Description of the study location
The study was carried out at the Kenya Agricultural and Livestock Research Organization’s [KALRO] Kiboko station, with planting being done on 7th November 2017, and harvesting done 15th August 2018. The station is situated 2o 10’S and 34o40’E, 975m above sea level, under eco-climatic zone V of Kenya [13]. The mean monthly rainfall ranged from 2.8 in October 2017 to 512 mm in March 2018, realizing a total of 1,207 mm, within 48 rainy days. The rainfall distribution was poor, with first quarter receiving 15% of the total rain, compared to quarter 2 and three, 47% and 38% respectively. Due to poor rainfall distribution at Kiboko, supplementary irrigation was done, which supplied extra 988 mm of water, within 38 times of irrigation. The mean temperature was 24oC, while the mean relative humidity was 83%. Both the rainfall, relative humidity and temperature were within the range required by pigeonpea for growth and development as reported previously [14]. The soils are well drained, very deep, dark reddish brown to dark red, friable sandy clay to clay [Acri-Rhodic Ferrassols] developed from undifferentiated basement system rocks [13;15]. The soil characteristics were within the pigonpea soil requirements for proper growth. Based on previous findings, it has been shown that pigeonpea tolerates pH values of 4.5 to 8.0 for its growth [16].
Description of pigeonpea germplasm
Five medium duration pigeonpea genotypes: ICEAP 00557, ICEAP 00554, KAT 60/8, KIONZA and MZ 2/9, were used in this study. ICEAP 00557 and ICEAP 00554 were originally selected from germplasm collected from Nachingewa in Tanzania by International Crop Research Institute for the semi-arid tropics (ICRISAT). The genotypes flower in about 85-90 days with maturity of 150-160 days. KAT 60/8 flowers in 95-120 days, has plant height of 85-130cm, a spreading growth habit and was developed in Kenya. MZ 2/9 was selected from Mozambique germplasm collection by ICRISAT. It has early flowering, 80-100 cm in plant height, and seed mass of 30-40 g/100 seed. KIONZA is a local genotype grown by many farmers in the Eastern Kenya for both dry and green vegetable peas. The seeds were sourced from ICRISAT’s Kiboko and Kambi ya Mawe stations.
Experimental design and field management
The genotypes were planted in a randomized complete block design (RCBD), replicated three times. Each plot had a total of 3 rows and a plot area of 4.5m2 with row spacing of 1.5m x 0.3m. Seeds were drilled at a soil depth of 10cm. Agronomic and crop protection measures were done based on cultural practices as recommended for each location [11].
Sample harvest and preparation
Pigeonpea was harvested at 32 days after 50% flowering [Aug 15, 2018]. [17] recommended harvest of green vegetable pigeonpea seeds at 26-32 days after flowering as most of the nutrients are at maximum. The harvested pods [plots and genotypes] were thoroughly mixed to replicate farmer practices in the region. The pods were immediately threshed [~ 20 Kgs of green peas], which were transported to the lab in a cool box (4oC), based on previous recommendation [4]. A ten kg of sample of un-threshed pods were also transported in a separate cool box container and to the Department of Food Science and Nutrition at University of Nairobi for processing. Pigeonpea samples were placed on aluminum trays to remove damaged and inert materials at ambient condition for 2h prior to experimentation. Threshed pigeonpea was separated into two samples of 5 Kgs each for blanched and un-blanched post-harvest treatments [18;19].
Post-harvest processing of pigeonpeas
The harvested vegetable pigeonpea samples were grouped into five treatments of podded [un-threshed], deep-frozen [fresh un-blanched], blanched, blanched + oven-dried, and oven dried [18]. All treatments were arranged in a completely randomized design with 3 replications. After treatments, samples were stored for 14 days prior to sensory assessment. The treatment descriptions were as follows:
Podded [fresh un-blanched]:
For this treatment, sample was bagged in a polythene bag after air removal by pressing out air [not vacuumed] and stored at room temperature (24oC).
