, depending on the type of canner. A pressure gauge registers the air pressure inside the canner. A dial gauge gives a reading of the actual pressure, while weighted gauges will rock gently when the canner is at the proper pressure. Ten pounds of pressure at 115 °C (240 °F) is recommended for canning vegetables.

Pressure Canners

Dial Gauge

Weighted Gauges

Source: Georgia Cooperative Extension Service. 1984. So Easy to Preserve. University of Georgia , Athens , Georgia .

There are three types of glass canning jars used for processing horticultural crops. The ball type jar and the zinc capped jar both require rubber rings as seals. These can sometimes be difficult to obtain, but if locally available, make excellent containers. Currently the canning jar with a two-pieced lid is the most commonly used container.

No matter which jar is used, when filling containers, it is important to leave a small amount of headspace to allow for expansion of the food while processing. If a jar is filled too full, it may explode. If too much headspace is left, the food may spoil, since all the extra air may not be driven out during processing.

Canning jars and lids, L to R: ball type jar, zinc lid and two-piece

Source: Georgia Cooperative Extension Service. 1984. So Easy to Preserve. University of Georgia , Athens , Georgia .

Juicing

Fruits

To process tomatoes or fruits to juices, fruits are simmered in water or their own juice in a stainless steel, glass or enamelware pot. When tender, the product is cut into pieces and pressed through a food mill, colander or several layers of cheesecloth. Sugar or lemon juice can be added, to taste.

The juices must then be either frozen or canned for storage. Juices can be frozen in jars or freezer containers (leave 1/2 inch headspace). Most fruit juices can be canned in a boiling water bath for 20 minutes, but apple and grape juices can be processed in hot water (82 °C or 180 °F) for 30 minutes.

Vegetables

Vegetables should be chopped or shredded, then simmered for 45 to 50 minutes until mushy. The juice can then be pressed or strained from the vegetable pulp, and frozen or canned. Canning vegetable juices requires processing at ten pounds of pressure in a pressure canner. Pints should be processed for 55 minutes, and quart jars for 85 minutes.

Source: Stoner, C.H. (Ed). 1977. Stocking Up. Emmaus, Penn: Rodale Press.

Other methods of processing

Freezing

Most vegetables should be blanched before freezing to prevent loss of flavor and color during storage. Freezing temperatures are best set between -21 to -18 °C (0 to 5 °F).

Packages for freezing should be moisture proof and vapor proof and contain as little air as possible to prevent oxidation during storage. Heavy plastic bags, heavy aluminum foil, glass freezer jars and waxed freezer cartons all make good containers.

Jellies, Jams and Preserves

Making jams, jellies and other high sugar preserves requires a balance of fruit, acid, pectin and sugar for best results. Under-ripe fruits contain more pectin than ripe fruits, and apple juice is a good source of natural pectin. If fruits are low in acid, lemon juice can be added to the mixture of fruit and sugar. Cane or beet sugar is better for making preserves than honey or corn syrup.

To preserve fruits, cook on medium heat until the mixture "sheets" from a spoon. Avoid overcooking since this will lower the jelling capacity of the mixture. Pour into containers and seal with paraffin wax (jellies only). The other preserves should be processed in a boiling water bath for five minutes.

Fermentation

When lactic acid bacteria in foods convert carbohydrates to lactic acid, food is preserved by the resulting low pH. Sauerkraut (cabbage) and wine (grapes) are two examples of thousands of fermented foods made around the world. For more information and recipes, see Chioffi and Mead (1991).

Acidification

Pickling is a simple processing method that can be used with many types of fruits and vegetables. Brine solution (9 parts cider or white vinegar, 1 part non-iodized salt, 9 parts water, plus flavorings and spices) is poured over the product into glass canning jars (leave 1/2 inch headspace). Brined pickles are sealed and left at ambient temperature for three or more weeks, while fresh pack pickles are processed in a boiling water bath for 10 minutes.

Source: USDA. 1977. Canning, Freezing and Storing Garden Produce. USDA Agricultural Information Bulletin 410.