The main purpose of preconditioning is to loosen the hull and facilitate its separation from the kernel, thereby reducing dehulling losses. Five types of preconditioning methods were used to dehull the pigeon pea.
The methods followed were:
- Drying with and without tempering (Treatment A),
- Soaking in distilled water, followed by drying and tempering (Treatment B),
- Soaking in 8% urea solution, drying with and without tempering (Treatment C),
- Soaking in 8% sodium bicarbonate solution, drying with and without tempering (Treatment D)
- Steam treatment followed by drying with and without tempering (Treatment E).
Drying and tempering (Treatment A):
About 150 g of each grain was taken and dried for 6 min at 120oC using a laboratory fluidized bed dryer, the dried samples were divided into 3 parts. Two parts were kept in two separate glass bottles and properly covered. One was tempered for 4 hours andthe other one for 24 hours at room temperature of 22±2oC before dehulling. The third part was dehulled without tempering.
Water soaking, drying and tempering (Treatment B):
A sample of 150 g each was taken from mung bean and pigeon pea and soaked separately in distilled water for 4 h at room temperature of 22±2oC. At the end of soaking, the water was drained completely. From each sample, about 50 g was taken and dried for 6 min at 120oC using the laboratory fluidized bed dryer. The dried samples were tempered for 4 hours. The rest of the soaked samples about 100 g each were dried for 10 min at 120oC. Each sample was divided into two parts. One part was tempered for 4 h and the other part for 24 h before dehulling.
Soaking in urea solution, drying and tempering (Treatment C):
About 8% urea solution was prepared using distilled water at room temperature (22±2oC). Approximately 150 g of each grain was placed separately in glass bottles. Grains were soaked in 8% urea solution for 4 h at room temperature. After soaking, the solution was drained properly and the samples were dried for 10 min at 120oC using the laboratory fluidized bed dryer. Before dehulling, 50 g of each sample was tempered for 4 h and the other 50 g was tempered for 24 h. Dehulling was carried out for the remaining sample without any tempering.
Soaking in sodium bicarbonate solution, drying and tempering (Treatment D):
About 100 g of each grain (mung bean and pigeon pea) was placed separately in glass bottles. The grains were soaked in 8% sodium bicarbonate solution for 4 h at room temperature. After soaking, the solution was drained and the samples were dried for 10 min at 120oC. Dehulling was carried out without tempering for one part of the dried sample and the other part was tempered for 24 h before dehulling.
Steaming, drying and tempering (Treatment E):
About 250 g of each grain type was taken and subjected to steam treatment at 97±1oC for 10 min using the laboratory steam conditioning system. After steaming, the samples were dried at 120oC for 10 min. The dried samples were divided into five parts. Four parts were tempered at room temperature (22±2oC) for 4 h, 8 h, 12 h and 24 h before dehulling. The fifth part was dehulled without any tempering process.
Grit Size Selection
To determine the appropriate grit size for dehulling pigeon pea, grit sizes 24, 36 and 50 were tested. Untreated samples, with moisture content of about 10% (wb), were used in the tests. The samples were dehulled for 60 s and the abraded materials were separated into hulls, whole dehulled, split dehulled, undehulled (whole and split) and fines. Since a good percentage of brokens were obtained using grit size 24 and time required for dehulling was long using grit size 50, 36 grit size was selected and used for the rest of the experiments to obtain optimum result.
Two batches of about 20 g (total 40 g) of good quality preconditioned grains were selected from each sample and placed in two sample cups of the TADD, which were located opposite to each other. Retention time in the TADD was set at one minute. After dehulling, the abraded samples were collected using a vacuum aspirating collector device from the cups, and the hulls, split dehulled, split undehulled and fines blown by the fan were collected through a cyclone separator device, connected to the TADD.
Dehulled Sample Separation
The collected, dehulled samples were separated manually into different fractions such as whole dehulled kernels, split dehulled kernels, partially dehulled kernels, broken and hull. A kernel was considered completely dehulled when there was no hull adhering to it. For treatments D1, D2, E3 and E4 for pigeon pea and mung bean, the kernels were considered dehulled when about 90% of the hulls had been removed. Hulls, fines and split dehulled and undehulled (split) that were blown into the receptacle attached to the TADD were also collected and separated. The material collected in the receptacle was first separated using a stack of sieves, consisting of Canadian Standard sieve numbers 14 (1.41 mm), 16 (1.19 mm), 18 (1.00 mm), 20 (850 μm) and pan. Each fraction on top of each sieve was then separated using a fractionating aspirator. The separated fractions in the fractionating aspirator were then manually separated. The hulls, split dehulled and undehulled seeds were added to earlier manually separated fractions and weighed. The fraction in the pan was considered as fines and it was also weighed.
About 30 g sample each of pigeon pea was soaked in distilled water and the seed coat was removed manually. The seed coat and the cotyledons were dried at 71.1oC for 2 nights and they were weighed. The test was duplicated.