| Management
Practices to Maintain Dry Bean Grain Quality Ken Hellevang, Bob Henson, and Sam Chang, North Dakota State University |
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Dry bean (Phaseolus vulgaris L.) quality deteriorates in storage. With time, seeds become harder, increasing cooking time, and light-colored seedcoats tend to darken, compromising appearance. Growers are interested in on-farm storage to improve marketing options, but need information on proper storage conditions to maintain quality. A survey of numerous dry bean growers and more than 50 handlers / processors revealed a wide variety of procedures for handling, processing, storing, and determining bean quality (K. Hellevang, unpublished data). Only 10% of the respondents use USDA grading standards when determining quality. This research was conducted to identify the principle factors which contribute to dry bean grain quality deterioration to quantify the effects of harvest moisture content, ultraviolet light exposure, and storage temperature and moisture content on quality. Pinto bean cv. Maverick was sown at the North Dakota State University Carrington Research Extension Center. The crop was harvested at 19% grain moisture for imposition of treatments. Sublots of grain were stored in darkness at 18%, 16%, or 14% moisture and 4°C (40° F) or 270C (80° F). Additional sublots were exposed to ultraviolet light. As indicated by the decrease in Hunter-L values (a measure of whiteness), beans exposed to ultraviolet light darkened dramatically within weeks (Table 1). Hunter-a values increased from 4.5 to 5.7, indicating an increase in the redness. The Hunter-b values indicated little change in the yellowness of the beans. These results indicate a need to minimize exposure to light in storage and possibly even before storage. Approximately 70% of bean handlers use flat storage, which exposes a large surface area to potential discoloration by exposure to light. After 120 d, beans stored at 27oC (summer temperature) were much darker than those stored at 4oC (Table 2). The degree of darkening increased with moisture content. When stored at 4°C, there was little difference in color quality after eight months storage at the various moisture contents. Beans stored at 18% moisture and 4oC were lighter-colored than those stored at 14% moisture and 27oC, indicating the importance of temperature in maintaining color. The storage treatments which resulted in darkening of the seedcoats had a similar effect on redness (Table 3). Storage had no effect on the yellowness of bean seedcoats (data not shown). A dramatic difference in pin cooking times was observed among treatments after four months of storage (Table 4). Beans that were stored at 27°C had cooking times that were two to four times longer than those stored at 4°C. The beans that had decreased in color quality (decreased lightness and increased redness) had much longer cooking times. At 27°C, the shortest cooking time was recorded for the beans stored at 14% moisture content. |
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Table 1. Hunter colorimeter values for beans exposed to ultraviolet light.
|
Date |
Hunter-L
(White) |
Hunter-a
(Red) |
Hunter-b
(Yellow) |
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10/18/99 |
49.2 |
4.5 |
9.3 |
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10/25/99 |
48.7 |
4.8 |
9.5 |
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10/29/99 |
47.9 |
5.0 |
9.4 |
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11/05/99 |
47.4 |
5.3 |
9.6 |
|
11/12/99 |
47.4 |
5.0 |
9.4 |
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11/19/99 |
46.5 |
5.3 |
9.5 |
|
11/26/99 |
46.2 |
5.5 |
9.7 |
|
12/03/99 |
45.5 |
5.8 |
9.6 |
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12/10/99 |
45.6 |
5.7 |
9.7 |
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12/17/99 |
45.1 |
5.7 |
9.6 |
Table 2. Hunter-L values (whiteness) for indicated storage temperatures and moisture contents.
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|
4.4°C (40°F) |
26.7°C (80°F) |
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Date |
14% |
16% |
18% |
14% |
16% |
18% |
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10/18/99 |
49.2 |
49.2 |
49.2 |
49.2 |
49.2 |
49.2 |
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11/05/99 |
49.0 |
48.2 |
48.6 |
48.2 |
47.0 |
46.8 |
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12/17/99 |
48.3 |
47.7 |
47.6 |
43.6 |
44.4 |
46.2 |
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01/14/00 |
48.3 |
47.7 |
48.2 |
45.6 |
43.5 |
42.0 |
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02/25/00 |
47.5 |
47.3 |
47.2 |
43.8 |
41.5 |
39.5 |
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03/17/00 |
47.5 |
45.7 |
45.6 |
43.4 |
40.3 |
38.7 |
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04/28/00 |
48.1 |
47.1 |
47.6 |
43.3 |
39.4 |
38.2 |
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05/15/00 |
47.9 |
46.3 |
47.1 |
42.2 |
39.7 |
37.3 |
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06/29/00 |
47.2 |
47.3 |
47.1 |
41.4 |
38.5 |
36.0 |
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Table 3. Hunter-a values (redness) for indicated storage temperatures and moisture content.
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|
4.4°C (40°F) |
26.7°C (80°F) |
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Date |
14% |
16% |
18% |
14% |
16% |
18% |
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10/18/99 |
4.5 |
4.5 |
4.5 |
4.5 |
4.5 |
4.5 |
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11/05/99 |
4.5 |
4.5 |
4.6 |
4.8 |
5.1 |
5.2 |
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12/17/99 |
4.7 |
4.9 |
4.9 |
6.6 |
6.1 |
5.6 |
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01/14/00 |
4.8 |
4.9 |
4.9 |
6.0 |
6.5 |
7.2 |
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02/25/00 |
4.9 |
5.0 |
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