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Heat Pump Drying System for Onions:
« on: April 11, 2008, 11:29:22 AM »
A heat pump drying system for onions

by Ma. Eloisa E. Hernandez
 April-June 2007
Volume 9 Issue No. 2 


Almost all our cooked foods have that distinct flavor of onions filling our tastebud.  It is hard to imagine missing that distinctively pungent smell and taste on our sautés, soups, and other cuisine.

Chopping it might bring a tear to our eyes but we cannot afford trading these tears for a blunt flavor of our food. Our kitchen would not be that exciting and delightful as it used to be.

Onion, scientifically known as Allium cepa, comes from the Latin word unio for "single" or "one" as the onion plant produces a single bulb compared to garlic, that produces many small bulbs.

More than 130 countries in the world have been cultivating onion.  In 2004, the world production reached 55,025,127 metric tons (mt).

In the Philippines, Central Luzon and Northern Luzon are the major onion producers.  Particularly, Nueva Ecija contributed to about 56.80% with Ilocos Norte and Ilocos Sur producing 15% and 13.70%, respectively.

Almost 78% of the total production is consumed as food or used as food ingredient, 10% is exported, 7% is used as seeds, while 5% is for other uses.

Onion, considered as a heat sensitive crop, requires favorable dehydration process to preserve its organoleptic properties.  This should be dried at low temperature of 30-50°C and relative humidity of 10-15%.  It needs proper drying before they can be marketed and processed.  Dried fruits and vegetables can be a good source of energy.  They contain concentrated natural vitamins and minerals such as A and C, thiamine, riboflavin, niacin, iron, and potassium.

After harvest, onions are laid out on the fields for a day or two to be partially dried or farmers used alternative ways by hanging in sheds or through air drying.

Some of the issues and concerns that plagued onion production are:  weight loss and rapid deterioration of bulb, lack of proper cleaning, and limited drying and storage facilities resulting to erratic supply of onions and high fluctuating market prices in the country.

Finding solutions
 The above scenario led Engr. Lorcelie B. Taclan and Engr. Samuel S. Franco from the Mariano Marcos State University (MMSU) in Batac, Ilocos Norte to develop a heat pump drying system to determine the drying kinetics of onion.  The heat pump drying system was designed to use recoverable energy, that is, latent heat was converted to sensible heat, thus, handles the product gently. The system operates according to a basic air conditioning cycle involving five major components: evaporator, compressor, condenser, expansion valve, and drying chamber.

A heat pump collectd heat from the condenser or the outside unit and discharges it inside through the air handler.  The expansion valve assists in the flow of refrigerant as it moves in the opposite direction.  The extracted heat from the air handler will be discharged to the drying chamber through the air handler.

The heat pump is a refrigeration machine that extracts heat from one place and discharges it into another where it is not wanted or makes little or no difference.  The system was found to: 1) ensure hygienic process of drying; 2) produce longer period in the retention of product flavors; 3) reduce the color degradation of the food product after drying under the most favorable conditions; and (4) reduce the loss of thermal sensitive vitamins embedded in the food product.

“Heat pump drying dehydrates the onions but maintains its color and its flavor and alkaloids.  Heat pump drying utilizes lesser energy than conventional mechanical drying,” Engr. Franco averred.

The system
The system used a fabricated drying tray with a dimension of 1.71 ft x 1.75 ft.  The drying chamber is able to accommodate a total of 21 kg of fresh onions.  Green onions were used and were trimmed and sliced to desired size f 1/8” ring.  The drying, thermal and dehumidifying efficiency of the heat pump drying system were analyzed.

Drying kinetics of onion was described using the different stages: initial stage, constant rate period, and falling rate.  The initial period transfers the sensible heat to the product and dramatically increases the evaporation rate resulting to high moisture content removed.  The constant stage showed high drying rates without detrimental effect to the product.

The dehydration results showed that moisture content of onion was from 83.0 % to 15.0% at moisture reduction rate of 0.029 kg/hr dried for 33.0 hours.  The drying efficiency of the system showed 37.83% calculated in terms of the specific moisture extraction rate.  This illustrates that performance of the rotary type, heavy duty compressor is efficient vaporizing moisture for onions.

The favorable drying, thermal, and dehumidifying efficiencies of the system contributed to a dehydrated onion with reduced color degradation safe for storage.

Personal note
On a personal note, Engr. Franco stated that this intervention in agricultural engineering is deemed important in increasing productivity.  “As this technology reduces waste and postharvest losses, the resulting output is a net gain, therefore, it increases productivity.”

One of the benefits of the system is that it can be utilized for a multicrop application; hence, it can be used to dry other commodities like taro, banana, ubi, and saluyot among others.

In conclusion, Engr. Franco sees the onion industry as one of the agricultural aspects under pressure. But he mentioned pointers on how to combat this issues.  Engr. Franco suggested that there should be a well-supported integrated breeding and varietal improvement program to come up with varieties adaptable to the country.  He therefore mentioned the great assistance of the Bureau of Agricultural Research (BAR) provided in this endeavor.  “MMSU, together with BAR through the Ilocos Integrated Agricultural Research Center (ILIARC), all the aspects from basic research, testing to cultural management and engineering, hence to commercialization will follow.”

This article was based on the study, “Heat Pump Drying of Onions” by Ms. Lorcelie B. Taclan and Engr. Samuel S. Franco, Mariano Marcos State University (MMSU), Batac, Ilocos Norte, Philippines.

Sources:, Market Manila, retrieved 09 July 2007., Philippine Postharvest Industry Profile:Onion, retrieved 09 July 2007.


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