MICROSTRUCTURE OF THE PROCESSING OF BLACK BEANS (PHASEOLUS-VULGARIS L.) FROM PUREE TO DRUM-DRIED FLAKES

Black bean puree and drum-dried flake microstructures were studied and compared with raw bean by light and scanning electron microscopies. R esults illustrate the value of microscopy as a tool for evaluating phy sical and chemical changes in bean components due to processing. Micro scopy was employed to follow the microstructural changes during the pr ocessing of black beans by looking at the intermediate product (the pu ree) and the end product (the flake). The consequences of processing o n storage components such as starch, proteins, lipids, and cell walls were elucidated. The bean puree contained starch granules within intac t cells and seed coat fragments throughout the product. The drum-dried flakes had a rough exterior, and starch granules were also found with in intact cells. Both starch granules and seed coat fragments were fou nd 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 puree and the drum-dried flake. Cell contents that no longer occured in intact cells consisted of fused protein bodies in both pro ducts. The seed coat did not change significantly during processing, a nd pieces of seed coat were easily differentiated throughout the bean puree and the drum-dried flakes. The endosperm, which is closely adher ent to the inner portion of the seed coat in the intact bean, was no l onger evident in the processed samples, presumably because the endospe rm tissue had completely solubilized following the addition of water. Lipids occurred as small droplets and were located throughout both the bean puree and the drum-dried flakes. The bean puree consisted mostly of clumps of intact cotyledon cells with very few broken cells. The c otyledon cells contained intact starch granules with no evidence of st arch gelatinization [i.e., the irreversible breakdown of the granule s tructure (Dengate 1984)]. Free starch granules in the puree, resulting from broken cells, were slightly swollen and had begun to gelatinize. Protein bodies were still evident in the puree, 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 we re difficult to detect and appeared to be a rather continuous protein matrix. The cotyledon cells also contained partially gelatinized starc h granules that remained mostly distinct but were beginning to fuse. S tarch granules interspersed between intact cells were fused and comple tely gelatinized. The drum-dried flake matrix consisted of a continuou s mixture of protein, starch, and cell wall material.