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ARS Home » Midwest Area » East Lansing, Michigan » Sugarbeet and Bean Research » Research » Publications at this Location » Publication #142649
Title: THE ROLE OF THE EPICUTICULAR WAX LAYER IN WATER MOVEMENT ACROSS THE BEAN SEED COAT

Author
item Bushey, Shawna
item Hosfield, George
item OWENS, SHIRLEY - MICHIGAN ST UNIVERSITY

Submitted to: Bean Improvement Cooperative Annual Report
Publication Type: Proceedings
Publication Acceptance Date: 3/1/2002
Publication Date: N/A
Citation: N/A

Interpretive Summary: Dry bean (Phaseolus vulgaris L.) with black seed coats lose their color during cooking and canning because of leaching of pigments that are anthocyanin compounds. Leaching and the loss of color in black beans gives an unappetizing product that is often rejected by the consumer. Seed coat shininess is also important in black beans. Beans with a shiny seed coat retain more of their color after cooking and canning, but take up less water during soaking and cooking and are less palatable than beans with matte seed coats. An experiment was conducted, with two cultivars with matte and one with shiny seedcoats to study the rate of water taken up by the beans over a two hour soaking period. A second study was conducted which seeds of three cultivars were soaked in xylene, a solvent that dissolves wax, to ascertain whether the wax layer of the upper seed coat surface might be involved in water uptake. Seeds in both experiments were examined with a scanning electron microscope (SEM) which objects are magnified thousands of times their original size. In the soaking experiment the two cultivars with matte seed coats imbibed water much more rapidly than the cultivar with a shiny seed coat. SEM revealed that matte seeds had a wax layer that was extremely textured while the wax layer of the shiny seeds was smooth and uniform. There were also cracks in the wax layer of the matte seeds that apparently favored rapid water penetration into the seed. SEM revealed that the xylene was causing a gradual breakdown of the seed coat wax layer in matte seed coats as well as the cultivar with a shiny seed coat. The results of this study provide plant breeders with information for the first time to use in their quest to breed new black bean cultivars with water uptake properties necessary to give a high quality soaked and cooked product and maintain their black color. Black bean cultivars that retain their color after canning will eliminate a major obstacle to quality in the canning plant, and, thus, lower operating costs, which can be passed on to consumers.

Technical Abstract: Water uptake influences a seed's hydration, texture, palatability, and cooking time. Water uptake also influences the leaching of pigments that occurs in color seeded and especially black dry beans (Phaseolus vulgaris L.). Black bean leaching is a major problem in commerce because consumers expect beans to retain their color after cooking or canning. The black bean cultivars: 'Raven' (matte seed coat and heavy leaching during thermal processing), 'Black Magic' (matte seed coat but little to moderate leaching during thermal processing), and 'Shiny Crow' (shiny seed coat and little to no leaching during thermal processing) were studied to see if seed coat micro structure plays a role in leaching. Seed of each cultivar were soaked in xylene for 72 hours to remove epicuticular wax. Scanning electron micrographs of xylene treated seed were then compared to non-treated seed to determine whether seed coat microstructure was related to the amount of water that was imbibed. When beans were soaked in water without xylene, 'Raven', 'Black Magic', and 'Shiny Crow' imbibed 42, 23 and 0.8%, respectively, of their initial weight. Significant differences were found in the amount of water imbibed among the cultivars after the seed was treated with xylene. 'Raven', 'Shiny Crow', and 'Black Magic' imbibed 75%, 43%, and 43%, respectively, of their initial weight after the xylene treatment. The increased water imbibition of seeds due to the xylene treatment compared to the non xylene treatment was 20, 33, and 42% for 'Black Magic', 'Raven', and 'Shiny Crow', respectively. Micrographs of the seed coat surface revealed that all three cultivars had a breakdown of the epicuticular wax layer due to the xylene. Untreated seed of 'Shiny Crow' had a smooth epicuticular wax layer that was evenly distributed over the surface of the seed coat, but after treatment with xylene, the seed coat showed regions of variability in the thickness of the wax layer much like that seen for untreated seed of 'Raven' and 'Black Magic'. The current study supports the hypothesis that it is the thickness and uniformity of deposition of the waxy epicuticular layer that influences the rate of water uptake and, ultimately, leaching in black bean.