Microstructure of the processing of black beans (Phaseolus vulgaris L. ) from purée to drum-dried flakes

Abstract

Black bean purée and drum-dried flake microstructures were studied and compared with raw bean by light and scanning electron microscopies. Results illustrate the value of microscopy as a tool for evaluating physical and chemical changes in bean components due to processing. Microscopy was employed to follow the microstructural changes during the processing of black beans by looking at the intermediate product (the purée) and the end product (the flake). The consequences of processing on storage components such as starch, proteins, lipids, and cell walls were elucidated. The bean purée contained starch granules within intact cells and seed coat fragments throughout the product. The drum-dried flakes had a rough exterior, and starch granules were also found within intact cells. Both starch granules and seed coat fragments were found in the drum-dried flake matrices. Protein bodies, occurring in the raw bean, were present (but less obvious than in the raw bean) in both the bean purée and the drum-dried flake. Cell contents that no longer occurred in intact cells consisted of fused protein bodies in both products. The seed coat did not change significantly during processing, and pieces of seed coat were easily differentiated throughout the bean purée and the drum-dried flakes. The endosperm, which is closely adherent to the inner portion of the seed coat in the intact bean, was no longer evident in the processed samples, presumably because the endosperm tissue had completely solubilized following the addition of water. Lipids occurred as small droplets and were located throughout both the bean purée and the drum-dried flakes. The bean purée consisted mostly of clumps of intact cotyledon cells with very few broken cells. The cotyledon cells contained intact starch granules with no evidence of starch gelatinization [i.e., the irreversible breakdown of the granule structure (Dengate 1984)]. Free starch granules in the purée, resulting from broken cells, were slightly swollen and had begun to gelatinize. Protein bodies were still evident in the purée, but no longer stained as in the raw, mature bean. The drum-dried flakes contained intact and broken cotyledon cells. Intact cells contained protein bodies that were difficult to detect and appeared to be a rather continuous protein matrix. The cotyledon cells also contained partially gelatinized starch granules that remained mostly distinct but were beginning to fuse. Starch granules interspersed between intact cells were fused and completely gelatinized. The drum-dried flake matrix consisted of a continuous mixture of protein, starch, and cell wall material.