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Maintaining post-harvest red lentil quality

With red lentils, quality is paramount, and as the red lentil industry grows, quality issues related to long-term storage need to be addressed.

“Canada has become an important producer and exporter of red lentils, and there is the potential for this production to keep growing if the quality of red lentils can be maintained during long-term storage. The problem that Canadian red lentil producers face is that the Western Canadian climate is very different from other lentil producing countries,” says Dr. Stefan Cenkowski, a professor of biosystems engineering at the University of Manitoba.

Canada is the leading exporter of lentils, and Saskatchewan producers grow 99 percent of Canada’s lentil crop. Lentils grown in Canada are often subjected to high temperatures in the summer, followed by cold winter temperatures. Depending on the crop year, lentils may also be dried while going into storage. When Canadian lentils are shipped to warmer climates, they can undergo physical changes, such as absorbing moisture in the ship’s hold and at the point of destination. Although producers have no control over the weather or shipping, by optimizing their storage conditions, they can better maintain the quality of the crop. With that in mind, the Saskatchewan Pulse Growers funded research examining the effects of post-harvest handling.

Cenkowski teamed up with Dr. Lope Tabil, a professor of chemical and biological engineering at the University of Saskatchewan. They obtained several hundred kilograms of Robin and Impact lentils from two different crop years for testing. The lentils were stored and pre-treated to mimic post-harvest operations.

Researchers wanted to mimic the industrial dehulling process as closely as possible, so for the first year they used two dehulling units. The first unit was a small Satake mill, which they ran at a speed of 110 r.p.m. for 40 seconds. The lentils were processed at a pre-milling moisture content of 10.4 percent to 15.3 percent wet basis.

For the second dehuller, researchers followed a protocol established by the Canadian International Grain Institute in Winnipeg. The seeds were pitted just enough to open the seed coat and expose cotyledon. The moisture was tempered to 15 percent for four hours, and then dried at 70 degrees Celsius for 20 minutes. The seeds, which now had a moisture content of about six per cent, were cooled overnight with ambient air. The next day the lentils were dehulled and split, and the loose hulls were collected.

The Satake mill was more comparable to industrial dehulling, so after the first year, they only used the second dehuller to verify the results of the Satake mill.

Once the lentils were processed, researchers measured the dehulling efficiency and the cooking time. The Manitoba researchers also used visual imaging technology to analyze seed wrinkling and staining.

Samples were stored for up to 12 months at various temperatures to mimic summer, fall and winter storage conditions. “We didn’t see any difference in cooking time of the material, as well as the dehulling efficiency, except that the dehulling efficiency would decrease with respect to storage time,” says Tabil. 

Researchers also found a method that may reduce the negative effects of long storage times on dehulling efficiency. “What we also noticed, based on our experiments, was that when adjusting moisture content of red lentils 48 hours before milling, that influences quality. The moisture adjustment step could be used partially to alleviate the effect of storage conditions. We could probably gain a couple of percentages on the dehulling efficiency,” says Cenkowski.

The factor that was most detrimental to seed quality was not storage time. “I think the most important of all the conditions was the drying and wetting cycles. That was the most critical,” says Dr. Tabil.

Wet and dry cycles occur in storage when the sun warms one side of the bin. As air warms, it rises and picks up moisture. The air then circulates to the cooler side of the bin, dumping the moisture. As temperatures drop at night, this process reverses. This natural convection causes a one or two per cent change in moisture, which is enough to wrinkle and stain the seed. It also decreases the dehulling efficiency of the lentils, which leads to more waste when the lentils are processed. The cooking time of the lentils also increases.

“Forced convection is needed to stop the natural convection, and also forced convection will remove the respiration heat, which would also be responsible for enhancing wet pockets if moisture in the bin is not uniform,” says Cenkowski. He adds that producers need to be careful when they ventilate. For example, they should not ventilate when it’s very humid or raining.
Ultimately, producers do have some tools to help them maintain the quality of their red lentil crops. Cleaning seeds as soon as possible and not mixing seeds from multiple years are also beneficial management practices, Cenkowski explains.

“It is not completely out of hand, if we can control the cyclic moisture changes,” says Cenkowski.

March 2, 2012
By Lisa Guenther