Hyacinth Beans

By

Paula Szilard


Some of you may have seen the hyacinth beans in hanging pots when you were at my home in July. Americans know the hyacinth bean as an ornamental leguminous vine with a profusion of pinkish purple blossoms and shiny purple pods.  Yet to people in the arid tropics this bean is a valuable source of food for livestock and humans alike.  Here, in contrast, many people persist in the erroneous belief that these beans are poisonous.


Colorado gardeners have learned to appreciate the hyacinth bean’s ability to withstand searing temperatures and drought once established.  Though potentially a short-lived perennial in warm climates, it is often grown as an annual or biennial in an astonishing variety of soils from clay to sandy loam.  It tolerates pH levels of 5.9-7.8 (acidic to alkaline). The one thing it does not tolerate is being waterlogged.


There are two major types of hyacinth beans (Dolichos lablab or Lablab purpureus):  A bush bean, planted as a field crop and the bean with the vining habit, which can have, green foliage, green pods and white blossoms or the more familiar purple leaves, purple pods and purplish pink blossoms.  The field variety is mainly grown for its seeds, which are used principally as a fodder for livestock.


The hyacinth bean was once thought to have originated in the Indian subcontinent, where it has been an important crop for thousands of years. However, researchers now think it is more likely to have come from tropical east Africa, where it has also been an important food source.


The vining garden variety responds to a soil enriched with composted manure and needs a bit more water than the field crop.  The immature pods are harvested continuously, like snap beans and are eaten as a green vegetable.  The mature, dried beans are also eaten.  The small seeds are rounded and have a protruding white hilum that runs along 1/3 of the circumference of the seed.  In India, like many leguminous seeds, they are split to reduce cooking time.  At the Honolulu Community Gardens where I once gardened, some gardeners kept their plants productive for years.  There people seemed to favor the green podded variety for cooking like snap beans.  I was unaware of anyone eating mature seeds.


The immature pods are high in water and therefore low in calories.  They are also high in vitamins A and C.  The dried beans have a nutrient profile not unlike kidney beans.  They are very low in fat, moderately high in protein, and high in iron.


Like most leguminous food plants, hyacinth beans have a few undesirable characteristics.  Among them are tryspsin inhibitors, also found in soybeans and mature common beans, like kidney beans.  These substances prevent the complete digestion of dietary proteins unless they are deactivated by heat.  Another “antinutrient” hyacinth beans share with soybeans and common beans is a class of compounds called haemagglutinins, which cause clumping of red blood cells and which are likewise deactivated by heat.  The haemagglutinins are serious business.  They consist of many compounds, specific to particular bean varieties.  Laboratory studies on rats fed purified haemagglutinin extracts from raw black beans have shown the animals lost weight and died within several weeks.


Many people suffer from flatulence as a result of eating beans and have learned to sprinkle them generously with Beano and introduce them into their diet gradually.  The oligosaccharides responsible for this unruliness are stachyose and raffinose, carbohydrates that our upper gastrointestinal tracts cannot digest.  When these carbohydrates reach the colon, they are gobbled up by anaerobic bacteria, which in turn release the gas.  It seems most beans are well supplied with these oligosaccharides, including hyacinth beans.


Hyacinth beans also contain compounds called cyanide-releasing glycosides.  Our bodies convert these into hydrocyanic acid, a cellular poison.  These compounds are not present in the green-podded beans in appreciable quantities.  The purple-podded varieties, on the other hand, seem to contain a substantial amount of this toxin. 


Now, finally for the good news!  To survive over the millennia, humans simply could not afford to be fussy eaters, so they used their ingenuity and developed very simple techniques to make such foods wholesome.


Let us start with the red blood cell clumping haemagglutinins.  You only need to cook the immature pods and the dried seeds to inactivate these substances.  Thorough cooking, most successfully under pressure also eliminates the hydrocyanic acid.  As for the oligosaccharides, the USDA has developed a method, which it says eliminates 90% of the oligosaccharides in beans:  Boil the beans for 10 minutes in 5-10 cups of water per 2 scant cups of beans.  Allow them to cool and soak for 24 ours at room temperature.  Discard the soaking water, rinse the beans and finish cooking.  Personally, I consider this a lot of trouble, so I soak them for at least 8 hours or overnight, changing the water once or twice during this period.  I then pressure cook them for roughly 10 minutes, depending on the type of bean.


The trypsin inhibitors are virtually eliminated by heat, either dry roasting or cooking in water.  This is particularly true of the green pods, which have only 1/3 as much of these substances as the dry beans.

What is the bottom line?  Forget eating the dry beans.  Most of us cannot grow enough to do anything with them anyway.  If you do plan to eat your hyacinth beans, then plant the green-podded variety.  Most importantly, never eat the immature pods raw.  In addition, remember to cook them thoroughly!


Sources:

Committee on Food Protection, Food and Nutrition Board, National Research Council.  Toxicants Occurring Naturally in Foods.  Wash., D.C.:  National Academy of Sciences, 1973.


Deka, R. K.  and Sarkar C.R.  1990  “Nutrient composition and antinutritional factors in Dolichos lablab beans.”  Food Chemistry 38 (4) 239-246.


Devaraj, V.R.  and Manjunatha, N.H.  1995  “The effect of cooking on proteinase inhibitors of Dolichos lablab beans.” Plant Foods in Human Nutrition 42(2): 107-112. (abstract only)


Duke, James A.  Handbook of Legumes of Economic Importance.  New York:  Plenum, 1981.


Ensminger, A. H., et al, eds.  Foods and Nutrition Encyclopedia. 2nd ed.  Boca Raton: CRC Press, 1994.  2 vols.


Purseglove, J.W.  Tropical Crops:  Dicolyledons.  v.2 .  London, Longmans,

1968.


Vijayakumari, K. et al. 1995. “Effects of various hydrothermal treatments on certain antinutritional compounds in the seeds of the tribal pulse, Dolichos lablab, var. vulgaris L.”  Plant Foods in Human Nutrition 48(1) 17-29: 17-29.  (abstract only)


Author’s Note:  I wrote an earlier version of this article 10 years ago when I saw a piece by a local garden writer, which stated that hyacinth beans are poisonous.  I knew from my experience in Hawaii that people were eating them without ill effects, so I did the required research and came up with this piece, which looks at the individual antinutrients and toxic substances contained in this bean.