Effect of storage duration on the nutritional and value of cleome gynandra traditional leafy vegetable

Abstract

The nutritive value of most leafy vegetables tends to be affected in storage. Cleome gynandra is seasonal African traditional leafy vegetable with nutraceutical and pharmaceutical benefits. During its off-season, the vegetable could be dried and stored for two years, to ensure its sustainability. However, storage duration of dried vegetables could pose serious challenges in retaining their nutritional value. The study was conducted to investigate the suitable storage duration of freeze-dried C. gynandra leafy vegetable in order to retain the vegetable’s nutritional and physicochemical properties. The objectives of the study were (1) to determine whether the nutritional composition of freeze-dried C. gynandra leafy vegetable will be affected by storage duration and (2) to evaluate whether the physicochemical properties of freeze-dried C. gynandra leafy vegetable will be affected by storage duration. Fresh C. gynandra (2 kg) tender leaves at full vegetative stage were harvested from forty-day old greenhouse raised plants at the Green Biotechnology Research Centre of Excellence, University of Limpopo. The fresh harvests were transported to Limpopo Agro-Food Technology Station for nutritional and physicochemical properties determination. In both the objectives, the harvested tender leaves each weighing 1 kg of C. gynandra were washed and divided into seven samples. The first sample served as the control (fresh leaves) and the 6 samples were freeze-dried, prior to analysis, then stored for 0, 2, 4, 6, 8 and 10 months, which served as treatments. These were arranged in a completely randomised design in triplicates. In objective 1, the selected essential elements, namely, nitrogen (N), sodium (Na), calcium (Ca), iron (Fe), potassium (K), magnesium (Mg), phosphorus (P) and zinc (Zn) were determined, whereas in objective 2, the physicochemical properties namely, moisture, dry matter, ash, crude fat, total carbohydrate, total dietary fibre and protein were also determined. All collected data was subjected to ANOVA at the probability level of 5%. The treatments had highly significant (P ≤ 0.00) effect on the nutritional and physicochemical contents of the stored freeze-dried C. gynandra when compared to the control (fresh leaves). In objective 1, treatments contributed 98, 100, 99, 100, 100, 100, 99 and 98% to total treatment variation (TTV) in N, Na, Ca, Fe, K, Mg, P and Zn, respectively. The tested nutrient concentrations were all increasing with storage duration and the highest concentration of 5.36 g/100 as well as 206,1844, 199, 3113, 476, 623 and 5.17 mg/100 g of N, Na, Ca, Fe, K, Mg, P and Zn, respectively, where achieved at 10- months of storage. In objective 2, the storage duration had a highly significant (P ≤ 0.00) effect on the physicochemical composition of freeze-dried, stored C. gynandra TLV, contributing 100, 100, 98, 100, 100, 98 and 100% to TTV in moisture, dry matter, ash, fat, total carbohydrate, total dietary fibre, and protein, respectively. The physicochemical properties showed different response depending on the storage durations. The moisture content was the highest in the control (78.94%), but drastically reduced (8.40%) when freeze-dried at 0- month of storage. However, during storage under ambient temperatures, the moisture content of freeze-dried C. gynandra TLV, showed some minor increase from 8.40% at 0- month to 8.73% at 8- months storage duration. The dry matter content increased significantly from 0.22% at the control (fresh leaves) to 0.92% at 0- month of storage in freeze-dried C. gynandra TLV. However, in storage, the dry matter content of freeze-dried C. gynandra remained significantly (P ≥ 0.00) the same (0.92%) from 0- month to 6- months of storage duration before it slightly decreased to 0.91% at 8- and 10- months of storage duration. The ash content of fresh leaves of C. gynandra at the control had the lowest (4%) ash than the ash content of freeze-dried C. gynandra (20.17% - 20.83%) at all the tested storage duration (0 – 10 months). The fat content also increased from 0.11% at the control to 0.49% when the TLV was freeze-dried at 0- month of storage. During storage, the fat content of freeze-dried C. gynandra continued to increase from 0.49% at 0-month of storage duration to 0.71% at 10-month storage duration. Similarly, the carbohydrate content of C. gynandra fresh leaves (control) was also lower (4.65%) than the carbohydrate content of freeze-dried C. gynandra at 0- month (32.92%) of storage. However, during storage, the total carbohydrate content of freeze-dried C. gynandra fluctuated with the increasing storage durations and the lowest (29.55%) was recorded at 10- months of storage, whereas the highest (43.32%) was achieved at 4- months storage duration. Cleome gynandra TLV accumulated the highest (8.35%) total dietary fibre when freeze-dried at 0- month of storage duration, when compared to the lowest (3.35%) total dietary fibre recorded from the fresh leaves at the control. Notably, in storage, the total dietary fibre content of freeze-dried C. gynandra decreased from 8.35% at 0-month to 7.70% at 10- months storage duration. The same was observed with the protein content of C. gynandra TLV. When compared to the control, the protein content of fresh C. gynandra TLV (9.30%) was also reduced when compared to the protein content of the freeze-dried C. gynandra at 0- to 10-months (29.50 to 33.50 g/100) of storage. In conclusion, the removal of moisture through freeze-drying retained the nutritive and physicochemical properties of C. gynandra leafy vegetable in storage and the tested storage durations showed some positive increase on the quality of the vegetable, which extended to 10- months of its storage life.