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The waxy cuticle layer is the outermost portion of the seed coat. Although the cuticle does allow some polar and nonpolar compounds to permeate the seed coat, its primary function is preventing water penetration due to its hydrophobic layers that form a waxy barrier (Bukovac et al. 1981). The palisade layer has been reported by many researchers to appear with a linear lucida, or light line, that gives the appearance of two layers of cells (Sefa‐Dedeh and Stanley 1979b). The cell layers immediately beneath the palisade layer are termed hourglass cells. Sefa‐Dedeh and Stanley (1979b) described this layer as the amorphous second layer in the palisade layer.

ssss1 SEM showing structural components of dry navy bean seed coat: C−Cuticle layer, PAL−Pallisade cells, HG−Hourglass cells, PRC−Parenchyma cells.

Source: Ruengsakulrach (1990).

The parenchyma layer cells have thick walls and are readily distinguishable after hydration, as they appear spongy and exhibit noticeable swelling. Once the parenchyma cells are hydrated, increased rates of water imbibition occur. The seed coat possesses high levels of structural carbohydrates (e.g., cellulose and hemicelluose) that contribute the vast majority of total dietary fiber of beans. Aguilera et al. (1982) reported the production of bean seed coat (hull) flours by cracking and air aspiration of seat coats from navy beans. The mass balance for this separation yielded a 7–13% seed coat fraction. Dietary fiber ranged from 31% to 50% with a mean of 40%. Crude fiber (primarily cellulose and lignin) was approximately 60% of total dietary fiber. The ash content of this flour fraction ranged from 6% to 7%. These data demonstrate the high level of fiber components within the seed coat; variability was likely due to nonuniformity in fractionation because of some contamination of cotyledon residue.