Rice drying in the American tropics

by Emily Wilson
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Globalization is making all commodity markets, including rice, more competitive. Processors in the rice-growing regions of the American tropics, from mid-Mexico to northern Chile, are trying to find ways to reduce high production costs and remain competitive.

There are two main differences between rice production systems in temperate and tropical areas: temperate regions produce higher average yields, but the tropics offer the ability to harvest two full crops and possibly continuous seeding and harvesting for a year-round crop.

Rice produced in the tropics usually has lower total yield (polished grains, whole and brokens), lower head yield (whole, non-broken grains) and higher contents of chalky and white-bellied kernels. From the millers’ perspective, chalky areas form weak spots with less mechanical resistance to drying, husking and whitening stresses.

The chalky and white-spotted kernels may be the result of the shorter time it takes to "fill the grains" in the tropics, which is 30 days on average — 15 to 30 days less than in temperate areas.

However, rice produced north of Peru (in tropical areas 4° to 10° latitude south) normally behaves as temperate-produced rice. The colder climate of Peru’s sea currents and the desert’s low relative humidity reduces chalkiness and increases mechanical resistance to most varieties.

Rice is a staple food in Andean countries, which includes Bolivia, Colombia, Ecuador, Peru and Venezuela. Per capita rice usage in this region averages 30 kilograms, varying from 50 kilos in Peru and Ecuador to 19 kg in Venezuela and 17 kg in Bolivia. Per capita rice usage in the United States is 14.21 kg.

Andean countries now form a loose common market that allows few restrictions to the movement of products between them, and the rice market among Colombia, Venezuela and Ecuador has increased substantially in recent years. Market globalization trends will soon force Andean countries to join Mercosur, a South American trading block consisting of Argentina, Uruguay, Paraguay and Brazil, and also the proposed Americas Free Trade Zone.

Rice production costs in Andean countries are relatively high, well above production costs in Thailand, Vietnam, Uruguay and Argentina. These costs must be reduced if the crop is to survive. Rice drying is one place to start evaluating costs.

DRYING METHODS. Various rice drying methods are used in Andean countries, ranging from sun drying, still used in the dry, coastal areas of Peru, to the drying pools used mostly in Colombia and Ecuador and the column dryers typical in Venezuela.

Only in Peru is sun drying feasible, due to its very dry climate and low labor costs. High relative humidity is constant throughout the rest of the tropics.

Drying pools are rectangular, flat bottom bins with internal and external walls made of clay or cement bricks. Grain layers are commonly between 1 to 1.5 meters, with airflow at about 3 to 4 cfm per bushel. Traditional pools are loaded and unloaded by hand and need around 60 to 70 hours to dry rice from 23% to 12% moisture.

In the late 1960s and early 1970s, a Colombian company introduced the U.S. concept of drying rice in several passes through a dryer, removing a few moisture points in each pass, followed by tempering for several hours.

However, trials in Colombia showed rice head yields were always 3 to 4 percentage points lower than ones obtained from traditional flat bottom pools. Most likely, the weaker tropical rice could not support internal stresses of the procedure.

In Venezuela, column drying technology was generally adopted. Higher industrial margins allowed its permanence, despite notorious differences with Colombian mills in head yields. Today, lower profit margins are forcing the industry to search for better alternatives.

Price differences of more than 50% between head and broken rice made the new alternative unfeasible in Colombia.

Dryer manufacturers soon learned that column units may be installed and used as an initial step for drying pools to successfully remove the first 4% to 5% of moisture, without impairing head yield. More than 100 installations were completed in the next 10 years.

In Colombia and Ecuador, traditional drying pools have been substantially improved. To load grain, a belt conveyor with a tripper is used, while another conveyor is used for unloading.

Air flows of 11 to 12 cfm/bu are now used. Air distributing tunnels have been redesigned to have lower air speed and reduce pressure losses. Centrifugal fans that initially had less than 20% efficiencies to convert electric HP (horsepower) into air HP now can convert more than 40%.

Fuel types include natural gas, coke coal and rice husk burned in automatic devices. The husk-fired units are being built with capacities equivalent to up to 25 gallons of fuel oil per hour.

In a typical installation, a column dryer is used to reduce moisture from 23% to 19%, using air temperatures of 55°C, with grain temperatures below 32°C. Without any tempering, grain is loaded into the drying pools to receive air between 35°C to 40°C for 24 to 30 hours. Several installations may handle more than 1,000 tonnes per day.

Final moisture is on average 12%, but grain on the "hot side" (in contact with the screen) may have up to 2% less moisture than grain on the "cool side" (in contact with the atmosphere). This moisture difference, if not equalized during storage, will affect the proper functioning of milling machines, particularly huskers and paddy separators.

The new "change of state" theory about rice drying and breakage, developed by the Rice Processing Program at the University of Arkansas, Fayetteville, Arkansas, U.S., may explain the results of the different drying systems. (Read more about the Rice Processing Program online by accessing the Article Archives at www.World-Grain.com.)

The high head yields obtained with drying pools is explained by low temperatures that do not force the starch of rice to change its state from glassy to rubbery at any time, and all drying is done in the glassy stage. The rice remains below the stress transition zone.

The higher amount of broken kernels produced in column drying systems may be explained by the change of state from glassy to rubbery. This change is produced especially in the last passes of grain where the cooling effect of evaporation reduces and grain may reach temperatures that put it in the middle of the "transition" zone, where part of the grain is glassy state and the other in rubbery state.

Basically, part of the grain tries to do one thing and the other the opposite, a situation that induces breaking stresses.

A CALCULATED COMPARISON. To measure which drying system is most efficient, it is best to make a calculated comparison of, for example, constructing a facility to dry 1,000 tonnes per day.

With a column dryer, the estimated drying plant itself (without receiving pits, scales or cleaning equipment) would cost U.S.$650,000, and require 900 square meters of land. It consumes electricity and fuel more efficiently than drying pools, in addition to having lower labor costs. Column dryers produce an average head yield of 55%.

Investment for a drying pool facility will total approximately U.S.$500,000, producing head yields around 58%. The drying pool is less efficient than the column dryer and requires considerably more space, 5,600 square meters. However, the 3% head yield difference amounts to an extra 25 tons each day, with a net gain of U.S.$220 per ton.

Despite the pool’s inherent lower efficiencies in fuel and electricity usage, its economic benefits outweigh those of the column drying system because of the larger percentage of head yield.

The estimated yearly result, figuring 200 working days per year and accounting for electricity, fuel, labor, cost of investment and head yield, results in a net gain of about U.S.$775,000 with drying pools. That net gain currently is equivalent to about 1.3% of sales value, a large part of the total profits made in rice mills in Colombia.

Rice drying costs are an expensive link in the tropical American countries’ rice production chain because of higher moisture content at harvesting time. In Colombia and Ecuador, drying costs are higher in fuel, power, labor and space requirements than in Venezuela. But in Colombia, where drying systems tend to be more energy efficient, rice drying affects grain quality and reduces its value.

There is an urgent need to search for a system that can reduce cost of inputs to the Venezuelan level or lower, while maintaining high yields of unbroken kernels. Perhaps the options are in the applied research of the University of Arkansas’ theory of the glassy to rubbery transition in rice or in new machinery, such as the fluidized bed dryers being developed in Brazil, Thailand and other Asian countries.

Alvaro Castillo is an enginner-economist-consultant specializing in rice milling in the tropics of the Americas. A resident of Colombia, Castillo also has published two books on grain drying and one on rice milling in the tropics, all in Spanish.

Rice production (paddy) in Andean countries, 1999



Production (paddy), in tonnes