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Quality Factors of Green Frosted Soybeans


Duane R. Berglund, NDSU Extension Agronomist

Every year, the question surfaces of what to do about green soybeans and/or green soybean oil if an early frost occurs. This year an early frost did occur and probably will occur again before the soybean crop is fully mature and safe.

The following should answer some of the usual questions and provide some options:

The green color, of course, comes from chlorophyll or chlorophyll-like compounds and relates to immaturity of the harvested soybeans. If harvested soybeans are compared to normal soybeans, the greenish color is quite apparent visually.

If the soybeans are only slightly green, there is a good possibility that drying and storage will fade out the green. On the other hand, slightly green soybeans give the refiner the most problems because the green color in the oil may be masked by the red and yellow. During normal processing, the red and yellows are reduced and the greens then may become apparent and are, of course, undesirable.

For the refiner, it is strongly suggested that all incoming oils be routinely analyzed for chlorophylls before processing. Once detected it is rather easy to simply increase the dosage of activated earth in the bleaching step to remove it. Activated carbon also can be used but it is more expensive and absorbs more oil than acid-activated earth.

To our knowledge, there is no way yet devised to remove the green pigments from crude oil per se.

There are discounts for green soybeans and these stem from the refining problems stated above. If soybeans do not lose the green color in storage, then soybeans destined for extraction inevitably will be discounted.

Other alternatives than extraction, such as utilization as full-fat soybean meal either by roasting or extrusion, should be considered to avoid discounts. The green color should have no impact on feeding values if the soybeans are normal in composition otherwise.

Seed quality is another issue with frosted beans. Usually the germination and seedling vigor is reduced to such a degree that saving it for seed is discouraged. One would always want to have seed tested for its viability and seedling vigor if its origin was an early-frosted soybean field.