Experiences with Drying Around the World

It’s not unusual for me to find myself on foreign shores working to share the knowledge of processing technology I’ve acquired over the years. In the course of two recent international assignments, I encountered some unusual dryers from which there are lessons to be learned. We have discussed drying in this column several times (Food Technology 60(12): 90-95; 61(07): 97-100; and 64(3): 70-72) so I will not repeat that material except for several significant points that are relevant.

Drying is a simultaneous heat and mass transfer unit operation in which water is converted to vapor and removed from a solid matrix. Drying rate, which largely dictates the productivity of a given system, is primarily affected by piece size, initial moisture content, and temperature. Temperatures are usually limited by the specific food’s tolerance of color and texture changes caused by high temperature. High temperature often causes discoloration and may aggravate shrinking, which is inevitable when large volumes of moisture are removed from a flexible matrix of carbohydrates and proteins.

Drying rate can also be affected by the velocity of gas used to deliver heat and remove moisture vapor. High velocity increases external heat and mass transfer coefficients, but since most of the resistance to heat and mass transfer occurs within the piece, there is a limit to the benefit that can be realized from high air velocity. In addition, as pieces become lighter due to the removal of moisture, they are more easily blown off the trays or belts on which they are held.

Most hot air atmospheric dryers for fruits and vegetables are either batch, using movable carts with perforated or woven wire trays, or continuous, using perforated or mesh belts that are usually stainless steel, but can be made of other materials. Fruits and vegetables can become sticky and leave residues on contact surfaces so cleanability is an important design feature for food dryers.

Multi-pass Dryer in Uzbekistan
On my fifth trip as a volunteer in the United States Agency for International Development (USAID) Farmer to Farmer (FTF) Program, managed by the Citizens Network for Foreign Affairs (CNFA), I spent two weeks in Uzbekistan trying to train a local firm on the operation of its multi-pass dryer purchased from a firm in Turkey.

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CNFA (www.cnfa.org) is a nongovernmental organization (NGO) based in Washington, D.C., that manages the Farmer to Farmer program in Eastern Europe, Central Asia, East Africa, and Southern Africa. Despite its name, the program sends volunteer experts for all elements of the food chain, from agriculture through processing to marketing and business management.

On previous FTF trips through CNFA, I had worked on breakfast cereal in Moldova (twice), food safety in Moldova, and hot sauce manufacture in Malawi. In Uzbekistan, the objective was to help a small firm that wanted to dry locally grown fruits and vegetables for export, primarily to Russia. Uzbekistan is a former Soviet republic whose major cash crop is cotton. It is doubly land bound (all the countries it borders, including Afghanistan, are land bound) and very arid.

The dryer that had been purchased from a Turkish firm had not been operated for about eight months because after its installation and a brief training period, the owners of the Uzbek drying firm had not felt confident in their ability to operate the equipment. In addition, there was a dispute between the Uzbeks and the equipment supplier, which was complicated by the fact that the drying company had Turkish investors. 

In my opinion, the dryer was of unusual design and poorly constructed. It was about 20 m long and had a belt width of 1.5 m. There were five belts arranged in tiers so that the feed was on the top belt and material dropped from one belt to the next. There were 10 heating zones, but each zone affected all five belts, so the only sensible temperature strategy, I felt, was to set all to the same temperature. Fresh air could be admitted at either end and discharged at the opposite end, but the best arrangement seemed to be to admit at the product discharge end.

Most of the dryer was stainless steel, but apparently the side walls had been made of insulated sandwich panels, which quickly scorched and were replaced. However, the ends and bottom of the dryer body were made of press board, a cheap construction material unable to resist water. This meant the plastic mesh belts and the dryer itself could not be cleaned with hot water.

The belts were geared so that they ran at different speeds, evidently to compensate for the loss in weight as drying proceeded and maintain even loading on the belts. The belt speed control was not calibrated, but by somewhat tedious testing, it was determined that residence time could be varied from two to six hours. Temperature could be set up to 80˚C, provided there was sufficient gas pressure. It was discovered that the gas supply company needed prior notice to provide adequate pressure. Electric power was also unreliable and failed at least once during a trial.

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The small laboratory had moisture measuring scales and an oven for estimating drying curves. While waiting during residence time studies, the laboratory technicians were trained on using the instruments.

A problem typical of economically poor developing countries is obtaining sufficient raw material to conduct full-scale trials, and that was the case here. By calculation, it takes over one metric ton of raw material just to fill the dryer. The dryer was claimed to have a capacity of 20 tons/day, but was more likely to be capable of about half that. The owners needed permission from the government to purchase two tons of chili peppers for a trial. Chili peppers have over 90% moisture so are poor candidates, from a yield point of view, for drying. Nonetheless, they were the least expensive available commodity. Conditions of 75˚C and 4 hr residence time eventually gave several hours worth of well-dried product. The Uzbeks were disappointed to learn that the first and last materials were poorly dried, illustrating that such equipment wants to operate continuously for long runs.

Drying Without External Heat
Catholic Relief Services (CRS) has been operating in Haiti for many years and is one of the more effective foreign aid organizations in that poor country. Two years after the earthquake of January 2010, hundreds of thousands of Haitians still are living in tents and very little rebuilding has occurred. CRS is assisting a farmers’ organization, ORE, in the Southern part of Haiti, where damage was less than in the capital city of Port au Prince.

The concept is to convince farmers that mango trees are more valuable producing fruit than as firewood. Most of Haiti’s trees have been cut for fuel, leading to soil erosion. Only about 30% of mangos are acceptable for export as fresh fruit, so CRS wants to help ORE dry and otherwise process mangos to increase farmer income.

ORE has an unusual dryer made in France that uses a refrigeration unit to dehumidify circulating air that passes over sliced fruit on racks in carts within a converted shipping container. The dryer is designed for, and achieves, about 1,000 lb/day fresh fruit to yield about 166 lb dried fruit. ORE had hoped that production could be increased somehow, and adding heat seemed like one possibility. ORE operates a diesel generator to supply the electricity for the refrigeration compressor and fans.

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A colleague, Will Burke, President of Sol Simple, operates a solar-assisted fruit dryer in Nicaragua (www.sol-simple.com) and is going to assist ORE in marketing its fruit. He also felt that the drying cycle should be shorter than 24 hours. However, upon deciphering the French operating manual, we learned that the dryer was, in fact, operating exactly as specified. It was intended to give very gentle, slow, low-temperature dehydration for delicate products such as fish. It produced nice dried fruit, but may have been gentler than necessary for mango, based on Burke’s experience. Together, we determined that ORE could increase its production by tighter scheduling and adding some people and space to permit preparation and packaging to occur at the same time. Will also suggested that ORE could bottle the very pure water discharged from the dehumidifier.

The ORE facility is well designed and constructed. Each worker showers and changes clothes before starting work. There are hand wash stations in the production area. The same people prepare fruit and then pack it the next day, but this means the dryer is idle. By adding carts and building some additional space, the dryer could be more fully utilized. In the future, ORE could add a more conventional hot air dryer if they need additional capacity or they could produce mango pulp by hot filling bulk bags.

Both of these experiences involved unusual dryers in developing countries, but the important lessons in each case were more about ancillary issues, such as markets for products, raw material supply, and utility reliability than about drying technology. Assisting such ventures, whether as volunteer or consultant, requires a holistic approach and a fair amount of patience and flexibility.

J. Peter Clark,
Contributing Editor,
Consultant to the Process Industries,
Oak Park, Ill. 
[email protected]