Evaluation of physical traits and chemical components associated with hard-to-cook phenomenon in Bambara groundnut (Vigna subterranea (L.) Verdc.)

Tan, Xin Lin (2023) Evaluation of physical traits and chemical components associated with hard-to-cook phenomenon in Bambara groundnut (Vigna subterranea (L.) Verdc.). MPhil thesis, University of Nottingham.

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Bambara groundnut (Vigna subterranea (L.) Verdc.) is a potential crop for future sustainable agri-food systems. However, utilisation of the crop is constrained by several factors, one of which is the hard-to-cook (HTC) phenomenon. The HTC seeds do not soften sufficiently after prolonged cooking time, thus demanding more energy for preparation. This, in turn, reduces the economic value and consumers acceptance of the pulse. The objective of this project was to explore the HTC traits in Bambara groundnut through the evaluation of physicochemical, microstructural, and technological properties to provide a basis to improve the processing efficiency and utilisation of the nutrient-dense pulse.

A screening study was first conducted to assess the variability and relationship between physical, microstructural, hydration and cooking characteristics among 12 Bambara groundnut genotypes. The physical traits of the seeds, which were characterised in terms of geometric, gravimetric, and seed coat properties, varied among genotypes. Thick seed coat (95.29-133.19 μm) and palisade layer (70.62-103.03 μm), compact cotyledon cells, narrow hilar groove, small tracheid bar, and occluded micropyle were among the factors contributing to the poor hydration behaviour of Bambara groundnut. During the soaking process, the seed coat, which exhibited moisture-dependent permeability, was the primary barrier to initial water uptake among the dry seeds. A sigmoidal model was applied to describe the hydration kinetics of the seeds. Three hydration parameters were subsequently estimated: (1) equilibrium moisture content (94.5-135.2 %), (2) hydration rate (0.095-0.272 h-1), and (3) time to achieve half saturation (9.6-24.8 h). The cooking times (CTs) also showed genotypic variation, ranging from 70-208 mins and 38-120 mins for partially and fully hydrated seeds, respectively. The CT of fully hydrated seeds was not correlated (p>0.05) with any of the physical, microstructural and hydration kinetics parameters. It was, however, significantly (p<0.05) correlated with leaching losses during soaking, supporting the cell membrane deterioration hypothesis.

The objective of the second stage of this study was to examine the differences in the characteristics of starch, protein, and cell wall materials between easy-to-cook (ETC) and hard-to-cook (HTC) genotypes. Two ETC genotypes (C_KARO and R_SONG; CT: 38-43 min) and two HTC genotypes (B_IPBB and N_ANAM; CT: 80-120 min) were selected for this study. Genotypic differences in cooking time could not be attributed to protein content and solubility. Additionally, the results of Fourier transform infrared spectroscopy indicated that there was no association between ease of cooking and molecular order of starch, secondary structure of protein, and molecular structure of cell wall materials. However, using a differential scanning calorimetry, a greater (p<0.05) thermal stability was observed among the HTC genotypes, as reflected by a higher thermal transition temperature and enthalpy of change. The HTC genotypes also exhibited a higher content of chelator-soluble pectin (p<0.05) and a lower content of water-soluble pectin (p<0.01) compared to the ETC genotypes, indicating the role of pectin solubility in strengthening intercellular adhesion and delayed cell separation during cooking.

In the final study, the response surface methodology was applied to identify the optimal soaking solution for Bambara groundnut genotype C_NAV4 to maximise the hydration capacity and cookability of seeds while minimising colour changes of the cooked seeds. Consequently, a soaking solution containing 0.25% NaHCO3 + 0.14% Na2CO3 was selected. A comparative study was then conducted to assess the impact of salt solution on the hydration and cooking behaviour of the seed. The most notable effects arising from the presence of alkaline salt in the soaking solution were: (1) an improved hydration behaviour during soaking, as evidenced by a shorter lag time (τ = 8.1 h) and a faster hydration rate (k = 0.211 h-1) than that of distilled water-soaked seeds (τ = 10.9 h and k = 0.181 h-1, respectively), (2) a greater leaching loss throughout soaking and cooking processes, (3) a significant (p<0.05) decrease in the level of chelator-soluble pectin, and (4) a 2.5-fold increase in the rate constant of cooking. These results indicate that the salt solution was effective in improving the hydration rate and shortening the cooking time of Bambara groundnut. The microstructural changes at various stages of cooking provided evidence for an association between cotyledon cell separation and texture softening of seeds.

Item Type: Thesis (University of Nottingham only) (MPhil)
Supervisors: Azam Ali, Susan
Massawe, Festo
Keywords: Bambara groundnut, Vigna subterranea, hard-to-cook, physical properties, hydration, microstructure, pectin, thermal properties
Subjects: S Agriculture > SB Plant culture
Faculties/Schools: University of Nottingham, Malaysia > Faculty of Science and Engineering — Science > School of Biosciences
Item ID: 72251
Depositing User: Tan, Xin
Date Deposited: 18 Feb 2023 04:40
Last Modified: 18 Feb 2023 04:40
URI: https://eprints.nottingham.ac.uk/id/eprint/72251

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