Deep-frozen [fresh un-blanched]:
The samples were bagged in polythene bag followed by air removal from the bag. Vegetable pigeonpea was then stored in a deep freezer at -18C. The frozen samples were thawed and subjected to sensory analyses after post treatment (22 days).
Blanched:
Steam blanching of vegetable pigeonpea was done at the pilot plant in the Department of Food Science and Nutrition, University of Nairobi, Kenya. Pigeonpeas were blanched at 72oC for 2 min, submerged in cold water then drained. A 100g sample was taken for sensory analysis.
Oven-dried:
The fresh vegetable pigeonpea placed in an aluminum tray was dried in the oven at 65oC for 8 hours. After oven drying, samples were evaluated for sensory characteristics.
Blanched + oven dried sample:
Blanched vegetable pigeonpea in an aluminum tray was placed in the oven at 65oC for 8 hours for oven drying. The sample was then stored in a deep freezer for evaluation of sensory characteristics.
Moisture content of pigeopeas
This was determined according to AOAC method specifications by using oven drying [21]. A 3g of sample was weighed in clean, dried crucible [W1]. The crucible and content were dried in an oven at 100oC for 12 hrs until a constant weight was obtained. The crucible plus content was cooled in a desiccator for 30 mins and weighed again [W2]. The percent moisture content was calculated at 0, 22 and 60 days of oven drying by the following formula:
Percent moisture [wet weight basis] = [[W1-W2]/Weight of Sample]] *100%
Where: W1=initial weight of crucible+sample; and W2=final weight of crucible+ Sample
Sensory evaluation of vegetable pigeonpeas
Panelist selection and training
The sensory assessment was used to evaluate acceptability of post-harvest pigeonpeas by panelists based on published methods [22]. Panelists for the sensory evaluation were made up of 7 individuals from the Department of Food Science and Nutrition, University of Nairobi, Kenya. A questionnaire was used to evaluate individual reaction, based on a 7–point hedonic scale was used to quantify the sensory attributes of post-harvest traits as follows: 1 = very highly unfavorable, 2 = highly unfavorable, 3 = moderately unfavorable, 4 = neither favorable nor unfavorable, 5 = moderately favorable, 6 = highly favorable, and 7 = very highly favorable [23 and 24]. The post-harvest traits or characteristics evaluated were appearance, color, taste, aroma [flavor], seed tenderness and overall acceptance. Preliminary screening was done based on knowledge of the desired sensory attributes of vegetable pigeon peas, in a focus group discussion before the actual testing. The panelists were trained on the sensory attributes of vegetable pigeonpea and on the use of appropriate descriptive terms in round-table discussions, prior to the sensory evaluation which was supervised by a panel leader.
Sample preparation
At twenty-two days of storage of post treatment of harvested pigeonpeas, samples of 200 grams were taken from each of the 5 treatments [podded, deep-frozen, blanched, oven-dried, blanched+oven-dried] as previously described [21, 10 and 25]. The deep-frozen storage samples [pigeonpea] were thawed for 20 mins. Pigeonpea samples from pods were threshed and weighed [200g]. The samples were fully cooked to a status of normal softness, based on the household procedure and consumer expectation of what is regarded as a cooked product.
Sensory evaluation
Fully cooked pigeonpeas were evaluated for tenderness [based on chewing], taste, aroma or flavor based on published procedure [26]. Similarly, evaluations were made for appearance and seed color. All samples for sensory evaluations were served at the same temperature and in equal sample size to minimize panel bias. The evaluations were done in the sensory testing room at the Department of Food Science, University of Nairobi. Testers rinsed their mouth after testing each genotype to reduce the lingering taste of the last tested genotype.
Statistical analysis of data
The effect of treatments were evaluated by performing a two-way ANOVA to test the impact of storage duration and treatment methods on sensory characteristics of vegetable pigeonpea samples at 5% level of significance, using by using GENSTAT 15th edition [27]. The mean values of treatment of each parameter were further compared by using the least significant difference [LSD] test at [P<0.05] level of probability, using Turkey’s method based on SAS Institute 2016, SAS User’s Guide Statistics, Cary, NC.